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 ANTAR ES-I 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 ti ghtly merged into one structure bordered to the north with the eastwar d extension of the Agulhas Return Current. The oxygen and salinity ext rema of the newly injected AAIW, at a depth of 1200 m, well north of t he STF, reach as high as 6.17 ml l(-1) and 34.15. These values coincid e almost exactly with those in the AAIW source, found at a depth of 50 0 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-f rontal injections of new AAIW were not continuous in space and time, b ut 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 Basi n, supporting previous observations. Moreover, our observations indica te more precisely the bottom water pathway, which is concentrated agai nst the continental rise west of 56 degrees E, following roughly the 4 000 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 dy namically transparent for the cross-frontal exchange of intermediate a nd deeper layer water masses of potential density anomalies greater th an sigma(0) = 27.1, including AAIW and AABW. (C) 1997 Elsevier Science Ltd.