ABSTRACT The effect of future climate change on the boreal winter response to strong El Niño is investigated using pacemaker simulations with the EC-Earth3-CC model constrained towards

observed tropical Pacific sea surface temperature anomalies. Under the Shared Socioeconomic Pathway 2-4.5, the surface temperature response to strong El Niño intensifies in North America,

northern Africa, Australia and the North Atlantic compared to present day. However, future strong El Niño has a weaker climate impact in southern America and Africa. Temperature extremes

under strong El Niño intensify in the future in some regions, with more cool days in eastern North America, while warm days in northern South America decrease. Assuming that the

THE MID-LATITUDES Article Open access 05 March 2021 RECENT CHANGES IN ENSO’S IMPACTS ON THE SUMMERTIME CIRCUMGLOBAL TELECONNECTION AND MID-LATITUDE EXTREMES Article Open access 14 January

2025 EMERGENCE OF CLIMATE CHANGE IN THE TROPICAL PACIFIC Article 07 March 2022 INTRODUCTION El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability across

the global tropics. Every 2-7 years, anomalously warm or cool sea surface temperature (SST) anomalies develop in the equatorial Pacific Ocean and trigger a global cascade of remote

effects1. The warm phase of ENSO, known as El Niño, leads to dry and hot conditions over Australia2, northern South America3, southern Africa4,5 and the Indian Summer monsoon region6, as

well as wet and cooler conditions over some parts of North America7 and Eastern Africa8. The strongest El Niño events since the late 19th century occurred in 1982/83, 1997/98, and 2015/16,

when SST anomalies in the Niño3.4 region (SST averaged over 5°S-5°N, 170 W°-120°W) reached 2.2oC, 2.4oC and 2.6oC, respectively (estimated as 3-month running means with the ERSST.v5

dataset9). These strong El Niño events led to widespread devastating impacts, including severe droughts in Southeast Asia, Northern and Southern Africa10, the Amazon11, Mexico and

Philippines, and a mangrove dieback in Northern Australia12; widespread fires in Indonesia13,14; storms in the US, flooding in Cuba, Peru, Bolivia and Ecuador15; large changes in the global

carbon cycle16,17; and food insecurity10,18. During strong El Niño events, SST and precipitation anomalies extend further east in the equatorial Pacific compared to moderate events19,20,21

particularly in December-January-February (DJF). The teleconnections of El Niño to the Indo-Pacific sector22, Australian23, Northern24 and South American25 and North Atlantic-European26

regions depend on the amplitude of the event, with the impacts of strong El Niño often more severe than weak or moderate events. The global impacts of strong El Niño events may change in the

future due to increases in anthropogenic radiative forcing and, as a direct consequence, continued global mean surface warming. Such changes could arise from two effects: 1) changes in the

frequency and/or characteristics of strong El Niño events; 2) changes in the processes that lead to teleconnections. Both effects depend on the interactions between ENSO and the mean climate

state. Regarding mechanism 1, in the tropical Pacific most climate models project a mean weakening of the Walker circulation and a reduction of the zonal SST gradient in the equatorial

Pacific by the end of the 21st century27. This change in mean state leads to an increase of ENSO variability under increased anthropogenic forcing in most models from the Sixth phase of the

Coupled Model Intercomparison Project (CMIP6)28,29, including an increase in the likelihood of strong El Niño events20,30 partially driven by a strong increase in temperature over the

central and eastern equatorial Pacific. However, there is uncertainty associated with this response, in part due to the generally poor representation of recent tropical Pacific SST trends in

historical model simulations31. In fact, despite most CMIP6 models simulating an “El Niño-like” SST state in the future, the models which more closely capture the observed the tropical

Pacific SST trend project a continuation of the “La Niña-like mean state”, which eventually might weaker the amplitude of El Niño events in the future32. Regarding mechanism 2, the Sixth

Assessment Report of the Intergovernmental Panel on Climate Change highlights a robust trend towards an increase in ENSO-driven precipitation variability in a warmer climate33. Following the

Clausius-Clapeyron relationship, absolute humidity increases by approximately 7% per Kelvin, hence the changes in tropical humidity driven by ENSO are expected to increase in the future34.

