E xtreme events such as droughts, floods, heat waves, cold waves, devastating thunder clouds, cloudbursts and intense cyclonic storms have been occurring frequently over the globe in recent decades. Climate change is realised all over the world and is significantly altering the structure and functioning of many ecosystems. Significant advances in the scientific understanding of climate change now make it clear that there has been a change in climate that goes beyond the range of natural variability.
It is evident from instrument (thermometer)-based observations that the planet’s average surface temperature has risen about 1.1 0Celsius since the late 19th century, a change driven largely by increased carbon dioxide and other human-made emissions into the atmosphere by fossil-fuel burning and deforestation. Most of the warming has occurred in the past 35 years, with the five warmest years on record taking place since 2010. This speed of warming is more than 10 times that has occurred on a global scale in the last one million years. There exists a strong relationship between CO2 concentration in the atmosphere and global temperature. Measurements of air in ice cores show that for the past 8,00,000 years up until the 20th century, the atmospheric CO2 concentration has stayed within the range of 170 to 300 parts per million (ppm); the recent rapid rise to nearly 412 ppm over 200 years is particularly remarkable. Measurements of different forms of carbon reveal that this increase is because of human activities.
Impact of climate change
Observational, theoretical and numerical modelling studies demonstrate increasing trends in temperature, precipitation and extreme events as realised by global warming in response to anthropogenic greenhouse gas (GHG) emission. In a warming world, differential heating between land and oceans and between polar and tropical regions will affect global circulation patterns and thereby change the intensity, frequency and seasonality of climate patterns and distribution of extreme weather events (for example, floods, droughts and storms).
The magnitude of extreme rainfall events is predominantly controlled by the increased atmospheric moisture content. Future climate will be warmer than the present climate. The response of this warming climate to precipitation is rather complex. However, the increasing capacity of warming atmosphere to hold more water vapour potentially favours intense rainfall and hence more heavy rain spells of shorter duration are expected in the future. Long-term variations in precipitation and temperature are considered common indicators of climate change over a region. Extreme rainfall events that occur very rapidly over a locality are a consequence of what meteorologists call the instability of moist atmosphere. Attribution of single extreme events to anthropogenic climate change still remains a challenging topic. Changing climate leads to change in the frequency, intensity, spatial extent, duration and timing of extreme weather events and can result in unprecedented extreme weather over many vulnerable regions. For instance, the biggest cause of droughts and floods around the world is the shifting of climate patterns between El Nino and La Nina events. On land, El Nino events lead to drought (for example, Indian summer monsoon) in many tropical and subtropical areas, while La Nina events promote wetter conditions in many places, as has happened in recent years. These short-term and regional variations are expected to become more extreme in a warming climate.
What is happening to our monsoon?
The Indian summer monsoon is part of the Asian summer monsoon, the strongest monsoon system in the world, and shapes the health and wealth of one-sixth of the world’s population. Though there exists large inherent variability of 10-20 per cent across the Indian land mass, monsoon has been considered a stable and robust system in the past. The Indian summer monsoon is highly sensitive to global warming, and increases in anthropogenic GHG emissions could further amplify its impact on the distribution of rainfall and temperature. In response to the warming basic state, monsoon rainfall over the Indian region has been showing erratic behaviour, especially during the last two decades, over different meteorological subdivisions. Apart from seasonal extremes, extreme weather and heavy rainfall events are becoming more frequent over the Indian region. According to the majority of the IPCC’s (Intergovernmental Panel on Climate Change) Climate Model Intercomparison Project Phase 5 (CMIP5) models, rainfall over India will become more and more erratic in nature with a plausible decrease in overall rainfall and increase in extreme weather events. The magnitude of extreme rainfall events is predominantly controlled by the increased atmospheric moisture content while the reduction of cumulative seasonal rainfall results from the weakening of the air flow in response to the reduced sea-land thermal contrast. Under the warming mean basic state, longer dry spells and shorter sudden heavy spells are increasing during monsoon season almost all over India.
Realised changes in the climate over Kerala
Studying long-term changes and associated extreme hydrological events over a small region like Kerala, a southern peninsular Indian State, is challenging because of the large heterogeneity in the complex terrain bounded by the Arabian Sea on the west and the Western Ghats on the east. Across the small average width of 100 kilometres in the east-west direction over the complex terrain of Kerala, there exists lots of heterogeneity in the land surface and land use, land cover characteristics. Hence, inadequate high-resolution observational network makes it difficult to quantify realised changes in the climate over Kerala. Based on the altitude, the terrain is mainly divided into three categories, namely low land, mid land and high land. The unique annual and seasonal distribution of rainfall across Kerala and along the Western Ghats region in particular is mainly accountable for the exceptional repository of a variety of flora and fauna. Kerala is known as “God’s own country” because of this unique climate, land cover and biodiversity.
