The Bermuda Sun reports that scientists at the Bermuda Institute of Ocean Sciences (BIOS) have found that the unusually low tides in Bermuda over the last several months are caused by a large cyclonic (anticlockwise) eddy that has caused a lowering of the sea level around the island. Here are excerpts with a link to the full article below:
BIOS scientists are constantly monitoring eddies in the Sargasso Sea using both remote sensing techniques and ship-based observations from the institute’s research vessel Atlantic Explorer. Formation of this recent cyclone was observed in late January 2010, some 300 to 400km east of Bermuda. The strength (noted by sea level depression) of this cyclone intensified in late March with a sea level depression of approximately 35cm at its center at which time the feature began to influence Bermuda’s water causing the notable low tides.
[. . .T]hese eddies do not typically travel as far south as Bermuda and instead eddies commonly found in Bermuda tend to originate in the eastern Sargasso Sea. The genesis of these Sargasso Sea mesoscale eddies (length scale of approximately 100’s of Km’s) still remains a research question, although their spin-up is generally associated with density instabilities.
Mesoscale eddies are ubiquitous to the Sargasso Sea and, beyond the seasonal cycle, are the principal mode of variability, dictating spatial patterns and modulating temporal changes at fixed locations. These eddies can be cyclonic (anti-clockwise rotating) or anti-cyclonic (clockwise) and the gradients associated with their density fields drive ocean currents in a similar manner to high and low pressure systems in the atmosphere forcing the wind field. For the cyclonic case, horizontal flows are divergent from the eddy center which causes the warmer surface waters to be replaced by colder deeper waters. Hence the core of a cyclonic eddy is anomalously cold which through thermal contraction creates a reduction in the water column height. In contrast anti-cyclones are convergent systems where the warmer surface waters are forced downwards at the eddy center resulting in a warmer water column with a subsequent rise in the sea level.
[. . .] On closer analysis of the ocean data (particularly data from June cruise), it seems that this anomalously low temperature was the combination of cooling from the severe winter storms and the uplifting of deeper cold water by the cyclonic eddy.
[. . .] “It is perhaps too early to make any conclusions,” noted Johnson. “But I think this analysis of eddies illustrates how our long-term studies of the open ocean at BATS and Hydrostation can shed light on questions we have about both global ocean dynamics and more localized affects such as the low tide levels that Bermuda has been experiencing.”