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Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current

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Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Balwada, Dhruv; Speer, Kevin G.; LaCasce, Joseph H.; Owens, W. Brechner; Marshall, John and Ferrari, Raffaele. “ Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current a .” Journal of Physical Oceanography 46, no. 7 (July 2016): 2005–2027. © 2016 American Meteorological Society As Published http://dx.doi.org/10.1175/JPO-D-15-0207.1 Publisher American Meteorological Society Version Final published version Citable link http://hdl.handle.net/1721.1/109082 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. JULY 2016 B A L W A D A E T A L . 2005 Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Currenta DHRUV BALWADA AND KEVIN G. SPEER Department of Earth, Ocean, and Atmospheric Science, and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida JOSEPH H. LACASCE Department of Geosciences, University of Oslo, Oslo, Norway W. BRECHNER OWENS Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts JOHN MARSHALL AND RAFFAELE FERRARI Department of Earth, Atmosphere and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts (Manuscript received 27 October 2015, in final form 4 April 2016) ABSTRACT The large-scale middepth circulation and eddy diffusivities in the southeast Pacific Ocean and Scotia Sea sectors between 1108 and 458W of the Antarctic Circumpolar Current (ACC) are described based on a subsurface quasi-isobaric RAFOS-float-based Lagrangian dataset. These RAFOS float data were collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The mean flow, adjusted to a common 1400-m depth, shows the presence of jets in the time-averaged sense with speeds of 2 2 6cms 1 in the southeast Pacific Ocean and upward of 13 cm s 1 in the Scotia Sea. These jets appear to be locked to topography in the Scotia Sea but, aside from negotiating a seamount chain, are mostly free of local topographic constraints in the southeast Pacific Ocean. The eddy kinetic energy (EKE) is higher than the mean kinetic energy everywhere in the sampled domain by about 50%. The magnitude of the EKE increases drastically (by a factor of 2 or more) as the current crosses over the Hero and Shackleton fracture zones into the Scotia Sea. The meridional isopycnal stirring shows lateral and vertical variations with local eddy diffu- 2 2 sivities as high as 2800 6 600 m2 s 1 at 700 m decreasing to 990 6 200 m2 s 1 at 1800 m in the southeast Pacific Ocean. However, the cross-ACC diffusivity in the southeast Pacific Ocean is significantly lower, with values of 2 690 6 150 and 1000 6 200 m2 s 1 at shallow and deep levels, respectively, due to the action of jets. The cross- 2 ACC diffusivity in the Scotia Sea is about 1200 6 500 m2 s 1. 1. Introduction and an overturning component that is more tightly linked to diabatic processes in the interior or at the polar The global ocean circulation is often divided into a extremes. The polar extremes of dense water formation nearly horizontal, or approximately isopycnal, component create water masses that spread and fill the global ocean, but this spreading depends on the topography of ocean a Geophysical Fluid Dynamics Institute Contribution Number 476. basins. The cold deep water formed in the northern polar regions of the Atlantic Ocean, North Atlantic Deep Water (NADW), flows south in a deep western Corresponding author address: Dhruv Balwada, Geophysical Fluid Dynamics Institute, Florida State University, 018 Keen boundary current and eventually spreads along the Building, 77 Chieftan Way, Tallahassee, FL 32306-4360. northern flank of the Antarctic Circumpolar Current E-mail: [email protected] (ACC) on its course to the Indian and Pacific Ocean DOI: 10.1175/JPO-D-15-0207.1 Ó 2016 American Meteorological Society 2006 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46 basins. A fraction of NADW is injected into the ACC in which was undertaken in 2009–14 to quantify the magni- layers below the Drake Passage sill depth and can be tude of isopycnal eddy diffusivities and diapycnal mixing. transported across the ACC in deep geostrophic boundary We present results from the deployment of RAFOS floats currents to upwell into regions of surface buoyancy loss (subsurface drifters tracked by a moored acoustic net- and to be transformed into Antarctic Bottom Water work) in the southeast Pacific Ocean and Scotia Sea sec- (AABW). This AABW and the other part of the NADW tors of the ACC. We focus here on velocity statistics that moves into the Indian and Pacific basins are trans- (section 3) and isopycnal mixing (section 4) derived from formed to Indian Ocean Deep Water (IDW) and Pacific the RAFOS float observations. Ocean Deep Water (PDW) via diapycnal processes (e.g., Talley 2013). 2. Overview of the DIMES RAFOS float The shallower portions of these deep water masses of experiment the Indian and Pacific Oceans, referred to as Upper Circumpolar Deep Waters (UCDW), form layers in the RAFOS floats were deployed as part of the DIMES Drake Passage latitude band that are above the sill experiment, primarily between the synoptically ob- depth, sill depth being a somewhat complicated con- served positions of the Subantarctic Front (SAF) and struct primarily due to the Scotia Arc and the Kerguelen Polar Front (PF) at 1058W. Additional floats were de- Plateau. In these layers, simple theory suggests that ployed downstream of this deployment site to supple- there is no mean geostrophic flow across the 500-km ment the dataset. The total number of floats deployed band of the ACC (Warren 1990). It is often argued that was 210. However, after failures, 140 float tracks com- the dynamics in these layers is like that of the atmo- prising 183 years of float data (66 795 float days) were sphere, where the action of eddies can produce a mean retrieved. Figure 1 shows a summary of the experi- residual flux that on large scales in the Southern Ocean mental design and regional geography, together with the is toward the south (Thompson 2008). To quantify the mean sea surface height (SSH) contour lines that en- transport of this residual flux, in the absence of accurate velope the extent of the initial float deployment relative deep velocity measurements, one needs to quantify the to the ACC and the climatological position of the SAF amplitude of the isopycnal eddy stirring (eddy diffusiv- and PF according to Orsi et al. (1995). These SSH and ity) and the large-scale gradient of thickness or potential frontal positions provide a general sense of the large- vorticity (PV). Indirect estimates with box model in- scale ACC flow in the region that was sampled. versions suggest a southward flux of order 10 Sv (1 Sv [ The duration of the RAFOS float experiment was 2 106 m3 s 1) in deep layers (Lumpkin and Speer 2007; from 2009 to 2011, with the highest number of float days Sloyan and Rintoul 2001; Naveira Garabato et al. 2014). sampled in 2010 (Fig. 2). The floats were originally One view of the ACC (Meredith et al. 2011) is that of a ballasted to stay near two isopycnal surfaces of neutral large-scale, latitudinally broad mean eastward flow, density 27.6 and 27.9s. However, because of technical with a transport of about 140 Sv. However, there are failures, the behavior was closer to that of isobaric floats. large meridional excursions in the regions where it goes For this reason, the analysis in this manuscript treats the over midocean ridges and approaches continents. On floats as quasi-isobaric floats. Some floats showed a slow 2 this broad, baroclinically unstable mean flow lays a sinking of about 100 m yr 1, which does not affect any convoluted structure of jets and eddies (Sokolov and results presented here. The distribution of float days in Rintoul 2009). The merging and splitting can at any in- depth shows a bimodal structure with peaks at 800 and stance be acting as a barrier to mixing and at another 1400 m corresponding to the mean positions of the bal- instance strongly mix fluid parcels (Thompson 2010). lasting isopycnals. As the floats did not maintain their This is in marked contrast to the Gulf Stream, for ex- target density, the float days distribution in temperature is ample, where a single primary jet exists. The ACC jets wider, showing only a single peak. A distribution of float can be locked to topography, and nearly stationary, or days over topographic depth following the float shows a more freely evolving typically in regions with less to- peak at 4500 m corresponding to the mean depth of the pographic control (Sallée et al. 2008a). southeast Pacific Ocean. This distribution also has a long Although the importance of the ACC to the adiabatic tail toward shallower depths corresponding to the passage closure of the meridional overturning circulation has been through the Scotia Sea, where topographic variability is inferred for some time, direct measurements of the greater and topographic features often reach within a strength and nature of this process have been lacking few hundred meters of the surface. Hancock and Speer (Marshall and Speer 2012). Here we analyze results from (2013a,b) provide a detailed report of this dataset.
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