Frequency Distribution of Daily ITCZ Patterns over the Western–Central Pacific

Abstract

This study attempts to explore a comprehensive and compact approach for delineating the multiscale and multivariate characteristics of the ITCZ over the western?central Pacific based on daily satellite observations of precipitation, SSTs, and surface winds. Essentially six distinct ITCZ spatial patterns?namely, the north, south, equator, double, full, and weak?are identified according to the daily percentage coverage of deep convection within different latitudinal bands on and off the equator over the western?central Pacific. The evolving structure of the ITCZ over the western?central Pacific is investigated with a focus on the transient statistical characteristics. The relationship between these daily ITCZ patterns and SSTs, and near-surface winds, is also examined. The north (37%), south (24%), and weak (24%) ITCZs represent the three major ITCZ daily patterns over the western?central Pacific, and combined they account for almost 85% of the total number of days within a 10-yr period. The other three ITCZ patterns, namely, the equator (3%), double (6%), and full (5%) ITCZs, occur infrequently. The climatology of the ITCZ, such as monthly, seasonal, and annual means, can be approximately determined by how often and intense these ITCZ daily spatial patterns occur within a specified period. Taking the long-term mean statistics for each ITCZ daily type into account, the double ITCZ deep convection typically observed over the western?central Pacific in monthly, seasonal, and annual mean plots appears to be mainly associated with the frequent occurrence of the north and south ITCZ patterns, instead of the double ITCZ pattern in which an ITCZ appears on each side of the equator simultaneously on a daily basis. Consistent with the strong seasonality in their frequency of occurrence, the three major ITCZ patterns indicate a dominant correspondence with the seasonal meridional migration of warm SSTs. In contrast, the three less frequent ITCZ patterns show a close relationship with the positive or negative SST anomaly over the equatorial central and eastern Pacific, namely, the extension and retraction of the equatorial cool tongue and its strength. Surface wind divergence/convergence does not show any distinct patterns for these ITCZ spatial patterns, suggesting that little relationship between low-level convergence and precipitation can be discerned from daily data. As an application of the method proposed, the frequency distribution of daily ITCZ patterns, as derived from a recent version of the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) general circulation model (GCM), is evaluated against observations. Preliminary comparisons indicate that the GEOS-5 GCM is capable of simulating the correct ITCZ spatial patterns, but their occurrence frequencies can be further improved, in particular, the weak ITCZ and the patterns with fewer occurrences, which may be associated with significantly different control mechanisms and/or feedbacks.