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This study investigates the variations in air–sea heat fluxes and temperatures in two ocean front regions, the Southwestern Atlantic Ocean (SWA) and the Drake Passage, widely recognized as hotspot areas with significant influences on South America weather and climate. We analyse means and trends of latent and sensible heat fluxes (LHF and SHF) and their associated air–sea temperatures (SAT and SST), based on monthly ERA5 (1985–2014) and eight CMIP6 models, for historical and long‐term simulations (2015–2099). The ERA5 trend for all parameters was positive over SWA, contrasting with the negative values observed in the Drake Passage, indicating surface warming and cooling, respectively. In the SWA, the ERA5 and CMIP6 multi‐model ensemble (MME) align in SST and SAT values, with a historical trend of 0.1°C·decade−1 and a significantly increasing trend in warming estimated by 0.4°C·decade−1 up to 2099. The ERA5 LHF and SHF trends were 4 and 0.6 W·m−2·decade−1, respectively. The MME shows historical (SSP5‐8.5) trends of 0.2 (1.3) W·m−2·decade−1 for LHF and −0.2 (−0.1) W·m−2·decade−1 for SHF. In the Drake Passage, the models accurately reproduced the air–sea mean temperatures; however, they failed to simulate negative trends observed in SAT and SST. Under the high emissions scenario, all CMIP6 models predict an increasing warming trends of 0.1–0.4°C·decade−1 and ocean heat gain of −0.2 to −1.2 (−1 to −2) W·m−2·decade−1 for LHF (SHF). For both regions, spatial analyses of SST and SAT highlight the coupling between the ocean and the atmosphere, alongside changes in air–sea heat fluxes. Our findings reveal inhomogeneous patterns of SST warming trends by the end of the 21st century, which are approximately 2–4 times greater than historical trends. The results suggest the persistence and enhancement of these regions as hotspots with significant potential to influence oceanic and atmospheric dynamics. [ABSTRACT FROM AUTHOR]

Under global climate changes, the temperature and precipitation on the Tibetan Plateau changed significantly, causing accelerated melting of glaciers and snow and changes in runoff. The response of runoff and its components to climate change is important for water resource management and ecology conservation in the region. Therefore, the Spatial Processes in Hydrology model, a distributed cold‐zone hydrological model that contains a glacial ablation module, was used to simulate runoff in the Yangtze River source located in the middle of the Tibetan Plateau during 2000–2050, and the Nash‐Sutcliffe efficiency coefficient, relative error and coefficient of determination (R2) for the calibration period and validation period revealed that the model performed well in most years. The results showed that rainfall runoff contributed the most to the total runoff (61%), followed by baseflow (23%), snowmelt runoff (12%) and glacier runoff (4%) in the Yangtze River source during 2000–2020. The runoff amounts of the three source rivers, Dangqu River, Tuotuo River and Chumar River, accounted for approximately 53% of the total runoff in the Yangtze River source basin, with rainfall runoff (52%) contributing the most and glacier runoff (6%) contributing the least. Compared to the contributions of glacial runoff in the Dangqu River (9%) and Tuotuo River (7%), the proportion of glacial runoff in the Chumar River is small (1%). Under the CMIP6 climate model, the mean runoff depth is predicted to increase by approximately 13.5 mm from 2020 to 2050 compared with that from 2000 to 2020. [ABSTRACT FROM AUTHOR]

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The Atlantic Subtropical Cells (STCs) consist of poleward Ekman transport in the surface layer, subduction in the subtropics, and equatorward transport in the thermocline layer that largely compensates the surface Ekman divergence and closes the STCs via equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients exchange. However, their representation in state‐of‐the‐art coupled models has not been systematically evaluated. In this study, we investigate the performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, that is, the meridional Ekman and geostrophic flow across 10°N and 10°S, and the Equatorial Undercurrent (EUC) at 23°W. We find that the model ensemble reveals biases toward weak Southern Hemisphere Ekman transport and interior geostrophic transports, as well as a weak EUC. We then investigate the large inter‐model spread of these key components and find that models with strong Ekman divergence between 10°N and 10°S tend to have strong mixed layer and thermocline interior convergence and strong EUC. The inter‐model spread of the EUC strength is primarily associated with the intensity of the southeasterly trade winds in the models. Since the trade‐wind‐induced poleward Ekman transports are regarded as the drivers of the STCs, our results highlight the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing. Plain Language Summary: This work systematically assesses how well state‐of‐the‐art climate models simulate the Atlantic Subtropical Cells (STCs). The STCs are part of the shallow (upper 200 m) ocean circulation. They connect the upper tropical and subtropical Atlantic Ocean in each hemisphere via poleward transport in the surface layer (surface to about 100 m) and equatorward transport in the subsurface layer (100–200 m). Therefore, the STCs are important for the exchange of heat, freshwater, dissolved oxygen, and nutrients between these regions. In this work, we find that the climate models generally simulate a too weak Southern Hemisphere STC which can be related to weaker southeasterly trade winds compared to observations. The results highlight the necessity to improve the ability of coupled climate models to realistically simulate the winds in the Southern Hemisphere, in order to better simulate the tropical‐subtropical ocean circulation. The results provide an important reference for studies on the mean state and variability of the Atlantic STCs, and for studies investigating the STCs' possible change in the future under global warming using coupled climate models. Key Points: The performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the mean Atlantic Subtropical Cells is evaluatedThe model ensemble shows large biases toward weak southern hemisphere subtropical cell and weak Equatorial UndercurrentThe large inter‐model spread of the subtropical cell and Equatorial Undercurrent strength is associated with the southeasterly trade winds [ABSTRACT FROM AUTHOR]

Here we examine the large‐scale transport characteristics of the new "Middle Atmosphere" NASA Goddard Institute for Space Studies (GISS) climate model (E2.2). First, we evaluate the stratospheric transport circulation in historical atmosphere‐only simulations integrated with interactive trace gas and aerosol chemistry. Compared to lower vertical resolution model versions, E2.2 exhibits improved tropical ascent and older stratospheric mean ages that are more consistent with observed values. In the troposphere, poleward transport to the Arctic and interhemispheric mean ages in E2.2 are comparable to models participating in the Chemistry Climate Modeling Initiative. In addition to validating E2.2, we also assess its "transport sensitivity" using the coupled atmosphere‐ocean abrupt 4xCO2 and transient 1%CO2 simulations submitted to the Coupled Model Intercomparison Project, Phase 6, along with a 2xCO2 simulation used to evaluate the linearity of the transport circulation's response to increased CO2. We show that decreases (increases) in a stratospheric mean age (idealized surface loss) tracer scale linearly with increased lower stratospheric upwelling, which also increases linearly with warming tropical sea surface temperatures (SSTs). Abrupt 2xCO2 and 4xCO2 experiments constrained with (fixed) pre‐industrial SSTs are also used to quantify the relative importance of rapid adjustments versus SST feedbacks to the transport circulation responses in the model. Finally, sensitivity experiments are presented to illustrate the impact of changes in the convective parameterization on stratospheric transport. Key Points: New GISS "high‐top" climate model features significantly improved stratospheric transport properties compared to previous model versionsThe stratospheric transport circulation responds more or less linearly to increased CO2SST feedbacks dominate the lower stratospheric (80 hPa) transport response to CO2 while rapid adjustments are more important above (10 hPa) [ABSTRACT FROM AUTHOR]