Wednesday, November 23, 2011

During our cruise, we sampled along Bransfield Strait as part of the survey for salps and krill. We entered the strait from the east by coming around Elephant and Clarence Islands after a series of stations along the Drake Passage north of the South Shetland Islands. The MOCNESS was towed obliquely to 1,000 meters at our Stns #14, #15, #17, #19, and #20 (Fig. 1).

Figure 1.Map of the stations in Bransfield Strait from which the temperature and salinity data were mapped.  The Strait lies between the Western Antartic Peninsula and the South Shetland Islands. Stations are indicated by number; islands are indicated by letter and named at right.
During each MOCNESS tow, data were collected on pressure (P), the temperature (T), and salinity (S). The data were used to create a longitudinal view of the physical oceanography of the Strait, called a hydrographic section, from the surface to 1,000 m. The pressure, temperature, and salinity values from both the down- and up-haul of the MOCNESS were assigned geospatial (latitude and longitude) coordinates. The values were then mapped in relation to Stn #14, which was at the northern end of our section (Fig. 2).

Figure 2. A) Temperature and B) salinity sections for Bransfield Strait. Distance from Stn #14 is shown on the x-axis; tracks of the MOCNESS are shown as white vertical lines; the values were interpolated to provide the views shown using EasyKrig3.0 (Chu, 2004 and ftp://globec.whoi.edu/pub/software/kriging/easy_krig).
Although the number of profiles was small and the spacing wide, the T and S sections provide a basis for comparison with previous studies of the physical oceanography of Bransfield Strait.
Relatively warm deep (above O C) water from the offshore Antarctic Circumpolar Current enters the southern portion of the Bransfield Strait through a channel between Snow and Smith Islands. This water, which flows past Low Island and into the Strait between Deception and Trinity Islands, is identified by being warmer than 0o C and with a salinity of about 34.5 PSU. Such water is evident at Stn #20, which was situated between Low, Trinity, and Deception Islands.

According to Stern and Heywood (1994), “Deep basins within the Strait contain only Bottom Water, which is colder and more saline than the Antarctic Bottom Water of the Drake Passage and the Scotia Sea, and which is formed in situ during the seasonal freeze of Surface Water.” This water, known as the Bransfield Strait Basin Bottom Water, is also evident in our sections as the less than -1.0o C water in the center of the section at Stns #17 and #19 below about 400 m (see the dark blue area in Figure 2A).  The cold (~ -0.5o C) less saline water at the surface is likely from Weddell Sea to the east of the Strait.

This type of analysis of the physical oceanography of the Southern Ocean regions we are sampling will be used to help us understand the ecology of the zooplankton we collect. In particular, the different origins of the water in the Bransfield Strait will have a strong influence on the distribution of the target species, salps and krill, that we are studying. 

-- Peter Wiebe, Woods Hole Oceanographic Institution

Reference:
Stern, M. and R.B. Heywood (1994) Antarctic environment - physical oceanography: the Antarctic Peninsula and Southwest Atlantic region of the Southern Ocean. In Southern Ocean Ecology: the BIOMASS Perspective, [Ed] S. Z. El-Sayed, Cambridge University Press, New York. Pages 11-24

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