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

Barbara M Johnston,Albert J Gabric

doi:10.1111/j.1600-0870.2010.00462.x

Barbara M Johnston, Albert J Gabric

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https://doi.org/10.1111/j.1600-0870.2010.00462.x

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Abstract

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.

Highlights

Insights into decadal-scale changes in the meridional circulation in the Southern Ocean (SO) are important from the point of view of understanding climate change in that region and globally
The region studied in this work is the Australian sector of the Southern Ocean (110-160◦E, 40-65◦S) and this is shown in Figure 1, which presents sea surface temperature, sea surface temperatures (SST), in January 2000, taken from Simple Ocean Data Assimilation (SODA), as well as mean positions for the Polar Front (PF) and Southern Boundary of the Antarctic Circumpolar Current (ACC) (Orsi and Ryan, 2001, updated 2006)
In order to assess SODA’s skill at representing hydrographic data in the Australian region of the SO, temperature and salinity plots were produced from SODA and compared with plots produced from hydrographic data collected along the SR3 track in November 2001 (Sokolov and Rintoul, 2007)

Summary

Introduction

Insights into decadal-scale changes in the meridional circulation in the Southern Ocean (SO) are important from the point of view of understanding climate change in that region and globally. The conventional view (Speer et al, 2000) regards the meridional circulation in the SO as consisting of upper and lower cells, where the upper (Deacon) cell involves water upwelling southwards to the near-surface along steeply tilted isopycnals, moving northward by surface Ekman transport and sinking to form Antarctic Intermediate Water (AAIW) or Subantarctic Mode Water (SAMW). This meridional upwelling takes place against a background of intense zonal flow in the Antarctic Circumpolar Current (ACC) and occurs south of the Polar Front (PF) (Hoppema et al, 2003; Sokolov and Rintoul, 2007)

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