CL2.2

The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project Six and the multimodel Large Ensembles archive.  Comparison is made using three metrics of SST trend - the east-west and north-south sea surface temperature (SST) gradients and a pattern correlation for the equatorial region - as well as change in thermocline depth.  It is shown that the latest generation of models persist in not reproducing the observed SST trends as a response to radiative forcing and that the latter are at the far edge or beyond the range of modeled internal variability.  The observed combination of thermocline shoaling and lack of warming in the equatorial cold tongue upwelling region is similarly at the extreme limit of modeled behavior.  The persistence over the last century and a half of the observed trend towards an enhanced east-west SST gradient, and in four of five observed datasets to an enhanced equatorial north-south SST gradient, is also at the limit of model behavior. It is concluded that it is extremely unlikely that the observed trends are consistent with modeled internal variability.  Instead, the results support the argument that the observed trends are a response to radiative forcing in which an enhanced east-west SST gradient and thermocline shoaling are key and that the latest generation of climate models continue to be unable to simulate this aspect of climate change.

How to cite: Seager, R., Cane, M., and Henderson, N.: Persistent discrepancies between observed and modeled trends in the tropical Pacific, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13182, https://doi.org/10.5194/egusphere-egu22-13182, 2022.

Changes in the tropical Pacific background state can affect interannual variability, i.e. the El Niño-Southern Oscillation (ENSO) by altering feedbacks that control ENSO’s characteristics. Here, the sensitivity of ENSO to the background climate is investigated utilizing two Community Earth System Model version 1 (CESM1) simulations in which the solar constant is altered by ±25 W/m². The resulting stable warm and cold climate mean state simulations differ in terms of ENSO characteristics such as amplitude, frequency, asymmetry and seasonality. Under warm mean state conditions, ENSO reveals a larger amplitude and occurs at higher frequencies than in the cold mean state and control run. The warm run also features an increased asymmetry and a stronger seasonal phase-locking. We relate these changes to the differences in the mean state and the amplifying and damping feedbacks. In the warm run, a shallower mean thermocline results in a stronger subsurface-surface coupling while the cold run reveals reduced ENSO variability due to a reduced Bjerknes Feedback in accordance with a deeper mean thermocline and enhanced mean surface wind stress. A strong zonal advective and Ekman feedback further contribute to the large ENSO amplitude in the warm mean state run. However, in light of the large temperature contrast between the simulations of up to 6 K in the tropical Pacific, the results also highlight the robustness of ENSO dynamics under vastly different climate mean states.

How to cite: Lübbecke, J., Siuts, T., and Bayr, T.: Changes in ENSO characteristics in CESM1 simulations with considerably altered background climate states, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1420, https://doi.org/10.5194/egusphere-egu22-1420, 2022.

Two atmospheric feedbacks play an important role in the dynamics of the El Niño/Southern Oscillation (ENSO), the amplifying zonal wind feedback and the damping heat flux feedback. Here we investigate how and why both feedbacks change under global warming in climate models of 5^th and 6^th phase of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively) under a “business-as-usual” scenario (RCP8.5 and SSP5-8.5, respectively). The amplifying wind feedback over the western equatorial Pacific (WEP) becomes stronger in most climate models (on average by 8 ± 8%) as well as the damping heat flux feedback over the eastern and central equatorial Pacific (EEP and CEP, respectively) (on average by 18 ± 11%). The simultaneous strengthening of both feedbacks can be explained by the stronger warming in the EEP relative to the WEP and the off-equatorial regions, which shifts the rising branch of the Pacific Walker Circulation to the east and increases mean convection and precipitation over the CEP. This in turn leads to a stronger vertical wind response during ENSO events over the CEP that strengthens both atmospheric feedbacks. Further, we separate the climate models into sub-ensembles with STRONG and WEAK ENSO atmospheric feedbacks, as 2/3 of the models underestimate both feedbacks under present day conditions by more than 40%, causing an error compensation. Despite both sub-ensembles show similar changes in the mean state and ENSO atmospheric feedbacks, the ENSO dynamics in WEAK remain weaker relative to STRONG under global warming. Due to the more realistic ENSO dynamics, we postulate that the ENSO predictions of the models in STRONG should be more reliable. Finally, we analyze the relation between changes in ENSO amplitude and ENSO atmospheric feedbacks. We find that models tending to simulate an eastward shift of the wind feedback and increasing precipitation response over the EEP during Eastern Pacific El Niño events also exhibit an increasing ENSO amplitude.

How to cite: Bayr, T. and Latif, M.: ENSO Atmospheric Feedbacks under Global Warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1347, https://doi.org/10.5194/egusphere-egu22-1347, 2022.

