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

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 m agnitude of the advection of water vapour for one event is calculated, and it is estimated that the time fur the atmosphere to recover to moist condit ions was similar to 10-20 days. From the magnitude of the drying and from t he frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region durin g the COARE, making it difficult for the atmosphere to reach a radiative-co nvective equilibrium. Intrusions, instead, can help to recharge the tropica l atmosphere by decreasing convective activity and, thus, driving the atmos phere toward unusually large values of convective available potential energ y. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on th is work and previous investigations. As in past studies, the recovery of th e atmosphere to moist conditions is accomplished through detrainment from c onvective clouds that began to form soon after the arrival of the dry air m ass and slowly deepen in height as the recovery progresses. Previous invest igators concluded that the entrainment of dry air into convective cells is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditio ns. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where th ere is little inhibition to deep convection and, when inhibition does occur , it can be removed by surface fluxes within hours. In contrast, it is foun d that convective inhibition can be large enough to suppress convection fol lowing dry intrusions, and that the diurnal variation in rainfall is due pa rtly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical prof iles 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 d iurnal variations in the surface fluxes, and often ignored convective inhib ition and diurnal variations in atmospheric radiative heating.