6D.7 Satellite-Observed Subseasonal Rainfall Variations in Tropical East Atlantic-West Africa

Tuesday, 29 April 2008: 11:45 AM
Palms I (Wyndham Orlando Resort)
Guojun Gu, NASA/GSFC and ESSIC, Univ. of Maryland, Greenbelt, MD

Seasonal-subseasonal variations in the tropical East Atlantic-West African region are examined by means of daily precipitation estimates based on the 9-year (1998-2006) TRMM Multi-satellite Precipitation Analysis (TMPA/3B42). Several other recently archived satellite measurements are also applied, including the TRMM Microwave Imager (TMI) sea surface temperature (SST), columnar water vapor, and cloud liquid water, and the QuikSCAT surface winds. Evident seasonal variations exist in these variables. Specifically, a focus here is a peak rainfall season along the coast of the Gulf of Guinea during boreal spring-early summer (March-June). Various time lags are seen prominently in the seasonal cycles for various physical components, showing their causal relationships.

In addition to a power peak at the period of about 6 days, spectrum analyses of daily rainfall during the mainrainfall season (March-June) show two other major power peaks on the subseasonal time scales within the period ranges of 10-18 and 30-70 days, respectively. Various lag-correlations for these three oscillation modes are estimated corresponding to the phases with maximum rainfall. It is of interest to note that the synoptic-scale (2-10 days) signals have certain properties of the Kelvin-type modes, and the 30-70-day ones have coherent associations among various components similar to the global tropical intraseasonal mode, i.e., the Madden-Julian Oscillation (MJO). The climatological features of the 30-70-day signals are further examined by a harmonic analysis of the 9-year-mean daily TRMM rainfall. The 4-15 harmonics, roughly corresponding to these signals, seem to be an important component of seasonal rainfall variations, which provide much more detailed subseasonal features than the first 3 harmonics usually applied to represent the seasonal cycle. This is further confirmed by the results from a relatively long-time-record satellite product: the 28-year (1979-2006) daily Outgoing Longwave Radiation (OLR) measurements.

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