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ENSO and its interactions with other tropical basins are the dominant source of interannual climate variability in the tropics and across the globe. Understanding the dynamics, predictability, and impacts of ENSO and tropical basins interactions, and anticipating their future changes are thus of vital importance for society. This session invites contributions regarding all aspects of ENSO and tropical basins interactions, including: dynamics, multi-scale interactions; low frequency, decadal and paleo variability; theoretical approaches; ENSO diversity; global teleconnections; impacts on climate, society and ecosystems; seasonal forecasting and climate change projections of ENSO and its tropical basins interactions. Studies aimed at understanding ENSO and its tropical basins interactions in models of a range of complexity are especially welcomed, including analysis of CMIP model intercomparisons.

Public information:
Dear all,
welcome to the virtual EGU 2020. This is just a reminder that we will have the "ENSO and Tropical Basins Interactions: Dynamics, Predictability and Modelling" (CL4.20/AS1.12/NP2.6/OS1.27) session Thu, 07 May, 14:00–15:45 (Vienna time zone). It will be an online chat only session.

In addition to the EGU chat session on Thursday we plan to do a video meeting for the "ENSO and Tropical Basins Interactions: Dynamics, Predictability and Modelling" session with presentations from the authors (you) some time later this year (e.g. June/July). More on this we will discuss on Thursday in the EGU chat of this session.

Best regards and hope to chat with you on Thursday!
Dietmar Dommenget, Antonietta Capotondi, Daniela Domeisen and Eric Guilyardi

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Co-organized by AS1/NP2/OS1
Convener: Dietmar Dommenget | Co-conveners: Antonietta Capotondi, Daniela Domeisen, Eric Guilyardi
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| Thu, 07 May, 14:00–15:45 (CEST)

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Chat time: Thursday, 7 May 2020, 14:00–15:45

D3497 |
EGU2020-11551
Pedro DiNezio

Paleoclimate records show pronounced changes in the El Niño/Southern Oscillation (ENSO) phenomenon over past climatic intervals, but the application of these results to understand future changes is not straightforward. To address this issue we propose the following mechanism controlling ENSO variability across altered climate states. Numerical simulations show that extreme El Niño – the warm phase of ENSO – could become more frequent in climatic states with a shallower ocean mixed layer, as predicted for the future, and extremely infrequent under climatic states with a deeper mixed layer, typical of glacial intervals. Wind fluctuations involved in the onset of El Niño transfer momentum more efficiently over a thinner ocean mixed layer, thus favoring stronger ocean currents and faster warming during the event. The robustness of this momentum coupling mechanism across climatic states, together with the evidence that ENSO was weaker under glacial conditions, increases our confidence in model predictions of more frequent extreme El Niño under greenhouse warming.

D3498 |
EGU2020-11931
Evgeniya Predybaylo, Georgiy Stenchikov, Andrew Wittenberg, and Sergey Osipov

To improve El Niño / Southern Oscillation (ENSO) predictions and projections in a changing climate, it is essential to better understand ENSO’s sensitivities to external radiative forcings. Strong volcanic eruptions can help to clarify ENSO’s sensitivities, mechanisms, and feedbacks. Strong explosive volcanic eruptions inject millions of tons of sulfur dioxide into the stratosphere, where they are converted into sulfate aerosols. For equatorial volcanoes, these aerosols can spread globally, scattering and absorbing incoming sunlight, and inducing a global-mean surface cooling. Despite this global-mean cooling effect, paleo data confirm remarkable warming of the eastern equatorial Pacific in the two years after a tropical eruption, with a shift towards an El Niño-like state. To illuminate this response and explain why it tends to occur during particular seasons and ENSO phases, we present a unified framework that includes the roles of the seasonal cycle, stochastic wind forcing, eruption magnitude, and various tropical Pacific climate feedbacks. Analyzing over 20,000 years of large-ensemble simulations from the GFDL-CM2.1 climate model forced by volcanic eruptions, we find that the ENSO response comprises both stochastic and deterministic components, which vary depending on the perturbation season and the ocean preconditioning. For boreal winter eruptions, stochastic dispersion largely obscures the deterministic response, being the largest for the strong El Niño preconditioning. Deterministic El Niño-like responses to summer eruptions are well seen on neutral ENSO and weak to moderate El Niño preconditioning and grow with the eruption magnitude. The relative balance of these components determines the predictability and strength of the ENSO response. The results clarify why previous studies obtained seemingly conflicting results.

D3499 |
EGU2020-7763
Antje Weisheimer, Magdalena Balmaseda, and Tim Stockdale

Motivated by the high skill in predicting ENSO on seasonal time scales with ECMWF’s seasonal forecasting system SEAS5 and by previous findings of multi-decadal variability in seasonal forecast skill of extratropical dynamics, we have carried out an extensive set of 24-month long coupled hindcasts from 1901 to 2010. The hindcasts were run with SEAS5 in reduced resolution and are initialised from, and verified against, reanalyses of the 20thCentury. They allow us to analyse ENSO forecast skill beyond the first year, to study how skill varies on decadal time scales and to test sensitivities to atmospheric wind forcings and the assimilation of ocean observations in the initial conditions.

First results show a substantial amount of multi-decadal variability in both ENSO mean state and forecast skill. We find periods in the early-to-mid 20thCentury with much reduced levels of skill, in particular after the spring barrier in the first forecast year. Periods at the beginning and at the end of the Century show broadly similar good performances with substantial skill even after the first year spring barrier. Combined effects of the wind forcing and the assimilation of ocean data on the initial state seem to play a crucial role in understanding this behaviour.

