The prevailing impression among many people is that the monsoon makes a stormy onset over the Kerala coast every year. This dramatic entry by a convective system formation is known as the monsoon onset vortex (MOV).
“The term ‘onset vortex' is a misnomer, however, and it forms only occasionally and has no influence on the monsoon onset date over Kerala,” says Dr. M.R. Ramesh Kumar, Senior Scientist, Physical Oceanography Division, National Institute of Oceanography, Goa.
He has recently lead-authored a paper on the onset vortex which has been published in the Journal of Theoretical and Applied Climatology .
Earlier studies by others have shown that the North Indian Ocean warms rapidly from March onwards. By May, a large area in the north Indian Ocean attains sea surface temperatures above 29.5 degrees C and this area can be called the Indian Ocean Warm Pool.
Moisture covergence
This region of warm sea surface temperatures can create large-scale moisture convergence, deep convective clouds and a lowering of surface pressure which are conducive for the formation of a vortex over this area.
In a study period extending from 1987-2005, an analysis of MOV and mini warm pool (an area with sea surface temperatures above 30 degrees C over the South Eastern Arabian Sea [SEAS]) revealed that a vortex formed only during 5 years (1998, 1999, 2001, 2002, 2004).
On the other hand a mini warm pool was observed during most of the years (70 per cent of the cases). In two cases, the vortex formed well in advance, whereas for 3 years, namely, 1998, 1999 and 2001 vortex formation was quite close to the monsoon onset date over Kerala.
The studies did not identify the reasons why the MOV formed only in certain years and was absent in other years.
In the recent study by Dr. Ramesh Kumar, an analysis of data for the three case study years (1998, 1999 and 2001) showed that in 1998, the mini warm pool formed over an extensive area of the Arabian Sea well in advance of the formation of an MOV there, but the MOV was only a deep depression.
In 1999, the mini warm pool was not as intense as that of 1998 but the MOV which formed became a very severe cyclonic storm, indicating little or no influence of the mini warm pool in this case. Similar was the case in 2001.
There was hardly any presence of a mini warm pool during 2000, when there was a normal monsoon onset over Kerala (June 1). There was also no MOV that season.
Atmospheric water vapour data obtained from satellite can be used for identifying MOV since mid-tropospheric humidity plays an important role in tropical cyclo-genesis. It was observed that in the onset vortex years (1998, 1999 and 2001) atmospheric moisture values were very high during vortex formation and growth.
Hindering effect
Dr. Ramesh Kumar states that the vortex sucks in moisture from the atmosphere, to sustain itself and grow, which could have otherwise been precipitated as monsoon rainfall over Kerala. Thus an onset vortex can actually hinder monsoon onset.
Another important factor in the initiation of monsoon onset over Kerala was the effect of the vortex on moisture-carrying westerlies prior to monsoon onset.
In the 2000 normal monsoon onset season the winds were much better organised prior to onset. For other seasons (1998 and 2001) the presence of the vortex over the Arabian Sea (a deep depression in 1998 and a very severe cyclonic storm in 2001) led to a change in the wind pattern prior to the monsoon onset.
Thus, Dr. Ramesh Kumar says that any convective system which forms over the South east Arabian Sea prior to monsoon onset does not help in the strengthening and deepening of the monsoon westerlies, necessary conditions for setting up of a monsoon onset over Kerala.
