Observed Eddy Heat Fluxes across the ACC in northern Drake Passage

D. Randolph Watts, Karen Tracey, Kathleen Donohue (Univ. Rhode Island)

and Teresa Chereskin (Univ. Calif. San Diego) for IAPSO – July 26, 2013

“cDrake”Dynamics and Transport of the ACC in Drake Passage

Black triangles are current and pressure recording inverted echo sounders (CPIES)

4 years,12/2007- 12/2011

• Local dynamics array: • maps the daily (u,v), T fields• in upper and deep levels• stable statistics in 4 yrs

co-PI’s: T. Chereskin, K. Donohue, R. Watts

cDrake is funded by US National Science Foundation

CPIES:current and pressure recording inverted echo sounder

Measures bottom current. (50 m off bottom)

Emits 12kHz sound pulses.Measures round trip travel times of acoustic pulses from sea floor to sea surface. Measures bottom pressure.

A CPIES array yields daily maps of current and temperature fields u(x,y,p,t), T(x,y,p,t)

Donohue et al. (2010 JTech); Firing et al. (2013, submitted)

Y. Firing et al., 2013 in review at JTech.

(u, v) and T are estimated well in LDA, top to bottome.g, comparison with French mooring (thanks to C.Provost)

520 dbar

1020 dbar

2540 dbar

u v T

(u’T’, v’T’) are also estimated well in LDA, top to bottome.g, again by comparison with French mooring (thanks to C.Provost)

520 dbar

1020 dbar

2540 dbar

u’T’ v’T’ T’

This indicates CPIES (u,v), T fields account well for eddy heat flux.

Four-year mean eddy heat flux fields in cDrake

<u’T’>

<u’T’> field in the cDrake LDA, -- mid-thermocline at 400 dbar -- superimposed with surface EKE

Total EHFs are large (~0.05oC m s-1) in this region of high-EKE

These 4-yr statistics are stable (even after 2-yrs)

But structure is complicated by a large nondivergent component, whose direction varies, equatorward and poleward, depending on location.

We next discuss how to eliminate this nondivergent part…

EKE:

• The ocean current structure measured by CPIES provides a straightforward way to identify and eliminate a large nondivergent part of eddy heat flux (EHF)…

φ-

φ+

200ubcb100020004000

u = ubcb + urefuref

Absolute geostrophic currents u are the vector sum ubcb+uref

ϕ is baroclinic stream function;ubcb is vertically-aligned baroclinic current, zero at the bottom; uref is reference current measured

at bottom.

baroclinic EHF is entirely nondivergent, ∇ <u’bcbT’> = 0 because ubcb parallels isotherms, ϕ=ϕ(T) at each pressure level (like Marshall & Shutts’81)

barotropic reference EHF, <u’refT’>, contains the divergent EHF

Separate baroclinic and reference parts of eddy heat flux

<u’T’>

EKE:

<u’refT’>

✚

The baroclinic EHF causes the complicated structure. It is nondivergent and recirculates along <T’2> contours; ignore it. (like Marshall & Shutts(‘81), Cronin & Watts (‘96))

The total mean EHF is composed of barotropic ref and baroclinic EHFs

✚<u’bcbT’>

The mean baroclinic and reference parts of eddy heat flux

<u’T’>

EKE:

<u’refT’>

✚<u’bcbT’>

BCB part recirculates along <T’2> contours; we will focus on BT REF part.

<u’T’>

EKE:

<u’refT’>Now focus on barotropic ref EHF

Barotropic ref EHF • (~ 0.025 oC m s-1)• strongest downstream of SFZ ridge • mainly poleward

<u’refT’>

Zonal averages show the main contribution to barotropic EHF is by transient eddiesrather than by stationary meanders.

[<u’refT’>][<u*ref><T*>]

<u’refT’>

purple shaded region: DEHF downgradient

<u’T’>div = DEHF

Contours: mean ϕ, baroclinic front

Extract the divergent part by optimal interpolation(Watts et al., 2013)

The barotropic ref EHF contains all the divergent contribution (DEHF)

<u’T’>div <T>∇ modifies its environment and feeds eddy growth

DEHF vertical structure (4-yr mean field)

Six locations in the 400m mean DEHF field, chosento examine vertical structure of <u’T’>div

• Downgradient flux all along PF and SAF interfrontal zone

• Peak in thermocline near 300 m, yet still ~20% as large between

1500-3500m• Important contrib’n top-to-bottom

with 50% of flux below 800m

• Median DEHF, sites 2,4,5 o 0.009 oC m s-1 = 38 kW m-2 at

400 mo vertical integral = 56 MW m-1

-0.01 0.020.010.0-4000

-3000

-2000

-1000

0

Time series of DEHF, and case-study 1

Rapid growth of a SAF meander crest

(A) jointly steepens with deep anticyclone.

(B) signature BC instability, upper crest & trough led by deep high & low

(C) grows more vertically aligned.

(D) mature vertically-aligned current structure and little DEHF.

A B C D

Meridional DEHF, site 1 is typical. Mean accumulates from many short-lived events.

