Indian Ocean

The Indian Ocean is the third-largest ocean in the world, after the Pacific and Atlantic oceans, and covers one-fifth of the world’s ocean surface area. It is bounded by 20 degrees east to 145 degrees east longitude and 60 degrees south to 31 degrees north latitude, and has a total area of 28 million sq. mi. (73.4 million sq. km). The coastline of the Indian Ocean is about 41,631 mi. (67,000 km). It is estimated that the oldest segment of the Indian Ocean’s seafloor was formed about 145 to 165 million years ago by the rifting of Africa and South America. The average depth of this ocean is 12,762 ft. (3,890 m), while the deepest point, near the southern coast of Java, is 24,442 ft. (7,450 m). There are about 36 coastal countries bordering the Indian Ocean.

The Indian Ocean is landlocked at the Northern Hemisphere and does not connect to Arctic waters. It does not appear to have a temperateto-cold zone and has fewer islands and narrow continental shelves compared to the Pacific and Atlantic oceans. It is the only ocean with a semiannually reversing surface circulation. Some studies, using observational data that began in 1958 and spanned 40 years, have found a dipole mode in the Indian Ocean as well as accompanying wind and precipitation anomalies like the Pacific and Atlantic oceans. The bottom water in the Indian Ocean originates away from its boundaries. The ocean has two sources of highly saline water: one in the Persian Gulf and the other in the Red Sea. The ocean’s water is extremely low in oxygen, especially in the north.

The most important climate process in the Indian Ocean is the monsoon. Monsoon dynamics are linked with the El Niño anomaly and with the Southern Oscillation atmospheric pattern of the South Pacific Ocean. But it is the Indian Ocean sea surface temperature (SST) that plays a key role in the regional climate dynamics in the Indian Ocean’s bordering regions. Although the Indian Ocean has been investigated for more than a century now, its role in the region’s monsoon variability has still not been fully analyzed because of complex issues as diverse as African orography, Himalayan snowfall, the El Niño-Southern Oscillation (ENSO), or ocean dynamics. Therefore, the Indian Ocean remains an active area of research among Earth and climate scientists.

Regional Climate Zones

Based on atmospheric circulation, the Indian Ocean can be divided into four latitudinal climatic zones: the monsoon (10 degrees south to 31 degrees north), the trade wind (10 degrees south to 30 degrees south), the subtropical (30 degrees south to 45 degrees south), and the Antarctic (45 degrees south to 60 degrees south).

The climate in the latitude band of 10 degrees south to the north is very much influenced by the land boundaries of the Asian and Indian subcontinents. This massive land in the north sets up the required land–sea thermal contrast that is instrumental for the development of the monsoon. Ocean circulation changes are seen over the entire north Indian Ocean in terms of surface current reversals, coastal upwelling, and sea level variability. There is also evidence of SST anomalies over the Arabian Sea and Bay of Bengal to influence the interannual variability of the monsoon and fuel the genesis and intensification of tropical cyclones. The response to the reversing wind field is most important in the Somali upwelling region in the western Arabian Sea, with the Somali current directed toward the equator during the winter monsoon and poleward during the summer monsoon. This zone also receives freshwater input from several major rivers, including the Gangas and the Brahmaputra, in addition to significant amounts of rainfall during the southwest monsoon season. While synoptic observational estimates show the formation of a freshwater-induced barrier layer, the impact of freshwater input on regional SST is not as evident.

Because of the presence of an Indonesian low pressure, the prevailing equatorial winds are westerlies in this band. The main consequence of the westerly winds and surrounding landmass is the absence of an equatorial upwelling zone and a seasonal cycle of winds and currents. Thus, the annual mean winds and currents are almost negligible north of 10 degrees south, but at their peaks these winds and currents are as strong as any in the tropical oceans. The Bay of Bengal has a history of destructive tropical cyclones like the Nargis, which made landfall in Myanmar in May 2008. These storms typically occur just before and after the southwest monsoon rains. The northwestern part of this zone has the driest climate, with some areas receiving less than 9.8 in. (250 mm) of rainfall annually. The equatorial regions are the wettest, with an average of more than 78 in. (2,000 mm). Precipitation decreases from north to south.

The air temperature over the ocean in the summer is from 77 to 82 degrees F (25 to 28 degrees C), but along the northeastern coast of Africa it drops to 73 degrees F (23 degrees C) because of the upwelling of deep, cold water. The winter air temperature drops to 71 degrees (22 degrees C) in the northern ocean, remaining almost unchanged along and south of the equator. Cloudiness is 60 to 70 percent in summer and 10 to 30 percent in winter in the monsoon region.

The next zone is the trade wind zone, which lies between 10 degrees and 30 degrees south. A steady southeasterly trade wind prevails throughout the year in this zone. In the northern part of the zone, the air temperature averages from 77 degrees F (25 degrees C) during the winter (May–October) to slightly higher the rest of the time; along latitude bands parallel to 30 degrees south, the temperature varies from about 60 degrees F (16 degrees C) in winter to 71 degrees F (22 degrees C) in the summer (November–April). In this zone, a warm ocean current increases the air temperature by 5.4 degrees F (3 degrees C) in the western trade-wind zone than in its eastern part.

The third zone lies in the subtropical and temperate latitudes between 30 degrees and 45 degrees south. In the northern part of the zone the prevailing wind is light and varies across the zone, while in the southern area, moderate to strong westerly winds prevail. The average air temperature decreases with increasing southern latitude: from 68 to 71 degrees F (20 to 22 degrees C) down to 50 degrees F (10 degrees C) in the Austral summer (December–February), and from 60 to 62 degrees F (16 to 17 degrees C) to 43 to 44 degrees F (6 to 7 degrees C) in winter (June–August), whereas the rainfall is moderate and uniformly distributed.

The Antarctic zone covers the region from 45 degrees south up to the continent of Antarctica. In this region, a steady westerly wind prevails, occasionally reaching to a strong wind with its passage through deep Antarctic low-pressure zones. The average winter air temperature varies from 43 to 44 degrees F (6 to 7 degrees C) in the north to about 3.2 degrees F (minus 16 degrees C) near the continent, while for the summer, it varies from 50 to 24 degrees F (10 to minus 4 degrees C). Precipitation is frequent and decreases in quantity southward, with snow common in the far south.