Seasonal and interannual variability of the mixed layer properties and steric height at station KERFIX, southwest of Kerguelen

Abstract

Monthly hydrographic data collected from May 1991 to December 1994 at station KERFIX (50°40′S, 68°25′E) were analysed to describe and understand the seasonal and interannual variability of the mixed layer properties and steric height for the first 500-m water column. The monthly mean mixed layer depth (determined from the density difference criterion of 0.02 σ θ ) varies from 60 m in January to 185 m in August, with corresponding temperature and salinity varying within a limited annual range of 2.3°C (around 4.1°C in March and 1.8°C in September) and 0.06 (around 33.825 in June and 33.886 in October). A noticeable interannual change in mixed layer properties has been detected: warm and low-salinity anomalies in 1992 and cold and high-salinity anomalies in 1994; the mixed layer depth was significantly deeper in more recent years. A possible linkage of these low-frequency variations to the Pacific El Niño Southern Oscillation (ENSO) is suspected. The 3 years' (1992–1994) mean monthly steric height varies from +2.2 cm in March to −2.3 cm in September, with the appearance of a secondary maximum in July and a secondary minimum in June. Consistent with the mixed layer properties, the steric sea level of the site reveals an important interannual change, with a positive anomaly (+1.3 cm) in 1992 and a negative anomaly (−1.3 cm) in 1994. Superimposed on this low-frequency drift are quasi-semiannual (5–6 months) variations, due to the halocline-layer salinity anomalies having the same periodicity. The commonly observed mesoscale eddy activity in the Kerguelen region seems to be the major cause. Finally, the monthly mixed layer salinity of the site is determined largely by the competing effects of evaporation–precipitation and horizontal advection. On an annual basis, however, the entrainment of saltier deep water mostly compensates the eventual freshening of the mixed layer which might otherwise be caused by the excess of precipitation over evaporation in this region.