A proposed mechanism for the intrusion of dry air into the Tropical Western Pacific region

Recent studies using data from the Tropical Ocean and Global Atmosphere pro gram's Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) have shown that synoptic-scale areas of extremely dry air can occur in the tropospher e over the equatorial western Pacific. These layers of extremely dry air mo dify convective activity and the vertical profile of radiation in clear air . At the present. time there is some disagreement as to the dynamic mechani sm responsible for these events and a number of their characteristics are r elatively unknown. In this study, the origin and characteristics of the dry air events were investigated through analysis of TOGA COARE rawinsonde dat a and examination of global analyses from two different forecast centers. T hese drying events were found to be Very common and evidence was presented that their intensity was underestimated in the global analyses. These dry e vents were shown to most often originate in the Northern (winter) Hemispher e as troughs associated with baroclinic waves intensified and expanded equa torward, leading to a process analogous to Rossby wave breaking. In these c ases, the dry air at the edge of the westerlies at upper levels was incorpo rated into the equatorward extension of thin NE-SW tropospheric troughs, wh ere it subsided and was subsequently advected equatorward. If sufficient su bsidence took place, the dry air continued flowing equatorward on the easte rn edge of well-defined anticyclones in the lower troposphere. The dry air ill one case originated in a Southern (summer) Hemisphere trough that was a ssociated with midlatitude baroclinic waves that propagated equatorward and developed into a series of distinct disturbances along a subtropical jet. In both the Northern and Southern Hemisphere events, the subsiding dry air in the midtroposphere was injected into the fringes of the Tropics, where i t was able to reach equatorial regions if it interacted with favorable meri dional flow in the Tropics. Past studies have proposed that these intrusion s of dry air could induce droughts in the Tropics through decreasing deep c onvective activity. The implication of this study is that these droughts ar e actually induced by midlatitude processes.