D3500 |
EGU2020-21603
Jing-Jia Luo, Fenghua Ling, Yoo-Geun Ham, and Jeong-Hwan Kim

Variations in the El Niño/Southern Oscillation (ENSO) are associated with a wide array of regional climate extremes and ecosystem impacts. Robust, long-lead forecasts would therefore be valuable for managing policy responses. But despite decades of effort, forecasting ENSO events at lead times of more than one year remains problematic. Here we show that a statistical forecast model employing a deep-learning approach produces skilful ENSO forecasts for lead times of up to one and a half years. To circumvent the limited amount of observation data, we use transfer learning to train a convolutional neural network (CNN) first on historical simulations and subsequently on reanalysis from 1871 to 1973. During the validation period from 1984 to 2017, the all-season correlation skill of the Nino3.4 index of the CNN model is much higher than those of current state-of-the-art dynamical forecast systems. The CNN model is also better at predicting the detailed zonal distribution of sea surface temperatures, overcoming a weakness of dynamical forecast models. A heat map analysis indicates that the CNN model predicts ENSO events using physically reasonable precursors. The CNN model is thus a powerful tool for both the prediction of ENSO events and for the analysis of their associated complex mechanisms.

D3501 |
EGU2020-4086
Jian Shi, Alexey Fedorov, and Shineng Hu

Ocean energetics is a useful framework for understanding El Niño development and diversity; however, its key element, available potential energy (APE), requires accurate ocean subsurface data that are hard to measure. However, sea surface heights (SSH) provide a useful alternative. In this study, we describe an SSH-based index, SSHI, that accurately captures APE variations and can be easily computed from satellite observations. Using SSHI we obtain an observation-based estimate of the APE damping timescale α-1 of approximately 1.7 years, slightly longer than previous ocean reanalysis-based estimates. We further show that SSHI records the relative strength of the thermocline feedback, serving as an indicator for El Niño “flavors”. SSHI demonstrates a clear decadal shift in El Niño-Southern Oscillation (ENSO) properties that occurred in early 2000s, with a more tilted mean thermocline and weaker thermocline slope variations indicative of the dominance of “Central Pacific” El Niño activity during the past two decades.

D3502 |
EGU2020-6240
Hanjie Fan, Bohua Huang, and Song Yang

This study investigates the mechanisms for the Pacific meridional mode (PMM) to influence the development of an ENSO event and its seasonal predictability. To examine the relative importance of several factors that might modulate the efficiency of the PMM influence, we conduct a series of prediction experiments to selected ENSO events with different intensity from a long simulation of the Community Earth System Model (CESM). Using the same coupled model, each of the ensemble prediction is conducted from slightly different ocean initial states but under a common prescribed PMM surface heat flux forcing. In general, the matched PMM forcing to ENSO, i.e., a positive (negative) PMM prior to an El Niño (a La Niña), plays an enhancing role while a mismatched PMM forcing plays a damping role. For the matched PMM-ENSO events, the positive PMM exerts greater influence than its negative counterpart does, with stronger enhancement of positive PMM events on an El Niño than that of negative PMM events on a La Niña. This asymmetry in ENSO influence largely originates from the intensity asymmetry between the positive and negative PMM events in the tropics, which can be explained by the nonlinearity in the growth and equatorward propagation of the PMM-related SST and surface zonal wind anomalies through both wind-evaporation-SST (WES) feedback and summer deep convection (SDC) response. Furthermore, the response of ENSO to an imposed PMM forcing is modulated by the preconditioning of the upper ocean heat content, which provides the memory for the coupled low-frequency evolution in the tropical Pacific.

D3503 |
EGU2020-10541
Soon-Il An and Shang-Ping Xie

The delayed negative feedback is a key process for the turnabout between El Niño and La Niña. Since the intensity of this dynamical negative feedback is determined by itself, the stronger event is supposed be strongly damped during the decaying phase. However, the extreme El Niño actually lived longer than the normal El Niño. Here, we propose that the far-eastward extension of the warm pool promotes the positive SST tendency during the decaying phase of El Niño so disrupting a strong decay. The warm pool expansion accompanies by the expansion of the convective threshold region toward the eastern Pacific. During and after the mature phase of the extreme El Niño, therefore the rainfall band and the enhanced westerly anomalies over the eastern Pacific move to the east, which enhances the upwelling. This eastward migration of surface winds also plays a role of out-of-phase relationship between SST anomaly and subsurface temperature anomaly. All these processes results in the positive SST tendency through the positive nonlinear dynamical heating. This positive SST tendency maintains the warm eastern Pacific until the following summer.

D3504 |
EGU2020-1391
Effects of Tropical Cyclones on ENSO
(withdrawn)
Tao Lian, Jun Ying, and Hong-Li Ren
D3505 |
EGU2020-7024
Yuming Zhang, Tobias Bayr, Mojib Latif, Zhaoyang Song, Wonsun Park, and Annika Reintges

We investigate the origin of the equatorial Pacific cold sea surface temperature (SST) bias and its link to wind biases, local and remote, in the Kiel Climate Model (KCM) with dedicated coupled and stand-alone atmosphere model experiments. In the coupled experiments, the National Centers for Environmental Prediction Climate Forecast System Reanalysis (NCEP/CFSR) wind stress is prescribed over four different spatial domains: globally, over the equatorial Pacific (EP), the northern Pacific (NP) and southern Pacific (SP). The corresponding cold SST bias over the equatorial Pacific is reduced by 94%, 48%, 11% and 22%, respectively. Thus, the equatorial Pacific SST bias is mainly attributed to the wind bias over the EP region, with small but not negligible contributions from the SP and NP regions. Biases in the ocean dynamics cause the EP SST bias, while the atmospheric thermodynamics counteract it.

To examine the origin of wind biases, we force the atmospheric component of the KCM in stand-alone mode by observed SSTs and simulated SSTs from the coupled experiments with the KCM. The results show that wind biases over the EP, NP and SP regions are initially generated in the atmosphere model and further enhanced by the biased SST in the coupled model.

We conclude that the cold SST bias over the equatorial Pacific originates from biases in the ocean circulation that are forced by the biased surface winds over the EP, NP and SP regions. On the other hand, the cold equatorial Pacific SST bias amplifies the wind biases over the EP, NP and SP regions, which in turn enhances the cold SST bias by ocean-atmosphere coupling.

