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Future changes in the seasonal evolution of the El Niño—Southern Oscillation (ENSO) during its onset and decay phases have received little attention by the research community. This work investigates the projected changes in the spatio-temporal evolution of El Niño events in the 21 st Century (21 C), using a multi-model ensemble of coupled general circulation models subjected to anthropogenic forcing. Here we show that El Niño is projected to (1) grow at a faster rate, (2) persist longer over the eastern and far eastern Pacific, and (3) have stronger and distinct remote impacts via teleconnections. These changes are attributable to significant changes in the tropical Pacific mean state, dominant ENSO feedback processes, and an increase in stochastic westerly wind burst forcing in the western equatorial Pacific, and may lead to more significant and persistent global impacts of El Niño in the future. The El Niño - Southern Oscillation can have global impacts, therefore assessing its future occurrence is needed. Here, the authors project that El Niño will grow at a faster rate, persist longer over the eastern and far eastern Pacific, and have stronger and distinct remote impacts in the 21st Century

According to the World Meteorological Organization, 2023 was ranked as the second warmest year in the global surface temperature record since 1850, setting warm surface temperature records over more than 20% of the global land surface. In particular, the southwestern United States (US) and Northern Mexico experienced their longest stretch of record-breaking heat wave, affecting over 100 million people, causing over 200 deaths, and $14.5 billion in economic loss. Here we show that the 2023 heat wave event was linked to a strong anticyclonic blocking pattern that persisted for more than six weeks across the western US. Regression analysis and atmospheric model simulations suggest that the anticyclonic pattern was ultimately forced by the extremely warm sea surface temperature in the Atlantic. The combination of a warm Atlantic and a developing Pacific El Niño significantly amplified regional heat waves, doubling their number, tripling their days, and increasing their duration by about 50%. The persistent 2023 heat wave that affected the Southwest U.S. and Northern Mexico was forced by the extremely warm sea surface temperatures in the Atlantic Ocean and a developing El Niño in the tropical Pacific Ocean.

At seasonal-to-interannual timescales, Atlantic hurricane activity is greatly modulated by El Niño–Southern Oscillation and the Atlantic Meridional Mode. However, those climate modes develop predominantly in boreal winter or spring and are weaker during the Atlantic hurricane season (June–November). The leading mode of tropical Atlantic sea surface temperature (SST) variability during the Atlantic hurricane season is Atlantic Niño/Niña, which is characterized by warm/cold SST anomalies in the eastern equatorial Atlantic. However, the linkage between Atlantic Niño/Niña and hurricane activity has not been examined. Here, we use observations to show that Atlantic Niño, by strengthening the Atlantic inter-tropical convergence zone rainband, enhances African easterly wave activity and low-level cyclonic vorticity across the deep tropical eastern North Atlantic. We show that such conditions increase the likelihood of powerful hurricanes developing in the deep tropics near the Cape Verde islands, elevating the risk of major hurricanes impacting the Caribbean islands and the U.S. Atlantic Niño, the Atlantic counterpart of the Pacific El Niño, increases the likelihood of powerful hurricanes developing near the Cape Verde islands, elevating associated risks for the Caribbean islands and the U.S.

An extreme Atlantic Niño developed in the boreal summer of 2021 with peak‐season sea surface temperature anomalies exceeding 1°C in the eastern equatorial region for the first time since global satellite measurements began in the early 1970s. Here, we show that the development of this outlier event was preconditioned by a series of oceanic Rossby waves that reflected at the South American coast into downwelling equatorial Kelvin waves. In early May, an intense week‐long westerly wind burst (WWB) event, driven by the Madden‐Julian Oscillation (MJO), developed in the western and central equatorial Atlantic and greatly amplified one of the reflected Kelvin waves, directly initiating the 2021 Atlantic Niño. MJO‐driven WWBs are fundamental to the development of El Niño in the Pacific but are a previously unidentified driver for Atlantic Niño. Their importance for the 2021 event suggests that they may serve as a useful predictor/precursor for future Atlantic Niño events. Plain Language Summary Atlantic Niño is the Atlantic counterpart of El Niño in the Pacific, often referred to as El Niño's little brother. It was previously thought to have only regional influence on rainfall variability in West Africa, but a growing number of studies have shown that Atlantic Niño also plays an important role in the development of El Niño–Southern Oscillation, as well as in the formation of powerful hurricanes near the coast of West Africa. This study investigates the development of an extreme Atlantic Niño in the summer of 2021. Here, we show that the 2021 event was preconditioned by warm waters piled up near the South American coast, and then directly triggered by a westerly wind burst event that drove the warm waters eastward. The westerly wind burst event was driven by a patch of tropical thunderstorms that formed across the Indian Ocean and moved slowly eastward across the Pacific, South America, and the Atlantic, also known as the Madden‐Julian Oscillation. Westerly wind bursts driven by the Madden‐Julian Oscillation are fundamental for the development of El Niño in the Pacific, but a previously unidentified driver for Atlantic Niño, and thus may improve our ability to predict future Atlantic Niño events. Key Points The extreme 2021 Atlantic Niño was preconditioned by a series of oceanic Rossby waves reflected into downwelling equatorial Kelvin waves One of the Kelvin waves was greatly amplified by an intense week‐long westerly wind burst event, initiating the 2021 Atlantic Niño The westerly wind burst was driven by the Madden‐Julian Oscillation, which is a previously unidentified driver for Atlantic Niño

