The northernmost latitude reached by the NECC is not influenced by El Niño-Southern Oscillation (ENSO) events, but easterly flow velocity evidently is accelerated at such times. Satellite-tracked drifter buoys from 1979 1993 demonstrated complex patterns of surface circulation with dominantly easterly flow (North Equatorial Counter Current, NECC), but also westerly currents (South Equatorial Current, SEC) that could transport propagules to Clipperton from both central and eastern Pacific regions. Three areas were surveyed in the Revillagigedo Islands (Mexico), and an intensive study was conducted on Clipperton Atoll (1,300 km SW of Acapulco), including macro-scale surface circulation, sea surface temperature (SST) climatology, geomorphology, coral community structure, zonation, and biogeography. This in turn results in the accumulation of calcareous sediment and coral rubble, and ultimately to the building of low-lying reef islands.Coral reef geomorphology and community composition were investigated in the tropical northeastern Pacific during April 1994. Much of the energy of incoming ocean swells is diminished by impact with the reef rim, but some of the wave energy crosses the reef flat and reaches island shores. Waves are the main factor leading to the building of reef islands. However, there is considerable natural variation to this basic pattern, along with (in many cases) human modification. Such islands usually consist of a distinct seaward ridge and a lesser lagoon-ward ridge, with an intervening depression (swale) in the middle. On many Indo-Pacific atolls, only about one-third of the island surface is more than 2 m above mean sea level. Reef islands often develop on reef rims.Islands are often more abundant on the higher energy, windward portions of perimeter rims and are characteristically low-lying land formations. In some cases, reef flats display a zonation pattern in which the algal ridge merges into a back reef zone formed by wave-scattered coral debris. Where islands are not present, a broad reef flat often comprises the bulk of the area behind the algal ridge, and extend all the way to the lagoon. On large atolls, reef flats occur in sections that often range from about 1,000 to 2,000 m in width. Reef Flats are shallow structures and often much of the flat is exposed during the lowest tides. In these high-energy environments, the seaward margin of the reef rims generally display a prominent algal ridge dominated by calcareous red algae. While waves break on all sides of those portions of the reef that reach sea level, the largest waves strike the windward margin. Where tidal exchange is strong, very large and well-developed patch reefs often occur. Numerous or large passages through the perimeter rim are effective at ensuring that the lagoon is flushed by daily tidal cycles, whereas atolls with almost continuous perimeter rims depend instead upon the far less effective process mechanism of wave-generated water exchange. The perimeter rim (also often simply called the reef rim) is highly variable in width, and generally contains two main structural elements: reef flats, and islands. The large-scale structure (geomorphology) of atolls has historically been mapped by in situ field studies, but more recently high-resolution satellite imagery and remote sensing algorithms have been used to map reef morphology and habitat distribution. An overview of the general process of coral reef formation as first described by Darwin is presented elsewhere on this website. This hypothesis was first developed and described by Charles Darwin and its basic tenets are now widely accepted. The typical structure of atolls derives from the most common method of the formation of such reefs specifically the gradual subsidence of tropical oceanic islands (see right). ![]() However, these reefs clearly differ in origin and geomorphology from superficially similar reef structures of the Indo-Pacific region, and most geologists do not consider them to be true atolls. Reefs such as Hogsty Reef and the Cay Sal Bank in the Bahamas, and Roncador Bank off the east coast of Nicaragua, have also sometimes been described as atolls. The three best examples (Glover’s, Lighthouse, and Turneffe) can be found just off the the Belize barrier reef. Atolls are relatively rare in the Caribbean, with counts ranging from 10 -27 depending upon the scientist doing the classification.
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