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

Olbers, Dirk; Lettmann, Karsten

doi:10.1007/s10236-007-0126-1

Cited by 22 publications

(26 citation statements)

References 30 publications

(73 reference statements)

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“…They concluded that there is a significant fraction of the variability (about 35%) in the total transport arising from variations in the baroclinic field. In agreement, a large variability in baroclinic transport was found from upperocean XBT measurements (Sprintall 2003) and from numerical models (Olbers and Lettmann 2007). Firing et al (2011) found from direct velocity observations that the depth-mean and shear components of the transport variability in the upper 1000 m are of similar size.…”

supporting

confidence: 73%

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

Chidichimo

Donohue

Watts

et al. 2014

Journal of Physical Oceanography

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The first multiyear continuous time series of Antarctic Circumpolar Current (ACC) baroclinic transport through Drake Passage measured by moored observations is presented. From 2007 to 2011, 19 current-and pressure-recording inverted echo sounders and 3 current-meter moorings were deployed in Drake Passage to monitor the transport during the cDrake experiment. Full-depth ACC baroclinic transport relative to the bottom has a mean strength of 127.7 6 1.0 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21) with a standard deviation of 8.1 Sv. Mean annual baroclinic transport is remarkably steady. About 65% of the baroclinic transport variance is associated with time periods shorter than 60 days with peaks at 20 and 55 days. Nearly 28% of apparent energy in the spectrum computed from transport subsampled at the 10-day repeat cycle of the ). The transport standard deviation in these layers decreases with depth (4.0, 3.1, 2.1, and 1.1 Sv, respectively). The transport associated with each of these water masses is statistically steady. The ACC baroclinic transport exhibits considerable variability and is a major contributor to total ACC transport variability.

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supporting

confidence: 73%

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

Chidichimo

Donohue

Watts

et al. 2014

Journal of Physical Oceanography

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“…We will show that the concept of transport determined directly by a southern mode survives intact when interpreted in terms of depth-averaged boundary pressure (bottom pressure on the Antarctic continental slope). However, baroclinic effects start to become important on timescales consistent with the predictions of Olbers and Lettmann (2007), producing a changing relationship between bottom pressure, sea level and transport which allows for the surface intensification required to reflect the geometry of the ACC at long periods. We also find that surface intensification of the meridionally integrated currents can in part be explained by a barotropic mechanism involving currents flowing on the continental slope.…”

Section: W Hughes Et Al: Antarctic Circumpolar Transport and Thementioning

confidence: 89%

“…On some timescale, presumably, variations must start to take a form comparable with the ACC, with more flow near the surface and a decay to smaller values at depth. This is an issue which was investigated by Olbers and Lettmann (2007) in the context of an idealized ocean model, with only two vertical modes, no eddies, and a smoothed representation of topography. They found a baroclinic timescale of about 16 years, and spectral analysis showed that the role of baroclinic terms was small at periods shorter than about 4 years, rising to play a major role at about a 7-year period.…”

Section: W Hughes Et Al: Antarctic Circumpolar Transport and Thementioning

confidence: 99%

Antarctic circumpolar transport and the southern mode: a model investigation of interannual to decadal timescales

et al. 2014

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Abstract. It is well-established that, at periods shorter than a year, variations in Antarctic circumpolar transport are reflected in a barotropic mode, known as the southern mode, in which sea level and bottom pressure varies coherently around Antarctica. Here, we use two multidecadal ocean model runs to investigate the behaviour of the southern mode at timescales on which density changes become important, leading to a baroclinic component to the adjustment. We find that the concept of a southern mode in bottom pressure remains valid, and remains a direct measure of the circumpolar transport, with changes at the northern boundary playing only a small role even on decadal timescales. However, at periods longer than about 5 years, density changes start to play a role, leading to a surface intensification of the vertical profile of the transport. We also find that barotropic currents on the continental slope account for a significant fraction of the variability, and produce surface intensification in the meridional-integral flow. Circumpolar sea level and transport are related at all investigated timescales. However, the role of density variations results in a ratio of sea level change to transport which becomes larger at longer timescales. This means that any long-term transport monitoring strategy based on present measurement systems must involve multiplying the observed quantity by a factor which depends on frequency.

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“…The ACC responds to changes in the zonal wind stress, and the response contains both barotropic and baroclinic processes [19,20]. In previous studies the SAM has been identified as the dominant mode of atmosphere variability with a zonally-symmetric dipole structure in the Southern Hemisphere [21].…”

Section: Discussionmentioning

confidence: 99%

Interannual variability of the Antarctic Circumpolar Current strength based on merged altimeter data

Zhang

Sun

2012

Chin. Sci. Bull.

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Interannual variability of the Antarctic Circumpolar Current (ACC) strength is studied in stream-coordinate with twenty-year Absolute Dynamic Topography data from satellite altimetry. The stream-coordinate projection method separates the ACC from adjacent subtropical and subpolar gyres, enabling consideration of the zonal asymmetry of the ACC rather than assuming that the ACC is a purely zonal flow. It is shown that the ACC strength has large interannual variations with two recent peaks around 2000 and 2009. The interannual variability appears mainly in the Indo-Pacific sector of the Southern Ocean and the strongest signal is located south of Australia. The intensification of the westerly wind in 1998 and 2008 appears to cause the strengthening of the ACC via baroclinic processes. The Antarctic Circumpolar Current (ACC) connects the world's three major ocean basins and is a key element in the global thermohaline circulation. The inter-basin exchange provided by the ACC redistributes heat, salt and other properties in the world's ocean. Therefore, changes in the ACC exert significant influences on the global climate system. Much work has attempted to quantify the transport and variability of the ACC based on hydrography and numerical models. (Figure 1), and therefore zonal transport estimates by previous studies are strongly influenced by adjacent subtropical and subpolar gyres and cannot solely represent the strength of the ACC. Moreover, the data sets used in previous studies are insufficient to map the interannual variability of the ACC, and sampling with a frequency significantly higher than weekly is required to obtain reliable estimates of the ACC variability at interannual periods [8]. Therefore, limited by the in-situ observations and data processing methods, few extant studies give reliable estimates of the interannual variability of the ACC strength. Recently a stream-coordinate method was developed to study the ACC by projecting hydrographic data into a stream function space [7,9]. The method isolates the ACC from its adjacent subtropical and subpolar gyres. It also considers the zonal asymmetry of the ACC path rather than simply assume that the ACC is a purely zonal current. It was found that the baroclinic structure and baroclinic transport of the ACC were very stable. But given a constant volume transport, the ACC strength can still change due to

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

Cited by 22 publications

References 30 publications

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

Antarctic circumpolar transport and the southern mode: a model investigation of interannual to decadal timescales

Interannual variability of the Antarctic Circumpolar Current strength based on merged altimeter data

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