Southern Annular Mode (SAM)

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

SAM – It refers to the north-south movement of Southern Westerly Winds over timescales of 10s to 100s of years.
They blow almost continuously in the mid- to high-latitudes of the southern hemisphere and so called as Antarctic oscillations.
It is usually defined as the difference in the zonal mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica).
Importance – It shapes the wave patterns over extended time frames and a climate driver by influencing rainfall and temperature.

Swell waves are the regular, longer period waves which are self-sustaining and generated by energy beneath the ocean’s surface, no longer needing local wind. The 2 crucial swell generation regions in the Indian Ocean are the extratropical southern Indian Ocean (ETSI) and the tropical southern Indian Ocean (TSIO).

Positive SAM phase	Negative SAM phase
There is lower anomalous air pressure over Antarctica, and higher anomalous air pressure over the mid-latitudes.	The Southern Westerly Winds expands northwards towards equator, bringing cold, wet weather to Patagonia and decreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf.
Increased wave activity is witnessed in Indian Ocean with increased wave height in the Arabian Sea.	Reduced wave heights are witnessed in the Arabian Sea.

Source:

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