It is a well known climatological fact that during pre-monsoon and post-monsoon seasons in the North Indian Ocean, more cyclones form in the Bay of Bengal compared with the Arabian Sea. Scientists have now discovered why in some years more cyclones form in the Arabian Sea than usual. This is due to a newly discovered Phenomenon (2007) El Nino Modoki — which causes warm moist conditions in the Central Pacific and dry cold conditions in Eastern and western pacific. A more familiar phenomenon, El Nino, was found to suppress cyclone formation in the Arabian Sea.

The findings are results of a study undertaken by a team led by Dr. M.R. Ramesh Kumar, Senior Scientist, National Institute of Oceanography, Goa. The study has been published in the Natural Hazards journal.

The reason why El Nino Modoki brings only fewer number of cyclones in the Bay of Bengal is because one of the two descending limbs of the Walker Cell is over the western Pacific and Bay of Bengal. The descending limb causes dry conditions not conducive for cyclone formation. The ascending limb of the Walker Cell, on the other hand, brings rain. Also, an El Nino Modoki creates stronger divergence over the western Pacific and Bay of Bengal compared to El Nino. Divergence (opposite of convergence) means surface winds move away from each other and result in low relative vorticity (rotational flow of winds). These conditions are not conducive for cyclones. This explains why Bay of Bengal region (close to western Pacific) has fewer cyclones during an El Nino Modoki.

On the other hand, there is large convergence over the Arabian Sea during an El Nino Modoki explaining the large number of cyclones in that region. A statistical analysis of the El Nino and El Nino Modoki years between 1979-2004 was conducted. It was found that there were four El Nino years and seven El Nino Modoki years during this period.

The number of cyclones per year show significant differences indicating that El Nino Modoki years are conducive for cyclone formation over Arabian Sea while El Nino is conducive for cyclones over the Bay of Bengal.

Only post-monsoon and pre-monsoon periods were chosen for the study. “Cyclones usually do not form during monsoon season,” Dr. Ramesh Kumar says in an email to this Correspondent. There are a few reasons for this.

“Atmospheric parameters — low-level relative vorticity, mid-tropospheric relative humidity, vertical wind shear — are not at values conducive for cyclone formation during monsoon,” Dr. Ramesh Kumar says. Second, during monsoon there is strong zonal (latitudinal) wind in the form of a jet at lower levels and this is not conducive for cyclone formation as the vertical shear between lower and upper troposphere will not be minimum.

Finally, the sea surface temperatures are too low for cyclogenesis.