The current state-of-the-art climate models when combined together suggest that the anthropogenic weakening of the Atlantic Meridional Overturning Circulation (AMOC) has already begun since the mid-1980s. However, continuous direct observational records during the past two decades have shown remarkable resilience of the AMOC. To shed light on this apparent contradiction, here we attempt to attribute the interdecadal variation of the historical AMOC to the anthropogenic and natural signals, by analyzing multiple climate and surface-forced ocean model simulations together with direct observational data. Our analysis suggests that an extensive weakening of the AMOC occurred in the 2000s, as evident from the surface-forced ocean model simulations, and was primarily driven by anthropogenic forcing and possibly augmented by natural variability. However, since the early 2010s, the natural component of the AMOC has greatly strengthened due to the development of a strong positive North Atlantic Oscillation. The enhanced natural AMOC signal in turn acted to oppose the anthropogenic weakening signal, leading to a near stalling of the AMOC weakening. Further analysis suggests that the tug-of-war between the natural and anthropogenic signals will likely continue in the next several years. The anthropogenic weakening of the Atlantic Meridional Overturning Circulation has begun since the mid-1980s. However, a tug-of-war between the natural and anthropogenic signals led to a near stalling of the weakening since the early 2010s.

This study presents a physical mechanism on how low-frequency variability of the South Atlantic meridional heat transport (SAMHT) may influence decadal variability of atmospheric circulation. A multicentury simulation of a coupled general circulation model is used as basis for the analysis. The highlight of the findings herein is that multidecadal variability of SAMHT plays a key role in modulating global atmospheric circulation via its influence on interhemispheric redistributions of momentum, heat, and moisture. Weaker SAMHT at 30°S produces anomalous ocean heat divergence over the South Atlantic, resulting in negative ocean heat content anomalies about 15–20 years later. This forces a thermally direct anomalous interhemispheric Hadley circulation, transporting anomalous atmospheric heat from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) andmoisture fromthe SH to the NH, thereby modulating global monsoons. Further analysis shows that anomalous atmospheric eddies transport heat northward in both hemispheres, producing eddy heat flux convergence (divergence) in the NH (SH) around 15°–30°, reinforcing the anomalous Hadley circulation. The effect of eddies on the NH (SH) poleward of 30° depicts heat flux divergence (convergence), which must be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream. This study illustrates that decadal variations of SAMHT could modulate the strength of global monsoons with 15–20 years of lead time, suggesting that SAMHT is a potential predictor of global monsoon variability. A similar mechanistic link exists between the North Atlantic meridional heat transport (NAMHT) at 30°N and global monsoons.

Ice sheet melting into the Southern Ocean can change the formation and properties of the Antarctic Bottom Water (AABW). Ocean models often mimic ice sheet melting by adding freshwater fluxes in the Southern Ocean under diverse spatial distributions. We use a global ocean and sea‐ice model to explore whether the spatial distribution and magnitude of meltwater fluxes can alter AABW properties and formation. We find that a realistic spatially varying meltwater flux sustains AABW with higher salinities compared to simulations with uniform meltwater fluxes. Finally, we show that increases in ice sheet melting above 12% since 1958 can trigger AABW freshening rates similar to those observed in the Southern Ocean since 1990, suggesting that the increasing Antarctic meltwater discharge can drive the observed AABW freshening. Plain Language Summary Previous research suggests that increased Antarctic ice sheet (AIS) melting may be driving freshening in the abyssal Southern Ocean, and Antarctic sea ice expansion since 1978. However, the main tools we have to assess abyssal ocean changes are ocean models, and they often misrepresent Antarctic ice melting by assuming it occurs uniformly along the Antarctic coast. In this study, we use a global ocean model to assess if correcting the spatial distribution of Antarctic ice melting (from uniform to spatially varying), and increasing its magnitude, can change the salinity in the abyssal Southern Ocean. We show that correcting the spatial representation of the AIS melting results in abyssal waters with higher salinities, correcting 30% the ocean model fresh bias in the abyssal cells. We also show that a 12% increase in Antarctic ice melting can trigger freshening of the abyssal Southern Ocean at rates similar to the trends observed since the 1990s, thus suggesting that enhanced melting of Antarctic land ice may be the main driver of the recently observed AABW freshening. Key Points An ocean and sea‐ice model is used to explore how spatiotemporal variations in meltwater fluxes affect the Antarctic Bottom Water (AABW) Spatially varying meltwater fluxes reduces the low‐salinity bias of AABW in the ocean and sea‐ice model used by 30% Increasing meltwater fluxes by 12% over 60 years induces AABW freshening at rates consistent to observed AABW trends since 1990