D3506 |
EGU2020-8835
Tobias Bayr, Dietmar Dommenget, and Mojib Latif

Many climate models strongly underestimate the two most important atmospheric feedbacks operating in El Niño/Southern Oscillation (ENSO), the positive (amplifying) zonal surface wind feedback and negative (damping) surface-heat flux feedback (hereafter ENSO atmospheric feedbacks, EAF), hampering realistic representation of ENSO dynamics in these models. Here we show that the atmospheric components of climate models participating in the 5th phase of the Coupled Model Intercomparison Project (CMIP5) when forced by observed sea surface temperatures (SST), already underestimate EAF on average by 23%, but less than their coupled counterparts (on average by 54%). There is a pronounced tendency of atmosphere models to simulate stronger EAF, when they exhibit a stronger mean deep convection and enhanced cloud cover over the western equatorial Pacific (WEP), indicative of a stronger rising branch of the Pacific Walker Circulation (PWC). Further, differences in the mean deep convection over the WEP between the coupled and uncoupled models explain a large part of the differences in EAF, with the deep convection in the coupled models strongly depending on the equatorial Pacific SST bias. Experiments with a single atmosphere model support the relation between the equatorial Pacific atmospheric mean state, the SST bias and the EAF. An implemented cold SST bias in the observed SST forcing weakens deep convection and reduces cloud cover in the rising branch of the PWC, causing weaker EAF. A warm SST bias has the opposite effect. Our results elucidate how biases in the mean state of the PWC and equatorial SST hamper a realistic simulation of the EAF.

D3507 |
EGU2020-7263
Seraphine Hauser, Christian M. Grams, Michael J. Reeder, Shayne McGregor, Andreas H. Fink, and Julian F. Quinting

The El Niño Southern Oscillation (ENSO) is typically associated with below-average rainfall in the winter-spring season in southeastern Australia. However, there is also a large case-to-case variability pointing to the non-linear relationship of El Niño strength and the impact on east Australian rainfall. Despite recent progress in understanding the linkage of remote climate drivers and this variability, the dynamical processes by which the drivers transmit their influence on rainfall are not fully understood. With this study, we aim to advance the dynamical understanding by relating patterns of monthly rainfall anomalies over southeastern Australia to a novel dataset of objectively identified weather systems derived from ERA-Interim reanalyses.

We find 4 rainfall anomaly patterns in the austral winter-spring season (JJASON) with above-average rainfall (Cluster 1), below-average rainfall (Cluster 2), above-average rainfall limited to the East Coast (Cluster 3) and above-average rainfall limited to the South Coast (Cluster 4) in southeastern Australia. Changes in the frequency of weather systems explain partly the rainfall anomalies in the clusters. Results indicate a significant increase of weather system activity in Cluster 1 and a weakening of weather system activity in Cluster 2. In Cluster 3, enhanced blocking favors the development of cut-off lows on its northeastern flank leading to increased rainfall along the East Coast. Positive rainfall anomalies along the South Coast are associated with frontal rainfall due to an equatorward shift of the midlatitude storm track (Cluster 4). Most of the rainfall is produced by warm conveyor belts and cut-off lows but the contributions strongly vary between the clusters. We further find that anomalies in rainfall result from changes in rainfall frequency more than in rainfall intensity. By calculating backward trajectories of warm conveyor belt and cut-off low rainfall, we point to the importance of moist air masses from the Coral Sea and the northwest coast of Australia for wet months. Air parcels, that end up in WCB or cut-off low rainfall, reach southeastern Australia from the dry remote areas to the north and not as one would expect from the Southern Ocean.

D3508 |
EGU2020-7527
Matyas Herein, Timea Haszpra, and Tamas Bodai

Traditionally we think that climate is a long term average of weather. It is true but only if the climate is stationary. However, in a changing climate, where one or more relevant system parameters are changing in time, there can be no stationarity by definition, whereas stationarity is crucial for the applicability of any temporal averaging techniques. To avoid this problem we redefine climate as the distribution of potential climate realizations characterized by the instantaneous statistics of an ensemble using the so-called snapshot attractor view. In this view, the relevant quantities of the climate system are the statistics taken over an ensemble of possible realizations evolved from various initial conditions. To illustrate the power and applicability of this method we investigate the changes in the El Niño–Southern Oscillation (ENSO) phenomenon and its precipitation-related teleconnections over the Globe under climate change in the Community Earth System Model’s Large Ensemble from 1950 to2100. For the investigation, a recently developed ensemble-based method, the snapshot empirical orthogonal function (SEOF) analysis is used. The instantaneous seasonal ENSO SST pattern is defined as the leading mode of the SEOF analysis carried out at a given time instant over the ensemble. The corresponding principal components (PC1s) characterize the ENSO phases. Considering regression maps, we find that the largest changes in the typical amplitude of SST fluctuations occur in the June–July–August–September (JJAS) season, in the Niño3–Niño3.4 region and in the western part of the Pacific Ocean. At the same time, the increase is also considerable along the Equator in December–January–February (DJF). The Niño3 amplitude shows also an increase of about 20% and 10% in JJAS and DJF, respectively. The strength of the precipitation-related teleconnections of the ENSO is found to be non-stationary, as well. For example, the anti-correlation with precipitation in Australia in JJAS and the positive correlation in Central and North Africa in DJF are predicted to be more pronounced by the end of the 21th century. Half-year-lagged correlations, aiming to predict precipitation conditions from ENSO phases, are also studied. The Australian, Indonesian precipitation and that of the eastern part of Africa in both JJAS and DJF seem to be well predictable based on ENSO phase, while the South Indian precipitation is in relation with the half-year previous ENSO phase only in DJF. The strength of these connections increases with time, especially from the African region to the Arabian Peninsula.