Atmospheric moisture is perturbed during ENSO variation, as can be quantified using regression with ENSO indices. Seasonal and annual anomalies of the water column, horizontal moisture flux, surface evaporation, rainfall, and other basic variables, associated with the sea-surface temperature indices NINO34 and Pacific-Indian Dipole are evaluated from ERA5 reanalyses over 1980-2019. The skill in the corresponding regression coefficients (at one standard deviation) from historical climate simulations by the ten (only) CMIP6 models for which the vertically integrated flux was submitted is assessed, subject to the statistical uncertainty in ENSO from 40-year series. The ten-model mean fields are encouragingly realistic, although ENSO anomalies in the equatorial Pacific extend farther westward. The future change for the period 2040-2079 under the SSP585 scenario of rising greenhouse gases is evaluated. There is generally little change in the standard deviation in the two indices or in the SST and wind anomalies. The water column, moisture flux, and rainfall anomalies tend to be amplified in the low latitudes, but with limited change in the teleconnections to higher latitudes. The climatological changes in rainfall and moisture flux resemble those of ENSO in the tropical Indo-Pacific, in part linked to a small positive shift in both the indices. Elsewhere, widespread increases in water column, evaporation, midlatitude surface pressure, and, of course, temperature are not ENSO-like. Implications for the reliability of future projected means and variability will be considered. An obvious recommendation is that the vertically integrated moisture fluxes be routinely output by climate models and be a requested variable in future CMIPs.

How to cite: Watterson, I.: Atmospheric moisture anomalies associated with ENSO and future changes in CMIP6 simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10905, https://doi.org/10.5194/egusphere-egu22-10905, 2022.

Atmospheric teleconnections are remote impacts associated with atmospheric processes transmitted through planetary-scale waves like the Rossby wave. Tropical heat sources like El Nino Southern Oscillation (ENSO) could force such planetary-scale wave responses. The El Nino events are classified into Non-OLR El Nino events and OLR El Nino events based on its convective signal over the central-eastern equatorial Pacific using an OLR based El Nino Index. The key purpose of this study is to analyse the difference in teleconnection patterns during these OLR based El Nino events and understand its baroclinic-to-barotropic mode responses using an intermediate complexity atmospheric circulation model called Quasi-equilibrium tropical circulation model (QTCM). The study analyses the difference in the distribution of atmospheric variables and Rossby wave source (RWS) anomalies during Non OLR El Nino events and OLR El Nino using QTCM experiments. It is seen that the OLR El Nino events have a larger barotropic contribution to the positive anomaly of SLP over the western Pacific and a larger baroclinic contribution to the negative anomaly of SLP over the eastern Pacific compared to Non-OLR El Nino events. This is due to stronger baroclinic Rossby waves from the eastern and central tropical Pacific that propagates towards western Pacific and force barotropic wave trains due to barotropic-baroclinic interactions. Also, on analysing the effective RWS forcing and its components over certain regions during OLR and Non OLR El Nino, we see a difference in their distribution due to contributions from the absolute vorticity advection by divergent wind flow and subtropics vortex stretching. We further investigates the baroclinic-to-barotropic interaction over the midlatitude and tropical teleconnection through baroclinic-barotropic interaction terms in barotropic Rossby wave during Non OLR El Nino and OLR El Nino. It was seen that among the barotropic Rossby wave source interaction terms, the shear advection term has the largest contribution and the mean baroclinic zonal wind that advects the baroclinic zonal wind anomaly due to tropospheric heating is the most relevant component. The effective RWS over the tropics and the subtropics arise from the mean state baroclinic flow that acts on the baroclinic wind structure arising due to the ENSO tropospheric heating that spreads over a scale of equatorial radius of deformation from the deep tropics to the subtropics. This baroclinic wind structure is stronger for OLR El Nino compared to Non OLR El Nino. The experiment is also extended to preindustrial and mid-Holocene periods using data from CESM. The mid-Holocene OLR El Nino has a weaker RWS response than the preindustrial OLR El Nino due to the relatively weaker tropospheric heating and temperature structure, resulting in a weaker baroclinic wind structure.

How to cite: Suresan, S. and Joseph Mani, N.: Understanding ENSO related tropical teleconnections using Quasi-equilibrium tropical circulation model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9861, https://doi.org/10.5194/egusphere-egu22-9861, 2022.

The El Niño-Southern Oscillation (ENSO) is linked with energy exchange between the ocean, atmosphere and space. It has a global impact on weather, agriculture and the economic system. In association with ENSO, we analyse the atmospheric energy export from the Tropical Pacific with the particle dispersion model FLEXPART using meteorological input data from the ERA5 reanalysis. In this Lagrangian model, the atmosphere was filled homogeneously with five million particles, which were traced forward in time and represent the global atmospheric mass transport. From this Lagrangian reanalysis dataset covering the years 1979-2017, air masses residing within the Nino3.4 + Nino3 region and below 1 km are selected and followed 30 days forward in time. We found that some of these relatively warm air masses are transported to the Atlantic Ocean where they are mainly located at upper layers. Furthermore, we found strong correlations between the mass transport and the Nino3.4 Index, thus more air is exported to the Atlantic Ocean during El Niño conditions. This transported air further releases energy, as shown by a negative energy divergence. Even over the Sahel zone there is a significant signal, which indicates a direct atmospheric connection between West Africa and the Tropical Pacific. Based on our findings, the transported air might support drier surface conditions during El Niño in that region. In summary, the Lagrangian technique provides new insights into how energy is exported from the Tropical Pacific via the atmosphere and clarifies the relevance of atmospheric transport associated with ENSO.

How to cite: Baier, K., Duetsch, M., Bakels, L., Mayer, M., Haimberger, L., and Stohl, A.: Energy Export from the Tropical Pacific via the Atmosphere - a Lagrangian Perspective, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4711, https://doi.org/10.5194/egusphere-egu22-4711, 2022.