Ying et al.35 found that an anthropogenically-forced signature in ENSO-related precipitation variability will emerge around 2040, decades before a robust change in ENSO SST can be detected

in models27. This means that we might expect changes in ENSO teleconnections to emerge before any changes in ENSO characteristics may be detectable from internal climate variability. While

several modelling studies have examined ENSO teleconnections in a future climate, they have generally used freely-evolving coupled climate models and therefore do not permit a separation of

mechanisms 1 and 229,36,37,38,39. McGregor et al.38 found significant changes in ENSO impacts over around one third of global land area in CMIP6 models, but it was unclear what factors

caused these changes. A small number of studies have isolated mechanism 2 by prescribing identical SST anomalies on top of present and future background climates40,41. These studies found an

amplification of ENSO teleconnections in boreal winter in the North Pacific and North Atlantic-European sectors under future climate conditions. However, Drouard and Cassou40 used ENSO SST

anomalies derived from a model, which may not be fully realistic, and Zhou et al.41 used an atmospheric model meaning any role of atmosphere-ocean coupling in modifying teleconnections was

excluded. Neither study assessed the global effects of future climate changes on El Niño impacts and did not specifically address the impacts of strong El Niño events, which as noted above

are often the most impactful. Our study follows a similar approach to Drouard and Cassou40 and Zhou et al.41 by exploring mechanism 2 and assuming that ENSO characteristics do not vary with

climate change. We specifically focus on strong El Niño and use observed SST anomalies combined with a pacemaker modelling approach to enable atmosphere-ocean coupled interactions outside of

the tropical Pacific (see Methods). We simulate three observed strong El Niño events (1982/83, 1997/98 and 2015/16) under present day and projected future climate states corresponding to

the year 2090 under the SSP2-4.5 scenario. We focus on responses in the boreal winter season (DJF) and find the events behave similarly in our pacemaker simulations, so in most of the

analyses we combine them into a multi-event mean (MEM). RESULTS CHANGES IN THE TROPICAL PACIFIC INDUCED BY STRONG EL NIÑO AND CLIMATE CHANGE Figure 1a shows the MEM El Niño SST anomalies

under present day. The anomalies closely resemble the pacemaker target SST pattern, with the canonical eastern Pacific structure of strong El Niño. In the tropical Pacific, strong El Niño

events drive an eastward shift of the precipitation maximum, leading to precipitation anomalies of above 10 mm day−1 over the central and eastern equatorial Pacific (Fig. 1b), similar to the

observed precipitation pattern during El Niño (Fig. S1). Those positive anomalies contrast with drier conditions off the equator around 10°N and in the South Pacific Convergence Zone. There

are very small differences in the El Niño SST anomalies in the future (Fig. 1c), indicating the pacemaker method works as intended (see “Methods” and Fig. S2). There are, however,

significant differences in El Niño-driven precipitation in the future (Fig. 1d), with higher precipitation rates in the eastern (more wetting) and western (less drying) equatorial Pacific of

up to 2 mm day−1 and negative anomalies (less wetting) of at least 2 mm day−1 over the central equatorial Pacific. We note that these differences do not fully match the projected change in

El Niño-driven precipitation anomalies in the tropical Pacific suggested by the literature, consisting of an eastward shift of precipitation during El Niño under greenhouse38 forcing42. This

might indicate an important role for El Niño-SST changes in the future El Niño-precipitation relationship seen in coupled climate models, or be specific to the effect of climate change on

the precipitation response to strong El Niño events, which are generally not isolated in other studies. It is also possible the pacemaker set–up influences the associated precipitation

response, for example because it constrains surface turbulent fluxes, but similar differences were found in an experiment with weaker flux restoring (not shown). In terms of mean state

changes, the tropical Pacific warms by around 2.5 K in the future (Fig. 1e), with the Niño4 (5°N-5°S, 160°E-150W) and Niño3 (5°N-5°S, 150°W-90°W) regions being 4.7% and 6.6% warmer,

respectively, compared with the present day. Consequently, the zonal east-west equatorial Pacific SST gradient decreases by 19.5%, similar to the response in other CMIP6 models43. Coincident

with the mean SST changes, there is a weakening of the Walker circulation, evidenced by a decrease in precipitation over the warm pool and an increase in precipitation over the Central and

Eastern Pacific (Fig. 1f). To understand how strong El Niño conditions would appear in the future compared to present day, we next consider the combined mean state plus El Niño anomalies

(Fig. 1g and 1h). Since the El Niño SSTs are largely unchanged in the future, the total near-surface temperature signal mainly reflects the change in tropical Pacific mean state, whereas for

the total precipitation anomaly, the substantial negative anomalies between 170°E-150°W seen as a difference in the response to El Niño in the future, expand the location of dry conditions

further towards the central Pacific. Such a change might be expected to alter the teleconnections from El Niño, which is explored in the next section. TELECONNECTIONS FROM STRONG EL NIÑO

EVENTS ARE MODULATED BY CLIMATE CHANGE In the present day, the modelled strong El Niño drives significant surface temperature impacts across the globe in DJF (Fig. 2a). Northern South

