Moisture budget in the tropics and the Walker circulation

Abstract

Moisture budget in the tropics is studied for two contrasting years, 1987 (El Niño) and 1988 (La Niña), and for two seasons, December, January, and February (DJF) and June, July, and August (JJA). To evaluate the moisture budget, data from the National Centers for Environmental Prediction (NCEP) reanalysis are used. First, the mean rainfall and evaporation characteristics of the NCEP reanalysis for January, April, July, and October are compared with other independent data. The general precipitation zones associated with the large‐scale features such as Intertropical Convergence Zone (ITCZ), South Pacific Convergence Zone (SPCZ), and South Atlantic Convergence Zone (SACZ) seem to be captured by the NCEP reanalysis. However, the seasonal variations, as seen in other data, are not reproduced well. Characteristics of ITCZ in the eastern Pacific is not well reproduced in the NCEP reanalysis data. The overall characteristics of latent heat flux (evaporation) in the NCEP reanalysis seem to be in qualitative agreement with other independent data. However, there are quantitative differences. The differences in rainfall over the western Pacific between DJF 1986/1987 (El Niño) and DJF 1988/1989 (La Niña) are associated with the differences in the position and intensity of SPCZ. The SPCZ is more intense in the El Niño year and is displaced northeastward. Northeast Brazil experienced higher rainfall in DJF 1988/1989 than in DJF 1986/1987. During the boreal summer (monsoon) season (JJA), rainfall was higher in 1988 over India than in 1987. The NCEP rainfall data are compared with the rainfall data from Schemm et al. and Huffman et al. The NCEP reanalysis seems to underestimate rainfall over the equatorial eastern Pacific. Higher precipitable water is found over the regions of intense convection such as SACZ, SPCZ, ITCZ, and the monsoon trough in JJA. The NCEP reanalysis seems to capture the general characteristics of vertically integrated moisture transport such as the general westward transport in the tropics, cross‐equatorial moisture transport over the Indian Ocean with high values near the Somali coast in JJA. Over the western Pacific the differences between the two periods are associated with differences in moisture convergence but not evaporation. The increment term (Dc) shows large values over the mountainous regions. The evaluation of moisture budget of the Walker circulation shows that over the western Pacific the large differences in precipitation between the two contrasting years are accounted mainly by the differences in moisture flux convergence. This verifies the important role of moisture convergence in this region as inferred indirectly by Cornejo‐Garrido and Stone. However, over the Amazon region, evapotranspiration seems to play an important role in the local precipitation.