Thermohaline and Wind Forcing of a Circumpolar Channel withBlocked Geostrophic Contours

Borowski, D.; Gerdes, Rüdiger; Olbers, Dirk

doi:10.1175/1520-0485-32.9.2520

Cited by 43 publications

(53 citation statements)

References 48 publications

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“…The quantity (1/ f )∇ E is approximately equal to the geostrophic transport relative to the bottom (see Olbers et al 2006). This correspondence has also been suggested by Borowski et al (2002). A reasonable proxy for the time-mean transport is the difference of E across Drake Passage, more specifically T −(E n − E s )/ f = −C.…”

Section: Variability and Trends From 197to 2005supporting

confidence: 54%

Barotropic and baroclinic processes in the transport variability of the Antarctic Circumpolar Current

Olbers

Lettmann

2007

Ocean Dynamics

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Synoptic scale variability of the Southern Ocean wind field in the high-frequency range of barotropic Rossby waves results in transport variations of the Antarctic Circumpolar Current (ACC), which are highly coherent with the bottom pressure field all around the Antarctic continent. The coherence pattern, in contrast to the steady state ACC, is steered by the geostrophic f/ h contours passing through Drake Passage and circling closely around the continent. At lower frequencies, with interannual and decadal periods, the correlation with the bottom pressure continues, but baroclinic processes gain importance. For periods exceeding a few years, variations of the ACC transport are in geostrophic balance with the pressure field associated with the baroclinic potential energy stored in the stratification, whereas bottom pressure plays a minor role. The low-frequency variability of the ACC transport is correlated with the baroclinic state variable in the entire Southern Ocean, mediated by baroclinic topographic-planetary Rossby waves that are not bound to f/ h contours. To clarify the processes of wave dynamics and pattern correlation, we apply a circulation model with simplified physics (the barotropicbaroclinic-interaction model BARBI) and use two types of wind forcing: the National Centers for Environmental Prediction (NCEP) wind field with integrations spanning three decades and an artificial wind field constructed from the first three empirical orthogonal We analyze the spinup, trends, and variability of the model runs. Particular emphasis is placed on coherence and correlation patterns between the ACC transport, the wind forcing, the bottom pressure field and the pressure associated with the baroclinic potential energy. A stochastic dynamical model is developed that describes the dominant barotropic and baroclinic processes and represents the spectral properties for a wide range of frequencies, from monthly periods to hundreds of years.

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Section: Variability and Trends From 197to 2005supporting

confidence: 54%

Barotropic and baroclinic processes in the transport variability of the Antarctic Circumpolar Current

Olbers

Lettmann

2007

Ocean Dynamics

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“…GA to Experiment GC, and the ACC strengthened (cf. Borowski et al 2002). In Experiment GC, the additional 1.0 SSS anomaly imposed on the Weddell Sea accelerated the ACC to almost 150% of its modern transport, as compared to Experiment M (Table 4).…”

Section: Circulationmentioning

confidence: 81%

The Atlantic Ocean at the Last Glacial Maximum: 2. Reconstructing the Current Systems with a Global Ocean Model

Paul

Schäfer-Neth

2003

The South Atlantic in the Late Quaternary

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Abstract:We use a global ocean general circulation model (OGCM) with low vertical diffusion and isopycnal mixing to simulate the circulation in the Atlantic Ocean at present-day and the Last Glacial Maximum (LGM). The OGCM includes δ 18 O as a passive tracer. Regarding the LGM sea-surface boundary conditions, the temperature is based on the GLAMAP reconstruction, the salinity is estimated from the available δ 18 O data, and the wind-stress is derived from the output of an atmospheric general circulation model. Our focus is on changes in the upper-ocean hydrology, the large-scale horizontal circulation and the δ 18 O distribution. In a series of LGM experiments with a step-wise increase of the sea-surface salinity anomaly in the Weddell Sea, the ventilated thermocline was colder than today by 2-3°C in the North Atlantic Ocean and, in the experiment with the largest anomaly (1.0 beyond the global anomaly), by 4-5°C in the South Atlantic Ocean; furthermore it was generally shallower. As the meridional density gradient grew, the Antarctic Circumpolar Current strengthened and its northern boundary approached Cape of Good Hope. At the same time the southward penetration of the Agulhas Current was reduced, and less thermocline-to-intermediate water slipped from the Indian Ocean along the southern rim of the African continent into the South Atlantic Ocean; the 'Agulhas leakage' was diminished by up to 60% with respect to its modern value, such that the cold water route became the dominant path for North Atlantic Deep Water (NADW) renewal. It can be speculated that the simulated intensification of the Benguela Current and the enhancement of NADW upwelling in the Southern Ocean might reduce the import of silicate into the Benguela System, which could possibly resolve the 'Walvis Opal Paradox'. Although δ 18 O w was restored to the same surface values and could only reflect changes in advection and diffusion, the resulting δ 18 O c distribution came close to reconstructions based on fossil shells of benthic foraminifera.

