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OS1.9

Observations and model simulation illustrate significant ocean variability and associated air-sea interactions from regional to global scale and on diurnal to inter-annual time scales. This session is devoted to the understanding of the tropical and subtropical ocean dynamics, its interaction with the overlying atmosphere from the equator to mid-latitudes and its climate impacts on adjacent to remote areas.
Relevant processes in the ocean include upper and deep ocean circulation, mild SST gradients to sharp fronts, eddies, filaments, tropical instability waves, warm pools, cold tongues and eastern boundary upwellings. Furthermore, we are interested in air-sea interactions related to both the seasonal cycle and the development of modes of variability from local to basin scale. Wind variations related to Madden-Julian Oscillation, cyclones, and convective systems, as well as those leading the air-sea coupled modes (e.g., the Meridional Mode and Atlantic Niño) are welcome. Finally, we also seek contributions examining the causes and impacts of systematic model errors in simulating the local to regional climate.
Studies based on direct observations, reanalysis, reconstructions as well as model simulations are welcome.

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Co-organized by AS2/CL2
Convener: Peter Brandt | Co-conveners: Alban Lazar, Marie-Lou BacheleryECSECS, Noel Keenlyside, Marta Martín-ReyECSECS, Teresa Losada, Ingo Richter
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| Mon, 04 May, 14:00–18:00 (CEST)

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Chat time: Monday, 4 May 2020, 14:00–15:45

D2757 |
EGU2020-5390
Shengping He and Tore Furevik

A fascinating character of the Arctic summer atmospheric circulation is the anomalous anticyclone circulation centered over the Arctic Ocean. Previous studies have related the underlying mechanisms to the atmospheric internal variability, the earlier spring Eurasian snowmelt, and the tropical Pacific forcing. Here we show that the Arctic summer anomalous anticyclone circulation is strongly associated with positive sea surface temperature anomalies (SSTAs) at midlatitudes of the extratropical North Pacific which are surrounded by significant negative SSTAs, resembling the negative phase of the Pacific Decadal Oscillation (PDO) but without significant signals in the tropics. The numerical experiments from the Whole Atmosphere Community Climate Model, prescribed with negative PDO-like SSTAs from May to August with the influence of El Niño-Southern Oscillation being reduced in advance, have simulated the observed positive air temperature and geopotential height anomalies over the Arctic and the circumpolar easterly anomaly at high latitudes in summer. The observational and simulated results strongly suggest that the extratropical SSTAs can influence the Arctic atmospheric temperature and circulation in summer.

D2758 |
EGU2020-20544
Nour-Eddine Omrani, Fumiaki Ogawa, Hisashi Nakamura, Noel Keenlyside, Sandro Lubis, and Katja Matthes

Semi-idealized Atmospheric General Circulation-Model (AGCM) experiments are used, in order to study the different aspects of the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation that are maintained by the North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs). Here we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport in agreement with the Bjerknes compensation. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. It is shown that the impact of OWBCs on northern hemisphere circulation has significant implication for stratosphere/troposphere dynamical coupling, time-scales on the NAM, frequency of Sudden stratospheric warming and potential formation of polar stratospheric clouds.

 

Reference:

Omrani et al., 2019: Key Role of the ocean Western Boundary currents in shaping the Northern Hemisphere climate, Scientific Reports, https://doi.org/10.1038/s41598-019-39392-y

 

D2759 |
EGU2020-12639
| solicited
Fei Zheng, Renping Lin, and Xiao Dong

Using observational data and the pre-industrial simulations of 19 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), the El Niño (EN) and La Niña (LN) events in positive and negative Pacific Decadal Oscillation (PDO) phases are examined. In the observational data, with EN (LN) events the positive (negative) SST anomaly in the equatorial eastern Pacific is much stronger in positive (negative) PDO phases than in negative (positive) phases. Meanwhile, the models cannot reasonably reproduce this difference. Besides, the modulation of ENSO frequency asymmetry by the PDO is explored. Results show that, in the observational data, EN is 300% more (58% less) frequent than LN in positive (negative) PDO phases, which is significant at the 99% confidence level using the Monte Carlo test. Most of the CMIP5 models exhibit results that are consistent with the observational data.

D2760 |
EGU2020-7603
Dirk Olonscheck, Maria Rugenstein, and Jochem Marotzke

A realistic representation of sea surface temperature (SST) patterns in climate models is important for constraining local climate change and estimates of climate sensitivity (Gregory et al, 2016; Zhou et al, 2016; Armour, 2017; Marvel et al, 2018; Andrews et al, 2018). However, it is debated whether global climate models are capable of simulating the observed local SST patterns (Zhou et al, 2016; Coats et al, 2017; Marvel et al, 2018; Kostov et al, 2018; Seager et al, 2019). Using seven single-model initial condition ensembles with 30-100 ensemble members and the two multi-model ensembles CMIP5 and CMIP6, we here show that observed and simulated regional trends in SST patterns are consistent when accounting for internal variability. Some individual ensemble members simulate SST trend patterns that resemble the observed patterns in large areas across different basis. We find that observed and simulated SST trends are also consistent in critical regions such as the Southern Ocean, the North Atlantic, and the equatorial Pacific east-to-west SST gradient. Observed regional trends that lie at the outer edge of the models' internal-variability range allow two non-exclusive interpretations: a) observed trends are unusual realizations of the Earth's possible behavior and/or b) the models are systematically biased but large local variability leads to some good matches with the observations. Furthermore, we find that the large internal variability influences the existing range of SST trends more strongly than differences in the model formulation or in the observational data set.

 

References:

Andrews, T., Gregory, J. M., Paynter, D., Silvers, L. G., Zhou, C., Mauritsen, T., Webb, M. J., Armour, K. C., Forster, P. M., & Titchner, H. (2018). Accounting for Changing Temperature Patterns Increases Historical Estimates of Climate Sensitivity. Geophysical Research Letters, 45 (16), 8490–8499.

Armour, K. C. (2017). Energy budget constraints on climate sensitivity in light of inconstant climate feedbacks. Nature Climate Change, 7, 331–335.

Coats, S., & Karnauskas, K. B. (2017). Are Simulated and Observed Twentieth Century Tropical Pacific Sea Surface Temperature Trends Significant Relative to Internal Variability? Geophysical Research Letters, 44 (19), 9928–9937.

Gregory, J. M., & Andrews, T. (2016). Variation in climate sensitivity and feedback parameters during the historical period. Geophysical Research Letters, 43 (8), 3911–3920.

Kostov, Y., Ferreira, D., Armour, K. C., & Marshall, J. (2018). Contributions of Greenhouse Gas Forcing and the Southern Annular Mode to Historical Southern Ocean Surface Temperature Trends. Geophysical Research Letters, 45 (2), 1086–1097.

Marvel, K., Pincus, R., Schmidt, G. A., & Miller, R. L. (2018). Internal Variability and Disequilibrium Confound Estimates of Climate Sensitivity From Observations. Geophysical Research Letters, 45 (3), 1595–1601.

Seager, R., Cane, M., Henderson, N., Lee, D.-E., Abernathey, R., & Zhang, H. (2019). Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases. Nature Climate Change, 9 (7), 517–522.

Zhou, C., Zelinka, M. D., & Klein, S. A. (2016). Impact of decadal cloud variations on the Earth’s energy budget. Nature Geoscience, 9 (12), 871–874.

D2761 |
EGU2020-146
| Highlight
Katherine Lisbeth Ccoica López, Ricardo Hallak, and Victor Raúl Chavez Mayta

The Tropical Pacific and Tropical Atlantic Ocean modulate the interannual precipitation over the Amazon region and the decadal and interdecadal variation as well. During El Niño Southern Oscillation (ENSO), below-average rainfall is recorded in the North and Northeast of the Basin, while deficit of precipitation is observed in the West and South. On the other hand, during La Niña years, rainfall is above of normal in the North and Northeast of Amazon Basin. However, there are also drought events, such as in 1964 and 2005, unrelated to the El Niño event, but influenced by warm conditions in the Tropical North Atlantic. In fact, the exceptional drought recorded in 2010 was influenced by a combined effect of the El Niño event during the peak of rainy season, followed by warm conditions in the Tropical North Atlantic during final of rainy season and dry season.