America experiences warm anomalies reaching up to 3 K over the Amazon, as well as large parts of the African continent, with the exception of near Lake Victoria. Western Australia

experiences hotter summers during strong El Niño, and south Asia also sees warm anomalies. There are some regions, though, where El Niño leads to cold anomalies in DJF, such as the southern

and western US and south-east South America (SESA). Note that the EC-Earth3-CC model captures the amplitude and spatial location of the winter surface response to El Niño over northern South

America, North America and Southern Africa similar to observations (Fig. S1). Alongside surface temperature changes, strong El Niño events drive dry anomalies of up to 2 mm day−1 over the

Amazon basin, although based on the model evaluation these anomalies may be overestimated (Fig. S1), and around 1 mm day−1 over Southern Africa. There is a dipole precipitation anomaly over

the North Atlantic, likely driven by a southward shift of the North Atlantic jet stream and storm track (Fig. 2b), characteristic of the link between El Niño and the negative phase of the

winter North Atlantic Oscillation (NAO)44. Over the Congo basin and the Horn of Africa, strong El Niño leads to weak but significant positive precipitation anomalies. Precipitation also

increases by 2 mm day−1 over SESA during strong El Niño, leading to wetter than usual summers in the north of Argentina, Uruguay and Paraguay. EC-Earth3-CC effectively captures the sign and

spatial location of positive precipitation anomalies during El Niño, but slightly underestimates its amplitude compared to observations (Fig. S1). In terms of the ability of the model to

represent the complexity of the amazon climate, it has shown to correctly simulate the annual cycle of temperature and precipitation, and outperforms most CMIP6 models in the representation

of the south American monsoon dynamics45. In the future, the DJF surface temperature response to strong El Niño increases in several regions (Fig. 2c). Strong El Niño drives even hotter

conditions in most of Africa (except in its southern part), and hotter summers in the south-west of the Amazon and Congo basins and eastern Australia, being the future response to El Niño in

the latter stronger than in the present. Over north-east North America, the cold anomalies driven by strong El Niño are also enhanced in the future. On the other hand, mean DJF temperature

responses driven by strong El Niño in the future weaken in other regions potentially due to shifts in atmospheric circulation. For example, cold anomalies over north-east Argentina, Paraguay

and south Brazil extend more towards the north, following the northward shift of precipitation shown in Fig. 2d. Over southern Africa, the temperature and precipitation responses to strong

El Niño weaken in our simulation, leading to milder summers. In Australia, the influence of future climate change on the response to strong El Niño shows dipole anomalies consisting of

higher precipitation rates over western Australia and drier conditions over the east in DJF, which is opposite to the projected precipitation trend in austral summer in the model (Fig. 2f).

We note that other climate models tend to show an increase in Australian summer rainfall in future projections33, which would constructively interfere with the altered strong El Niño signal

in western Australia. In a limited number of regions, the precipitation response to strong El Niño events weakens in the future compared to present day. For instance, in the present day

strong El Niño drives dry anomalies over the Amazon basin (Fig. 2b), but this changes in the future, where the meteorological drought induced by strong El Niño becomes significantly weaker

(Fig. 2d). Figure 2e, f show the mean state surface temperature and precipitation changes between the future and present day. By the late 21st century, EC-Earth3-CC simulates an overall

statistically significant increase in temperature across the globe, especially in the Northern Hemisphere (in agreement with Lee et al.33), where land areas experience a warming of above 4 K

in DJF. In the Southern Hemisphere, regions like western Australia, southern Africa and the Amazon basin are warming faster than others. In terms of precipitation, besides the projected

eastward shift in tropical Pacific precipitation resulting from a future weakening of the Walker circulation, there’s an overall increase in precipitation across the tropics, in agreement

with CMIP6 projections33. Regions like Australia, North America and East Asia are expected to experience a slight increase precipitation in DJF, in agreement with multimodel projections,

although the change in precipitation is not as evident as in temperature due to the higher variability of the former and the larger model uncertainty regarding dynamical changes as a

response to climate change. The total anomalies during strong El Niño in the future compared to present day (Fig. 2g, h) show constructive interference and exacerbated El Niño-driven warm

anomalies over eastern Australia, Northern South America and Central Africa during the austral summer months, which would lead to increased heat-related risks46,47. The mean warming under

climate change masks any enhancement of the cooling effect of strong El Niño, such as over North America or East Asia. Nevertheless, over South America, Fig. 2h highlights the contribution

of changes in strong El Niño impacts to the absolute response over the Amazon basin, which drives an increase in precipitation in DJF, opposing the mean drying under global warming.