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“…Gnanadesikan and Hallberg (2000), MacCready and Rhines (2001) and Tansley and Marshall (2001). A notable collection of eddy resolving QG channel simulations is found in Walsteijn (1996), and Borowski (2003) discusses a huge number of sensitivity studies with coarse resolution channel models. In these studies the relation between the transport and the wind strength and its meridional shape is in foreground, not the dependence on the height of topography which is in the center of the present study and the Bryan-Cox scenario.…”

Section: The Bryan-cox Scenario For a Zonal Channel Flowmentioning

confidence: 99%

“…Olbers and Völker 1996, Völker 1999, Borowski 2003, Olbers et al 2004, and the more fundamental work of Charney and DeVore 1979). We refrain from such further truncation of the spatial dependence of the model equations because it is found by direct integration (see next sections) that the yprofiles of the system components have a rather rich structure and, moreover, the following analysis can mostly be done without further simplifications.…”

Section: A Truncated Wave Modelmentioning

confidence: 99%

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Wave-induced topographic formstress in baroclinic channel flow

2007

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Large-scale zonal flow driven across submarine topography establishes standing Rossby waves. In the presence of stratification the wave pattern can be represented by barotropic and baroclinic Rossby waves of mixed planetary-topographic nature which are locked to the topography. In the balance of momentum the wave pattern manifests itself as topographic formstress. This wave-induced formstress has the net effect of braking the flow and reducing the zonal transport. Locally it may lead to acceleration and the parts induced by the barotropic and baroclinic waves may have opposing effects. This flow regime occurs in the circumpolar flow around Antarctica. The different roles which the wave-induced formstress plays in homogeneous and stratified flows through a zonal channel is analyzed with the BARBI model. It is used in complete form and in a low-order version to clarify the different regimes. It is shown that the barotropic formstress arises by topographic locking due to viscous friction and the baroclinic one due to eddy-induced density advection. For the sinusoidal topography used in this study the transport obeys a law in which friction and wave-induced formstress act as additive resistances, and windstress, the effect of Ekman pumping on the density stratification, and the buoyancy forcing (diapycnal mixing of the stratified water column) of the potential energy stored in the stratification act as additive forcing functions. The dependence of the resistance on the systems parameters (lateral viscosity , lateral diffusivity K of eddy density advection, Rossby radius λ, and topography height δ) as well as the dependence of transport on the forcing functions are determined. While the current intensity in a channel with homogeneous density decreases from the viscous flat bottom case in an inverse quadratic law ∼ δ −2 with increasing topography height and always depends on , a stratified system runs into a saturated state in which the transport becomes independent of δ and and is determined by the density diffusivity K rather than the viscosity: K/λ 2 acts as a vertical eddy viscosity and the transport is λ 2 /K times the applied forcing. Critical values for the topographic heights in these regimes are identified.

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Thermohaline and Wind Forcing of a Circumpolar Channel withBlocked Geostrophic Contours

Cited by 43 publications

References 48 publications

Barotropic and baroclinic processes in the transport variability of the Antarctic Circumpolar Current

Barotropic and baroclinic processes in the transport variability of the Antarctic Circumpolar Current

The Atlantic Ocean at the Last Glacial Maximum: 2. Reconstructing the Current Systems with a Global Ocean Model

Wave-induced topographic formstress in baroclinic channel flow

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