Therefore, the main aim of this study is exploring the Atlantic Sea Surface Temperature (SST) condition in modulating patterns that influence the development of drought and flood events in the Amazon Basin. First of all, the Atlantic Ocean is divided into Tropical North Atlantic (TNA), Tropical South Atlantic (TSA) and Subtropical South Atlantic (STSA), to analyze the behavior of each region separately. Atlantic Index, in each region, is the first principal component (PC1) time series, which comes from the empirical orthogonal function (EOF) analysis applied to Hadley Center Global Sea Ice and Sea Surface Temperature (HadISST) dataset for the 1870-2107 period. The Tropical North Atlantic, Tropical South Atlantic and Subtropical South Atlantic indices show the main years when drought and flood events reaching the Amazon Basin (droughts in 2005, 2010 and 2015, and floods in 2009 and 2012, mainly), and 5-years moving correlations indicate that these three ocean basin have been coupled and decoupled periodically each other in the last century.

The equatorial Pacific, North Atlantic and South Atlantic indices were also correlated with rainfall over the Amazon for three databases: the Tropical Rainfall Mission Measurements (TRMM), the Global Precipitation Climatology Centre (GPCC) and the HyBAm Observed Precipitation. All three databases showed the same results. An increase of the SST in Eastern Pacific influences in low precipitation over the central and west of the Amazon Basin during the rainy season (December to February), increase of the SST in Central Pacific influences for droughts over the northeast region and the TSA influences in the central Amazon. Increase of the SST in TNA and STSA influences mainly in the dry season (May to September), intensifying it. TNA is responsible for precipitation below normal over the central and west Amazon Basin, while STSA only influences in the central region of the basin. Finally, analysis of extreme events indicate that droughts and floods in the Amazon are intensified (de-intensified) if we consider warm (cold) phases of the AMO (Atlantic Multidecadal Oscillation) and the PDO (Pacific Decadal Oscillation).

D2762 |
EGU2020-10222
Wonsun Park, Mojib Latif, and Arielle Stela Imbol Nkwinkwa Njouodo

Mean state and internal variability in the tropics are crucially linked to air-sea interactions. State-of-the-art climate models exhibit long-standing problems not only in simulating tropical mean climate, such as too cold sea surface temperature (SST) over the central tropical Pacific and too warm SST over the eastern tropical Pacific and Atlantic, but also with respect to seasonal and longer variability. These biases question the credibility of future climate projections with the models, and it has not been shown to date whether or how such SST biases affect the projections. Here we focus on the tropical Atlantic (TA) and investigate how the mean state influences climate projections over the region.

We use two versions of the Kiel Climate Model (KCM) in global warming simulations, in which only atmosphere model resolution differs: one version carries ECHAM5 with a horizontal resolution of T42 (~2.8°) and 31 vertical levels, and the other ECHAM5 with a horizontal resolution of T255 (~0.47°) and 62 levels. Although only the atmospheric resolutions differ, the two KCM versions exhibit very different mean states over the tropical TA, with the higher-resolution version, among others, featuring much reduced warm SST bias over the eastern basin.

The response to increasing atmospheric carbon dioxide levels is found to be sensitive to the mean state. The model employing high atmospheric resolution and featuring a small SST bias projects an eastward-amplified SST warming over the TA, consistent with the pattern of interannual SST variability simulated under present-day conditions and in line with the observed SST trends since the mid-20th century. The model employing low-resolution and exhibiting a large SST bias projects more uniform SST change. Atmospheric changes also vastly differ among the two model versions.

Analysis of models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) support the KCM’s results: models with small SST bias project stronger warming over the eastern TA, while models with large SST bias either project uniform warming across the equator or largest warming in the west. This study suggests that reducing model bias may enhance global warming projections over the TA sector.

D2763 |
EGU2020-7848
Hyacinth Nnamchi, Mojib Latif, Noel Keenlyside, and Wonsun Park

Although the globally averaged surface temperature of the Earth has considerably warmed since the beginning of global satellite measurements in 1979, a warming hole, with hardly any surface warming that is most pronounced in boreal summer, has been observed in the equatorial Atlantic region during this period. The warming hole occurs in an extended area of the equatorial Atlantic that includes the cold tongue, the region of locally cooler ocean surface waters that develops just south of the equator in boreal summer, partly reflecting the upwelling of deep cold waters by the action of the southeasterly trade winds. This lack of surface warming of the cold tongue denotes an 11% amplification of the mean annual cycle of the sea surface temperature during the satellite era. The warming hole is driven by an intensification of the equatorial upwelling of cold waters into the ocean surface layers and damped by the surface heat flux. In observations, the tendency for surface cooling appears to reflect intrinsic variability of the climate system and is not unusual during the instrumental period. The warming hole is associated with wind-induced ocean circulation changes to the south and north of the northward of the equator. Coupled model ensembles forced by the observed varying concentrations of atmospheric greenhouse gases and natural aerosols as well as unforced runs were analyzed. The ensembles suggest a strong role for atmospheric aerosols in the warming hole. However, although aerosols can cause a cooling of the cold tongue, intrinsic climate variability as represented in the unforced experiment can potentially cause larger cooling than has been observed during the satellite era. This study highlights the difficulty in reconciling observations and the climate models for the attribution of the warming hole.

D2764 |
EGU2020-499
Chloé Prodhomme, Javier García-Serrano, Noel Keenlyside, Eleftheria Exarchou, Ingo Richter, and Aurore Voldoire

The Atlantic Niño is the leading mode of interannual variability in the Tropical Atlantic, which has impacts not only on the African monsoon but also in remote regions. In the present study, we investigate the predictability of the Atlantic Niño's mature phase (June-July) at seasonal time-scale, as well as its conditioning. We analyze a large set of state-of-the art forecasts systems from the North American Multi-Model Ensemble (NMME) and Copernicus Climate Change Service (C3S) multi-models. The prediction skill of the ATL3 index has considerably improved as compared to previous forecast quality assessments, with Anomaly Correlation Coefficient (ACC) reaching up to 0.8 for the May start date. Most of the models achieve skillful prediction of the Atlantic Niño from the May start-date, and some outperform persistence. For the start-dates of April, March and February, most of the models perform better than persistence and some achieve significant correlation skill for ATL3. While there has been improvement in forecasting capability, overall the warm SST bias and associated drift remain large in the equatorial Atlantic in most of the systems. Our results suggests that the skill in predicting the Atlantic Niño in summer is weakly related to the local SST drift during the first month of the forecast, but not to the magnitude of the SST bias during the peak. In addition, we find evidence that the skill in the equatorial Atlantic is related to the ability of the models to properly represent the large-scale atmospheric circulation pattern in the South Atlantic (i.e. St. Helena high).

D2765 |
EGU2020-2172
Serena Illig, Marie-Lou Bachèlery, and Joke Lübbecke

We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. It is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds. Yet, we observe that coastal SST anomalies off Angola tend to precede the ones in the equatorial cold tongue region by ~1 month.

We explain this counter-intuitive behavior using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we simulate warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor its seasonal variability controls the timing between events. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños.

Both warmings originate from a reduction in the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern equatorial SST. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely-forced coastal warming prematurely.

In conclusion, this study shows evidence that Atlantic and Benguela Niños are connected via an ocean teleconnection associated with equatorial and coastal wave propagations, but they are also tied by a large-scale atmospheric circulation and ocean-atmosphere interactions.

D2766 |
EGU2020-11198
Audrey Delpech, Claire Ménesguen, Frédéric Marin, Sophie Cravatte, and Yves Morel

The deep tropical ocean circulation is dominated by systems of vertically and meridionally alternating zonal jets, known as the Equatorial Deep Jets (EDJs) and Extra-Equatorial Jets (EEJs) respectively. The energy sources and physical mechanisms responsible for this circulation are still poorly understood. Recent studies have suggested the importance of intra-annual equatorial waves to transfer their energy to the EDJs.

