Abstract Canonical El Niño (EN) events typically peak in boreal winter and then rapidly decay in the ensuing months, transitioning to a La Niña (LN) event or a neutral condition by the following winter. Strikingly, an EN event that peaked in the 1986/87 boreal winter unexpectedly persisted into the following year, generating a rare 2-yr consecutive EN event. However, the 1986/87–1987/88 2-yr EN event receives little attention. This study reveals that this 2-yr EN event encompasses some critical yet commonly overlooked processes for the formation and development of an EN event. Specifically, the high-frequency (HF) westerly wind anomalies, induced by the tropical cyclones (TCs) and Madden–Julian oscillation (MJO) events, were the pivotal drivers of the unexpected reignition in the second year. During the 1986/87 winter, the unexpected emergence of four TCs induced vigorous westerly wind anomalies over the western equatorial Pacific (WEP), disrupting the anomalous anticyclone circulation over the western North Pacific anticyclone (WNPAC) and the associated easterly wind anomalies over the WEP that were anticipated during the 1986/87 winter. Such unexpected westerly wind anomalies helped maintain the EN warming through December 1986–February 1987. Subsequently, a series of HF westerly wind anomalies, induced by TCs and MJO events in April, May, and July 1987, reinvigorated the waning warming, pulling it back into a fledged EN event by the end of 1987. Gaining insights into the formation mechanism behind this unique 2-yr EN event can deepen our understanding of El Niño–Southern Oscillation (ENSO) dynamics and provide implications for enhancing the accuracy of EN prediction. Significance Statement Considering the critical impacts of multiyear El Niño–Southern Oscillation (ENSO) events on global weather and climate, understanding the formation dynamics of multiyear ENSO events is vital for improving the accuracy of ENSO prediction. Canonical El Niño (EN) events typically peak in boreal winter and then decay rapidly; however, the 1986/87 EN event persisted unexpectedly for two consecutive years, unlike typical single-year EN events. A revisit of 1986/87–1987/88 2-yr EN event reveals critical yet commonly overlooked processes in EN formation and development. Observational analyses reveal that the high-frequency westerly wind anomalies induced by the TCs and MJOs are crucial to the formation of this 2-yr consecutive EN event. Understanding the mechanism of this unique 2-yr EN event can enhance our knowledge of ENSO dynamics.

Abstract Prolonged compound hot–dry events (CHDEs) have been linked to surging emergency medical demand, especially among vulnerable populations, making them a growing public health concern in a warming climate. However, the seasonal and regional drivers of high health-risk CHDEs (HHR–CHDEs) remain unclear, limiting effective public health responses. Here, we integrate a temperature–humidity-based health-risk index, ambulance dispatch records, and distributed lag nonlinear models to obtain high health-risk temperature–humidity thresholds. During HHR–CHDEs exposure, particularly higher emergency dispatch demands are observed among males, the elderly, and individuals suffering from trauma or alcohol poisoning. We further use ERA5 reanalysis data to examine the dominant modes and physical mechanisms of HHR–CHDEs across China during the early and late warm seasons. In the early warm season, HHR–CHDEs are concentrated in northwest China and are jointly driven by enhanced surface heating (51% contribution) and intensified moisture loss (37%), underpinned by the synergistic effect of an upstream Rossby wave source over eastern Europe and anomalous northwest Pacific sea surface temperature (SST) warming. This coupling promotes a quasi-stationary high pressure anomaly that blocks synoptic disturbances and reinforces regional heat–dryness feedbacks. By contrast, in the late warm season, HHR–CHDEs are centered over the middle–lower Yangtze River region and are dominated by persistent heat accumulation (65%) with a secondary drought contribution (25%). These events are driven by large-scale circulation anomalies resulting from upstream Rossby wave energy originating in the North Atlantic and enhanced by downstream SST warming over the subtropical northwest Pacific. This Atlantic–Pacific coupling induces pronounced adjustments in the Walker and Hadley circulations, promoting subsidence and suppressed monsoonal moisture transport, and sustaining a health-threatening hot–dry atmosphere over densely populated regions. These findings reveal seasonally distinct remote forcing pathways and highlight the value of integrating large-scale diagnostics into climate–health early warning systems. Significance Statement High health-risk compound hot–dry events (HHR–CHDEs) are a growing climate–health threat, yet their physical drivers remain poorly understood. This study identifies seasonally distinct large-scale atmospheric and oceanic precursors of HHR–CHDEs across China, revealing key Rossby wave pathways and land–atmosphere feedbacks that amplify these events. By integrating climate diagnostics with health-relevant thresholds, our findings bridge the gap between meteorological extremes and public health outcomes, offering a foundation for targeted early warning systems under climate change.