The subarctic front (SAF) in the pelagic Pacific Ocean is the northernmost front that separates the Oyashio Current, which marks the southern boundary of the subpolar gyre, from the Kuroshio Current, the northern boundary of the subtropical gyre. Its strong sea surface temperature (SST) gradient is not a stable and permanent feature but shifts on timescales from interannual to glacial/interglacial. Yet the complex interplay of different driving mechanisms for this phenomenon is not yet entirely understood. In this study, we present newly retrieved data from the Emperor Seamount chain that reveals a link between long-term ENSO (El Niño /Southern Oscillation) dynamics in the tropics and shifts of the SAF. Here, we use marine sediment core SO264-45-2 (46°33.792’N, 169°36.072’E), recovered from the Emperor Seamount Chain during R/V SONNE Cruise SO264 in 2018 to reconstruct changes in (sub-) surface temperature and salinity via a combined Mg/Ca and δ¹⁸O analyses of the shells of the shallow living planktic foraminifera Globigerina bulloides and the near thermocline living Neogloboquadrina pachyderma. This reveals that SST and salinity do not show a clear glacial/interglacial pattern during the last 280 ka and thus we assume that the SAF was south of the core site during this time interval. Prior to 280 ka, SSTs were significantly higher and show greater amplitudes than after 280 ka, while the subsurface temperature stayed relatively constant. Such high SSTs together with the observed higher sea surface salinities prior to 280 ka indicate that water from the Kuroshio-Oyashio transition zone temporarily reached the core site in form of a warm surface water lens. This points to a northward displacement of the SAF of at least 5° so that it was located right above the core site. This way very small north and southward displacements e.g. in relation to glacial/interglacial periods would have caused SST changes as high as we observe them in the time interval 280-700 ka. Notably, this assumed shift of the SAF at 280 ka occurs simultaneously to a change from more La Niña-like to more El Niño-like conditions in the tropical Pacific. Moreover, warm phases in the time interval 280-700 ka seem to occur during times of more La Niña-like conditions in the tropics, while cold phases seem to be related to more El Niño-like conditions. As our study area is linked to the subtropical gyre via the Kuroshio Current, we assume that the observed shifts of the SAF at our study site were caused by the enhancement of the Kuroshio Current in time intervals of more La Niña-like like conditions.

How to cite: Jacobi, L., Chao, W., Nürnberg, D., Lembke- Jene, L., and Tiedemann, R.: ENSO induced shifts of the Subarctic Front in the North Pacific over the past 700 ka: Evidence from planktic foraminiferal proxy data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4869, https://doi.org/10.5194/egusphere-egu22-4869, 2022.

El Niño-Southern Oscillation (ENSO) is the main predictor of global climate variability at interannual time scales. Its impact on European precipitation variability has been deeply studied, but not so much its impact on temperature. Recent studies suggest that the increasing intensity in heatwaves seems to be related to the interannual variability of the mean temperature. Therefore, the predictability of temperature could be very useful for the future adaptation to potentially severe heatwaves. In this study, we investigate the impact of ENSO on maximum and minimum temperature throughout the whole seasonal cycle with the aim of finding some predictability and trends. Due to the observed changing teleconnection between ENSO and remote regions, we consider the possible nonlinear and nonstationary relationship as well. For our study, we choose a region in western Europe that has experienced intense heatwaves, and which is also the main region of air temperature interannual variability in Europe. We found a nonseasonal, nonlinear and nonstationary impact. During decades prior to 1980s, warmer conditions are related to La Niña events in summer. Nevertheless, El Niño events seem to be linked to the increase in fall temperatures during decades after the 1980s. These warmer conditions are found to be correlated as well with ENSO characteristics from previous seasons, which suggest a potential source for improving the seasonal forecast. We analyze the underlying dynamical mechanisms of the detected teleconnection, and we found a circumglobal response for summer and an arching-like pattern in fall. Finally, we investigate the possible reasons explaining this variable impact among seasons and decades.

How to cite: Martija-Diez, M., Rodríguez-Fonseca, B., and López-Parages, J.: ENSO Impact on Summer and Fall Temperatures in Western Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6877, https://doi.org/10.5194/egusphere-egu22-6877, 2022.

Future climate change will lead to both dynamical and thermal changes to the atmosphere, and these changes will affect the transmission and impact of ENSO-related teleconnections. As the dynamical atmospheric changes are a response to the radiatively-forced temperature changes, it is difficult to separate these effects. In this study we use a novel nudging technique to separately apply the future thermal and dynamical changes from CMIP6 models to the ECHAM6 atmospheric model.

First there is a training stage where the atmospheric model is nudged to a chosen future climate, and the nudging tendencies are recorded. In the second stage the nudging tendencies for temperature and winds can be applied individually or together to replicate different aspects of the future climate. During the second stage the nudging tendencies are independent of the current model state. This means that idealised ENSO SST experiments can be performed within the constructed future climates, and the model can respond to those perturbations. The study focuses on the how ENSO teleconnections, particularly relating the northern hemisphere polar vortex, will respond to future thermal and dynamical changes.

How to cite: Tyrrell, N. and karpechko, A.: The response of ENSO teleconnections to future dynamical and thermal changes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13397, https://doi.org/10.5194/egusphere-egu22-13397, 2022.