* bcb geopotential ϕ0 re 4000 (solid contours)* bt ref-pressure anomaly, P’4000 (colorbar)* daily DEHF u’T’div (green arrows)

* bcb geopotential ϕ0 re 4000 (solid contours)* bt ref-pressure anomaly, P’4000 (colorbar)* daily DEHF u’T’div (green arrows)

Context – eddy heat flux and air-sea fluxes in Southern Ocean

• Mean downgradient DEHF at 400 m is 0.009 oCm/s ≈ 38 kW/m2

• Vertically-integrated downgradient DEHF is 56 MW/m • If integrated circumglobally, 1.2 PW poleward heat transport• i

What is the meaning of these numbers? Heat budget in zonal mean…* Eddy heat transport could provide twice the total heat needed to balance heat lost from ocean to atmosphere south of 58oS.* Mean transport in regional and global stationary meanders is likely to play an important role, but in cDRAKE the eddy transports are 2-to-4 times larger* South of 58oS the Ekman and MOC contributions are small in zonal mean

Wind stress and momentum budget… * Poleward eddy heat flux corresponds to downward transfer of eastward momentum. Our confirmation that poleward EHFs extend with large magnitude throughout the water column is consistent with eddy transfer of wind stress downward to the sea floor.

cDrake findings review …•4 yr mean divergent eddy heat fluxes (DEHF) observed between the ‐Subantarctic Front (SAF) and Polar Front (PF) in Drake Passage are mainly down-gradient. They are stable estimates,

– arising from several poleward-DEHF transient eddies per year.

•Strongest DEHFs occur immediately downstream of Shackleton Fracture Zone (SFZ, topographic ridge), where

– meanders grow jointly with deep-reference barotropic eddies,

•Baroclinic current component is parallel to isotherms and produces only nondivergent EHFs

•Deep-reference eddy currents cross the baroclinic front, and produce divergent eddy heat flux throughout water column

– their phase-offset in growing baroclinic instability events favors downgradient flux

• END • QUESTIONS? • Thanks for your attention.

The cDrake experiment 11/2007 – 11/2011 had a local dynamics array of 24 current and pressure recording inverted echo sounders (CPIES) centered near 57oS, 63oW. It spanned a maximum eddy kinetic energy region between the Subantarctic Front and Polar Front. The CPIES array provides full water-column estimates of velocity and temperature, mapped daily with mesoscale resolution to quantify and characterize eddy heat flux. The dynamically important component of eddy heat flux is the divergent field, which modifies its environment and transfers energy from mean to eddy field; the non-divergent component just recirculates. Optimal-interpolation mapping was used to separate the total eddy heat fluxes into nondivergent and divergent parts. The observed time-average total eddy heat fluxes are large and rotate around elevated mean temperature variance regions. In contrast, divergent eddy heat fluxes (DEHFs) are oriented down-gradient and have magnitudes a few times smaller than the total eddy heat fluxes. DEHFs arise from nearly depth-independent geostrophic currents that can cross the temperature front. The mapped annual-mean DEHFs have spatial structure that is remarkably consistent from year-to-year, exhibiting strong poleward flux just downstream of a prominent topographic ridge (Shackleton Fracture Zone). The vertical structure of DEHFs is maximum near 300 m depth with typical value 40 kW m-2 and decreases to 7 kW m-2 by 1200 m depth, below which the DEHFs are relatively constant. Vertically-integrated poleward divergent fluxes are 60 MW m-1. Time series of daily heat flux show that the means accumulate from many poleward pulses that last just a few days; they arise during interactions between the deep barotropic eddies and the upper baroclinic jet in actively growing baroclinic instability events. Consequently the time-averages of DEHF are rather stable after only 1-2 years.

That was the “comfort slide” – now, what do we not know?

• The “zonal integral” of eddy heat flux based on one or two observational sites is not satisfactory;

• is it a small overestimate or a qualitative overestimate?

• Role of stationary meanders (regional scales, and global wavenumber 1)• Role of rings in poleward heat flux• Role of topography

• Driving deep mean flow across baroclinic jet• Causing strong meanders downstream

• How to use models to integrate heat flux zonally around globe?• OFES, HYCOM, POM, SOSE (ECCO), OCCAM, … • Initial look at <Vref’T’> distribution shows models differ qualitatively in their

relationship to EKE, SSH variability, topographic features…

Marshall and Shutts (JPO, 1981) discussed the importance of separating non-divergent and divergent eddy fluxes. They presented a method to separate them if the mean flow is nearly along temperature contours, satisfying approximately, ψ ≈ ψ(T). The nondivergent part of EHF recirculates around <T’2> = EPE extrema.

In the EPE equation (neglecting triple correlations), (multiplied by (gα/θz )) U ( <T’∇ 2> /2) + <u’T’> <T>∇ + <w’T’> θz = 0 ,

when the eddy heat flux is separated into nondivergent and divergent parts,

U ( <T’∇ 2> /2) + <u’T’>nondiv <T> ∇ = 0 , and <u’T’>div <T> + <w’T’> ∇ θz = 0

i.e., the nondivergent EHF term balances lateral advection of EPE by the mean flow,and the divergent EHF term balances <w’T’> θz , which produces EPE in eddies.

Important first step in decomposing eddy heat fluxes

3-year and 2-year mean EHF, EHFbtref , and DEHF fields provide nearly steady estimates