Syllabus Section: Climate Issues (GS Paper I)

Importance: UPSC Prelims and UPSC Mains

Why in News?

North India is experiencing a severe cold wave, with temperatures plunging below freezing levels in some areas.

What is the Cold Wave?

• According to the National Disaster Management Authority (NDMA), “is distinguished by a well-defined and protracted period of lower temperatures" and is an extreme weather phenomenon that prominently occurs between the months of November and February. 
• A ‘severe cold day’ occurs when the maximum decreases by 6.5 degrees or above the normal.
• Cold waves are linked to "transient disturbances in the mid-latitude westerlies" in the northern sections of India, particularly the hilly regions and adjacent plains,

What causes India's cold waves?

The reasons are:

1) A build-up of a ridge, or a protracted area of relatively high atmospheric pressure, in the jet stream across northwest Asia is one of the causes of cold waves in India.

• Jet streams are narrow bands of high wind that travel across the globe from west to east. A triggering mechanism "like a strong westerly breeze approaching northwest India".

2) The vast snow cover in the northwest Himalayas also contributes to the formation of a cold wave.

• Experts claim that during the winter months, the passage of western disturbances causes "dry, cold north-westerly winds" to blow into north and central India, causing minimum temperatures to plummet over these areas, resulting in cold-wave conditions.

3) According to studies, a western disturbance occurs when moisture-laden winds replace chilly and dry north-westerly winds with warm and wet easterly breezes.

• Harsh winter as bitter winds blow in from the northwest, indicates that the La Nina effect is lowering temperatures. 

What Is La Nina?

La Nina," which means "the girl" in Spanish, refers to the winds that blow over the Pacific Ocean and cause changes that affect people all around the world.

La Nina, the cooling phase of the phenomena known as El Nino Southern Oscillation (ENSO), causes stronger winds above the Pacific Ocean, causing more warm ocean water to be pushed west toward Indonesia than usual, resulting in more cold water coming to the surface near South America.

This modest difference in ocean temperatures is enough to cause a global change in weather.

Consequences of La- Nina:

• Below-normal temperatures in the northern hemisphere.
• La Nina can cause an increase in moisture over the Indian subcontinent, resulting in an abundance of snowfall and rainfall.

Source: Indian Express