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Abstract El Niño/Southern Oscillation variability has conspicuous impacts on ecosystems and severe weather. Here, we probe the effects of anthropogenic aerosols and greenhouse gases on El Niño/Southern Oscillation variability during the historical period using a broad set of climate models. Increased aerosols significantly amplify El Niño/Southern Oscillation variability primarily through weakening the mean advection feedback and strengthening the zonal advection and thermocline feedbacks, as linked to a weaker annual cycle of sea surface temperature in the eastern equatorial Pacific. They prevent extreme El Niño events, reduce interannual sea surface temperature skewness in the tropical Pacific, influence the likelihood of El Niño/Southern Oscillation events in April and June and allow for more El Niño transitions to Central Pacific events. While rising greenhouse gases significantly reduce El Niño/Southern Oscillation variability via a stronger sea surface temperature annual cycle and attenuated thermocline feedback. They promote extreme El Niño events, increase SST skewness, and enlarge the likelihood of El Niño/Southern Oscillation peaking in November while inhibiting Central Pacific El Niño/Southern Oscillation events. 

Abstract Deep convection associated with large-scale tropical atmospheric circulations governs tropical precipitation. Under anthropogenic warming, the weakened Walker and Hadley circulations alter tropical rainfall. Ocean circulations are also expected to change due to global warming, impacting tropical atmospheric circulation systems. From the perspective of ocean heat uptake, we investigate how ocean circulation change modulates tropical atmospheric circulation and vertical motion under CO₂warming by comparing fully coupled and slab-ocean simulations. We find that the slowed South Equatorial Current and subtropical cells in the Pacific induce anomalous advective warming, reducing ocean heat uptake in the central-western tropical Pacific. This, combined with increased downward radiation at the top of atmosphere and horizontal moisture advection, escalates the moisture static energy in the air column and promotes ascent in this region, shifting the Pacific Walker circulation eastward and strengthening the Pacific Hadley circulation. Across the tropical Indian Ocean, ocean heat uptake shows a dipole-like change, increasing in the eastern Indian Ocean and seas surrounding marine continents while decreasing in the western Indian Ocean. The former ocean heat uptake increase is triggered by anomalous oceanic vertical advective cooling, which abates the moisture static energy in the air column and inhibits the ascent in the area. The latter ocean heat uptake decrease is prompted by anomalous oceanic advective warming from both horizontal and vertical directions, which enhances the moisture static energy in the air column, resulting in anomalous upward motions. Over most of the tropics, ocean dynamics help attenuate the strengthening of the gross moist stability due to CO₂increase, thereby promoting ascent or weakening descent in the atmosphere. Significance StatementLarge-scale tropical atmospheric circulations are expected to weaken as a result of global warming, having a significant impact on tropical precipitation. Because the atmosphere and oceans are inextricably linked, any subtle change in one can affect the other. For this reason, it is critical to understand the role of ocean circulation change in steering the response of large-scale tropical atmospheric circulation to anthropogenic warming. This study approaches the aforementioned scientific question from the novel perspective of ocean heat uptake. It demonstrates how changes in ocean circulation affect heat uptake over tropical oceans, modifying vertical motion and the Walker and Hadley cells in the tropical atmosphere in a warming climate. 

Abstract Most oceans over the globe have experienced surface warming during the past century, but the subpolar Atlantic is quite otherwise. The sea surface temperature cooling trend to the south of Greenland, known as the North Atlantic Warming Hole, has raised debate over whether it is driven by the slowing of the Atlantic Meridional Overturning Circulation. Here we use observations as a benchmark and climate models as a tool to demonstrate that only models simulating a weakened historical Atlantic overturning can broadly reproduce the observed cooling and freshening in the warming hole region. This, in turn, indicates that the realistic Atlantic overturning slowed between 1900 and 2005, at a rate of −1.01 to −2.97 Sv century⁻¹(1 Sv = 10⁶ m³ s⁻¹), according to a sea-surface-temperature-based fingerprint index estimate. Particularly, the Atlantic overturning slowdown causes an oceanic heat transport divergence across the subpolar North Atlantic, which, while partially offset by enhanced ocean heat uptake, results in cooling over the warming hole region. 

