What caused the May 2004 Arabian Sea cyclone

January 19, 2011 11:37 pm | Updated November 28, 2021 09:08 pm IST

As a first time for National Institute of Oceanography, Goa scientists, several sensors (satellite, land and air, buoys etc.) have been utilised to look at a cyclone by them. Each sensor has different resolution and capability as well as potential.

By complementing several sensors, the scientists — led by Dr. M. R. Ramesh Kumar, Senior Scientist, Physical Oceanography Division, had a holistic view of a cyclone. They looked at different parameters that affected the formation, track and intensification of the cyclone which formed in the Arabian Sea near the Kerala coast on May 5, 2004. The cyclone moved north-westward over the next few days before dissipating off the Sourashtra coast on May 9.

The findings were published recently in the International Journal of Remote Sensing .

The difference

A marked difference between the western and eastern Arabian Sea in the case of mid-tropospheric (troposphere is the lowest layer of the atmosphere extending up to 15 km height) relative humidity was observed during the formation of the cyclone.

If the mid-tropospheric relative humidity values are low, a cyclone which is quite huge or vast in dimensions cannot sustain. Under such conditions, vertically developing clouds such as cumulonimbus will not develop or grow, noted Dr. Ramesh Kumar in an email to this correspondent. Before the May 2004 cyclone formation, a very dry condition existed throughout the northwestern Arabian Sea.

However, the southeastern part of the sea (off Kerala and Karnataka) showed a mid-tropospheric relative humidity value of 70–80 per cent, where the cyclone formed. It was also seen that a similar amount of mid-level moisture existed in the Bay of Bengal.

The cause

The higher amount of moisture over the southeastern Arabian Sea may have been because of the westward flow of the moisture from Bay of Bengal or because there was a strong convection over the eastern Arabian Sea, the paper notes. An atmospheric disturbance with strong winds was created that was favourable for cyclone formation. During the cyclone period, the mid-level moisture increased from 90 per cent to 100 per cent.

On the morning of May 5, the resultant shearing action of mutually opposing winds (vertical wind shear) — one at 1.5 km above sea level and the other at 12 km above sea level — was very less and helped the updraft of moisture and hence enhanced convection.

Thus, low shear values trapped the energy necessary for the cyclone formation without much loss to the surrounding environment. This helped the tropospheric warming, enhancement of moisture ascent from the lower layer and resulted in addition of more energy to the atmosphere. All these increased the instability and led to the cyclonic vortex ormation.

The study found that another factor which played an important role was the rotational flow of wind (relative vorticity) whose gradient was high in the northwest direction and caused the formation, intensification and subsequent movement in the northwest direction where the relative vorticity values were high.

A measure of reflected radiation from cloud top — outgoing longwave radiation — had low values in this case for the study period (May 3 to 8). This corresponded to the regions of maximum convection.

Outgoing longwave radiation is used as a proxy for convection. Low outgoing long wave radiation values mean greater convection and high value of outgoing long wave radiation means less convection.

Thus, monitoring the centre of maximum convection helped in predicting the intensification and the movement of the cyclone.

However, in order to be used as a predictor for the intensification and the movement of the convective system, the outgoing long wave radiation data must have a much better spatial and temporal resolution than the ones used in this study say the authors.

Cyclones are considered natural phenomena, which leave death and destruction in their wake. However a silver lining was seen in the May 3-8 cyclone. A large-scale upwelling (transport of cold nutrient-rich water to the surface layer from the deep layers) in the wake of the cyclone due to wind and wave action was seen.

This upwelling of nutrient-rich water led to the attraction of fish in large numbers and enabled a bountiful catch for fishermen.

The authors of the study, in a concluding note state: “These results are preliminary in nature and we propose to use a larger dataset for a longer period (30 years) to draw conclusions for more than one cyclone. We also feel that the recently launched OCEANSAT II and the forthcoming Megha Tropiques satellite mission will provide more information.”

Speaking over the telephone, Dr. Kumar said: “Only multi-sensor data which includes remotely sensed data in conjunction with reanalysis data and other means can provide information about the various ocean atmospheric processes involved in formation of the cyclonic storm from a depression to storm intensity. This is the first multi-sensor study undertaken by us and we would like to use this technique for future cyclogenesis studies.”

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