Monday, May 31, 2010

"Brewing low may prove the monsoon's undoing"

The Indian monsoon is the hottest topic among international weather experts, who do not seem to accept India Meteorological Department's (IMD) onset forecast at its face value.
The IMD had said, in its newly-introduced, two-week forecast on Friday, that the onset might happen around Monday, followed by an orderly northward progression of rains along the West coast.

Full-scale onset

The full-scale onset of monsoon would have to wait until June 10, to time with the arrival of the next convective (wet or rain-generating) phase of a Madden-Julian Oscillation (MJO) wave that travels periodically from west to east.
The MJO wave has a major role in precipitating a copy-book onset as distinct from a ‘false onset,' which is likely to unravel around the timeline fixed by the IMD.
The MJO has an alternating ‘dry' (suppressed rain) phase, which is currently on play over the equatorial Indian Ocean and the Arabian Sea, at large.

Onset vortex

The IMD had said that the onset would be facilitated by the formation of an ‘onset vortex,' expanded on Saturday to a full-blown low-pressure area.
The system might move away from the West coast to North-Northwest, but still would be able to draw in the flows and cause rains; it had said quoting numerical weather prediction models.
However, this may not be the case, according to the scientists. They believe that the brewing ‘low' might just prove the monsoon's undoing.
The onset, at best, would be transient and may not last longer than a couple of days.
This is because the ‘low' might strengthen rapidly and move away, robbing the monsoon system of much of its energy, denying the mainland any significant precipitation.
In fact, these scientists see the system developing as an intense cyclone (to be named ‘Phet') and moving initially West-Northwest and away in a near replication of Super Cyclone ‘Gonu' of 2007.

Gujarat for landfall?

Unlike in the Bay of Bengal, June is known for the strong cyclones in the Arabian Sea. Several factors, including warm waters and longer stay in the seas, may help strengthen the system many times over.
The European Centre for Medium-Range Weather Forecasts (ECMWF) continues to consider the south-west Gujarat coast as likely a target for landfall as the Oman coast on the other side, depending on the arrival of a westerly trough from across the border and dipping in.
One expert told Business Line on condition of anonymity that the ‘onset' phase as signalled by the IMD may end sooner than later, under the double whammy of the rogue Arabian Sea cyclone and a ‘dry' MJO phase.

Kinetic energy

Overall, kinetic energy is seen as only a fourth of what is required for the Arabian Sea to precipitate the onset, and may not improve substantially even with the formation of the vortex.
The kinetic energy could reach the threshold level only with the support of a wet MJO phase, and may do so only from June 8, according to the ECMWF.
Only this can ensure sufficient moisture flow rising to a level of five to six km, with the minimum threshold being four km, to sustain the onset phase for a week or so.
Another major facilitator is the formation of the east-west shear zone, which should be seen clearly demarcated at three km in height, but of which no specific forecast has been made.
This is the height at which the ‘monsoon front' moves to the North.

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