Observations of kinetic energy levels in the antarctic circumpolar current at Drake Passage

Abstract

Based on 31 nearly year-long records of current in Drake Passage the kinetic energy levels in the deep water (below 2000 m) across the passage and through the water column in the central passage are examined. The energy spectra show no significant temporal variability; by contrast, the spatial differences are pronounced, with more fluctuation kinetic energy ( K F ) in the northern than the southern passage and more above 1000 m than below. Partitioning by frequency bands shows that approx. 28% of the K F results from fluctuations with periods between 2 h (the Nyquist period for the sampling rate) and 2 days and that the energy level for this band is rather uniform across the passage. Large values of K F at northern passage locations result primarily from more activity in the period band between 2 and 50 days. Although that band accounts for almost half of the total K F , a large fraction (23%) of observed kinetic energy is associated with longer periods. The long-term records allowed examination of the representativeness of results—cumulative plots of K F and kinetic energy of mean motion ( K M ) vs time indicate that the kinetic energy densities reach equilibrium values (for specific long-term records) only after intervals of the order of 4 months. As in the case for the Gulf Stream system, abyssal K F values in the Antarctic Circumpolar Current system at Drake Passage are one or two orders of magnitude greater than K F values in the interior of the North Atlantic subtropical gyre. Moreover, deep K F values south of Cape Horn equal deep water values beneath the Gulf Stream. Values of K M from records deeper than 2500 m increase southward from northern to central locations in Drake Passage. In the central passage K M increases upward from values near 10 cm 2 s −2 at 2700 m to over 300 cm 2 s −2 at 300 m. K F increases from deep water values near 25 cm 2 s −2 to just over 200 cm 2 s −2 near 300 m. Relative to mid depths, ratios of K F to K M increase near the surface and near bottom in the central passage.