Regulation of Moist Convection over the West Pacific Warm Pool

Abstract

The mechanisms that regulate moist convection over the warm tropical oceans are not well understood. One school of thought holds that convection is caused by the convergence of moisture, which in turn is produced by an independent dynamical mechanism. Another school maintains that convection occurs as needed to just balance the production of convective instability and that the timescales to establish this balance is much less than the timescales of tropical disturbances. This is called the quasiequilibrium hypothesis. This paper explores how convection is actually governed over the west Pacific warm pool. Convection appears to be initiated there when the boundary-layer equivalent potential temperature exceeds a threshold value that is determined by conditions just above cloud base. Given known surface flux values and the propensity for convection to inject low equivalent potential temperature air into the boundary layer, it is shown that under most circumstances convection is regulated by a balance between the respective tendencies of surface fluxes and convective downdrafts to increase and decrease boundary-layer equivalent potential temperature. This regulatory mechanism is operative on timescales of one-half of a day or greater and is denoted boundary-layer quasiequilibrium. Given additional information about the ratio of downdraft to updraft volume fluxes and the equivalent potential temperature deficit in downdrafts, it appears to be possible to infer the mean vertical velocity at cloud base over timescales for which the clear-air vertical velocity is radiatively governed. On the basis of this analysis it is hypothesized that moisture convergence and low-level vertical motion over the west Pacific warm pool are largely a consequence rather than a cause of convection, at least on timescales of one-half of a day or greater. Externally imposed vertical motion should result in significant additional latent heat release only where the atmosphere is saturated. This typically occurs in the Tropics in the middle and upper levels of regions that are already convectively active.