Large-scale features of the Antarctic Circumpolar Current (ACC) are described using all historical hydrographic data available from the Southern Ocean. The geopotential anomaly of the sea surface relative to 1000 db reveals the highly-sheared eastward flow of the ACC and the strong steering of the current by the ridge system around Antarctica. The near-surface property distributions differentiate the ACC waters from the warmer and saltier waters of the subtropical regimes. The Subtropical Front (STF), interrupted only by South America, marks the northern most extent of subantarctic waters. Distributions of properties on isopycnal surfaces show an abrupt end to the characteristic signal of the Upper Circumpolar Deep Water (UCDW), as this water mass shoals southward and is entrained into the surface mixed layer. This sharp water mass boundary nearly coincides with the southernmost circumpolar streamline passing through Drake Passage. To its south are the weakly-sheared circulations of the subpolar regime. Inspection of many hydrographic crossings of this transition reveals that the poleward edge of the UCD W signal is a reasonable definition of the southern boundary of the ACC. At Drake Passage, three deep-reaching fronts account for most of the ACC transport. Well-established indicators of the Subantarctic Front and Polar Front are traced unbroken around Antarctica. The third deep-reaching front observed to the south of the Polar Front at Drake Passage also continues with similar characteristics as a circumpolar feature. It is called here the southern ACC front. Stations from multiple synoptic transects of these circumpolar fronts are used to describe the average property structure within each ACC zone. Between the STF and the southern boundary of the ACC, the shear transport of the circumpolar current above 3000 m is at all longitudes about 100 Sv (1 Sv = 106 m3 s−) eastward.

On the meridional extent and fronts of the Antarctic Circumpolar Current

Upper-level circulation in the South Atlantic Ocean

Circulation, mixing, and production of Antarctic Bottom Water

The California current system—hypotheses and facts☆

Coastal Upwelling in the California Current System

Three-week average speeds from an array of current meter moorings which spanned Drake Passage were used in conjunction with geostrophic calculations to estimate the short-term transport of the Antarctic Circumpolar Current. Closely spaced hydrographic stations show that the current consists of three vertically coherent bands of relatively high speed within the generally eastward flow. These bands separate four water mass regimes which have distinct T-S relationships at depths above the core of the Circumpolar Deep Water. The geostrophic transport relative to 3000 db averaged 95×106 m3 s−1 for five transects of the Passage and is consistent with previous measurements. Referencing the geostrophic transport to the current meter measurements gives an adjusted transport of 124×106 m3 s−1 to the east. This estimate is about midway between values obtained in the two previous attempts to adjust relative transport through Drake Passage to observed velocities. The previous estimates are reconsidered and co...

Structure and Transport of the Antarctic Circumpolar Current at Drake Passage from Short-Term Measurements

Preliminary Observations from Long-Term Current Meter Moorings Near the Ross Ice Shelf, Antarctica

Measurements were made of wind, surface circulation, hydrography, nutrients, and phytoplankton and zooplankton rate processes in a time series at 27/sup 0/N during April 1973. A string of current meters and a towed acoustic body were also used to assess the distribution of subsurface currents and red crabs. The seasonal onset of upwelling had already occurred. Diatoms were initially the dominant primary producers. Two perturbations of the upwelling state were observed in response to relaxation of the winds. A comparison of this study with a preceding one in March 1972 indicates that the rate of upwelling, the nutrient content, and the phytoplankton specific growth rate were about the same in both years. The total productivity and biomass of the phytoplankton were less in 1973, implying that greater grazing stress was associated with a switch from smaller dinoflagellates to larger diatoms. Local temporal variability of upwelling off Baja California is postulated to have led to the replacement of clupeids by the red crab, Pleuroncodes planipes, as a facultative herbivore.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1977.22.2.0264

Further transition states of the Baja California upwelling ecosystem1,2

Current observations over the continental shelf at locations off central Oregon and southern Washington had the period from July 18 to September 18, 1972, in common. Low-frequency fluctuations (less than one cycle per day) in the currents are compared by means of visual display, linear regression, and spectral analysis. The currents are found to be highly coherent over an alongshore separation of 200 km. Coherent signals occur at 0.16, 0.3, and 0.44 cpd. The signal at 0.16 cpd occurs with high mutual coherence in wind, current, and sea level and may be a forced shelf wave driven by the wind. The signal at 0.3 cpd has high coherence between current and sea level and may be a free shelf wave generated by the wind. Correlation between observations is higher for separations in the alongshore direction than in the offshore direction in spite of greater separations.

/pdf/alongshore-coherence-at-low-frequencies-in-currents-observed-2nwdmeni7l.pdf

Alongshore coherence at low frequencies in currents observed over the continental shelf off Oregon and Washington

To investigate the reasons for the wide variation in previous estimates of transport of the Antarctic Circumpolar Current through the Drake Passage, an analysis of the spatial and temporal variability of currents at 2700 m depth is made from year-long current measurements on six moorings in the Drake Passage. The currents are found to vary over time scales of about two weeks and over spatial scales shorter than 80 km. An average of the six down-channel velocity components is used to estimate the spatially averaged down-channel velocity, or mean flow, at 2700 m. This mean flow varies from 7.6 to–2.9 cm s−1 and has a root-mean-square (rms) amplitude of 2.0 cm s−1 about its time-averaged value. Provided the geostrophic transport relative to 2700 m depth remains constant in time, these variations may be interpreted as temporal variations of 2 60×106 m3 s−1 in total transport with an rms amplitude of 50×106 m3 s−1. The wide variation in previous estimates of transport from short-term measurements can ...

R. Dale Pillsbury

Papers

Structure and Transport of the Antarctic Circumpolar Current at Drake Passage from Short-Term Measurements

Preliminary Observations from Long-Term Current Meter Moorings Near the Ross Ice Shelf, Antarctica

Further transition states of the Baja California upwelling ecosystem1,2

Alongshore coherence at low frequencies in currents observed over the continental shelf off Oregon and Washington

Variability of Deep Flow in the Drake Passage from Year-Long Current Measurements