D3509 |
EGU2020-11318
Muhammad Adnan Abid, Fred Kucharski, Franco Molteni, In-Sik Kang, Adrian Tompkins, and Mansour Almazroui

El Niño Southern Oscillation (ENSO) have a weak influence on the seasonal mean Euro-Atlantic circulation anomalies during the boreal winter (Dec-Feb) season. Therefore, monthly ENSO teleconnections to Euro-Atlantic region were studied during the boreal winter season for the period 1981-2015 using reanalysis and hindcast dataset. It is shown that the ENSO-forced signal to the Euro-Atlantic circulation anomalies does not persist throughout the boreal winter season. During earlier winter, a positive ENSO phase strongly enforces rainfall dipole anomalies in the tropical Indian Ocean, with increased rainfall over the western tropical Indian Ocean, and reduced in the eastern tropical Indian ocean.  This Indian Ocean rainfall dipole weakens in late winter. During early winter, the Indian Ocean rainfall dipole modifies the subtropical South Asian jet (SAJET) which forces a wavenumber-3 response projecting spatially onto the positive North Atlantic Oscillation (NAO) pattern. On contrary, during late winter, the response in the Euro-Atlantic sector is dominated by the well-known ENSO wavetrain from the tropical Pacific region, involving Pacific North American (PNA) pattern anomalies that project spatially on the negative phase of the NAO. Atmospheric General Circulation Model (AGCM) numerical experiments forced with an Indian Ocean heating dipole anomaly support the hypothesis that the Indian Ocean modulates the SAJET that enforces the Rossby wave propagation to the Euro-Atlantic region in early winter. Moreover, the ECMWF-SEAS5 hindcast dataset reproduces the observed ENSO-forced inter-basin tropical teleconnections transition from early to late winter and their response to the Euro-Atlantic circulation anomalies quite well. Therefore, it is important to understand the tropical inter-basin transition, which may lead to improve the sub-seasonal to seasonal variability and predictability of the Euro-Atlantic circulation anomalies. 

D3510 |
EGU2020-16656
| Highlight
Andréa S. Taschetto, Caroline C. Ummenhofer, Malte F. Stuecker, Dietmar Dommenget, Karumuri Ashok, Regina R. Rodrigues, and Sang-Wook Yeh

The warming of the equatorial Pacific associated with the El Niño–Southern Oscillation (ENSO) causes profound impacts on rainfall and temperature in the tropics and extratropics. El Niño drives changes in the Walker and Hadley circulations, warms the tropics and affects the neighboring ocean basins, favoring a short-term rise in global temperatures. We will present an overview of the atmospheric teleconnections driven by ENSO and its diversity focusing on the impacts over land and remote ocean basins. During El Niño, dry conditions are generally observed in the Maritime Continent, northern South America, South Asia and South Africa, while wet weather typically occurs in southwestern North America, western Antarctica, and east Africa. Global effects during La Niña are overall the opposite to El Niño, however this assumption is not true for all regions. ENSO atmospheric teleconnections are non-linear in part due to different locations of the anomalous equatorial warming (Eastern versus Central Pacific events) superimposed on the Pacific mean state, as well as interactions with the annual cycle, off-equatorial regions, remote ocean basins, and other modes of climate variability. Adding to this complex behavior, ENSO teleconnections are non-stationary either due to deterministic low-frequency modulations or stochastic variability, the latter being a factor generally overlooked in the literature. As the world warms in response to greenhouse gas forcing, ENSO atmospheric teleconnections are expected to change, despite large uncertainties in ENSO projections. We will discuss the limitations of climate models in representing realistic teleconnections from the tropical Pacific to remote regions and some of the challenges for future projections.

D3511 |
EGU2020-8340
Goratz Beobide, Tobias Bayr, Annika Reintges, and Mojib Latif

The possible change of ENSO amplitude during the 21st century in response to global warming has been analyzed in models participating in the Coupled Model Intercomparison Phase 5 (CMIP5). Three types of uncertainties are investigated: scenario uncertainty, model uncertainty, and uncertainty due to internal variability.

The ENSO response obtained from the CMIP5 models is highly uncertain, leading to an ensemble-mean amplitude change of close to zero until the end of the 21st century, with an uncertainty exceeding 0.3 °C. The internal variability is the main contributor to the uncertainty during the first two decades of the projections. The inter-model differences dominate thereafter, while scenario uncertainty is relatively small throughout the whole 21st century. The zonal wind-SST feedback has been identified as an important factor of ENSO amplitude change: the global warming signal in the ENSO amplitude and zonal wind-SST feedback are highly correlated across the CMIP5 models, with correlation coefficients of 0.87, 0.84 and 0.78 for the RCP4.5, RCP6.0 and RCP8.5 scenarios, respectively.

The CMIP5 models with realistic ENSO dynamics have been analyzed separately. In this sub-ensemble, the global warming signal is strengthened with a mean ENSO amplitude decrease of approximately 0.1°C by the end of the 21st century. When only considering models with large decadal ENSO amplitude variability, the decrease in ENSO amplitude amounts to 0.1°C and 0.2°C for the RCP4.5 and RCP8.5 scenarios, respectively.

D3512 |
EGU2020-8916
Jonathan Beverley, Mat Collins, Hugo Lambert, and Rob Chadwick

El Niño–Southern Oscillation (ENSO) has major impacts on the weather and climate across many regions of the world. Understanding how these teleconnections may change in the future is therefore an important area of research. Here, we use simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to investigate future changes in ENSO teleconnections in the North Pacific/North America sector.

Precipitation over the equatorial Pacific associated with ENSO is projected to shift eastwards under global warming as a result of greater warming in the east Pacific, which reduces the barrier to convection as the warm pool expands eastwards. As a result, there is medium confidence (IPCC AR5 report) that ENSO teleconnections will shift eastwards in the North Pacific/North America sector. In the CMIP6 models, the present day teleconnection is relatively well simulated, with most models showing an anomalously deep Aleutian low and associated positive temperature anomalies over Alaska and northern North America in El Niño years. In the future warming simulations (we use abrupt-4xCO2, in which CO2 concentrations are immediately quadrupled from the global annual mean 1850 value), in agreement with the IPCC AR5 report, the North America teleconnection and associated circulation change is shifted eastwards in most models. However, it is also significantly weaker, with the result that the positive temperature anomalies in El Niño years over North America are much reduced. This weakening is seen both in models with a projected increase and projected decrease in the amplitude of future El Niño events. The mechanisms related to these projected changes, along with potential implications for future long range predictability over North America, will be discussed.

