85th AMS Annual Meeting

Thursday, 13 January 2005
Role of Indian Ocean on Pakistan Summer Rainfall
Muhammad Jawed Iqbal Sr., I am a Ph.D. student in Institure of Space and Palanetary Astrophysics, University of Karachi, Karachi, Pakistan
Abstract: The oceans play critical roles in the physical climate system of Earth. As seawater is a fluid, currents in the ocean can move water over great distances and carry heat and other ocean properties from one geographic area to another. The equator-to-pole energy transport by the ocean is important in reducing the pole-to-equator temperature gradient. Horizontal and vertical transport of energy by the ocean can also alter the nature of regional climates by controlling the local sea surface temperature (SST). Gradients of SST within the oceans are important in determining the location of precipitation over the tropics, including the monsoon regions. As a first approximation, one may assume that the distribution of SST in the Indian Ocean plays a role in determining monsoon rainfall variability. Models and empirical evidence for this suggest the influence of the tropical oceans on the Asian monsoon. We examine relationship between Indian Ocean sea surface temperature (SST) variability and the Pakistan monsoon, including analysis of potential long-lead predictions of Pakistan rainfall by regional SST and the influence of ENSO and decadal variability on the stability of the relationships. The basic SST data set used in this study is the Global Sea-Ice and Sea-Surface Temperature (GISST, version 2.2) dataset compiled from ship records. All Pakistan Monsoon Rainfall index (APSMRI), the rainfall over Pakistan averaged annually from June through September has been computed for the period 1961 to 1997. To determine the monthly SST-APSMRI relationship preceding and following the monsoon, we correlate the SST time series for each month at each grid point in the Indian Ocean with the APSMRI. When correlations of monthly mean SST are computed with the APSMRI, the strongest correlation of an individual month SST with the APSMRI occurs in the Arabian Sea in December at the 4 x 4 grid cell centered at 29 N, 65 E (Arabian Sea index or ASI) is 0.52. There is also a high correlation of April SST in the central Indian Ocean ( 25S and 85E) with the APSMRI. The April SST at this location (the central Indian Ocean, or CIOI index ) correlates to the APSMRI at 0.64. We find that the correlation of the ASI and CIOI SST indices with the Pakistan summer rainfall are largely unaffected by the removal of the ENSO signal.

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