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Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990 A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980 This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening

The tropical Southeast Indian Ocean (SEIO) is a key area linking the global freshwater and heat exchanges. The Indian Ocean Dipole (IOD) fundamentally modulates the Indian Ocean circulation and thus regulates the basin‐wide freshwater balance. However, our knowledge of this effect remains limited. Using observational‐based data sets, this study suggests that extreme positive IOD events have notable signatures on the three‐dimensional freshwater content of the SEIO, leading to the vertically opposite salinity anomalies in the surface and subsurface layers. The wind changes led to the northwestward extension of the South Equatorial Current and intensified Sumatra‐Java upwelling. The changing horizontal and vertical currents jointly result in the complicated salinity anomalies. The Equatorial Undercurrent serves as the conduit for water exchange between the equator and the SEIO. This work highlights a strong coupling between the equatorial circulation and the three‐dimensional freshwater inventory of the SEIO within the framework of the IOD. Plain Language Summary The tropical Southeast Indian Ocean (SEIO) connects the three oceans and contributes to the global mass and heat exchanges. The changing freshwater storage and heat content in the SEIO impact regional and global climates. Positive Indian Ocean Dipole (pIOD) is a pattern of internal variability with anomalously low sea surface temperature off Sumatra and high sea surface temperature in the western Indian Ocean, with accompanying wind and precipitation anomalies. The IOD‐related wind anomalies drive the ocean circulation and thus regulate the freshwater content. However, the related process is not clear. This study suggests a framework of IOD regulating the three‐dimensional freshwater content in the SEIO. During the extreme pIOD phase, the abnormal horizontal and vertical currents jointly modulate mass and material distributions in the SEIO, leading to the salting anomalies in the surface layer but the freshening anomalies in the subsurface layer. This study also provides evidence for water exchange between the subsurface layer of the Indian Ocean equator and the surface layer of the SEIO. These findings are a crucial step toward fully understanding the three‐dimensional freshwater content in this critical area. Key Points Extreme positive Indian Ocean Dipole has notable signatures on the three‐dimensional freshwater content in the Southeast Indian Ocean (SEIO) The combined effect of horizontal and vertical currents results in the vertically opposite salinity anomalies The Equatorial Undercurrent serves as an important conduit for water exchange between the equator and the SEIO

The structure of the equatorial atmospheric circulation, as defined by the zonal mass streamfunction (ZMS), computed using the new fifth-generation ECMWF reanalysis for the global climate and weather (ERA-5) and the National Centers for Environmental Prediction NCEP–US Department of Energy reanalysis (NCEP-2) reanalysis products, is investigated and compared with Coupled Model Intercomparison Project Phase 6 (CMIP 6) ensemble mean. The equatorial atmospheric circulations majorly involve three components: the Indian Ocean cell (IOC), the Pacific Walker cell (POC) and the Atlantic Ocean cell (AOC). The IOC, POC and AOC average monthly or seasonal cycle peaks around March, June and February, respectively. ERA-5 has a higher IOC intensity from February to August, whereas NCEP-2 has a greater IOC intensity from September to December; NCEP-2 indicates greater POC intensity from January to May, whereas ERA-5 shows higher POC intensity from June to October. For the AOC, ERA-5 specifies greater intensity from March to August and NCEP-2 has a higher intensity from September to December. The equatorial atmospheric circulations cells vary in the reanalysis products, the IOC is weak and wider (weaker and smaller) in the ERA-5 (NCEP-2), the POC is more robust and wider (feebler and teensier) in NCEP-2 (ERA-5) and the AOC is weaker and wider (stronger and smaller) in ERA-5 (NCEP-2). ERA-5 revealed a farther westward POC and AOC compared to NCEP-2. In the CMIP 6 model ensemble mean (MME), the equatorial atmospheric circulations mean state indicated generally weaker cells, with the IOC smaller and the POC greater swinging eastward and westward, respectively, while the AOC is more westward. These changes in equatorial circulation correspond to changes in dynamically related heating in the tropics.

On 15 January 2022, the Hunga Tonga-Hunga Ha'apai (HT) eruption injected SO2 and water into the middle stratosphere. Shortly after the eruption, the water vapor anomaly moved northward toward and across the equator. This northward movement appears to be due to equatorial Rossby waves forced by the excessive infrared water vapor cooling. Following the early eruption stage, persistent mid-stratospheric water vapor and aerosol layers were mostly confined to Southern Hemisphere tropics (Eq. to 30°S). However, during the spring of 2022, the westerly phase of the tropical quasi-biennial oscillation (QBO) descended through the tropics. The HT water vapor and aerosol anomalies were observed to again move across the equator coincident with the shift in the Brewer-Dobson circulation and the descent of the QBO shear zone.

Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.

El Niño‐Southern Oscillation (ENSO) triggers variations of the global Hadley circulation (HC), while the latter may potentially feedback to ENSO events. Previous studies mainly investigated the interactions between ENSO and the global zonal‐mean HC. Here, we present a regional perspective of HC variability by introducing zonal variations. Results show that El Niño intensifies the regional HC over the central‐eastern Pacific, while weakening the regional HC over both the Indo‐Pacific warm pool and the tropical Atlantic. The background seasonal cycle modulates the equatorial‐asymmetric component of HC, with an anticlockwise (clockwise) asymmetric circulation over the central equatorial Pacific before (after) El Niño peaks. Remarkably, the asymmetric HC in boreal spring leads ENSO with a lead correlation of up to 0.68, mediated by the wind‐evaporation‐sea surface temperature (SST) feedback and other atmosphere‐ocean dynamics. The antecedent HC anomaly may contribute to ENSO predictability. Plain Language Summary By decomposing the atmospheric circulation to its irrotational and nondivergent components, we integrated the three‐dimensional structure of the Hadley circulation (HC), and investigate its interactions with El Niño‐Southern Oscillation (ENSO). In particular, we focus on the zonal variation of the HC and its equatorial‐symmetric and asymmetric components associated with ENSO variability. We find that an El Niño event may intensify the HC over the central‐eastern Pacific, but weaken the circulation over the Indo‐Western Pacific and the tropical Atlantic. The intensity of the ENSO‐related asymmetric HC anomaly is as large as that of the symmetric HC. The asymmetric HC anomaly is characterized by a clear seasonal feature. In boreal spring (MAM) before an El Niño signal becomes clear, a strong anticlockwise (asymmetric) HC across the central equatorial Pacific usually lead the El Niño event, with an 8‐month lead correlation with the November‐December‐January (NDJ, the peak season) Niño‐3.4 index of up to 0.68, implying that the MAM asymmetric HC and its associated atmosphere‐ocean interactions may largely contribute to the development of ENSO events. Further analysis indicated that the wind‐evaporation‐sea surface temperature feedback and the Bjerknes feedback play important roles in the interactions between the ENSO events and HC. Key Points The three‐dimensional structure of El Niño‐related Hadley circulation (HC) anomaly shows a strong zonal variation The direction of the equatorial‐asymmetric HC associated with El Niño events shows a clear seasonal feature The asymmetric HC over equatorial central Pacific in spring may contribute to the development of an El Niño event

Coastal El Niño is an extreme situation of El Niño‐Southern Oscillation (ENSO) with sea surface temperature warming confined in the far‐eastern equatorial Pacific. Some coastal El Niños evolve into a basin scale El Niño, and some don't, implying a diversity in ENSO evolutions after a coastal El Niño event. In this study, the coastal El Niños in 2017 and 2023 are selected to examine their subsequent evolution. Both coastal El Niños developed after a La Niña, with the former followed by a La Niña and the latter by a basin‐scale El Niño. The cold (warm) subsurface temperatures in 2017 (2023) were key factors leading to the divergent ENSO evolution. Convection over the western tropical Pacific and the atmospheric circulation anomalies across the equatorial Pacific also contributed to the differences. Model predictions suggest that differences in ENSO evolution after a coastal El Niño are associated with differences in ENSO predictability. Plain Language Summary Compared with the global impact of basin‐scale El Niño–Southern Oscillation (ENSO) events, coastal El Niño impacts are mainly focused along the South American coast. They are less studied, especially, in terms of temporal evolution and longer‐term development. Here, we examine the divergent evolution of ENSO conditions in the tropical Pacific after the coastal El Niños in 2017 and 2023. This subsequent divergent evolution of these events was associated with both preceding subsurface ocean heat content levels, convection over the western tropical Pacific, and concurrent atmospheric circulation. Specifically, preceding subsurface ocean cooling combined with low‐level easterly wind anomalies led to the growth of La Niña after the coastal El Niño in 2017, while strong preceding subsurface ocean warming led to the growth of El Niño after the coastal El Niño in 2023. These differences in the evolution of tropical Pacific Ocean conditions after a coastal El Niño were associated with different levels of ENSO predictability. Key Points The coastal El Niño in 2017 was followed by a La Niña, while the coastal El Niño in 2023 evolved into a basin‐scale El Niño Subsurface ocean heat content levels, western Pacific convection, and off‐equatorial circulation differences affect El Niño‐Southern Oscillation (ENSO) evolution Divergent evolution of conditions in the tropical Pacific after a coastal El Niño is associated with differences in ENSO predictability

