Context
Heat waves on land and depressions in the sea this year have started early in and around India, most probably because of an unexpected climatic anomaly which could, in turn, be linked to global warming.
- The India Meteorological Department (IMD) declared the season’s first heat wave and severe heat wave March 11 and the first depression March 3.
What are Heat Waves?
- Heat waves are periods of unusually hot weather that can impact human health adversely.
- According to the India Meteorological Department (IMD), a region is classified as witnessing a heat wave when the maximum temperature reaches at least 40º C or more if it is located in the plains, or at 30º C or more in the hills.
- The IMD also declares a region to be experiencing a heat wave when its temperature departs from normal by 4.5º 6.4º C; if the departure from normal is more than 6.4º C, the region is said to be witnessing a ‘severe’ heat wave.
Reason behind the current phenomenon
- The reason behind early heat waves, early depressions and the weird dust storms is the continued persistence of a north-south low pressure pattern that forms over India during winters when a La Niña phenomenon is occurring in the equatorial Pacific Ocean.
- The sea surface temperatures over the east and central Pacific Ocean become cooler-than-average during La Niña.
- This affects the trade winds flowing over the ocean surface through change in wind stress.
- The trade winds carry this weather disturbance elsewhere and affect large parts of the world.
- In India, the phenomenon is mostly associated with wet and cold winters. Therefore, the current effect of La Niña is completely unexpected.