Abstract Observations reveal Antarctic sea ice expansion and Southern Ocean surface cooling trends from 1979 to 2014, whereas climate models mostly simulate the opposite. Here I use historical ensemble simulations with multiple climate models to show that sea-ice natural variability enables the models to simulate an Antarctic sea ice expansion during this period under anthropogenic forcings. Along with sea-ice expansion, Southern Ocean surface and subsurface temperatures up to 50^oS, as well as lower tropospheric temperatures between 60^oS and 80^oS, exhibit significant cooling trends, all of which are consistent with observations. Compared to the sea-ice decline scenario, Antarctic sea ice expansion brings tropical precipitation changes closer to observations. Neither the Southern Annular Mode nor the Interdecadal Pacific Oscillation can fully explain the simulated Antarctic sea ice expansion over 1979–2014, while the sea-ice expansion is closely linked to surface meridional winds associated with a zonal wave 3 pattern. 

Abstract Wind-driven and thermohaline circulations, two major components of global large-scale ocean circulations, are intrinsically related. As part of the thermohaline circulation, the Atlantic Meridional Overturning Circulation has been observed and is expected to decline over the twenty-first century, potentially modulating global wind-driven circulation. Here we perform coupled climate model experiments with either a slow or steady Atlantic overturning under anthropogenic warming to segregate its effect on wind-driven circulation. We find that the weakened Atlantic overturning generates anticyclonic surface wind anomalies over the subpolar North Atlantic to decelerate the gyre circulation there. Fingerprints of overturning slowdown are evident on Atlantic western boundary currents, encompassing a weaker northward Gulf Stream and Guiana Current and a stronger southward Brazil Current. Beyond the Atlantic, the weakened Atlantic overturning causes a poleward displacement of Southern Hemisphere surface westerly winds by changing meridional gradients of atmospheric temperature, leading to poleward shifts of the Antarctic Circumpolar Current and Southern Ocean meridional overturning circulations. 

Abstract In the era of escalating climate change, understanding human impacts on marine heatwaves (MHWs) becomes essential. This study harnesses climate model historical and single forcing simulations to delve into the individual roles of anthropogenic greenhouse gases (GHGs) and aerosols in shaping the characteristics of global MHWs over the past several decades. The results suggest that GHG variations lead to longer-lasting, more frequent, and intense MHWs. In contrast, anthropogenic aerosols markedly curb the intensity and growth of MHWs. Further analysis of the sea surface temperature (SST) probability distribution reveals that anthropogenic GHGs and aerosols have opposing effects on the tails of the SST probability distribution, causing the tails to expand and contract, respectively. Climate extremes such as MHWs are accordingly promoted and reduced. Our study underscores the significant impacts of anthropogenic GHGs and aerosols on MHWs, which go far beyond the customary concept that these anthropogenic forcings modulate climate extremes by shifting global SST probabilities via modifying the mean-state SST. 

Abstract The Pacific Decadal Oscillation has been suggested to play an important role in driving marine heatwaves in the Northeast Pacific during recent decades. Here we combine observations and climate model simulations to show that marine heatwaves became longer, stronger and more frequent off the Northeast Pacific coast under a positive Pacific Decadal Oscillation scenario, unlike what is found during a negative Pacific Decadal Oscillation scenario. This primarily results from the different mean-state sea surface temperatures between the two Pacific Decadal Oscillation phases. Compared to the cool (negative) phase of the Pacific Decadal Oscillation, warmer coastal sea surface temperatures occur during the positive Pacific Decadal Oscillation phase due to reduced coastal cold upwelling and increased net downward surface heat flux. Model results show that, relative to the background anthropogenic global warming, the positive Pacific Decadal Oscillation in the period 2013–2022 prolongs marine heatwaves duration by up to 43% and acts to increase marine heatwaves annual frequency by up to 32% off the Northeast Pacific coast. 

Abstract The Arctic sea ice has been rapidly dwindling over the past four decades, significantly impacting the Arctic region and beyond. During the same period, the Atlantic meridional overturning circulation (AMOC) was also found in a declining trend. Here we investigate the role of the AMOC in the recent Arctic sea ice changes by comparing simulations from the Community Climate System Model version 4 with decelerated and stationary AMOCs under anthropogenic climate change. We find that the weakened AMOC can slow down the decline rates of Arctic sea ice area and volume by 36% and 22% between 1980 and 2020, respectively. The decelerated ocean circulation causes a reduction of northward Atlantic heat transport and hence a general interior ocean cooling in the Arctic Mediterranean, which helps alleviate the Arctic sea ice loss primarily through thermodynamic processes occurring at the base of the sea ice.