Warm Arctic waves, La Niña to blame for early heat waves, depressions: Experts

Context

The India Meteorological Department (IMD) declared the season’s first heat wave and severe heat wave and the first depression in early march.

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.

About

What are heatwaves?

A heat wave is a period of unusually hot weather that typically lasts two or more days.
To be considered a heat wave, the temperatures have to be outside the historical averages for a given area.
The India Meteorological Department (IMD) defines a heatwave when the maximum difference in temperature is between 4.5 and 6 degrees.
- For example, if a locality's typical temperature is 40 degrees and the actual documented temperature is 45 degrees, the area is experiencing a heatwave.

If the highest temperature in an area or location hits or surpasses 40 degrees Celsius in the plains, or 30 degrees Celsius in hilly areas, it is regarded to be under the impact of a heatwave.
The highest temperature criterion for the coastal zones is 37 degrees.

Similarly, a severe heatwave is proclaimed when the maximum temperature recorded in a location deviates from normal by more than 6.4 degrees.

What are the reasons?

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 are air currents closer to Earth's surface that blow from east to west near the equator.

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.

La Niña’s effect on other regions of the world:

The temperatures over western Russia, down to Kazakhstan and over Pakistan, Afghanistan, all the way into Spain and Portugal were more than 10 degree Celsius warmer during February.

About La Niña:

La Nina is a climate pattern that describes the cooling of surface ocean water along the tropical west coast of South America.
Together, La Niña and El Niño are the "cold" (La Niña) and "warm" (El Niño) phases of the El Nino-Southern Oscillation (ENSO). ENSO is series of linked weather- and ocean-related phenomena.
Besides unusually warm or cool sea-surface temperatures, ENSO is also characterized by changes in atmospheric pressure.
La Niña events sometimes follow El Niño events, which occur at irregular intervals of about two to seven years.
- The local effects on weather caused by La Niña ("little girl" in Spanish) are generally the opposite of those associated with El Niño ("little boy" in Spanish).
La Niña is caused by a build-up of cooler-than-normal waters in the tropical Pacific, the area of the Pacific Ocean between the Tropic of Cancer and the Tropic of Capricorn.
Unusually strong, eastward-moving trade winds and ocean currents bring this cold water to the surface, a process known as upwelling.
Upwelling can cause a drastic drop in sea-surface temperature. Coastal sea-surface temperatures near Ecuador and Peru dropped nearly 4 degrees Celsius (7 degrees Fahrenheit) during the 1988-89 La Niña events.

Effects of La Niña

La Niña is characterized by lower-than-normal air pressure over the western Pacific.
- These low-pressure zones contribute to increased rainfall.
Rainfall associated with the summer monsoon in Southeast Asia tends to be greater than normal, especially in northwest India and Bangladesh.
- This generally benefits the Indian economy, which depends on the monsoon for agriculture and industry.
However, strong La Niña events are associated with catastrophic floods in northern Australia.
La Niña events are also associated with rainier-than-normal conditions are over south-eastern Africa and northern Brazil.
La Niña is also characterized by higher-than-normal pressure over the central and eastern Pacific.
- This results in decreased cloud production and rainfall in that region.
Drier-than-normal conditions are observed along the west coast of tropical South America, the Gulf Coast of the United States, and the pampas region of southern South America.