Context:
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.
About SAM:
- It refers to the north-south movement of Southern Westerly Winds over timescales of 10s to 100s of years.
- Known as Antarctic oscillations because – they blow almost continuously in the mid- to high-latitudes of the southern hemisphere.
- Difference in the zonal mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica).
- Importance –
- Shapes the wave patterns over extended time frames.
- Drives climate by influencing rainfall and temperature.
- Identifying the fair Sea state windows in monsoons that can be utilised by oil and shipping industries
- Helps in better coastal planning, resource management and also in disaster preparedness
Phases of SAM:
- Positive SAM phase –
- Anomalous air pressure: lower over Antarctica and higher over the mid-latitudes.
- Increased wave activity is witnessed in Indian Ocean
- New swell generation (Swell waves) region along the east African coast increases wave height in the Arabian Sea.
- Negative SAM phase –
- Southern Westerly Winds expands northwards towards equator, bringing cold, wet weather to Patagonia
- Decreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf.
- Eastern tropical southern Indian Ocean becomes the main region for generating swells, resulting in reduced wave heights in the Arabian Sea.
Swell waves:
- They are the regular, longer period waves which are self-sustaining and generated by energy beneath the ocean’s surface, no longer needing local wind.
- 2 crucial swell generation regions in the Indian Ocean –
- Extratropical Southern Indian Ocean (ETSI)
- Tropical Southern Indian Ocean (TSIO)
Source: The Hindu
Previous Year Question
With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?
1. IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.
2. An IOD phenomenon can influence an EI Nino’s impact on the monsoon.
Select the correct answer using the code given below:
[UPSC Civil Services Exam – 2017 Prelims]
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Answer: (b)
