Low-Frequency Variability of the South Pacific Subtropical Gyre as Seen from Satellite Altimetry and Argo

Low-frequency variability of the South Pacific Subtropical Gyre is investigated using satellite altimeter and Argo data. In most of the region studied, both sea surface height and steric height exhibit a linearly increasing trend, with its largest amplitude in the western part of the basin. Analysis of the Argo data reveals that the steric height increase north of 30 degrees S is primarily caused by variations in the upper 500 m, while the steric height increase south of 30 degrees S is determined by variations in the whole depths from the sea surface to 1800 m, with contributions from below 1000 m accounting for about 50% of the total variance. Most of the steric height increase is due to thermal expansion, except below 1000 m where haline contraction is of comparable magnitude with thermal expansion. Correspondingly, the South Pacific Subtropical Gyre has strengthened in the past decade. Within the latitude range between 10 degrees and 35 degrees S, transport of the gyre circulation increased by 20%-30% in the upper 1000 m and by 10%-30% in the deeper layers from 2004 to 2013. Further analysis shows that these variations are closely related to the southern annular mode in the South Pacific.