India is a land with a unique climatic regime, that includes, two monsoon seasons (south-west and north-east); two cyclone seasons (pre- and post-monsoon cyclone seasons); hot weather season characterised by severe thunderstorms, dust storms and heat waves; and, cold weather season characterised by violent snow storms in the Himalayan regions, cold waves and fog. Since agricultural output in India is primarily dependent on rainfall, the variability in rainfall-particularly the monsoon rainfall during June to September (June-July, August-September- JJAS) that occurs over a range of temporal scales from intra-seasonal to inter-decadal, dominated by inter annual variations, can have adverse impacts, such as crop failures.
This is particularly so since India receives nearly 75-90 per cent of its annual rainfall during the JJAS monsoon season. Hence, any shortfall in rain during this season can create drought conditions in the country, particularly in the drought-prone regions—which comprise one sixth of the total area of the country.
On the other hand, extremely heavy rainfall can result in disaster at another end of the scale, wherein floods can cause heavy damage to crops, life and property. With about 40 million hectares (mha) in India prone to floods, and an average of 7.5 mha hit by floods every year—particularly in the plains, the variability of monsoon rainfall within a season, especially its onset, withdrawal, active and dry spells are very crucial indeed.
India is frequently affected by cyclones, hailstorms, cloudbursts, dust storms and severe thunderstorms on a regular basis. Of these, cyclones are the most significant, given the fact that India has a 7516 km coastline covering ten states. On an average, 5 to 6 cyclones form over the Bay of Bengal and Arabian Sea and hit the coasts every year.
As per the 4th Assessment Report of The Intergovernmental Panel on Climate Change (IPCC) (AR4; IPCC, 2007) more frequent and intense weather events have been projected in the 21st century. This is likely to further degrade the resilience and coping capacities of poor and vulnerable communities. Thus it is all the more necessary to undertake spatio-temporal analytical studies, and hence plan well in advance to reduce losses to life and property.
Variability in weather extremes
The southwest monsoon has a stranglehold on agriculture, the Indian economy and consequently, the livelihoods of a vast majority of the rural populace. An overwhelming majority of cropped area in India (around 68 per cent) falls within the medium and low rainfall ranges regions. The All India Summer Monsoon Rainfall (AISMR) during JJAS has a unique identity due to its large inter annual variability. The long period average (LPA; 1951-2000) of summer monsoon rainfall is found to be about 89 cm with a coefficient of variability of ≈8.9 cm (10 per cent of LPA) as shown in Fig. 1a and b. The inter-annual variation of AISMR during 1901-2014 is shown in Fig. 2 and Table 1. As seen from Fig. 1 the period from 1901 to 2014 witnessed many deficient and excess monsoon years. The deficient or excess years are identified based on the rainfall departure of ±1 coefficient of variability.
Although there have been many studies linking deficient monsoon to the El Niño and Southern Oscillation, K Krishna Kumar et al., in their publication, ‘On the weakening relationship between the Indian monsoon and ENSO’, Science,1999, have suggested that the link with El Niño has weakened in the last decade, and in fact the All India Summer Monsoon Rainfall (AISMR) anomaly was positive in the recent intense warm event of 1997.
Many other studies have also linked various teleconnection patterns other than ENSO to explain the observed inter annual variability. In fact, even in a year of excess rainfall, there may be pockets of deficient rainfall and vice-versa. This can lead to drought or flood situations, as the case may be.
Active and break cycles of the monsoon
Analysis of seasonal monsoon rainfall shows intra seasonal variations with active and break phases of monsoon, which is very important for the agriculture sector and water management. The convective activity over the monsoon trough region and that over the equatorial south Indian ocean oscillates in sea-saw fashion with one active and the other inactive and vice versa. These influence monsoon activity over the Indian region on an intra seasonal scale. The phenomenon of `break in monsoon’ is of great interest because long intense breaks are often associated with poor monsoon seasons. Such breaks have distinct circulation characteristics (heat trough type circulation) and have a large impact on rain fed agriculture.
Although interruption of the monsoon rainfall is considered to be the most important feature of the break monsoon, traditionally breaks have been identified on the basis of the surface pressure and wind patterns over the Indian region. In K Ramamurthy’s 1969 paper, ‘Monsoon of India: Some aspects of the ‘break’ in the Indian southwest monsoon during July and August’ Forecasting Manual, No. IV , India Meteorological Department, a break situation is defined as one in which the surface trough (the ‘monsoon trough’) is located close to the foothills, and easterly winds disappear from the sea level, provided the break is for over two days.
Incidentally, the coupled convective phenomenon like the phase and magnitude of Madden Julian Oscillation (MJO) has a direct influence on intra seasonal oscillation of monsoon rainfall over India. The excess (deficient) rainfall years are mainly associated with more active (break) spells of monsoon.
