ABSTRACT
The atolls of the central Indian Ocean are divided into three archipelagoes, all of which are atop the north–south oriented, 2,500-km-long Chagos–Laccadive Ridge. The northernmost of these archipelagoes are the Lakshadweep Islands among which are 13 atolls that align on the eastern and western sides of the ridge. Half of these form submerged rims, especially to the west where they face the open Arabian Sea, whereas the east-facing atolls gain some protection from the Indian subcontinent and develop closed rims with islands. Monsoonal winds and currents move sediment, especially in the summer monsoon which is stronger than the winter phase. Tides range by roughly 1.5 m and the most energetic waves are from the west.
The Maldive Islands form farther to the south and include 21 atolls as well as four whose lagoons have been filled by sediment transport. Three of the atolls develop submerged rims, whereas the remaining 18 are closed. One atoll with two names, Tiladummati-Miladummadulu, forms the largest lagoon area of any atoll worldwide, and the city of Malé on the atoll of the same name is one of the most overdeveloped and crowded of any atoll. The northern and central Maldives are more affected by monsoonal reversals than the southern atolls and that may correspond with the unusual rims that are built by faroes, small circular reefs with their own lagoons that can be as deep as 30 m. Faroes also form in the lagoon of many atolls. Tides range by roughly 1 m, and large waves are most commonly from the west, although monsoons can generate waves from the east. In addition, the central and southern Maldives are in the path of the westward flow of the Equatorial Current.
The island group that forms farthest south is the Chagos Archipelago, with eight atolls that include a mixture of submerged and open rim structures as well as one with a semi-closed rim. These atolls are in the geographic center of the Indian Ocean. During the summer monsoon, an Equatorial Countercurrent follows the Intertropical Convergence Zone northward, and while its easterly flow is stronger to the south, it can be felt as far north as the northern Chagos Islands. Tides range up to 1.6 m at Diego Garcia.