TROPOSPHERIC CIRCULATION RESPONSE TO STRONG EL NIÑO AND ITS CHANGES UNDER FUTURE CONDITIONS To understand future changes in teleconnections of strong El Niño, we explore tropospheric

geopotential height at 500 hPa (z500) and zonal wind at 200 hPa (u200) anomalies in DJF. The extratropical teleconnections of El Niño depend on the background flow, specifically on the upper

tropospheric zonal wind48, which is stronger in the Northern hemisphere in DJF due to a stronger subtropical jet. In the present day, strong El Niño generates positive z500 anomalies of up

to 60 m off the coast of Japan (Fig. 3a), a deepening of the Aleutian low and barotropic Rossby wave trains that project onto the Pacific South American (PSA1) and Pacific North American

(PNA) patterns. As opposed to moderate and weak El Niño events, strong episodes shift the PNA further east24, making these events unique and incomparable with El Niño events of lesser

amplitude. Outside of the Pacific sector, strong El Niño drives negative geopotential anomalies over the south-east USA, which extend eastward over the subtropical Atlantic Ocean. Those

negative anomalies contrast with positive anomalies spanning from the north-east of Canada to the Norwegian Sea. This corresponds to a negative phase of the NAO and a southward shift of the

North Atlantic westerly jet stream, in agreement with the southward shift of precipitation anomalies shown in Fig. 2b. Under future conditions, strong El Niño drives a weaker PNA response,

with the simulations showing a relatively stronger Aleutian low in boreal winter with positive z500 anomalies of above 40 m (Fig. 3b). The eastward shift of z500 anomalies in the North

Pacific and North America, in agreement with Johnson et al.49 and Geng et al.50 coincide with below normal temperatures shown in Fig. 2c. Similar conditions are shown over central and

northern Europe, where strong El Niño events drive relatively low pressure anomalies associated with colder winters. There is a high pressure anomaly in the Caribbean region during strong El

Niño events in the future, which translates into the drying trend shown in Fig. 2c. In the Southern hemisphere, strong El Niño triggers an anomalous wavetrain spanning the extratropics and

subpolar regions and driving a dipole of positive and negative z500 anomalies in the west and eastern coasts of South America, respectively. The projected changes in El Niño-driven z500 over

South America, do not resemble those shown in Johnson et al.49 possibly due to the weaker climate change signal in our simulations (compared to SSP5-8.5 used in their study) and the fact

that changes in El Niño anomalies modulated by climate change are only expected to emerge at the end of the century. Also, as it is the case of the teleconnection towards north America, the

fact that amplitude of the regional response to El Niño is not linearly related to the strength of the event makes it hard to compare with studies based on El Niño composites of all

strengths as in Johnson et al.49 and McGregor et al.38. In terms of mean state changes, Fig. 3c shows a deepening of the Aleutian low by 20 m in the future climatology, compared to

present-day conditions. In the South Pacific Ocean, there is a strengthening of the zonal low pressure anomalies across SESA, and positive z500 anomalies in the Southern Ocean off the coast

of South America. It is important to note that the amplitude of future changes in El Niño-driven z500 anomalies (Fig. 3b) is twice as large as the mean state changes (Fig. 3c), highlighting

the significance of changes in ENSO teleconnections in the future relative to changes in the background flow. The absolute changes in z500 and zonal wind during strong El Niño events in the

future are dominated by changes in El Niño teleconnections rather than by mean state changes, as suggested by Beverley et al.51. REGIONAL EXTREME TEMPERATURE RESPONSES TO STRONG FUTURE EL

NIÑO EVENTS Building on the evidence in the previous sections showing regional signatures of the modulation of strong El Niño teleconnections under future climate change, we now investigate

regional changes in meteorological extremes under strong El Niño, in North America, southern South America and southern Africa. Special attention is paid to shifts in the distribution of

daily maximum temperature from the present to the future considering both cool and warm days (see Methods). We first consider cool days in North America using a cool day index (see Methods)

to understand changes to the distribution of cold anomalies driven by El Niño in the future. Note this index is percentile-based and therefore effectively removes the climatological shift in

cool days driven by anthropogenic forcing. In the present day, strong El Niño drives an increase of cool days in North America of up to 7% per winter, especially along the Rocky Mountains

and central-eastern US (Fig. 4a), and a 6% reduction in cool days per winter over eastern Canada. There is a relative increase in cool days per winter during strong El Niño events over most

of the USA in the future compared to present day (Fig. 4b), meaning an intensification of cold anomalies in the east and the appearance of cool anomalies over the northern US and Alaska. The

mean climate change signal shifts the local climatological 10th percentile of maximum daily temperature towards higher values in the whole North America (See Fig. S3c), especially over the

