El Niño-Southern Oscillation (ENSO) teleconnections exhibit a strong dependency on seasonally and intraseasonally varying mean states, which leads to impactful short-term variations in regional climate. The North Atlantic Oscillation (NAO)-ENSO relation is a typical example, in that its phase relationship reverses systematically between the early and late winter. However, the details and underlying mechanisms of this relationship transition are not well understood yet.

Here based on observations and an ensemble of atmosphere-only climate model simulations, we first reveal that this NAO phase reversal occurs synchronously in early January, which indicates strong abruptness. We demonstrate that this abrupt NAO phase reversal is caused by the change in ENSO-induced Rossby wave-propagating direction from northeastward to southeastward over the northeastern North American region, which is largely governed by a climatological alteration of the local jet meridional shear. We also provide evidence that the North Atlantic intrinsic eddy–low-frequency flow feedback further facilitates and amplifies the NAO responses. This abrupt NAO phase reversal signal is strong enough during the ENSO winter to be useful for intraseasonal climate forecasting in the Euro-Atlantic region.

How to cite: Geng, X., Zhao, J., and Kug, J.-S.: ENSO-driven abrupt phase shift in North Atlantic Oscillation in early January, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1944, https://doi.org/10.5194/egusphere-egu23-1944, 2023.

Increased weather and climate extreme events are often attributed solely to either human-induced climate change or internal variability, under the assumption that external forcing does not influence the internal variability. However, with the development of single-model initial-condition large ensembles, recent research shows the impact of global warming on internal variability. This study investigates how global warming influences the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, which are the dominant large-scale circulation/teleconnection modes in the North Atlantic sector.

The study analyzes the geopotential height data of the Max Planck Institute Grand Ensemble (MPI-GE)  with 100 ensemble members. The internal variability is quantified as the deviation from the ensemble mean. The influence of global warming on the internal variability is checked with a 1pcCO2 experiment, where the  concertation is increased by 1% every year. This experiment provides a scenario for relatively strong global warming based on increasing greenhouse gas concentration alone. The extreme NAO and EA are defined as those years where the indexes are above (positive extremes) or below (negative extremes) 2 standard deviations.

The results show increases in extreme events, especially negative extremes, for both NAO and EA during wintertime, in a warmer climate. While NAO extremes increase consistently across the whole troposphere, EA extremes increase more at higher altitudes (500hpa-200hpa) than at lower altitudes. The warming effect of positive extreme NAO over northern Eurasia gets weaker, while the cooling effect of negative extreme NAO over northern Eurasia gets stronger. The effects of both, positive and negative extremes of EA, extend eastward till Eastern Asia. Overall, this study underlines the impact of global warming onto the internal variability of NAO and EA.

How to cite: Liu, Q., Jungclaus, J., Matei, D., and Bader, J.: Global warming induces more internally generated extremes of North Atlantic Oscillation and East Atlantic pattern, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5176, https://doi.org/10.5194/egusphere-egu23-5176, 2023.

This paper reports the structure of an interhemispheric atmosphere–ocean coupling pattern, which occurs over the Atlantic Ocean from January to February, and refers to it as the Atlantic symmetric pattern (ASP). The ASP occurs in the middle–upper troposphere, with two trains of cyclonic–anticyclonic–cyclonic anomalous circulations aligned meridionally over the Atlantic Ocean. The sea surface temperature (SST) signature of the ASP, which is composed of a distinct SST dipole, is the leading mode of the interannual SST of the Southwest Atlantic Ocean. Experiments with the linear baroclinic model shows that the interhemispheric wave trains of the ASP can be excited as a Gill-type response to convection in the South American monsoon system and the South Atlantic convergence zone. Further studies are warranted to elucidate other aspects of the ASP, including teleconnection in the Northern Hemisphere and interactions with other climatic modes.

How to cite: Tseng, W.-L., Wang, Y.-C., Lee, Y., Hsu, H.-H., and Keenlyside, N.: An Atlantic interhemispheric teleconnection established by South American summer monsoon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16755, https://doi.org/10.5194/egusphere-egu23-16755, 2023.

