One of the most prominent features of the tropical atmosphere on which researchers have focused time and attention is a narrow, east-west band of cumulonimbus clouds that more or less encircles the globe within a few degrees of the equator. In terms of the climatological mean flow, this feature results from the meeting of the trade winds from the Northern and Southern Hemispheres and thus has been termed the Intertropical Convergence Zone (ITCZ). The introduction of reliable global analyses in the last decade presents researchers with an excellent opportunity to study the ITCZ observationally. This study utilizes both the global analysis fields as well as satellite and sea surface temperature data to study the structure and variability of the ITCZ of the western North Pacific over a five-year time period (1989-1993).
In the western North Pacific, the ITCZ can be divided into two separate flow regimes: the trade wind trough and the monsoon trough. The trade wind trough represents the convergence of the southeasterly and northeasterly trade winds. The monsoon trough generally occurs when the ITCZ is several degrees from the equator so that the trade winds from the opposite hemisphere have curled around to a westerly direction. The location, intensity, and structure of the ITCZ have been observed to vary greatly with both time and space. Of particular interest are the fluctuations of the ITCZ of the western North Pacific Ocean where the monsoon trough has been shown to play a significant role in tropical cyclogenesis.
Twice-daily objective analyses of the horizontal wind, temperature, and mean sea level pressure fields from the European Centre for Medium-range Weather Forecasting (ECMWF) as well as cloud top temperature data from the Geostationary Meteorological Satellites (GMS) are examined for the years 1989-1993. The location of the ITCZ for each time period from May to December is determined subjectively using both the 1000 hPa horizontal wind field and the satellite data. A climatology of the locations of the trade wind trough and the monsoon trough as well as the occurrence of any tropical cyclones is assembled. Time series of and correlations between the mean relative vorticity, cloud top temperature, and sea surface temperature in the ITCZ, trade wind trough, and monsoon trough are calculated. These time series are combined with the climatology of the locations of the ITCZ and the climatology of tropical cyclones in order to categorize the ITCZ for each observation time based on its location, its basic characteristics, and the presence of tropical cyclones. Composites of the wind, temperature, vorticity, cloud top temperature, and sea surface temperature fields are then constructed for each category of ITCZ. These composites are examined for differences that may explain the variability in the frequency and location of tropical cyclogenesis