• Western Disturbances are weather systems that move eastward and bring rainfall. They develop in regions west of India, mainly around Afghanistan and Iran, and gain moisture from water bodies such as the Mediterranean Sea, Black Sea, Caspian Sea, and Arabian Sea.
• These systems are generally extra-tropical cyclones formed due to the interaction between cold polar winds and warm tropical winds, creating low-pressure conditions.
• They are carried by the subtropical westerly jet stream, a fast-flowing, high-altitude air current that travels from west to east above the Himalayan and Tibetan regions.
• Western Disturbances occur most frequently during the winter season in the Northern Hemisphere, especially from December to March, and significantly affect the weather of northwestern India along with neighbouring countries like Pakistan, Afghanistan, and Tajikistan

3. El Niño Southern Oscillation (ENSO)

• The El Niño–Southern Oscillation (ENSO) is a major climatic process characterised by variations in sea surface temperatures in the eastern Pacific Ocean along with associated changes in atmospheric conditions above it.
• These shifts can disturb global atmospheric circulation patterns and thereby affect weather conditions across different parts of the world. ENSO has three phases — the warm phase known as El Niño, the cool phase called La Niña, and a neutral phase — and generally recurs at irregular intervals ranging from two to seven years.
• Under neutral conditions, the eastern Pacific Ocean near the western coast of South America remains cooler compared to the western Pacific around Indonesia and the Philippines.
• This temperature difference is maintained by easterly trade winds that push warm surface waters westward, allowing colder water from deeper layers to rise to the surface in the eastern Pacific.
• During an El Niño event, the eastern Pacific experiences abnormal warming of surface waters. This weakens the moisture-laden winds reaching India and often leads to a weaker or delayed monsoon, causing dry conditions in several agricultural regions.
• Studies have also linked El Niño with an increased occurrence and intensity of heatwaves in India.
• In April, the U.S. Climate Prediction Center estimated a 61% probability of El Niño developing between May and July and expected it to continue through the year.
• La Niña represents the reverse situation, where the eastern Pacific surface waters become cooler than usual.
• This strengthens the moisture-carrying winds towards India and generally enhances the southwest monsoon. However, in severe cases, the resulting heavy rainfall can trigger floods and damage crops

A heat wave refers to a prolonged spell of abnormally high temperatures that are significantly above the average climatic conditions of a particular region. Since normal temperature levels differ across regions based on historical weather patterns, the definition of a heat wave also varies accordingly. Its intensity can further increase due to factors such as high humidity, strong winds, and the length of the hot spell.

According to the criteria used by the India Meteorological Department, a heat wave may be identified when the maximum temperature reaches at least 40°C in plain areas and 30°C in hilly regions. After this threshold is crossed, a heat wave can be officially declared on two main bases:

• Departure from normal temperature:
  If the temperature is 4.5°C to 6.4°C above the seasonal average, it is classified as a heat wave. When the temperature exceeds this range, it may be categorised as a severe heat wave.
• Absolute maximum temperature:
  A heat wave is automatically declared when the maximum temperature touches or surpasses 45°C. If the temperature reaches 47°C or more, it is considered a severe heat wave.

• The temperature commonly recorded using a standard thermometer is known as the Dry Bulb Temperature. It represents the actual air temperature but does not take atmospheric moisture into consideration.
• In contrast, Wet Bulb Temperature measures the lowest temperature that air can achieve through the evaporation of water, thereby indicating the efficiency of evaporative cooling under humid conditions.
• Generally, the Wet Bulb Temperature remains lower than the Dry Bulb Temperature, and the gap between the two becomes larger when the air is drier.
• According to the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change, continuous exposure to wet bulb temperatures above 35°C can be life-threatening.
• Even temperatures exceeding 31°C can pose serious risks during strenuous physical activity. Individuals with existing health conditions such as diabetes or cardiovascular diseases are particularly vulnerable, while outdoor workers and labourers face greater danger because of dehydration and prolonged heat exposure.
• At the same time, the AR6 notes that the possibility of such extreme wet bulb temperatures persisting for more than a few hours remains very low

• The frequent occurrence of heat waves also adversely affects different sectors of the economy.
• For instance, the livelihood of poor and marginal farmers is negatively impacted due to the loss of working days.
• Heatwaves also have an adverse impact on daily wage workers' productivity, impacting the economy.
• Crop yields suffer when temperatures exceed the ideal range.
• Farmers in Haryana, Punjab, and Uttar Pradesh have reported losses in their wheat crop in the past rabi season. Across India, wheat production could be down 6-7% due to heat waves.
• Mortality due to heat waves occurs because of rising temperatures, lack of public awareness programs, and inadequate long-term mitigation measures.
• According to a 2019 report by the Tata Center for Development and the University of Chicago, by 2100 annually, more than 1.5 million people will be likely to die due to extreme heat caused by climate change.
• The increased heat wave will lead to an increase in diseases like diabetes, circulatory and respiratory conditions, as well as mental health challenges.
• The concurrence of heat and drought events is causing crop production losses and tree mortality. The risks to health and food production will be made more severe by the sudden food production losses exacerbated by heat-induced labor productivity losses.
  These interacting impacts will increase food prices, reduce household incomes, and lead to malnutrition and climate-related deaths, especially in tropical regions.