Canonical understanding based on general circulation models (GCMs) is that the large-scale circulation responds only weakly to extratropical sea-surface temperature (SST) anomalies, compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, which ranges from roughly 200 km to 25 km, is too coarse to fully resolve mesoscale atmospheric processes such as weather fronts. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in a variable resolution version of the Community Atmospheric Model (CAM6), with regional grid refinement of 14 km over the North Atlantic, and compare it to versions with 28-km regional grid refinement and global 111-km resolution. The high-resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 1-standard-deviation NAO anomaly for 2°C SST anomalies. The lower-resolution simulations show a much weaker response, and in some cases, a different spatial structure of the response. The enhanced large-scale circulation response at high resolution results from an increase in resolved vertical motions, which enables SST forcing to have a larger influence on transient-eddy heat and momentum fluxes. In response to positive SST anomalies, these processes contribute to a stronger North Atlantic jet that varies less in latitude, as is characteristic of the positive phase of the NAO. Our results suggest that the atmospheric circulation response to extratropical SST anomalies is fundamentally different at higher resolution. Regional refinement in key regions offers a potential pathway towards improving simulation of the atmospheric response to extratropical SST anomalies and thereby improving multi-year regional climate predictions.

How to cite: Jnglin Wills, R., Herrington, A., Simpson, I., and Battisti, D.: Resolving weather fronts increases the large-scale circulation response to Gulf Stream SST anomalies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2125, https://doi.org/10.5194/egusphere-egu23-2125, 2023.

In this communication, we will present results from an analysis of the variability of the vertically averaged (i.e., barotropic) atmospheric circulation simulated by climate models, which integrate the current Coupled Model Intercomparison Project (CMIP6). The variabilities in two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations were compared with the variabilities in two ensembles of fully coupled simulation counterparts of the current CMIP6 (Castanheira and Marques, 2022).

The atmospheric models simulate less variability of the barotropic atmospheric circulation over the Northern Atlantic and more variability over the North Pacific when compared with the corresponding variabilities in the ERA5 reanalysis (“observations”), at intraseasonal and interannual scales. When integrated over the whole globe, the variability in the coupled climate simulations is smaller than the variability in the corresponding AMIP simulations. The smaller global variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the Northern Atlantic. The results suggest that the reduction of the biases in the barotropic atmospheric variability over the North Pacific, in the coupled climate simulations, is achieved through compensating biases in the mean Sea Surface Temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic region.

 Acknowledgements: The CESAM is supported by FCT/MCTES through the project UIDP/50017/2020+ UIDB/ 50017/20201+LA/P/0094/2020.

References

Castanheira, J. M., Marques, C. A. F. (2022). Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models. Journal of Climate,  Vol. 35, 5071–5085,  DOI: 10.1175/JCLI-D-21-0581.1. 

How to cite: Castanheira, J. M. and Marques, C. A. F.: How can the most unstable barotropic mode of atmospheric models contribute for the explanation of atmospheric variability biases of climate models in the North Atlantic?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13716, https://doi.org/10.5194/egusphere-egu23-13716, 2023.

Previous studies have shown that the response of the Atlantic Meridional Overturning Circulation (AMOC) to increasing greenhouse gas forcing is a key driver of inter-model uncertainties. While all models project an AMOC decline, the inter-model spread in the decline rate drives very different climate change impacts, including temperature, precipitation, and large-scale atmospheric circulation patterns. Here we investigate the impacts of a weakened AMOC by performing idealized climate model experiments using EC-Earth3, a state-of-the-art GCM participating in CMIP6. We compare results from a control experiment run under preindustrial forcing, with an experiment in which we force a weakened AMOC by applying a virtual salinity flux in the North Atlantic/Arctic basin. Here we analyze previously unexplored aspects of the climate response to a weakened AMOC, focusing on impacts on wintertime daily timescales in the Euro-Atlantic region.

We find that a weakened AMOC forces an overall drier climate over most of Europe; however, some regions especially in northwestern Europe experience an increase in the number of very wet days. We investigate drivers of precipitation changes by performing a moisture budget and analyzing the association with changes in weather regimes at daily timescales. We find that an increase in the occurrence of the NAO+ days (going from a frequency of ~26% of occurrence to above 42%) together with an enhanced and more central jet, favors drier conditions over southern Europe and wetter conditions over northwestern Europe. Further, enhanced but drier storms cause dryness over Europe while thermodynamic processes per se, namely the Clausius-Clapeyron constraint on temperature, play a second role. Finally, we explore these relationships in additional experiments in which we keep the AMOC constant in a forced 4xCO2 experiment by applying a reversed virtual salinity flux, which allows us to separate the effects of 4xCO2 forcing from the weakened AMOC on climate change impacts. Our results have broader implications for understanding the role of the AMOC response on future climate change, allowing us to separate the impacts of the AMOC from those of the CO2 increase.

