According to the Aridity Anomaly Outlook Index for July, issued by the India Meteorological Department (IMD) this year, at least 85% of districts faced arid conditions across India. Also, around 21.06 percent of India was facing drought-like conditions, according to the Drought Early Warning System. Aridity is defined, in meteorology and climatology, as the degree to which a climate lacks effective, life-promoting moisture. Drought is a period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance.

The differences between the two include:

• Difference in measurement: Aridity is measured by comparing long-term average water supply (precipitation) to long-term average water demand (evapotranspiration). If demand is greater than supply, on average, then the climate is arid. Drought refers to the moisture balance that happens on a month-to-month (or more frequent) basis. If the water supply is less than water demand for a given month, then that month is abnormally dry and if there is a serious hydrological impact, then a drought is occurring that month.
• Difference with respect to time scale: Drought is a recurrent and temporary aberration, unlike aridity, which is a permanent feature of climate.

Multi-dimensional impacts of droughts are the following:

• Water supply: During droughts, communities may have limited access to water for household use, including drinking, cooking, cleaning, etc. Further, it affects transportation and power generation.
• Agriculture: Droughts affect livestock and crops thereby having a devastating effect on farming and food production, which contributes to food price instability. In countries already facing food insecurity, cost spikes can lead to social unrest, migration, and famine.
• Energy: Droughts can raise concerns about the reliability of electricity production from plants that require cooling water to maintain safe operations. Hydroelectric power may also become unavailable during droughts. Further, when heat waves coincide with droughts, electricity demands can grow, compounding stress on the grid.
• Public health: Reduced flows in rivers and streams due to droughts can lead to a concentration of pollutants, thereby threatening the quality of water used for drinking and recreation. Also, drought-fuelled wildfires can expose nearby communities to smoke and pollutants, which can exacerbate chronic respiratory illnesses.
• Social impacts: Due to frequent droughts, there can be outmigration of the population from drought-affected areas leading to greater indebtedness, alienation from land and livestock assets, malnutrition, starvation, etc. There needs to be regular monitoring of droughts in states by setting up Drought Monitoring Centres (DMCs), which will be staffed by a multi-disciplinary team of meteorologists, hydrologists, and agriculture scientists. Also, state governments and businesses need to identify their vulnerability to droughts and improve resilience by practicing and promoting water conservation and enhancing water efficiency.

The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents, like a conveyor belt, driven by differences in temperature and salinity. It is a thermocline circulation that carries warm surface waters from the tropics towards the Northern Hemisphere, where it cools and sinks. It then returns to the tropics and then to the South Atlantic as a bottom current. From there it is distributed to all ocean basins via the Antarctic circumpolar current. This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the earth. However, the Intergovernmental Panel on Climate Change (IPCC) in its recent report highlighted that AMOC is losing its stability and is very likely to decline over the 21st century due to the following reasons:

• Melting of Arctic ice sheets: Global warming has led to an increase in freshwater due to the melting of Greenland and Arctic ice sheets. It can make circulation weaker as it reduces the salinity and density of the water, making it unable to sink to the bottom.
• Weakening of the Gulf Stream: Anthropogenic factors such as global warming have led to a weakening of the Gulf Stream, which has further led to instability and decline of AMOC.
• Increasing precipitation and river run-off: It can change the salinity and density of the ocean water, leading to unstable or not-so-strong AMOC circulation.

Impact of the Decline of AMOC

• Change in regional climate: The weakening of AMOC and Gulf Stream will trigger a cooling effect on climate and a decrease in rainfall over the North Atlantic region, as the northward heat supply is slowed down. It may also lead to an increase in winter storms over Europe and stronger hurricanes in the US.
• Sea level rise: The northward surface flow of the AMOC leads to the deflection of water masses to the right, away from the US east coast. As the current slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise.
• Severe consequences for Atlantic marine ecosystems: The North Atlantic ecosystem is adapted to the existence of the overturning circulation, which sets the conditions like the seasonal cycle, the temperature, and the nutrient conditions. Any changes in these conditions would disrupt fish populations and other marine life.
• Other impacts: The collapse of the AMOC may induce changes in ENSO (El Nino-Southern Oscillation) characteristics, dieback of the Amazon rainforest, and shrinking of the West Antarctic Ice Sheet due to the seesaw effect, etc.

There is a need to reconcile climate models with the presented observational evidence to assess how far or how close the AMOC really is to its critical threshold. Further, there is an urgent need to ensure the effective implementation of environmental commitments under the Paris climate deal by every country to address climate change and slow down the weakening of AMOC.

The relative position of the Earth with respect to the sun changes within a year due to Earth’s revolution. Further, due to the inclination of the Earth on its axis, there are differences in the heating of the continents and oceans, and as a result, the pressure conditions in January and July vary greatly. This consequently results in the shifting of the wind belts.

Shifting of pressure and wind belts:

• Summer and Winter Solstice:
  • The sun during the summer solstice is vertical over the Tropic of Cancer and therefore, all the pressure belts shift northward, including the Equatorial Low-Pressure Belt, and Subtropical High-Pressure Belt except the Northern Polar High-Pressure Belt. The wind belts associated with the said pressure belts also shift northward.
  • Similarly, during the winter solstice, the pressure and wind belts shift southward except the Southern Polar High-Pressure Belt.
• Autumnal and Vernal Equinox: The sun becomes vertical over the Equator and hence all the pressure belts occupy their normal positions.

These seasonal changes in the relative positions of the wind belts introduce the following typical climatic conditions: (i) The Mediterranean climatic regions are found in the western parts of the continents within the latitudinal zone of 30°-45° in both hemispheres. The Sub-Tropical High-Pressure Belts extending between 30°-35° latitudes are characterized by dry trade winds during the summer season and anti-cyclonic conditions. This belt extends up to 40° latitudes in the Northern hemisphere at the time of summer solstice and in the Southern hemisphere at the time of winter solstice. Thus, the western parts of the continents within the zone of 30°-40° latitudes do not receive rainfall during the summer season. On the other hand, the Sub-Tropical Belt shifts towards the Equator at the time of winter solstice in the Northern hemisphere and at the time of summer solstice in the Southern hemisphere. Consequently, the zone is characterized by the Westerlies, which lead to precipitation during the winter season. The Mediterranean type of climate is thus characterized by dry summers and wet winters. (ii) The regions lying between 60°-70° latitudes are characterized by two types of winds in a year. With the northward migration of the sun at the time of summer solstice, the Polar Easterlies are weakened because the Westerlies extend over these areas due to the northward shifting of Sub-Polar Low-Pressure Belts. The situation is reversed at the time of winter solstice when there is southward migration of the sun. The Polar Easterlies are re-established between 60°-70°N because of the shifting of the belt of the Westerlies southward. Consequently, it creates a climate characterized by wet summers through the Westerlies and associated cyclones and dry winters due to Polar Easterlies. (iii) Monsoon climate is also the result of the shifting of pressure and wind belts. Due to the northward migration of the sun in the Northern hemisphere at the time of summer solstice, the North Inter-Tropical Convergence Zone (NITCZ) is extended up to 30°N latitude over the Indian subcontinent, Southeast Asia, and parts of Africa. Thus, the Equatorial Westerlies are also extended over the aforesaid regions, which become the southwest or summer monsoons. These southwest monsoon winds bring much rain because they come from over the ocean and are associated with tropical cyclones. The NITCZ is withdrawn from the Indian subcontinent and Southeast Asia because of the southward shifting of pressure and wind belts due to the southward migration of the sun at the time of winter solstice. Thus, north-east trades are re-established which leads to the north-east or winter monsoons. Since they come from over the lands, they are dry.