141

Global Warming, Climate Change and Cyclone Related Destructive
Winds – Discussion of Results from Some Selected Studies with
Emphasis on the North Indian Ocean
1

G.S. MANDAL and 2Prem KRISHNA

1

Former Additional Director General, India Meteorological Department (Retd)
248, Aravali Apartment, Alaknanda, New Delhi 110019, India
2
Former Professor and Head, Civil Engineering Department, University of Roorkee (Now IIT), Roorkee
e-mail: gsmandal2009@gmail.com

Abstract
Global warming due to concentration of greenhouse gases is a reality. Consequent changes are likely inthe ocean and earth-atmosphere systems. This raises the apprehension of future increases in the frequency ofhigh-velocity winds generated by weather systems, like tropical cyclones, and their destructive potentials.
Our present knowledge on the subject is inadequate and incomplete. The projections available throughdifferent climate models are at variance. Nevertheless, if past behaviour is an indication of the future, then it
should be possible to project likely changes in high-velocity winds generated by tropical cyclones throughthe analysis of past data.
With this in view, we analyzed historical data on tropical cyclones, which are one of the sources ofhigh velocity winds in the tropics, for the period of 1891 to 2008 along with recent sea-surface temperature
data of the North Indian Ocean region (the Bay of Bengal and Arabian Sea). It has been observed thatthough the sea-surface temperature in the North Indian Ocean region (NIO) is increasing (as projected by
global circulation models), the frequency of tropical cyclones, including intense ones, is decreasing.
Analysis of estimated/observed highest maximum winds associated with TCs of very severe intensity(winds with 64 knots or more) observed every year during 1965 to 2008 in the NIO region also do not show
any increasing trend. This result, though tentative (short data series), led us to conclude that there may not
be any significant increase in high velocity winds and consequent coastal impact due to global warming in
the NIO region. However, this does not exclude the occurrence of some individual cyclones of catastrophic
nature that has been observed also in the past.
Key words: destructive winds, global warming, North Indian Ocean, tropical cyclone

1. Introduction
According to the United Nations Intergovernmental
Panel on Climate Change (IPCC), Fourth Assessment
Report (2007), “The warming of climate systems is unequivocal. Most of the observed increase in globally
averaged temperatures since the mid-20th century isvery likely due to the observed increase in anthropogenic greenhouse gas concentrations.” The report notes
many observed changes in the Earth’s climate including
atmospheric composition, global average temperatures,
ocean conditions and other climate changes. There areapprehensions that these changes in atmospheric andoceanic conditions due to global warming could lead tovariations in high-velocity winds and their destructive
potential.
It is well known that high-velocity winds are destrucGlobal Environmental Research
13/2009: 141-150
printed in Japan

tive. They cause loss of lives and damage or destruction
to infrastructure along with trees, plantations, crops, etc.,
and increased coastal inundation and erosion. Theirvariation in frequency and intensity is therefore of natural human concern. The sources of high-velocity surface
winds in the tropics are meteorological phenomena like(i) tropical cyclones and their generic systems (depressions, etc.) and (ii) thunderstorms and their off-shoots
like tornadoes, dust storms, etc. It is therefore relevant
to study the impact of global warming on the genericsystems causing high-velocity winds, like tropical
cyclones, thunderstorms, etc., to understand the impact
of global warming on high-velocity winds. The intensities of these systems are measured by their associatedwind speeds which are inversely related to pressure, i.e.,
the lower the pressure in the centre of the system, thehigher the wind speed.
©2009 AIRIES142

G.S. MANDAL and P. KRISHNA

In this paper the impact of global warming andclimate change on tropical cyclones in different ocean
basins has been analyzed with an emphasis on the NIO
region to draw conclusions about changes in surface
wind speeds associated with these weather systems. The
trends in sea-surface temperature in the Bay of Bengal
and Arabian Sea have also been analyzed and the results
are presented here. Required data for the NIO region
have been obtained from Mandal (1991) and different
publications of the India Meteorological Department(IMD). Data for other oceanic areas considered here arefrom published literature or from the Internet.

