Estimates of Eddy Heat Flux Crossing the Antarctic Circumpolar Current from Observations in ...

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D. Randolph Watts
University of Rhode Island, [email protected]
Karen L. Tracey
University of Rhode Island
Kathleen A. Donohue
University of Rhode Island
Teresa K. Chereskin
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Watts, D. R., Tracey, K. L., Donohue, K. A., & Chereskin, T. K. (2016). Estimates of Eddy Heat Flux Crossing
the Antarctic Circumpolar Current from Observations in Drake Passage. Journal of Physical
Oceanography, 2083-2102.
Available at: http://dx.doi.org/10.1175/JPO-D-16-0029.1
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Obs_EHF_Drake.tex
Estimates of eddy heat flux crossing the Antarctic Circumpolar Current
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from observations in Drake Passage
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D.RandolphWatts∗,KarenL.Tracey,KathleenA.Donohue,
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GraduateSchoolofOceanography,UniversityofRhodeIsland,Narragansett,RhodeIsland
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TeresaK.Chereskin
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ScrippsInstitutionofOceanography,UniversityofCaliforniaSanDiego
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∗Correspondingauthoraddress:D.RandolphWatts,GraduateSchoolofOceanography,University
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ofRhodeIsland,SouthFerryRd,Narragansett,RI02882.
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E-mail: [email protected]
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Generatedusingv4.3.2oftheAMSLATEXtemplate 1
ABSTRACT
Four-year measurements by current- and pressure-recording inverted echo
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sounders in Drake Passage produced statistically stable eddy heat flux esti-
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mates. Horizontal currents in the Antarctic Circumpolar Current (ACC) turn
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with depth when a depth-independent geostrophic current crosses the upper
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baroclinic zone. The dynamically important divergent component of eddy
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heat flux is calculated. Whereas full eddy heat fluxes differ greatly in mag-
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nitude and direction at neighboring locations within the Local Dynamics Ar-
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ray (LDA), the divergent eddy heat fluxes are poleward almost everywhere.
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Case studies illustrate baroclinic instability events that cause meanders to
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grow rapidly. In the southern passage where eddy variability is weak, heat
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fluxes are weak and not statistically significant. Vertical profiles of heat flux
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are surface-intensified with ∼50% above 1000 m and uniformly distributed
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with depth below. Summing poleward transient eddy heat transport across
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the LDA of −0.010±0.005 PW with the stationary meander contribution
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of −0.004±0.001 PW yields −0.013±0.005 PW. A comparison metric,
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−0.4 PW, represents the total oceanic heat loss to the atmosphere south of
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60◦S. Summed along the circumpolar ACC path, if the LDA heat flux oc-
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curred at six ‘hot spots’ spanning similar or longer path segments, this could
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account for 20% to 70% of the metric, i.e., up to −0.28 PW. The balance of
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ocean poleward heat transport along the remaining ACC path should come
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fromweakeddyheatfluxesplusmeancross-fronttemperaturetransports. Al-
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ternatively,themetric−0.4PW,havinglargeuncertainty,maybehigh.
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2
1. Introduction
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The Southern Ocean heat balance affects Antarctic climate and glacial melting directly and
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global climate in general through its effect upon down- and upwelling across the Antarctic Cir-
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cumpolar Current (ACC). These processes in turn facilitate the biological productivity around
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Antarctica and govern the sequestration and release of CO . A proper understanding of how heat
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crosses the ACC is crucial to correctly model the ocean’s influence upon climate and has become
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critically needed owing to uncertainties about how the ACC system responds to changes in atmo-
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spheric forcing. Southern Ocean heat losses from ocean to atmosphere and northward heat losses
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by wind-driven Ekman transport must, in a slowly-changing mean state, be balanced by ocean
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processes. Transient-eddyprocessesandhorizontalandvertical-overturningcirculationcontribute
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topolewardheattransport. Therelativeroleoftheseheat-transfermechanismsremainsuncertain.