Extreme rainfall and its variability
During the southwest monsoon season (JJAS), many parts of India including the west coast of the peninsula, northeast and central region of the country receive heavy to very heavy rainfall during the active spells of the summer monsoon. Yet, within the season, spells with heavy rainfall alternate with spells of little or no rainfall. Occasionally the rainfall is exceptionally heavy at some stations associated with an active monsoon trough and passage of low pressure areas and depressions.
These heavy rainfall events with daily rainfall of the order of 10 cm or more at some stations over the west coast and other parts of India cause extensive damage to life and property every year during JJAS through landslides and flash floods. There are many past instances of Indian stations having recorded as much as half of their annual mean rainfall, and sometimes even more than their annual mean rainfall, in one single day, such as 944 mm over Mumbai (Santacruz) on July 26-27, 2005, which resulted in massive urban flooding.
Climate change studies have indicated increasing frequency of extreme weather events including the frequency of heavy rainfall over the globe. Modelling studies indicate changes in intense precipitation are more likely as global temperature increases. There have also been a few studies over the Indian region highlighting different aspects of mean and extreme rainfall events during the southwest monsoon season.
Like the frequency of extreme rainfall events, the contribution of extreme rainfall to the total rainfall in a season is also showing a highly significant increasing trend during the JJAS monsoon on seasonal scale and during June and July on monthly scale, as per the 2010 study of D R Pattanaik and M Rajeevan, titled ‘Variability of extreme rainfall events over India during southwest monsoon season’, published in Meteorological Applications.
Data for 64 years of tropical cyclones from 1951-2014, documents monthly total cyclonic disturbances; formed over the northern Indian Ocean (Fig. 3). These data sets are collected from the storm e-atlas published by the India Meteorological Department (IMD), and indicate that the months of October-November-December (OND) produce cyclonic disturbances of severe intensity.
The strong winds, heavy rains and large storm surges associated with cyclonic disturbances are the factors that eventually lead to loss of life and property. Rains associated with cyclones are another source of damage. However, one does not discern much of a trend in the data sets indicating the frequency of cyclones over the Bay of Bengal, the northern Indian Ocean and Arabian Sea.
These systems form initially as low pressure areas (when the maximum sustained surface wind is less than 7 knots) over the northern Indian Ocean and then intensify into depressions (DD; where maximum sustained surface wind is between 17 and 33 knots) and occasionally become tropical cyclones (TCs). These may be cyclonic storms (CS); when surface wind speed is between 34 and 47 knots and severe cyclonic storm (SCS) when surface wind speed is between 48 knots and 63 knots; very severe cyclonic storm (VSCS) when it exceeds 63 knots till 119 knots; super cyclone when surface wind speed exceeds 119 knots.
The incidence of TCs with intensity of severe to very severe cyclones across Tamil Nadu and Andhra Pradesh is high during the northeast monsoon season (OND), whereas, the highest annual number of CSs and SCSs occur along the Odisha-West Bengal coast.
Surface air temperature
The surface air temperature is an important factor that affects agriculture and food security. The surface air temperature drives crop growth duration; influences milk production in animals and spawning in fish. Temperature in conjunction with relative humidity can influence pest and diseases incidence on crops, livestock and poultry. Surface air temperature during the pre-monsoon season also influences soil moisture and hence the performance of the ensuing monsoon. Thus, variability of surface air temperature plays an important role in the national economy.
There are some other aspects of surface air temperature which have assumed significance of late. According to a study by D R Kothawale and K Rupa Kumar., 2005, ‘On the recent changes in surface temperature trends over India’, in Geophys. Res. Lett., the mean maximum temperature (Tmax) over India increased during the 1901-1987 period. Currently, there is a significant warming trend in annual mean temperature over India due to increasing Tmax during 1901-2003. Associated with high Tmax in summer, most areas in India have experienced episodes of heat waves causing sunstroke, dehydration and death. Extreme heat waves can also result in losses in standing crops, livestock and fisheries, affecting a majority of the population. On the other hand, cold waves and frost in winter can play havoc with winter crops.
Heat wave and abnormal high temperature
Many areas of West Rajasthan, Punjab, Haryana, northern parts of East Rajasthan, Madhya Pradesh, Chhattisgarh, Vidarbha, western Uttaranchal, East Uttar Pradesh, western parts of Jharkhand and Bihar, Gangetic West Bengal, northern parts of Odisha, Telangana, Coastal Andhra Pradesh, eastern parts of Rayalaseema and north Tamil Nadu on an average have experienced ≥ 8 heat wave days (Fig 4a and b).
Although the maximum number of severe heat wave days have always been reported in the north-western north and eastern parts of India, heat waves in June could also occur due to the stagnation of movement of the monsoon. The heat wave during the middle of June in 2005 was due to such stagnation in monsoon progress over the region. During the period from June 14-22, 2005 the Tmax anomaly reported was of the order of 5 to 6 °C in some eastern states of India, where maximum deaths were reported during this period.
