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Interactions among Cloud, Water Vapor, Radiation, and Large-Scale Circulation in the Tropical Climate. Part II: Sensitivity to Spatial Gradients of Sea Surface Temperature

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

The responses of the large-scale circulation, clouds, and water vapor to an imposed sea surface temperature (SST) gradient are investigated. Simulations compare reasonably to averaged observations over the Pacific, considering the simplifications applied to the model. The model responses to sinusoidal SST patterns have distinct circulations in the upper and lower troposphere. The upper circulation is sensitive to the heating from deep convection over the warmest SST. Stronger SST gradients are associated with stronger longwave cooling above stratus clouds in the subsidence region, stronger lower-tropospheric large-scale circulation, a reduction of the rain area, and larger area coverage of low clouds. A similar SST gradient with a warmer mean temperature produces slightly weaker lower-tropospheric circulation, and slightly reduced low cloud coverage.

The outgoing longwave radiation (OLR) is not sensitive to the mean SST or the range of the imposed sinusoidal SST gradient. The positive feedbacks of water vapor and decreasing high cloud OLR compensate for the increase in longwave emission with increasing mean temperature in these simulations. As the SST gradient is increased keeping the mean SST constant, the positive high cloud feedback is still active, but the air temperature increases in proportion to the maximum SST in the domain, increasing the clear-sky OLR value and keeping the average OLR constant.

The net absorbed shortwave radiation (SWI) is found to be extremely sensitive to the SST gradient. The stronger lower-tropospheric large-scale circulation produces a higher water content in the high and low clouds, increasing the absolute magnitude of the shortwave cloud forcing. A 25% increase in the maximum zonal mass flux of the lower circulation of the 300-K mean, 4-K SST range simulation leads to a 7.4 W m−2 decrease in SWI. Increasing the mean SST creates a positive feedback in these simulations because of the decrease in the lower-tropospheric large-scale circulation and the resultant decrease in cloud optical depth.

Corresponding author address: Dr. Kristin Larson, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. klarson@atmos.washington.edu

Abstract

The responses of the large-scale circulation, clouds, and water vapor to an imposed sea surface temperature (SST) gradient are investigated. Simulations compare reasonably to averaged observations over the Pacific, considering the simplifications applied to the model. The model responses to sinusoidal SST patterns have distinct circulations in the upper and lower troposphere. The upper circulation is sensitive to the heating from deep convection over the warmest SST. Stronger SST gradients are associated with stronger longwave cooling above stratus clouds in the subsidence region, stronger lower-tropospheric large-scale circulation, a reduction of the rain area, and larger area coverage of low clouds. A similar SST gradient with a warmer mean temperature produces slightly weaker lower-tropospheric circulation, and slightly reduced low cloud coverage.

The outgoing longwave radiation (OLR) is not sensitive to the mean SST or the range of the imposed sinusoidal SST gradient. The positive feedbacks of water vapor and decreasing high cloud OLR compensate for the increase in longwave emission with increasing mean temperature in these simulations. As the SST gradient is increased keeping the mean SST constant, the positive high cloud feedback is still active, but the air temperature increases in proportion to the maximum SST in the domain, increasing the clear-sky OLR value and keeping the average OLR constant.

The net absorbed shortwave radiation (SWI) is found to be extremely sensitive to the SST gradient. The stronger lower-tropospheric large-scale circulation produces a higher water content in the high and low clouds, increasing the absolute magnitude of the shortwave cloud forcing. A 25% increase in the maximum zonal mass flux of the lower circulation of the 300-K mean, 4-K SST range simulation leads to a 7.4 W m−2 decrease in SWI. Increasing the mean SST creates a positive feedback in these simulations because of the decrease in the lower-tropospheric large-scale circulation and the resultant decrease in cloud optical depth.

Corresponding author address: Dr. Kristin Larson, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. klarson@atmos.washington.edu

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