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.  

 
Figure 1a. 850mb (about 5,000 ft) wind vectors over the Arabian Sea and India on 2007-05-30 at 12Z. Notice the positions of the Somali Low Level Jet (SLLJ) and the onset vortex. Courtesy ESRL. Figure 1b. Infrared Satellite image of the Arabian Sea and India from 2007-05-30 06Z. Notice the high, cold cloud tops (bright whites) associated with strong convection occurring along the southwest coast. Courtesy Donnees Satellitaires. Large version.

 

  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.   

 

 
Figure 2a. 850mb vector winds over the Arabian Sea and India on 2007-06-02 at 12Z. Notice that much of the strongest wind flow of the SLLJ is now being diverted into the cyclone and away from the west Indian coast, reducing the amount of warm moist air flow into southern & western India. Courtesy ESRL. Figure 2b. Infrared Satellite image of the Arabian Sea and India from 2007-06-02 12Z, corresponding to Figure 2a. Notice the moisture being diverted into the cyclone as it has lowered over southern India. Courtesy Donnees Satellitaires. Large version.

 

 From here, the story became anything but textbook. Gonu continued to strengthen, and by June 4th became the strongest cyclone ever recorded in the Arabian sea, with sustained winds of 160 mph, gusts to 195 mph, and a central pressure of 920 mb. Even though the winds had weakened to 60 mph when it crossed the northwestern tip of Oman, that still made it the strongest tropical cyclone to ever strike the Arabian Peninsula. A considerable amount of destruction and loss of life due to heavy flooding occurred in northern Oman and southern Iran. Here's the complete account of Gonu. Figure 3 at right shows the powerful cyclone's near perfect symmetry and eye as it approached the Gulf of Oman early on June 4th.  

  In addition to setting these records, Gonu had the distinction of completely shutting off the Indian monsoon of 2007 for over a week. Here are the 06-04 850mb vector winds, 06-04 850mb RH and 06-04 IR satellite. Notice how the SLLJ flow is still diverted northward into the cyclone along with the low level moisture and convection. This condition continued until finally, on June 6th, Gonu moved into the Gulf of Oman, and the entrainment of very dry air and interaction with land caused it to weaken rapidly. As it did so, its interaction with the SLLJ finally began to ease (see Figures 4a and 4b below), and the SLLJ began to return to a position more normal for early June, allowing the monsoon flow to resume over southern and western India. But notice in the 06-09 monsoon progress chart how the monsoon had made no advancement at all in over a week due to Gonu, and had, in fact fallen way behind schedule. We can finally see a resumed advance a few days later in this 06-13 monsoon progress chart.   

 

Figure 3. MODIS AQUA visible satellite image of powerful Tropical Cyclone Gonu at full strength on 2007-06-04 09Z. Courtesy MODIS. 

 

 
Figure 4a. 850mb vector winds over the Arabian Sea and India on 2007-06-07 at 12Z. Notice that stronger winds associated with the SLLJ are once again reaching the west Indian coast and southern India. Courtesy ESRL. Figure 4b. IR Satellite image from 2007-06-07 12Z. The moisture flow has returned to southern portions of the Indian subcontinent. Notice the remnant moisture of Gonu (top center) being blown eastward by the mid latitude westerlies. Courtesy Donnees Satellitaires. Large version.