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

• Published in: Climate dynamics 2013-09, Vol.41 (5-6), p.1651-1669
• Main Authors: Sharmila, S., Pillai, P. A., Joseph, S., Roxy, M., Krishna, R. P. M., Chattopadhyay, R., Abhilash, S., Sahai, A. K., Goswami, B. N.
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric forcing; Atmospheric models; Climate science; Climate system; Climatology; Climatology. Bioclimatology. Climate change; Convection; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Environmental aspects; Exact sciences and technology; External geophysics; Geophysics/Geodesy; Marine; Meteorology; Monsoons; Ocean-atmosphere interaction; Oceanography; Physics of the oceans; Propagation; Sea surface temperature; Sea-air exchange processes; Storms; Summer; Topical Collection on Climate Forecast System Version 2 (CFSv2); Wind; Wind shear
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-013-1854-1

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.