The dominant intraseasonal mode of intraseasonal South Asian summer monsoon

Abstract

From June through September, the intraseasonal variability of the Asian summer monsoon is dominated by the so-called “monsoon intraseasonal oscillation (MISO).” This paper provides a comprehensive description of the MISO based on outgoing longwave radiation (OLR) data. The MISO is characterized by alternating active periods, in which the primary rain area of the Asian summer monsoon that stretches from the northern Arabian Sea east southeastward almost all the way to the northwest Pacific Intertropical Convergence Zone is relatively intense, and break periods, in which the heaviest rainfall shifts from south Asia to the central and eastern equatorial Indian Ocean. The MISO is attended by well-defined but weak sea surface temperature (SST) perturbations whose phase is indicative of a negative feedback upon the atmospheric perturbations. Meridional profile of variables on the various regression maps shown in this paper averaged along a set of tilted axes parallel to the west-northwest to east-southeast (WNW-ESE) sloping lines in empirical orthogonal function 1 of OLR have been made, and it is found that the strongest westerly 850 hPa wind anomalies are located two grid points (5° of latitude) to the south of the reference latitude. At the 150 hPa level, the meridional profile of divergence is closely aligned with the OLR profile. SST profile is lowest at approximately 2.5° of latitude to the south of the minimum OLR and 2.5° to the north of the strongest westerly 850 hPa wind anomalies. The sea level pressure profiles and the midlower tropospheric geopotential height profiles are almost in phase. It is observed that in most years, there are two–three bands of intensified and suppressed rainfall that cross the reference line from south to north (northward propagating) at the interval of 30–60 days over South Asia. The degree of correspondence between the MISO and active and break spells of the Indian summer monsoon rainfall is also documented.