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. See More
