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

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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

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.