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Global Environmental Research ©2009 AIRIES 13/2009: 141-150 printed in Japan

Global Warming, Climate Change and Cyclone Related Destructive

Winds – Discussion of Results from Some Selected Studies with Emphasis on the North Indian Ocean

1G.S. MANDAL and 2Prem KRISHNA

1Former Additional Director General, India Meteorological Department (Retd)

248, Aravali Apartment, Alaknanda, New Delhi 110019, India 2Former 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 in

the ocean and earth-atmosphere systems. This raises the apprehension of future increases in the frequency of high-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 through different 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 through the analysis of past data.

With this in view, we analyzed historical data on tropical cyclones, which are one of the sources of high 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 that though 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 un-equivocal. Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropo-genic 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 are apprehensions that these changes in atmospheric and oceanic conditions due to global warming could lead to variations in high-velocity winds and their destructive potential.

It is well known that high-velocity winds are destruc-

tive. They cause loss of lives and damage or destruction to infrastructure along with trees, plantations, crops, etc., and increased coastal inundation and erosion. Their variation in frequency and intensity is therefore of natu-ral human concern. The sources of high-velocity surface winds in the tropics are meteorological phenomena like (i) tropical cyclones and their generic systems (depres-sions, 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 generic systems causing high-velocity winds, like tropical cyclones, thunderstorms, etc., to understand the impact of global warming on high-velocity winds. The intensi-ties of these systems are measured by their associated wind speeds which are inversely related to pressure, i.e., the lower the pressure in the centre of the system, the higher the wind speed.

142 G.S. MANDAL and P. KRISHNA

In this paper the impact of global warming and climate 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 are from 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) by examining the reasons the increase in sea-surface tem-perature over tropical ocean, as suggested by global circulation models, might lead to increases in (a) fre-quency and (b) intensity of tropical cyclones, (ii) by analyzing 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 the case with other oceanic areas there are some uncertain-ties in the historical data related to frequency and inten-sity of cyclones of the pre-satellite era. This is more so in the NIO region from where aircraft reconnaissance data from cyclone field is not available in operational mode 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 are also available for the recent period. Presuming that the past trends in tropical cyclones may be a good reflection of the future, past data are used to project the expected changes in the frequency and intensity of tropical cyclones. Good data on the history of tropical cyclones for the recent period are also available from other cyclone basins. The emphasis of the present study is to examine changes in tropical cyclone frequency and intensity vis-à-vis changes in sea-surface temperature during the recent past and to draw conclusions on sur-face 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 to Tropical 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 to seven decades, can be broadly summarized as follows: (i) substantial warming in the earth’s climate, (ii) an anticipated 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 least sea-surface temperature increase occurring over the tropical oceans, (iv) a far-from-global global warming; which will have an exceedingly regional distribution instead, (v) a likely increase in the ocean area with sea-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 the subject of which a few selected studies have only been considered. These can broadly be divided into three main 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 tropical oceans 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 con-straints limit the upper bound of tropical cyclone inten-sity, 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, the destructive potential of tropical cyclones would increase. Evan (1993) observed that the maximum intensity of tropical cyclones (wind-speed scale) and sea-surface temperature of that location and time where the maxima were observed bear no significant relationship. This sug-gests 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 143

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 fre-quency due to global warming. Broccoli and Manabe (1990) and Haarsma et al (1992) using low-resolution climate models concluded that there will be an increase in 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 on high-resolution (T106, L19) global circulation models. Their simulated studies generated a realistic distribution of tropical cyclone-like vortices over the globe. Their final conclusion was a decrease in the frequency of tropical cyclones with an even greater decrease in the southern hemisphere under doubled-CO2 conditions. The latest study by Bengtsson et al. (2007) summarized the modeling efforts of various scientists. This indicated that the majority of such exercises to date show a decrease in frequency of tropical cyclones globally, although some studies show a regional increase.

Emanuel, in collaboration with other scientists, pub-lished several papers in recent times in various reputed meteorological and climatological journals strongly sup-porting the view that global warming would have significant impact on tropical cyclone frequency and intensity by increasing the number of more intense tropical cyclones. In his latest work, Emanuel et al. (2008), however, he concluded, “A new technique for deriving 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 an eminent scientist like Emanuel, who has long been a strong supporter of global warming causing more hurri-canes. Emanuel ran several computer models designed to recreate past hurricane activity. According to the study, “The model consensus has slightly decreased fre-quency of events in the Caribbean and western North Atlantic.” Regarding the intensity of future storms, Emanuel reports the models are split. Some indicate that hurricane and tropical storm intensity will remain stable or decline, whereas others predict the cyclones’ overall intensity will increase somewhat.