The El Niño-Southern Oscillation (ENSO) teleconnection towards the Tropical North Atlantic (TNA) represents a robust response, where sea surface temperatures (SST) are positively correlated with ENSO. Following the peak of TNA SST anomalies (SSTAs) in the decaying phase of ENSO, the TNA can influence the local Walker circulation, creating a Rossby Wave Source (RWS) over the Caribbean region in boreal spring and summer. Additionally, when combined with the Pacific SSTAs, this Walker cell perturbation forms the Pacific-Caribbean Dipole (PCD), acting predominantly in the developing phase of ENSO and impacting the North Atlantic European (NAE) region. However, the influence of the TNA SSTAs on the Caribbean RWS and resulting NAE perturbation in the decaying phase of ENSO remains unclear. Thus, we use a series of sensitivity experiments with a simplified atmospheric general circulation model to determine how the TNA modulates the inter-basin teleconnection and how this modulation can influence the NAE response. We find that the NAE region is modulated by the TNA SSTA and Caribbean region in the boreal spring and summer. In boreal spring, a propagating Rossby wave train modulates the NAE region, while in boreal summer, the influence is nonlinear and tends to strengthen ENSO’s influence in the NAE region. Overall, our analysis presents a deeper understanding of the inter-basin Walker cell interactions in the decaying phase of an ENSO event and the TNA’s modulation of the teleconnection to the NAE region.

How to cite: Casselman, J. W., Jiménez-Esteve, B., and Domeisen, D. I. V.: Tropical Atlantic modulation of the ENSO teleconnection to the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8519, https://doi.org/10.5194/egusphere-egu22-8519, 2022.

Tropical modes of variability, such as El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), exert a strong influence on the interannual variability of Australian precipitation. Nevertheless, commonly used indices of ENSO and IOD variability display significant co-variability that prevents a robust quantification of the independent contribution of each mode to precipitation anomalies. This co-variability issue is often addressed by statistically removing ENSO or IOD variability from the precipitation field before calculating teleconnection patterns. However, by performing a suite of coupled and uncoupled modelling experiments in which either ENSO or IOD variability is physically removed, we show that ENSO-only-driven precipitation patterns computed by statistically removing the IOD influence significantly underestimate the impact of ENSO on Australian precipitation variability. Inspired by this, we propose a conceptual model that allows one to effectively separate the contribution of each mode to Australian precipitation variability.

How to cite: Liguori, G., McGregor, S., Singh, M., Arblaster, J., and Di Lorenzo, E.: Revisiting ENSO and IOD contributions to Australian Precipitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10456, https://doi.org/10.5194/egusphere-egu22-10456, 2022.

Colombia is the world’s third largest coffee exporter. The high altitude and rich soils of Colombia’s mountains and valleys create ideal conditions for growing coffee plants. The coffee industry in Colombia mostly consists of small, family-owned farms, and provides many hundreds of thousands of jobs in rural areas. Climatic conditions during the growing season strongly influence the quality and overall yields of coffee beans. Links between the ENSO cycle and coffee production will be investigated. Additionally, coffee crops in Colombia face a variety of threats originating from climate change, including loss of quality and increased prevalence of pests (e.g., the coffee berry borer, Hypothenemus hampei) and diseases (e.g., the coffee leaf rust, Hemileia vastatrix). High resolution climate data are needed to assess how the climate of the coffee growing areas could change and assist growers to adapt to these changes. The ability of three regional climate models (RCA4, RegCM4.3 and CRCM5) to reproduce observed teleconnections between the ENSO cycle and climate in coffee-growing areas of Colombia is also assessed. These regional climate model simulations were produced for the Coordinated Regional Dynamical Experiment (CORDEX) for the Central America, Caribbean, and Mexico (CAM) domain. They represent the highest resolution climate data available for Colombia. Projected changes in the ENSO cycle and possible impacts on coffee production will also be investigated. This study is believed to be the first to explicitly use the CAM-CORDEX results for Colombia.

How to cite: Sanderson, M., fox, C., Hodge, K., Cure, J. R., Rodríguez, D., Ponti, L., and Gutierrez, A. P.: Impacts of the ENSO cycle on climate and coffee production in Colombia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13513, https://doi.org/10.5194/egusphere-egu22-13513, 2022.

The meridional modes (MM) in the Pacific are the conduit by which mid to high-latitudes external forcing (NPO/SPO) can trigger or influence ENSO; While for the Northern Hemisphere the MM (NPMM) is considered a precursor of ENSO, the MM-ENSO relationship in the Southern Hemisphere (SH) is more uncertain. Here we show that, rather than acting as a precursor, strong MMs of the SH (SPMM) are dominantly (~66%) triggered by strong El Niño events in observations and the historical simulations of the Large Ensemble CESM (LENS-CESM). In the LENS-CESM simulations, strong ENSO-induced SPMMs are associated with a precursor signal (warm SST anomalies) of the coast off northern central Chile (20°S-35°S) resulting from the combined effect of the propagation of oceanic downwelling coastal Kelvin waves and the reduction in upwelling favorable winds due to an activated Pacific South American (PSA) pattern during the development of coincident ENSO cycle. The analysis of the simulations of the Coupled Intercomparison Project phases 5 and 6 (CMIP5/6) indicate a large diversity in terms of the ENSO-SPMM relationship, which can be interpreted as resulting from the spread in the meridional location of the center of action of the SPMM and of the seasonality of the SPO variance. We further discuss how ENSO-induced SPMM interferes with the coincident ENSO cycle and contributes to its asymmetry.