Northeast of the United States, Canada and Alaska, where the threshold for cool days increases by up to 8 °C. The changes in relative occurrence of cold days during strong El Niño could have

societal implications, since a focus on adaptation to a warmer climate may reduce the focus on preparedness for cold days. At the same time, percentile-based indicators might not be as good

a proxy for climate impacts on health in the future as they are in the present, i.e. cold days in the future will be less harmful for human health. This difference overrides the

intensification of the cold El Niño anomalies expected at the end of the century, since the overall temperature will be higher due to the dominance of the global warming signal for the total

temperature anomaly. Northern South America is one of the regions where strong El Niño events have the largest impacts on daily temperature extremes. From December to February, strong El

Niño leads to an increase of warm days of up to 40%, especially around the Amazon basin in Brazil, Venezuela and the Pacific coast of Colombia and Ecuador. In the future, the striking impact

of strong El Niño events in the region gets muted in the Amazon basin, where the percentage of warm days decreases up to 15%. In southwest Brazil and Bolivia, however, there is a

significant strengthening of the El Niño signal, with up to 15% more warm days from December to February. Changes in extreme temperature impacts driven by El Niño in the future will play a

key role in the frequency and intensity of forest fires in the Amazon region, fueled by a negative precipitation and soil moisture trend52. According to Fig. 6, during strong El Niño, areas

of Africa south of the equator experience a strong increase in warm days in summer, with the largest increases over Equatorial Guinea, Republic of the Congo and Angola, with up to 25%

additional warm days per summer. In the future experiment, there is a relatively smaller increase in warm days per summer due to strong El Niño (approximately 7% less in some areas). In

addition, there are enhanced warm anomalies towards the northern regions of lakes Tanganyika and Malawi, Tanzania and the Democratic Republic of the Congo, where the number of anomalous warm

days per summer during strong El Niño is found to increase by up to 10%. Figure S3g-i shows the shift of the 90th Tmax percentile towards higher values in southern Africa, especially over

the west and southwest of the region, where the temperature threshold for a day to be considered warm will rise from 30 °C to 35 °C. The smaller increase in warm days in the future relative

to present-day conditions is caused by a raise of the Tmax threshold for warm days. Nevertheless, in the future strong El Niño still leads to an increase in warm days in southern Africa,

further exacerbating the underlying signal of climate change consisting of a shift towards higher temperatures in the region in austral summer. In the coastal areas of Mozambique and

Tanzania there is a significant increase of up to 10% in the number of El Niño-driven warm days in summer, which constructively interfere with climate change leading to an exacerbation of

warm summer days in the region. As shown in Figure S3i, the largest shift in the climatological warm day threshold is found in Namibia, Botswana and Mozambique. DISCUSSION This study has

used idealised pacemaker simulations with the EC-Earth3-CC model following the SSP2-4.5 scenario to quantify changes in the global impacts of strong El Niño events in a warmer climate. This

approach isolates the role of changing background climate conditions on the atmospheric response to El Niño, which has been a limitation of some previous studies (e.g., McGregor et al.38).

The experiments show precipitation anomalies during strong El Niño increase in the eastern equatorial Pacific and warm pool under future climate change, and decrease over the central Pacific

(Fig. 1d). The changes in the strong El Niño precipitation response in the eastern Pacific are comparable to the changes from mean climate change. Alongside the impact of climate change on

the local Pacific response to strong El Niño, the large-scale global atmospheric teleconnections are also modified. The Rossby wave trains emanating from the western tropical Pacific into

the northern and southern hemispheres show differences in amplitude and phase that modify regional climate hazards associated with strong El Niño. The change in frequency of cool days per

winter under strong El Niño is larger in the future over eastern North America. It is important to note that due to the overall background warming from climate change, the percentile-based

threshold adopted for cool days increases from −5 °C to 0 °C between present and future meaning the absolute temperatures are less severe but still below freezing. The number of cool days

per winter slightly decreases in the future climatology. Owing to the altered teleconnection associated with the PNA pattern, there is a larger frequency of cool days driven by El Niño in

the future (17.1%) than in the present (9.5%), as suggested by Lieber et al. 5. In agreement with Meehl et al.53 we find an eastward shift of cold extremes in future El Niño events and a

decrease in cool days over the west coast of the USA. This shift might be linked to the strengthening of the teleconnection of El Niño to the North Atlantic sector54. Finally, we show that

despite most of the regions with significant changes in strong El Niño impacts, a limited number of areas, like northern South America and southern Africa, experience a weakening of the

signal driven by El Niño under future conditions. In the Amazon basin, a region where warm days roughly double in frequency when there is a strong El Niño event, our experiments suggests a

weakening of this teleconnection translated into a decrease in warm days over most of the region. Figure 5 shows a weakening of the El Niño-driven warm days in most of the region except

southeast Brazil and Bolivia, which might be the result of a saturation of the impacts of El Niño under events of high amplitude such as the ones considered in this study. The weakening of