D3513 |
EGU2020-12812
Zoe Thomas, Chris Turney, A. Peter Kershaw, Richard Jones, Ian Croudace, Patrick Moss, Timothy Herbert, Mark Grosvenor, Raphael Wust, Joanne Muller, Malin Kylander, Susan Rule, Sophie Lewis, Sarah Coulter, and Manfred Mudelsee

The El Niño-Southern Oscillation (ENSO) is a driver of global atmosphere-ocean dynamics, but projections of frequency and magnitude in different climate states remain uncertain. Palaeoclimate records offer the potential to improve our understanding of ENSO behaviour but most are fragmentary, suffer low resolution, and/or typically do not cover periods warmer than present day. The Last Interglacial (129-116 kyr BP) was the most recent period during which global temperatures were close to 21st century projections, and potentially provides insights into operation of climate modes of variability in the future. Here we report a continuous, inter-annually resolved record of hydroclimate spanning 220-80 ka from Lynch’s Crater in tropical northeast Australia, a region highly sensitive to ENSO. Our reconstruction is based on a micro-X-ray fluorescence (XRF)-generated elemental profile at 200 µm resolution, combined with loss-on-ignition, magnetic susceptibility, and pollen analysis. We find that during globally warmer periods (including super-interglacial Stage 5e, and 5c), there are significantly larger amplitudes in high-frequency ENSO spectral range (3-8 years), which are absent from the record during the glacial stages MIS6 and MIS4. Our results imply an ENSO dependence on mean climate, with enhanced ENSO variance during interglacials globally warmer than present. These results are consistent with climate model projections for a future slowdown of the Walker circulation and more extreme El Niño events under greenhouse warming.

D3514 |
EGU2020-21341
Mario Rodrigo, Javier Garcia-Serrano, Ileana Bladé, Froila M. Palmeiro, and Bianca Mezzina

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 (ENSO). Three 100-year  long experiments with fixed radiative forcing representative of the present climate are compared: one with the top at 0.01hPa and 91 vertical levels (HIGH-TOP), another with the top at 5hPa and 62 vertical levels (LOW-TOP), and another high-top experiment with the stratosphere nudged to the climatology of HIGH-TOP from 10hPa upwards (NUDG). The differences in vertical resolution between HIGH-TOP and LOW-TOP start at around 100hPa. This study focuses on the canonical ENSO phenomenon, which is the most important source of variability and predictability on seasonal-to-interannual timescales.

 

Preliminary results indicate that EC-EARTH realistically simulates the ENSO SST pattern in the tropical Pacific regardless of vertical resolution, although HIGH-TOP (LOW-TOP) overestimates (underestimates) the SST variability during boreal summer (winter). In both configurations, the SST tongue is narrower meridionally and slightly shifted towards the central-western Pacific compared to observations, a common bias of climate models. Resolving the stratosphere has a clear effect on the power spectrum of the Niño3.4 index: as compared to observations where there is a well-known frequency range of 2-7 years, HIGH-TOP and LOW-TOP have a prominent peak centered at 4-5 years but additionally both simulations display another peak, towards higher (~ 2yrs) and lower (~ 7yrs) frequencies, respectively. Another impact of including a well-resolved stratosphere is to systematically enhance the amplitude of the SST, wind and convective anomalies in the tropical Pacific throughout the entire ENSO cycle. Finally, similar differences are obtained when comparing HIGH-TOP and NUDG, suggesting an active role of the tropical stratospheric variability on ENSO.

D3515 |
EGU2020-3983
Yuqiong Zheng

This study evaluates the ability of 35 climate models, which participate in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical climate simulations, in reproducing the connection between boreal spring Arctic Oscillation (AO) and its following winter El Niño-Southern Oscillation (ENSO). The spring AO-winter ENSO correlations range from -0.41 to 0.44 among the 35 models for the period of 1958-2005. Ensemble means of the models with significant positive and negative AO-ENSO correlations both show strong spring sea surface temperature (SST) cooling in the subtropical North Pacific during a positive phase of spring AO, which is conducive to occurrence of a La Niña event in the following winter. However, the models with positive AO-ENSO relations produce a pronounced spring cyclonic anomaly over the subtropical northwestern Pacific and westerly anomalies over the tropical western Pacific (TWP). These westerly wind anomalies would lead to SST warming and positive precipitation anomalies in the tropical central-eastern Pacific (TCEP) during the following summer, which could maintain and develop into an El Niño-like pattern in the following winter via a positive air-sea feedback. By contrast, the models with negative AO-ENSO connections fail to reproduce the spring AO-related cyclonic anomaly over the subtropical northwestern Pacific and westerly wind anomalies in the TWP. Thus, these models would produce a La Niña-like pattern in the subsequent winter. Difference in the spring AO-associated atmospheric anomalies over the subtropical North Pacific among the CMIP5 models may be attributed to biases of the models in simulating the spring climatological storm track.

D3516 |
EGU2020-10693
Michael Mayer and Magdalena Alonso Balmaseda

In 2014 the scientific community and forecasters were expecting a major El Nino event, which was suggested by physical indicators and predicted by several seasonal forecasting systems. However, only moderately warm El Nino – Southern-Oscillation (ENSO) conditions materialized in 2014, but one year later in boreal winter 2015/16, one of the strongest El Ninos on record occurred. Moreover, the 2015/16 El Nino exhibited very unusual energetics: Despite warm conditions in the tropical Pacific in 2014 and especially 2015, its ocean heat content (OHC) did not decrease during that period, which usually is the case during El Nino events. Overall, the 2014-16 evolution of the tropical Pacific was quite different from the evolution during the 1997/98 El Nino, which exhibited exceptionally strong Pacific OHC discharge. This discrepancy was attributed at least partly to the anomalously warm Indian Ocean and the exceptionally weak Indonesian Throughflow transports during 2015-16 that kept Pacific OHC at high levels.