In this study, we use idealized numerical simulations forced with a wave-like surface momentum flux to investigate how intra-annual variability can be relevant to the formation of the EEJs. It is shown that the amplitude of the jets, their meridional scales and their vertical and latitudinal extensions are sensitive to the period and wavelength of the forced wave. Short intra-annual waves with periods around ~70 days and wavelength ~300 km are found to reproduce the observed circulation most realistically. Focusing on the dominant barotropic mode, the underlying physical processes are detailed. A spectral analysis reveals that the energy transfer between the forced waves and the jet-structured circulation is compatible with a decay instability occurring in waves triadic interactions.

In parallel, a statistical analysis is performed on observations of the 1000m-velocities inferred from Lagrangian Argo floats drifts to document the amplitude and scales of the deep intra-annual variability in the tropical Pacific and Atlantic oceans. It gives evidence for the presence of short intra-annual waves that share common properties with the most unstable waves found for the EEJ generation.

D2767 |
EGU2020-1961
Kelvin Richards, Andrei Natarov, and Yanli Jia

Our focus is the way various scales of motion in the tropical ocean are linked through mixing and its modification by larger scales. Enhanced mixing caused by small vertical scale features (SVSs) in the equatorial thermocline is known to impact the state of the ocean and its interaction with the atmosphere, in particular the sea surface temperature of the Pacific cold tongue and ENSO variability. The SVSs are produced by wind variability (from, for instance, the MJO) and instabilities, with an equatorial enhancement caused by a combination of factors including the characteristics of the forcing and propagation of internal waves and near-equator inertial and sub-harmonic parametric instabilities. Numerous scale interactions are at play. For instance, an eastward extension of the warm(fresh) pool in the western tropical Pacific, typical under El Niño conditions, stratifies the upper ocean. This stratification can produce a dramatic decrease in the downward propagation of wind-generated inertia-gravity waves and a decrease in the mixing in the main thermocline. The associated changes to the thermocline are advected to the east and impact the eastern cold tongue and hence the coupling with the atmosphere. Using a combination of observations and models we investigate the properties of SVS activity, its impact on mixing, and interaction with larger scales. Of particular interest is the dependency on stratification, the spatial and temporal variability of wind forcing, the impact on larger scales, and the resolution of both observations and models. The good news is that with enough resolution the relevant scales can be captured in both observations and models.

D2768 |
EGU2020-12170
Kevin Lamb, Peter Brandt, and Marcus Dengler

The Angolan and Peruvian shelves are located in upwelling regions along the eastern boundaries of the tropical Atlantic and Pacific Oceans. They are sites of important fisheries supported by high productivity which is driven by fluxes of nutrients from deep to near surface water along the coast. Mixing associated with internal waves is believed to play a role in this process. Recent field observations have shown the presence of an active internal wave field that includes internal solitary waves. In this talk results of high-resolution two-dimensional simulations of internal wave generation by tide-topography interactions on the Angolan and Peruvian shelves are presented. The simulations show the generation of internal wave beams at near-critical slopes and the generation of high-frequency internal solitary waves. The high-frequency IW spectrum is enhanced when small scale bathymetric ripples are included. Wave generation during winter and summer stratifications will be compared.

D2769 |
EGU2020-20036
| Highlight
Alice Pietri, François Colas, Rodrigo Mogollon, Dante Espinoza-Morriberón, Adolfo Chamorro, Jorge Tam, and Dimitri Gutiérrez

Rapidly developing extreme events such as anomalously warm water events, known as Marine Heat Waves (MHWs), have received considerable attention in the past few years due to the significant impact they have on regional ecosystems and socioeconomic activity. The Peruvian Coastal Upwelling System (PCUS), one of the most productive ecosystem in the world in terms of fisheries, is highly exposed to climate variability in particular because of its geographic location close to the equator, and the influence of the subtropical high pressure cell variability.

The PCUS is highly influenced by El Niño events, which have been intensively studied, and whose variability is related to the longest and most intense MHWs in the region. However the very visible El Niño events probably overshadowed the MHWs of shorter duration that also have an important impact on the coastal environment as they can often go with other extreme events such as nearshore hypoxia. To date, a census of MHWs of shorter duration (less than 30 days) is lacking in the region.

Here, we investigate the characteristics (spatial variability, frequency, intensity and duration) and evolution of such MHWs in the South Tropical Eastern Pacific, with a focus on the PCUS coastal area where the ecological vulnerability is higher. Several sea surface temperature satellite products are compared to test the sensitivity of the results.

The distinction between El Niño events and regular MHWs has a major impact on the statistical distribution of MHWs properties in the South Equatorial and South Tropical Eastern Pacific as well as on their evolution over the last 35 years. First results indicate that in the equatorial region and along the Peruvian coast, fewer MHWs and of shorter duration are observed north than south of 15°S. The observed trend is an increase of MHWs occurrences, duration and intensity in the South Tropical Eastern Pacific over the last 35 years, with the exception of the coastal region off Peru where the trend in occurrences and duration is the same but the average temperature anomaly associated to MHWs has decreased. It also seems that there is no apparent preferential season for the occurrence of MHWs. A study of the possible drivers is performed in an attempt to disentangle the role of the local (wind stress, heat fluxes) and remote (equatorial wave activity) forcing.

D2770 |
EGU2020-21190
Martin Schmidt, Hadi Bordbar, Fernanda Nascimento, and Claudia Frauen

High resolution regional ocean circulation models are needed to investigate regional ecosystem dynamics. However, these models may suffer from biases due to shortcomings in reanalysis datasets like NCEP or ERA-Interin, that have traditionally been used as atmospheric forcing. More realistic results can be achieved by replacing the reanalysed wind with scatterometer based winds. However, inconsistencies between different scatterometers like ASCAT and QuikSCAT introduce new uncertainty, which prevents a discussion of long-term trends in these models. The ERA-5 reanalysis offers a new consistent data set to force highly resolving regional ocean models. Based on such a simulation we analyse trends and anomalies in poleward currents in the Eastern Boundary Current off Southern Africa and Northern Benguela upwelling intensity due to changing wind stress and wind stress curl. Model results are validated with remote sensing as well as shipborne and mooring data. Further, variability of oxygen conditions in the Northern Benguela and the Angola Gyre oxygen minimum zone is discussed. 

D2771 |
EGU2020-846
João Bettencourt and Carlos Guedes Soares

The Azores Current-Front system coincides with the northern limit of the subtropical gyre in  the Eastern North Atlantic. The mean zonal jet is positioned south of the Azores archipelago  and extends from west of the mid-atlantic ridge to the Gulf of Cadiz, where it partially  turns south. North of the main jet, a sub-surface counter-current is found, flowing westwards. The associated thermal front separates the warm subtropical waters from the colder subpolar waters. The instantaneous flow in the Azores Current/Front system is characterized by the presence of meandering currents with length scales of 200 km that regularly shed anticyclonic warm water and cyclonic cold water eddies to the north and south of the mean jet axis, respectively, due to vortex stretching and the planetary beta effect. The time scale of eddy shedding is 100-200 days. On the meandering arms of the current, downwelling 
and upwelling cells are found and sharp thermal gradients are formed and a residual poleward heat transport is observed. The instability cycle that originates the mesoscale meanders and the eddies is well-known from quasi-geostrophic and primitive equation models initialized from a basic baroclinic state: a first phase of baroclinic instability feeds on available potential energy to raise eddy kinetic energy levels, that, in a second phase feed the mean kinetic energy by Reynolds stress convergence. The cycle repeats itself as long as the APE reservoir is filled at the end of each cycle.

However, seasonal variability of the zonal jet dynamics has not been addressed before and it can provide valuable insights in to the variations of the Eastern North Atlantic between the subtropical and subpolar gyres. We use a primitive equation regional ocean model of the Eastern Central North Atlantic with realistic climatological wind and thermal forcing to study the yearly cycle of meandering, eddy shedding and restoration of the mean jet in the Azores/Current system. We observe an semi-annual cycle in the jet's kinetic energy with maxima in Summer/Winter and minima in early Spring/Autumn. Potential energy conversion by baroclinic instability occurs throughout the year but is predominant in the first half of the year. The mean kinetic energy draws from the turbulent kinetic energy through Reynolds stress convergence in periods of 50 - 100 days, that are followed by short barotropic instability periods. During Winter, Reynolds stress convergence, and thus mean jet reinforcement from the mesoscale eddy field, occurs along the jet meridional extent, in the top 500 m of the water column, but from Spring to Autumn it is observed only in the southern flank of the mean jet axis.

