Low tides explained by eddy patterns in the Sargasso Sea

TUESDAY, AUGUST 31:  The unusually low tides in Bermuda over the last several months are caused by a large cyclonic (anticlockwise) eddy in local waters. This eddy has caused a lowering of the sea level around Bermuda.

Scientists at the Bermuda Institute of Ocean Sciences (BIOS) are constantly monitoring eddies in the Sargasso Sea using both remote sensing techniques and ship-based observations from BIOS’ research vessel Atlantic Explorer. Most of the monitoring takes place at two long-term open-ocean observation sites, Hydrostation ‘S’ and Bermuda Atlantic Time-series Study (BATS), near Bermuda.

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.

Large eddies may be formed when an ocean current, such as the Gulf Stream, develops an instability. The current ‘wanders’, and is eventually pinched off from the main current into a roughly circular motion around the center of the eddy. These types of eddies are sometimes referred to as cold core eddies where the core of the eddy contains water from north of the gulf stream and is thus foreign to the interior waters of the Sargasso Sea.

However, these 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.

Our understanding of ocean dynamics has improved dramatically since the early 90’s with the advent of satellite imagery (especially from satellite altimeters which measure sea surface height) which combined with shipboard observations has helped oceanographers detail the three-dimensional structure of ocean eddies. For Bermuda, the classical picture of a weak Gulf Stream recirculation current (flows less than 0.1 knot) to the southwest no longer reflects reality on timescales of days to years. Instead, reality is a complex system of rotating eddies mostly propagating from the east through Bermuda in a semi-chaotic manner. In addition to the more notable dynamical effects (sea level change and currents) these eddies may also impact Bermuda’s local waters in other ways since they tend to have varying biological and chemical signatures.  As observed in the tidal records off Bermuda, the island is constantly being impacted by the passage of these ocean eddies.

During the June BATS cruise, BIOS scientists including Dr. Rod Johnson collected ocean profiles from areas some 50km to the east of the BATS location (approximately 40 miles SE of Bermuda).

“Using satellite imagery provided by colleagues from WHOI using altimeter products produced by AVISO (http://www.aviso.oceanobs.com/) as part of the Ssalto ground processing segment, we were able to identify waters outside of the influence of this large cyclone, enabling us to contrast with observations from stations within the cyclonic eddy,” Johnson said.

All Bermudians can attest to the fact that the past winter has been particularly severe. Data from the BIOS ocean time-series projects reveal the intensity of ocean mixing this winter, where the upper 300m of the ocean has been overturned and the temperature of this 300m ‘mixed layer’ has been as low as 18.3 °C. Typically, annual minimum temperatures off Bermuda are above 19 °C. This past winter was one of coldest recorded by BIOS scientists in the Hydrostation time-series, which has been collecting and analysing ocean temperature data at a point 12 miles South east of Bermuda since 1954, the longest continuous record of open-ocean measurements in the world. However an intriguing aspect of this year’s winter mixing is that the ocean temperature is lower than what might be expected for a 300m mixed layer. 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.

Scientists are constantly evolving their understanding of ocean dynamics and improving their knowledge about cycling patterns to help determine inter-annual and longer term variability. According to Johnson, this year’s sampling efforts off Bermuda have helped improve our understanding of “how eddies can cause significant variability in winter mixing from year to year. Winter mixing for much of the ocean is the primary mechanism for nutrient input to the upper ocean and quantifying this process is a key factor for determining upper ocean carbon budgets in relation to the ocean’s role in global carbon sequestration.

Johnson and his BIOS colleagues collaborate closely on this eddy research with Dr. Dennis McGillicuddy and Valery Kosnyrev of the Woods Hole Oceanographic Institution in Massachusetts.