Met. Monograph No. Cyclone Warning - 3/2009
VARIABILITY AND TREND IN THE CYCLONIC
STORMS OVER NORTH INDIAN OCEAN
BY
N.T. NIYAS, A.K. SRIVASTAVA and H.R. HATWAR
Chapter
I
Introduction
A tropical cyclone is an intense atmospheric vortex of low pressure with a
huge mass of revolving moist air. In general, tropical cyclones form over the warm
tropical oceanic regions away from the equator, where the moist air converges and
weak vertical wind shear prevails. The horizontal extent of a well developed tropical
cyclone is of the order of 100 to 1000 Km and its vertical extension is about 10 to 15
Km. The word, ‘Cyclone’ is derived from the Greek word ‘Cyclos’, meaning the coil
of a snake. Tropical cyclones are referred to as ‘Hurricanes’ over Atlantic Ocean,
‘Typhoons’ over Pacific Ocean, ‘Willy-Willy’ over Australian Seas and simply as
‘Cyclones’ over Indian Seas. The low pressure systems are classified according to
the wind speed over the oceanic areas.
India Meteorological Department classifies the various low pressure systems
forming over the North Indian Ocean, as following:
Sr.No. Types of Disturbances
Associated wind speed
1
Low Pressure Area
< 17 knots
< 32 km/hr
2
Depression
17-27 knots
32-50 km/hr
3
Deep Depression
28-33 knots
51-59 km/hr
4
Cyclonic Storm
34-47 knots
60-90 km/hr
5
Severe Cyclonic Storm
48-63 knots
90-119 km/hr
6
Very Severe Cyclonic Storm 64-119 knots 119-220 km/hr
7
Super Cyclonic Storm
> 119 knots
> 220 km/hr
Tropical cyclones occur predominantly over tropical oceans where observed
meteorological data are sparse. In addition, the destructive nature of tropical
cyclones makes their observations difficult and expensive. Reconnaissance aircraft,
satellite
observations,
radar
observations,
rawindsonde
observations,
and
conventional surface observations are used in monitoring the genesis, intensity and
movement of tropical cyclones. The best method of observing a tropical cyclone is by
direct observations from reconnaissance aircraft, particularly for monitoring location
3
and intensity. However, it is very expensive and such observations are not used
routinely except only over the North Atlantic Ocean (Henderson-Sellers et al., 1998).
Satellite data, although extremely useful and widely used, are not a complete
substitute for reconnaissance aircraft observations because of the difficulties
involved in translating radiances into required meteorological parameters. However,
the Dvorak technique (Dvorak 1984) in combination with spiral overlays and
subjective interpretations is commonly applied to estimate the intensity of tropical
cyclones from satellite imageries.
Tropical cyclones derive energy primarily from evaporation from the ocean
and the associated condensation in convective clouds concentrated near their center
(Holland 1993).
Cyclones are influenced, greatly, by the underlying ocean surface
over which they form and travel. As long as cyclone remains over warm water, the
energy is unlimited. Warm and highly humid Equatorial and Maritime Tropical air
spirals inward towards the centre of the low pressure to replace the heated and
rapidly ascending air. Ascending air releases heat into the atmosphere, cools and
are condensed into cloud. Additionally, tropical cyclones are characterized by a
“warm core” (relatively warmer than the environment at the same pressure level) in
the troposphere. The greatest temperature anomaly generally occurs in the upper
troposphere around 250 hPa. It is this unique warm-core structure within a tropical
cyclone that produces very strong winds near the surface and causes damage to
coastal regions and islands through extreme wind storm surge, wave action and
torrential rains (Henderson-Sellers et al., 1998).
Tropical cyclones form over all tropical oceanic areas except the South
Atlantic and the Southeast Pacific. Western North Pacific accounts for the largest
number of Tropical Cyclones (Typhoons), averaging 26 per year, followed by
Eastern North Pacific (17), South Indian Ocean (10) and North Atlantic (10). In
comparison, North Indian Ocean basin has an average of only 5.5 cyclones per year.
Further, their frequency in the Indian Seas shown a bi-model character, with a
maximum peaks, one from mid-April to mid-June and second one from October to
December.
4
The cyclonic disturbances are 5 to 6 times more frequent over the Bay of
Bengal than over the Arabian Sea. One third of the Bay disturbances and half of the
Arabian Sea disturbances intensify into tropical storms.
