Moisture Variability over the Tropical Western Pacific Ocean

Abstract

In this paper, we study the characteristics of moisture variability over the tropical western Pacific warm pool region using sounding data obtained through the use of research vessels. Data used here were collected in both boreal summer and winter seasons over a nine-year period from 1993 through 2001. Sounding data are categorized into five zonal wind vertical shear modes, and then examined for common and singular features of moisture variability. The results show frequent occurrences of high relative humidity centered around 945 hPa, and 560 hPa, for any wind shear conditions, as previously found in the Tropical Ocean and Global Atmosphere/ Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) studies. We also found another small peak at 845 hPa in low-level easterly conditions. The degree of these features is modulated by wind shear mode differences. An interesting feature is that the variation of total precipitable water is well represented by the synoptic scale mid-tropospheric moisture variation. However, a close relation between the total moisture budget and the moisture variation in the boundary layer is also obtained for a time scale of less than a few days (especially the diurnal cycle). We investigated extremely dry air conditions as one factor modulating mid-tropospheric moisture. Backward air parcel trajectories show that there are two major routes for the intrusion of dry air over the warm pool. In the first, the air parcel could be traced back to higher latitudes and the upper troposphere. For this type, anticyclonic equatorward flow associated with developing mid-latitude baroclinic waves, such as Rossby wave breaking, is responsible for injecting subtropical dry air into the Tropics—as proposed by Yoneyama and Parsons for TOGA COARE dry events. In the second type, the air parcel could be traced back to the upper troposphere, but within lower latitudes in the central-eastern Pacific. For this case, the air parcel originates from the trade winds that passed over the region where dry air was injected by the anticyclonic equatorward flow associated with the baroclinic wave development in the subtropical region. The relationship between dry air conditions and large-scale features is also discussed.