27 June 2007

Arabian Sea cyclones

June has seen the formation of two tropical cyclones in the Arabian Sea. The first, the formation of Cyclone Gonu in early-June was a highly unusual storm being the first Category 5 TC to be observed over the northern Arabian Sea. As seen in the satellite image at left, the cyclone had a quite distinct eye and was quite powerful.

The second storm, named Cyclone 03B, initially formed in the Bay of Bengal, dissipated and reformed over the Arabian Sea. Its strength was more typical of Arabian Sea TC, reaching only tropical storm strength.

At this time of the season, two events this close together, along with the intensity of Gonu is quite rare
. Arabian Sea SSTs have recently been 1-2 C warmer than normal, particularly in early-June, which likely contributed to the strength of Gonu. These anomalies have been decreasing in strength since that time.

Tropical cyclone climatology suggests a small peak in TCs in May/June and a stronger peak in October/November for the entire north Indian Ocean basin...(see Fig 1.23). More of the storms reaching cyclone strength (65 kts; Fig 1.10) occur in the Bay of Bengal. Over the Arabian Sea, typically there are 5-20 cyclones per 100 years from any given point within the sea. The main focus of that activity lies in the eastern portion of the Sea. This figure, showing TC tracks over the N. Indian basin for the previous 25 years or so, summarizes these aspects of the climatology nicely.

Is this event an impact of climate change? The answer must be a qualified maybe. There is no doubt that Gonu, in particular, was an unusual event. We cannot be absolutely sure as to how unprecedented this event truly is. The data just aren't there. That said, it is just one more event in the (apparent) global trend of more frequent extreme events consistent with the hypothesis of anthropogenic climate change.

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