D2772 |
EGU2020-11183
Tina Georg, Maria C. Neves, Paulo Relvas, and Kate Malmgren

Sea surface temperature differences between coastal and offshore waters and Ekman transport inferred from the wind velocity have been used to construct upwelling indices. Those indices have been widely used in climatological studies. In the present research we look to the upper layer structure of the ocean, down to 500 m depth, to infer relations between climate and the upwelling regimes. In particular, we explore the links between climate variability and the three-dimensional spatial structure of the upwelling activity along the Canary Current Upwelling System (CCUS) sector limited to 25-35° N, where upwelling is permanent, but intensified during the summer. The vertical structure of the CCUS is studied using vertical profiles of temperature, salinity, density and spiciness from the World Ocean Atlas (WOA). Monthly grids are retrieved for the past 30 years and vertical profiles exported at selected locations. The aim is to identify inter-annual and seasonal changes in the thermocline and the mix layer depth and link them to the upwelling characteristics. We then relate periods of strong upwelling with large-scale modes of climate variability, namely the North Atlantic Oscillation (NAO) and Eastern Atlantic pattern (EA). Time series of winter composites of NAO and EA are separated into positive and negative phases and their signatures quantified through composites of SST, salinity and density. The results provide the first assessment of inter-annual variability of the Canary upwelling current at both the surface and throughout depth and contributes towards understanding the connection between the vertical ocean structure and the large-scale climate modes. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

D2773 |
EGU2020-1433
Geidy Rodríguez-Vera, Rosario Romero-Centeno, Christopher L. Castro, and Víctor Mendoza Castro

This work describes dominant patterns of coupled interannual variability of the 10-m wind and sea surface temperature in the Caribbean Sea and the Gulf of Mexico (CS&GM) during the period 1982–2016. Using a canonical correlation analysis (CCA) between the monthly mean anomalies of these fields, four coupled variability modes are identified: the dipole (March–April), transition (May–June), interocean (July–October), and meridional-wind (November–February) modes. Results show that El Niño–Southern Oscillation (ENSO) influences almost all the CS&GM coupled modes, except the transition mode, and that the North Atlantic Oscillation (NAO) in February has a strong negative correlation with the dipole and transition modes. The antisymmetric relationships found between the dipole mode and the NAO and ENSO indices confirm previous evidence about the competing remote forcings of both teleconnection patterns on the tropical North Atlantic variability. Precipitation in the CS and adjacent oceanic and land areas is sensitive to the wind–SST coupled variability modes from June to October. These modes seem to be strongly related to the interannual variability of the midsummer drought and the meridional migration of the intertropical convergence zone in the eastern Pacific. These findings may eventually lead to improving seasonal predictability in the CS&GM and surrounding land areas.

D2774 |
EGU2020-6635
Zhichun Zhang and huijie Xue

            Based on a nonlinear reduced gravity model simulation, formation cause of Subtropical Countercurrent(STCC) in the Pacific Ocean are investigated. The model reproduces well the characteristics of circulation of thermocline in the North pacific Ocean. The results suggest that the variation of the west boundary topography, especially the witdh of the luzon strait, play a key role on the formationg of STCC as well as the wind sress meridional gradient. When the witdh of the luzon strait gradually decrease, the STCC increase . the model results also reveal that the wind stress dipole curl of west ot the hawaii islands is key to the HLCC formation.

D2775 |
EGU2020-2333
Yilong Lyu

Mooring measurements at ~140°E in the western equatorial Pacific documented greatly intensified eastward subsurface currents, which largely represents the nascent Equatorial Undercurrent (EUC), to ~67 cm s-1 in boreal summer of 2016. The eastward currents occupied the entire upper 500 m, with the westward surface currents nearly diminished. Similar variations were also observed during previous El Niño events, as suggested by historical in-situ data. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140°E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along New Guinea coast. A 1.5-layer reduced-gravity ocean (RGO) model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments are performed to understand the underlying dynamics. The observed circulation anomalies are largely the delayed ocean response to the strong equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Niño (September-April). Downwelling equatorial Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly wind anomalies in the eastern Pacific. Upon reaching western Pacific, the Southern Hemisphere lobe of Rossby waves encounter the slanted New Guinea island and deflects equatorward, establishing a local sea surface height maximum near the equator and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.

D2776 |
EGU2020-1759
Guidi Zhou and Xuhua Cheng

The decadal variability of the Kuroshio Extension (KE) is investigated using altimeter observations (AVISO) and the output of an ocean model (OFES). It is shown that the KE decadal variability is manifested in its strength, latitudinal position, and zonal extent, as well as the associated mesoscale eddy activity. Two differences between the two datasets are identified: (a) In OFES, the eddy activity positively correlates with the KE mode index when it leads by a few years, whereas in AVISO the two are negatively and concurrently correlated. (b) In OFES, the positive KE mode is associated with large meanders of the Kuroshio south of Japan, but in AVISO they are irrelevant. These differences indicate that the generation mechanism of KE's decadal variability is different in OFES and the real ocean. The sea surface height anomaly (SSHA) is then decomposed into major components including the wind-driven Rossby waves and residual (intrinsic) variability. The relationship between the two components are virtually the same in OFES and in AVISO, showing a negative correlation when the wind-driven part leads by a few years. Further diagnostics based on OFES reveals that the residual SSHA originates from the downstream region over the Shatsky Rise, slowly propagates westward, and is driven by eddy potential energy transfer. The OFES results partly conform to the intrinsic relaxation oscillation theory put forth by idealized model analyses, but in the latter the SSHA signal originates from the upstream Kuroshio. A new mechanism is then proposed for OFES: the decadal variability of the KE is first a result of the intrinsic relaxation oscillation probably excited by wind forcing, which regulates the strength of the KE’s inflow and thus modulates the downstream topography interaction, resulting in different downstream mesoscale eddy activity that further feeds back on the mean-flow. The mechanism for the real ocean is also reassessed.

D2777 |
EGU2020-6666
Sok Kuh Kang, Sebastian Landwehr, Eun Jin Kim, Kyeong Ok Kim, and Jae Hyoung Park

A summertime field survey in 2018 has been carried out to investigate the air-sea interaction over warm eddy in the Northwest Pacific. The strongest Supertyphoon Mangkhut in the Northwest Pacific got rapidly intensified from 80 kts to 120 kts during Sep.10, 2018, and it lasted as category 5 typhoon during Sep. 11 to 14. The maximum wind speed during intensification reached 100 kts on Sep.10. At distance 500 km off typhoon track the observations by sensors equipped on wave gliders were carried out in order to measure air-sea interaction parameters, as well as using instruments equipped on research vessel Isabu.

During the intensification of typhoon Mangkhut the enhanced bulk latent heat fluxes (LHF) were estimated over the several days. The latent heat flux was estimated from gust wind and 1 minute mean parameters. Peak LHF from gust wind reached over 1,100 W/m2, while 1 minute mean LHF reached about 900 W/m2. Data analysis reveals that the enhanced heat flux appears to exist due to an increase in moisture disequilibrium between the ocean and atmosphere. This supports the hypothesis that enhanced buoyant forcing from the ocean is likely to be an important mechanism in tropical cyclones over warm oceanic mesoscale eddy (Jaimes et al., 2016).

This work was supported by the project of “Study on air-sea interaction and process of rapidly intensifying typhoon in the Northwestern Pacific” funded by the Ministry of Ocean and Fisheries, Korea,

D2778 |
EGU2020-2033
Yang Mengzhou, Yuan Chaoxia, Li Wenmao, and Zhong Yahan

Using a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), South China Sea (SCS), adjacent land region (LD) and Pacific Ocean (PO) to the interannual summer precipitation variations in Southwest China (SWC) are investigated. Results show that on average, IO, SCS, LD, and PO contribute 46.8%, 25.3%, 21.8% and 2.3% of total moisture release in SWC in summer. In the above-normal precipitation summers, the moisture from IO and LD is increased by 48.2% and 28.8%, whereas that from SCS is decreased by 37.2%. In the below-normal precipitation summers, the moisture from IO and LD is decreased by 34.6% and 25.2%, while that from SCS is increased by 23.7%. In addition, the moisture anomalies from the four source regions can explain 85% of the total variances of the SWC summer precipitation.

