IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002
Orissa coast near Paradeep on 29" between 0400 and 0530
UTC. The cyclone track starting from its development stage to
land fall near Paradeep has been presented in figure 1. To
compare the sea surface temperatures during cyclone period
with those prier to cyclone, SSTs from first week of October
1999 have been chosen as reference, since there was another
cyclone just two weeks prior to this cyclone. The colour coded
SST image generated from the TMI data during first week of
October 1999 has been presented in figure 2. The temperatures
during the cyclone period (last week of October 1999) have
been presented in figure 3. To examine the effect of cyclone on
sea surface temperatures, the difference in SSTs between the
last week and first week of October 1999 obtained from TMI
data and MSMR data are presented in figures 4 and 5
respectively. The sea surface temperatures along the cyclone
track during the last and first weeks of October 1999 are
presented in figure 6. To make sure the reduced temperatures
are due to cyclone not as regular seasonal cooling, we have also
presented the SST variations along the cyclone track during
first and last weeks of 1998 and 2000 in figures 7 and 8
respectively. The observed cooling (warming) along the
cyclone track during October 1998, 1999 and 2000 are
presented in figure 9. It can be seen from figure 2 that the
entire northern bay is covered with warm waters, temperatures
ranging from 29° to 30° C, where as southern bay temperatures
are from 27° to 29° C. This was the situation during the first
week of October 1999 (prier to cyclone). By last week of
October 1999 (during the cyclone), the northern bay become
cooler than south (figure 3). The more cooling is observed
along the cyclone track (figs. 4 & 5). MSMR observations are
not available at two-foot prints closer to the coast. Hence a
detailed pattern similar to the one observed in TMI is not
observed in the SST anomalies. Thus the coastally trapped
Kelvin waves could not be reproduced in the MSMR-SST
anomalies. However, the cooling to the west of the track is
clearly seen (figure 5). The lowering of temperatures from first
week to last week is clearly seen from figure 6. The
temperatures are ranging from 28.8° to 30.6° C along the track
during first week of October 1999 and are correspondingly
lowered by 27.6° to 27.05° C during the last week of October
1999 (during cyclone). The similar cooling is not observed
along the track during 1998 and 2000 (figs. 7 & 8). The
temperature is lowered by 1.2° C at locations 1 and 2 ( during
initial phase of the cyclone) and the cooling is observed to be
2.0° to 2.5° C at other locations except at location 7 (near
coast), where the cooling is observed to be 3.45° C (fig. 9).
The upwelling caused by the cyclonic storm has to be balanced
by the downwelling at the adjoining place. Since the storm is
closer to the coast, the downwelling might have probably taken
place to the west of the storm, which is evident from the
positive SST anomalies towards the west of the cyclonic track.
The storm also has generated coastally trapped Kelvin waves
with cooler waters that propagated towards the east coast of
India. The extent of these cooled waters near the east coast of
India is also prominent in 1999 compared to the other years.
The Sea Surface Temperatures cooled significantly along the
storm track in the open ocean, but the cooling is more
substantial along the coast, near Paradeep. The cooling is very
less during low pressure at Gulf of Siam. There are two
possible explanations for surface cooling due to cyclones:
extraction of thermal energy from the surface due to
evaporation, or wind induced vertical mixing of the water
column. From these images (figures 2, 3, 4 and 5), it was
deduced that the cooling was due to vertical mixing. This
400
argument is based upon viewing the ocean as a two-layer fluid;
a warm upper layer, and a cold deeper layer. (The interface is
referred to as the main thermocline.). The upper layer is
relatively thick in October, so there must be a great deal of
vertical mixing to draw deep cold water to the surface. Along
the shelf, however, which the storm encounters as it approaches
Paradeep, the upper layer is relatively thin. Therefore, with the
same amount of vertical mixing, more cold water can be drawn
to the surface, and cooling is more substantial. In Gulf of
Siam, where the cyclone is originated by forming low, the
vertical mixing is very less, since winds are less. Vertical
mixing, due to the winds associated with the periphery of the
tropical cyclone result in a change in SST.
4. CONCLUSIONS
The cyclone induced cooling along the cyclone track is clearly
seen in the satellite derived Sea Surface Temperatures during
the super cyclone 1999 in the Bay of Bengal.
The surface cooling is mainly due to the vertical mixing, which
brings cooler water from deep to surface by strong wind
induced turbulence.
The storm generated coastally trapped Kelvin waves with
cooler waters that propagated towards the east coast of India is
prominent during 1999.
Substantial cooling observed before the landfall of the cyclone
near Paradeep is due to relatively thin upper layer near coast
hence, with the same amount of vertical mixing, more cold
water can be drawn to the surface.
ACKNOWLEDGEMENTS
The authors are extremely thankful to Dr. R. R. Navalgund
Director, NRSA for his keen interest and encouragement.
Thanks are due to Dr. A. Bhattacharya, Deputy Director
(RS&GIS), Sri A. K. Chakraborthi, Group Director (WR&OG)
for their support. TMI data provided by NASA's TRMM
Science Team under the sponsored ship of NASA’s Earth
Science Information Partnerships (ESIP) is greatly
acknowledged.
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