HR: 15:25h
AN: A33B-08 INVITED     [Abstracts]
TI: Role of ENSO-induced Indian Ocean SST on the Asian Monsoons
AU: * Annamalai, H
EM: hanna@hawaii.edu
AF: IPRC, University of Hawaii, 1680 East West Road, Honolulu, HI 96822 United States
AB: Recent studies point to dynamic air-sea interaction over the eastern equatorial Indian Ocean during July-November and over the southwest Indian Ocean during December-May, leading to considerable local SST anomalies. The effect of these SST anomalies in conjunction with ENSO-related SST anomalies on the Indian Summer Monsoon and East Asian Winter Monsoon are examined. An Atmospheric General Circulation Model (AGCM) is used to elucidate the relative importance of local and remote forcing, and model solutions were sought for experiments where SST anomalies are inserted in the (i) tropical Indo-Pacifc, (ii) tropical Pacific and (iii) tropical Indian Ocean. A 10-member ensemble simulation is carried out for each of the forcing scenarios. Diagnostics from observations reveal that after the 1976-77 climate shift in the Pacific despite stronger El Ni¤o events the Indian Summer Monsoon (ISM) during July-August was above normal. Based on 1950-1975 (PRE76), and 1977-2001 (POST76) El Ni¤o composites we note that the major differences between the two epochs, in terms of observed SST during July-August, are the presence of cold SST anomalies over the Indo-Pacific warm pool and the intensity of warm SST anomalies in the central Pacific in POST76. Model solutions demonstrate that low-level anticyclonic circulation anomalies that develop as a Rossby wave response to these convective anomalies increase the south-westerlies over the northern Indian Ocean to favor a stronger ISM. During boreal winter, model solutions show that precipitation variations over the southwest Indian Ocean are tied to local SST anomalies and insensitive to changes in model initial conditions. Changes in the Indian-Ocean Walker Circulation suppress precipitation over the tropical west Pacific - Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine and South China Seas. This anticyclone increases precipitation along the East Asian Winter monsoon front from December to May. The anomalous subsidence over the Maritime Continent in conjunction with persistent SST anomalies and precipitation over the southern Indian Ocean in spring prevent the north-northwestward migration of a deep moist layer, causing a significant delay in the ISM onset in June by 6-7 days. Our results indicate that Indian Ocean SST anomalies that develop in response to El Ni¤o and/or due to internal air-sea interaction processes need to be considered for a complete understanding of regional climate variability, particularly around the Indian Ocean rim.
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting