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

Hughes, Chris W.; Williams, Joanne; Coward, Andrew C.; Cuevas, B. A. de

doi:10.5194/os-10-215-2014

Cited by 9 publications

(11 citation statements)

References 17 publications

(23 reference statements)

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“…[] suggests that the baroclinic component accounts for about 70% of the total, and that the variability due to the barotropic component is probably only important on time scales of a few years or less. A recent model study [ Hughes et al ., ] supports the suggestion of Olbers and Lettmann [] that barotropic effects cease to dominate at periods longer than about 5 years.…”

Section: Introductionsupporting

confidence: 88%

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Forced and intrinsic variability in the response to increased wind stress of an idealized Southern Ocean

2015

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We use ensemble runs of a three layer, quasi-geostrophic idealized Southern Ocean model to explore the roles of forced and intrinsic variability in response to a linear increase of wind stress imposed over a 30 year period. We find no increase of eastward circumpolar volume transport in response to the increased wind stress. A large part of the resulting time series can be explained by a response in which the eddy kinetic energy is linearly proportional to the wind stress with a possible time lag, but no statistically significant lag is found. However, this simple relationship is not the whole story: several intrinsic time scales also influence the response. We find an e-folding time scale for growth of small perturbations of 1-2 weeks. The energy budget for intrinsic variability at periods shorter than a year is dominated by exchange between kinetic and potential energy. At longer time scales, we find an intrinsic mode with period in the region of 15 years, which is dominated by changes in potential energy and frictional dissipation in a manner consistent with that seen by Hogg and Blundell (2006). A similar mode influences the response to changing wind stress. This influence, robust to perturbations, is different from the supposed linear relationship between wind stress and eddy kinetic energy, and persists for 5-10 years in this model, suggestive of a forced oscillatory mode with period of around 15 years. If present in the real ocean, such a mode would imply a degree of predictability of Southern Ocean dynamics on multiyear time scales.

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Section: Introductionsupporting

confidence: 88%

“…In primitive equation simulations, it was found [ Hughes et al ., ] that transport fluctuations were dominated by a barotropic response to winds on time scales shorter than 5–10 years, with baroclinic changes becoming dominant at longer time scales, so 15–20 years seems to be a time scale associated with baroclinic processes.…”

Section: Model and Experimentsmentioning

confidence: 99%

Forced and intrinsic variability in the response to increased wind stress of an idealized Southern Ocean

2015

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“…The effect of rapid propagation of coastal trapped waves is seen in the coastal sea-level response to El Niño events along the eastern boundary of the Pacific (Enfield and Allen 1980;Kurapov et al 2017). It has also been noted indirectly via the uniformity of sea-level signals around Antarctica as discussed by Hughes et al (2014) and references therein, and by an analogous mode in the Arctic (see Fukumori et al 2015 and references therein). These polar modes are manifestations of the response to near-coastal winds being trapped and rapidly propagated along the coast, a phenomenon also seen in the eastern North Atlantic (Calafat et al 2012(Calafat et al , 2013, and discussed in more detail elsewhere in this volume.…”

Section: The Disparity Between Coastal and Open-ocean Wave Speedsmentioning

confidence: 89%

Sea Level and the Role of Coastal Trapped Waves in Mediating the Influence of the Open Ocean on the Coast

et al. 2019

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The fact that ocean currents must flow parallel to the coast leads to the dynamics of coastal sea level being quite different from the dynamics in the open ocean. The coastal influence of open-ocean dynamics (dynamics associated with forcing which occurs in deep water, beyond the continental slope) therefore involves a handover between the predominantly geostrophic dynamics of the interior ocean and the ageostrophic dynamics which must occur at the coast. An understanding of how this handover occurs can be obtained by considering the combined role of coastal trapped waves and bottom friction. We here review understanding of coastal trapped waves, which propagate cyclonically around ocean basins along the continental shelf and slope, at speeds which are fast compared to those of baroclinic planetary waves and currents in the open ocean (excluding the large-scale barotropic mode). We show that this results in coastal sea-level signals on western boundaries which, compared to the nearby open-ocean signals, are spatially smoothed, reduced in amplitude, and displaced along the coast in the direction of propagation of coastal trapped waves. The open-ocean influence on eastern boundaries is limited to signals propagating polewards from the equatorial waveguide (although a large-scale diffusive influence may also play a role). This body of work is based on linearised equations, but we also discuss the nonlinear case. We suggest that a proper consideration of nonlinear terms may be very important on western boundaries, as the competition between advection by western boundary currents and a counter-propagating influence of coastal trapped waves has the potential to lead to sharp gradients in coastal sea level where the two effects come into balance.

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“…Even less is certain about variability on interannual and longer time scales. In situ bottom pressure measurements have been used to study transport variability and to reveal how it relates to sea level around Antarctica (e.g., Hughes et al, 1999), with connections to Southern Hemisphere westerly winds summarized by correlations with the Southern Annular Mode (SAM) index (e.g., Meredith et al, 2004), which numerical model results predict will persist over a range of time scales (Hughes et al, 2014). Atmospheric measurements indicate that the westerly winds in the Southern Ocean have intensified since the 1970s in conjunction with a higher SAM index (Thompson & Solomon, 2002); yet, none of the long ACC transport time series (Koenig et al, 2014;Meredith et al, 2011) show a clear long-term trend.…”

Section: Introductionmentioning

confidence: 99%

Antarctic Circumpolar Current Transport Through Drake Passage: What Can We Learn From Comparing High‐Resolution Model Results to Observations?

Chassignet

Firing

et al. 2020

JGR Oceans

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Uncertainty exists in the time‐mean total transport of the Antarctic Circumpolar Current (ACC), the world's strongest ocean current. The two most recent observational programs in Drake Passage, DRAKE and cDrake, yielded transports of 141 and 173.3 Sv, respectively. In this paper, we use a realistic 1/12° global ocean simulation to interpret these observational estimates and reconcile their differences. We first show that the modeled ACC transport in the upper 1,000 m is in excellent agreement with repeat shipboard acoustic Doppler current profiler (SADCP) transects and that the exponentially decaying transport profile in the model is consistent with the profile derived from repeat hydrographic data. By further comparing the model results to the cDrake and DRAKE observations, we argue that the modeled 157.3 Sv transport, that is, approximately the average of the cDrake and DRAKE estimates, is actually representative of the time‐mean ACC transport through the Drake Passage. The cDrake experiment overestimated the barotropic contribution in part because the array undersampled the deep recirculation southwest of the Shackleton Fracture Zone, whereas the surface geostrophic currents used in the DRAKE estimate yielded a weaker near‐surface transport than implied by the SADCP data. We also find that the modeled baroclinic and barotropic transports are not correlated; thus, monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport.

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

Cited by 9 publications

References 17 publications

Forced and intrinsic variability in the response to increased wind stress of an idealized Southern Ocean

Forced and intrinsic variability in the response to increased wind stress of an idealized Southern Ocean

Sea Level and the Role of Coastal Trapped Waves in Mediating the Influence of the Open Ocean on the Coast

Antarctic Circumpolar Current Transport Through Drake Passage: What Can We Learn From Comparing High‐Resolution Model Results to Observations?

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