Thursday, May 14, 2009

Monsoon lag in Bay, Kerala may spring a surprise

Thiruvananthapuram, May 13

India Meteorological Department (IMD) has started an unofficial `count-down' prior to declaring monsoon onset conditions even as the Bay of Bengal weather kept forecasters on their tenterhooks.

Indications are that the flagging winds may delay the Bay onset until May 20 (five days behind schedule) while a `low' could spring up at the crucial monsoon `gateway' in the south-east Arabian Sea.


But the latter might not trigger the conventional onset over adjoining Kerala coast despite an expected scale-up of rain showers in the region, according to Dr Akhilesh Gupta, Senior Adviser to the Ministry of Science and Technology.

The wind flow in the lower levels is predominantly north-northwesterly whereas the flow over the Bay remains weak and is not representative of typical monsoonal flow required for onset over the Andaman Sea region, he said.

Based on medium range predictions for next seven days, a gradual build up of monsoon flow over southeast Bay and Andaman sea regions is likely to take place beginning May 18 (Monday next).


This build up may be seen in the form of strong westerly winds in the equatorial zone (in the lower levels) over south-west Bay which appears to be converging with a southerly cross-equatorial flow (from southern hemisphere) across southeast Bay of Bengal.

This flow is expected to strengthen further and result in greater moisture incursion into the region.

This may eventualy cause large-scale rainfall activity over the region during subsequent 2-3 days.

"This analysis suggests that we can expect onset of monsoon over Andaman and Nicobar Islands around May 20," Dr Gupta told Business Line speaking in his individual capacity.


Both the Arabian Sea and the Bay get `lighted up' as an enhanced convective phase of an itinerant Madden-Julian Oscillation (MJO) wave, a major monsoon onset enabler, makes a fortuitous call.

The MJO wave is a periodical wave transiting east Africa into the southwest Indian Ocean and the north Indian Ocean (Arabian and Sea and Bay of Bengal combined) and has alternating wet and dry phases.

The wet phase has been known to set up monsoon onsets and intra-seasonal surplus rain events while the dry phase is identified with the feared `break monsoon' (when the rains dry up mid-season).


In the instant case, the upper-level MJO is not forecast to enjoy `ground-level' support around the Somali coast where southeast trades cross the equator to become monsoon-carrier southwesterly flows, currently lacking in strength.

The Somali orientation becomes important given that it is the low-level monsoon `jet' (LLJ) extending from this African destination that powers the entry of the seasonal rains into the mainland

Given this, the early rains over the peninsula might just peter out unless the monsoon manages to switch itself to `auto-pilot,' says Dr Gupta.

The auto-pilot is best achieved best when the `east-west shear zone' fully evolves and manages to migrate to the northern latitudes (up to around the 20 deg North) beyond the ken of MJO influence.


The east-west shear zone manifests itself in the middle levels of the atmosphere and is set up by opposing wind regimes (westerly as against southwesterly at the lower levels) and triggers formation of massive rain-bearing clouds.

While migrating to the northern latitudes, the shear zone can ignite the Bay and set up the usual monsoon `low' which can take the rains farther into the west and the northwest in near copybook fashion.


An MJO-assisted onset is fraught with the risk of untimely and season-spoiler storms in the Arabian Sea, as in Super Cyclone Gonu among others that took the winds out of monsoon sails during the past few years.

But this is unlikely to happen this time round if only for the fact that the MJO wave is forecast to stay anchored for a while - even up to June 12 though progressively reducing in intensity, according to coupled Linear Inverse Modelling by the Earth Systems Research Laboratory (ESRL) at US National Oceanic and Atmospheric Administration (NOAA).

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