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Because the ACC encircles the globe with several fronts that signify partial barriers to cross-
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frontalexchange,meandersandeddiesmustplayanimportantroleinproducingmeridionalfluxes
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in the Southern Ocean. An early study by de Szoeke and Levine (1981) suggested that along a
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mid-ACC path defined by the 2◦C isotherm, transient eddies were almost entirely responsible for
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cross-frontal heat fluxes. Exchange across ACC fronts is thought to be particularly concentrated
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in just a handful of locations with energetic eddies and steep stationary meanders, facilitated by
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bottom topography (Thompson and Naveira Garabato 2014). Estimates of eddy heat fluxes from
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observations have been made in a small subset of these exchange ‘hot spots’, notably Drake Pas-
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sage(Bryden1979;Nowlinetal.1985;Walkdenetal.2008;Lennetal.2011;Ferrarietal.2014),
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south of Tasmania (Phillips and Rintoul 2000) and south of New Zealand (Bryden and Heath
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1985). Combining these estimates to elucidate the global role of transient eddies is challenging,
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not only because of uncertainties in the along-ACC heterogeneity of the transient eddy heat flux
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3
magnitude, but also due to the different techniques used to calculate the fluxes themselves: Two
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examplesarecompensationformooringdraw-downinenergeticregimes(eg.,Nowlinetal.1985)
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andtheseparationofrotationalanddivergentcomponentsofoceaneddyheatfluxes(e.g.,Marshall
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and Shutts 1981; Jayne and Marotzke 2002). More recently, analysis of an eddy permitting ocean
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modelbyVolkovetal.(2010)showedthattheinfluenceoftransienteddieshasastronglatitudinal
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dependence, weakening substantially south of ∼60◦S. Furthermore, Volkov et al. (2010) showed
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thatstationarymeandersareanimportantconduitforheattransport. Inastationarymeander,both
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local and circumglobal, a zonal-mean meridional heat flux ensues if portions flowing northward
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andothersflowingsouthwardhavedifferenttemperatures(SunandWatts2002).
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Interaction of the ACC with topography that leads to turning of the mean along-stream ACC
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with depth provides another mechanism for poleward heat transport. The exact nature of this
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mean cross-ACC flow is not well understood. It has been hypothesized that this occurs at a few
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isolated locations by Sekma et al. (2013) based upon their observations in the narrow channel
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at Fawn Trough. Yet other studies indicate it may occur more ubiquitously. Chereskin et al.
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(2012) showed that recirculations are common in the Polar Frontal Zone within Drake Passage
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with components crossing the upper baroclinic zone of the ACC both northward and southward.
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Analysis of ocean circulation models shows that mean-flow contributions could result from the
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accumulation of weak mean cross-ACC flow (Pen˜a Molino et al. 2014) or from the accumulation
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oflargelocalizedpositiveandnegativecontributions(Ferrarietal.2014).
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The purpose of this work is to quantify poleward heat fluxes across the ACC by transient and
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stationary eddies using observations in Drake Passage. Our study, called cDrake, deployed an
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extensivearrayofcurrent-andpressure-recordinginvertedechosounders(CPIES)tomeasurethe
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current and temperature structure through the full water column for four years in Drake Passage.
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Itincludedatransectspanningthechannelplusalocaldynamicsarraysitedinaregionofelevated
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4
eddy kinetic energy (Lenn et al. 2007; Firing et al. 2011) in the Polar Frontal Zone between the
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SubantarcticFrontandthePolarFront. Sections2and3presentthedetailsoftheobservationsand
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methodology. Inparticular,Section2presentstechniques,developedinGulfStreamandKuroshio
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studies (Cronin and Watts 1996; Bishop et al. 2013), that remove a large rotational nondivergent
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contribution from the full eddy heat flux in order to identify the dynamically important divergent
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eddyheatflux.
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Section 4 presents our findings: mapped mean eddy heat flux in central Drake Passage, case
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studies of cyclogenesis responsible for the observed eddy heat flux pattern, and estimates of eddy
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heat flux on a transect that spans Drake Passage. Section 5 returns to the issues raised in this
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introduction with a more thorough treatment of the relative contribution and role of the various
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ocean heat-transfer mechanisms. cDrake results and existing estimates of eddy and mean-flow
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heat fluxes are discussed in the context of the global heat budget. Section 6 provides a summary
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ofourresults.
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2. ObservationsandData
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The cDrake experiment deployed current- and pressure-recording inverted echo sounders in
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Drake Passage for four years (December 2007 through November 2011), arranged in two con-
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figurations(Fig.1). One,theClinetransectthatspannedthe800kmpassage,included20CPIES
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spaced by 40–60 km. In addition, a local dynamics array (LDA), centered upon the region of
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highest eddy kinetic energy (EKE) between the Polar Front (PF) and Subantarctic Front (SAF),
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included 24 CPIES sites in a two-dimensional grid with 40 km spacing. During the final year, a
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closely-spaced third array of five CPIES was moored at the base of the Shackleton Fracture Zone
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(SFZ).Atotalof43CPIESweredeployedinthethreearrays.
