Abstract The response of tropical precipitation to extratropical thermal forcing is reexamined using an idealized moist atmospheric GCM that has no water vapor or cloud feedbacks, simplifying the analysis while retaining the aquaplanet configuration coupled to a slab ocean from the authors’ previous study. As in earlier studies, tropical precipitation in response to high-latitude forcing is skewed toward the warmed hemisphere. Comparisons with a comprehensive GCM in an identical aquaplanet, mixed-layer framework reveal that the tropical responses tend to be much larger in the comprehensive GCM as a result of positive cloud and water vapor feedbacks that amplify the imposed extratropical thermal forcing. The magnitude of the tropical precipitation response in the idealized model is sensitive to convection scheme parameters. This sensitivity as well as the tropical precipitation response can be understood from a simple theory with two ingredients: the changes in poleward energy fluxes are predicted using a one-dimensional energy balance model and a measure of the “total gross moist stability” [Δm, which is defined as the total (mean plus eddy) atmospheric energy transport per unit mass transport] of the model tropics converts the energy flux change into a mass flux and a moisture flux change. The idealized model produces a low level of compensation of about 25% between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics regardless of the convection scheme parameter. Because Geophysical Fluid Dynamics Laboratory Atmospheric Model 2 (AM2) with prescribed clouds and water vapor exhibits a similarly low level of compensation, it is argued that roughly 25% of the compensation is dynamically controlled through eddy energy fluxes. The sensitivity of the tropical response to the convection scheme in the idealized model results from different values of Δm: smaller Δm leads to larger tropical precipitation changes for the same response in the energy transport.
Abstract Changes in ocean salinity are essential for the stratification of the upper ocean and the regional marine ecosystem. In this study, 10 sets of large ensemble experiments and multi‐model ensembles from the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used to investigate the effect of anthropogenic forcing on upper ocean salinity in the South China Sea (SCS). In most models, surface salinity increases during the historical period due to external forcing. Using the salinity budget, we find that a decrease in freshwater flux, particularly precipitation, is responsible for the increase in salinity, although horizontal advection also contributes to the change. Single forcing experiments reveal that the change in salinity in the SCS is mainly attributed to anthropogenic forcing, while the influence of natural forcing is relatively small. Anthropogenic aerosols (AAs) can decrease the dynamic and thermal components of precipitation, resulting in a considerable increase in salinity. In contrast, anthropogenic greenhouse gases (GHGs) have less effect on long‐term trend in SCS salinity because the GHG forcing leads to an increased thermal response of precipitation but a decreased dynamic response. Additionally, we use the Community Earth System Model version 1 (CESM1) to evaluate the role of different aerosol emission sources in modulating the salinity change in the SCS. The experimental results show that aerosol emissions from Asia dominate the salinity change in the SCS by changing the local Hadley circulation. In contrast, the contribution of aerosol emissions from North America and Europe (NAEU) is much smaller.
Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO2) forcing, sea ice changes, and regional ocean heat uptake. The CO2 forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes.
Recent studies have indicated that ocean circulation damps the atmospheric energy transport response to hemispherically differential energy perturbations, thereby muting the shifts of the Inter-Tropical Convergence Zone (ITCZ). Here, we focus on the potential role of Ekman heat transport in modulating this atmospheric response. An idealized representation of Ekman-driven heat transport (FE) is included in an aquaplanet slab ocean coupled to a gray radiation atmospheric model. We first alter the strength of FE in the control climate by tuning the gross stability of the Ekman layer SE. For a wide range of FE, the total poleward transport of energy remains nearly unchanged, but the ocean transports an increasing share for larger SE. The control climate is then perturbed by adding surface cooling in the Southern Hemisphere and warming in the Northern Hemisphere. The Ekman coupling damps the atmospheric energy transport response, as in previous coupled model experiments with full ocean dynamics. The ratio of the changes in Ekman to atmospheric energy transport is determined by the ratio of the gross stability in the Ekman layer to the atmosphere in the control climate, and is insensitive to the amplitude and location of forcing. We find that an unrealistically large SE is needed to reproduce the ratio of the changes in cross-equatorial oceanic to atmospheric energy transport in fully coupled models. The limited damping effect of Ekman transport highlights the need to examine the roles of deep circulation and subtropical gyres, as well as ocean heat uptake processes.
Since the beginning of the satellite era, Southern Ocean sea surface temperatures (SSTs) have cooled, despite global warming. While observed Southern Ocean cooling has previously been reported to have minimal impact on the tropical Pacific, the efficiency of this teleconnection has recently shown to be mediated by subtropical cloud feedbacks that are highly model-dependent. Here, we conduct a coupled model intercomparison of paired ensemble simulations under historical radiative forcing: one with freely evolving SSTs and the other with Southern Ocean SST anomalies constrained to follow observations. We reveal a global impact of observed Southern Ocean cooling in the model with stronger (and more realistic) cloud feedbacks, including Antarctic sea-ice expansion, southeastern tropical Pacific cooling, northward-shifted Hadley circulation, Aleutian low weakening, and North Pacific warming. Our results therefore suggest that observed Southern Ocean SST decrease might have contributed to cooler conditions in the eastern tropical Pacific in recent decades.
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