Frequency distribution of daily ITCZ patterns over the Western/Central Pacific

Using multi-year satellite measurements of daily precipitation, the transient structure of the inter-tropical convergence zone (ITCZ) over the western-central Pacific is described by not only its time averages, but also its occurrence frequency and duration. Six distinct spatial patterns, namely, the North ITCZ, the South ITCZ, the Equator ITCZ, the Double ITCZ, the Full ITCZ and the Weak ITCZ, are identified based on the area percentage coverage of deep convection over different latitudinal bands on and off the equator. The relationship between these daily ITCZ patterns and SSTs, as well as near-surface winds, is also examined.

The most frequent ITCZ types are the North and South ITCZs, which mostly appear in their own summers. Closely associating with the annual latitudinal migration of SSTs, these ITCZ patterns indicate an evident seasonal cycle in month-by-month variations of occurrence frequencies. Another major type is the Weak ITCZ, which occurs remarkably frequently and the daily SST shows overall weak negative anomalies and a minimum along the equator. Compared with the three major ITCZ types, the other three ITCZ patterns occur more sporadically. Among those three less frequent ITCZ patterns, the Double ITCZ exhibits certain regularity; its occurrence becomes more frequent during the boreal fall and winter and less frequent during the boreal spring and summer. In contrast with the three major ITCZ types, quite large daily SST anomalies (more than ±0.5oC) are corresponding to the Equator, Double and Full ITCZs indicating a strong interconnection between day-by-day SST anomalies and these ITCZ patterns that have lower frequencies of occurrence. Composite surface wind vector indicates that the precipitation maxima appear to be coinciding with the local surface wind convergence centers and with surface wind speed minima. However, from surface wind convergence there is no distinct pattern found to bear any resemblance to any defined precipitation pattern suggesting that little relationship between low-level convergence and precipitation can be discerned from daily data.

As an application of the approach proposed, the simulation from a current version of the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) general circulation model (GCM) has been evaluated. The results indicate that the GEOS GCM has a great capability to simulate the correct ITCZ spatial patterns but not their occurrence frequencies, especially for those patterns with fewer occurrences. The problems may lead to large biases in their simulation of seasonal and monthly means.