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Since the early 2010s, anthropogenic aerosols have started decreasing in East Asia (EA) while have continued to increase in South Asia (SA). Yet the climate impacts of this Asian aerosol dipole (AAD) pattern remain largely unknown. Using a state-of-the-art climate model, we demonstrate that the climate response is distinctly different between the SA aerosol increases and EA aerosol decreases. The SA aerosol increases lead to ~2.7 times stronger land summer precipitation change within the forced regions than the EA aerosol decreases. Contrastingly, the SA aerosol increases, within the tropical monsoon regime, produce weak and tropically confined responses, while the EA aerosol decreases yield a pronounced northern hemisphere warming aided by extratropical mean westerly and positive air-sea feedbacks over the western North Pacific. By scaling the observed instantaneous shortwave radiative forcing, we reveal that the recent AAD induces a pronounced northern hemisphere extratropical (beyond 30°N) warming (0.024 ± 0.010 °C decade⁻¹), particularly over Europe (0.049 ± 0.009 °C decade⁻¹). These findings highlight the importance of the pattern effect of forcings in driving global climate and have important implications for decadal prediction.

 

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 CO₂forcing 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.

 

Abstract The Hadley cell response to globally increasing CO 2 concentrations is spatially complex, with an intensified rising branch and weakened descending branch. To better understand these changes, we examine the sensitivity of the Hadley cell to idealized radiative forcing in different latitude bands. The Hadley cell response is, to first order, governed by the latitudinal structure of the forcing. The strengthening of the upward branch is attributed to tropical forcing, whereas the weakening of the descending branch is attributed to extratropical forcing. These direct radiatively-forced Hadley cell responses are amplified by changes in atmospheric eddy heat transport while being partially offset by changes in gross moist stability and ocean heat uptake. The radiative feedbacks further modulate the Hadley cell response by altering the meridional atmospheric energy gradient. The Hadley cell projections under global warming are thus a result of opposing – and thus compensating – effects from tropical and extratropical radiative forcings. 

Excessive precipitation over the southeastern tropical Pacific is a major common bias that persists through generations of global climate models. While recent studies suggest an overly warm Southern Ocean as the cause, models disagree on the quantitative importance of this remote mechanism in light of ocean circulation feedback. Here, using a multimodel experiment in which the Southern Ocean is radiatively cooled, we show a teleconnection from the Southern Ocean to the tropical Pacific that is mediated by a shortwave subtropical cloud feedback. Cooling the Southern Ocean preferentially cools the southeastern tropical Pacific, thereby shifting the eastern tropical Pacific rainbelt northward with the reduced precipitation bias. Regional cloud locking experiments confirm that the teleconnection efficiency depends on subtropical stratocumulus cloud feedback. This subtropical cloud feedback is too weak in most climate models, suggesting that teleconnections from the Southern Ocean to the tropical Pacific are stronger than widely thought. 

The atmospheric Green's function method is a technique for modeling the response of the atmosphere to changes in the spatial field of surface temperature. While early studies applied this method to changes in atmospheric circulation, it has also become an important tool to understand changes in radiative feedbacks due to evolving patterns of warming, a phenomenon called the “pattern effect.” To better study this method, this paper presents a protocol for creating atmospheric Green's functions to serve as the basis for a model intercomparison project, GFMIP. The protocol has been developed using a series of sensitivity tests performed with the HadAM3 atmosphere‐only general circulation model, along with existing and new simulations from other models. Our preliminary results have uncovered nonlinearities in the response of the atmosphere to surface temperature changes, including an asymmetrical response to warming versus cooling patch perturbations, and a change in the dependence of the response on the magnitude and size of the patches. These nonlinearities suggest that the pattern effect may depend on the heterogeneity of warming as well as its location. These experiments have also revealed tradeoffs in experimental design between patch size, perturbation strength, and the length of control and patch simulations. The protocol chosen on the basis of these experiments balances scientific utility with the simulation time and setup required by the Green's function approach. Running these simulations will further our understanding of many aspects of atmospheric response, from the pattern effect and radiative feedbacks to changes in circulation, cloudiness, and precipitation.

 

This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO₂with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQ_net). We demonstrate that thetransientpatterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by theequilibriumsolutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQ_netdistribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO₂-induced thermodynamic warming effect and the ΔQ_net(ocean heat uptake)–induced cooling effect. As ΔQ_netis largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO₂warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQ_net-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO₂warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQ_net, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQ_neteffect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern.

 

Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude–to–Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns.