The Eastern Pacific ITCZ during the Boreal Spring

Guojun Gu, Robert F. Adler, Adam H. Sobel
2005 Journal of the Atmospheric Sciences  
The intertropical convergence zone (ITCZ) 1~ a narrow band of tropical convection a m rainfall in the Pacific and Atlantic, generally staying in the north hemisphere. The variations in its latitudinal locations and intensity can effectively influence weather and climate around the world. In the eastern Pacific, two ITCZs across the equator are often observed during March-April. Why and how this unusual phenomenon appears in this region are not very clear. The purpose of this study is to
more » ... study is to document this phenomenon using the 6-year (1998-2003) rainfall products from the Tropical Rainfall Measuring Mission (TRMM). The double ITCZ phenomenon exists during March-April in the four TRMM years (1999)(2000)(2001)(2002). There seems to be a competition for rainfall between the south and north hemispheres during this season. When one of the two rainfall maxima representing the ITCZ is strong, the other tends to be weak. However, the total rainfall over the two hemispheres (from the equator to about 15"N or 15"s) shows much less variation. A similar result exists for SST, but it is not very convincing. Thus, at least some variations in the relative strength of the two rainfall maxima or the appearance of the double ITCZ may be independent of SST, which means that the atmosphere itself can modulate or determine the rainfall distribution to a certain extent. Another result is the importance of the cold SST over the equator. A cold equator is essential to the existence of the double ITCZs. However, too cold the equator is not preferred by the double ITCZs. These conclusions are generally confirmed by a 25-year (1979-2003) monthly rainfall dataset from the Global Precipitation Climatology Project (GPCP). Abstract The &year (1998-2003) rainfall products from the Tropical Rainfall Measuring Mission (TRMM) are used to quantify the Intertropical Convergence Zone (ITCZ) in the eastern Pacific (defined by longitudinal averages over 9OoW-13O0W) during boreal spring (March-April) . The double ITCZ phenomenon, represented by the occurrence of two maxima with respect to latitude in monthly mean rainfall, is observed in most but not all of the years studied. The relative spatial locations of maxima in sea surface temperature (SST) , rainfall, and surface pressure are examined. Interannual and weekly variability are characterized in SST, rainfall, surface convergence, total column water vapor, and cloud water. There appears to be a competition for rainfall between the two hemispheres during this season. When one of the two rainfall maxima is particularly strong, the other tends to be weak, with the total rainfall integrated over the two varying less than does the difference between the rainfall integrated over each separately. There is some evidence for a similar competition between the SST maxima in the two hemispheres, but this is more ambiguous, and there is evidence that some variations in the relative strengths of the two rainfall maxima may be independent of SST.
doi:10.1175/jas3402.1 fatcat:ctyxsinjhbdwxeo6nxcwxyf7iu