CoAuthors

John Gilson (Scripps Institution of Oceanography, USA); Philip Sutton (National Institute of Water and Atmospheric Research, New Zealand); Nathalie Zilberman (Scripps Institution of Oceanography, USA)

Multi-decadal trends in ocean heat and freshwater content are well-documented. Much less evidence exists for long-term changes in ocean circulation, although these could be equally informative of a changing climate. In previous work, a 12-year increase in the circulation of the South Pacific subtropical gyre interior (from 1993 to 2004) was described (Roemmich et al., J. Phys. Oceanogr., 2007). That analysis was based on comparing early Argo results with 1990’s hydrographic data, and on changes in satellite sea surface height. Here we show that the increasing circulation trend continues, and is seen distinctly within the Argo decade, 2005 to 2014. Patterns that indicate an increase in the equatorward circulation of the eastern portion of the interior South Pacific Gyre are seen in Argo temperature and steric height, Argo trajectory data, sea surface height, sea surface temperature and salinity, sea level pressure, and wind stress. Between 2005 and 2014 the circulation was enhanced by about 5 Sv of anomalous equatorward flow extending from 160°W to the South American coast along 35°S. The northward anomaly was balanced by an equal southward transport anomaly between the dateline and 160°W. Thus a 5 Sv anticlockwise circulation anomaly is seen centered on 35°S, 160°W. Corresponding temperature anomalies during the Argo decade span the 2000 m depth range of Argo observations, with warming maxima seen in thermocline and in intermediate water layers. The 22-year trend in sea surface height is 8 cm/decade, centered at 35°S, 160°W. Trends in sea surface temperature over 35 years show a very similar spatial pattern to that of the 22-year sea surface height record, with an increase of 0.5 °C/decade at 35°S, 160°W since 1980. While there are aspects of South Pacific trends in circulation and ocean properties that are not understood, it is demonstrated that the Argo dataset has sufficient coverage and duration to accurately describe the signatures of climate variability and change.
Figure: The linear trend in SSH (cm/decade) in the South Pacific, 1993 - 2014