JASON and Argo reveal a decadal spin-up of the South Pacific gyre

Dean Roemmich, John Gilson, Russ Davis, Phil Sutton, Susan Wijffels, Steve Riser

Scripps Institution of Oceanography
University of California San Diego
La Jolla CA 92093-0230 USA

The combination of satellite altimetry with the now-global Argo float array enables unprecedented observations of seasonal-to-decadal variability in the global ocean circulation. The combined measurements describe the evolution in sea surface height (SSH) together with the subsurface temperature, salinity and velocity variability that are responsible for large-scale SSH changes. In the present work, Argo profile and trajectory data, along with altimetric height and WOCE/CLIVAR hydrography, reveal a decadal spin-up of the South Pacific gyre.

The increase in circulation of the deep subtropical gyre is observed over 12 years. The signal in SSH is a 12 cm increase between 1993 and 2003, centered at 40oS, 170oW and on large spatial scale. The subsurface datasets show that this signal is predominantly due to density variations in the water column, extending to depths of at least 1800 m. The increases in SSH and dynamic height are co-located with the deep center of the subtropical gyre, representing an increase in the counter-clockwise geostrophic circulation of the gyre by ~ 25% at 1000 m. At the sea surface, the corresponding decadal increase in SST is 0.4o C. A comparison of WOCE and Argo float trajectory data at 1000 m provides agreement with the gyre spin-up during the 1990&Mac226;s as seen in SSH and dynamic height. Similarly, all of the datasets indicate that the spin-up signal peaked and began to reverse around 2003. The 1990s&Mac226; increase in wind-driven circulation resulted from decadal intensification of wind-stress curl east of New Zealand, variability associated with an increase in the southern hemisphere annual mode (SAM). It is suggested (based on SSH) that mid-latitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal timescales. A major advance from the altimeters and the Argo array could be the global description of this climate variability signal.