The El Niño/Southern Oscillation (ENSO) represents the most consequential fluctuation of the global climate system, with dramatic societal and environmental impacts. Its general dynamics are reasonably well understood in terms of ocean–atmosphere interactions that modify the Walker circulation in the equatorial Pacific. However, some of its space–time features remain stubbornly elusive, such as its event-to-event diversity and asymmetry. Here we show that zonal shifts in the Walker circulation control ENSO space–time complexity in a low-dimensional theoretical framework. We encapsulate these movements in a conventional recharge–discharge oscillator for ENSO by replacing the regionally fixed sea surface temperature index with a warm-pool edge index that displaces the Walker circulation structures. By doing so, we can model essential ingredients of ENSO diversity and nonlinear behaviour without increasing the complexity of the dynamical model. The simple framework is able to reproduce the continuum of ENSO flavours and the asymmetry in the amplitude of warm and cold events. The spatial shifting concept paves the way for a more unified understanding of ENSO and its associated climate teleconnections.A simple conceptual model suggests that the complex behaviour of the El Niño/Southern Oscillation can be explained by zonal shifts in the Walker circulation.

BRAN2020 (2020 version of the Bluelink ReANalysis) is an ocean reanalysis that combines observations with an eddy-resolving, near-global ocean general circulation model to produce a four-dimensional estimate of the ocean state. The data assimilation system employed is ensemble optimal interpolation, implemented with a new multiscale approach that constrains the broad-scale ocean properties and the mesoscale circulation in two steps. There is a separation in the scales that are corrected in the two steps: the high-resolution step corrects the mesoscale dynamics in the same way as previous versions of BRAN, while the extra coarse step is effective at correcting biases that develop at large scales. The reanalysis currently spans January 1993 to December 2019 and assimilates observations of in situ temperature and salinity, as well as of satellite sea-level anomaly and sea surface temperature. BRAN2020 is planned to be updated to within months of real time after this initial release, until an updated version of BRAN is available. Reanalysed fields from BRAN2020 generally show much closer agreement to observations than all previous versions with misfits between reanalysed and observed fields reduced by over 30 % for some variables, for subsurface temperature and salinity in particular. The BRAN2020 dataset is comprised of daily averaged fields of temperature, salinity, velocity, mixed-layer depth and sea level. Reanalysed fields realistically represent all of the major current systems within 75∘ S and 75∘ N, excluding processes relating to sea ice but including boundary currents, equatorial circulation, Southern Ocean variability and mesoscale eddies. BRAN2020 is publicly available at https://doi.org/10.25914/6009627c7af03 (Chamberlain et al., 2021b) and is intended for use by the research community.

We evaluate and compare the simulation of the main features (low-level westerlies (LLWs) and the Congo basin (CB) cell) of low-level circulation in Central Equatorial Africa (CEA) with eight climate models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) and the corresponding eight previous models from CMIP5. Results reveal that, although the main characteristics of the two features are reasonably well depicted by the models, they bear some biases. The strength of LLWs is generally overestimated in CMIP5 models. The overestimation is attributed to both divergent and rotational components of the total wind with the rotational component contributing the most in the overestimation. In CMIP6 models, thanks to a better performance in the simulation of both divergent and rotational circulation, LLWs are slightly less strong compared to the CMIP5 models. The improvement in the simulated divergent component is associated with a better representation of the near-surface pressure and/or temperature difference between the Central Africa landmass and the coastal Atlantic Ocean. Regarding the rotational circulation, and especially for HadGEM3-GC31-LL and BCC-CSM2-MR, a simulated higher 850 hPa pressure is associated with less pronounced negative vorticity and a better representation of the rotational circulation. Most CMIP5 models also overestimate the CB cell intensity and width in association with the simulated strength of LLWs. However, in CMIP6 models, the strength of key cell characteristics (intensity and width) are reduced compared to CMIP5 models. This depicts an improvement in the representation of the cell in CMIP6 models and this is associated with the improvement in the simulated LLWs.