Floating marine debris surface drift: Convergence and accumulation toward the South Pacific subtropical gyre

AbstractLow-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°S is primarily caused by variations in the upper 500 m, while the steric height increase south of 30°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° and 35°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.

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Decadal Spinup of the South Pacific Subtropical Gyre

Journal of Physical Oceanography ◽

10.1175/jpo3004.1 ◽

2007 ◽

Vol 37 (2) ◽

pp. 162-173 ◽

Cited By ~ 177

Author(s):

D. Roemmich ◽

J. Gilson ◽

R. Davis ◽

P. Sutton ◽

S. Wijffels ◽

...

Keyword(s):

Ocean Circulation ◽

World Ocean ◽

South Pacific ◽

Sea Surface Height ◽

Subtropical Gyre ◽

Sea Surface ◽

The South ◽

Maximum Increase ◽

Large Spatial Scale ◽

Dynamic Height

Abstract An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand—variability associated with an increase in the atmosphere’s Southern Hemisphere annular mode. It is suggested (based on altimetric height) that midlatitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales.

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