Expedition blogs and news from the Shallow Marine Surveys Group

Oceanography studies

Ascension Island – a lonely piece of land in a seemingly borderless ocean turned out to be the centre of an oceanographic 'collision'  which does not happen very often in featureless seas. Here, the central branch of the Southern Equatorial Current that normally goes on surface meets the Southern Equatorial Counter-Current that normally goes in subsurface layers but right here, between 7 and 8°S, it travels to the surface. Interactions of these two streams give rise to high water turbulence, numerous gyres and eddies and other kinds of water unrest. Those, combined with upwelling areas in inshore waters caused by the bottom topography, are responsible to the high productivity of the area that attracts numerous large predators close to shore that might be seen filleted on Georgetown pier almost every night.

[caption id="attachment_547" align="aligncenter" width="584"] Getting picked up by our research vessel for the circumnavigation of the island, the Queen of Atlantis.

To study the local oceanographic features, a total of 16 oceanographic stations with manually deployed CTD (Conductivity – Temperature – Density) devices were carried out. To complete the picture around the island, Vlad Laptikhovsky, Steve Cartwright, Wetjens Dimmlich, Frithjof Kuepper and Kostas Konstantinos circumnavigated Ascension in the comfort of the Queen of Atlantis. Generally conditions in the voyage were good but did become quite rough along the more exposed coast near Boatswain Bird Island.

The results reveal a complicated oceanographic structure even in the upper 50-m layer, where waters of both the major oceanic currents combined with a mixed layer of local origin.

[caption id="attachment_548" align="aligncenter" width="584"] Vlad deploying the CTD during the 4-hr trip around Ascension.

During the past two weeks the interaction of these currents was quite mobile. The cold productive Counter-Current eventually occupied the surface layer around most of the island, excluding the small offshore part in the north around English Bay. The more saline (because of evaporation) Equatorial Current moved its water mostly deeper than 20 m revealing expected phenomenon of temperature increase with depth, and surfaced only in the very north of the studied area.

- Contributed by Vladimir Laptikhovsky

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