This contribution aims to elucidate the role of the Indian Ocean in the tropical Pacific Ocean evolution during ENSO for the two periods February 1997-1999 and February 2014-2016. For this purpose, we perform initialized two-year predictions using the ECMWF seasonal forecasting system. To isolate the role of the Indian Ocean, we carry out hindcasts with unperturbed ocean initial conditions and hindcasts with swapped Indian Ocean initial conditions, where the 2014 (1997) hindcasts use Indian Ocean initial conditions from 1997 (2014). We first investigate the impact of the Indian Ocean on the strength of the Indonesian Throughflow and the evolution of the tropical Pacific heat budget. Second, we seize these experiments to explore the impact of the Indian Ocean state on two-yearly ENSO evolution, especially on the probability of extreme events, and which role the atmospheric bridge plays versus the oceanic bridge.

D3517 |
EGU2020-5119
Xinjia Hu, Holger Kantz, and Eberhard Faust

The El Niño Southern Oscillation (ENSO) is one of the most important inter-annual climate phenomena with worldwide impacts. It can influence daily temperature and rainfall, as well as cause extreme weather events and natural disasters. Therefore, early and reliable prediction of the onset and magnitude of ENSO is crucial for different stakeholders. In order to overcome the “spring predictability barrier” in ENSO prediction, recent studies have developed some analysis tools and put forward some forecasting indices based on climate network, claiming they have achieved the long-lead-time (over 6 months) forecasts. However, there are few kinds of research to quantitatively compare the predictive power of these methods. Thus developing a method to measure the quality of these forecasts and compare their predictive power is necessary and meaningful for the improvement of ENSO prediction skills. In these existing researches, in order to set the threshold or estimate the accuracy of the prediction, the standard El Niño indices such as Oceanic Index (ONI), Niño 3.4 Index and etc., are often used to be compared with the invented indices series. In this research, we look into these comparisons and results, and use the receiver operating characteristic curve (ROC) to quantitatively compare these recent analysis tools. Additionally, for demonstrating that the results are not accidental, randomized series obtained by reshuffling the temperature records are analyzed. In this paper, we use the method of surrogate data instead of using shuffle data in the evaluation procedure of the prediction to further improve the evaluation method of the El Nino prediction.

 

(This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 813844.)

D3518 |
EGU2020-4828
Gabriella Satori, Tamas Bozoki, Earle Williams, Colin Price, Anirban Guha, Ciaran Beggan, Mariusz Neska, and Mike Atkinson

Schumann resonances (SR) are the global electromagnetic resonances of the Earth-ionosphere cavity and constitute the extremely low frequency (< 100 Hz) radiation of the worldwide lightning activity (Schumann, 1952). The recording of SR intensity at a few distant SR stations is an efficient tool to monitor the global lightning. We present the variations of SR intensity in the transition months preceding the warm ENSO episodes for the two super El Niño events in 1997/98 and 2015/16 as well as for the two medium size El Niño periods in 2001/02 and 2008/09  based on SR observations at multiple locations.: Nagycenk, Hungary (47.6N, 16.7E);  Hornsund, Svalbard (77.0N, 15.6E);  Eskdalemuir, UK (55.3N, 3.2W);  Alberta, Canada (51.9N, 111.5W);  Boulder Creek, USA (37.2N, 122.1W).

A remarkable increase in SR intensity is documented two-three months before or just at the beginning of El Niño episodes as compared with the SR intensity in the same months of the preceding La Niña (or non-ENSO) phase for all cases studied here. The percentage increase in SR intensity depends on the amplitude of the warm ENSO period, and is consistently higher for the two super El Niño events. The enhanced SR intensity indicates a worldwide response of global lightning activity. Increased atmospheric instability due to the land-ocean thermal interaction during the transition interval could be responsible for the intensification of lightning activity. This systematic behavior may have been overlooked in earlier studies that compared lightning activity in the integrated ‘cold’ and the ‘warm’ phases, but without exploring the transitional variation. Our results suggest that the SR intensity variation on the interannual time scale acts a precursor for the occurrence of warm ENSO episode.

D3519 |
EGU2020-7407
Chen Li, Dietmar Dommenget, and Shayne McGregor

A robust eastern tropical Pacific surface temperature cooling trend along with the strengthening of Pacific trade wind is evident across different observations since late 1990s, which is considered as a pronounced contributor to the slowdown in global surface warming. However, most CMIP5 historical simulations failed to reproduce this La Niña-like change. Previous studies have attributed this discrepancy between the multi-model simulations and the observations to the underrepresentation of Pacific low-frequency variability together with the misrepresentation of inter-basin forcing response. The underlying reasons remain unclear. Here, we investigate a hypothesis that common Pacific mean SST bias may diminish the Pacific-Atlantic atmospheric teleconnection and further contribute to the underestimated eastern Pacific cooling. Model results suggest that the CMIP5-like Pacific bias acts to reduce the Atlantic heating response by strengthening the atmospheric stability over the Atlantic region and therefore weaken the trans-basin variability. In addition, the Pacific bias simulation with a strong SST cold tongue substantially undermined the positive zonal wind feedback, which also contributes to the underestimated Pacific cooling response. Future efforts aim at reducing the model mean state biases may significantly help to improve the simulation skills of the trans-basin teleconnection, Pacific decadal variability, and the associated Pacific dynamics.      

D3520 |
EGU2020-3235
Ruiqiang Ding, Yu-heng Tseng, and Jianping Li

Variations in the sea surface temperature (SST) field in both the North Pacific [represented by the Victoria mode (VM)] and the South Pacific [represented by the South Pacific Quadrapole (SPQ) mode] are related to the state of the El Niño-Southern Oscillation (ENSO) three seasons later. Here, with the aid of observational data and numerical experiments, we demonstrate that both VM and SPQ SST forcing can influence the development of ENSO events through a similar air–sea coupling mechanism. By comparing ENSO amplitudes induced by the VM and SPQ, as well as the percentages of strong ENSO events followed by the VM and SPQ events, we find that the VM and SPQ make comparable contributions and therefore have similar levels of importance to ENSO. Additional analysis indicates that although VM or SPQ SST forcing alone may serve as a good predictor for ENSO events, it is more effective to consider their combined influence. A prediction model based on both VM and SPQ indices is developed, which is capable of yielding skillful forecasts for ENSO at lead times of three seasons.