Abstract Oceanic meridional heat transport (MHT), a key component of Earth's energy flow, fundamentally shapes climate variability by redistributing heat between hemispheres and across latitudes, thereby regulating surface energy exchange, atmospheric circulation, and hydrological patterns. However, accurate quantification of the global MHT remains challenging. Here, we use the energy budget approach to derive MHT from 1985 to 2023 based on sea surface heat fluxes (Fs), ocean heat content tendency (OHCT), and heat changes related to sea ice melt/formation (Q ice ), with objective adjustment applied to these components under an ocean energy budget constraint. This approach enables an improved estimate of the MHT climatology, variability, and trend for the global, Indo-Pacific and Atlantic oceans. With good agreement with RAPID and OSNAP mooring array observation time series, our estimates indicate realistic northward MHT at all latitudes in the North Atlantic with a peak of 1.22 ± 0.07 PW at 24.5°N. In contrast, the Indo-Pacific MHT is poleward with a peak of −1.67 ± 0.06 PW at 13.5°S. Regional energy budgets reveal that Fs dominates the mean state of MHT, while OHCT controls its interannual variability. After 2000, when the data quality of both OHC and Fs improved a lot, the Indo-Pacific Ocean exhibited a statistically significant increase in MHT, whereas the Atlantic Ocean showed a basin-wide weakening of MHT, particularly between 25°S and 18°N. The derived ocean MHT data in this study provide a basis to evaluate model and reanalysis data, and support a better understanding of the Earth’s energy flow.

Abstract Autumn precipitation plays a crucial role in agricultural irrigation and hydropower generation in southwestern China (SWC) and the Yangtze River Basin (YRB), yet its long-term changes and underlying mechanisms remain unclear. Based on the ERA5 reanalysis and station observation data, this study reveals significant declines in autumn precipitation across the SWC and YRB regions from 1973 to 2023. These declines are attributed to weakened water vapor transports from the Bay of Bengal and the South China Sea to China land areas. Further investigation indicates that increasing surface pressure over the Tibetan Plateau and decreasing pressure over the northwestern Pacific in the past several decades have led to a decreased ocean-to-land pressure gradient over East Asia during the autumn. This change in pressure gradient induces easterly and northerly trends in water vapor fluxes over East Asia, reducing moisture transport from the tropical oceans and resulting in decreased precipitation in SWC and the YRB. Both statistical analyses and model sensitivity experiments demonstrate that these pressure changes are associated with the tropical Pacific and Atlantic sea surface temperature (SST) warmings. Specifically, warm SSTs in the western Pacific trigger northeastward-propagating Rossby waves, increasing (decreasing) surface pressure over the Tibetan Plateau (northwestern Pacific). Meanwhile, warm SSTs in the tropical North Atlantic can influence the upstream Rossby wave patterns, lowering surface pressure over the northwestern Pacific. These findings improve our understanding of the changes and variability in autumn precipitation across China, providing important implications for predicting and managing regional droughts under global warming.