2. Methodology
There are three possible ways to address the impact
of global warming on tropical cyclones, viz., (i) byexamining the reasons the increase in sea-surface temperature over tropical ocean, as suggested by global
circulation models, might lead to increases in (a) frequency and (b) intensity of tropical cyclones, (ii) byanalyzing the historical records of tropical cyclone
variations with sea-surface temperature variations and(iii) by simulation of tropical cyclones through global
circulation models.
Though there have been some encouraging results(Bengtsson et al., 1996), at the present state of climate
modeling, reasonably realistic simulation of cyclonic
disturbances in different years at different locations is
not possible. Lighthill et al. (1994) elaborated on this.
As a result, at present the problem can only be
addressed with the help of the methodology suggested
under ways (i) and (ii).
Reasonably good historical records of tropical
cyclones in the North Indian Ocean region are available
since 1891 and it would have been quite straightforward
to determine the trends in tropical cyclone frequency
and intensity by analyzing these data. However, as is thecase with other oceanic areas there are some uncertainties in the historical data related to frequency and intensity of cyclones of the pre-satellite era. This is more soin the NIO region from where aircraft reconnaissancedata from cyclone field is not available in operationalmode till today. This is a distinct constraint in drawing
conclusions about trends in tropical cyclones based on
historical data before the 1960s.
Reasonably good sea-surface temperature records arealso available for the recent period. Presuming that thepast trends in tropical cyclones may be a good reflectionof the future, past data are used to project the expectedchanges in the frequency and intensity of tropical
cyclones. Good data on the history of tropical cyclones
for the recent period are also available from othercyclone basins. The emphasis of the present study is to
examine changes in tropical cyclone frequency andintensity vis-à-vis changes in sea-surface temperatureduring the recent past and to draw conclusions on surface wind variations associated with such systems. Brief
mention of important results from a few selected studies

has also been made.

3. Impact of Global Warming Relevant toTropical Cyclones (TCs)
In spite of limitations and gaps in our understanding,the global circulation models are the best available
means for projecting future scenarios of global warming.
The impact of global warming relevant to tropical
cyclones, based on some global circulation model
results for doubled CO2 conditions during the next six toseven decades, can be broadly summarized as follows:(i) substantial warming in the earth’s climate, (ii) ananticipated sea-surface temperature increase of about
1°C or at most 2°C; with estimations ranging from 0°C
to 2°C (Carson, 1992), (iii) expectations of the leastsea-surface temperature increase occurring over thetropical oceans, (iv) a far-from-global global warming;
which will have an exceedingly regional distribution
instead, (v) a likely increase in the ocean area withsea-surface temperatures of 26°C or more, (vi) a likely
decrease in the pole-to-equator temperature gradient and(vii) a likely increase in the vertical wind shear.

4. Past Studies
4.1. Categories of studies
There are large number of studies available on thesubject of which a few selected studies have only been
considered. These can broadly be divided into threemain categories, viz., (i) climatological/empirical, (ii)theoretical and (iii) numerical. In addition to these, there
are status reports.
4.2 Some earlier studies
Hobbgood and Corverny (1988) assumed that during
the ice ages, oceans were much cooler. They estimated a
reduction of sea-surface temperature by 3°C in tropicaloceans and concluded that tropical cyclones were
weaker than at present during the ice ages and cyclones
were less frequent or might have been totally absent.
Using global circulation models of the Goddard
Institute of Space Studies (GISS), Emanuel (1988, 1989)
concluded there would be more intense cyclones under
doubled-CO2 conditions. According to Miller (1958),
Meril (1987) and Emanuel (1987), theoretical constraints limit the upper bound of tropical cyclone intensity, and minimum pressure cannot exceed a certain
limit (880 hPa for the North Indian Ocean region).
According to emanual’s estimate, the minimum pressure
drop could be 50% more than at present. As such, thedestructive potential of tropical cyclones would increase.
Evan (1993) observed that the maximum intensity oftropical cyclones (wind-speed scale) and sea-surface
temperature of that location and time where the maxima
were observed bear no significant relationship. This suggests that the maximum intensity does not depend
significantly on local sea-surface temperature.
Schneider (1990) concluded that there will be an