D3521 |
EGU2020-6492
Na-Yeon Shin, Jong-Seong Kug, Felicity S. McCormack, and Neil J. Holbrook

   In the past decades, our understanding of the ENSO phenomenon increased steadily. Especially, one of the most interesting topics was the El Niño type because of the different global impacts. The classic classification is the two types of the El Niño and there are various terms to refer this. The conventional El Niño is called the Cold tongue El Niño or the Eastern pacific El Niño. And the other type of the El Niño is called the Warm pool El Niño, the Central pacific El Niño, the El Niño Modoki or the dateline El Niño. However, in Coupled Model Intercomparison Project version 5 (CMIP5) Coupled General Circulation Models (CGCMs) results, those have been shown the Double peaked El Niño events which are the new type of the El Niño due to the climatological cold tongue bias. Double peaked El Niño events are defined as a positive sea surface temperature anomalies are separated into two centers (in Western and Eastern Pacific) and grow individually and simultaneously, and the peak of SST anomalies exceeds the threshold.

   Double peaked El Niño events are found in not only the models, but also the observations. But there are no dynamical analysis of observations. In this study, the mechanism giving rise to Double peaked El Niño in observation is examined by analyzing the mixed layer heat budget equation and comparing with the Warm Pool El Niño and Cold tongue El Niño.

   The warm SST anomalies of the western peak and the eastern peak are caused by different dynamic mechanism. Western peaks of Double peaked El Niño are similar to the Warm Pool El Niño. Those can be developed by Zonal advection feedback terms and negative anomalous wind speed, whereas eastern peaks of Double peaked El Niño are different from Warm pool El Niño. Thermocline feedback term considerably contribute to the occurrence of eastern peak. Differences of intensity of the precipitation(4-8N, 195-225E) derive other significant differences of the zonal wind stress(5S-5N, 170-200E), sea level(5S-5N, 230-250E) and zonal current(5S-5N, 230-250E). Thus, the process above can induce the eastern peak of the Double peaked El Niño.

D3522 |
EGU2020-17743
Valentina Pivotti, Annalisa Cherchi, Alessio Bellucci, and Bruce Anderson

<p>Previous research has extensively established that, during the last decades, the Trade Wind Charging (TWC) mechanism is a fundamental precursor of El Ni&ntilde;o Southern Oscillation (ENSO). Moreover, recent results suggest that its relevance as an ENSO driver varies when a longer time interval is included the analysis. This article investigates whether TWC is isolated as a significant ENSO precursor; and how the internal variance of their coupling behaves, on a CMIP6 multi model ensemble. In particular, we consider the models participating to the CMIP6 HigResMIP, specifically designed to investigate the role that model resolution plays in simulating climate processes. For each model, we have included in the analysis 100-year long integrations of the present climate that are forced with constant radiative forcing representative of the 1950s at standard and enhanced resolutions.</p>
<p>The analysis follows two steps for each experiment. First, through a combination of Empirical Orthogonal Function (EOF) and Canonical Correlation Analysis (CCA) it isolates ENSO and TWC. Then, in order to study their mutual relation, the combination of EOF and CCA is repeated over shifting time intervals.</p>
<p>The analysis indicates TWC as a strong ENSO precursor for at least one model, and the coupling between the modes shows signs of internal variability. Also, the ways in which the models reconstruct the TWC, in its intensity and shape, and its coupling with ENSO appear to be affected by the changes in resolution. These results provide an insight over the different degrees at which HigResMIP model experiments are able to characterize the features of a fundamental process like ENSO. Moreover, they cast a light over the impacts that a change in oceanic or atmospheric resolution can have when simulating a coupled mode.</p>

D3523 |
EGU2020-21756
Lander R. Crespo, Belen Rodriguez-Fonseca, Irene Polo, Noel Keenlyside, and Dietmar Dommenget

We use a simple conceptual recharge oscillator model for the tropical Pacific to identify multidecadal changes in El Niño-Southern Oscillation (ENSO) statistics and dynamics during the observational record. The model, defined by only two variables, sea surface temperature (SST) and warm water volume (WWV), is fitted to the observations for the period 1901-2010. The variability of ENSO has increased during the 20th century. The model simulates similar changes in variance of SST and WWV. The cross-correlation between SST and WWV also shows significant changes during the observational record. From the 1970s onwards, both observations and model output show that the SST drives WWV anomalies with a lead-time of 10 months and the WWV feedbacks onto the SST with a lead-time of about 8 months. The latter is reminiscent of a recharge-discharge mechanism of the upper ocean heat content. Before the 1970s only the impact of SST on WWV, through implied wind changes, is observed and is reproduced by the model. The periodicity of ENSO has also changed; ENSO has become more frequent changing from a 7-yr periodicity in the beginning of 20th century to a 5-yr periodicity in the recent decades. We find that the full recharge-discharge mechanism of the equatorial upper ocean heat content that characterizes the dynamics of the ReOsc model is only observed from the 1970s onwards and is likely to be a consequence of a stronger observed coupling between WWV and SST and of the leading role of the thermocline feedback. The degrading quality in the observations for earlier periods can also partly explain the decadal changes in the ENSO interactions. We find that the Atlantic Multidecadal Variability and global warming can partly explain the observed and simulated multidecadal changes in ENSO properties.

D3524 |
EGU2020-9584
David Webb

An analysis of archived data from the NEMO 1/12th degree global ocean model shows the importance of the North Equatorial Counter Current (NECC) in the development of the strong 1982–1983 and 1997–1998 El Niños.  The model results indicate that in a normal year the coreof warm water in the NECC is diluted by the surface Ekman transport, by geostrophic inflow and by tropical instability waves. During the development of the 1982–1983 and 1997–1998 El Niños, these processes had reduced effect at the longitudes of warmest equatorial temperatures. During the autumns of 1982 and 1997, the speed of the NECC was also increased by a stronger-than-normal annual Rossby wave and other changes in sea level in the western Pacific.  The resulting increased transport of warm water by the NECC resulted in water with temperatures above 28C reaching the eastern Pacific.  This appears to have been a major factor in moving the centre of deep atmospheric convection eastwards across the Pacific.