El Niño-driven temperature extremes in northern South America in the future coincides with the expected changes in mean temperature (Fig. 2c). Figure 6 suggests that El Niño-driven warm days

in summer will be lower in the future, especially over southern Namibia, Botswana and central South Africa. While the threshold to compute warm days as simulated by EC-Earth3-CC increases

from 28.8 °C in the present to 31.2 °C in the future, in agreement with future CORDEX-Africa climate projections over the region55, the El Niño-driven warm days do not increase in frequency

in a warmer climate. In absolute terms, the background warming induced by climate change will increase the frequency of heatwaves over Southern Africa, independent of whether there is a

strong El Niño event happening or not. Interestingly, the projected changes in El Niño teleconnections resemble those identified in previous studies, suggesting that simulated changes in

future impacts of El Niño in coupled climate models are dominated by changes in the mean state, rather than by changes in ENSO SSTs. In agreement with McGregor et al.38, we find a

strengthening and southward shift of the North Atlantic jet stream in boreal winter during El Niño events in the future, highlighting the reinforcement of the El Niño-negative NAO

relationship. This is also in agreement with previous literature (i.e. Drouard and Cassou40, Johnson et al.49), although this response is not captured in CMIP5 models37. Our results also

align with the overall CMIP5 and CMIP6 trends in the northward shift of precipitation anomalies north of SESA and the drying trend to the south of Brazil, although our pacemaker reveals a

weakening of the dry anomalies driven by strong El Niño events over the Amazon basin. Future work will assess whether there is any saturation of dry anomalies over this area with El Niño

events of large amplitude such as the ones we are focusing on. In terms of future changes in surface temperature anomalies driven by strong El Niño, we find similarities between our results

and the overall trend encompassing El Niño events of any amplitude. Some examples are a hot-gets-hotter trend over western Brazil, and Northern Africa, and while CMIP5 projections do not

show a significant change over Australia37, our results agree with overall CMIP6 projections38 and large ensemble studies36. Some additional features arise when focusing on strong El Niño

events that are not seen in the overall projections of ENSO impacts based on composite analysis, that indicate either a nonlinear climate response to strong El Niño events or a disagreement

of the EC-Earth3-CC model with the CMIP6 multi-mean results (i.e. McGregor et al.38). Our results reveal potential nonlinearities in the future climate response to El Niño events (e.g. the

Amazon basin), where increasing SST anomalies in the Niño3.4 region do not further amplify the global surface impacts associated with a canonical El Niño event. This nonlinearities might be

accompanied by shifts in circulation induced by climate change, with South America being the most illustrative example. According to our results, dry anomalies over the Amazon basin driven

by very strong El Niño will get muted in the future, and wet and cool anomalies over the SESA are expected to shift northwards in DJF. Further work is needed to assess the nature of these

shifts and nonlinear behaviours. We find an intensification of cold anomalies driven by strong El Niño over northeast North America and a significant weakening of warm anomalies over

northern South America and southern Africa. By comparing changes in the teleconnections of strong El Niño events with mean state changes due to climate change, we emphasise that although

strong El Niño might drive cool anomalies in the future in some regions, the underlying signature of climate change largely dominates the global sign of temperature anomalies in the future,

masking potential cooling effects of natural climate variability. The fact that changes in warm days driven by strong El Niño in the future over northern South America and Southern Africa

are not very pronounced is caused by the use of fully moving thresholds (based on climatological values of each period) to compute these extreme heat indices. In the examples shown in this

study, this indicates that thermodynamic changes modulated by climate change are the dominant source of the shift in the distribution of maximum daily temperature values, in agreement with

Vogel et al.56. Future work is needed to address potential nonlinearities in the impacts driven by very strong El Niño events in the future. As seen in most CMIP6 models, EC-Earth3-CC shows

a long-term future trend resembling an El Niño-like warming in the tropical east Pacific, decreasing the zonal SST gradient by almost 20% towards the end of the 21st century. This feature is

consistent with a weakening Fof the Walker circulation and is similar to the trend projected in most CMIP557 and CMIP6 models27, but does not match observed the Walker circulation

strengthening and SST trends over recent decades58,59. Human-caused climate change is warming the planet at unprecedented rates. Natural climate variability can either amplify or mute the

signature of climate change in the future. Strong El Niño events are projected to increase in frequency20, and given the severity and reach of their impacts, it is crucial to understand how

the impacts of these extreme events will change in the future to facilitate adaptation and mitigation policies in the areas affected. Our study suggests that the future climate conditions

during strong El Niño events will be more impacted by the underlying signature of climate change than by changes in ENSO itself. Future studies are needed to improve our understanding of the

mechanisms of such amplification and weakening of the future impacts of strong El Niño events. Additionally, a multi-model approach would enable our results to be verified for a larger set

of projections, as well as an exploration of other seasons beyond the canonical boreal winter season. METHODS Pacemaker simulations are performed with the EC-Earth3-CC model60 to understand

changes in the impacts of strong El Niño events between present and future climates. MODEL EC-Earth3-CC is an Earth System model with an approximate atmosphere and ocean resolution of 1°.