How to cite: Dewitte, B., Concha, E., Sepulveda, D., Pizarro, O., Martinez-Villalobos, C., Ramos, M., and Montecinos, A.: The ENSO-induced South Pacific Meridional Mode, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6181, https://doi.org/10.5194/egusphere-egu22-6181, 2022.

In 1929, Dr Friedrich Ritter and his mistress Dore Strauch left their spouses and the turmoil of post-World War I Germany for the remote, rugged and uninhabited volcanic island of Floreana in the Galapagos archipelago.  Their dream was to live self-sufficiently in an idyllic tropical setting unspoiled by civilization. Yachts stopping at Floreana after Ritter and Strauch established a homestead reported on their pioneering enterprise to the outside world in the early 1930s. The news created a sensation that subsequently attracted other settlers to the island, one of whom, a mysterious Austrian faux baroness, vexed Ritter and Strauch to the point of open hostility. Not all the participants in this drama survived the experience of colonizing Floreana though. A prolonged drought that gripped the island from 1933 to 1935 led to food shortages and ultimately the death of Dr. Ritter, who unwittingly ate tainted chicken out of desperation. The bizarre intrigues, extraordinary adventures, and struggles to endure on Floreana were chronicled in Strauch’s 1936 memoir “Satan Came to Eden” and a 2013 Hollywood documentary based on it.  A story that has not been told is how climate variability, and in particular an extended period of cold La Niña conditions in 1933-35, led to the drought that caused food shortages on the island and the untimely demise of Dr. Ritter.  We will use atmospheric reanalyses, contemporaneous marine meteorological observations in the vicinity of islands, and historical accounts from the broader Pacific basin, to describe the evolution of the 1933-35 La Niña and how it affected the human drama as it unfolded on Floreana Island. This protracted La Niña event had impacts felt in other parts of the globe as well and in particular was a major influence on development of the 1930s Dust Bowl in the southern plains of the United States.

How to cite: McPhaden, M. J. and Karamperidou, C.: La Niña Came to Eden, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6460, https://doi.org/10.5194/egusphere-egu22-6460, 2022.

We combine sea surface temperature (SST) and OHC indices derived from the gridded datasets to construct a phase space for data-driven ENSO models. Using a Bayesian optimization method, we construct linear as well as nonlinear models for these indices. We show that the joint SST-OHC optimal models yield significant benefits in predicting both the SST and OHC as compared with the separate SST or OHC models. It is shown that these models substantially reduces seasonal predictability barriers in each variable – the spring barrier in the SST index and the winter barrier in the OHC index. We find significant nonlinear relationships between the ENSO variables manifesting on interannual scales, which opens prospects for improving yearly ENSO forecasting. The work is supported by he Russian Science Foundation (grant #20-62-46056).

How to cite: Mukhin, D. and Seleznev, A.: Studying ENSO nonlinearity using observations of SST and ocean heat content, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11745, https://doi.org/10.5194/egusphere-egu22-11745, 2022.

El Niño Southern Oscillation (ENSO) dynamics are best described by the recharge oscillator model, in which the eastern tropical Pacific sea surface temperatures (T) and subsurface heat content (thermocline depth; h) have an out-of-phase relationship. This defines a 2-dimensional phase space diagram between T and h. In an idealized damped oscillator, the phase space diagram should be a perfectly symmetrical circle with a clockwise propagation over time. However, the observed phase space shows strong asymmetries in this diagram. In this study we will illustrate how the ENSO phase space can be used to discuss the phase-dependency of ENSO dynamics. The normalized spherical coordinate system allows to define a phase-depending ENSO growth rates and phase transition speeds. Based on these we discuss the implications of the observed asymmetries are for the dynamics and predictability of ENSO, with a particular focus on the variations in the growth rate and coupling of ENSO along the oscillation cycle.  Using linear and non-linear recharge oscillator models we will show how noise and internal dynamics are driving ENSO at different phases of the ENSO cycles. We will illustrate that a non-linear growth rate of T can explain most of the observed non-linear phase space characteristics.

How to cite: Dommenget, D. and Al Ansari, M.: Asymmetries in the ENSO phase space, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3263, https://doi.org/10.5194/egusphere-egu22-3263, 2022.

We assess dynamical nonlinearities and asymmetries in the atmospheric branch of the Bjerkness feedback. The atmospheric response to El Nino-related sea surface temperature anomalies (SSTAs), consists of developing deep convection with low-level westerlies west of the anomalous heating. Simple models of ENSO usually assume this process to be linear and symmetric. In this study, we use a global primitive equation model to evaluate nonlinearities and asymmetries that can potentially modify the Bjerkness mechanism.

The Dynamical Research Empirical Atmospheric Model (DREAM) is a global primitive equation model. It explicitly calculates advection and linear diffusion but physical processes are represented by an external empirical forcing deduced from ERA-Interim data. DREAM can be used as a stationary wave model where a perturbation develops on a fixed basic state. Both linear and nonlinear perturbations can be simulated by controlling the amplitude of the anomaly. DREAM can also be used as a simple GCM. In this case the model develops transient eddy fluxes and we obtain a full nonlinear solution. ENSO perturbations are introduced as a deep convective heating based on precipitation anomalies. Both idealised and realistic heating anomalies are studied, deduced from observed precipitation composites or calculated from SSTAs using an empirical transfer function.

