The evolution of the tropical western Pacific atmosphere‐ocean system following the arrival of a dry intrusion

Abstract

Abstract Recent studies using TOGA COARE data have found that extremely dry air from middle‐latitude waves frequently intrudes into the equatorial troposphere over the western Pacific. Using sounding data taken during the COARE, the magnitude of the advection of water vapour for one event is calculated, and it is estimated that the lime for the atmosphere to recover to moist conditions was ∼ 10‐20 days. From the magnitude of the drying and from the frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during the COARE, making it difficult for the atmosphere to reach a radiative‐convective equilibrium, intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on this work and previous investigations. As in past studies, the recovery of the atmosphere to moist conditions is accomplished through detrainment from convective clouds that began to form soon after the arrival of the dry air mass and slowly deepen in height as the recovery progresses. Previous investigators concluded that the entrainment of dry air into convective ceils is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditions. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where there is little inhibition to deep convection and, when inhibition does occur, it can be removed by surface fluxes within hours. In contrast, it is found that convective inhibition can be large enough to suppress convection following dry intrusions, and that the diurnal variation in rainfall is due partly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes, and perhaps in large‐scale subsidence, leading to a minimum in convective inhibition during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes, and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.