Note:  This is based on the paper published in Ocean Science.  An oral presentation is possible.

D3525 |
EGU2020-18388
Srinivas Gangiredla, Jerome Vialard, Takeshi Izumo, Matthieu Lengaigne, and Eric Guilyardi

Understanding the key physical processes involved in the development and diversity of El Niño events is essential to anticipate their multiple impacts. Current El Niño theories generally assume that wind stress responds linearly to El Niño Sea Surface Temperature (SST) anomalies. Yet, the deep atmospheric convection that energizes this wind stress response has obvious nonlinear features. Observations indeed indicate that rainfall (a proxy of the tropospheric heating) increases slowly with SST up to 26.5OC, followed by a sharp increase of rainfall at higher SSTs. In this study, we use that mean observed relation to derive a nonlinear relation between SST and rainfall anomalies, that depends on the background climatological SST. This relation performs much better to explain rainfall anomalies in the eastern Pacific (Niño3 region) than a linear relation, which underestimates rainfall during most extreme El Niños events. On the other hand, it underestimates rainfall anomalies in the western Pacific (Nino4), because it only considers the local SST forcing and neglects the atmospheric convergence feedback. Our observational results are in line with previous modeling studies, who have also underlined the importance of the nonlinearity of the convective response to SST anomalies for large El Niño events in coupled models. We end by discussing other possible sources of nonlinearity in the wind stress and heat flux responses to SST, which play a strong role in the most essential El Niño feedbacks.

D3526 |
EGU2020-1163
Jake Casselman and Daniela Domeisen

El Niño-Southern Oscillation (ENSO) influences the weather around the globe. These so-called ‘teleconnections’ occur on sub-seasonal-to-seasonal timescales, and can be useful for weather and climate predictions. ENSO teleconnections can reach as far as the North Atlantic-European (NAE) region, where ENSO influences remain insufficiently understood. ENSO teleconnections to the NAE region can travel through a range of different pathways, with differences in seasonality for each pathway. We here focus on determining the importance of the North Atlantic Oscillation (NAO) for establishing the connection between the tropical Pacific and the tropical Atlantic, following an ENSO event. We use reanalysis data in combination with a simplified atmospheric global circulation model with ENSO-like sea surface temperature (SST) forcing to investigate both the isolated and combined influences from these different pathways. Initial results suggest that the NAO’s influence onto the tropical Atlantic may play a minor role, as surface wind impacts are likely too far north to contribute to a wind-evaporation-SST (WES) feedback within the tropical Atlantic. Shifts in the longitudinal position of ENSO may, however, cause changes in the influence from the NAO onto the tropical Atlantic. Such changes may help in explaining the presence of significantly different spatial patterns of SST in the tropical Atlantic, following different ENSO flavors.

D3527 |
EGU2020-19162
Fred Kucharski, Manish K. Joshi, and Mohammad Adnan Abid

In this study the role of an Indian Ocean heating dipole anomaly in the transition of the North Atlantic circulation response to ENSO from early to late winter is analysed using 20th century observations and simulations from the fifth Coupled Model Intercomparison Project (CMIP5). It is shown that in early winter a warm (cold) ENSO even is connected trough an atmospheric bridge with positive (negative) rainfall anomalies in the western and negative (positive) anomalies in the eastern Indian Ocean. The early winter heating dipole teleconnected to a warm (cold) ENSO event can set up a wavetrain emanating from the south Asian subtropical jet region that reaches the North Atlantic and leads to response that spatially projects onto the positive (negative) phase of the North Atlantic Oscillation (NAO). The Indian Ocean heating dipole is partly forced as an atmospheric teleconnection by ENSO, but can also exist independently and is not related to local Indian Ocean SST forcing. The Indian Ocean heating dipole response to ENSO is much weaker in late winter (February and March) and not able to force significant signals in the North Atlantic region. CMIP5 models reproduce the early winter heating dipole reponse to ENSO and the ENSO response transition in the North Atlantic region to some extend, but with weaker amplitude. Generally models that have a strong early winter ENSO heating dipole teleconnection to the Indian Ocean also reproduce the North Atlantic response. If an Indian Ocean vertical velocity dipole index is defined, overall CMIP5 models are able to reproduce the extratropical responses in early winter reasonably well.

D3528 |
EGU2020-4187
Lina M. Estupinan-Suarez, Alexander Brenning, Fabian Gans, Guido Kraemer, Carlos A. Sierra, and Miguel D. Mahecha

The response of tropical vegetation to El Niño Southern Oscillation (ENSO) is considered a main driver of global annual atmospheric CO2 concentrations at inter-annual time scales. ENSO warm and cold phases, El Niño and La Niña respectively, cause contrasting climatic conditions along tropical South America. While some regions experience wetter conditions during El Niño, such as  the Pacific coast, others regions such as the Amazon are exposed to warmer and drier climates. Besides this spatial variation, the biospheric response also differs between ENSO type and intensity, overruling of local conditions and ecosystems types. Due to this complexity, there is a lack of understanding on what ecosystems and regions are systematically affected by ENSO and how biospheric variables respond. Here, we analysed the Northern region of tropical South America covering tropical savannas, forests, and mountainous ecosystems in several countries. To do this, we assessed different land surface (e.g. GPP, NDVI,  FPAR, LST) and climate data streams compiled in the regional Earth System Data Lab (ESDL, https://www.earthsystemdatalab.net/) at 1 km and 10 km pixel size from 2001 to 2015. We applied Isomap, a non-linear dimensionality reduction method in the time domain for high dimensional dynamical systems. Our analysis was constrained to the fourth order continental basins and dominant land cover types. Land use change pixels were disregarded. Further, a comparison of ENSO indexes was conducted among basins. We found that isomap components  are able to capture the biosphere variability related to ENSO in basins that have been historically affected such as Magdalena-Cauca valleys and the Caribbean region. Implementation of non-linear methods increases our understanding of ENSO impacts spatially in regions where events intensity and frequency is increasing, and effective ecosystems management is urgent.