The linear response to equatorial heating is strongly dependent on the position of the heating. Stationary equilibrium responses to idealised equatorial heating show that the response to a source in the East Pacific is larger than for the West Pacific. Stationary wave solutions are weakly nonlinear in heating amplitude, and the modification of the linear response is well explained as a linear response to stationary nonlinear fluxes. In fully turbulent simple GCM experiments, the response to equal and opposite heating perturbation shows a much larger asymmetry. This is diagnosed in terms of the linear response to anomalous transient eddy fluxes. The degree of nonlinearity in the response depends on the region and the variable under consideration. Nonlinearity is strong in the extratropics but less likely to be a significant factor locally and so probably of secondary importance for the Bjerkness feedback. However, the sensitivity of the response to diverse and asymmetric perturbation forcing implies that even when nonlinearity is weak, the asymmetric nature of the atmospheric response must be taken into account.

How to cite: Beniche, M., Hall, N., Vialard, J., and Lengaigne, M.: Nonlinearity and asymmetry in the atmospheric branch of ENSO with a simple GCM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5079, https://doi.org/10.5194/egusphere-egu22-5079, 2022.

ENSO features prominent asymmetries, in terms of amplitude, spatial pattern and phase-transition between warm and cold events. Here we examine the contribution of atmospheric nonlinearities to ENSO asymmetries through a set of forced experiments with the CNRM-CM6 AGCM and the NEMO OGCM. Control experiments can reproduce the major atmospheric and oceanic asymmetries of ENSO, with stronger signals east of the dateline for strong El Niño events, and west of it for strong La Niñas. Ensemble atmospheric experiments forced by observed ENSO SST anomalies and their opposites allow diagnosing asymmetries in air-sea heat and momentum fluxes that are directly attributable to atmospheric nonlinearities. They indicate that atmospheric nonlinearities are largely attributable to nonlinearities in the rainfall-SST relation and act to enhance El Niño atmospheric signals east of the dateline and those of La Niña west of it. An ocean simulation where the non-linear signature of air-sea fluxes is removed from the forcing reveals that asymmetries in the ENSO SST pattern are primarily due to atmospheric nonlinearities, and result in a doubling of eastern Pacific warming during the peak of strong El Niño events and a 33% reduction during that of strong La Niña events. Atmospheric nonlinearities also explain most of the observed prolonged eastern Pacific warming into boreal summer after the peak of strong El Niño events. Atmospheric nonlinearities  also appear to contribute strongly to phase transition asymmetries. They indeed result in stronger, more equatorially focussed zonal wind stress anomalies during strong El Niño events. The resulting enhanced off-equatorial wind stress curl doubles the discharge rate after the peak of strong El Niño events, thereby promoting a more systematic transition  to La Niña through the recharge oscillator mechanism. Overall, these results imply that properly simulating the nonlinear relationship between SST and rainfall in CGCMs is essential to accurately simulate asymmetries in ENSO amplitude, spatial pattern and phase transition. Finally, we discuss the inherent limitations to our two-tier forced approach. 

How to cite: Gangiredla, S., Vialard, J., Lengaigne, M., Voldoire, A., Izumo, T., and Guilyardi, E.: Atmospheric nonlinearities strong contribution to the skewed ENSO amplitude and phase transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8534, https://doi.org/10.5194/egusphere-egu22-8534, 2022.

The European Consortium EC-EARTH climate model version 3.1 is used to assess the effects of a well-resolved stratosphere on the representation of El Niño-Southern Oscillation, and in particular on the simulation of extreme El Niño events, known as super El Niños. Three 100-year long experiments with fixed radiative forcing representative of present climate are compared: one with the top at 0.01hPa and 91 vertical levels (HIGH-TOP or HT), another with the top at 5hPa and 62 vertical levels (LOW-TOP or LT), and another high-top experiment but with the stratosphere nudged to the climatology of HT from 10hPa upwards (NUDG). The differences in vertical resolution between HT and LT start at around 100hPa. By comparing HT with LT we explore the influence of increased vertical resolution above the tropopause on ENSO, while by comparing HT with NUDG we evaluate the influence of stratospheric variability, with special emphasis on the Quasi-Biennial Oscillation (QBO). No extreme ENSO events occur in the two simulations without QBO (LT and NUDG), while HT is able to simulate such extreme events. These super El Niños coincide with a positive Indian Ocean Dipole (IOD) and the westerly phase of the QBO in the lower stratosphere during boreal summer and fall. Previous studies have proposed an interaction between El Niño and IOD-related sea surface temperature anomalies to explain the existence of super El Niños. Our work suggests that this interaction alone is not enough in our climate model to simulate super El Niños. We postulate that changes in the upper tropospheric circulation over the Indian Ocean-Maritime Continent during boreal summer and fall, related to the westerly phase of the QBO, establish favourable conditions for the development of El Niños, increasing the probability of having super El Niños.

How to cite: Rodrigo, M., García-Serrano, J., and Bladé, I.: Is there an impact of resolving the stratosphere on ENSO? A first approach from EC-EARTH, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10063, https://doi.org/10.5194/egusphere-egu22-10063, 2022.