Abstract The El Niño–Southern Oscillation (ENSO) Transition Mode (ETM) is a distinct Southern Hemisphere mode characterized by a multidecadal seesaw in boreal spring sea level pressure about the date line in the southern Pacific Ocean. ETM significantly influences the equatorial winds and plays a key role in shaping ENSO’s boreal winter-to-summer seasonal transition. This study combines observational data and climate model simulations to examine how ETM affects global rainfall during boreal summer at multidecadal time scales. During its positive phase, ETM is linked to a cooler Northern Hemisphere, especially the northern Pacific, and a warmer Southern Hemisphere with maximum warming over the southern Indian Ocean. This creates an unfavorable interhemispheric sea surface temperature (SST) gradient, weakening the large-scale atmospheric circulation by reducing cross-equatorial mass flux and decreasing rainfall over the regions such as South Asia and the Sahel. These results underscore the significant impact of the Southern Hemisphere variability on the global climate patterns at multidecadal time scales. Significance Statement Understanding the monsoon variability in the Northern Hemisphere at decadal-to-multidecadal time scales can benefit billions of climate vulnerable people. While some studies have reported a significant increasing trend in the monsoons over the Northern Hemisphere in recent decades, it is unclear whether these changes are due to natural climate variability or anthropogenic changes, especially from the Southern Hemisphere. Using observations and atmospheric general circulation model (AGCM) simulations, we demonstrate that the natural El Niño–Southern Oscillation (ENSO) Transition Mode (ETM) from the Southern Hemisphere plays a crucial role in modulating boreal summer monsoons at multidecadal time scales. During its positive phase, a colder northern Pacific and a warmer southern Indian Ocean intensify the negative interhemispheric sea surface temperature (SST) gradient, which weakens the flow from the Southern Hemisphere and leads to a decline in rainfall over South Asia and the Sahel.

Abstract The Indian summer monsoon (ISM) is crucial to over a billion people since it supplies over 75% of the country’s annual precipitation. Significant intraseasonal variability in rainfall affects people, with breaks responsible for causing water shortage. It is known that dry intrusions play a role in breaks; however, it is not well understood compared to their role during progressions of the onset and withdrawal of the ISM. In this study, we use observations and the ERA5 reanalysis to understand the role of dry intrusions in breaks during 1940–2023. We develop an index based on moisture deficit to identify dry intrusions, and find that most breaks are associated with dry intrusions emanating from arid regions to the west and northwest of India. These dry intrusions begin to enter India around a week prior to the middle day of breaks, reaching their peak strength over northwest India and adjoining eastern Pakistan two to three days prior to the middle day of breaks. Vertical profiles reveal that these are mid-level dry intrusions, which are similar to those driving the direction of the withdrawal of the ISM. As breaks evolve, these dry intrusions deepen throughout their horizontal extent and descend into the country, stabilising the troposphere and creating an unfavourable environment for deep convection. We also find that extended breaks have stronger dry intrusions as precursors. Thus, this work helps establish a causal relationship between mid-level dry intrusions and breaks. The results could help improve forecasts of breaks, ultimately benefiting stakeholders in improving long-term planning.

Abstract Stratospheric temperature trend is a fingerprint for anthropogenic climate change. Since 1980, observational records have shown a negative trend in the Arctic lower stratospheric temperature (ALST). However, it remains inconclusive the extent to which the observed changes are driven by external forcing versus internal variability, and which modes of interdecadal internal variability significantly affect ALST. Moreover, providing a solid answer to how ALST will change in the near future remains a challenge. Using five large-ensemble simulations, this study shows that both external forcing and internal variability associated with the Interdecadal Pacific Oscillation (IPO) modulate the ALST changes. Among, the positive-to-negative phase transition of the IPO from 1980 to 2014 enhances both the upward propagation of planetary waves into the stratosphere and the strength of the Brewer-Dobson (BD) circulation, contributing to the Arctic stratospheric warming. The quantitative results suggest that the phase transition of IPO could offset approximately 15% (4%–26%) of the Arctic lower stratospheric cooling. Furthermore, the IPO evolution substantially affects the near-term projection of the ALST, introducing large uncertainty to the near-term projection. In conclusion, the findings highlight that the interaction between external forcing and internal variability, especially that relating to IPO, significantly influences ALST changes.

