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Structures and mechanisms of the first-branch northward-propagating intraseasonal oscillation over the tropical Indian Ocean |
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| Authors: | Kuiping Li Weidong Yu Tim Li V S N Murty Somkiat Khokiattiwong T R Adi S Budi |
| Institution: | 1. Center for Ocean and Climate Research, First Institute of Oceanography, SOA, Qingdao, 266061, China 2. IPRC and Department of Meteorology, University of Hawaii, Honolulu, HI, USA 3. National Institute of Oceanography Regional Centre, Visakhapatnam, 530017, India 4. Phuket Marine Biology Center, Phuket, Thailand 5. Agency for Marine and Fishery Research and Development, Center for Marine and Coastal Resources Research and Development, Jakarta, Indonesia |
| Abstract: | 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. |
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