There have been several studies that have indicated extreme temperatures all over India.
G S P Rao et al. 2005, ‘Climate change over India as revealed by critical extreme temperature analysis,’ in Mausam, have reported that 80 per cent stations in peninsular India and 40 per cent stations in northern India showed increasing trend in temperatures during the day with critical extreme maximum temperature. D R Kothawale et al. (2010), in their article, ‘Recent trends in pre-monsoon daily temperature extremes over India’, Journal of Earth Systems Science, have found widespread increasing trend in the frequency of occurrence of hot days and hot nights and widespread decreasing trend in those of cold days and cold nights in the pre-monsoon season. In their article, S K Dash and Ashu Mamgain (2011), ‘Changes in the frequency of different temperature extremes in India’, in Journal of Applied Meteorological Climatology, have pointed out that seven homogeneous regions during the period 1969-2005 indicated a significant increasing trend in the number of warm days in summer in the interior peninsula. On the eastern and western coasts, the maximum number of warm days in summer was noticed only during 1996-2005. These results, thus, broadly suggest warming trends in large parts of India.
Cold wave, abnormal low temperatures and fog
Cold wave conditions observed in the hilly regions in the north of India and adjoining plains are usually influenced by western disturbances. These systems are transient winter disturbances in the mid-latitude westerlies with weak frontal characteristics. U S De et al. (2005), ‘Heat and Cold Waves Affecting India During Recent Decades’, International Journal Of Meteorology, have inferred that the occurrence of cold wave conditions in the last century was at a maximum in the Jammu and Kashmir region followed by Rajasthan and Uttar Pradesh. Results of D S Pai et al. 2004, in ‘Decadal variation in the heat and cold waves over India during 1971-2000’, Mausam, show that cold wave conditions were most often experienced in west Madhya Pradesh in the decade 1971-80, in Jammu and Kashmir in 1981-90, and in Punjab in 1991-2000. The 2011 study by S K Dash et al, cited earlier indicates a significant decrease in the frequency of occurrence of cold nights in winter months in India and its homogeneous regions in the north except in the western Himalaya. Southern regions showed a drastic decrease in the frequency of cold nights relative to 1969-75.
As regards fog, R K Jenamani, 2007, ‘Alarming rise in fog and pollution causing a fall in maximum temperature over Delhi’, Current Science, has shown that due to the rapid urbanisation in cities like Delhi, fog occurrences have increased with the rise in pollution causing a fall in Tmax (maximum temperature) in winter.
Thunderstorm and hailstorm
A thunderstorm occurs when a giant cumulus cloud develops into a towering dark cumulonimbus giving off lightning flashes during the pre-monsoon season. While farmers welcome thunderstorms, due to the accompanying rain and other benefits, a thunderstorm that develops into a hailstorm can result in severe crop losses, and cause harm to standing crops. Gangetic West Bengal and Bangladesh record between 80 and 100 days of thunderstorm annually. Kerala records the highest (80-100 days) thunderstorm frequency over the peninsula. Udhampur observatory (132 days) in Jammu sub-division records the highest number of thunderstorms in the country followed by Kumbhigram (Silchar) observatory (129 days) in south Assam and Hasimara (123 days) in Sub Himalayan West Bengal.
In 2015, hailstorm and unseasonal rain lashed India during February, March and April. A total of 182.38 lakh hectares of cropped area was affected, worst amongst which were Uttar Pradesh, Rajasthan and Haryana with 95.17, 30.57 and 22.24 lakh hectares respectively.
Also, during the first half of December, 2014 unprecedented hailstorms struck several villages in Nashik district, in Maharashtra. The untimely rain caused a lot of damage to farmers. Almost 8 lakh hectares of crop was damaged, particularly standing crops like wheat, jowar, pulses, and sunflower, as also grapes, pomegranate, and vegetables.
Cloudburst and landslides
The Western Himalayas sometimes witness cloudbursts under active monsoon conditions. In 2010, a cloudburst resulted in flash floods and mudflow in Leh and surrounding areas causing severe damage to human lives and property. It left 196 persons dead with 65 missing, 3,661 houses damaged and affected 27,350 hectares of crop area.
Heavy rainfall events in the western Himalayas often produce landslides; nearly 30 per cent of the world’s landslides occur in these regions. The Himalayan mountains, which constitute the youngest and most dominating mountain system in the world, comprise a series of seven curvilinear parallel folds running along a grand arc for a total of 3400 km. In fact, in north Sikkim and Garhwal regions in the Himalayas, there are an average of two landslides per square kilometre.
Given the widespread damage that weather extremes like extreme heavy rainfall events, cloudbursts, hailstorms, drought and cyclones regularly cause in India, it is extremely important that monitoring and forecasting of these be undertaken on a regular basis. This can help keeping casualties and agricultural losses to a minimum.