Summary
Circumpolar Deep Water (CDW) is the most extensive water mass in the Australian Circumpolar Current (ACC). Upper CDW (UCDW) is studied here over the period 1958-2005 using Simple Ocean Data Assimilation (SODA) reanalysis data. UCDW is supplied mainly by recirculated North Atlantic Deep Water/Lower CDW, which travels from the ACC through the Indian and Pacific Oceans and then back into the ACC. UCDW is characteristically high in nutrients and is entrained in the ACC as it shoals meridionally southwards, upwelling between the Polar Front and the Southern Boundary of the ACC. The delivery of nutrients, in particular iron, means that changes in the upwelling of UCDW are significant from the point of view of primary production and hence the drawdown of CO2 via the biological pump. Three sectors of the Southern Ocean are considered in this work, which covers the latitudinal range 40-65/70degreesS – Indian (20-60degreesE), Australian (110-160degreesE) and Pacific (130-80degreesW). Climatological properties for UCDW variables such as temperature, vertical velocity, upwelling depth and so on, as well as any trends in these variables, are presented for each of the three sectors. Differences between these results are discussed in terms of possible forcings associated with the meridional circulation of UCDW. A study examining the intrusion of UCDW into the mixed layer (ML) finds results that are consistent with UCDW upwelling and then moving north in the Ekman layer. The data show that UCDW is found in the Pacific sector ML less than half as often as in the other two sectors and that, in the Australian sector, there is a trend towards more overlaps between UCDW and the ML. Possible reasons for these results are discussed, along with implications for trends in primary production