News Excerpt:

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

About Southern Annular Mode (SAM)

The SAM refers to the (non-seasonal) north-south movement of the strong westerly winds that blow almost continuously in the mid-to-high latitudes of the southern hemisphere. 

• This belt of westerly winds is also associated with storms and cold fronts moving west to east, bringing rainfall to southern Australia.

• These are also called Antarctic Oscillation.

• The SAM has three phases: neutral, positive and negative. 

• Each positive or negative SAM event lasts one to two weeks, though longer periods may also occur. 

• The time frame between positive and negative events is random but typically ranges from a week to a few months.

• The SAM's effect on rainfall varies greatly depending on season and region.

Phases of SAM

Positive Phase:

• band of westerly winds contracts toward Antarctica

• higher pressures over southern Australia

• can relate to stable, dry conditions.

Negative Phase:

• band of westerly winds expands towards the Equator

• more (or stronger) low-pressure systems over southern Australia

• can mean increased storms and rain.

Impact of SAM across the Indian Ocean

• SAM impact expands beyond swells, influencing the Northern Indian Ocean wave climate by altering wind seas through Hadley cell (HC) circulation shifts.

During a Positive SAM phase, 

• A cyclic pattern of warm sea surface temperature anomalies can be seen, and strong winds cause increased wave activity in the Indian Ocean. 

• Since the early 1990s, changes in the atmospheric fields have evoked anomalous vertical motion over the continent and the Indian Ocean, enhancing the southerly cross-equatorial flow by increased land–sea thermal contrast, thereby increasing decadal rainfall in the region.

• A new swell generation region along the East African coast caused an increase in wave height in the Arabian Sea.

During a Negative SAM phase, 

• The eastern tropical southern Indian Ocean becomes the central region for generating swells, resulting in reduced wave heights in the Arabian Sea.

Significance of the Study for Indian Ocean:

• It could help the country with better coastal planning, resource management, and disaster preparedness.

• It can also contribute to advancing the accuracy of wave predictions.

• It can help identify fair weather windows, thus benefitting the fisherfolk community and blue economy stakeholders such as shipping, maritime boards, and the oil industry for their multi-million dollars' worth of operations at sea.

• • Inland vessel operations and oil exploration activities have a blanket ban during the monsoon season.

  • One can identify ‘Fair Sea state windows’ in monsoons by predicting the SAM phases in the Ocean Forecasting System.

  • Oil and shipping industries can utilise these fair windows for their operations, thus significantly impacting the blue economy activities on the Indian coast.

• It is important to study ocean surface waves as they are crucial in shaping coastal processes and influencing numerous aspects of society, including shoreline erosion, sediment transport, coastal engineering, and recreational activities.

Conclusion:

Extreme wave events in the Indian Ocean can significantly impact coastal livelihoods, infrastructure, and ocean activities. Understanding Southern Annular Mode can help forecast fair weather windows, benefiting maritime trade and fishermen. Accurate forecasting of future wave amplitude is crucial for timely warning and coastal planning.