Intraseasonal Variability of the Low-Level Jet Stream of the Asian Summer Monsoon

The strong cross-equatorial low level jet stream (LLJ) with its core around 850 hPa of the Asian summer monsoon (June–September) is found to have large intraseasonal variability. During the monsoon onset over Kerala, India, and during break monsoon periods, when the convective heating of the atmosphere is over the low latitudes of the Indian Ocean, the axis of the LLJ is oriented southeastward over the eastern Arabian Sea and it flows east between Sri Lanka and the equator and there is no LLJ through peninsular India. This affects the transport of moisture produced over the Indian Ocean to peninsular India and the Bay of Bengal. In contrast, during active monsoon periods when there is an east–west band of strong convective heating in the latitudes 10°–20°N from about longitude 70° to about 120°E, the LLJ axis passes from the central Arabian Sea eastward through peninsular India and it provides moisture for the increased convection in the Bay of Bengal and for the monsoon depressions forming there. The LLJ does not show splitting into two branches over the Arabian Sea. Splitting of the jet was first suggested by Findlater and has since found wide acceptance as seen from the literature. Findlater's findings were based on analysis of monthly mean winds. Such an analysis is likely to show the LLJ of active and break monsoons as occurring simultaneously, suggesting a split. Strengths of the convective heat source (OLR) over the Bay of Bengal and the strength of the LLJ (zonal component of wind) at 850 hPa over peninsular India and also the Bay of Bengal between latitudes 10° and 20°N have the highest linear correlation coefficient at a lag of 2–3 days, with OLR leading. The LLJ crossing the equator close to the coast of East Africa will pass through India only if there is active monsoon convection in the latitude belt 10°–20°N over south Asia. The position in latitude of the LLJ axis between longitudes 70° and 100°E is decided by the south–north movement of the east–west convective cloud band of the monsoon in its 30–50-day oscillation. When there is little convection over south Asia in the latitude belt 10°–20°N, the LLJ crossing the equator curves clockwise over the Arabian Sea under conservation of potential vorticity and bypassing India passes east close to the equator. It is speculated that the cyclonic vorticity associated with this low-latitude LLJ causes convergence in the boundary layer and consequent upward motion in the atmosphere resulting in the formation of a convective cloud band there that later moves into the Bay of Bengal as part of the monsoon's 30–50-day oscillation. Since LLJ is very important in monsoon dynamics, monsoon modelers should take adequate care to see that LLJ and the associated deep convection and their intraseasonal variability are properly simulated in their models.