Tropical cyclone (TC) can pump heat downward through inducing intense vertical mixing. Many efforts have been made to estimate the magnitude of TC-induced ocean heat uptake (OHU), but the spatiotemporal variability of TC-induced OHU remains unclear. This study uses satellite-observed sea surface temperature (SST), subsurface temperature profiles, and turbulent heat fluxes to investigate the spatiotemporal variability of TC-induced OHU and its potential impacts on ocean heat content (OHC) during the period 1985-2018. It is found that category 3-5 TCs dominate the TC-induced OHU, accounting for ~70% of overall amount of TC-induced OHU globally each year. The time series of TC-induced OHU in global and regional oceans exhibit evident interannual-to-interdecadal variability, which is closely related to the TC power dissipation index (PDI). We further decompose PDI into TC intensity, frequency, and duration and find that category 3-5 TC frequency, annually averaged TC intensities, and durations all contribute to the variability of TC-induced OHU except that the averaged TC intensities have no significant relations with the TC-induced OHU in the North Indian Ocean, South Indian Ocean, and Southwest Pacific. In addition, the TC-induced OHU is shown to be responsive to equatorial SSTs rather than tropical SSTs, implying that the TC-induced OHU is modulated by El Niño-Southern Oscillation (ENSO). The TC-induced OHU might have the potential to influence OHC variability, particularly in the equatorial Pacific, where there is significant TC-induced OHU convergence. It has an important implication that TC-induced OHU might have potential effects on ENSO evolution.

How to cite: Fan, K., Wang, X., and Shao, C.: Spatiotemporal Variability of Tropical Cyclone Induced Ocean Heat Uptake and Its Effect on Ocean Heat Content, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2966, https://doi.org/10.5194/egusphere-egu22-2966, 2022.

Extreme precipitation in the western tropical Andes have significant socio-economic impacts in northern Peru and Ecuador. Previous investigations have shown that high impact episodes were caused by atmospheric moisture flux convergence associated with strong El Niño events in the eastern Pacific Ocean, identifying two patterns: the one emerging during the 1982/1983 and 1997/1998 events, and the one emerging during the 2015/2016 event.

In this contribution, we discuss the ability of CMIP6 global climate models to represent these two types of extreme El Niño events, by analyzing the associated atmospheric moisture transport patterns. Based on SST observations, we identified historical extreme El Niño events using the relative Niño34 index, an index recently proposed for addressing ENSO in a warming climate. We also use ERA5 to compare with the moisture flux of CMIP6. We compared 13 CMIP6 models with the historical record (1901-2014). We found the following: (1) six of the models simulated the two extremes El Niño patterns; (2) 62% of the models identify 4.5 extreme El Niño events; and (3) only 27% of the models represent the seasonality of the moisture flux convergence overestimating the moisture flux convergence branch located to the south (4° S) of its normal position (4° N).

Our results provide a starting point to investigate the impacts of climate change and its impacts on atmospheric dynamics and associated extreme events at the regional level in tropical South America.

How to cite: Sanabria, J., Calanca, P., Neukom, R., Salzmann, N., and Carrillo, C.: Representation of extreme El Niño events and associated atmospheric moisture flux convergence patterns in observations and CMIP6 global climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3833, https://doi.org/10.5194/egusphere-egu22-3833, 2022.

Multi-station observations of Schumann resonance (SR) intensity document common behavior in the evolution of continental-scale lightning activity in two super El Niño events, occurring in 1997/98 and 2015/16. The vertical electric field component of SR at Nagycenk, Hungary and the two horizontal magnetic field components in Rhode Island, USA in 1997, and in 2014-2015, the two horizontal magnetic field components at Hornsund, Svalbard and Eskdalemuir, United Kingdom as well as in Boulder Creek, California and Alberta, Canada exhibit considerable increases in SR intensity from some tens of percent up to a few hundred percent in the transition months preceding the two super El Niño events. The UT time distribution of anomalies in SR intensity indicates that in 1997 the lightning activity increased mainly in Southeast Asia, the Maritime Continent and India, i.e. the Asian chimney region. On the other hand, a global response in lightning is indicated by the anomalies in SR intensity in 2014 and 2015. SR-based results are strengthened by comparison to independent lightning observations from the Optical Transient Detector and the World Wide Lightning Location Network, which also exhibit increased lightning activity in the transition months. The increased lightning is attributable to increased instability due to thermodynamic disequilibrium between the surface and the mid-troposphere during the transition. Our main conclusion is that variations in SR intensity may act as a precursor for the occurrence and magnitude of these extreme climate events, and in keeping with earlier findings, as a precursor to maxima in global surface air temperature. As a continuation of our research we plan to set up a webpage dedicated to monitor the actual state of global lightning activity based on SR measurements which may contribute to the early identification of increased instability preceding the next super El Niño event. 

How to cite: Bozoki, T., Williams, E., Satori, G., Beggan, C. D., Price, C., Steinbach, P., Guha, A., Liu, Y., Neska, A., Boldi, R., and Atkinson, M.: Predicting the occurrence of extreme El Nino events based on Schumann resonancemeasurements?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-95, https://doi.org/10.5194/egusphere-egu22-95, 2022.

Seasonal to interannual forecasts made by coupled general circulation models (CGCMs) undergo strong climate drift and initialization shock, driving the model state away from its long-term attractor. Here we explore initializing directly on a model’s own attractor, using an analog approach in which model states close to the observed initial state are drawn from a “library” obtained from prior uninitialized CGCM simulations. The subsequent evolution of those “model-analogs” yields an ensemble forecast, without additional model integration. This technique is applied to CGCMs either used operationally by NCEP or as part of the CMIP6 dataset. By selecting from these long control runs those model states whose monthly SST and SSH anomalies best resemble the observations at initialization time, hindcasts are then made for leads of 1-36 months during 1958-2019. Deterministic and probabilistic skill measures of these model-analog hindcasts are comparable to, and in some regions better than, traditionally assimilation-initialized CGCM hindcasts after 1982, for both the individual models and the multi-model ensemble.

