Cross-frontal exchange of Antarctic Intermediate Water and Antarctic Bottom Water in the Crozet Basin

The hydrographic survey made across the Antarctic Circumpolar Current (ACC) in the southwestern Crozet Basin during the April–May 1993 ANTARES-1 cruise revealed unprecedented strong cross-frontal injections of newly formed Antarctic Intermediate Water (AAIW) into the subtropical zone north of the ACC. The ACC in this area is mostly associated with the Subantarctic Front (SAF) and Subtropical Front (STF), which are tightly merged into one structure bordered to the north with the eastward extension of the Agulhas Return Current. The oxygen and salinity extrema of the newly injected AAIW, at a depth of 1200 m, well north of the STF, reach as high as 6.17 ml l−1 and 34.15. These values coincide almost exactly with those in the AAIW source, found at a depth of 500 m, just south of the SAF, but surpass considerably (oxygen by + 1.35 ml l−1 and salinity by −0.23 psu) those values corresponding to the old AAIW at the nearby subtropical stations. Such spectacular cross-frontal injections of new AAIW were not continuous in space and time, but were found to be rather local, intermittent and impulsive. This was closely correlated with the highly perturbed, meandering frontal zone due to the frequent passages of strong atmospheric depressions during the cruise. There is also clear evidence of the northward penetration of Antarctic Bottom Water (AABW) through the Crozet-Kerguelen gap and its northward flow along the deep western boundary of the Crozet Basin, supporting previous observations. Moreover, our observations indicate more precisely the bottom water pathway, which is concentrated against the continental rise west of 56°E, following roughly the 4000 m isobath. It is concluded that the Crozet Basin frontal zone acts as a barrier only for the upper layer water masses, and that it is dynamically transparent for the cross-frontal exchange of intermediate and deeper layer water masses of potential density anomalies greater than σϱ = 27.1, including AAIW and AABW.