Sitting at the crossroads of weather and climate, the Madden‐Julian Oscillation (MJO) is considered a primary source of subseasonal predictability. Despite its importance, numerical models struggle with MJO prediction as its convection moves through the complex Maritime Continent (MC) environment. Motivated by the ongoing effort to improve MJO prediction, we use the System for High‐resolution prediction on Earth‐to‐Local Domains (SHiELD) model to run two sets of forecasts, one with and one without a nested grid over the MC. By efficiently leveraging high‐resolution grid spacing, the nested grid reduces amplitude and phase errors and extends the model's predictive skill by about 10 days. These enhancements are tied to improvements in predicted zonal wind from the Indian Ocean to the Pacific, facilitated by westerly wind bias reduction in the nested grid. Results from this study suggest that minimizing circulation biases over the MC can lead to substantial advancements in skillful MJO prediction. Plain Language Summary The Madden‐Julian Oscillation, a large‐scale system that moves west to east from the Indian Ocean to the Pacific Ocean over the course of 2–3 months, exerts significant influence on atmospheric circulation worldwide. Nestled somewhere between near‐term weather and long‐term climate, accurate prediction of the Madden‐Julian Oscillation can be used to extend forecasts into the subseasonal, or 3–4 weeks range. In this study the global‐scale System for High‐resolution prediction on Earth‐to‐Local Domains model is used to run two sets of forecasts, one with and one without an advanced grid integrated into the global model over and around Indonesia. This region, known as the Maritime Continent, is a troublesome area for Madden‐Julian Oscillation prediction in numerical models due to its complex environment, which makes it a prime target for the advanced grid. Integrating the advanced grid into the forecasts is shown to extend the skill of Madden‐Julian Oscillation predictions by about 10 days and improve their accuracy. These enhancements are tied to a better representation of the wind field in the advanced grid area, which suggests that substantial advancements in predicting the Madden‐Julian Oscillation can be made by simply minimizing circulation biases over the Maritime Continent in numerical models. Key Points Integrating a nested grid over the Maritime Continent (MC) in SHiELD extends Madden‐Julian Oscillation (MJO) prediction skill by about 10 days with enhanced reliability Positive impacts imparted by the two‐way nested grid on the MJO extend beyond the confines of the nested region An improved representation of the mean circulation over the MC in the nested grid contributes to substantial advancements in MJO prediction

This study examines the large-scale atmosphere-ocean environments that led to the winter tornado outbreak across the Ohio Valley on 10–11 December 2021, also known as the Quad-States Tornado Outbreaks. Here, we show that the Quad-States Tornado Outbreaks occurred under an exceptionally strong and prolonged negative Pacific-North American pattern (PNA), which developed around December 1 and persisted for a month. This unusual PNA produced a strong atmospheric ridge along the south and eastern US seaboard, which in turn helped warm the Gulf of Mexico and produced large-scale environments conducive for tornadogenesis across the Ohio Valley. Further analysis shows that a broad region across the Ohio Valley is particularly vulnerable to extensive winter tornado outbreaks during long-lived negative PNA, whereas a limited region in the central US is exposed to winter tornado activity during short-lived negative PNA. Finally, although the PNA is a mode of internal variability that occurs with or without El Niño—Southern Oscillation, the occurrence of prolonged negative PNA is more frequent during La Niña than during El Niño.

Observational records during the past several decades show a marked increase in boreal winter extreme US hydroclimate events, with extreme floods and droughts becoming more common. Coincidentally, El Niño-Southern Oscillation (ENSO), a key driver of US precipitation and associated extreme hydroclimate on interannual time scales, has also increased in amplitude and is projected to continue increasing throughout the 21st century. This study examines future changes in ENSO and its impacts on the US winter extreme hydroclimate events (e.g., drought and flood) by using a large ensemble simulation. Results in this study show that both the amplitude of ENSO and ENSO-induced atmospheric teleconnections are projected to strengthen, leading to a significant increase in US precipitation variability and extreme hydroclimate events, albeit with notable regional differences. Signal-to-noise ratio analysis shows that the ENSO signal explains a significantly increased fraction of the total variance in US winter precipitation compared to non-ENSO factors (i.e., noise), suggesting a growing role of ENSO in future US extreme hydroclimate events. Further analysis shows that while both the increase in ENSO amplitude and the atmospheric response to ENSO have a similar impact on the hydroclimate over the Southeast and Southwest US, the amplification of the atmospheric response to ENSO plays a more dominant role in the Northeast US.