A decade of oceanographic variability in summertime near Elephant Island, Antarctica

The oceanography of near-surface (to 750 m) summertime waters surrounding the Elephant Island area of the South Shetland Islands has been surveyed each year from 1990 to 1999 as part of the U.S. Antarctic Marine Living Resources (AMLR) program. Using data from a grid of conductivity-temperature-depth stations occupied twice each summer and surface meteorological data collected continuously in the survey area, the mean (and annual departures from the mean) hydrography, circulation, and overlying wind field have been determined. The area is an important krill fishing ground and includes several water mass zones and a major frontal boundary. Differences between middle and late summer ("seasonal") conditions and interannual variability have been investigated. The mean summer condition is quite representative of conditions encountered in each of the 10 years, and little change occurs from middle to late summer, other than a warming of Antarctic Surface Water, including the Winter Water temperature minimum at 100 m, but there is no discernible trend from middle to late summer at the depth of the Circumpolar Deep Water. Interannual variability of sea temperature, winds, and sea level pressure as measured during these surveys showed no correlation with a sea ice index and a recently devised pressure index, the Drake Passage Oscillation Index (DPOI) [Naganobu et al., 1999], despite the correlations found in that study between AMLR chlorophyll a and krill recruitment indices, sea ice, and the DPOI. No correlation was found between DPOI and annual summertime sea temperature, wind, and surface pressure anomalies in the survey area. The implication is that global-scale climatic oscillations with periodicities measured in years may not be adequately detected by small-scale oceanographic surveys even though the surveys may extend over several years unless the surveys are specifically designed for that purpose.