Long-term trends in upper ocean structure and meridional circulation of the Southern Ocean south of Australia derived from the SODA reanalysis

Decadal-scale changes in the meridional circulation of the Southern Ocean south of Australia are studied, over the period 1958–2005, using Simple Ocean Data Assimilation (SODA) reanalysis data. Upper Circumpolar Deep Water (UCDW) is found to upwell nearer to the surface over time, while the mixed layer (ML) is found to deepen, leading to an increase in the number of times that UCDW intrudes into the ML. This entrainment of nutrients, especially iron, into the ML from UCDW, is crucial for primary production and appears to occur predominantly in summer/autumn, contrary to previous reports.

ML temperature, density and salinity all show increasing trends in almost all seasons and latitudinal zones within the study region. A notable exception to the general increase in temperature occurs in the most southerly zone 60–65°S in summer. An explanation for this apparent anomaly could be related to increased winds (in conjunction with the increasing trend in the Southern Annular Mode), which mix remnant winter water into the ML, negating the surface temperature increase.

Unlike trends in ML variables, trends in UCDW variables appear to be decoupled from the surface trends and occur on time-scales that may be centennial rather than decadal.