Realised changes in the climate over Kerala received more attention during recent times as evident from back–to-back drought conditions in 2015 and 2016, and then in 2017, hit by the first-ever cyclonic storm “Ockhi” though the main damage occurred at sea, and followed by the historic flood episodes during the 2018 monsoon season which claimed more than 500 lives. After a sluggish start of monsoon this year and associated drought conditions, now we are facing havoc because of extreme rainfall.
Kerala witnessed one of the most distressing droughts in 2016 which posed a severe threat to both agriculture and hydrology. It is also realised that the frequency of drought years has been increasing over Kerala in recent decades. Each deficit rainfall year is unique in the sense that its impact on agriculture, hydrology and socioeconomic sector does vary. Though the southwest monsoon in 2015 was bad for Kerala, the annual water stress and severity of drought in 2015 did not scale up to the level of 2016. Interestingly, all India summer monsoon rainfall (ISMR) was deficit in 2015, and Kerala also received deficit rainfall during the summer monsoon season. Contrary to that, Kerala received deficit summer monsoon rainfall during the 2016 season even when ISMR was normal. This clearly exhibits the large spatial heterogeneity in the rainfall distribution during the monsoon season across the country and its inherent inter-annual variability. Agricultural operations over Kerala are mostly rain-fed and around 30 per cent of the agricultural field is irrigated.
Warmer atmosphere provides high potential for intense storms and similar severe weather events. Consistent with theoretical expectations, heavy rainfall (which increases the risk of flooding) and heat waves are generally becoming more frequent all over the globe, and Kerala is no longer an exception. The impact of climate change on cyclone frequency remains a subject of ongoing studies. While changes in cyclone frequency remain uncertain, basic physical understanding and model results suggest that the strongest cyclones (when they occur) are likely to become more intense and possibly larger in a warmer, moister atmosphere over the oceans. The north Indian Ocean, including the Arabian Sea and the Bay of Bengal, is warming at a higher rate (of about 1.1 0C/116 years) compared with other ocean basins such as the Atlantic Ocean and the Pacific Ocean. As a result, the number of cyclones and their intensity are expected to increase in future in the Arabian Sea. A warming ocean along with ocean acidification and hypoxic regions are increasing at a faster rate and that is going to affect fish abundance at sea and hence the lives and livelihoods of coastal communities.
Some conditions favourable for strong thunderstorms that spawn hails and lightning are expected to increase with warming, but uncertainty exists in other factors that affect tornado formation, such as changes in the vertical and horizontal variations of winds. However, such types of vertically developing deep convective clouds are rare during the southwest monsoon season over Kerala. There have been many reports of increasing incidence of lightning and small-scale tornado-like swirling winds and water spout in recent years. During the extreme rainfall events of the 2019 monsoon season, there have been reports of tornado-like rotating updraft winds and water spout from some regions. This also signals that, along with rainfall pattern and distribution, cloud structure is also undergoing drastic changes. In places of shallow clouds (< 7 km), which are usually found to occur during a monsoon season along the Western Ghats region, in this season clouds vertically grew up to a height of 12-14 km and produced lightning and thunder. Owing to this, cloud drops are getting bigger in size. This type of towering convective clouds extending from 1 to 14 km can hold more water in its vertical depth and act like an overhead “water tank”. When they discharge as rain, they have huge potential to produce extremely heavy rain within a short duration. It is therefore important to look into the presence of such deep convective clouds during the extremely heavy rainfall events of 2018 and 2019.
Similarities and differences in extreme rainfall events over Kerala in 2018 and 2019
Compared with the early arrival of monsoon on May 29, 2018, monsoon onset occurred over Kerala late by about one week, on June 8, 2019, as against the normal onset date of June 1. Then the presence of the very severe cyclonic storm “Vayu” during June 10-17 drew all moisture towards it and literally paused the northward propagation of monsoon surge beyond Kerala. After the dissipation of Vayu, monsoon gained strength and reached central Maharashtra by June 24 as against the normal onset date of June 10. By July 31, 2019, rainfall deficit over Kerala loomed around -31.5 per cent. On the other hand, July 31, 2018, ended up with an excess rainfall of 18 per cent. By August 18, 2018, monsoon rain over Kerala became large excess by about 42 per cent. On the other hand, by August 18, 2019, rainfall deficit over Kerala improved from a rain deficit of -32 per cent to 0.9 per cent above average. However, when we compare the quantum of rainfall during the extremely active spells, the total rainfall received during August 7-11, 2019, was around 477 millimetres as against the normal of 78 mm during the period with an excess of 613 per cent during the period. However, during the extremely heavy spell of August 13-17, 2018, Kerala received 431 mm of rainfall as against its normal value of 73, and excess rainfall was around 494 per cent. Hence, in terms of the quantum of rainfall received and departure from normal, the 2019 heavy rainfall event received more rain compared with the 2018 heavy rain event.
