Heat Waves
Extreme heat might not seem as dramatic as hurricanes or floods, but the National Weather Service has deemed it the deadliest weather phenomenon in the US over the past 30 years, on average.
India suffers from major heatwaves since March-June. The rising trend of the number of intense heatwaves in recent decades has been vaguely attributed to global warming. Since heat waves have a serious effect on human mortality, the root causes of these heatwaves need to be clarified. Based on the observed patterns and statistical analyses of the maximum temperature variability, we identified two types of heatwaves. The first type of heatwave over north-central India is found to be associated with blocking over the North Atlantic. The blocking over North Atlantic results in a cyclonic anomaly west of North Africa at upper levels. The stretching of vorticity generates a Rossby wave source of anomalous Rossby waves near the entrance of the African Jet. The resulting quasi-stationary Rossby wave-train along the Jet has a positive phase over the Indian subcontinent causing anomalous sinking motion and thereby heatwave conditions over India. On the other hand, the second type of heatwave over coastal eastern India is found to be due to the anomalous Matsuno-Gill response to the anomalous cooling in the Pacific. The Matsuno-Gill response is such that it generates northwesterly anomalies over the landmass reducing the land-sea breeze, resulting in heatwaves.
Heatwaves affect human comfort and are associated with marked short-term increases in mortality have shown the urban areas across the globe to have a significant increase in the number of heatwaves during the recent decades. The recent increasing trend in the frequency and intensity of heatwaves is often attributed to climate change. However, we need more detailed analyses because year-to-year variations are large. The European heatwave of August 2003 caused total deaths of about 35000 with more than 14,800 deaths in France alone. According to the report of the Japan Ministry of Environment, deaths due to heatstroke during the unusually hot summer in 2010 amounts to 1745 in Japan. Deaths were also reported in India due to heatwaves over the years.
The heatwave of 1988 caused an estimated number of 1300 deaths and likewise, the heatwaves of 1998 and 2003 caused deaths of about 2042 people 7 and 3054 people, respectively. According to EM-DAT, the international disaster database, the heatwave of 2015 caused the deaths of 2248 people in various parts of India.
The heatwaves over India are projected to be more intense and occur more frequently in the future. Climatologically heatwaves occur during March to June, with high frequency over north, northwest, central, and the eastern coastal regions of India. The heatwaves over India have been linked with the climate modes such as El Niño-southern oscillation (ENSO) and also to the variations in the sea surface temperatures in the Bay of Bengal. Some studies also linked the heatwaves to the re-curving tropical cyclones in the Bay of Bengal. The re-curving tropical cyclones before the onset of the heatwaves could change the direction of the winds and cut-off moisture to the inland regions leading to heatwaves. Despite the large societal impact, there has been no systematic attempt to understand the principal mechanism of heatwaves over India.
During the summer in the Northern Hemisphere, the northern half of the planet is tilted toward the sun, which increases daylight hours and warms the hemisphere. The impact of this additional exposure to solar radiation is cumulative, which is why temperatures generally peak weeks after the longest day of the year.
Heatwaves begin with a high-pressure system (also known as an anticyclone), where atmospheric pressure above an area builds up. That creates a sinking column of air that compresses, heats up, and oftentimes dries out. The sinking air acts as a cap or heat dome, trapping the latent heat already absorbed by the landscape. The high-pressure system also pushes out cooler, fast-moving air currents and squeezes clouds away, which gives the sun an unobstructed line of sight to the ground.
The ground — soil, sand, concrete, and asphalt — then bakes in the sunlight, and in the long days and short nights of summer, heat energy quickly accumulates and temperatures rise.
Heatwaves are especially common in areas that are already arid, like the desert Southwest, and at high altitudes where high-pressure systems readily form. Moisture in the ground can blunt the effects of heat, the way evaporating sweat can cool the body. But with so little water in the ground, in waterways, and in vegetation, there isn’t as much to soak up the heat besides the air itself.
Urban areas further exacerbate this warming. As roads, parking lots, and buildings cover natural landscapes, cities like Los Angeles and Dallas end up absorbing more heat than their surroundings and can become as much as 20 degrees Fahrenheit warmer. This is a phenomenon known as the urban heat island effect.
Eventually, the high-pressure system will start to weaken, allowing in cooler air and precipitation that can bring the heatwave to an end. However, as the warm season continues, more high-pressure systems can settle in and restart the heating process.
