Influence of PDO on South Asian summer monsoon and monsoon–ENSO relation
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This study has investigated the possible relation between the Indian summer monsoon and the Pacific Decadal Oscillation (PDO) observed in the sea surface temperature (SST) of the North Pacific Ocean. Using long records of observations and coupled model (NCAR CCSM4) simulation, this study has found that the warm (cold) phase of the PDO is associated with deficit (excess) rainfall over India. The PDO extends its influence to the tropical Pacific and modifies the relation between the monsoon rainfall and El Niño-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Niño (La Niña) on the monsoon rainfall is enhanced (reduced). A hypothesis put forward for the mechanism by which PDO affects the monsoon starts with the seasonal footprinting of SST from the North Pacific to the subtropical Pacific. This condition affects the trade winds, and either strengthens or weakens the Walker circulation over the Pacific and Indian Oceans depending on the phase of the PDO. The associated Hadley circulation in the monsoon region determines the impact of PDO on the monsoon rainfall. We suggest that knowing the phase of PDO may lead to better long-term prediction of the seasonal monsoon rainfall and the impact of ENSO on monsoon.
KeywordsIndian monsoon PDO ENSO CCSM4 Monsoon circulation
This work was supported by National Science Foundation (grants ATM-0830062 and ATM-0830068), National Oceanic and Atmospheric Administration (grant NA09OAR4310058), and National Aeronautics and Space Administration (grant NNX09AN50G). We thank National Center for Atmospheric Research for the model data and Ben Kirtman for helpful discussions. This work formed a part of the Ph.D. thesis of Lakshmi Krishnamurthy at George Mason University.
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