2014 Ocean Sciences Meeting | 23–28 February 2014 | Honolulu, HI

Submesoscale Water-Mass Spectra in the Sargasso Sea1 Applied Physics Laboratory, U of Washington, Seattle, WA 98105; e-mail: kunze@apl.uw.edu; ph: 206.221.26162 SEOS, U of Victoria, Victoria, BC, V8W-3P6, CANADA3 MIT, Cambridge, MA 021394 Marine Science and Technology, U of Massachusetts Dartmouth, New Bedford, MA 02744

E. Kunze 1, J. Klymak 2, C.M. Lee 1, R. Ferrari 3, L. Goodman 4, and R.-C. Lien1

Poster 1506

During ONR’s Lateral Mixing DRI June 2011 field program, multiple-platform nested surveys in the Sargasso Sea captured submesoscale water-mass variability. To filter out internal waves that dominate dynamic signals and focus on subinertial stirring, these data are synthesized into a spectrum for salinity (water-mass, spice) anomalies on isopycnals. Gradient spectra are flat within ±1/3 slope over horizontal wavelengths of 0.1–30 km consistent with fronts. They cannot be explained by:

• quasigeostrophic turbulence theory predictions of blue (k 1) Batchelor (1959) gradient spectra above either the interior or surface Rossby wavenumber

• diapycnal influences due to mixing by internal-wave breaking or nocturnal mixed-layer deepening

• internal-wave horizontal strain which is an order of magnitude too small

It is suggested that subinertial horizontal shears are not limited by mode-one interior or surface Rossby deformation scales of quasigeostrophic theory but have a broadband spectrum extending to smaller horizontal scales associated with eddy straining.

ABSTRACT

Gradient spectra from 20–60 m depth in the summer Sargasso Sea are flat to within slopes ±1/3 over horizontal wavenumbers k = 0.03–10 cpkm (horizontal wavelengths 0.1– 30 km) (Fig. 5).

Flat gradient spectra in the 0.03–10 cpkm wavenumber range are not consistent with either interior or surface quasigeostrophic turbulence theories (e.g., Charney 1971; Scott 2006) which predict a k1 Batchelor (1959) spectrum above the Rossby wavenumber. The redder observed spectrum suggests significant horizontal straining at smaller lengthscales.

The k 0 (flat) tracer-gradient spectra appear to be more universal, extending to higher wavenumber (Fig. 5, Ferrari and Rudnick 2000; Cole et al. 2010; Callies and Ferrari 2014) and greater depth (Cole and Rudnick 2012), than can be explained by quasigeostrophic theories, calling for alternative explanations.

7. SUMMARY

Batchelor, G.K., 1959: Smallscale variation of convected quantities like temperature in turbulent fluid 1: General discussion and the case of small conductivity. J. Fluid Mech., 5, 113–133.

Callies, J., and R. Ferrari, 2014: Interpreting energy and tracer spectra of upper-ocean turbulence in the submesoscale range (1-200 km). J. Phys. Oceanogr., submitted.

Capet, X., J.C. McWilliams, M.J. Molemaker and A.F. Shchepetkin, 2008: Mesoscale to submesoscale transition in the California Current system I: Flow structure, eddy flux and observational tests. J. Phys. Oceanogr., 38, 29–43.

Charney, J.G., 1971: Geostrophic turbulence. J. Atmos. Sci., 28, 1087–1095. Cole, S.T., D.L. Rudnick and J.A. Colosi, 2010: Seasonal evolution of upper-ocean horizontal structure and the remnant

mixed layer. J. Geophys. Res., 115, doi: 10.1029/2009JC005654. Cole, S.T., and D.L. Rudnick, 2012: The spatial distribution and annual cycle of upper ocean thermohaline structure. J.

Geophys. Res., 117, doi: 10.1029/2011JC007033.Ferrari, R., and D.L. Rudnick, 2000: Thermohaline variability in the upper ocean.

J. Geophys. Res., 105, doi: 10.1029/2000JC900057. Klymak, J.M., W. Crawford, M.H. Alford, J.A. MacKinnon and R. Pinkel, 2014: Along-isopycnal variability of spice in the

North Pacific. J. Geophys. Res., submitted. Scott, R.K., 2006: Local and nonlocal advection of a passive tracer. Phys. Fluids, 18, 1– 8. Tulloch, R., J. Marshall, C. Hill and K.S. Smith 2011: Scales, growth rates and spectral fluxes of baroclinic instability in the

ocean. J. Phys. Oceanogr., 41, 1057–1076.

REFERENCES

NOT:(i) internal-wave stirring • horizontal wavenumber spectrum for horizontal strain from the Garrett-and-Munk

model spectrum is an order of magnitude below observed spectra (ii) diapycnal processes removing spice variance at smaller horizontal lengthscales

to redden the slope • diapycnal mixing will remove variance on horizontal lengthscales L ~ 10 m, beyond

the scales resolved(iii) nocturnal mixed-layer deepening • only extends to 20-m depth based on EM float profile time-series

8. EXPLANATIONS

MAYBE:(iv) measurements capture an unrepresentative part of the field that does not reflect

the full turbulent statistics • this does not explain similar results from much more extensive measurements that

were not so biased (Ferrari and Rudnick 2000; Cole et al. 2010; Cole and Rudnick 2012; Callies and Ferrari 2014; Klymak et al. 2014)

(v) subinertial turbulent field is nonstationary or nonhomogeneous beyond assumptions in QG turbulence theory and models

(vi) subinertial shears are not limited by the Rossby deformation wavenumber as in quasigeostrophic turbulence theory

• Tulloch et al.’s (2011) numerical simulations find that, in the absence of a deep PV-gradient reversal that typifies western boundary currents, that is, conditions that typify the Sargasso Sea, baroclinic instability fills out the submesoscale

• Capet et al. (2008) report flat tracer gradient spectrum at all depths in California Current primitive equation simulations, even though HKE and APE transition from k –2 surface QG to k –1 interior QG at depth

Submesoscale stirring is important for the cascade of water-mass (spice) and other tracer variability to small scales and mixing. These lengthscales represent an observational challenge because internal gravity waves dominate dynamic (HKE, APE) signals, leaving only passive tracers as possible signatures of subinertial submesoscale dynamics.

