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Why does the Indian Ocean rise and fall?

May 13, 2017 05:18 pm | Updated 05:21 pm IST

How is global warming affecting oceans?

There are two broad mechanisms at work. Heat trapped in the atmosphere due to rising sea levels makes water expand and separately, melting ice sheets begin to add water to the world’s oceans. Were you to peruse NASA’s satellite data on the average rise and fall in sea levels, it shows that the seas on average have risen 85 mm since 1993, adding about 3.5 mm annually.

Why is the Indian Ocean peculiar?

Since 2004, it has been known that the Indian Ocean has been rising particularly rapidly. However, it turned out that this was specific to a smaller stretch called the North Indian Ocean, which consists of the Bay of Bengal, the Arabian Sea and a large part of the Indian Ocean until the 5 degree S latitude. This is an imaginary line cutting through Indonesia, central Africa and Peru. More surprisingly, as a team of oceanographers observed in a report published in the March edition of the peer-reviewed Climate Dynamics , the North Indian Ocean sea levels actually dipped between 1993 and 2004, at about 0.3 mm per year, but after 2004, the rise was 6 mm annually. Such a fluctuating trend hasn’t been observed for the Pacific Ocean and the Atlantic Ocean.

Why did this happen?

Unlike the Pacific Ocean and the Atlantic Ocean, the North Indian Ocean is hemmed in on all sides, except an outlet on the southern side. This influences the rate at which heat is absorbed and flushed out from within the system. According to their calculations, heat was moving out slower after 2004 than during the 1990s. Moreover, wind flows, which led to warm water welling up on the Indian Ocean surface, changed directions every decade and probably influenced sea level patterns.

What does this imply?

This means a rise in average global temperature doesn’t mean a concurrent rise in sea levels everywhere. Every year in the last decade has broken temperature records that have held for over a century but researchers associated with this study are willing to wager that North Indian Ocean levels may see a fall over the next decade (like seen between 1993 and 2004). This points to a need for more research to understand the inherent variability of the Indian Ocean. This could help sharpen monsoon forecasts and predicting coastal erosion patterns. Better understanding of sea level undulations could also inform future reports by the Intergovernmental Panel on Climate Change.

Does this challenge conventional science?

It doesn’t challenge it but certainly complicates it. Researchers use various models to extrapolate future trends on sea level rise and quantify the risk it poses to coastal populations. Several of these model, however, lack the resolution power to capture the vagaries of local climate and it is assumed that what is true for one sea will broadly apply to the others too. For this study the scientists relied on new data sources--from argo floats and satellite-based measurements — and it indicated numbers at variance from previous measurements, from tide gauges. More micro-level data with improved computing power would mean better local-level forecasts.

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