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The climatology and interannual variability of the East Asian summer monsoon (EASM) simulated by 34 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated. To estimate the role of air–sea coupling, 17 CGCMs are compared to their corresponding atmospheric general circulation models (AGCMs). The climatological low-level monsoon circulation and mei-yu/changma/baiu rainfall band are improved in CGCMs from AGCMs. The improvement is at the cost of the local cold sea surface temperature (SST) biases in CGCMs, since they decrease the surface evaporation and enhance the circulation. The interannual EASM pattern is evaluated by a skill formula and the highest/lowest eight models are selected to investigate the skill origins. The observed Indian Ocean (IO) warming, tropical eastern Indian Ocean (TEIO) rainfall anomalies, and Kelvin wave response are captured well in high-skill models, while these features are not present in low-skill models. Further, the differences in the IO warming between high-skill and low-skill models are rooted in the preceding ENSO simulation. Hence, the IO–western Pacific anticyclone (WPAC) teleconnection is important for CGCMs, similar to AGCMs. However, compared to AGCMs, the TEIO SST anomaly is warmer in CGCMs, since the easterly wind anomalies in the southern flank of the WPAC reduce the climatological monsoon westerlies and decrease the surface evaporation. The warmer TEIO induces the stronger precipitation anomaly and intensifies the teleconnection. Hence, the interannual EASM pattern is better simulated in CGCMs than that in AGCMs.

The climatology and interannual variability of EastAsian summer monsoon (EASM) are investigated by using 13 atmospheric general circulation models (AGCMs) from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and 19 AGCMs from CMIP5. The mean low-level monsoon circulation is reasonably reproduced in the multimodel ensemble mean (MME) of CMIP3 and CMIP5 AGCMs, except for a northward shift of the western Pacific subtropical high. However, the monsoon rainband known as mei-yu/baiu/changma (28°–38°N, 105°–150°E) is poorly simulated, although a significant improvement is seen from CMIP3 to CMIP5. The interannual EASM pattern is obtained by regressing the precipitation and 850-hPa wind on the observed EASM index. The observed dipole rainfall pattern is partly reproduced in CMIP3 and CMIP5 MME but with two deficiencies: weaker magnitude and southward shift of the dipole rainfall pattern. These deficiencies are closely related to the weaker and southward shift of the western Pacific anticyclone (WPAC). The simulation skill of the interannual EASM pattern has been significantly improved from CMIP3 to CMIP5 MME accompanied by the enhanced dipole rainfall pattern and WPAC. Analyses demonstrate that the tropical eastern Indian Ocean (IO) rainfall response to local warm SST anomalies and the associated Kelvin wave response over the Indo–western Pacific region are important to maintain the WPAC. A successful reproduction of interannual EASM pattern depends highly on the IO–WPAC teleconnection. The significant improvement in the interannual EASM pattern from CMIP3 to CMIP5 MME is also due to a better reproduction of this teleconnection in CMIP5 models.

SignificanceThe Earth has warmed by 1.2 ± 0.1 °C since the preindustrial era. The most common metric to measure the ongoing global warming is surface air temperature since it has long and reliable observational records. However, surface air temperature alone does not fully describe the nature of global warming and its impact on climate and weather extremes. Here we show that surface equivalent potential temperature, which combines the surface air temperature and humidity, is a more comprehensive metric not only for the global warming but also for its impact on climate and weather extremes including tropical deep convection and extreme heat waves. We recommend that it should be used more widely in future climate change studies. Trends in surface air temperature (SAT) are a common metric for global warming. Using observations and observationally driven models, we show that a more comprehensive metric for global warming and weather extremes is the trend in surface equivalent potential temperature (Thetae_sfc) since it also accounts for the increase in atmospheric humidity and latent energy. From 1980 to 2019, while SAT increased by 0.79°C, Thetae_sfc increased by 1.48°C globally and as much as 4°C in the tropics. The increase in water vapor is responsible for the factor of 2 difference between SAT and Thetae_sfc trends. Thetae_sfc increased more uniformly (than SAT) between the midlatitudes of the southern hemisphere and the northern hemisphere, revealing the global nature of the heating added by greenhouse gases (GHGs). Trends in heat extremes and extreme precipitation are correlated strongly with the global/tropical trends in Thetae_sfc. The tropical amplification of Thetae_sfc is as large as the arctic amplification of SAT, accounting for the observed global positive trends in deep convection and a 20% increase in heat extremes. With unchecked GHG emissions, while SAT warming can reach 4.8°C by 2100, the global mean Thetae_sfc can increase by as much as 12°C, with corresponding increases of 12°C (median) to 24°C (5% of grid points) in land surface temperature extremes, a 14- to 30-fold increase in frequency of heat extremes, a 40% increase in the energy available for tropical deep convection, and an up to 60% increase in extreme precipitation. Description