The ratio of tropical
cyclones between the Bay of Bengal and the Arabian Sea is 4:1. This is probably
due to the fact that SST over the Arabian Sea is cooler than that of over the Bay of
Bengal. Moreover, passage of westward moving remnants of the tropical cyclones
forming in the west Pacific Ocean over the Bay of Bengal are also helps in more
cyclogenesis over the region. Presence of the Inter Tropical Convergence Zone
(ITCZ) near the Equatorial region of the Bay of Bengal due to either advancement or
retreat of monsoon (South west or North east) during these periods help to intensify
low level cyclogenesis into cyclone. The Bay water maintains the critical ocean
temperature of 26-27°C needed for cyclogenesis. The sensible heat maintains the
vertical coupling between the lower and upper troposphere flow pattern in the
cyclone. The absence of sensible heat leads to the degeneration of cyclone.
Cumulus convection acts as prime mechanism for vertical coupling.
A tropical cyclone unleashes its highest destructive potential when it makes
landfall in the coastal belt. Violent winds, torrential rains and storm surge are the
three major causes of destruction. The storm surge which is not properly understood
by common people, is in fact, responsible for nearly 80% of the loss of lives. Though
fewer tropical cyclones occur in the North Indian Ocean compared to the other
oceanic basins, the shallow depth of the Bay of Bengal and the low flat coastal
terrain produce much larger storm surge and take a very heavy toll of life.
One of the expected fall out of the global warming is increase in the frequency
and intensity of the extreme events like tropical cyclone. A little enhancement in the
frequency and intensity of tropical cyclones may have more lethal and widespread
damage potential. Therefore, it is desirable that the trend in the tropical cyclone and
its rate of intensification be examined.
Subsequent chapters besides giving an account of various related works,
examine the variability and trend in tropical cyclones forming over the North Indian
Ocean for the period 1891-2008.
5
Chapter
II
Review of the studies examining variability and trend
in the tropical cyclones forming over the North Indian Ocean
Shyamala and Iyer (1996) studied cyclonic disturbances in Arabian Sea
including cyclonic storms for 118 year period from 1877-1995 with respect to the
decadal variability. They found that maximum number of cyclonic storm and severe
cyclonic storm in Arabian Sea occurred during the decade 1901-1910 (15) followed
by 1961-1970 (14), 1971-1980 (13), 1981-1990 (3) and 1991-2000 (7). The decade
1981-1990 had the lowest frequency of cyclonic storm and severe cyclonic storm (3).
These are statistically significant decadal variabilities with decreasing tendency in
decadal frequency of cyclonic storms in Arabian Sea since the last 3 decades from
1971-2000.
Singh and Rout (1999) examined the variabilities in the frequency of cyclonic
disturbances during ENSO/non-ENSO and excess/deficient monsoon years. The
study revealed that the annual frequency of depressions and cyclones was
significantly higher during excess/normal monsoon years as compared to that during
deficient monsoon years and the annual frequency of cyclones and depressions was
higher during non-ENSO years as compared to that during ENSO years.
Joseph and Xavier (1999) studied the time series of Indian Summer Monsoon
Rainfall (ISMR) and the frequencies of monsoon depressions and tropical cyclones
using harmonic analysis. The study showed that frequency of tropical cyclones had
very little long-term trend (like ISMR) but it had an oscillation of period close to 36
years. Monsoon depression frequency has a 36-year oscillation superimposed on a
prominent long term decreasing trend.
Singh et al. (2000) studied the changes in the frequency of tropical cyclones
developing over the Arabian Sea and the Bay of Bengal (Indian Seas) using 122
years (1877-1998) data. Examination of the frequencies of severe cyclones with
maximum sustained winds 48 knots and more revealed that these cyclones had
become more frequent in the North Indian Ocean (Bay of Bengal) during intense
6
cyclone period (May & November) of the year. The rates of intensification of tropical
disturbances to severe cyclone stage also showed an upward trend.
Srivastava et al. (2000) studied trends in annual cyclonic disturbances for the
period 1891-1997 over the Bay of Bengal and the Arabian Sea. It was noticed that
there was a significant decreasing trend in the annual frequency of storms over both
the basins and the slopes of decreasing trend were maximum during last four
decades.
