CLIMATIC-SCALE SPACE-TIME VARIABILITY OF TROPICAL PRECIPITATION

More than 15 years of monthly microwave sounding unit rainfall data ov er the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5 degrees x 2.5 degrees grid s quare. Regions of highest rainfall variability tend to be characterize d by a powerful annual cycle. The semiannual cycle is generally a triv ial component of the seasonal cycle, except in some regions where eith er the mean climatological precipitation is low or where the total sea sonal cycle is weak. An interesting exception, in this respect, is a b and of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highes t climatological mean rainfall are characterized by weak seasonality b ut strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via princ ipal component analysis in the time domain. The two dominant modes tog ether describe precipitation variability associated with the El Nino-S outhern Oscillation: they outline the evolution of warm- and cold-even t precipitation anomalies and contrast the intense 1982-1983 warm even t with the moderate events of 1986-1987 and 1992-1993. These two modes display oscillations with predominantly quasi-biennial and similar to 5-year periods. Another coherent mode summarizes intraseasonal variab ility which, although inadequately resolved by the monthly average rai nfall data, displays typical signs of the 40- to 50-day oscillation. A ll coherent modes, despite having much of their energy concentrated ar ound rather different frequencies, show signs of interaction.