Energetic eddies at the Northern edge of the Antarctic circumpolar current in the Southwest Pacific

Abstract

Two-year time series measurements of current and temperature were made at the northern edge of the Antarctic Circumpolar Current in the southwestern Pacific from 1978 to 1980 to investigate eddy heat and momentum fluxes in the circumpolar region. The measurement site near 49° 30′S 170°W is at a transition zone or front between subantarctic and subtropical waters in which meridional temperature and salinity gradients and, hence, vertical shear in eastward velocity extend down to 2,000 m depth. Energetic eddies are found to have typical amplitudes of 20 cm s −1 at 1,000 m depth, to be vertically coherent over the depth range from 1,000 to 5,000 m, to vary over temporal scales of 20 days and horizontal scales of 60 km and to propagate southeastward at about 12 cm s −1. Observed eddy kinetic energy of 169 cm 2 s − at 1,000 m depth is larger than that at comparable depths in the Gulf Stream and Kuroshio extension regions and in central Drake Passage. Like Drake Passage eddies, these eddies transport a significant amount of heat poleward in the band of periods between 20 and 50 days, but this band does not dominate the spectrum the way it does in Drake Passage. Energetic fluctuations with periods longer than 50 days effect an eddy heat flux whose sign is ambiguous so that the net meridional heat flux is not statistically significant. Eddy momentum fluxes transport eastward momentum northward away from the Circumpolar Current. These fluxes are statistically significant, but are only about one fourth of the size required to balance the eastward momentum put into the water column by the wind. An estimate of the standing eddy momentum flux due to large spatial scale variations in the Circumpolar Current is also too small so that pressure forces against bottom topography is likely to be the mechanism by which the wind input is balanced.