Impact of eastern equatorial Indian ocean during positive tropical dipole on regions over Tamilnadu and coastal Andhra Pradesh

摘要

The main objective of the present work is to investigate the influence of eastern Sea surface temperature (SST) anomalies during the Positive Tropical Indian Ocean dipole (PTIOD) events on regional summer monsoon rainfall especially over Tamilnadu and Coastal Andhra Pradesh (A. P.) for the period of 1980-2009. There were 7 strong PTIOD events recorded during the period of study which are accompanied by significant increase in the seasonal summer monsoon rainfall over India. Averaged SST anomalies (SSTA) over the EQ.ESIO (Equatorial Eastern South Indian Ocean) during AMJ (April, May, June) shows that colder (warmer) EQ.ESIO during AMJ and positive TIOD events were associated with the decreasing (increasing) tendency of summer monsoon rainfall over Tamilnadu and coastal A.P. The statistical analysis shows the correlation coefficient of 0.59 and 0.79 between AMJSST anomalies and the rainfall over Tamilnadu and Coastal A. P. respectively. Wind anomalies during PTIOD years with negative AMJSSTA, transports the cold upwelled oceanic current off Sumatra-Java towards the South West Bay of Bengal (SWBOB) and caused anomalous SST cooling in the region. Thus, the low level southerly wind reaching over the SWBOB during AMJ enforced the colder oceanic surface temperature of the region during the month of maximum solar insolation and thereby weak thermal gradient was observed with the shifting of the convective activity towards the southeastorthern Bay of Bengal (BOB). Moreover, less moisture advection with less convective activity in SWBOB region during pre-monsoon season may be the caused of the below-normal rainfall over Tamilnadu and coastal A. P. subdivisions during June, July,August and September (JJAS). Higher outgoing longwave radiation (OLR) in this region was associated with the weak low level convergence and cloud formation. The wet years have large negative OLR Indices whereas dry years have relatively higher values. Due to ocean-atmosphere thermal coupling in the lower troposphere, thickness anomalies developed over the SWBOB region with the same sign as the underlying SST anomalies. The lowest negative thickness anomaly during July-August has been found in 1994 (dry year). Anomalous cold (warm) SST leads to negative (positive) thickness anomalies over the SWBOB, extending over the lower eastern coast of India in dry (wet) cases. Thermal winds associated with the weakening (intensifying) of thermal gradient have greater impact on seasonal winds over the SWBOB leading to decrease (increase) in the latent heat loss. On analyzing the variations in the heat budget component latent heat flux (LHF), positive LHF anomaly in the range of 20-40 w/m~2 was found during May in the driest year of 1994 over the domain of the SWBOB. Warm AMJSSTA over the EQ. ESIO during PTIOD years, like 2007,low level enhanced westerly wind reached over the central BOB due to thermal contrast caused strong convection in the region and resulting huge band of convective cloud formation leads to high rainfall over the Eastern Ghats of India.