The Hindu Business Line : Experts for studying Low Level Jetstream

THE dynamics of the genesis of monsoon depression needs to be studied in relation to the behaviour of the strong cross-equatorial Low Level Jetstream (LLJ), according to experts.

The LLJ, the main conduit for transporting moisture, is an important component of the Indian summer monsoon, say Dr P.V. Joseph and Dr S. Sijikumar of the Department of Atmospheric Sciences, Cochin University of Science and Technology (CUSAT), in a paper titled `Low Level Jetstream in relation to the variability of the Indian Summer Monsoon'.

The LLJ is a strong cross-equatorial stream with a core around 850 hPa prevailing over the Indian Ocean and South Asia during the summer monsoon season June to September.

It transports of most of the moisture required to maintain the monsoon rainfall over south Asia. This jet accounts for nearly half the inter-hemispheric transport of air in the lower troposphere around the globe.

The LLJ has been seen to split into two branches over the Arabian Sea, one branch passing southeastwards towards Sri Lanka and the other eastwards through peninsular India. The section through central Arabian Sea had only one core for the LLJ, but the section through peninsular India had two cores at the same level.

During `active' monsoon spells, the LLJ carries the bulk of the moisture to India while during `break' monsoons, it avoids India and the moisture generated over the Indian Ocean is instead carried to the west Pacific Ocean to feed the convection and typhoons that become augmented there by that time.

A related study have shown that the east African highlands and a land-sea contrast in surface friction are essential prerequisites for the concentration of the cross equatorial low level flow into full-fledged LLJ. This study identified the strong sensitivity of the LLJ to changes in convective heating over the Indian Ocean.

At the time of onset of monsoon over Kerala, the area of convection and the axis of LLJ are close to the equator.

The east-west elongated convection bands over the equatorial Indian Ocean is called the Maximum Cloud Zone (MCZ). The MCZ and LLJ axis are closely linked.

In this scenario, the MCZ bands of convection, which form in the low latitude regions of the Indian Ocean, along with the slow progression of the associated LLJ, call for detailed investigation.

The sudden two-way transitions between the equatorial areas and the central Bay of Bengal also have great importance and research must be stepped up on their dynamics and numerical simulation, the paper said.

It is important to know in this context that during the monsoon season there is a narrow band of warm waters near the equator between longitudes 70oE and 100oE with surface temperature of close to 29oC (not cooled by the monsoon) and featuring a mixed layer more than 50 metres deep. It is over this warm pool of water that a fresh MCZ forms at the end of an `active' monsoon spell.

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