Global Oceanic Precipitation Derived from TOPEX and TMR: Climatology and Variability

Abstract

A detailed study on global oceanic precipitation is carried out using the simultaneous TOPEX and TMR (TOPEX Microwave Radiometer) data. It is motivated by the success of a series of feasibility studies based on a few years of TOPEX?TMR data, and the availability of a decade-long new dataset that spans 1992?2002. In this context, a previously proposed rain probability index is improved by taking into account the difference of the dynamic range of the TOPEX-measured backscatter coefficients at the Ku and C bands and the latitudinally complementary sensitivities of the TOPEX and TMR rain detections, leading to a refined joint precipitation index, which is generally consistent and quantitatively comparable with existing precipitation climatologies from the Global Precipitation Climatology Project (GPCP) and the Comprehensive Ocean?Atmosphere Data Set (COADS). The new TOPEX?TMR precipitation climatology, on the one hand, confirms the fundamental features of global oceanic rainfall with additional details, and, on the other hand, reveals a number of interesting characteristics that are previously unknown or poorly defined. 1) The spatial variability of the western Pacific ?rain pool? (the atmospheric counterpart of the oceanic warm pool) is characterized by an interannual zonal migration, an annual cycle of meridional seesaw, and a semiannual cycle of expansion and shrinking. 2) The Pacific, Atlantic, and Indian Ocean intertropical convergence zones (ITCZs) all have an annual cycle of cross-basin oscillation with east and west stops in JJA and DJF, respectively. 3) A well-defined prominent rainy zone is observed in the southeast China Seas around Taiwan Island, connecting with the Pacific rain pool in the south. 4) Between El Niño and La Niña years, there is a systematic sign reversal of the geographical distribution of precipitation anomaly, which exists globally rather than in the tropical oceans only. 5) On a global basis, interannual and annual precipitation variabilities are of the same magnitude, but the interannual (annual) component is more important for the Southern (Northern) Hemisphere. 6) For the tropical oceans, ?season? defined by rainfall usually has a one-quarter delay with respect to the corresponding meteorological season. For the ?marine deserts? in the subtropical oceans, however, the rain-based season is found to be anticorrelated with the meteorological season. In addition, the annual cycle of the Atlantic precipitation is nearly 180° out of phase with respect to that of the Pacific and Indian Ocean for the same hemisphere.