I moved from the beautiful coast of Anse a La Mouche to the foothills of the Trois Freres Mountain which overlooks the Seychelles’ capital city Victoria because I thought sea level rise and other climatic events would flood the area. Now I am told that that there are no sea level rises around the Seychelles. In fact, since the 1960s there have been substantial decreases in sea levels in the south tropical region of the Indian Ocean including the Seychelles and Zanzibar according to a study just released.The study was published this week in the journal Nature Geoscience by scientists at the University of Colorado at Boulder and the National Center for Atmospheric Research in the United States. Sea levels have risen across the world as a result of the ocean water expanding as it heats up and as melting ice adds more water volume. However, the rises are not uniform across the world and are affected by changes in atmospheric or oceanic currents. The study combined direct sea surface measurements as well as satellite observations of Indian Ocean sea level with climate-model simulations to identify a distinct pattern of sea-level rise since the 1960’s.
In contrast to Seychelles, sea level rises have been much higher along the coastlines of East Africa, Mauritius, La Reunion, the Bay of Bengal, the Arabian Sea, Sri Lanka and the Indonesian islands of Sumatra and Java. The sea level rise in these areas, which may aggravate monsoon flooding in Bangladesh and India, could have far-reaching impacts.
The key player in the process is the Indo-Pacific warm pool, an enormous, bathtub-shaped area of the tropical oceans stretching from East Africa to the International Date Line in the Pacific. The warm pool has heated by about 0.5 degrees Celsius, in the past 50 years, primarily caused by human-generated increases of greenhouse gases.
The patterns of sea level change here are driven by the combined increase in two atmospheric wind patterns known as the Hadley circulation and the Walker circulation. The Hadley circulation in the Indian Ocean works as follows: hot tropical waters near the equator cause air currents to rise and flow South, cooling and sinking to the ocean in the subtropics and causing surface air to flow back. The Indian Ocean's Walker circulation causes hot air to rise and flow westward, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool.
The whole effect is like giant fans blowing at the water in a bathtub and causing uneven distribution of the water level. The new study shows that in order to document sea level change on a global scale we need to know the specifics of regional sea level changes.
Well, what a relief for us in Seychelles and our friends in Zanzibar. But for others the prediction is dire. In fact, the complex circulation patterns in the Indian Ocean may also affect precipitation by forcing even more atmospheric air down to the surface in Indian Ocean subtropical regions especially in the eastern tropical regions of the Indian Ocean and increase drought in the western equatorial Indian Ocean region including East Africa.
Nirmal Shah is Nature Seychelles Chief Executive
Patterns of Indian Ocean sea-level change in a warming climate
Weiqing Han, Gerald A. Meehl, Balaji Rajagopalan, John T. Fasullo, Aixue Hu, Jialin Lin, William G. Large, Jih-wang Wang, Xiao-Wei Quan, Laurie L. Trenary, Alan Wallcraft, Toshiaki Shinoda & Stephen Yeager
Nature Geoscience. Published online: 11 July 2010 | doi:10.1038/ngeo901