Sea Level Variability and Trends in the North Indian Ocean
Anant Parekh, C. Gnanaseelan, J.S. Deepa, Ananya Karmakar and J.S. Chowdary
Sea level is a crucial oceanic parameter which shows variability on different timescales including the long-term trends. The reported increasing trend in sea level has very strong socioeconomic consequences as it inundates to the surrounding coastal area (e.g., Nicholls et al. 2007). Sea level rise depends on the following processes: (1) thermal expansion due to temperature rise (which is known as thermosteric effect), (2) salinity change (halosteric effect) (temperature- and salinity-induced ocean volume change results in the sea level change and is known as “steric change”), (3) melting of sea ice, (4) terrestrial water storage (different components of water reservoir on the Earth such as rivers, lakes, ice sheets, glaciers, and precipitation) exchanges water mass with oceans, they modify the volume of the ocean water time to time, which is known as a “eustatic” and (5) sediment deposition at the coast. The role of above processes to the sea level change is highly uncertain due to the lack of information on contributions by the natural and anthropogenic activities. Church et al. (2001) using global coastal tide gauge observations reported that sea level rise during the twentieth century was
1.5 ± 0.5 mm/day. In the past, several attempts were made to estimate sea level changes by incorporating different processes. The resultant estimate of total sea level change was 0.7 ± 1.5 mm/day, suggesting more uncertainty in such computations (Fig. 1). Thus, it is easy to observe the sea level change from observations, but it is difficult to quantify the role of individual processes to the resultant sea level change, probably due to the existence of some unknown processes. Several studies reported a sea level rise of 3.2 ± 0.4 mm/year during 1993-2009 (e.g., Church and White 2011) using satellite observations, much of which is due to steric effect (Cabanes et al. 2001b). This recent trend is much higher than the
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M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and Change Over the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_11
Fig. 1 Contribution of different component to sea level rise and level of uncertainty for individual component estimation (adopted from Cazenave and Nerem 2004)
twentieth-century trend. Southern subtropical Indian Ocean displayed stronger sea level rise compared to the northern counterpart (Cabanes et al. 2001a). It is important to note that oceans absorb about 80 % of the heat supplied to the climate system, leading to the upper ocean (up to 3000 m depth) temperature rise. The global warming induced melting of glacier and sea ice caps also contribute to the sea level change. Satellite data confirms that the ice cover of the Greenland and polar region displays decreasing trend, suggesting its possible role on sea level rise.
Sea level does influence the ecosystems and humans in many ways. The inundation of coastal area due to the sea level change is closely related to the slope of the beach as well. It is estimated that sea level rise to beach inundation ratio is about hundred, and hence, sea level rise of 1 cm can contribute to the inundation by 1 m. A sea level rise also affects freshwater availability to the coastal and island population as the freshwater sources may get contaminated. Also, the increase in sea level and the associated vertical rise in water column result in waterlogging, thereby destroying the coastal ecology such as mangroves and dependent biota. Sea level rise accelerates the impact of flooding and cyclone-driven storm surges and giant waves. Sea level rise is dramatically changing the physical character of the low-lying river deltas, for example, the Ganges, Mississippi, Nile, and numerous others. Sea level rise generally creates fears of possible submerge of islands and low-lying coastal cities, which invites the international attention. The present chapter provides the current status and knowledge of Indian Ocean sea level change and variability. Further, it includes a brief description on future projections of sea level changes over the Indian Ocean.