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5
CPIES measure hourly near-bottom horizontal currents (u ,v ), 50 m above the seafloor in
101 ref ref
order to be outside the benthic boundary layer, bottom pressure (p ), and surface-to-bottom
102 bot
round-tripacoustictraveltime(τ). Mostsitesreturnedhourlydataforthefouryears(93%to97%
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data-return for different variables). Low-pass filtering used a 4th order Butterworth filter, passed
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forward and backward, and cut-off periods of 3 d or 7 d as noted. These data are documented in a
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comprehensivetechnicalreportbyTraceyetal.(2013).
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Full-depth CTD casts were collected for calibration purposes at the CPIES sites during five
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annualcruisesthatvisitedthearraysites. TheCPIESdataprocessingproducedverticalprofilesof
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thetime-varyingcurrentandtemperaturefields,astreatedinthefollowingSection3.
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The Ssalto/Duacs daily sea level anomaly products were produced by the Copernicus Marine
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and Environment Monitoring Service, and the mean dynamic topography MDT CNES CLS13
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wasproducedbyCLSSpaceOceanographyDivision. Bothaltimeterproductsweredistributedby
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AvisowithsupportfromCnes(http://aviso.altimetry.fr/).
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3. Methods
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a. Measurementsoftemperatureandcurrentfields
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CPIES determine temperature and horizontal velocity profiles (T(p),u(p),v(p)) following
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methods presented in Donohue et al. (2010), which were expanded and applied to Drake Passage
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byFiringetal.(2014). Instrongandeddyingcurrentsystems,τ hasbeenappliedtodetermineden-
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sityprofilesusingagravestempiricalmodelook-uptablebasedonlocalhydrography(Meinenand
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Watts 2000; Watts et al. 2001). Applying geostrophy, laterally separated pairs of density profiles
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produce vertical profiles of baroclinic velocity relative to a near-bottom reference level, chosen
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here to be 3500 dbar. In a two-dimensional array, the velocities determined are two-dimensional
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baroclinic current profiles relative to the bottom, designated u = (u ,v ). The velocities
123 bcb bcb bcb
and temperatures are mapped at half-daily intervals by optimal interpolation as described by Fir-
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ingetal.(2014)withcarefulattentionpaidtoerrorestimates.
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Deeppressureanomaliesareleveledtoaconsistentmean3500dbargeopotential,assumingthat
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long time averages of near-bottom currents and bottom pressures are geostrophic. Multivariate
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mappingwhichcombinesthedeeppressureandcurrentmeasurements(Firingetal.2014)provides
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the reference velocities, u =(u ,v ), used to render the baroclinic velocity profiles absolute
129 ref ref ref
(Fig.2):
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u = (u,v) = (u ,v )+(u ,v ) = u +u (1)
bcb bcb ref ref bcb ref
It is important to note that, while the u component flows unidirectionally parallel to the front,
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thevectorsumutotalcurrentturnswithdepthduetotheu contribution. Theturningillustrated
132 ref
in Fig. 2 can occur instantaneously as well as in the time-mean, because deep eddies and mean
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topographicallysteeredcurrentsandrecirculationscancrosstheupperbaroclinicACCstructure.
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Velocity and temperature time series at depths distributed through the water column from the
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CPIES LDA agree well with contemporaneous moored current meter measurements on French
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mooring M4 (Ferrari et al. 2012, 2014), whose location was near site E02 (Fig. 1). Firing et al.
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(2014) showed this comparison, which we repeat in Fig. 3 with correlations for 3 d and 7 d low-
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pass filtering noted. The current meters measuring u,v,T moved vertically in the water column
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as the mooring drew down in response to variable drag by the currents. So at each time-sample,
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theCPIESu,v,T measurementswerecomputedtocoincidewiththepressurelevelofeachcurrent
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meter, in order to conduct a comparison without introducing uncertainties due to mooring motion
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compensation.