Global Warming, Climate Change and Cyclone Related Destructive Winds

increase in tropical cyclone frequency due to global
warming. Idso et al (1990) are of the opinion that there
may be no apparent change in tropical cyclone frequency due to global warming. Broccoli and Manabe(1990) and Haarsma et al (1992) using low-resolution
climate models concluded that there will be an increasein the number of tropical vortices.
4.3 Some recent studies
A landmark work in simulation studies during recent
times, however, is by Bengtsson et al (1996) based onhigh-resolution (T106, L19) global circulation models.
Their simulated studies generated a realistic distributionof tropical cyclone-like vortices over the globe. Their
final conclusion was a decrease in the frequency oftropical cyclones with an even greater decrease in thesouthern hemisphere under doubled-CO2 conditions.
The latest study by Bengtsson et al. (2007) summarized
the modeling efforts of various scientists. This indicatedthat the majority of such exercises to date show adecrease in frequency of tropical cyclones globally,
although some studies show a regional increase.
Emanuel, in collaboration with other scientists, published several papers in recent times in various reputed
meteorological and climatological journals strongly supporting the view that global warming would havesignificant impact on tropical cyclone frequency andintensity by increasing the number of more intense
tropical cyclones. In his latest work, Emanuel et al.
(2008), however, he concluded, “A new technique forderiving hurricane climatologies from global data,applied to climate models, indicates that global warming
should reduce the global frequency of hurricanes,
though their intensity may increase in some locations.”
The study comes as a tremendous concession from aneminent scientist like Emanuel, who has long been astrong supporter of global warming causing more hurricanes. Emanuel ran several computer models designed
to recreate past hurricane activity. According to thestudy, “The model consensus has slightly decreased frequency of events in the Caribbean and western North
Atlantic.” Regarding the intensity of future storms,
Emanuel reports the models are split. Some indicate thathurricane and tropical storm intensity will remain stableor decline, whereas others predict the cyclones’ overall
intensity will increase somewhat.
Knutson et al. (2008) through their modeling studyof future hurricanes reported, “Global warming is likely
to reduce the number of hurricanes that occur every
year.”
4.4 International workshop on tropical cyclones
The subject was initially discussed at an International Workshop on Tropical Cyclones (IWTC – II) heldin Manila in 1989, where it was concluded that morediscussion would be needed on the subject to come to aconclusion. Accordingly, the subject was extensively
debated by the participants (which included cyclone
modelers, researchers, operational forecasters and policy

143

planners) of the WMO/ICSU Third International Workshop on Tropical Cyclones (IWTC-III) held in Hua
Tulco (Mexico) in 1993, and a status report on the subject was prepared (Lighthill et. al 1994). Probably, thiswas the first “status paper” of its kind on the subject
based on combined wisdom of different scientificcommunities. Summarized results of this paper are as
follows:
Out of the three methodologies listed under“Methodology” the general consensus was that at thepresent state of climate modeling, reasonably realistic
simulation of cyclonic disturbances in different years is
not yet possible. As regards the first methodology, it wasargued that one of the six important conditions favourable for the formation and development of tropical cyclones was elevated sea-surface temperatures. Other
conditions were (i) large low-level cyclonic relative
velocity; (ii) some critical value of the Coriolis parameter, (iii) weak vertical shear of zonal winds, (iv) somedegree of convective instability and (v) large values ofrelative humidity. Here, condition (iii) on low vertical
shear is particularly important.
Accordingly, it is not certain that a future wideningof the oceanic area where sea-surface temperatures exceed 26°C, will substantially widen the area liable totropical cyclone formation. Again, whereas condition(ii) above imposes a lower limit on those latitudes (N orS) where tropical cyclone formation can occur, there isan upper limit on such latitudes imposed by condition(v). This upper limit between 25° latitude and 30° latitude is set by the lower latitude boundary of that region
where subsidence in the Hadley cell generates the trade
inversion (where large-scale vertical ascents areopposed). The features of Hadley circulation are, however, resistant to climate change. Therefore, if tropical
cyclone frequency depends essentially on the frequencyof occurrence of situations in which all six conditions
for tropical cyclone formation are satisfied, then there
can be little direct effect from global warming on it
since such warming will lift sea-surface temperatures
above 26°C mainly in regions where some of the otherknown conditions may not be satisfied.
4.5 The IPCC fourth assessment report
The IPCC Fourth Assessment Report (AR4) 2007
indicates, “There is no clear trend in the annual numbersof tropical cyclones. Tropical cyclone intensity is likely
to increase. Evidence is insufficient to determine
whether trends exist in small-scale weather phenomena
such as tornadoes, hail, lightening and dust storms”.
4.6 Latest WMO update
The WMO Expert Team (Update, Feb 23, 2010) on
climate change impact on tropical cyclone concluded“that, if twenty-first century warming occurs as projected, there will likely be an increase, on averageworldwide, in the maximum wind speed of tropical
cyclones and in rainfall rate within 100 km of storm
centre. The team concluded that the total number of

