|Severe heat wave over the Indian subcontinent in 1998, in perspective of global climate|
spells of high temperature sometimes claim heavy toll of human lives as well as live stock. Spells of these abnormally hot weather conditions are sometimes observed to move progressively from
one region to another and are therefore, called heat wave. In India, a heat wave is considered to be severe when maximum temperature remains 7° C or more above its long term normal value for a
Figure 1. Daily maximum temperature of selected stations
during 1531 May 1998.
indicates normal maximum temperature.
station having normal maximum temperature £
40° C or remains 5° C or
more above its long term normal value for a station having normal temperature > 40° C. The above-mentioned criteria are laid down by India
Department (IMD) for declaring the heat wave.
In 1998, major parts of the country extending from north India, parts of north-east India and the northern parts of peninsular India experienced severe heat wave during the second half of May. Even south interior Karnataka and some stations of Tamil Nadu were under the grip of severe heat wave conditions during this period. Chennai recorded the second highest maximum temperature (44° C) of the present century on 24 May, which was 8° C above normal. The highest ever recorded maximum temperature (45° C) had occurred in Chennai on 21 May, 1910.
This severe heat wave condition initially prevailed over north-west India. It then extended south-south-east towards Orissa and coastal Andhra Pradesh. According to the press information, summer of 1998 was the worst in the past fifty years. It took a toll of nearly 1300 human lives of which 650 lives were lost in Orissa alone. Millions of people soaking in sweat. The heat wave took its toll in almost every state in the country, the worst affected being Orissa. During this period a few stations in Rajasthan experienced temperature in excess of 49° C. Delhi recorded the second highest maximum temperature in the past 54 years of 46.5° C on 28 May. However, the highest maximum temperature of this century (47.2° C) was recorded in Delhi on 29 May, 1994.
We know that the people residing at a place for a sufficiently long
time get more or less acclimatized to the normal weather conditions of that place. Thus,
though, the maximum temperature of north-west India especially Rajasthan was nearing 50° C, the death toll in
Rajasthan and north-west part of the country was less, compared to that of Orissa. The large number of deaths in Orissa was perhaps due lack of adaptability to such extreme conditions.
Figure 1 depicts the variation of maximum temperature of some selected stations from 1531 May 1998, while Figure 2 depicts the composite circulation pattern of 500 hPa level during the period 1931 May 1998. This is the period when most parts of the country especially southern India experienced heat wave conditions. The ridge line at 500 hPa passed through Veraval, Aurangabad, Visakhapatnam and then eastwards. This indicates that the subsidence prevailed over the central parts of the country which in turn resulted in clear sky conditions and strong solar insolation over this part and subsequent steep rise in the maximum temperature.
Figure 2. Composite wind pattern at 500 hPa during 1931 May 1998.
Figure 3. OLR anomaly May 1998. The contour interval
is 20 Wm2 with area below
20 Wm2 shaded.
This is further supported by the mean monthly Outgoing Longwave Radiation (OLR) anomaly for the month of May 1998, depicted in Figure 3. OLR anomaly has been calculated using mean monthly OLR data for a 5-year period (19871991). The positive OLR anomaly prevailed throughout the country including Bay of Bengal and Arabian Sea. The maximum positive anomaly (50 Wm2) is observed over the Bay, while OLR anomalies (³ 20 Wm2) covered the whole area south of 20° N indicating absence of convective activity during May over that part of the country. This again indicates clear sky and the large solar insolation in the peninsular region and the possible cause of abnormal heat over Orissa and coastal Andhra Pradesh.
The number of heat wave days that occurred during May and June in the Indian subcontinent along with loss of human lives in the last 20 years is presented in Table 1. It has been prepared with the help of All India Weather Summary, Weekly Weather Reports and Disastrous Weather Events published by IMD.
Table 1 shows that the number of the heat wave days as well as number of human lives lost due to heat wave during May and June in the present decade (19891998) are comparatively higher than in the previous decade (19791988). In the present decade, 1995 and 1998 were in the years when the magnitude of heat wave was maximum. During 1995, nearly 357 people lost their lives due to severe heat wave condition which prevailed over north-west India on 2 and 3 June. Dholpur recorded the maximum temperature 50° C. Also, the duration of heat wave condition was most prolonged in 1995.
