Subantarctic Mode Water

Immediately north of the circumpolar subantarctic front, deep {400-600 m) well-mixed Jayers are found in late winter. Spring and summer heating isolates {but does not completely erase) these layers beneath the seasonal thermocline as thermostads. The zone in which this active renewal is found is several hundred kilometers wide, but the associated thermostad can be traced much further north-on the order of 2000 km. The thermostads and the often associated dissolved oxygen maxima can be found as far north as the south equatorial current regions of each southem hemisphere subtropical gyre. This water mass formation and spreading process is equivalent to that occurring east and south of the Gulf Stream and Kuroshio currents, where the thermostads are called Subtropical Mode Water {STMW). In light of the association of these southern ocean thermostads with the circumpolar subantarctic front, rather than the subtropical fronts {westem boundary currents such as the Agulhas current) the name Subantarctic Mode Waier (SAMW) is suggested. In common with STMW. SAMW contributes substantial volumetric modes to the central water masses. indicating SAMW tobe the renewal agent ofthe high oxygen parts ofthe main thermoc!ine water of the southern hemisphere subtropical gyres. Finally, it is noted that the specific types of SAMW formed in the southeast Pacific and Scotia Sea areas are identical in temperature and salinity to the South Pacific and South Atlantic varieties of Antarctic Intermediate Water {AAIW). The renewal process for AAIW is hence indicated as taking place 11orth of the polar front zone, in the southeast Pacific and Scotia Sea parts of the subantarctic zone. The actual process is late winter convective overturning of the somewhat warmer and more saline waters advecting into the region from the west along the subantarctic front. The low salinity of AATW is due to the pronounced excess of precipitation over evaporation in the subantarctic zone. This process is quite different from the traditional concept of circumpolar cross-polar-frontal mixing of Antarctic Surface Water with Subantarctic Surface Water