Roel Neggers, J. David Neelin and Bjorn Stevena
J. Climate, 20, 2623-2642, 2007.
Paper (PDF 2.6 MB)
© Copyright 2007 by the American Meteorological Society.
Abstract. Subtropical shallow cumulus convection is shown to play an important role in tropical climate dynamics through a chain of indirect feedbacks. It is found that the presence of shallow convection in the subtropics contributes to setting the width and intensity of oceanic intertropical convergence zones (ITCZs), a mechanism here termed the {\em shallow cumulus humidity throttle}. These conclusions are reached after investigations based on a tropical climate model of intermediate complexity, with sufficient vertical degrees of freedom to capture i) boundary layer physics and ii) feedbacks between tropospheric humidity and deep convection. Using this modeling framework, the mechanisms of interaction between small-scale shallow cumulus convection and large-scale tropical and subtropical climate dynamics are identified and quantified. A simple first-order parameterization for shallow cumulus convection is used in which the time scale of shallow cumulus adjustment can be varied to assess sensitivity. A moist static energy budget analysis is then performed that reveals the chains of relevant feedback mechanisms acting in the tropics.
The first direct impact of reduced shallow cumulus convection is to increase boundary layer humidity in the subtropics. This locally reduces surface evaporation and suppresses tropospheric humidity. Horizontal transport of this drier air by humidity transients and dry intrusions significantly reduces the precipitating deep convection at the edges of the oceanic convection zones. As a result, the temperature is reduced by several degrees throughout the tropics. In turn, the combination of this temperature change and the subtropical drop in water vapor path triggers a compensating increase in net longwave warming and surface heat flux. In the central axis of the oceanic convection zones this acts to increase the moist static energy forcing, which results in stronger large-scale convergence producing more intense precipitation.
Citation. Neggers, R., J. D. Neelin and B. Stevens, 2007: Impact mechanisms of shallow cumulus convection on tropical climate dynamics. J. Climate, 20, 2623-2642. doi:10.1175/JCLI4079.1
Acknowledgments.
This research was supported in part by National Science Foundation grants DMS-0139666
and ATM-0082529. The members of the Focused Research Group on tropical atmospheric
dynamics are thanked for discussions that helped stimulate this research. We thank A.
Sobel for the helpful feedback on a draft version of this paper.
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