Mann et al.1 concluded, with the help of proxy climate indicator along with very long instrumental record, that the present century is the warmest in six hundred years and the three warmest years of 90s are hotter than any other period since the middle ages. According to World Meteorological Organization2 (WMO), the global average surface temperature of 1995 was 0.4° C above in 19611990 average, according to the observation made at land stations and sea surface temperature measured from ships and buoys. The previous warmest year since 1861 was 1990, which had an anomaly 0.36° C. Parts of Siberia were about 3° C warmer than normal in 1995. It may be worth mentioning that a volcanic eruption of Mt. Pinatubo in Phillippines in June, 1991, had somewhat arrested the increasing trend of northern hemispheric temperature anomaly.
According to WMO Global Climate System Review3, the warmest 7 years have been recorded since 1982. The years are 1983, 1987, 1988, 1990, 1991, 1994 and 1995. Of these, 1982, 1987, 1991 and 1994 are the warm ENSO years over the eastern Equatorial
Table 1. Heat wave days and deaths in the Indian subcontinent
No. of heat
Year wave days States affected Lives lost
1979 16 Orissa, Andhra Pradesh, 365
Gangetic West Bengal, Uttar Pradesh, Bihar
1980 7 Uttar Pradesh, Maharashtra, Tamil Nadu 106
and Andhra Pradesh
1981 7 Uttar Pradesh, Madhya Pradesh, Bihar, 63
1982 4 Uttar Pradesh 11
1983 11 Bihar, Punjab, Uttar Pradesh, Maharashtra 185
1984 11 Uttar Pradesh, Punjab, Maharashtra 58
1985 5 Punjab, Bihar 141
1986 8 Andhra Pradesh, Punjab, Rajasthan, 155
Himachal Pradesh, Bihar
1987 6 Orissa, Punjab, Haryana, Uttar Pradesh 90
1988 21 Rajasthan, Gujarat, Saurashtra and Kutch 924
Uttar Pradesh, Punjab
1989 15 Rajasthan, Madhya Pradesh, Maharashtra 43
1990 6 Rajasthan
1991 10 Rajasthan, Madhya Pradesh, Gujarat, Maharashtra 250
1992 13 Rajasthan, Madhya Pradesh, Haryana, Bihar 114
Uttar Pradesh, Maharashtra
1993 13 Punjab, Haryana, Rajasthan, Uttar Pradesh 73
1994 25 Punjab, Haryana, Rajasthan, Madhya Pradesh 234
Uttar Pradesh, Maharashtra
1995 29 Haryana, Rajasthan, Punjab, Uttar Pradesh 410
1996 9 Rajasthan, Haryana, Uttar Pradesh 17
1998 27 Punjab, Rajasthan, Gujarat, Uttar Pradesh, 1300
Maharashtra, Orissa, Andhra Pradesh,
South Tamil Nadu
Surprisingly during 1997, no heat wave condition prevailed over any part of the country.
Pacific. From Table 1, it can be seen that the maximum number of the heat wave days and the human lives lost during May and June over the Indian
subcontinent are comparatively large during the years 1983, 1988, 1995 and 1998. These years are preceded by warm ENSO years. 1997 was the severest
warm ENSO year of the century. Hot and dry weather conditions were not only experienced in India in the 1998 summer but the drought and deluges prevailed in almost every part of the globe. The first five months of 1998 were the planets hottest on the record according to the scientists of the USA-based National Oceanographic and Atmospheric Administration. The average global temperature from January through May was about 17.2° C, marginally hotter than the previous record set in 1997.
Further studies in this subject are needed for understanding and predicting such extreme events which are hazardous to life on earth.
ACKNOWLEDGEMENT. We thank the Director General of Meteorology, India Meteorological Department for facilities to undertake the study.
U. S. DE
R. K. MUKHOPADHYAY
India Meteorological Department,
Shivajinagar, Pune 411 005, India
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