How to cite: Bellomo, K., Meccia, V., D'Agostino, R., Fabiano, F., Larson, S., von Hardenberg, J., and Corti, S.: Impacts of a weakened AMOC on the European climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5775, https://doi.org/10.5194/egusphere-egu23-5775, 2023.

A potential future slowdown or acceleration of the Atlantic Meridional Overturning Circulation (AMOC) would have profound impacts on global and regional climate. Recent studies have shown that AMOC responds, among many other processes, to anthropogenic changes in tropical Indian ocean (TIO) temperature. However, internal unforced co-variations between these two basins are largely unexplored as of yet. Here, we use the ERSST v5, HadISST v1, and COBE v2 gridded observational products for the period 1870-2014, as well as dedicated simulations with coupled climate models, and show that internal changes in sea surface temperature gradients between the Indian and Atlantic Ocean (SSTgrad) can drive teleconnections that influence internal variations of North Atlantic climate and AMOC.

We separate the unforced observed component (i.e., internal signal) from the forced signal following the residuals method presented by Smith et al. (2019). In the absence of direct AMOC observation we estimate AMOC variability from an SST index (SST_AMOC; Caesar et al., 2018). We find a robust observed relationship between the unforced tropical SSTgrad and SST_AMOC when TIO leads by ~25 years. This time-lag is in line with a recently described mechanism of anomalous tropical Atlantic rainfall patterns that originate from TIO warming and cause anomalously saline tropical Atlantic surface water which slowly propagate northward into the subpolar North Atlantic, ultimately altering oceanic deep convection and AMOC (Hu and Fedorov, 2019; Ferster et al. 2021). Our study now suggests that it is the tropical SSTgrad that drives those AMOC changes, with a limited role for the western tropical Pacific. Pre-industrial control simulations with the IPSL-CM6A-LR model confirm this relationship, indicating a time lag of ~25 years between SSTgrad and SST_AMOC variations. These simulations also confirm that the SST_AMOC is representative of unforced AMOC variations when SST_AMOC leads by 5 years. This work therefore indicates that an unforced pathway between tropical ocean temperature and AMOC exists with a ~20 year lag, which opens the potential for using SSTgrad as precursor to predict future AMOC changes.

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196.

Ferster, B. S., Fedorov, A. V., Mignot, J., & Guilyardi, E. (2021). Sensitivity of the Atlantic meridional overturning circulation and climate to tropical Indian Ocean warming. Climate Dynamics, 1-19.

Hu, S., & Fedorov, A. V. (2019). Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature climate change, 9(10), 747-751.

Smith, D. M., Eade, R., Scaife, A. A., Caron, L. P., Danabasoglu, G., DelSole, T. M., ... & Yang, X. (2019). Robust skill of decadal climate predictions. Npj Climate and Atmospheric Science, 2(1), 1-10.

How to cite: Ferster, B., Borchert, L., Mignot, J., and Fedorov, A.: AMOC variations modulated by Tropical Indio-Atlantic SST Gradient, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7011, https://doi.org/10.5194/egusphere-egu23-7011, 2023.

Arctic amplification of global warming is accompanied by a dramatic decline in sea ice. This, in turn, has been linked to cooling over the Eurasian subcontinent over recent decades, most dramatically during the period 1998-2012. Such a coherent and pronounced cooling is a counterintuitive impact under global warming. Some studies have proposed a causal teleconnection from Arctic sea ice retreat to Eurasian wintertime cooling; others argue that Eurasian cooling is mainly driven by internal variability. Overall, there is an impression of strong disagreement between those holding the “ice-driven” versus “internal variability” viewpoints. We offer an alternative framing that shows that the sea ice and internal variability views can be compatible. Key to this is viewing Eurasian cooling through the dual lens of dynamics (linked primarily to internal variability with a small contribution from sea ice; cools Eurasia) and thermodynamics (linked to sea ice retreat; warms Eurasia). This framing, combined with recognition that there is uncertainty in the hypothesized mechanisms themselves, allows both viewpoints (and others) to co-exist and contribute to our understanding of Eurasian cooling. A simple autoregressive model shows that strong Eurasian cooling is consistent with internal variability, with some periods being more susceptible to strong cooling than others. Rather than posit a “yes-or-no” causal relationship between sea ice and Eurasian cooling, a more constructive way forward is to consider whether the cooling trend was more likely given the observed sea ice loss, as well as other sources of low-frequency variability. Taken in this way both sea ice and internal variability are factors that affect the likelihood of strong regional cooling in the presence of ongoing global warming. Improving our understanding of the underlying mechanisms is critical for quantifying regional responses and impacts as well as producing reliable near-term climate predictions. 