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The two time series for each variable at each depth compare point current meter measurements
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and optimally mapped geostrophic currents. These variables would have small intrinsic differ-
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7
ences even if both measurements were perfect. Temperatures agree within 0.3, 0.1, and 0.1◦C at
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approximately520,930,and2540dbar,respectively,andFiringetal.(2014)reportthebaroclinic
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velocities at the corresponding levels agree within 0.11, 0.08, and 0.05 m s−1. Firing et al. (2014)
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accounted carefully for the observed differences as the sum of measurement and mapping error
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plus rapid small-scale ageostrophic processes present in the water column. The high correlation-
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squaredvaluesimprovefurtherwith7dlow-passfiltering(r2ranging0.74to0.90),whichremoves
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some of the variability from small-scale processes. Some short-period variability in the moored
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pointmeasurementscanbenotedthatisnotpresentintheCPIESrecords. Forexample,inthetem-
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peraturerecords,fluctuationsofsmallverticalscale,suchasfrominternalwavesorfromfilaments
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advected past individual current meters, would have insignificant effect upon τ and geopotential.
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These results are consistent with the high correlations also found south of Australia during the
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SubantarcticFrontDynamicsExperiment(SAFDE, Wattsetal.2001).
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b. Estimatesofeddyheatfluxfields
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Eddy heat flux computed from the total velocity time series is denoted (u(cid:48)T(cid:48),v(cid:48)T(cid:48)), where (·)(cid:48)
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indicatesdifferencefromthetemporalmean,(·),forexample,T(cid:48)=T−T. Timeseriesoffulleddy
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heat flux agree well between the CPIES and the moored current meter measurements at this same
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suite of depths through the water column (Fig. 4). This further comparison demonstrates that the
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covariances between eddy current and temperature are dominated by low-frequency geostrophic
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variability of large vertical scale rather than by higher vertical modes or by ageostrophic fluc-
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tuations. Just as for Fig. 3, both of these time series follow the varying depth of the moored
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measurements,inordertoavoidintroducinguncertaintiesfrommooringmotioncompensation.
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Short-period variability in the moored eddy heat fluxes does not contribute substantially to the
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time-mean eddy heat flux, (u(cid:48)T(cid:48),v(cid:48)T(cid:48)). Small-scale temperature features that quickly pass the
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pointmeasurementsareuncorrelatedwiththecurrents,whicharepredominantlygeostrophic. For
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current meter measurements, as well as for CPIES, the fraction of u(cid:48)T(cid:48) covariance in periods
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shorter than 7 d is only 4% to 9%. The u(cid:48) versus T(cid:48) coherences (not shown) are typically greater
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than0.9forperiodslongerthan10dandfallsharplybelow0.4to0.2forperiodsshorterthan7d.
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Overall, eddy heat fluxes are mapped and measured well by the CPIES, accounting for 80% to
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90%oftheeddyheatfluxvarianceatalldepths.
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c. Thedivergentcomponentofeddyheatflux
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MarshallandShutts(1981)demonstratedthatthefulleddyheatflux,u(cid:48)T(cid:48),containsarotational
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component that, although it may be large, recirculates around contours of temperature variance,
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(T(cid:48))2. As they noted, it is advantageous to remove that rotational contribution because the dy-
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namically important part of eddy heat flux is the divergent component. Cronin and Watts (1996)
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appliedtheMarshallandShutts(1981)methodtodatafromanarrayofcurrentmetersmooredbe-
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neath the Gulf Stream. Their application removed a large rotational contribution of the eddy heat
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flux. Additionally, they discussed the dynamical importance of the divergent residual component.
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Bishopetal.(2013)studieddivergenteddyheatfluxesintheKuroshioExtensionusingdatafrom
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anarrayofCPIES.TheyshowedanaturaloutcomeofexpressingtheCPIESgeostrophicvelocities
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as above in equation 1 is that the divergent eddy heat flux arises entirely from joint interaction of
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theupperbaroclinicfrontandthedeepreferencecurrent,asweillustratenext.
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Referring again to Fig. 2, the velocity component u is geostrophic and flows along the time-
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varyingfront,paralleltogeopotentialcontours(φ). ForthecDrakegravestempiricalmodelook-up
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table, temperature and geopotential are both functions of τ on pressure surfaces, so isotherms are
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parallel to geopotential contours. Firing et al. (2014, their Fig. 13) show these cDrake gravest
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empirical mode relationships for T, S, δ, and φ versus τ. Chidichimo et al. (2014) also illustrate
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