What is Southern Annular Mode (SAM)?

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What is Southern Annular Mode (SAM)? Blog Image


Recently, the Indian National Centre for Ocean Information Services (INCOIS) has discovered that the Southern Annular Mode (SAM), an important climate pattern, plays a pivotal role in shaping the sea conditions across the Indian Ocean.

About Southern Annular Mode

  • It is the north-south movement of Southern Westerly Winds over timescales of 10s to 100s of years. 
  • It is also known as the Antarctic Oscillation.
  • They blow almost continuously in the mid- to high-latitudes of the southern hemisphere.
  • It is usually defined as the difference in the zonal mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica).
  • Positive Southern Annular Mode
    • In this mode, there is lower anomalous air pressure over Antarctica and higher anomalous air pressure over the mid-latitudes.
    • The belt of strong westerly winds strengthens and contracts towards Antarctica.
  • Negative Southern Annular Mode
    • In this mode the belt of strong Southern Westerly Winds expands northwards towards the equator, bringing cold, wet weather to Patagonia and glacier advance, and decreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf. 
  • Significance
    • The study contributes to advancing the accuracy of wave predictions and can help identify fair weather windows.
    • It will benefit the fisher folk community and blue economy stake holders such as shipping, maritime boards and oil industry for their multi-million dollars worth operations at sea.

Q1) What are westerlies?

The westerly winds, also known as the westerlies, occur at two regions on Earth: between 30 and 60 degrees latitude in the northern hemisphere and between 30 and 60 degrees latitude in the southern hemisphere. The name of these unique winds comes from the direction of their origin; the westerlies run west to east while other winds run east to west.

Source: INCOIS ‘SAM’ study to understand fair weather patterns on seas for maritime activities