144

G.S. MANDAL and P. KRISHNA

tropical cyclones worldwide will likely either decreaseor remain unchanged. However, a likely increase intropical cyclone intensity means that the frequency ofthe strongest tropical cyclones will more likely than notincrease under the projected warming scenarios”. The
Expert Team was co-chaired by Thomas R. Knutson of
the NOAA in the U.S. and by John McBride of the
Bureau of Meteorology, Australia.
4.7 Historical evidence
Neumann et al (1993) concluded that the long seriesof tropical cyclones in the North Atlantic Ocean for theperiod of 1871 to 1992 shows no increase in the numberof storms, except large variations from year to year.
Chris Landsea (2005, 2007), analyzing historical tropical cyclone data of the Atlantic concluded that there was
no evidence of global warming having any effect on
hurricanes. Gray (2008, International Conference on
Climate Change) indicated that real-world evidence
never supported the assertion that global warming wascausing an increase in hurricane activity. He noted, “Science tells us that we should trust the real-world evidence.”

5. Analysis of Tropical Cyclones and
Sea-Surface Temperature Variation
5.1 Analysis of historical records of tropical
cyclones in the North Indian Ocean
Tropical cyclones in the NIO region has four subclassifications viz., cyclonic storm (wind speed between
34 knots and 47 knots), severe cyclonic storm (windspeed between 48 knots and 63 knots), very severe
cyclonic storm (wind speed between 64 knots to
120 knots) and super cyclonic storm (wind speed of
121 knots or more). Besides, low intensity low pressure
systems with wind speed between 17 knots and 33 knots
are known as Tropical Depressions. Cyclonic Disturbance is the term used to represent depressions and allclasses of tropical cyclones. The frequency of tropical
cyclone in the NIO region is the list in the globe (about
5.4 annually) and bi-model in character with a primary
peak in the post-monsoon month and a secondary peakin pre-monsoon months.
5.1.1 Variability of Tropical Cyclones
Like in other oceanic areas, variability of tropical
cyclones has been observed in the North Indian Ocean
in all time scales–monthly, seasonal, inter-annual anddecadal. There are some recent studies on the trends andvariability of cyclonic disturbances in North Indian
Ocean region. Mandal (1991) indicated large year-toyear variability in tropical cyclone frequency andperiods of increasing and decreasing frequencies on amulti-decadal scale. Mandal (1995) focused on thedecreasing trend in cyclonic disturbances in the North
Indian Ocean. After analyzing short period global
historical data of tropical cyclones, Mandal (2002) concluded that the global tropical cyclone frequency
showed no obvious trend. His conclusion about individ-

ual basins for North Atlantic hurricanes and Northwest
Pacific typhoons was also similar but, analysis brought
out a decreasing trend in cyclonic disturbances in the
NIO region and an increasing trend in the frequency ofNortheast Pacific hurricanes. This increase in hurricane
frequency in the Northeast Pacific, he mentioned, may
be due to improvement in systems of monitoring tropical cyclones after the sixties.
Srivastava et al (2000) indicated a significant decreasing trend in the frequency of storms in the North
Indian Ocean region. Rao et al. (2001) indicated epochsof increasing and decreasing trends in the frequency oftropical cyclones and a consistent decreasing trend after
1950. Singh (2001) while studying the long-term trends
in the frequency of cyclonic disturbances and tropical
cyclones forming over the North Indian Ocean region
during the monsoon season indicated significant decreasing trends in both frequencies.
5.1.2 Yearly distribution of cyclonic disturbances(CDs) and tropical cyclones (TCs)
The yearly distribution of cyclonic disturbances (lowpressure areas with wind speeds of 17 knots/31 kmph ormore) and tropical cyclones (low pressure areas withwind speeds of 34 knots/62 kmph or more) along with
10-year moving averages for the period of 1891 to 2008
in the North Indian Ocean are shown in Figs.1 and 2,
respectively.

Fig. 1 Cyclonic Disturbances; NIO Region (1891-2008).

Fig. 2 Tropical Cyclone; NIO Region (1891-2008).

