TROPICAL CYCLONE GONU AND THE 2007 ONSET VORTEX
by Philip Lutzak – June 2007
The summer monsoon in India, also called the southwest monsoon, begins between mid May and mid June each year in the southern regions, bringing heavy rain, and then advances northward, eventually covering most of the subcontinent by mid July. As the season begins, the Somali Low Level Jet, a river of air at about 5,000 feet, develops and begins blowing very humid air from the coast of Somalia towards the Indian west coast (more detail on the Somali Low Level Jet (SLLJ)). As this humid air flow starts causing thunderstorms to form on the Indian west coast, there is a particular weather feature that often develops and helps enhance the start of the monsoon: the onset vortex. This relatively weak mid-level low pressure system usually develops over or near the west Indian coast on the cyclonic shear (in this case, poleward) side of the SLLJ. Once formed, the southwesterly winds around the bottom of the low combine with the winds of the SLLJ (Figure 1a) in bringing a flow of very warm, moist unstable air into southwestern India around Kerala state. This flow allows strong convection (thunderstorms) to form, giving a big boost to the onset of the southwest monsoon.
In 2007, the Indian Monsoon got off to an early start on May 29, as a textbook case onset vortex developed over the eastern Arabian Sea. Looking again at Figure 1a, notice the eastward wind flow along the southern Arabian Sea into southern India. This 05-30 850mb Relative Humidity analysis shows how this flow has pushed air of very high moisture content into the southwest Indian coast. The corresponding infrared satellite image in Figure 1b, below right, shows the convection (bright whites) over the southeast Arabian Sea and west Indian coast associated with the SLLJ and onset vortex. Together, they are working in tandem to push high theta-e air onto the southwest coast, kicking off the 2007 monsoon season in southern India.
Here is another view of the flow showing the 850 mb streamlines from 05-30. And here are the 05-30 600mb analysis and the 05-30 500mb analysis, showing the classic signature of a shallow Indian Ocean mid-level depression (in this case the onset vortex), where the circulation is distinct at 850mb, still discernable at 600mb, but either no longer or barely closed at the 500mb level. Although these onset vortices can develop into a tropical depression or stronger, they usually remain weak, drift to the north and west, and eventually dissipate. But during the next two days, due to unusually favorable conditions, this weak, mid level low began to intensify, and was classified a tropical depression on June 1st. By the afternoon of June 2nd it had strengthened further and was designated Tropical Cyclone Gonu by the India Meteorological Department (IMD). Figure 2a below shows how the strengthening spiral of the tropical cyclone began diverting the air flow of the SLLJ into its own circulation, consequently cutting down the SLLJ moisture flow into southwestern India - see the 06-02 850mb RH. Notice that while the winds were blowing from 6 to 10 knots or better into southwestern India on May 30th (Figure 1a), they have now dropped to less than 5 knots. Figure 2b shows how the moisture content and convective activity has dropped off over the extreme southeast Arabian Sea and southern India, and is now mostly associated with the cyclone. The monsoon flow had ground to a halt.