Knutson et al. (2008) through their modeling study of 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 Interna-tional Workshop on Tropical Cyclones (IWTC – II) held in Manila in 1989, where it was concluded that more discussion would be needed on the subject to come to a conclusion. Accordingly, the subject was extensively debated by the participants (which included cyclone modelers, researchers, operational forecasters and policy

planners) of the WMO/ICSU Third International Work-shop on Tropical Cyclones (IWTC-III) held in Hua Tulco (Mexico) in 1993, and a status report on the sub-ject was prepared (Lighthill et. al 1994). Probably, this was the first “status paper” of its kind on the subject based on combined wisdom of different scientific communities. Summarized results of this paper are as follows:

Out of the three methodologies listed under “Methodology” the general consensus was that at the present state of climate modeling, reasonably realistic simulation of cyclonic disturbances in different years is not yet possible. As regards the first methodology, it was argued that one of the six important conditions favour-able for the formation and development of tropical cy-clones was elevated sea-surface temperatures. Other conditions were (i) large low-level cyclonic relative velocity; (ii) some critical value of the Coriolis parame-ter, (iii) weak vertical shear of zonal winds, (iv) some degree of convective instability and (v) large values of relative humidity. Here, condition (iii) on low vertical shear is particularly important.

Accordingly, it is not certain that a future widening of the oceanic area where sea-surface temperatures ex-ceed 26°C, will substantially widen the area liable to tropical cyclone formation. Again, whereas condition (ii) above imposes a lower limit on those latitudes (N or S) where tropical cyclone formation can occur, there is an upper limit on such latitudes imposed by condition (v). This upper limit between 25° latitude and 30° lati-tude 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 are opposed). The features of Hadley circulation are, how-ever, resistant to climate change. Therefore, if tropical cyclone frequency depends essentially on the frequency of 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 other known 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 numbers of 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 pro-jected, there will likely be an increase, on average worldwide, 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 decrease or remain unchanged. However, a likely increase in tropical cyclone intensity means that the frequency of the strongest tropical cyclones will more likely than not increase 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 series of tropical cyclones in the North Atlantic Ocean for the period of 1871 to 1992 shows no increase in the number of storms, except large variations from year to year. Chris Landsea (2005, 2007), analyzing historical tropi-cal 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 was causing an increase in hurricane activity. He noted, “Sci-ence tells us that we should trust the real-world evi-dence.”

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 sub- classifications viz., cyclonic storm (wind speed between 34 knots and 47 knots), severe cyclonic storm (wind speed 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 Distur-bance is the term used to represent depressions and all classes 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 peak in 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 and decadal. There are some recent studies on the trends and variability of cyclonic disturbances in North Indian Ocean region. Mandal (1991) indicated large year-to- year variability in tropical cyclone frequency and periods of increasing and decreasing frequencies on a multi-decadal scale. Mandal (1995) focused on the decreasing trend in cyclonic disturbances in the North Indian Ocean. After analyzing short period global historical data of tropical cyclones, Mandal (2002) con-cluded 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 of Northeast Pacific hurricanes. This increase in hurricane frequency in the Northeast Pacific, he mentioned, may be due to improvement in systems of monitoring tropi-cal cyclones after the sixties.

Srivastava et al (2000) indicated a significant de-creasing trend in the frequency of storms in the North Indian Ocean region. Rao et al. (2001) indicated epochs of increasing and decreasing trends in the frequency of tropical 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 de-creasing trends in both frequencies. 5.1.2 Yearly distribution of cyclonic disturbances

(CDs) and tropical cyclones (TCs) The yearly distribution of cyclonic disturbances (low

pressure areas with wind speeds of 17 knots/31 kmph or more) and tropical cyclones (low pressure areas with wind 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 145

The data show large year-to-year variability of cyc-lonic disturbances and tropical cyclones with a slight decreasing trend in the frequency of cyclonic distur-bances 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 of trend in seasonal scale. The seasonal distributions for the four seasons, viz., winter (January-February), pre- monsoon (March-May), monsoon (June-September) and post monsoon (September-December) of cyclonic dis-turbances and tropical cyclones of the NIO region have been analysed with a view to examine the presence of trend if there is any. It may be mentioned that, in this region, the frequencies of cyclonic disturbances are maximum in the monsoon season and tropical cyclones in the post monsoon season. The frequencies of CDs and TCs, in terms of 10 years moving averages of the NIO region for the monsoon and post-monsoon seasons are shown in Figs. 3(a) and 3(b) respectively. 5.1.4 Severe cyclonic storms (Trend)

The yearly distribution and 10-year moving averages of severe cyclonic storms (wind speeds of 48 knots/89

kmph or more) are shown in Figs. 4(a) and 4(b). The data show slight increase in the frequency of

severe cyclonic storms in the NIO region, especially after 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 the uncertianity in deciding intensity of tropical cyclones of lower category (T3 or below in Dvorak’s scale) and forecaster’s bias to classify boarder line cases to a higher 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, slight increasing trend of frequency, observed in the case of severe cyclonic storm could be due to more of uncertainity in deciding intensity of boarder line cases of TCs in absence of actual observation from the cyclone field, rather than the actual variability and impact 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)

012345678

1880 1900 1920 1940 1960 1980 2000 2020Years

Frequency

(b)

00.5

11.5

22.5

33.5

44.5

1880 1900 1920 1940 1960 1980 2000 2020Years

10 Yr MovingA

(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).