Singh (2001) examined long term trends in the frequencies of cyclonic
disturbances (i.e. depression) and the cyclonic storms forming over the Bay of
Bengal and the Arabian Sea during southwest monsoon season. The study reveals
that frequency of cyclonic disturbances has decreased at the rate of about 6 to 7
disturbances per hundred years in the monsoon season whereas frequency of
cyclonic storms in monsoon season has decreased at the rate of about 1 to 2
cyclonic storms per hundred years.
7
Chapter
III
Climatology
Tropical cyclones are seasonal phenomena: most tropical ocean basins have
a maximum frequency of cyclone formation during the late summer to early autumn
period. This is associated with the period of maximum sea surface temperature
(SST), although other factors, such as the seasonal variation of the ITCZ/monsoon
trough location, are also important (Frank 1987; McBride 1995).
15% of the total tropical cyclones form over the North Indian Ocean and their
frequency shows a bimodal maxima peaking once from mid-April to mid-June and
again from October to mid-December. Over the Bay of Bengal and the Arabian Sea,
during the southwest monsoon season (June to September), the intense systems
usually do not develop due to shift of the convergence zone northward over the land.
On an average, each year, 4 cyclones form in the Bay and 1 in the Arabian Sea,
although there have been exception in individual years.
An analysis of the data for the last 118 year period, 1891-2008 shows that
(i)
Out of the total 618 cyclones, 485 (i.e. 78%) formed over the Bay of
Bengal, while 133 (i.e. 22%) formed over the Arabian Sea (Fig. 1)
(ii)
Taking the North Indian Ocean as a whole, on an average 5.2 cyclonic
storms and 2.4 severe cyclonic storms form per year.
(iii)
For the North Indian Ocean as a whole, the number of cyclonic and severe
cyclonic storms showed distinct decadal variability. The maximum number
(67) occurred in the decade 1921-30 and minimum (38) during 1981-90
(Fig. 2).
(iv)
The maximum number (18) of cyclonic storms and severe cyclonic storms
occurred in the month of November during the decade 1921-30. Similarly
in the pre-monsoon season, maximum number (14) of cyclonic storms and
severe cyclonic storms occurred in the month of May during 1961-70.
8
Chapter
IV
Trend in the frequency of Tropical Cyclones
over the North Indian Ocean
Long term linear trend (1891-2008) in frequency of tropical cyclones over the
North Indian Ocean as a whole, the Bay of Bengal and the Arabian Sea for different
seasons, generally, show a significant decreasing trend. However, an increasing
trend in the frequency of tropical cyclones forming over the Bay of Bengal in the
months of May and November, the principal cyclone months, was observed. These
trends are more discernible once time series of the frequency of tropical cyclones
are smoothened for small period oscillations by taking 5 and 11 year moving total of
the frequency of cyclonic storm. These results are shown in Fig. 3. It may be
observed that rate of decrease in frequency of tropical cyclone, is maximum for the
monsoon season. Similar results were obtained, when the series were subjected to
Spearman rank test for examining the trend.
Interestingly, cyclone frequency data for the last four decades (1961
onwards), since when significant monitoring tools are available, show a significant
decreasing trend for all the months and seasons; once again the maximum decrease
was noticed in the monsoon season. These results are shown in Fig. 4. Further,
data for the period 1891-1960 do not show any significant trend except for the
monsoon season which shows that the increasing trend over the Bay of Bengal for
May and November months for the data period 1891-2008 is not consistent
throughout the time series. It may be mentioned that these results are consistent
with findings of Srivastava et al. (2000) and Singh et al. (2000). We have also
examined the trend in the rate of intensification of cyclones into severe cyclones over
the Indian seas. A significant increasing trend has been noticed and this trend is
very sharp for the recent decades (Fig. 5).
Thus, the cyclone frequency over the Indian seas, has shown a decreasing
trend in the last four decades since when better monitoring tools are available,
however, their intensity appears to have increased.
9
We have also examined the trend in the frequency of cyclones dissipating
over the sea itself for the period 1891-2008.
Although, monthly, seasonal and
annual frequency of such cyclones does not show any significant linear trend over
the Bay of Bengal, a significant increasing trend was found over the Arabian Sea for
the annual frequency as well as for the month of May. Further, 11 year running total
of such cyclones over the Bay of Bengal, however, exhibits significant increasing
trend for the November month and a significant decreasing trend for the annual
frequency.