We can now briefly examine what the common features and differences between extreme rainfall events over Kerala in August 2018 and 2019 are. The lower-level wind and circulation pattern reveals that the extended monsoon trough which runs from Pakistan to head Bay of Bengal was active during the period and there was the presence of a monsoon depression in head Bay of Bengal during both the events. There were typhoons over the Western Pacific during both the periods. Hence, the presence of a depression over the Bay of Bengal, which moved north-westwards along the monsoon trough, and the presence of typhoons in the Western Pacific together helped in strengthening the low-level jet stream off the west coast and made it to blow perpendicular to the Western Ghats. Hence we can assume that since these two features were almost similar in both the years, there must be some other factor that might have contributed to the more intense rain spell during August 7-11, 2019, compared with August 13-17, 2018.
There are some who believe that a warmer Arabian Sea is mostly responsible for this extreme spell. Let us look into it in detail. Sea Surface Temperature was abnormally high (Positive SST Anomaly: Red colour) over the Arabian Sea and the Equatorial Indian Ocean during both June and July of 2019. If this is the reason for the extreme rainfall events over Kerala during August 7-11, 2019, then naturally a question arises as to why June and July had not seen any such heavy rainfall activity. It is true that warmer SST may lead to the development of intense tropical cyclones as the energy supply for tropical cyclones is mostly from warmer ocean surface. However, there exists no one-to-one relationship between SST over ocean and land precipitation. Rainfall over the Indian land region during the monsoon season is mostly controlled by the strong south-westerly jet stream known as the Somali jet, which helps in more evaporation over the ocean surface, and the moisture-laden air is transported to the Indian subcontinent by the Somali jet.
Hence warmer SST in the Arabian Sea and the Equatorial Indian Ocean may not be the sole reason for the extreme rainfall event over Kerala. During the 2018 extreme rainfall events, the Arabian Sea and the Equatorial Indian Ocean were cooler than in 2019, and still we got many extremely heavy rain spells during the 2018 monsoon season; the quantum of rainfall received during the entire season was much higher than that received during 2019 when SST in June and July of 2019 was much higher than that in 2018. Other scientific results also suggest that it is not the absolute value of SST but its spatial inhomogeneity (in other words, spatial SST gradient) that is actually driving convection (rainfall). Hence, when the entire ocean basin is warmer than normal, that may not help in producing convection. There is also scientific evidence to suggest that there exists a critical value of SST, beyond which convection and cloud formation are found to be decreasing with increasing SST. At the same time, a warmer ocean supports more evaporation. Hence, it may be assumed that a warmer SST alone is not responsible for causing intense downpour. But of course, it can be one of several contributing factors for the extreme rainfall event over Kerala during August 7-11, 2019.
Satellite observations of the cloud top height in terms of cloud top temperature indicate the presence of vertically developing deep convective clouds during August 7-11, 2019, with Infra-Red Brightness temperature as low as below 200 Kelvin (the lower the brightness temperature, the higher the height of the clouds). The cloud ice optical thickness derived from satellites also confirms the presence of ice in the clouds. Ice inside the cloud will only form when the clouds grow deeper, above the height of 5 km (close to 0 degree isotherm), and the presence of ice favours the occurrence of lightning and occasional incidence of small tornadoes under favourable wind conditions. As discussed earlier, the structure of clouds also seems to have undergone drastic changes during recent years and these vertically developing deep convective towers might have contributed to intense rain spells over north Kerala. The spatial distribution of high resolution rainfall observations during August 7-11, 2019, also confirms intense rainfall of greater than 600 mm over north Kerala along with lower cloud top temperature and presence of ice. These pieces of scientific evidence suggest that it is possibly some unusual cloudburst event that occurred over these places during one of those days of heavy rain. However, such type of deep convective cloud development remained totally absent during August 13-17, 2018. As expected, the changes in the rainfall pattern along with structural changes in the cloud properties are a consequence of climate change induced by global warming. We are now on the path towards more such short-duration extreme rainfall events with decreasing number of rainy days and prolonged breaks. That is going to become the new norm for Kerala and India as a whole, and the same region may experience havoc from floods and droughts sometimes within a season. Managing a flood while dealing with a drought and vice versa is challenging.
We may have to deal with the frequent incidence of both ends of extreme weather events in future. Along with changes in the spatial and temporal distribution of rainfall, seasonality is also found to be changing in recent decades. In recent decades, monthly rainfall distribution over Kerala has been showing slight reduction in the June rainfall and an increase in the rainfall during the second half of the southwest monsoon season in August and September. Accordingly, many corrections are to be done in agriculture, water resources management and disaster management. In recent decades, ‘the stability of the monsoon system has been found to be decreasing and the monsoon will become more chaotic in nature with increased year-to-year (inter-annual) variability along with increased intra-seasonal (within season) variability. Though, not much change is expected for the seasonal mean monsoon rainfall, the variability on shorter time-scale manifested as short duration floods or prolonged break spells. The increased variability and uncertainty driven by global warming have made monsoon climate more unpredictable in the future.
Dr Abhilash S. is Associate Director, Advanced Centre for Atmospheric Radar Research (ACARR), Cochin University of Science and Technology.