Freq

uenc

y Fr

eque

ncy

10 Yrs. Moving

Global Warming, Climate Change and Cyclone Related Destructive Winds 147

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 of intense tropical cyclones in the NIO region. The data length is short (43 years), but the data being for the satellite 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 made to examine maximum winds associated with TCs after 1965. Maximum winds here relate to the highest inten-sity 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 their peak intensity prior to their landfall. In most cases high-est winds mentioned here are satellite estimated winds of the TCs. A series has been constructed considering all the tropical cyclones (74 in total) of the NIO region with wind 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 discern-able. Table 1 below shows the decadal frequency of tropical cyclones with maximum winds ≥ 100 knots in the NIO region for the period 1965 to 2004.

It may be seen that high intensity cyclones are almost equally distributed in 4 decades considered, con-firming 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 to global warming. However, this conclusion may not be true for the other oceanic areas.

5.2 Global scenario

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

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 the length of the data series is short, the data are from the satellite era and are more reliable both in terms of fre-quency and intensity.

The year to year variations with 5 year moving aver-age for the North Atlantic and Northeast Pacific hurri-canes 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 fre-quency for the North Atlantic and Northwest Pacific, there is an increasing trend in the frequency of hurri-canes 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 north

Indian 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 and Arabian Sea is shown in Figs. X and Y.

Table 1 Decadal Frequency of TCs with winds ≥ 100 knots.

Decade No. of Cases 1965-1974 5 1975-1984 4 1985-1994 4 1995-2004 4

(a)

(b)

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

The year-to-year variations and 5-year moving aver-ages of sea-surface temperature in the Arabian Sea are shown 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 indi-cates systematic warming of sea-surface temperatures over the NIO region during that period.

From the above analysis, it may be concluded that though sea-surface temperatures are showing an increas-ing trend in the NIO region, there is no obvious trend in tropical 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 distur-

bances and tropical cyclones in the NIO region shows a decreasing trend in the frequency of cyclonic distur-bances mainly due to a significant decreasing trend in tropical depressions during the monsoon season. Though, there is no obvious trend in the overall frequency of tropical 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 boarder line cases of different sub-groups of tropical cyclones of lower intensity (T3 or below) in the beginning of

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

Fig. 7 NW Pacific Typhoons (Data Source Internet).

0

5

10

15

20

1960 1965 1970 1975 1980 1985 1990 1995Years

Frequency

Frequency5 Yr moving Av.

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).

Frequency 5 Yrs. Moving Av.

Frequency 5 Yrs. Moving Av.

Global Warming, Climate Change and Cyclone Related Destructive Winds 149

satellite era. However, recent data of 43 years (satellite era) show a decreasing trend in the frequency of tropical cyclone of higher category like very severe cyclonic storms 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 not show any increasing trend.

Short period data, that has been analysed also did not show any obvious trend in the global total frequency of TCs. The same is the case for North Atlantic hurricanes and Northwest Pacific typhoons, but there is an in-creasing trend in the frequency of Northeast Pacific hurricanes. The increasing frequency in NE Pacific hurricanes 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 a warming 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 to global warming. Increase in TC related high velocity winds due to global warming, as projected by many, is also not confirmed by the past data. However, above conclusion related to high velocity winds is based on

limited data of 43 years, as such, it is tentative. Also, it is not valid for other TC basins as, wind data for other areas 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 Meteor-ological Department (IMD) in various capacities and retired from Govt. services as Additional Director General of Meteor-ology (ADGM), in August, 1995. After his retirement, he served as a Consultantin 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 in1959, and, spanning approximately 50years to date, Dr. Prem Krishna has had aunique opportunity of studying andteaching at Civil Engineering Depart-ments of the highest quality and reputa-tion, namely, the University of Roorkee(Now Indian Institute of TechnologyRoorkee), the University of Illinois,Urbana, USA, and the Imperial College of

London, UK in addition to extensive R&D work and industrial consultancy related to engineering structures, wind engineering & disaster mitigation. He has had a major role in developing the area of Wind Engineering and related Disaster Mitigation in India, while also interacting with leading professionals in the field the world over, which lead to his nomination as President of the International Association of Wind 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-Pacific Regional Conference on Wind Engineering, which has now become a 4-yearly event, and its example has been followed with similar regional conferences in other regions too. Dr. Krishna continues to be active in working on various aspects of Wind Engineering, both Nationally as well as Internationally. Duly recognized for his Professional contributions, he is currently Vice-President of the Indian National Academy of Engineering.

(Received 11 November 2009, Accepted 25 December 2009)