The variations in the moisture from IO, SCS, and LD to SWC are not independent to one another and strongly influenced by the large-scale atmospheric circulation anomalies in the lower troposphere analogous to the Pacific-Japan (PJ) pattern and further studies showed that the PJ pattern was stimulated by the SST anomaly in the equatorial Atlantic. The anomalous warming in the tropical Atlantic that can modify the Walker circulation and introduce an anomalous descending over the central Pacific, thus inducing the anomalous anticyclone in the western North Pacific as the classical Matsuno-Gill response. The resultant suppressed precipitation in the western North Pacific excites the PJ pattern. The observed impacts of the tropical Atlantic SSTs on the atmospheric circulation can be well reproduced in an atmospheric general circulation model and the ability of the CMIP5 and CMIP6 models to reappear this relationship is verified, which will help the models to improve the simulation performance of summer large-scale circulation anomalies and precipitation in East Asia.

D2779 |
EGU2020-6293
Joong-Bae Ahn and Yeon-Woo Choi

This study investigates the relationship between the preceding late spring Sea Surface Temperature (SST) over the tropical Atlantic and the East Asian Summer Monsoon (EASM) based on the observational data and Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations. The results show that warm (cold) tropical Atlantic SST (TASST) during May tends to be followed by a strong (weak) EASM with positive (negative) precipitation anomalies over the subtropical frontal area. Evidence is also provided that the atmospheric teleconnections propagating in both east and west directions are the key mechanisms linking the EASM with the preceding May TASST. That is, the warm TASST anomaly during late spring can persist through the subsequent summer, which, in turn, induces the Gill-type Rossby wave response in the eastern Pacific, exciting the westward relay of the Atlantic signal, as well as the eastward propagation of the Rossby wave along the jet stream. Furthermore, the westward (eastward) propagating teleconnection signal may induce the anomalous anticyclone in the lower troposphere over the Philippine Sea (anomalous tropospheric anticyclone with barotropic structure over the Okhotsk Sea). The anomalous anticyclonic circulation over the Philippine Sea (Okhotsk Sea) brings warm and humid (cold) air to higher latitudes (lower latitudes). These two different types of air mass merge over the Baiu-Meiyu–Changma region, causing the enhanced subtropical frontal rainfall. To support the observational findings, CMIP5 historical simulations are also utilized. Most state-of-the-art CMIP5 models can simulate this relationship between May TASST and the EASM.

Reference: Choi, Y., Ahn, J. Possible mechanisms for the coupling between late spring sea surface temperature anomalies over tropical Atlantic and East Asian summer monsoon. Clim Dyn 53, 6995–7009 (2019) doi:10.1007/s00382-019-04970-3

Acknowledgment: This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2018-01213.

 

D2780 |
EGU2020-1751
Linfang Zhang, Yaokun Li, and Jianping Li

            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.

Chat time: Monday, 4 May 2020, 16:15–18:00

D2781 |
EGU2020-21343
Marie-Lou Bachelery, Serena Illig, and Mathieu Rouault

Several studies suggest that the mean atmospheric and oceanic features in south-eastern Atlantic have experienced changes over the last few decades with, in particular, a shift in the mean position of the Saint Helena Hight Anticyclone and an increase in the mean ocean stratification. Modification of the wind forcing and the mean state at the equator and along the south-western coast of Africa will most likely impact the characteristics of the eastward propagating interannual Equatorial Kelvin Waves (EKW) and subsequent Coastal Trapped Waves (CTW) in the south-eastern Atlantic. These changes will also affect the interannual variability in the Benguela Upwelling System, especially since the remote equatorial ocean dynamics is instrumental in the development of extreme warm and cold Benguela Niño/Niña events. The objective of this study is to document the low-frequency change in the characteristics (amplitude, duration and timing) of the interannual Benguela Niño/Niña events. Using model solutions and sensitivity experiments, we investigate the mechanisms that control the low-frequency modulation of the coastal interannual variability off the coasts of Angola/Namibia. Our results reveal that the decadal modulation of the interannual variability off the Angolan coast is controlled by change in the EKW activity. In the Southern Benguela, the modulation of the interannual is dominated by the influence of the local alongshore winds. However, periods during which the equatorial forcing is intensified, EKW propagate and imprint the oceanic variability off the coast of Namibia.

 

D2782 |
EGU2020-3039
Swantje Bastin, Martin Claus, Peter Brandt, and Richard J. Greatbatch

Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal jets that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models, most likely due to a lack of vertical resolution. However, when the vertical resolution is sufficiently high, EDJ oscillating at a period of about 4.5 years are present in idealised model configurations driven by steady wind forcing. We have used such a model for a basin-wide reconstruction of the intraseasonal eddy momentum flux convergence, which has recently been shown to be a large contributor to the EDJ maintenance (Greatbatch et al., 2018). When we apply the diagnosed momentum flux convergence that is oscillating at the EDJ period as forcing in a model without wind forcing, EDJ develop, allowing us to verify the mechanism proposed in Greatbatch et al. (2018). Additionally, we can isolate the nonlinear effect that the EDJ have on the time mean zonal flow at intermediate depths. We can show that the EDJ drive time mean zonal flow that is similar in structure to the mean flow measured by Argo floats at 1000 m depth, contributing (in our idealised setup) about one fifth of the total magnitude of the observed flow, the rest likely resulting from direct forcing by downward propagating intraseasonal waves as shown before in other studies.

 

References:

Greatbatch, R. J., M. Claus, P. Brandt, J.-D. Matthießen, F. P. Tuchen, F. Ascani, M. Dengler, J. Toole, C. Roth, J. T. Farrar, 2018: Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability. Geophysical Research Letters, 45, 1923-1929.

D2783 |
EGU2020-10934
Peter Brandt, Johannes Hahn, Schmidtko Sunke, Franz Philip Tuchen, Robert Kopte, Rainer Kiko, Bernard Bourlès, and Marcus Dengler

The upper ocean circulation of the tropical Atlantic experiences long-term changes associated with different climate modes, but is at the same time expected to adjust to changes in the meridional overturning forced by climate warming. While observations of decadal variability of the surface circulation are generally based on satellite altimetry, direct observations of subsurface circulation mostly rely on very few long-term mooring sites typically covering energetic currents such as the Atlantic Equatorial Undercurrent (EUC). Here we focus on the period 2006 to 2018 that was covered by an intense field program including oxygen and circulation observations in the equatorial and tropical North Atlantic. During the observational period, a strengthening of the EUC of about 20% was detected based on data of an equatorial current meter mooring at 23°W. The EUC strengthening is related to a similar strengthening of the subtropical cells (STC). These STC changes were forced by a trade wind intensification in both hemispheres, however, more pronounced in the north and in the western basin. The STC strengthening is found to be consistent with the observed 12-year oxygen increase in the equatorial band (i.e. south of about 5°N) in the upper 400m obtained from repeat ship sections along 23°W. Such strongly enhanced oxygen levels relative to climatological mean were also observed in the upper 300-400m during a recent cruise along the whole Atlantic equator from Africa to South America. Our results are discussed with regard to the superposition of internal climate variability likely associated to a recent phase shift in the Atlantic multidecadal variability and changes due to global warming including ocean deoxygenation and enhanced thermocline stratification.

D2784 |
EGU2020-5693
Angelo Rubino, Davide Zanchettin, Francesco de Rovere, and Michael J. McPhaden

On large (global and hemispheric) scales, sea surface temperature (SST) anomalies are considered to be good surrogates for marine air temperature (MAT) anomalies. Here we investigate how MAT and SST anomalies from instrumental measurements compare regarding a few crucial aspects of their variability including seasonality and multiannual trends. We make use of MAT and SST data acquired by moored buoys constituting the Tropical Atmosphere Ocean (TAO) array. Buoys are managed by the Pacific Marine Environmental Laboratory (PMEL), which is part of the National Oceanic and Atmospheric Administration (NOAA) agency of the United States of America. We  aim at answering the following questions: How do the monthly average anomalies of SST and MAT compare? Do observed MAT and SST data contain significantly different multiannual trends?

