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Role of ocean-atmosphere interaction on northward propagation of Indian summer monsoon intra-seasonal oscillations (MISO)

Sharmila, S and Pillai, PA and Joseph, S and Roxy, M and Krishna, RPM and Chattopadhyay, R and Abhilash, S and Sahai, AK and Goswami, BN (2013) Role of ocean-atmosphere interaction on northward propagation of Indian summer monsoon intra-seasonal oscillations (MISO). Climate Dynamics, 41 (5-6). pp. 1651-1669.

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Atmospheric dynamical mechanisms have been prevalently used to explain the characteristics of the summer monsoon intraseasonal oscillation (MISO), which dictates the wet and dry spells of the monsoon rainfall. Recent studies show that ocean-atmosphere coupling has a vital role in simulating the observed amplitude and relationship between precipitation and sea surface temperature (SST) at the intraseasonal scale. However it is not clear whether this role is simply 'passive' response to the atmospheric forcing alone, or 'active' in modulating the northward propagation of MISO, and also whether the extent to which it modulates is considerably noteworthy. Using coupled NCEP-Climate Forecast System (CFSv2) model and its atmospheric component the Global Forecast System (GFS), we investigate the relative role of the atmospheric dynamics and the ocean-atmosphere coupling in the initiation, maintenance, and northward propagation of MISO. Three numerical simulations are performed including (1) CFSv2 coupled with high frequency interactive SST, the GFS forced with both (2) observed monthly SST (interpolated to daily) and (3) daily SST obtained from the CFSv2 simulations. Both CFSv2 and GFS simulate MISO of slightly higher period (~60 days) than observations (~45 days) and have reasonable seasonal rainfall over India. While MISO simulated by CFSv2 has realistic northward propagation, both the GFS model experiments show standing mode of MISO over India with no northward propagation of convection from the equator. The improvement in northward propagation in CFSv2, therefore, may not be due to improvement of the model physics in the atmospheric component alone. Our analysis indicates that even with the presence of conducive vertical wind shear, the absence of meridional humidity gradient and moistening of the atmosphere column north of convection hinders the northward movement of convection in GFS. This moistening mechanism works only in the presence of an 'active' ocean. In CFSv2, the lead-lag relationship between the atmospheric fluxes, SST and convection are maintained, while such lead-lag is unrealistic in the uncoupled simulations. This leads to the conclusion that high frequent and interactive ocean-atmosphere coupling is a necessary and crucial condition for reproducing the realistic northward propagation of MISO in this particular model.

Item Type: Article
Additional Information: Copyrights of this article belongs to Taylor and Francis.
Uncontrolled Keywords: air-sea interaction; atmosphere-ocean coupling; atmospheric dynamics; atmospheric forcing; convective system; monsoon; observational method; precipitation intensity; sea surface temperature; seasonal variation; summer; weather forecasting; wind shear, India
Subjects: Meteorology and Climatology
Depositing User: IITM Library
Date Deposited: 20 Jun 2014 06:24
Last Modified: 20 Jun 2014 06:24

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