Recent advances in the observation and analysis of large-scale oceanic precipitation

Oceanic precipitation, particularly in the tropics, is an important component of the fresh water flux in the ocean. The associated condensation heating is probably the most significant factor in forcing seasonal to interannual anomalies in atmospheric circulation. Dynamical models that attempt to simulate or predict variations in oceanic and atmospheric circulation, such as those associated with the El Niño/Southern Oscillation (ENSO), must accurately reproduce observed variations in precipitation. Until the past decade, the observation and analysis of variations in oceanic precipitation was exceedingly difficult. In this paper, we will discuss the recent advances in observing systems, analytic techniques, and international cooperation that have succeeding in producing data sets that can be used to describe climatic variations in oceanic precipitation, to monitor those variations and their impact on other aspects of the climate system, and to validate dynamical prediction models. Significant advances in the use of passive microwave radiometric observations from satellites since the first availability of such instruments in 1987 have provided much more accurate estimates of oceanic precipitation. The development of techniques to combine those measurements with the better sampled infrared –based estimates from geostationary satellites have yielded the first complete time series of oceanic maps of monthly precipitation. The Global Precipitation Climatology Project (GPCP) of the World Climate Research Programme was largely responsible for bringing together the international partnership of satellite operators and scientists that made this possible. The Tropical Rainfall Measuring Mission (TRMM) satellite, launched in 1997 through a joint effort of the US and Japan, provided the first radar observations of oceanic precipitation. While the results from TRMM are only beginning to be fully utilized, they have already demonstrated their enormous potential for describing the full 3-dimensional structure of oceanic precipitation, and, in conjunction with the products of the GPCP, greatly improving the accuracy of oceanic precipitation analyses. These much more accurate products are being used together with new analytic techniques that utilize empirical orthogonal functions of precipitation variability to produce a 50-year time series of maps of monthly precipitation. Taken together, these advances are making possible the detailed description of oceanic precipitation variations associated with ENSO and other climate variations, and the verification of extended time series of model simulations and forecasts.