Global Warming, Climate Change and Cyclone Related Destructive Winds

The data show large year-to-year variability of cyclonic disturbances and tropical cyclones with a slightdecreasing trend in the frequency of cyclonic disturbances and tropical cyclones. It is also apparent that
there are periods of increasing and decreasing tropical
cyclone frequencies on a multi-decadal scale.
5.1.3 Seasonal Frequency
In view of very low frequency of tropical cyclones in
the NIO region, it is difficult to see a clear picture oftrend in seasonal scale. The seasonal distributions forthe four seasons, viz., winter (January-February), premonsoon (March-May), monsoon (June-September) andpost monsoon (September-December) of cyclonic disturbances and tropical cyclones of the NIO region havebeen analysed with a view to examine the presence oftrend if there is any. It may be mentioned that, in thisregion, the frequencies of cyclonic disturbances are
maximum in the monsoon season and tropical cyclones
in the post monsoon season. The frequencies of CDs andTCs, in terms of 10 years moving averages of the NIO
region for the monsoon and post-monsoon seasons areshown in Figs. 3(a) and 3(b) respectively.
5.1.4 Severe cyclonic storms (Trend)
The yearly distribution and 10-year moving averagesof severe cyclonic storms (wind speeds of 48 knots/89

145kmph or more) are shown in Figs. 4(a) and 4(b).
The data show slight increase in the frequency ofsevere cyclonic storms in the NIO region, especiallyafter the sixties.
In view of contradictory trend (decresing frequency)
seen in the case of tropical cyclones (Fig. 1b) and very
severe cyclonic storm (Fig. 4c), it is difficult to explain
the result of slight increasing trend seen in the case of
severe cyclonic storms. Opinion may differ, but only
explanation that could be advanced for this is theuncertianity in deciding intensity of tropical cyclones oflower category (T3 or below in Dvorak’s scale) andforecaster’s bias to classify boarder line cases to ahigher category in absence of any ground truth towards
the beginning of satellite era. In view of clear decresing
trend in the frequency of very severe cyclonic storms(T4 and above where subjective error is less) this
explanation seems to be reasonable. Therefore, slightincreasing trend of frequency, observed in the case of
severe cyclonic storm could be due to more of
uncertainity in deciding intensity of boarder line casesof TCs in absence of actual observation from thecyclone field, rather than the actual variability andimpact of global warming.

(a)

(b)

Fig. 3 (a) Frequencies of CDs and TCs (10 Yrs. moving avgs); monsoon season.
(b) Frequencies of CDs and TCs (10 Yrs. moving avgs); post monsoon season.

146

G.S. MANDAL and P. KRISHNA(a)Frequency

Frequency

8
7
6
5
4
3
2
1
0
1880

1900

1920

1940

1960
Years

1980

2000

2020

1960

1980

2000

2020

(b)

4.5
4

10 Yrs.
Yr Moving
10
MovingA

Frequency

3.5
3

2.5
2
1.5
1

0.5
0
1880

1900

1920

1940

Years

(c)

(d)

Fig. 4 (a) Severe Cyclonic Storms; NIO (1891-2000).
(b) Severe Cyclonic Storms; NIO (10 Yrs. Moving Average).
(c) Number of VSCS (Wind Speed 64 knot/119 kmph or more) in the NIO Region (1965-2008).
(d) TC Associated Highest Max. Winds (knots); NIO Region (1965-2008).

Global Warming, Climate Change and Cyclone Related Destructive Winds

5.1.5 Clear decreasing trend in frequency
The number of Very Severe Cyclonic Storms (VSCS),
wind speeds of 64 knots/119 kmph or more in the NIO
region for the period of 1965 to 2008 along with 5-year
moving averages have been presented in Fig. 4(c). The
data reveal a clear decreasing trend in the frequency ofintense tropical cyclones in the NIO region. The data
length is short (43 years), but the data being for thesatellite era confidence in quality of data is high.
5.1.6 Tropical cyclone associated highest maximum
winds
Prior to satellite era (1965), reliable data on TC
associated maximum winds in the NIO region are either
not available or even if available in some cases, those
may not be reliable. As such, an attempt has been madeto examine maximum winds associated with TCs after
1965. Maximum winds here relate to the highest intensity attended by the cyclones considered anytime during
their life period and do not represent highest winds
along the coast, as many of these weakened from theirpeak intensity prior to their landfall. In most cases highest winds mentioned here are satellite estimated windsof the TCs. A series has been constructed considering all
the tropical cyclones (74 in total) of the NIO region withwind speed ≥ 64 knots (VSCS and above) for 43 years
for the period 1965 to 2008. During this period there
were only 36 years with one or more number of TCs
with wind speed ≥ 64 knots. The plots of 36 highest
maximum winds related to above mentioned 36 years
along with 5 years moving averages are shown in Fig. 4(d). These data do not show any specific trend in TC
associated maximum winds in the NIO region, at least
increasing trend as apprehended by many, is not discernable. Table 1 below shows the decadal frequency oftropical cyclones with maximum winds ≥ 100 knots in
the NIO region for the period 1965 to 2004.
It may be seen that high intensity cyclones arealmost equally distributed in 4 decades considered, confirming no increasing trend. Thus, it may be concluded
that if the past trend is an indications of their future
trend, then there may not be any significant increase in
TC related high velocity winds in the NIO region due toglobal warming. However, this conclusion may not betrue for the other oceanic areas.

