THE DELAY OF THE MONSOON

The 2007 southwest monsoon season got off to a fairly normal start around May 29, even a few days early on the southwest
coast.  By this time, the Somali Low level Jet really began to establish itself, as seen in the 850 mb vector winds figure below.  
The increasingly strong flow over the Arabian Sea brought moist, unstable air into southwest India.  The infrared satellite image
below from May 29 shows an eruption of showers and thunderstorms over southern India as the onset vortex began to form,            
consistent with the beginning of the summer monsoon.

This map of 850 mb vector winds shows the Somali Low level Jet (SLLJ) beginning its summer                                        This satellite image shows showers and thunderstorms over the
onslaught into India at 00Z on May 29, courtesy of ESRL.                                                                                                       Arabian Sea moving inland at 00Z on May 29, courtesy of SATMOS.

As I mentioned earlier, the onset vortex began to form at the end of May, as the cyclonic flow shows on the 600 mb wind analysis to the left. This vortex normally increases the moisture laden flow of air into India which enhances convection. Once in a while, an onset vortex turns into a tropical depression or stronger, but usually it remains weak and moves to the northwest away from India and eventually dissipates; however, due to very favorable conditions, this vortex intensified dramatically.  By June 2, Tropical Cyclone Gonu formed from this onset vortex. Satellite imagery from this date shows Gonu strengthening over the Arabian Sea.  As a result, thunderstorms over India were only scattered at best.

This 600 mb vector wind analysis shows the wind becoming cyclonic off the west coast of India at 00Z on May 29,
signaling the onset vortex, courtesy of ESRL.

How could a cyclone over the Arabian Sea affect thunderstorms over India?  The answer lies with the Somali Low level Jet.  
The images below are from June 4 when Gonu was at its strongest, with winds of 160 mph.  As you can see on the 850 mb
vector wind image below, Gonu diverted the moist flow of the jet into its own circulation, cutting off the the flow into India.  
As a consequence, the corresponding satellite image shows very little rain over the Indian mainland.  This resulted in a halt to
the northward progression of the Indian monsoon.

This figure shows the Somali Low level Jet (SLLJ) at 850 mb being directed into the circulation                                 This satellite image shows Cyclone Gonu over the Arabian Sea and the
of Cyclone Gonu and away from India at 00Z on June 4, courtesy of ESRL.                                                                    lack of convection over India at 00Z on June 4, courtesy of SATMOS.

Cyclone Gonu made landfall on the Arabian peninsula on June 6.  Once the storm weakened over land, its effects on the
Somali Low level Jet stopped.  The 850 mb vector wind figure below shows that by June 19 the normal position of the jet
resumed, directing the super moist air back into India.  As a result, the satellite image below from that day shows the
resumption of widespread showers and thunderstorms.  Now, the Indian monsoon continued its northward progression because
there was no longer anything in the way.

This figure shows the Somali Low level Jet (SLLJ) at 850 mb returning to its normal position,                                    This satellite image shows numerous showers and thunderstorms back
at 00Z on June 19, courtesy of  ESRL.                                                                                                                                     in the Arabian Sea and India at 00Z on June 19, courtesy of SATMOS.

As it turns out, the Indian summer monsoon is a very powerful force; therefore, it took something equally as powerful to
temporarily bring it to a halt: a strong cyclone.  Gonu was the second most powerful cyclone ever to form in the Arabian Sea.
Once it diminished over land, it was back to business as usual for the summer monsoon and the effects of the Somali Low
level Jet.


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