Over the Arabian Sea, 11 year running total shows a significant
increasing trend for the May month, pre-monsoon and monsoon season and for the
annual frequency (not shown).
It may be mentioned that cyclones dissipating over the sea are not frequent
and the series of any particular month contain zero frequency for many years.
Therefore, results obtained may be misleading and no definite conclusion can be
drawn.
10
Chapter
V
Summary of results and the studies examining
trend and variability over other ocean basins
Nicholls et al. (1998) studied cyclones in Australian regions and found that
there was a numerical decline since the late 1980s, but the trend is not statistically
significant.
Authors have attributed the decrease in moderate cyclones to more
frequent occurrences of El-Nino during the 1980s and 1990s.
Chan et al. (1996) found that the frequency of typhoons and the total number
of tropical storms and typhoons over the western North Pacific Ocean have been
more variable since about 1980. There was an increase from 1981 to 1994, which
was preceded by a nearly identical magnitude of decrease from about 1960 to 1980.
Studies by Neumann (1993) and Lander & Guard (1998) reveal that the
northeast sub-tropical pacific experienced a significant upward trend in tropical
cyclone frequency for a short period further followed by no appreciable trend. There
is no appreciable long-term variation in the total number of tropical storm strength
cyclones observed in the North Indian, southwest Indian and southwest Pacific
Oceans east of 1600E.
Landsea et al. (1996) found that there have been periods with a strong mean
intensity of the North Atlantic tropical cyclones (mid 1940s to the 1960s and 1995 to
1999) and a weak intensity (1970s to early 1990s). The study revealed that there
has been no significant change in the peak intensity reached by the strongest
hurricane each year.
Fernandez-Partgas and Diaz (1996) have estimated that overall Atlantic
tropical storm and hurricane activity for the year 1851 to 1890 was 12% lower than
the corresponding forty year period of 1951 to 1990, although little could be said
regarding the intense hurricanes. They based this assessment upon a constant ratio
of USA land falling tropical cyclones to all-basins activity, which is likely to be valid
for multi-decadal time-scale.
11
Conclusions
1. Taking the North Indian Ocean as a whole, on an average 5.2 cyclonic storms
and 2.4 severe cyclonic storms form per year.
2. For the North Indian Ocean as a whole, the number of cyclonic and severe
cyclonic storms showed distinct decadal variability. The maximum number
(67) occurred in the decade 1921-30 and minimum (38) during 1981-90.
3. Long term linear trend (1891-2008) in frequency of tropical cyclones over the
North Indian Ocean as a whole, the Bay of Bengal and the Arabian Sea for
different seasons, generally, show a significant decreasing trend.
4. However, an increasing trend in the frequency of tropical cyclones forming
over the Bay of Bengal in the months of May and November, the principal
cyclone months, was observed,
5. It may be observed that rate of decrease in frequency of tropical cyclone, is
maximum for the monsoon season.
6. Cyclone frequency data for the last four decades (1961 onwards), since when
significant monitoring tools are available, show a significant decreasing trend
for all the months and seasons; once again the maximum decrease was
noticed in the monsoon season, however, their intensity appears to have
increased.
7. Results of the studies examining trend in the frequency / intensity of cyclones
over the other oceanic basins also highlight decadal to multi-decadal
variability.
8. IPCC TAR concludes that there are no discernible global trends in number,
intensity or location of tropical cyclone due to impact of climate change.
9. Analysis of sea surface temperature data of the North Indian Ocean region for
the past fifty years on the other hand clearly shows a warming trend. The
analysis of the past data therefore do not support the view of increase in the
frequency or intensity of tropical cyclone, though it seems the sea surface
temperature is rising.
12
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ACKNOWLEDGEMENT
Authors are thankful to the Dr. Ajit Tyagi, Director General of Meteorology, for
encouragement and providing support & facility for this research study. We are also
thankful to Mrs. R. V. Yadav, Mrs. P. V. Mahajani, Mr. C.N Shaligram, Mr. S.W. Sonparote
and Mr. B.P. Patkar for data collection, processing and other technical support. We are also
thankful to Mrs. Chandrachood for typing the report. We are also thankful to Shri. Philipose
Abraham and other staff members of the DTP unit of ADGM (R) Office, Pune, for designing,
type setting, printing and publication of the Met. Monograph.
35