Abstract The onset and withdrawal of the summer monsoon over the Bay of Bengal and the South China Sea (B–S) mark the beginning and end of the Asian summer monsoon, respectively, and their changes are therefore of great concern to society. Compared to the onset of the B–S monsoon, its withdrawal has received limited attention, so its future change under global warming remains to be elucidated. This study investigates how the withdrawal of the B–S monsoon will change in the future under the very-high-emission scenario [shared socioeconomic pathway 5–8.5 (SSP5-8.5)] based on multimodel results from the Coupled Model Intercomparison Project phase 6 (CMIP6). We show that the majority of models analyzed (>75%) project that the B–S summer monsoon will be delayed in the future, due to a longer persistence of the low-level westerly wind over this region during the transition season (September–October). The enhanced low-level westerlies over this region are jointly contributed by the sea surface temperature (SST) cooling over the southeastern Indian Ocean (SEIO), SST warming in the tropical eastern Pacific, and the surface warming in the northern mid–high latitudes (NMHL). Furthermore, we find that the intermodel uncertainty in the projected changes in the B–S monsoon withdrawal is dominated by the changes in SEIO SST and NMHL surface temperature, with each factor having a similar contribution. These findings provide valuable insights into the expected change in the duration of the Asian rainy season under a future warming climate.

Abstract Understanding changes in global oceanic precipitation remains challenging due to observational limitations and model deficiencies, particularly in representing cloud and precipitation properties over oceans. In this paper, climatologies and trends in oceanic precipitation are examined using a collection of 27 state-of-the-art satellite and reanalysis datasets available on a uniform daily 1° × 1° resolution from the Frequent Rainfall Observations on Grids (FROGS) database between 2001 and 2020. Reanalysis datasets generally report higher annual-mean daily precipitation than satellite datasets. The humid tropics exhibit the greatest absolute discrepancies in precipitation rates, while arid regions such as the southeast Pacific and Atlantic show substantial relative differences among products. An upward ocean-mean trend is observed in most of latest-version satellite products, whereas reanalyses suggest declining trends. We also assess the precipitation changes in different locations against the “wet gets wetter, dry gets drier” (WWDD) hypothesis. Reanalyses on average show a pronounced decrease over the intertropical convergence zone, whereas the latest-version satellite products on average more closely follow the WWDD pattern, with agreements over more than half of the oceanic regions. The precipitation trend in averaged reanalyses also exhibits the weakest consistency with sea surface temperature trends in wet regions (34.2%), compared with dry regions in the averaged reanalyses (53.4%) and both wet (59.6%) and dry (58.5%) regions in averaged latest-version satellite products. Overall, the latest-version satellite trends, while diverse across products, tend to align better on average with this simple hypothesis than reanalyses. We recommend using the latest-version satellite products for investigating global oceanic precipitation while exercising greater caution when utilizing reanalysis datasets.

Abstract Satellite microwave observations of Antarctic sea ice started in 1973, just in time to capture a massive open water area enclosed in winter sea ice in the Weddell Sea, known as the Weddell Polynya. This polynya was roughly the size of the United Kingdom and it lasted through the winters of 1974–76 with observed ocean mixed layer depths exceeding 3000 m. This study evaluates the impacts of the 1970s Weddell Polynya on Antarctic Bottom Water trends and volume transports. We use two global ocean simulations at eddying resolutions to create polynyas similar in size and duration to the Weddell Polynya. These are initiated with a brief, localized wind perturbation near Maud Rise. A comparison of simulated water-mass trends to available full-depth hydrographic data reveals that warming and oxygen decline in the bottom layers in the Weddell and Scotia Seas can be explained by a multidecadal recovery from the Weddell Polynya that continues today. The observed decline of Antarctic Bottom Water (AABW) outflow from the Scotia Sea since the 1990s can also be attributed to the multidecadal recovery from a large spike in abyssal transport created by the polynya. However, the model simulations do not show a substantial change in the Antarctic Circumpolar Current transport or lower cell overturning transports north of the South Scotia Ridge, and the water-mass trends do not substantially propagate north of ∼55°S in the Atlantic sector. Recently observed bottom water trends in the Pacific and East Indian sector of the Southern Ocean are likely controlled by factors other than the Weddell Polynya, such as increased glacial melt.