On average, ENSO skill of AC>0.5 exists for forecast leads of 18 months for forecasts initialized in summer. More important, we find that not only were some notable ENSO events predictable two years (or more) ahead of time, but that we can both identify forecast “hits” and avoid “false alarms” -- at the time of forecast -- by using a simple forecast signal-to-noise metric (SNR; root-mean-squared ensemble mean divided by ensemble spread), determined from the large (O(100) member) model-analog ensemble. That is, our analog ensemble approach can be used to make actionable ocean predictions, where forecasts of opportunity can be identified well in advance.

Since these long-lead hindcasts do not require full-field initialization, they have also been extended back prior to 1900. We find that while there has been considerable multi-decadal variation in seasonal ENSO skill, there has been no long-term trend for leads up to about 6-9 months. However, while multi-year ENSO skill appears to have also occurred in the past for a few large ENSO events, in the past thirty years it has occurred with considerably greater frequency, raising the possibility that it is a more recent phenomenon.

How to cite: Newman, M., Ding, H., Lou, J., Lillo, S., Alexander, M., Hoell, A., and Wittenberg, A.: Mining Large Climate Model Datasets to Make Multi-Year Initialized ENSO Forecasts with Actionable Skill, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10892, https://doi.org/10.5194/egusphere-egu22-10892, 2022.

A consensus has not yet been reached when it comes to the long-term changes in ENSO diversity. Indeed, for models that simulate larger warming in the East Pacific, some studies show an increase of Eastern Pacific (EP) events, and a decrease in Central Pacific (CP) events, or the opposite. Similar apparent contradictions also emerge from analyses of the changes in EP versus CP El-Niño events in the Holocene. In this study, we consider the Holocene period as a means to study long-term El Niño variability in a context relatively close to the present. Indeed, the Holocene period allows studying the changes related to the long-term trend induced by the long-term evolution of the Earth’s orbit and seasonal evolution induced by the orbital forcing. We use two 6,000-year-long transient simulations of the IPSL model and two different indicators to characterize El Niño events. 

This study shows that we can have opposite results on the behavior of EP and CP events depending on the type of indicator used to characterize El Niño. We will discuss the reasons for these contrasting results, as seen in two previous studies. Moreover, we will test the extent to which the types of events are induced by changes in the tropical Pacific’s thermocline.

How to cite: Abdelkader Di Carlo, I., Braconnot, P., Marti, O., Carré, M., and Elliot, M.: El Niño diversity during the Holocene in relation to mean state changes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5921, https://doi.org/10.5194/egusphere-egu22-5921, 2022.

The El Niño Southern Oscillation (ENSO) exhibits a large diversity of events characterized by the location of extreme sea surface temperature anomalies either in the Eastern Pacific (EP) or Central Pacific (CP). While broadband stochastic wind forcing is one of the known drivers of ENSO, the relative influence of its high- and low-frequency component on ENSO diversity remains unclear. 

We conduct a spectral analysis of westerly wind anomalies, yielding high- and low-frequency wind components for six different regions in the equatorial Pacific. The influence of these high- and low-frequency westerly wind anomalies, as well as their spatial location, is used for predicting ENSO diversity at different lead times. Using causal network discovery combined with multiple linear and non-linear regression analyses, we obtain different predictors for the different types of ENSO. Our results identify causally relevant, spatial and frequency-band restricted westerly wind anomalies at different lead times, which might improve early forecasting of El Niño events and improve our understanding of stochastic wind forcing energizing ENSO. 

How to cite: Unterholzner, J., Schlör, J., and Goswami, B.: Spatial and frequency dependence of westerly wind events causing ENSO diversity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10331, https://doi.org/10.5194/egusphere-egu22-10331, 2022.

Coastal El Niño events —instances of anomalous surface ocean warming in the eastern Tropical Pacific not associated to basin-wide events— have received a great deal of attention following the strong coastal event of early 2017. This event was associated to large increases in precipitation and widespread damage in Ecuador and Northern Peru comparable to that during the 1997/98 El Niño event. Despite their importance, it is currently not well understood whether these events are essentially driven by local dynamics or are a local manifestation of large-scale modes of climate variability, a possibility that may increase their predictability prospects. We identify three Coastal El Niño events and 7 Coastal La Niña events occurring in the last 40 years. We show that these events are at least partially driven by large-scale processes and can be grouped in two types. The first type is driven by westerly wind bursts in the western Pacific and occur in the initial stages of the development of basin-wide El Niño events. The second type occurs in association with active phases of the North Pacific Meridional Mode and are characterized by large-scale positive wind-evaporation-SST (WES) feedback. We develop a simple model that provides theoretical underpinnings for the WES feedback-driven type of events. Finally, we show that these two types of events have counterparts in the CESM Large Ensemble and discuss their projected change under global warming.

How to cite: Martinez-Villalobos, C., Dewitte, B., Garreaud, R. D., Loyola, L., and Concha, E.: Two types of Coastal El Niño events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6406, https://doi.org/10.5194/egusphere-egu22-6406, 2022.