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Deep Convection Helps Tropics Keep Their (Relative) Cool
Reference
Sud, Y.C., Walker, G.K. and Lau, K.-M.  1999.  Mechanisms regulating sea-surface temperatures and deep convection in the tropics.  Geophysical Research Letters 26: 1019-1022.

What was done
Based on first principles of moist convection and data from the Tropical Ocean Global Atmosphere - Coupled Ocean-Atmosphere Response Experiment, the authors set out to investigate two questions: (1) why deep atmospheric convection is initiated when sea surface temperatures (SST) reach 28°C and (2) how the observed upper SST limit of approximately 30°C is maintained.

What was learned
Sud et al. demonstrate that deep convection in the tropics acts as a thermostat to keep SST vacillating between, approximately, 28 and 30°C.  Initially, the tropical ocean acts as a net energy receiver in its warming phase; but as the SST reaches 28-29°C, the cloud-base airmass is charged with the moist static energy needed for clouds to reach the upper troposphere.  At this point, the cloud cover thus formed reduces the amount of solar radiation received at the sea surface; and cool and dry downdrafts produced by the moist convection tend to promote ocean surface cooling by increasing sensible and latent heat fluxes at the surface that cause temperatures there to decline.

What it means
This "thermostat-like control," as the authors put it, tends "to ventilate the tropical ocean efficiently and help contain the SST between 28-30°C."  It would also be expected to similarly operate to keep tropical ocean surface temperatures from rising any higher in response to enhanced CO2-induced radiative forcing.


Reviewed 15 December 1999