D2785 |
EGU2020-4893
Yoania Povea Perez

The Atlantic Warm Pool (AWP) is a big body of warm water with SST greater or equal to 28.5◦ C, that appears in the Gulf of Mexico, the Caribbean and the western tropical North Atlantic and it is a key element of the climate system. Previous studies have focused on climate variability within the AWP, but did not take into account the distinctive properties of AWP sub-regions. In other cases, obtained results had not been tested against selected databases. This work will try to deal systematically with these limitations. Ocean reanalysis databases have been used in order to detect AWP climate variability, mechanisms through which thermal component of ocean-atmosphere interactions operates and the effect of remote phenomena such as El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO).  Empirical Orthogonal Functions, spectral analysis, linear correlation and composites analysis techniques have been used. A large portion of AWP variability comes from Caribbean Sea and Gulf of Mexico while North tropical Atlantic contains a large internal variability. The thermal component of ocean-atmosphere interactions appears partitioned in Gulf of Mexico and Atlantic from Caribbean Sea. SST/latent heat feedback mechanism operates not globally in the AWP but stronger in the open Atlantic sub-region. ENSO+ enhances AWP development, while ENSO- is opposite to both development and decay of AWP. NAO effect is stronger in its negative phase by enhancing the AWP decay.

D2786 |
EGU2020-9788
Markus Dunkel, Martin Claus, and Richard Greatbatch

A new model for the tropical ocean is described in which variations in the vertical are taken care of using vertical normal modes and the horizontal structure is taken care of by a set of linear shallow water models. The code is written in python and run in parallel and uses domain splitting in the horizontal. The model can be run in fully nonlinear mode and using generalized vertical mixing. The advantage of this approach is an almost continuous representation in the vertical and the ability to easily diagnose mode-mode interactions. The model performance is illustrated using the model of McCreary. In its original form, the McCreary model is linear and uses a very special form for the vertical mixing. Here we show preliminary results using more general vertical mixing and including nonlinearity.

D2787 |
EGU2020-22633
Yaci Alvarez and Andre Luiz Belem

The western boundary regime of the tropical South Atlantic Ocean is the main pathway of an important meridional transfer of warm and cold water masses that balances the global temperature on Earth, known as Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system that depends on a delicate balance of temperature and salinity effects on density, and is considered one of the main elements of the terrestrial system. The objective of this work was to study the variability of the salinity in the Western Tropical Atlantic Ocean, in order to identify salt transport anomalies in the circulation of the Atlantic Meridional Overturning Circulation as a result of climate change. Based on 3 decades of hydrographic observations of the Northern Brazilian Current and of the Deep Western Boundary Current, neutral density surfaces, salinity anomalies, geostrophic transport and salt transport were calculated. In general, the results reveal a coherent decadal change in salinity in 5°S and 11°S. In the upper ocean, both water masses, the South Atlantic Central Water and the Antarctic Intermediate Water, presented an increase of the salinity. The Antarctic Intermediate Water shows small trends with a decrease in salinity values in the upper part of the layer and an increase at the border to the North Atlantic Deep Water. In the deep ocean, the North Atlantic Deep Water layers the salinity generally decreases and, as expected for a warmer ocean in the Southern Hemisphere, the Antarctic Bottom Water layer shows an increase in salinity. The geostrophic and salt transports suggest a multidecadal variability and the changes in upper layer salinity are consistent with an increased Agulhas leakage, as described in literature. In the deep ocean, water mass changes seem to be likely related to changes in weather patterns in the North Atlantic as well as in tropical circulation changes.

D2788 |
EGU2020-4047
Sabeerali Cherumadanakadan Thelliyil, Ravindran Ajayamohan, and Praveen Veluthedathekuzhiyil

Atlantic Zonal Mode (AZM) and Indian summer monsoon rainfall (ISMR) are known to have an inverse relationship, which means that the cold (warm) phases of AZM result in strong (weak) ISMR. The realistic simulation of AZM and its teleconnection with ISMR in coupled models is important for the better seasonal prediction of ISMR. Here, we evaluated the performance of 26 CMIP6 models in simulating the AZM-ISMR teleconnection using 40 years of historical simulations. The skill of most CMIP6 models in simulating the teleconnection between AZM and ISMR is poor. Out of the 26 models analyzed, only 10 models show the correct sign of AZM related rainfall response over central India. The underlying mechanism responsible for the models' failure in capturing AZM-teleconnection is studied using the large-scale dynamical/thermodynamical variables. By choosing a set of good and bad models we unravel the common biases responsible for the wrong teleconnection between AZM-ISMR. This study highlights the importance of correcting AZM‐ISMR teleconnection in climate models for better seasonal monsoon prediction.

D2789 |
EGU2020-12722
Ingo Richter and Hiroki Tokinaga

General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic, as well as its linkage to the tropical Pacific. While, on average, mean state biases have improved little relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.

D2790 |
EGU2020-22388
Sadegh Yari and Volker Mohrholz

The Humboldt (Peruvian) Upwelling System (HUS) is the most productive among the main Eastern Boundary Upwelling Systems (EBUS), namely California, North West Africa, Benguela and itself. In spite of comparable upwelling intensity its fisheries production exceeds that of the other upwelling systems considerably (Chavez and Messie 2009). Wind is the major driving force of the coastal and curl driven upwelling, that controlls the nutrient supply from the deep water pool to the euphotic surface layer. Strength, spatial and temporal variability of the wind forcing are subjected to seasonal and interannual changes. The core of this study is describe the wind driven upwelling cells in the Peruvian coastal area in detail using long-term data which is not well understood. A better understanding of the state and dynamics of HUS seems essential for fututre regional climate predictions. ASCAT wind stress data for the period of 11 years (2008-2018) is analyzed to assess the spatio-temporal variations of the wind stress field, coastal upwelling and Ekman pumping along the Peruvian coast. The meridional component of wind stress off the peruvian coast, which is the main driver of offshore transport, has been marginally inensified over the entire priod. However, a high level of interannual variability is evident. The El-Niño years show anomalously high wind stress and associated Ekman transoprt. Our results indicate that the southern sector is more influenced by ENSO cycle than the northern sector. Additionally, a strong seasonality in the wind stress is observed. During the austral summer (December-February) the wind stress show the minimum value while the high values are observed in July-September.

D2791 |
EGU2020-16996
Fangfang Kuang, Junpeng Zhang, Aijun Pan, and Dayong Zhu

In this work, the intra-seasonal oscillation of the abyssal currents in the Middle East Pacific Ocean is investigated using direct observations from ADCP instruments, which are mounted on a subsurface mooring deployed at 154oW,10oN. The observation shows that the intra-seasonal (20-100 days) oscillation part of the kinetic energy accounts for more than 40% of the low-frequency flow kinetic energy between 200~2000m, while accounts for more than 50% under 2000m; the intra-seasonal oscillation of meridional flow is more obvious than that of zonal flow. The meridional velocity in the upper layer (100-1000m) shows an oscillation at periods of 50~90 days, which is most obvious at the depth of 500m; from 200m to the bottom layer currents shows an synchronous oscillation at a period of 30 days lasting for several months, and the oscillation signal is the strongest in the deep layer (4600m); The correlation is good between the 20~40 day band passed meridional current at the bottom layer and that of the geostrophic current. The observed temperature of 4000m and 5000m also shows similar characteristics of 30 days period oscillation, which has good correlation to the sea level height. The reanalysis data shows the 30 days oscillation of the abyssal currents is propagated from west to east at a speed of about 0.29m/s while the 40~100 day oscillation is propagated at a speed of about 0.1m/s; the intensity of the intra-seasonal oscillation has obvious interannual variations, which may be related to the change of the eddy energy of the sea surface.

