Modal Decomposition of the Low-Frequency Currents and Baroclinic Instability at Drake Passage

Abstract

Recently collected hydrographic data show that each of the four water mass zones previously identified in Drake Passage is characterized by a distinctly different vertical profile of Brunt-Väisälä frequency. Stratification in Drake Passage is rather weak, resulting in small values of the first baroclinic radius of deformation varying from 17.3 km for the Subantarctic Zone in the north to 7.7 km for the Continental Zone in the south. Using current meter records collected in 1979 at nine moorings, the vertical structure of the low-frequency currents is described in terms of dynamic normal and empirical modes. Dominance of the barotropic and first baroclinic modes was evident irrespective of mooring location, accounting for typically 83?98% of record variance. The first empirical mode, which explains more than 90% of record variance at most moorings, is surface-intensified, and appears to be a superposition of the barotropic and first baroclinic modes due to the observed modal coupling between the barotropic and first baroclinic modes. Current variability in Drake Passage is characterized by red spectra with time scales of 20?50 days. At mooring NT in the northern passage, however, currents vary more quickly with time scales of 12?15 days while the vertical structure is richer probably due to topographic waves trapped near the shelf break or generated by an upstream ridge. The second empirical mode is bottom-trapped with short time scales of 7?20 days, indicating the possible effects of bottom topography. At mooring MS5, the bottom-trapped mode in the cross-passage direction is significant and has a time scale comparable to that of the surface-intensified mode, indicating that those two modes are related probably due to current-topography interaction in the rough bottom area of the central passage. The effect of various stratification profiles on baroclinic instability were considered. For the stratification profiles and mean currents characteristic of Drake Passage, the currents appear to be quite unstable. The zonal wave length of the fastest growing wave varies from 183 km in the Subantarctic Zone to 91 km in the Continental Zone, consistent with the observed eddy scales. Barotropic and first baroclinic modes dominate the fastest growing waves, explaining the observed dominance of the two modes and the modal coupling. The major conclusion of this study is that the vertical structure of the low-frequency currents at Drake Passage appears to be a manifestation of very active baroclinic instabilities.