147

of 64 knots or more for the globe excluding the South
Pacific are shown in Figs. 5(a) and 5(b) respectively.
The data show no discernable trends. Though thelength of the data series is short, the data are from thesatellite era and are more reliable both in terms of frequency and intensity.
The year to year variations with 5 year moving average for the North Atlantic and Northeast Pacific hurricanes are shown in Figs. 6 and 8 respectively. The
yearly variations of Northwest Pacific tropical storms(TS), typhoons and major typhoon are presented in Fig 7.
While there is no obvious trend in tropical cyclone frequency for the North Atlantic and Northwest Pacific,
there is an increasing trend in the frequency of hurricanes in the Northeast Pacific. The increase in frequency
in the Northeast Pacific could be attributed to better
monitoring of tropical cyclones after the sixties.
5.3 Trends in sea-surface temperatures in the northIndian ocean region
To examine sea-surface temperature variations over
the North Indian Ocean region, weekly high-resolution(1° × 1°) global sea-surface temperature data from 1982
to 1999 (Reynolds & Smith, 1994) are utilized for
averaging. The averaging area for the Bay of Bengal andArabian Sea is shown in Figs. X and Y.
(a)

(b)

5.2 Global scenario
The yearly variations and 5-year moving averages oftropical cyclone (tropical storms) with wind speeds of
34 knots or more, and tropical cyclones with higher
intensities (hurricanes, typhoons, etc.) with wind speeds
Table 1

Decadal Frequency of TCs with winds ≥ 100 knots.
Decade1965-1974
1975-1984
1985-1994
1995-2004

No. of Cases
5
4
4
4

Fig. 5 (a) TC Frequency (Global); (data source: WMO: TCP-30).
(b) TC Frequency (Global); (5 years moving averages).

148

G.S. MANDAL and P. KRISHNA

Frequency
5 Yrs. Moving Av.

Fig. 6 North Atlantic Hurricanes; 1944-2001(Data Source :WMO: TCP-30).

The year-to-year variations and 5-year moving averages of sea-surface temperature in the Arabian Sea areshown in Figs. 9(a) and 9(b) and that for the Bay of
Bengal, in Figs. 10(a) and 10(b), respectively.
The analysis of sea-surface temperature data indicates systematic warming of sea-surface temperatures
over the NIO region during that period.
From the above analysis, it may be concluded thatthough sea-surface temperatures are showing an increasing trend in the NIO region, there is no obvious trend intropical cyclone frequency. However, the frequency of
cyclones with high wind speeds (119 kmph or more) has
shown a decreasing trend.

6. Summary and Conclusions
Examination of historical records of cyclonic disturbances and tropical cyclones in the NIO region shows adecreasing trend in the frequency of cyclonic disturbances mainly due to a significant decreasing trend intropical depressions during the monsoon season. Though,there is no obvious trend in the overall frequency oftropical cyclones, there are large year-to-year variations
with periods of larger and smaller numbers of cyclonic
disturbances and tropical cyclones. A slight increasing
trend in the frequency of severe cyclonic storms in the
NIO region (wind speeds of 48 knots or more) is noticed
especially for a small period after the sixties. This could
be due to uncertianity in deciding intensity of boarderline cases of different sub-groups of tropical cyclones
of lower intensity (T3 or below) in the beginning ofFig. 7 NW Pacific Typhoons (Data Source Internet).