D2792 |
EGU2020-11616
Jingxuan Wei, Robert Reece, Will Fortin, and Tanner Acquisto

South Atlantic water masses and circulation significantly influence the dynamics and water mass structure of the Atlantic Meridional Overturning Circulation (AMOC). Previous research in the South Atlantic has mostly focused on energetic regions such as the Brazil/Malvinas Confluence Zone along the western boundary and the Agulhas retroflection to the east. However, it is also important to understand water circulation and diapycnal mixing within the South Atlantic Basin (SAB). Previous studies have observed low salinity patches of the Antarctic Intermediate Water within the western side of the SAB at 30o S, but the temporal variability of the scales, locations and structures of these low salinity patches are still uncertain. Former studies also show an increased level of mixing within the SAB above the Mid-Atlantic Ridge, but did not evaluate mixing on smaller scales such as mesoscale and sub-mesoscale.

Here we present a water mass structure analysis at 30o S from Rio Grande Rise to the Mid-Atlantic Ridge by using Seismic Oceanography (SO). SO is being applied around the world to image mesoscale water mass structures using the seismic reflection method. Reflections in the seismic images are essentially temperature gradients that are proxies for isopycnal surfaces. We paid particular attention in seismic processing to imaging of structures that characterize the boundary between water masses. We imaged the upper South Atlantic Central Water, and identified discontinuous water boundaries (about 150 km long) between the Antarctic Intermediate Water and the North Atlantic Deep Water that could correspond to the intermittent appearance of low salinity patches. We combine seismic images with previous hydrographic measurements to investigate the temporal change of these low salinity patches. We use a horizontal slope spectra to quantify mixing rate from tracked seismic horizons to evaluate mesoscale and sub-mesoscale mixing events such as internal waves and eddies. Through SO, we hope to better constrain South Atlantic circulation and contribute to the understanding of AMOC as a whole.

D2793 |
EGU2020-17853
Noel Keenlyside, Lander Crespo, Shunya Koseki, Lea Svendsen, and Ingo Richter

The tropical Atlantic SST have warmed by about 1 degree over the historical period, with greatest warming in the east, along the African coast and in the Gulf of Guinea. Experiments performed from the Coupled Model Intercomparison Projects (CMIP) indicate that models fail to reproduce this warming pattern, instead showing a rather uniform warming. Future projections with these models also tend to show rather uniform warming. In constrast. results from anomaly coupled models indicate that model biases impact the ability of climate models to simulate warming patterns in the tropical Atlantic. Here we investigate the role of model biases on climate change in the tropical Atlantic in the CMIP experiments. In addition, we have analyzed impacts of global warming on tropical Atlantic climate variability, and we assess the sensitive of the results are to model biases.

D2794 |
EGU2020-18792
Xiaohui Tang, Fan Wang, and Yilong Lyu

Based on direct current measurements by ADCP moorings conducted during 2014-2018, seasonal-to-interannual variabilities of the Western Equatorial Pacific currents in different depth layers are analyzed. GODAS, Tropflux and NCEP reanalysis2 data are used to study the climatological factors influencing the current variabilities. The results show that both Equatorial Under Current (EUC) and Equatorial Intermediate Current (EIC) have significant seasonal-to-interannual variabilities. Both are closely related to the ENSO cycle, but through different mechanisms. Variations of the zonal velocity of Western Pacific EUC have noticeable correlations with subtropical SST, SLP and wind velocity, suggesting an influence of the Pacific meridional mode. The EIC, however, changes basically in corresponding to the Pacific zonal mode (ie. canonical ENSO mode). ENSO signals of the Eastern Equatorial Pacific might impact the Western Pacific EIC through vertical propagation of Rossby wave. This study gives an example on how atmospheric signals influence the subsurface ocean currents up to 800m depth.

D2795 |
EGU2020-20615
Antonio Castaño-Tierno, Belén Rodríguez-Fonseca, Elsa Mohino, and Teresa Losada

The Northwest Africa (NWA) upwelling region is located along the Senegalese and Mauritanian coast, between 10°N and 25°N and in a very narrow longitudinal band. In this region, most of the upwelled waters are due to alongshore surface winds through Ekman pumping.

The rapid increase in the upper ocean upwelling in this region along the 20th century and the contradictions found about future projections put forward the need for a better understanding of model’s ability to simulate Ekman induced upwelling processes.

In this work we assess intermodel variability to better understand the causes of different responses and spread among a set of CMIIP5 models.  

Results suggest that the seasonal cycle of NWA upwelling is qualitatively well simulated by CMIP5 models, although models tend to show strong biases for the permanent upwelling latitudes (north of 20°N) and the seasonal upwelling area (around 15°N in boreal spring). The maximum vertical temperature gradient shown by CMIP5 models is higher than that of SODA reanalysis and prevents cold waters from deeper layers to reach the surface, thus making coastal upwelling less effective in affecting sea surface temperatures.

Most of the intermodel variance is explained by the two first EOF modes of intermodel variability. The first mode shows a latitudinal structure, with a maximum in the permanent upwelling season, while. the second one is more seasonal. Both modes are very related to changes in the North-West Africa land-sea surface pressure gradient. In the case of the leading mode, incoming solar radiation differences between the North African desert and the ocean are the cause of the pressure gradients. For the second mode pressure changes in the Atlantic Ocean are driven by ITCZ shifts in response to interhemispheric differential warming.

D2796 |
EGU2020-21321
Fernanda P. S. Nascimento, Martin Schmidt, and Volker Mohrholz

An understanding of Benguela Nino events is important for local economy, ecosystem and ocean dynamics. Aiming to see if Benguela Nino events can be seen in observations and reproduced by the models, an investigation of sea surface temperature (SST) temporal and spatial variability was done throughout the Southwest African coast.  Using SST obtained from satellite observations and from four different numerical models, a coastal strip of 1o width from 8S to 28S was calculated and averaged longitudinally. Even though models were warmer than the observations, variability seen on observations were reproduced by the models. Highly anomalous warm and cold periods that coincides with years of Benguela Niño and Niña were found both on observations and in the models, as well as SST weakening after 2000.

D2797 |
EGU2020-7291
Marta Martín-Rey, Jose Luis Pelegrí, Emilia Sánchez-Gómez, and Christophe Cassou

Traditionally, the interannual Tropical Atlantic variability (TAV) is thought to be governed by two air-sea coupled modes denoted as Meridional Mode (MM) and Equatorial Mode (EM), peaking in boreal spring and summer respectively. Several studies have proposed a possible connection between the MM and EM, but without reaching a consensus about its frequency, type and associated mechanisms. Remarkably, recent findings brought to light decadal changes in the structure, intensity and teleconnections of the EM along the observational record. In particular, new overlooked equatorial modes called ‘non-canonical EM’ and ‘Horse-Shoe mode’ have been reported, which exhibit significant sea surface temperature anomalies in the north tropical Atlantic region. This gives robustness to the connection between the boreal spring and summer interannual modes.

Here, using observational and CMIP6 model datasets, we demonstrate the existence of distinct interannual modes in the tropical Atlantic basin along the record. Furthermore, the emergence of these modes is not stationary on time and varies from some decades to the others.  In this study, using observations and coupled climate models we explore the connection between the MM and EM to generate the diverse of tropical Atlantic variability reported in previous works. Moreover, the air-sea mechanisms and ocean dynamics involved in the evolution of these modes and the role of the mean state in the connection between them is assessed.

D2798 |
EGU2020-21599
Nan Zang

The dimensional and temporal distribution of Antarctic Intermediate Water (AAIW) and North Pacific Intermediate Water (NPIW) in the Philippines Sea were explored using Argo profiles. As the salinity minimum of intermediate water from mid-high latitude of the southern and northern hemisphere of the Pacific Ocean, the properties of AAIW and NPIW merges at about 10°N with different properties in the Philippine Sea. The core of AAIW is located below 600dbar with potential density of 27≤σθ≤27.3 kg m-3 and salinity of 34.5≤S≤34.55 psu. The core of NPIW is located between 300-700dbar with potential density of 26.2≤σθ≤27 kg m-3 and salinity of 34≤S≤34.4 psu. The volume of AAIW and NPIW during January 2004 to December 2017 is negative correlated.  The time series of AAIW and NPIW is dominated by semi-annual signals. The variations of AAIW and NPIW was mainly affected by volume transport through 130°E section by North Equatorial Current (NEC) and North Equatorial Undercurrent (NEUC).

