Michael McCARTNEY

Woods Hole Oceanographic Institution

Molly O'Neil BARINGER

National Oceanic and Atmospheric Administration

Antarctic Intermediate Water (AAIW) is a low salinity stratum found throughout the tropical, subtropical and subantarctic circulation systems of the southern hemisphere. The most common hypothesis for its formation assumes an Antarctic source: either by sinking of Antarctic Surface Water across the Antarctic convergence, or by isopycnal mixing across the polar frontal zone (PFZ). In the late 1970's, McCartney offered an alternate hypothesis, attributing the formation of AAIW to a local Subantarctic Zone (SAZ) process - convectively produced Subantarctic Mode Water (SAMW) - which culminates in the southeastern South Pacific Ocean with a variety fresh enough to function as the AAIW source without the need for an explicit Antarctic component.

We are interested in combining historical data with newer WHP data to characterize the formation and circulation of AAIW/SAMW in the South Pacific, and look for evidence of long term shifts in its characteristics. So far our focus has been on

• late winter/early spring historical data taken in 1980 (Altantis II) in the southeast Pacific to characterize the formation and connection between SAMW and AAIW. In the SE South Pacific SAZ, the SAMW, near 4°C and often fresher than 34.2, is in essence the AAIW core because the salinity increases with depth below the winter mixed layer. At this convective density (about 27.15) the waters represent the end product of the circumpolar flow and transformation of warmer more saline waters along the SAZ. Below this density, diffusion between the freshest SAMW types and denser levels freshens the part of the AAIW stratum below its core; a core that is circumpolar, but that is freshest through this vertical exchange in the SE Pacific and Drake Passage, and elsewhere in the circumpolar band is more saline through mixing with subtropical waters. In the South Pacific, AAIW circulates as a ventilated tube around the subtropical gyre and is comprised of an upper part (the core and shallower) that is formed by surface convection in the SAZ and a lower part beneath the core that is formed by vertical diffusion from the SAMW to the denser strata.
• zonal WHP data along 32°S (P6) and 17°S (P21) which has provided this sharp characterization of the SAMW and AAIW flowing around the subtropical gyre. Comparison of this transported water with winter Atlantis II data is consistent with McCartney's formation hypothesis. At 32°S (P6), we've found about 5 Sv of renewed AAIW sweeping northward through the eastern third of this section; the transport of the upper part of the AAIW is about 3.3 Sv, and the lower part is about 1.7 Sv along the ventilated tube.

In the future we would like to expand this analysis by

• including other WHP sections in the South Pacific to fully describe the surface and intermediate water circulation and ventilation. Historical data has shown that much of the east to west progression of SAMW types across the South Pacific SAZ are fresher than the AAIW core that advects into the South Pacific from south of Australia, so the freshening of the core occurs along a large fraction of the South Pacific SAZ by a diffusion between the convectively formed SAMW and the underlying AAIW core.
• examing more historical data near the Drake Passage to look at the time variability of SAMW/AAIW formation. In particular, some historical data shows a subsurface salinity maximum (SAMW) north of the SAZ that has led to the hypothesis that the low slainity AAIW just below must be formed by cross frontal mixing. Under McCartney's hypothesis this salinity maximum is simply an artifact of warmer, saltier varieties of SAMW being advected through Drake Passage after winter convection allows SAMW to ventilate the AAIW core. Property time series through Drake Passage could provide further confirmation of this mechanism.
• examing the time history of SAMW/AAIW along the SAZ to possibly link to the White and Peterson circumpolar wave. We have noted that Roether's January 1990 section across the Drake Passage showed SAMW 0.5°C warmer that historical data. The preceding winter is when the circumpolar wave showed maximum warm anomaly there, which is consistent with the SAMW convection running warm. This could indicate an rms variability approaching 1°C for SAMW convection temperatures along the SAZ. The 1990 section also showed the water below SAMW much more saline than older data on the same isopycnals, suggesting that AAIW characteristics had also been altered.

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