Frequency
Frequency
Yrs.moving
MovingAv.
Av.
55 Yr

20
Frequency

15
10
5
0
1960

1965

1970

1975

Years

1980

1985

1990

1995

Fig. 8 NE Pacific Hurricanes; 1966-1992 (Data Source: WMO: TCP-30).

Fig. X

SST Averaging Area (Arabian Sea).

Fig. Y SST Averaging Area (Bay of Bengal).

Global Warming, Climate Change and Cyclone Related Destructive Winds
(a)

149

(b)

Fig. 9 (a) Yearly Mean SST Distribution (Arabian Sea), (b) Mean SST; 5 Years Moving Average (Arabian Sea).
(a)

(b)

Fig. 10 (a) Yearly Mean SST Distribution (Bay of Bengal), (b) Mean SST; 5 Years Moving Average (Bay of Bengal).

satellite era. However, recent data of 43 years (satelliteera) show a decreasing trend in the frequency of tropical
cyclone of higher category like very severe cyclonicstorms and above (wind speeds of 119 kmph or more) in
the NIO region. Analysis of estimated/observed highest
maximum winds associated with TCs of very severe
intensity (winds with 64 knots or more) observed every
year during 1965 to 2008 in the NIO region also do notshow any increasing trend.
Short period data, that has been analysed also did notshow any obvious trend in the global total frequency ofTCs. The same is the case for North Atlantic hurricanes
and Northwest Pacific typhoons, but there is an increasing trend in the frequency of Northeast Pacifichurricanes. The increasing frequency in NE Pacifichurricanes may again be due to improvement in systems
of monitoring tropical cyclones after the sixties.
An analysis of past sea-surface temperature data of
the NIO region, on the other hand, clearly shows awarming trend.
In summary, based on the analysis of historical data,
it may be concluded that the frequency of tropical
cyclone in the NIO region may not increase due toglobal warming. Increase in TC related high velocity
winds due to global warming, as projected by many, isalso not confirmed by the past data. However, above
conclusion related to high velocity winds is based onlimited data of 43 years, as such, it is tentative. Also, it
is not valid for other TC basins as, wind data for otherareas have not been analysed.
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G.S. MANDAL
Born on August 2, 1937 in West Bengal in
India, Dr. Gouri Sankar Mandal served
more than 31 years in India Meteorological Department (IMD) in various
capacities and retired from Govt. services
as Additional Director General of Meteorology (ADGM), in August, 1995. After
his retirement, he served as a Consultant
in various organizations (Govt. as well as
Private) including UNDP in India and provided advisory services related
to meteorology and early warnings. Currently, Dr Mandal is a Specialist in
National Disaster Management Authority, Government of India. Dr
Mandal earned his Master’s in Physics and Ph.D. in Science. He received
his Professional training in India and in the National Hurricane Centre,
Miami, USA. He has more than 30 years of experience in the field of
Weather Forecasting & Warnings. He is a permanent member of Indian
Meteorological Society and Indian Society of Wind Engineers. He has
more than 65 research papers/publications to his credit (published in India& abroad), mostly on cyclones, monsoons, tsunami, tornadoes and early
warning systems of natural hazards. He presented papers and delivered
invited lectures in several National and International Workshops/
Symposia/Seminars held in India and abroad.

Prem KRISHNA
In an engineering career commencing in
1959, and, spanning approximately 50
years to date, Dr. Prem Krishna has had aunique opportunity of studying andteaching at Civil Engineering Departments of the highest quality and reputation, namely, the University of Roorkee(Now Indian Institute of Technology
Roorkee), the University of Illinois,
Urbana, USA, and the Imperial College ofLondon, UK in addition to extensive R&D work and industrial
consultancy related to engineering structures, wind engineering & disastermitigation. He has had a major role in developing the area of Wind
Engineering and related Disaster Mitigation in India, while alsointeracting with leading professionals in the field the world over, whichlead to his nomination as President of the International Association ofWind Engineering (1991-95) and organisation, for the first time in India,of the World Conference on Wind Engineering at Delhi, India in1995. He
was responsible for initiating,in1985, the organization of the Asia-PacificRegional Conference on Wind Engineering, which has now become a4-yearly event, and its example has been followed with similar regional
conferences in other regions too. Dr. Krishna continues to be active inworking on various aspects of Wind Engineering, both Nationally as well
as Internationally. Duly recognized for his Professional contributions, heis currently Vice-President of the Indian National Academy ofEngineering.

(Received 11 November 2009, Accepted 25 December 2009)