The atmospheric model is the European Center for Medium-range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) version 36r4, which is based on the ECMWF seasonal prediction

system 4, and has a resolved stratosphere. NEMO3.6 and LIM3 are the ocean and sea ice components; biogeochemical processes in the ocean and over land are simulated by the PISCES and

LPJ-GUESS models, respectively. The EC-Earth3-CC model generally reproduces observed ENSO teleconnections. Figure S1 compares the global ENSO precipitation and surface temperature

teleconnections in the December-February (DJF) season in EC-Earth3-CC with observations from the GPCP v2.361 and Berkeley Earth62 datasets. For the model-data comparison we use output from a

time-dependent tropical pacemaker simulation with EC-Earth3-CC covering the period 1900-2018, where historical SSTs are forced with observed SST anomalies in the tropical Pacific ocean.

This ensures the ENSO events simulated in the model broadly follow those observed during the historical period. The model captures the sign and amplitude of ENSO-related precipitation

anomalies over the tropical Pacific, but tends to overestimate the precipitation dipole response over the Indian Ocean (See Fig. S1). Over the equatorial Atlantic, the model displays a weak

zonal precipitation dipole consisting of dry conditions over the Amazon and western equatorial Atlantic and wet anomalies over the eastern Atlantic and the Gulf of Guinea, whereas

observations show an overall negative precipitation anomaly that peaks off the east Brazilian coast. Regardless of these differences, the overall patterns of global response in temperature

and precipitation are well captured by EC-Earth3-CC. EXPERIMENTAL PROTOCOL Six experiments are performed in which the model SSTs in the equatorial eastern Pacific are restored towards the

observed SST anomalies from the 1982-1983, 1997-98 and 2015-16 strong El Niño events superimposed on either the present day or future model climatological SST. Observed monthly SST anomalies

are extracted from the ERSSTv5 dataset9 calculated relative to the period 1981-2010. Present day climatological values are estimated from the historical simulations of EC-Earth3-CC using

the period 2005-2014. For the future, we use the SSP2-4.5 scenario, a middle-of-the-road greenhouse gas emissions pathway where global mean surface temperature in EC-Earth3-CC increases by

2.8 K in 2100 relative to preindustrial times. This SSP is consistent with the climate that would be achieved by the end of this century if countries deliver on their current climate

targets. Acknowledging that a stronger SSP scenario would provide a larger climate change signal, we performed large ensemble experiments with each El Niño event being simulated 30 times in

both present and future climates, ensuring that changes in El Niño in the future are distinguishable from the climate change signal. The period 2085-2094 is used to define the future

climatology. For each 10-year climatological period, we average over 10 initial condition ensemble members, meaning that the climatological states are calculated over 100 years. The observed

ERSSTv5 SST anomalies for the 3 strong El Niño cases are interpolated to the model’s ocean grid (ORCA1) and added to the present or future model climatology to produce the target restoring

fields. For each experiment, a 30 member ensemble is generated by using 3 sets of initial conditions from each of the 10 historical or SSP2-4.5 simulations at 5 year intervals (for example

for present-day we use 2005, 2010 and 2014). The SST target pattern is imposed through surface flux restoring with a restoring coefficient strong enough to achieve atmosphere-only conditions

in the restoring domain, which spans across the tropical Pacific from latitudes is 10°S - 10°N and longitudes from 130°E - 70°W, with a 10° buffer zone on each side except the eastern

margin which coincides with the American continent. All variables outside of the restoring region evolve freely in a coupled configuration. Each member is integrated for 2 years starting

from January 1st and the SST restoring is activated from the 1st of January of the first year until the end of December of the second year, thus having a 2-year simulation for each ensemble

member where SST anomalies are restored monthly to match the target observed pattern. The result is a suite of 30-member ensemble simulations for 3 strong El Niño events in present and

future climates. The analysis showed very similar changes in impacts for the 3 El Niño cases, so the analysis focuses on the “multi-event mean”, hereafter MEM. We focus on the DJF season

when the El Niño events peak and compare the anomalous response to strong El Niño in the future with that of the present. Anomalies from each period are calculated relative to their

corresponding climatological values. CLIMATE SIGNAL DECOMPOSITION To differentiate between changes to El Niño itself and changes in El Niño impacts added on top of the global warming signal,

the figures from this paper follow a similar structure. We first introduce the image showing present-day anomalies during very strong El Niño events in DJF (Eq. 1), consisting of