How to cite: Sobolowski, S., Outten, S., and Li, C.: Reconciling conflicting evidence for the cause of the observed early 21st century Eurasian Cooling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5279, https://doi.org/10.5194/egusphere-egu23-5279, 2023.

The Hadley circulation (hereafter HC) is one of the most prominent meridional overturning circulations in the climate system. In addition to maintaining energy balance and momentum exchange in tropics and extratropics, it can also shape the Intertropical Convergence Zone (ITCZ) and subtropical dry arid zones by regulating the hydrological cycle in tropical and extratropical regions. Weakening and expanding HC and narrowing of the ITCZ are projected with human greenhouse gas emissions. However, no consensus has been achieved regarding the relative importance of direct CO₂ radiative effect and indirect effects via SST changes in shaping the future HC changes. This limits our deep understanding of the climate impacts imposed by changes in the HC. Here we analyze a broad range of CMIP5 experiments and show that future changes in SST patterns play the leading role in the determining the future changes in HC and ITCZ. In addition, a series of individual basin perturbation experiments were conducted at 1.5°C, 2°C, and 3°C temperature thresholds to identify key basins that determine HC strength, edges, and ITCZ locations. Our work highlights the overwhelming role of future tropical Indian Ocean warming on the HC and ITCZ changes.

How to cite: Sun, Y., Ramstein, G., Fedorov, A. V., Ding, L., and Liu, B.: Impacts of oceanic warming patterns versus CO2 radiative forcing on the Hadley Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10582, https://doi.org/10.5194/egusphere-egu23-10582, 2023.

During the last decades, CMIP5 models simulate a warming trend in the tropical eastern Pacific that has not been present in observations (Seager et al., 2019). Associated with this, the Walker circulation has experienced a westward migration while CMIP5 models simulate an eastward migration. This mismatch is still present in CMIP6 models and might affect climate projections worldwide. In the Caribbean region, CMIP6 models project a strong drying at the end of the 21st century. El Niño-like changes in the Walker circulation are the dominant teleconnections driving the Caribbean drying. The models that project a strong Caribbean drying also simulate generally a strong equatorial eastern Pacific warming trend over the recent decades. Thus, the mismatch between observed and simulated warming trends over the equatorial eastern Pacific questions the reliability of the Caribbean precipitation projections. The warming bias might also have implications for tropical cyclones’ projections in the Atlantic and Pacific through the effect of vertical wind shear, which is related to shifts in the Walker circulation. In addition, the double Intertropical Convergence Zone (ITCZ) bias might be influenced by the mismatching trends. The strong influence of El Niño-Southern Oscillation (ENSO) dynamics on the world’s climate demands more in-depth studies addressing the drivers of the Walker circulation and the equatorial Pacific warming bias.

How to cite: Brotons Blanes, M., Haarsma, R., and Bloemendaal, N.: Impact of tropical eastern Pacific warming bias on Caribbean climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5034, https://doi.org/10.5194/egusphere-egu23-5034, 2023.

It is well established that the positive phase of El Niño Southern Oscillation (ENSO) tends to weaken the Northern Hemisphere stratospheric polar vortex (SPV), promoting a negative North Atlantic Oscillation (NAO). Pacific Decadal Variability (PDV) is characterised by a pattern of sea surface temperatures similar to ENSO, but its impacts are more uncertain: some studies suggest similar impacts of ENSO and PDV on the SPV and NAO, while others find the opposite. We use climate model experiments and reanalysis to find further evidence supporting opposite interannual and decadal impacts of Pacific variability on the extratropics. We propose that the decadal strengthening of the SPV in response to positive PDV is caused by a build-up of stratospheric water vapour leading to enhanced cooling at the poles, an increased meridional temperature gradient and a strengthened extratropical jet. Our results are important for understanding decadal variability, seasonal to decadal forecasts and climate projections.

How to cite: Seabrook, M., Smith, D., Dunstone, N., Eade, R., Hermanson, L., Scaife, A., and Hardiman, S.: Opposite Impacts of Interannual and Decadal Pacific Variability in the Extratropics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6966, https://doi.org/10.5194/egusphere-egu23-6966, 2023.