1. INTRODUCTION

LATMIX11 14 JUN 2011 Hammerhead S'0.29

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Hammerhead towyo path in Eulerian (left) and Lagrangian (right) coordinates from its 14 June deployment (Fig. 4). Transformation centered on a Gateway buoy reduces space-time aliasing.

4. LAGRANGIAN FRAME

Redder than k 1 gradient spectra have been reported in other observations.

1. Based on wavelet analysis, Ferrari and Rudnick (2000) reported white spice-gradient spectra on wavelengths of 0.01–10 km (~0.1–100 cpkm).

2. Cole et al. (2010) found flat or slightly blue gradient spectra for k < 0.03 cpkm.

3. Callies and Ferrari (2014) report consistency with interior QG for k > 0.5 cpkm in the Sargasso Sea, but bluer spectra with depth independence over 130–250 m inconsistent with the expected decay of surface QG in the North Pacific.

4. In 2 years of mid North Pacific glider data, Cole and Rudnick (2012) report k 0 gradient spectra for 0.003 < k < 0.07 cpkm to a depth of 800 m where surface quasigeostrophy should not apply.

5. Klymak et al. (2014) found blue spice-gradient spectra with slopes 0.1–1 over 0.01–0.25 cpkm in the summer eastern North Pacific subpolar gyre, and 0.25–0.55 in the summer central subtropical gyre. Shallower spectra were bluer while deeper spectra redder (slopes 0.1– 0.3), inconsistent with the predicted trend from surface QG theory (Scott 2006). They reported vertically coherent eddies of nonlocal origin; coherent eddy features are not included in homogeneous QG turbulence theory.

6. OTHER SUBMESOSCALE TRACER SPECTRA

Hammerhead was towyoed within ± 5 m of the target dye injection density to resolve the smallest possible horizontal scales. Temperature, conductivity and pressure from all 4 instruments were transformed into isopycnal (σθ) temperature T, salinity S and displacement ξ.

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• Triaxus measured a 35-km radiator grid (red)• U of Victoria’s MVP repeated 15-km cross-shaped surveys (blue) • Hammerhead towyoed in 2-km radius circles (red loops centered on 32°5’N,

73°7’W) around • 1-km T-REMUS boxes (green) centered on a Gateway buoy

Figure 3. Illustration of the nested sampling over 3 days at site 1.

3. MEASUREMENTS

Acknowledgments: The co-authors were all supported for this work under the ONR Lateral Mixing DRI.

All four instruments (Fig. 5) show a flat (k 0) gradient spectra within slope ±1/3 for salinity variability on isopycnals for wavenumbers of 0.03–10 cpkm (horizontal wavelengths 0.1–30 km).

A k 0 (flat) gradient spectrum is consistent with a step or front.

MVP and Triaxus found higher levels at site 2 and MVP still higher in the zonal direction, consistent with the front (Figs. 2 and 4).

With less data, Hammerhead and T-REMUS spectra had to be averaged over both sites — as well as both zonally and meridionally— to produce stable spectra. Higher spectral levels for the zonal spectra at site 2 were also found by Hammerhead and T-REMUS (not shown), consistent with MVP.

5. SPECTRA

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Figure 5. Horizontal wavenumber gradient spectra k 2S [S’ ](k) synthesizing salinity anomalies S’ along isopycnals from Triaxus (solid, site 1, site 2), MVP (dot and dash, site 1, site 2), Hammerhead (black) and T-REMUS (green). Dotted black curves correspond to: surface quasigeostrophic turbulence theory predications at (i) the surface (0-m, k1/3) and (ii) upper pycnocline (60-m) for which a k1 Batchelor spectrum is predicted above the Rossby

wavenumber (left gray shading), and (iii) internal-wave (GM) model spectra. Wavenumber is in cycles per kilometer (cpkm).

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Two Sargasso Sea sites were sampled:

1. 2–10 June 2011: very weak eddy field with 1-km confluences less than 0.01f and little water-mass variability;

• confluence (or rate of strain, or rate of deformation) is [(ux – vy)2 + (vx + uy)2]1/2 2. 13 –19 June 2011: moderate 0.1f confluence sharpened a meridional water-mass

front (Figs. 1–2) and accelerated a NNW jet over 13–16 June.

2. BACKGROUND ENVIRONMENT

Figure 2. MVP time-series of sharpening of a water-mass front during 12–15 June 2011 by ~0.1f confluence. Plotted variable is salinity S. Vertical axis is an isopycnal coordinate, average depth of an isopycnal <z(ρ)>. Horizontal axis is zonal distance from the front axis. Pink contour shows the extent of the dye patch. Sharpening is not monotonic but oscillates due to near-inertial vertical shear.

Figure 1. Evolution of AVHRR SST between 13 and 14 June 2011 at the onset of the site 2 occupation of the eastern side of the weak warm intrusion near 32°30’N, 73°10’ W (circles).