Sunday, 22 January 2012
Influence of Decadal Variability of Global Oceans on South Asian Monsoon and ENSO-Monsoon Relation
Hall E (New Orleans Convention Center )
The Indian monsoon rainfall (IMR) is known to be influenced by several factors among which the equatorial Pacific Ocean sea surface temperature (SST) exhibits high correlation. The negative correlation between the SST anomalies over the equatorial central/eastern Pacific and the IMR is the conventional El Niņo Southern Oscillation (ENSO)-monsoon relationship. However, not all El Niņo (La Niņa) years are associated with drought (flood) conditions over India and the failure of ENSO-monsoon relation is not well understood. This study investigates the possible role of decadal modes associated with global oceans in either enhancing or decreasing the conventional ENSO-monsoon relationship. Specifically, this study explores the role of the decadal oscillations of the North Pacific and North Atlantic Oceans. An analysis of long record of observed Indian monsoon rainfall shows decadal oscillations at 52, 21 and 13 years. One of them is associated with the North Pacific (Pacific Decadal Oscillation (PDO)) and the other two with North Atlantic (Atlantic Multidecadal Oscillation (AMO) and Atlantic Tripole). The PDO exhibits significant negative correlation with the IMR similar to the of ENSO-IMR relation. Further, when the ENSO and PDO are in (out of) phase, they complement (counteract) each other to enhance the conventional ENSO-monsoon relationship. The physical mechanism is explained in terms of the circulation associated with PDO which affects the Walker circulation in the equatorial pacific and, in turn, in the Indian monsoon region. The two Atlantic decadal modes have opposite relations with monsoon, with AMO having positive correlation and Tripole having negative correlation with IMR. When AMO and ENSO are out of (in) phase, they enhance (counteract) the conventional ENSO-monsoon relationship. The mechanisms through which AMO and Tripole could influence IMR are being explored. Similar decadal modes associated with the North Pacific and North Atlantic are also found in SST and IMR in NCAR CCSM 4 model. In order to further substantiate the combined influence of PDO, AMO, Atlantic Tripole and ENSO on the Indian monsoon and to explain the physical mechanisms involved, regionally uncoupled model experiments (with prescribed climatological SST) with CCSM 4 are conducted.