Intense tropical cyclones (TCs), which often peak in autumn 1 , 2 , have destructive impacts on life and property 3 , 4 – 5 , making it crucial to determine whether any changes in intense TCs are likely to occur. Here, we identify a significant seasonal advance of intense TCs since the 1980s in most tropical oceans, with earlier-shifting rates of 3.7 and 3.2 days per decade for the Northern and Southern Hemispheres, respectively. This seasonal advance of intense TCs is closely related to the seasonal advance of rapid intensification events, favoured by the observed earlier onset of favourable oceanic conditions. Using simulations from multiple global climate models, large ensembles and individual forcing experiments, the earlier onset of favourable oceanic conditions is detectable and primarily driven by greenhouse gas forcing. The seasonal advance of intense TCs will increase the likelihood of intersecting with other extreme rainfall events, which usually peak in summer 6 , 7 , thereby leading to disproportionate impacts. We identify a seasonal advance of intense tropical cyclones that is closely related to the seasonal advance of rapid intensification events, favoured by the observed earlier onset of favourable oceanic conditions.

Changes in the seasonal cycle of sea surface temperature (SST) have far-reaching ecological and societal implications. Previous studies have found an intensified SST seasonal cycle under global warming, but whether such changes have emerged in historical records remains largely unknown. Here, we reveal that the SST seasonal cycle globally has intensified by 3.9 ± 1.6% in recent four decades (1983–2022), with hotspot regions such as the northern subpolar gyres experiencing an intensification of up to 10%. Increased greenhouse gases are the primary driver of this intensification, and decreased anthropogenic aerosols also contribute. These changes in anthropogenic emissions lead to shallower mixed layer depths, reducing the thermal inertia of upper ocean and enhancing the seasonality of SST. In addition, the direct impacts of increased ocean heat uptake and suppressed seasonal amplitude of surface heat flux also contribute in the North Pacific and North Atlantic. The temperature seasonal cycle is intensified not only at the ocean surface, but throughout the mixed layer. The ramifications of this intensified SST seasonal cycle extend to the seasonal variation in upper-ocean oxygenation, a critical factor for most ocean ecosystems. The authors reveal a 3.9% intensification in the sea surface temperature seasonal cycle over the past four decades, with hotspot regions experiencing intensification of up to 10%. This intensification extends throughout the mixed layer, amplifying the seasonal cycle of upper-ocean oxygenation.

Marked uncertainty in California (CA) precipitation projections challenges their use in adaptation planning in the region already experiencing severe water stress. Under global warming, a westerly jet extension in the North Pacific analogous to the El Niño-like teleconnection has been suggested as a key mechanism for CA winter precipitation changes. However, this teleconnection has not been reconciled with the well-known El Niño-like warming response or the controversial role of internal variability in the precipitation uncertainty. Here we find that internal variability contributes > 70% and > 50% of uncertainty in the CA precipitation changes and the El Niño-like warming, respectively, based on analysis of 318 climate simulations from several multi-model and large ensembles. The Interdecadal Pacific Oscillation plays a key role in each contribution and in connecting the two via the westerly jet extension. This unifying understanding of the role of internal variability in CA precipitation provides critical guidance for reducing and communicating uncertainty to inform adaptation planning. There is a strong disagreement between climate models on how precipitation in California will change. Here, the authors show that much of this uncertainty originates from internal variability with the Interdecadal Pacific Oscillation linking the precipitation changes with the El Niño-like warming pattern through the westerly jet extension

High Mountain Asia (HMA) has experienced a spatial imbalance in water resources in recent decades, partly because of a dipolar pattern of precipitation changes known as South Drying–North Wetting 1 . These changes can be influenced by both human activities and internal climate variability 2 , 3 . Although climate projections indicate a future widespread wetting trend over HMA 1 , 4 , the timing and mechanism of the transition from a dipolar to a monopolar pattern remain unknown. Here we demonstrate that the observed dipolar precipitation change in HMA during summer is primarily driven by westerly- and monsoon-associated precipitation patterns. The weakening of the Asian westerly jet, caused by the uneven emission of anthropogenic aerosols, favoured a dipolar precipitation trend from 1951 to 2020. Moreover, the phase transition of the Interdecadal Pacific Oscillation induces an out-of-phase precipitation change between the core region of the South Asian monsoon and southeastern HMA. Under medium- or high-emission scenarios, corresponding to a global warming of 0.6–1.1 °C compared with the present, the dipolar pattern is projected to shift to a monopolar wetting trend in the 2040s. This shift in precipitation patterns is mainly attributed to the intensified jet stream resulting from reduced emissions of anthropogenic aerosols. These findings underscore the importance of considering the impact of aerosol emission reduction in future social planning by policymakers. Dipolar precipitation change in High Mountain Asia during summer is primarily driven by weakened westerly jet and decadal variations in the South Asian monsoon, and the dipolar pattern is projected to shift to a monopolar wetting trend in the 2040s.

This study investigates the role of internal variability in modulating the East Asian summer monsoon (EASM)–ENSO relationship using Twentieth-Century Reanalysis (20CR) data and simulations from phase 5 of CMIP (CMIP5). Analysis of 20CR data reveals an unstable EASM–ENSO relationship during the twentieth century. During the high-correlation periods of 1892–1912 and 1979–99, an evident western Pacific anticyclone (WPAC) and dipole sea level pressure (SLP) pattern are present in the decaying El Niño summer, accompanied by Indian Ocean warming and a tropospheric temperature Matsuno–Gill pattern. However, these are weaker or absent during low-correlation periods (1914–34 and 1958–78). After removing the external forcings based on historical simulations from 15 CMIP5 models, all the above features remain almost unchanged, suggesting the crucial role of internal variability. In a 501-yr preindustrial control (piControl) simulation without external forcing variation from CCSM4, the EASM–ENSO relationship also shows significant decadal variation, with a magnitude comparable to the 20CR data. The analysis demonstrates that the EASM–ENSO relationship’s variation is modulated by the interdecadal Pacific oscillation (IPO). Compared to negative IPO phases, the warmer East China Sea in positive IPO phases weakens the western North Pacific subtropical high (WNPSH), inducing more precipitation. Thus, the Kelvin wave–induced interannual divergence suppresses more mean-state precipitation and leads to a stronger WPAC. Hence, the IPO modulates the EASM–ENSO relationship through the WNPSH, which is evident in both 20CR and the piControl simulation.

Land monsoon rainfall has a distinct annual cycle. Under global warming, whether the phase and amplitude of this annual cycle would be changed is still unclear. Here, a global investigation is conducted using 34 CMIP6 and 34 CMIP5 models under a high emission scenario. Seasonal delays would occur in the Southern Hemisphere (SH) American (3.43 days), Northern Hemisphere (NH) African (5.98 days) and SH African (3.76 days) monsoon regions, while no robust signal is found in other monsoon regions. Except NH American monsoon, amplitude is enhanced in all the monsoon regions. Compared to amplitude, the phase changes dominate the future changes of precipitation in the SH American, NH African and SH African monsoon regions. In these phase‐dominated regions, atmospheric energetic framework is proved to be reliable at regional scale and the enhanced effective atmospheric heat capacity is found to be the dominant factor. Plain Language Summary Monsoon rainfall sustains nearly two‐thirds of the world's population. The arrival timing and the difference between maximum and minimum of monsoon are the main features of concern. We revealed that rainfall would be delayed over the Amazon, Sahel and South Africa and enhanced in most monsoons except North American monsoon under global warming. In the Amazon, Sahel and South Africa, future changes of the arrival timing can be more remarkable compared to the changes of range. These regional changes can be resulted from physical constraints related to atmospheric energy under global warming, which was previously proposed at larger scale and has been proved to be reliable here at regional monsoons. Key Points Precipitation would be robustly seasonally delayed over the South American, North African and South African monsoons under global warming The phase delays would dominate precipitation annual cycle changes in the above three regions, while amplitude changes dominate the others The enhanced effective atmospheric heat capacity under warming is key to the seasonal delay of precipitation at the phase‐dominated regions

Record rainfall and severe flooding struck eastern China in the summer of 2020. The extreme summer rainfall occurred during the COVID-19 pandemic, which started in China in early 2020 and spread rapidly across the globe. By disrupting human activities, substantial reductions in anthropogenic emissions of greenhouse gases and aerosols might have affected regional precipitation in many ways. Here, we investigate such connections and show that the abrupt emissions reductions during the pandemic strengthened the summer atmospheric convection over eastern China, resulting in a positive sea level pressure anomaly over northwestern Pacific Ocean. The latter enhanced moisture convergence to eastern China and further intensified rainfall in that region. Modeling experiments show that the reduction in aerosols had a stronger impact on precipitation than the decrease of greenhouse gases did. We conclude that through abrupt emissions reductions, the COVID-19 pandemic contributed importantly to the 2020 extreme summer rainfall in eastern China. In the summer of 2020, China experienced record rainfall and flooding. Here, the authors investigate the connection between the rainfall and the abrupt decrease in emissions due to Covid-19 and find that the emission decrease may have contributed to the rainfall.