Decadal Climate Variability (DCV) modes perturb regional climatic parameters across the globe at multi-year timescales. Precipitation is one such climatic parameter of socio-economic importance.

Our study examines the ability of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in representing the observed teleconnection of DCV modes; Pacific Decadal Oscillation (PDO) and Tropical Atlantic SST Gradient with the global precipitation. We chose the subset of CMIP6 models that participate in both historical and hindcast experiments.

In this study we examine the relationship between the model's ability to simulate the long-term DCV pattern and its ability to simulate the teleconnection between DCV mode and global precipitation.

HadGEM3-GC31-MM and MPI-ESM-1-2-HR, which simulate the observed global SST anomaly pattern in the warm phase of PDO considerably well, also simulate observed global precipitation patterns during the warm phase of PDO quite well in regions like  central India, Europe, North- and South-America, Eastern Africa, Eastern Australia etc. However, BCC-CSM2-MR and NorCPM1 fail to effectively simulate observed precipitation patterns in the warm phase of PDO in regions like, North- and South-America, Africa etc. 

Hence, we found that models that are able to simulate the PDO pattern of SST are also able to represent the teleconnection between PDO modes and precipitation across the globe. We also examined the regression pattern of wind circulation, and the regression pattern of converging and diverging parts of the wind with PDO index. Models that better represent the observed warm phase of PDO pattern, also well represent the observed circulation pattern in respective phases of PDO. Similar analysis is also performed for TAG.

Keywords: Decadal Climate Variability (DCV), CMIP6, historical experiments, teleconnection, precipitation.

How to cite: Dixit, J., Mehta, V. M., and AchutaRao, K. M.: Representation of relationship between PDO and global precipitation in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15027, https://doi.org/10.5194/egusphere-egu23-15027, 2023.

The central role of tropical sea surface temperature (SST) variability in modulating Northern Hemisphere (NH) extratropical climate has long been known. However, the prevailing pathways of teleconnections in observations and the ability of climate models to replicate these observed linkages remain elusive. Here, we apply maximum covariance analysis between atmospheric circulation and tropical SST to reveal two co-existing tropical-extratropical teleconnections albeit with distinctive spatiotemporal characteristics. The first mode, resembling the Pacific-North American (PNA) pattern, favors a Tropical-Arctic in-phase (warm-Pacific-warm-Arctic) teleconnection in boreal spring and winter. The second mode, predominant in summer and autumn, is manifested as an elongated Rossby-wave train emanating from the tropical eastern Pacific that features an out-of-phase relationship (cold-Pacific-warm-Arctic) between tropical Pacific SST and temperature variability over the Arctic. This Pacific-Arctic teleconnection (PARC) mode partially explains the observed summertime warming around the Arctic. The reliability of climate models to replicate these leading teleconnections is of primary interest in this study to improve decadal prediction on regional climate. While climate models participating in CMIP6 appear to successfully simulate the PNA mode and its temporal characteristics, the majority of models’ skill in reproducing the PARC mode is obstructed by apparent biases in simulating low-frequency SST and rainfall variability over the tropical eastern Pacific and the summer climatological mean flow over the North Pacific. Considering the contribution of the PARC mode in shaping low frequency climate variations over the recent decades from the tropics to the Arctic, improving models’ capability to capture the PARC mode is essential to reduce uncertainties associated with decadal prediction and climate change projection over the NH.

How to cite: Feng, X.: Possible causes of model biases in simulating Tropical-Arctic teleconnections in CMIP6, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6017, https://doi.org/10.5194/egusphere-egu23-6017, 2023.

The amplitude and spatial distribution of low-frequency natural variability is determinant for regional climate projections, but it is still poorly understood. 

In a previous study, pollen-based temperature reconstructions were used to quantify spatial patterns of millennial temperature variability. This showed an inverse relationship across timescales with sub-decadal variability from instrumental data in extra-tropical regions over land (Hébert et al., 2022, under review). We concluded that due to varying marine influence, regions characterized by stable oceanic climate at sub-decadal timescales experience stronger long-term variability while continental regions with higher sub-decadal variability show weaker long-term variability. Indications of this relationship could also be inferred from instrumental data alone as regions of low sub-decadal variability were more likely to exhibit a steeper increase of variability over multi-decadal timescales and vice versa. 

