Temperature fluctuations and current shear in Antarctic Bottom Water at the Vema Sill

doi:10.1016/j.pocean.2006.05.006

Progress in Oceanography

Volume 77, Issue 4, June 2008, Pages 276-284

https://doi.org/10.1016/j.pocean.2006.05.006 Get rights and content

Abstract

The Vema Channel acts as a major conduit for the equatorward spreading of Antarctic Bottom Water between the Argentine and Brazil Basins. For almost two years the thermal stratification above its saddle depth (4660 m) – called Vema Sill – was recorded by moored thermistors and current meters. The lowest 490 m of the water column was instrumented to monitor the well-developed benthic boundary layer of Antarctic Bottom Water. The latter can be subdivided into Weddell Sea Deep Water on the sea bed and lower Circumpolar Deep Water above it. The data show fluctuations on various scales including periods, each about 1–2 weeks long, when the abyssal stratification virtually disappeared. Assuming a stable ratio between density and temperature, time series of bulk Richardson numbers are estimated from temperature and current shear data. The results suggest a potential for intermittent episodes of locally generated vertical mixing.

Section snippets

Motivation and observational strategy

The intricate spreading of Antarctic Bottom Water, a fundamental component of the global thermohaline circulation, is strongly controlled by the ocean’s bathymetry. The Rio Grande Rise acts as a major topographic obstacle for the equatorward bottom water flow on the western side of the South Atlantic (Fig. 1). A restricted exchange of abyssal waters between the Argentine and the Brazil Basins is accomplished by deep flow through the Vema Channel. Its narrowest spot, known as Vema Sill, is only

Instrument performance and calibration

Here, we discuss the data handling of the five recording instruments looking upward from the near-bottom position. Their overall performance was quite satisfactory. The CTD recorder (V389105 in Table 2) was brand new. Because of a short-dated delivery we had to trust the manufacturer’s calibration from early 1998. The recorder was mounted on the rope in a vertical position. Technically the moored CTD instrument performed excellently. Short-term salinity fluctuations of O(0.02) occur in the

Temperature distribution across the sill

Prior to deployment of mooring V389, a short CTD section (Fig. 2) across the sill was occupied. Due to lack of ship time efforts were concentrated on the eastern side of the Vema Channel (for detail see cruise report, Schulz et al., 1999). Similar sections from previous occupations are published in the literature (i.e. Hogg et al., 1982, Speer and Zenk, 1993, Jungclaus and Vanicek, 1999). Three characteristics of the thermal stratification are noted which are typical for deep passages in the

Discussion

By having a closer look at the isotherm diagram (Fig. 5) we detect in the gradient region (above ∼4330 m) a series of stratification collapses. If we take fluctuations of the 0 °C-isotherm as an indicator of advected temperature signals, we count O(10) abrupt displacements. They are not at all restricted to warm Circumpolar Deep Water (CDW) layers but could also be triggered by a near-bottom signal of penetrating cold Weddell Sea Deep Water (WSDW). Such episodic changes can reach almost

Acknowledgements

This note on temperature fluctuations of Antarctic Bottom Water is dedicated to Matthias Tomczak, respected author of topic contributions to oceanic fronts and mixing. Both METEOR cruises were funded by the Deutsche Forschungsgemeinschaft, Bonn. Additional support came from the Bundesministerium für Bildung und Forschung, Berlin (03F0377B). Many thanks go to the captains and crews of the two METEOR cruises as well as to the technical staff of the former IfM department of Marine Physics. The

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In the post-WOCE phase, i.e. in the beginning of CLIVAR a further experiment with moored current meters and two vertical thermistor chains was conducted on the Vema Sill (1998–2000). Again two stacked current meters (#52, 54) were deployed in the cold core of AABW close to the eastern wall of the sill (Zenk, 2008). Finally several 3.4-year-long records from two mooring sites on opposite sides of the Vema Sill were gathered between 2003 and 2007.
The Vema Channel represents the only major conduit through which the deepest and coldest (<0.2 °C potential temperature) Antarctic Bottom Water (AABW) flows from the Argentine into the Brazil Basin. From 2003 to 2007 two current meter moorings were present on each side of the Vema Sill, close to the narrowest spot of the Vema Channel. The data from the moorings are compared with earlier current and temperature observations. On average the maximum current core lies ∼100 m above the bottom of the sill with a mean northward speed of 0.3 m s⁻¹. Farther up in the water column where Lower Circumpolar Deep Water and North Atlantic Deep Water prevail, one finds a level of sluggish currents with a southward tendency in the sub-centimeter-per-second range. The lower boundary of a layer of ‘no’ motion was observed at ∼3700 m depth where the mean potential temperature amounts to 1.5 °C.
The evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the Argentine and the Brazil Basin differs from the more stable attitude of deep horizontal currents. Starting with CTD observations in 1972 we find a steady increase of temperatures of the coldest AABW in the Vema Channel. This general trend of rising abyssal potential temperatures of almost 2 mKelvin per year is based on mostly annual CTD observations. The overall warming trend is fully compatible with our three-year moored temperature series in agreement with earlier records with high temporal resolution.
Distinct frequently fluctuating horizontal current shear between the western and eastern sides of the Vema Sill may be explained by two different catchment areas for AABW at the mouth of the Vema Channel. One pathway originates at the American continental rise and advects bottom water in form of the deep western boundary current. A second pathway is supplied by an eastern boundary current along the Mid Atlantic Ridge in the Argentine Basin. Both source waters merge at the channel entrance, mix, and their respective strengths can alternate within the sill area.
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Temperature fluctuations and current shear in Antarctic Bottom Water at the Vema Sill

Abstract

Section snippets

Motivation and observational strategy

Instrument performance and calibration

Temperature distribution across the sill

Discussion

Acknowledgements

Deep-Sea Res. I

Deep-Sea Res. I

Deep-Sea Res.

Deep-Sea Res. I

Deep-Sea Res. I

Deep lenses of circumpolar water in the Argentine Basin

J. Geophys. Res.

Ocean turbulence. Part I: one-point closure model – momentum and heat vertical diffusivities

J. Phys. Oceanogr.

Circulation, transport and bottom boundary layers of deep currents in the Brazil Basin

J. Mar. Res.

Antarctic Bottom Water flux in the equatorial western Atlantic

J. Phys. Oceanogr.

Hydraulic control and flow separation in a multi-layered fluid with application to the Vema Channel

J. Phys. Oceanogr.

Long-period changes in the bottom water flowing through Vema Channel

J. Geophys. Res.

On the transport and modification of Antarctic Bottom Water in the Vema Channel

J. Mar. Res.

Circulation in the deep Brazil Basin