RAINFALL VARIATIONS IN THE TROPICS DURING 1986-1989, AS ESTIMATED FROM OBSERVATIONS OF CLOUD-TOP TEMPERATURE

Large-scale shifts in rainfall in the tropics, particularly the western tropical Pacific, were observed during the transition from "warm" (1986-1987) to "cold" (1988-1989) El Nino/Southern Oscillation (ENSO) conditions. Differences in the estimated rainfall rate in excess of 20 mm d-1 were observed south of the equator, near the international date line, between the northern winter seasons of 1986-1987 and 1988-1989. The distribution of zonal mean rainfall varied significantly during the study period, as did monthly accumulated rainfall in the global tropics, where the difference between the wettest and driest months for the period January 1986 through May 1989 was 24%. No direct measurements (rain gage, radar) of large-scale areal-averaged rainfall are available for the global tropics, hence rainfall estimates, as inferred from satellite observations of cloud-top temperature, are used in this paper. A qualitative comparison of the behavior of the rainfall estimates during the 1986-1989 period with the expected rainfall anomaly patterns in regions with known rainfall responses to both the low ("warm") and high ("cold") Southern Oscillation Index (SOI) phases of ENSO phenomena is presented. The comparison indicates that rainfall variation during 1986-1989 in these regions behaved as expected, for the most part, for both phases of the ENSO cycle. The rainfall patterns are also compared to observed moisture and vertical motion fields in the atmosphere and sea surface temperature (SST) during this period.