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Does Humidity’s Seasonal Cycle Affect the Annual-Mean Tropical Precipitation Response to Sulfate Aerosol Forcing?

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  • 1 McGill University, Montreal, Quebec, Canada
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Abstract

Sulfate aerosol radiative forcing alters the distribution of tropical precipitation in climate model simulations. The annual-mean tropical precipitation change is typically described as arising from an annual-mean change in the mean atmospheric circulation. However, the seasonality of the climatology of tropical humidity may modulate the annual-mean precipitation response. Here, the role of seasonality of tropical humidity is assessed using reanalysis and idealized atmospheric general circulation model (GCM) simulations perturbed by sulfate aerosol radiative forcing. When coupled to an aquaplanet slab ocean with low thermal inertia, the seasonal cycle in GCM simulations of the “continental” regime is large and the annual-mean precipitation change depends on both the seasonally varying perturbation mean meridional circulation and the seasonally varying climatological specific humidity. When coupled to an aquaplanet slab ocean with a higher thermal inertia, the seasonal cycle in GCM simulations of the “oceanic” regime is smaller and the annual-mean precipitation change can be approximated by considering the perturbation convergence of the water vapor flux of the annual-mean perturbation mean meridional circulation and the annual-mean climatological specific humidity. The results of the aquaplanet simulations taken together with the magnitude of the seasonality of humidity in an atmospheric reanalysis suggest that the simplest forms of energetic arguments for the tropical precipitation response to perturbations in the atmospheric energy budget, which neglect an explicit role for the seasonality of the radiative forcing and the climatological specific humidity, are not quantitatively accurate for Earth’s tropical land regions.

Corresponding author address: Timothy M. Merlis, Dept. of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal QC H3A 2K6, Canada. E-mail: timothy.merlis@mcgill.ca

Abstract

Sulfate aerosol radiative forcing alters the distribution of tropical precipitation in climate model simulations. The annual-mean tropical precipitation change is typically described as arising from an annual-mean change in the mean atmospheric circulation. However, the seasonality of the climatology of tropical humidity may modulate the annual-mean precipitation response. Here, the role of seasonality of tropical humidity is assessed using reanalysis and idealized atmospheric general circulation model (GCM) simulations perturbed by sulfate aerosol radiative forcing. When coupled to an aquaplanet slab ocean with low thermal inertia, the seasonal cycle in GCM simulations of the “continental” regime is large and the annual-mean precipitation change depends on both the seasonally varying perturbation mean meridional circulation and the seasonally varying climatological specific humidity. When coupled to an aquaplanet slab ocean with a higher thermal inertia, the seasonal cycle in GCM simulations of the “oceanic” regime is smaller and the annual-mean precipitation change can be approximated by considering the perturbation convergence of the water vapor flux of the annual-mean perturbation mean meridional circulation and the annual-mean climatological specific humidity. The results of the aquaplanet simulations taken together with the magnitude of the seasonality of humidity in an atmospheric reanalysis suggest that the simplest forms of energetic arguments for the tropical precipitation response to perturbations in the atmospheric energy budget, which neglect an explicit role for the seasonality of the radiative forcing and the climatological specific humidity, are not quantitatively accurate for Earth’s tropical land regions.

Corresponding author address: Timothy M. Merlis, Dept. of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal QC H3A 2K6, Canada. E-mail: timothy.merlis@mcgill.ca
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