Eddy heat fluxes from direct current measurements of the Antarctic Polar Front in Shag Rocks Passage

Determining meridional heat flux in the Southern Ocean is critical to the accurate understanding and model simulation of the global ocean. Mesoscale eddies provide a significant but poorly-defined contribution to this transport. An eighteen-month deep-water current meter array deployment in Shag Rocks Passage (53°S, 48°W) between May 2003 and November 2004 provides estimates of the eddy flux of heat across the Polar Front. We calculate a statistically nonzero (99% level), vertically coherent local poleward heat flux of 12.0 ± 5.8 kW m-2 within the eddy frequency band at ~2750 m depth. Exceeding previous deep-water estimates by up to an order of magnitude, this highlights the large spatial variation in flux estimates and illustrates that constriction of circumpolar fronts facilitates large eddy transfers of heat southwards

A compilation of observations from moored current meters. Vol. 12. Currents, temperature and pressure in the Drake Passage during FDRAKE 77, 78, January 1977-January 1979

Internally recording instruments were installed in the Drake Passage in January 1977 and recovered in December 1977. Additional instruments were installed at this time and recovered in January 1979. The first array consisted of 21 Aanderaa current meters on five moorings. The second array was one mooring with three current meters. The data were taken at one hour intervals. In this report the measurements are summarized through pertinent statistics, progressive vector diagrams, real time plots, stick figures, rotary spectra, and conventional power spectra

Temporal Variability of Diapycnal Mixing in Shag Rocks Passage

Diapycnal mixing rates in the oceans have been shown to have a great deal of spatial variability, but the temporal variability has been little studied. Here we present results from a method developed to calculate diapycnal diffusivity from moored Acoustic Doppler Current Profiler (ADCP) velocity shear profiles. An 18-month time series of diffusivity is presented from data taken by a LongRanger ADCP moored at 2400 m depth, 600 m above the sea floor, in Shag Rocks Passage, a deep passage in the North Scotia Ridge (Southern Ocean). The Polar Front is constrained to pass through this passage, and the strong currents and complex topography are expected to result in enhanced mixing. The spatial distribution of diffusivity in Shag Rocks Passage deduced from lowered ADCP shear is consistent with published values for similar regions, with diffusivity possibly as large as 90 × 10-4 m2 s-1 near the sea floor, decreasing to the expected background level of ~ 0.1 × 10-4 m2 s-1 in areas away from topography. The moored ADCP profiles spanned a depth range of 2400 to 1800 m; thus the moored time series was obtained from a region of moderately enhanced diffusivity. The diffusivity time series has a median of 3.3 × 10-4 m2 s-1 and a range of 0.5 × 10-4 m2 s-1 to 57 × 10-4 m2 s-1. There is no significant signal at annual or semiannual periods, but there is evidence of signals at periods of approximately fourteen days (likely due to the spring-neaps tidal cycle), and at periods of 3.8 and 2.6 days most likely due to topographically-trapped waves propagating around the local seamount. Using the observed stratification and an axisymmetric seamount, of similar dimensions to the one west of the mooring, in a model of baroclinic topographically-trapped waves, produces periods of 3.8 and 2.6 days, in agreement with the signals observed. The diffusivity is anti-correlated with the rotary coefficient (indicating that stronger mixing occurs during times of upward energy propagation), which suggests that mixing occurs due to the breaking of internal waves generated at topography

Variability in the location of the Antarctic Polar Front (90°-20°W) from satellite sea surface temperature data

The path of the Antarctic Polar Front (PF) is mapped using satellite sea surface temperature data from the NOAA/NASA Pathfinder program. The mean path and variability of the PF are strongly influenced by bathymetry. Meandering intensity is weaker where the bathymetry is steeply sloped and increases in areas where the bottom is relatively flat. There is an inverse relationship between meandering intensity and both the width of the front and the change in temperature across it There is a persistent, large separation between the surface and subsurface expressions of the PF at Ewing Bank on the Falkland Plateau

Velocity Comparisons from Upward and Downward Acoustic Doppler Current Profilers on the West Florida Shelf

Current observations are compared from upward- and downward-looking acoustic Doppler current profilers (ADCPs) deployed on the West Florida Shelf (WFS). Despite regional differences, statistical analyses show good agreement between all sets of observations throughout the water column except in the upper few meters where all downward-looking ADCPs exhibit small, but significant, reduction in rms speed values. Evidence suggests that this reduction is mooring related. It is possible that the presence of near-surface bubbles caused by wave activity could bias the near-surface observations. Otherwise, either the upward- or downward-looking mooring systems produce equivalent observations with differences due to spatial variations