In the current work, the relationship found in the instrumental data was further investigated using different instrumental products. In addition, a large multi-model ensemble of CMIP6 models, as well as single-model ensembles, were considered for analysis and it was found that they do not systematically reproduce the relationship found in the instrumental data. This indicates a fundamental deficiency in the model simulations with regard to the mechanism driving the emergence of low-frequency climate variability. This characteristic being related to multi-decadal variability thus has important significance for multi-decadal regional climate projections and might be used as an emergent constraint in model evaluation and inter-comparison.

How to cite: Hébert, R. and Laepple, T.: Emergence of Low-Frequency Temperature Variability in Instrumental Data and Model Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16668, https://doi.org/10.5194/egusphere-egu23-16668, 2023.

How to cite: Rauch, M., Bliefernicht, J., Laux, P., and Kunstmann, H.: Classification of Atmospheric Circulation Patterns That Trigger Rainfall Extremes in the Sudan-Sahel Region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12678, https://doi.org/10.5194/egusphere-egu23-12678, 2023.

The Intertropical Convergence Zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. But recently, this paradigm was challenged. To find out what are the main drivers of the annual cycle of rainfall over central Africa, we present a simple comprehensive paradigm with both local forcings and regional-scale processes playing crucial role. Due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year–round. The deep mode is associated with poleward branches at upper levels that transport the atmospheric energy. The shallow mode is characterized by a meridional return flow in the mid-troposphere that transports the water vapour instead of lower branches as widely reported. This favours the building-up of the mid-tropospheric moisture flux convergence with a limited contribution of the midlevel easterly jet, conducive to deep convection. Embedded in this strong rising branch of Hadley cells at midlevels, the intense convective rainfall, and with it the rainfall maximum position, is seasonally controlled by the dynamics of the midlevel shallow meridional return flow. This highlights the interhemispheric rainfall contrast over central Africa and outlines its unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes – (i) the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season and (ii) the local evaporation-controlled regime with drizzle and (iii) the precipitable-water-controlled regime, with exponential increase of rainfall that both occur during the dry season.

How to cite: Longandjo, G.-N. T., Monyela, B. M., and Rouault, M.: Drivers of the Annual Cycle of Rainfall over Central Africa: The Role of Water Vapor and the Mid-Tropospheric Meridional Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13299, https://doi.org/10.5194/egusphere-egu23-13299, 2023.

Indian Ocean Dipole (IOD) is an air-sea coupled variability in the Tropical Indian Ocean (TIO), which strongly impacts climate variability over the Indian Ocean rim countries. Though many positive IODs co-occurred with El Niño Southern Oscillation (ENSO), IODs do evolve independently, suggesting the possible role of internal dynamics of the Indian Ocean. In this study, the subtropical IOD (SIOD) is reported as one of the triggers for non-ENSO IODs. The study highlights the existence of cyclic feedback between IOD and SIOD through tropical subtropical interaction, a possible mechanism for the biennial tendency of both IOD and SIOD modes. The positive SIOD induce warming in the southwest of the Subtropical South Indian Ocean (SSIO) during April-May months and creates a meridional cell with subsidence over the southwestern TIO region (10^oS). The subsidence expands the existing anticyclonic circulation over SSIO towards the equator and develops easterlies along the equator, warming the western TIO region. A zonal-vertical cell with convection over the western TIO and subsidence over the eastern TIO originates during June-July, which subsequently generates positive IOD in the following months. The positive IOD triggers negative SIOD by developing a stationary Rossby wave train in the midlatitudes. The southeastern anticyclonic circulation develops during the IOD peak season as Gill’s response initiates warm SST anomalies in the northeastern subtropics. As a result of the warming, the evolution of upper-level divergence and high absolute vorticity gradient over the subtropics generate an equivalent barotropic Rossby wave number 3 pattern in the extratropics. The cyclonic circulation over the southwest SSIO related to this Rossby wave pattern creates cold SST anomalies there. The cooling in the southwest and the warming in the northeast SSIO persisted from the IOD peak season, which strengthened the cyclonic circulation over SSIO, reinforcing the existing negative and positive SST anomalies through a positive feedback mechanism and generating negative SIOD, which peaks in the following January-March months.

How to cite: Sebastian, A. and Gnanaseelan, C.: Coupled feedback between the tropics and subtropics of the Indian Ocean with emphasis on the coupled interaction between IOD and SIOD, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-373, https://doi.org/10.5194/egusphere-egu23-373, 2023.