Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean
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The first-branch northward-propagating intraseasonal oscillation (FNISO) over the tropical Indian Ocean (IO) often triggers the onset of the Asian summer monsoon. In this study we investigate the structures and mechanisms associated with FNISO through the diagnosis of ERA-Interim reanalysis data for the period of 1990–2009. A composite analysis is conducted to reveal the structure and evolution characteristics of the FNISO and associated background circulation changes. It is found that the FNISO convection originates from the southwestern IO and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern Bay of Bengal (BoB). Two possible mechanisms may contribute to the northward propagation of the FNISO. One is the meridional asymmetry of the background convective instability. A greater background convective instability over the northern BoB may destabilize Rossby waves and cause convection to shift northward. The other is the meridional phase leading of perturbation humidity in the planetary boundary layer (PBL). Maximum PBL moisture appears to the north of the convection center, which promotes a convectively unstable stratification ahead of the convection and leads to the northward propagation of the FNISO. A PBL moisture budget analysis reveals that anomalous zonal advection is a dominant process in contributing to the moisture asymmetry.
KeywordsIntraseasonal oscillation Northward propagation Monsoon onset
This work was supported by Chinese MoST grants 2009DFA21000, 2010CB950303, NSFC grant 41005032, SOA grant 2011246 and FIO grant 2008T02. This research is part of the Southeast Asian Global Ocean Observation System (SEAGOOS) pilot project entitled Monsoon Onset Monitoring and its Social and Ecosystem Impacts (MOMSEI) under the Sub-Commission for Western Pacific of the Intergovernmental Oceanographic Commission (IOC-WESTPAC). Tim Li was supported by NSF AGS-1106536 and by the International Pacific Research Center that is sponsored by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), NASA (NNX07AG53G) and NOAA (NA17RJ1230). This research is also part of the collaboration program between the National Institute of Oceanography/Council of Scientific and Industrial Research (NIO/CSIR) and the National Science Foundation of China (NSFC). This is SOEST contribution number 8725, IPRC contribution number 905 and NIO contribution number 5220.
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