D2799 |
EGU2020-22282
Mohammad Hadi Bordbar, Volker Mohrholz, and Martin Schmidt

Several long-established upwelling indices derived from the observed wind fields, Chlorophyll-a concentration, sea surface temperature (SST) are used to investigate the climatology annual cycle of Benguela Upwelling System (BUS). Chlorophyll-a concentration is taken as an indicator of ocean primary production. In addition, we analyze a multi-decadal simulation of a state-of-the-art eddy resolving ocean model which was forced by observed atmospheric heat and momentum fluxes. We take the vertically averaged of simulated vertical velocity in water column as a direct measure of upwelling strength.

The Ekman offshore transport tends to have two distinctive upwelling cells near the coast of Lüderitz (26.3°S) and Cape Frio (17°S) with large seasonal cycles. The former peaks between September and December. The latter features a biannual cycle with two peaks over April-June and September-December, which is concurrent with meridional migration of Angola-Benguela SST front. The offshore (30-200 km) vertical velocity, primarily induced by Ekman transport divergence, depicts a similar annual cycle, but with smaller magnitude. It becomes broader from south to north with four distinctive upwelling cells located near the coast of Cape Columbine (33°S), Orange River (28°S), Walvis Bay (23°S) and northern part of Cape Frio (16°S). The spatial and temporal variation of Ekman pumping and Chlorophyll-a, as measures of upwelling, show a clear correlation. However, such a correlation is not evident when Ekman coastal transport is taken. SST-based index depicts a very similar spatial pattern. However, the seasonal cycle does not match with other observational and simulated indices. Our finding suggests that the local SST anomalies are strongly influenced by horizontal heat advection and surface heat flux anomaly which can dominate over the anomalies associated with the upwelling; meaning that SST-index alone may not give a realistic estimate of upwelling strength over the region.

D2800 |
EGU2020-22335
Adama Badiane, Belén Rodríguez-Fonseca, Teresa Losada, Abdou Lahat Dieng, and Saidou Moustapha Sall

The impact of ENSO (El Niño Southern Oscillation) events on the cyclogenesis of the eastern tropical North Atlantic is highlighted, focusing on decadal variations of the interannual relationship at the Senegalese coast, which is the main cyclone development region (MDR). SST anomalies in the Equatorial Pacific associated with ENSO events affect vertical wind shear over the eastern Atlantic, by inducing strong subsidence of dry air over the eastern Atlantic which tends to inhibit deep convection and thus be unfavorable to cyclonic activity. Based on 20yr- correlations between the number of cyclones that are born in the MDR and ENSO index, we have selected two different periods of study (period1: 1950-1969; and period2: 1996-2015).The results show that period2 presents the highest scores of negative correlations between ENSO and tropical Atlantic cyclogenesis. Although there is an intensification of ENSO events during period2 compared to period1, we have found that decadal changes in climatology have a more significant effect on the MDR than the interannual changes. Additionally, the changes in the interannual signal appear to be related to the concomitant action of interannual SST anomalies over the whole tropical basins.

D2801 |
EGU2020-10459
Muhammad Azhar Ehsan, Dario Nicolì, Fred Kucharski, Mansour Almazroui, Michael Tippett, Alessio Bellucci, Paolo Ruggieri, and In-Sik Kang

Middle East surface air temperature (ME−SAT), during boreal summer (June to August: JJA), shows robust multidecadal variations for the period 1948−2016. Here using observational and reanalysis datasets as well as coupled atmosphere−ocean model simulations, we linked the observed summer ME−SAT variability to the multidecadal variability of sea surface temperature (SST) in the North Atlantic Ocean (AMV). This Atlantic−ME connection during summer involves ocean−atmosphere interactions through multiple ocean basins, with an influence from the Indian Ocean and the Arabian Sea. The downstream response to Atlantic SST is a weakening of the subtropical westerly jet stream that impacts summer ME−SAT variability through a wave−like pattern in the upper tropospheric levels. The Atlantic SST response is further characterized by positive geopotential height anomalies in the upper levels over the Eurasian region and dipole−like pressure distribution over the ME lower levels. For positive Atlantic SST anomalies, this pressure gradient initiates anomalous low−level southerly flow, which transports moisture from the neighboring water bodies toward the extremely hot and dry ME landmass. The increase in atmospheric moisture reduces the longwave radiation damping of the SAT anomaly, increasing further ME−SAT. A suite of Atlantic Pacemaker experiments skillfully reproduces the North Atlantic−ME teleconnection. Our findings reveal that in observations and models the Atlantic Ocean acts as a critical pacemaker for summer ME−SAT multidecadal variability and that a positive AMV can lead to enhanced summer warming over the Middle East.

D2802 |
EGU2020-4858
Ana Trindade, Marta Matín-Rey, Marcos Portabella, Eleftheria Exarchou, Pablo Ortega, and Iñigo Gómara

The Atlantic Ocean has suffered tremendous warming during recent decades as a consequence of anthropogenic forcing, modulated by the natural low frequency variability. Special attention should be paid to the high temporal frequency of warm interannual events in the North Tropical Atlantic (NTA) since the early 2000s, resulting in the most intense hurricane seasons on record (Hallam et al., 2017; Lim et al., 2018; Murakami et al., 2018; Klotzbach et al., 2018; Camp et al., 2018). Moreover, NTA sea surface temperature anomalies during boreal spring have been suggested as a potential precursor to the Equatorial Mode (Foltz and McPhaden, 2010ab; Burmeister et al., 2016; Martín-Rey and Lazar, 2019; Martín-Rey et al., 2019). 

This study aims to investigate the development of the 2017 NTA spring-summer warming event, which was the strongest of the last decade, as well as the importance of an accurate ocean forcingin the simulation of this event. For such purpose, a set of four simulations using distinct ocean wind forcing products, namely from the EC-Earth model, ERA-Interim (ERAi) reanalysis and a new ERAi-corrected ocean wind product (ERAstar), have been performed and analysed.The latter consists of average geolocated scatterometer-based corrections applied to ERAi output (Trindade et al., 2019).In this sense, ERAstar includes some of the physical processes missing or misrepresented by ERA-i, and corrects for large-scale NWP parameterization and dynamical errors.

The air-sea processes underlying the onset and development of the warm 2017 NTA event and the wave activity present in the equatorial Atlantic will be explored to determine their possible connection with the equatorial sea surface temperature variability. Furthermore, the comparison between the different experiments allows us to validate the new surface wind dataset and evaluate the importance of accurate, high-resolution ocean forcing in the representation of tropical Atlantic variability.

D2803 |
EGU2020-22337
Belen Rodriguez-Fonseca, Irene Polo, Elsa Mohino, Teresa Losada, Marta Martín-Rey, Noel Keenlyside, and C. Roberto Mechoso

Observational studies have reported that interannual variability of sea surface temperature in two tropical Atlantic regions can act as ENSO predictors in different seasons and periods: boreal summer Atlantic Nino (AN) in negative phases of the Atlantic Multidecadal Variabil- ˜ ity (AMV); and boreal spring tropical north Atlantic (TNA) in positive AMV. The robustness of the AMV role in the interbasin connection remains an open question due to the short observational record. Using observations and pre-industrial climate model simulations, we demonstrate for the first time that latitudinal displacements of the Atlantic ITCZ act as a switch for the type of inter-basin teleconnection. During periods in which the Atlantic ITCZ is further equatorward (northward) AN (TNA) impacts ENSO. This ITCZ location can be 1 affected by several factors, including the inter-hemispheric SST gradients associated with AMV.Coupled models success in capturing the AN-ENSO connection. Nevertheless, they have difficulties in reproducing the TNA-ENSO connection because they overestimate rainfall in the southern tropical Atlantic. The TNA-ENSO connection occurs sporadically during periods when the ITCZ is shifted further northward in association with strong heat transports by the AMOC. Weaker AMOC periods in coupled models don't present the TNA-ENSO connection. State-of-the-art models still need to improve for correctly representing tropical Atlantic impact on ENSO.