subtracting the present-day climatology to the absolute climate impacts during El Niño: $$ElNi\tilde{{\rm{n}}}oimpacts={ElNi\tilde{{\rm{n}}}o}_{p}-{Meanclimate}_{p}$$ (1) We then explore

changes in El Niño impacts in the future and present, removing the mean climate state and thus accounting just for changes to El Niño teleconnections regardless of the background climate

change signal (Eq. 2): $$Changes\in ElNi\tilde{{\rm{n}}}oimpacts=({ElNi\tilde{{\rm{n}}}o}_{f}-{Meanclimate}_{f})-({ElNi\tilde{{\rm{n}}}o}_{p}-{Meanclimate}_{p})$$ (2) Changes in the mean

climate state are also quantified and displayed in the plots (Eq. 3): $$Meanstatechanges={Meanstate}_{f}-{Meanstate}_{p}$$ (3) Adding up Eqs. 2 and 3 we obtain Eq. 4, which illustrate the

changes in absolute impacts of El Niño in the future, considering both changes in El Niño teleconnections as well as changes in the mean state. $$Totalchanges\in

ElNi\tilde{{\rm{n}}}oimpacts={ElNi\tilde{{\rm{n}}}o}_{f}-{ElNi\tilde{{\rm{n}}}o}_{p}$$ (4) CLIMATE INDICES El Niño is known to drive weather extremes over North America15,53, northern South

America and Southern Africa2, among other regions, so these are used as three regional case studies to examine the influence of future climate change on El Niño impacts. In North America, we

quantify the change in cool days per winter, which is computed as the number of days when maximum daily surface temperatures (Tmax) during the DJF season lay below the 10th percentile based

on the model climatology63. Secondly we explore changes in the amount of warm days per summer, which is again computed using maximum daily temperature and calculated the percentage change

in days when maximum temperature exceeds the 90th percentile. We focus on northern South America and Southern Africa to explore changes in the frequency of warm days per summer. Note that

the thresholds to compute cool and warm days in the present and future are based on the present (1995-2014) and future (2085-2094) climatologies, respectively, using fully moving

thresholds51. Therefore, the indices capture similar relative anomalies in the two periods. While we do not link the meteorological conditions to climate impacts (e.g. on human health), this

choice is most relevant to a situation in which society undertakes some climate adaptation to accommodate mean climate change. An alternative approach would be to use the present day

threshold in both periods, which would be more relevant to a situation in which climate adaptation is not undertaken. To assess whether changes are statistically significant, we performed a

two sided Student’s t-test. CODE AVAILABILITY The code used to calculate the indicators and prepare the figures will be available upon request. The experimental protocol can be accessed in

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HadEX2 dataset. _J. Geophys. Res.: Atmospheres_ 118, 2098–2118 (2013). Article  Google Scholar  Download references ACKNOWLEDGEMENTS PTC was funded by a PhD studentship from the NERC

PANORAMA Doctoral Training Partnership (NE/S007458/1) and a CASE award from the Barcelona Supercomputing Center (BSC). YRR received the support of a fellowship from "la Caixa"

Foundation (ID 100010434) and from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No 847648. ACM was supported by

the EU H2020 CONSTRAIN project under Grant Agreement No. 820829. Supercomputing resources were provided by BSC. We acknowledge Eric Ferrer for his technical support and Etienne Tourigny, the

Computational Earth Sciences at BSC and the EC-Earth3-CC developer team for developing the model that was used in this study. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Earth Sciences

Department, Barcelona Supercomputing Center, Barcelona, Spain Paloma Trascasa-Castro & Yohan Ruprich-Robert * School of Earth and Environment, University of Leeds, Leeds, UK Paloma

Trascasa-Castro & Amanda C. Maycock Authors * Paloma Trascasa-Castro View author publications You can also search for this author inPubMed Google Scholar * Yohan Ruprich-Robert View

author publications You can also search for this author inPubMed Google Scholar * Amanda C. Maycock View author publications You can also search for this author inPubMed Google Scholar

CONTRIBUTIONS P.T.C., Y.R.R. and A.C.M. conceptualised the experiments and goals of the study. Y.R.R. and P.T.C. prepared the experimental setup and P.T.C. ran the model. P.T.C. performed

the analysis, produced the figures and wrote the initial draft. Y.R.R. and A.C.M. contributed to writing the following versions of the manuscript. CORRESPONDING AUTHOR Correspondence to

Paloma Trascasa-Castro. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER’S NOTE Springer Nature remains neutral with

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A.C. Future climate response to observed strong El Niño analogues. _npj Clim Atmos Sci_ 8, 116 (2025). https://doi.org/10.1038/s41612-025-01003-1 Download citation * Received: 21 November

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