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Enhanced upward heat transport at deep submesoscale ocean fronts

Abstract

The ocean is the largest solar energy collector on Earth. The amount of heat it can store is modulated by its complex circulation, which spans a broad range of spatial scales, from metres to thousands of kilometres. In the classical paradigm, fine oceanic scales, less than 20 km in size, are thought to drive a significant downward heat transport from the surface to the ocean interior, which increases oceanic heat uptake. Here we use a combination of satellite and in situ observations in the Antarctic Circumpolar Current to diagnose oceanic vertical heat transport. The results explicitly demonstrate how deep-reaching submesoscale fronts, with a size smaller than 20 km, are generated by mesoscale eddies of size 50–300 km. In contrast to the classical paradigm, these submesoscale fronts are shown to drive an anomalous upward heat transport from the ocean interior back to the surface that is larger than other contributions to vertical heat transport and of comparable magnitude to air–sea fluxes. This effect can remarkably alter the oceanic heat uptake and will be strongest in eddy-rich regions, such as the Antarctic Circumpolar Current, the Kuroshio Extension and the Gulf Stream, all of which are key players in the climate system.

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Fig. 1: Study region (22 December 2014).
Fig. 2: Characteristics of the strongly turbulent area (red in Fig. 1).
Fig. 3: Strain field, frontogenesis and VHT.
Fig. 4: FSLE and horizontal buoyancy gradients (\(| {b}_{x}|\)).
Fig. 5: Temperature anomaly, vertical velocity and VHT.

Data availability

The marine mammal data were collected and made freely available by the International MEOP Consortium and the national programs that contribute to it, and is available at www.meop.net/database/meop-databases/meop-sms-database-submesosc.html. The Ssalto/Duacs altimeter products were produced and distributed by the Copernicus Marine and Environment Monitoring Service with support from CNES, and is available at http://marine.copernicus.eu/services-portfolio/access-to-products/.

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Acknowledgements

We thank K. Richards for his insightful comments, F. d’Ovidio for providing the code to compute FSLE. The elephant seal work was supported as part of the SNO-MEMO and by the CNES-TOSCA project Elephant seals as Oceanographic Samplers of submesoscale features led by C. Guinet with support of the French Polar Institute (programmes 109 and 1201). This research was carried out, in part, at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). High-end computing resources for the numerical simulation were provided by the NASA Advanced Supercomputing Division at the Ames Research Center. This work was partly funded by the CNES (OSTST-OSIW) and the Laboratoire d’Excellence LabexMER (ANR-10-LABX-19). L.S. is a NASA-JVSRP affiliate and is supported by a joint CNES-Région Bretagne doctoral grant. P.K. is supported by the NASA-CNES SWOT mission and a NASA Senior NPP Fellowship. A.F.T. is supported by the David and Lucille Packard Foundation and NASA grant NNX16AG42G. M.F. is supported by NASA grant NNX15AG42G.

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Contributions

L.S. and P.K. conceived the experiments, analysed the results and wrote the manuscript. D.M. and H.S.T. ran the numerical simulations. H.S.T. helped with analysing the regional simulation. L.S., P.K., P.R., A.F.T., H.S.T. and M.F. reviewed the manuscript.

Corresponding author

Correspondence to Lia Siegelman.

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The authors declare no competing interests.

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Peer review information Primary Handling Editor(s): Heike Langenberg.

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Extended data

Extended Data Fig. 1 Weakly turbulent and southern eddy edge areas.

Same as Fig. 2 but for i) 2014/11/24 to 2014/12/20 with the SSH snapshot in a) taken on 2014/12/07. The seal crosses a large anti-cyclonic region (grey trajectory in Fig. 1) characterized by weaker currents (smaller SSH anomalies) and referred to as the weakly turbulent area. ii) 2014/12/22-29 with the SSH snapshot in a) taken on 2014/12/26. The seal follows the edges of mesoscale eddies over a distance of ~600 km. This region is referred to as the southern eddy (in orange in Fig. 1). Bold black arrows indicate the direction of the seal.

Extended Data Fig. 2 Lateral gradient of buoyancy and Richardson number in the strongly turbulent area.

a) RMS of lateral gradients of buoyancy, |bx|, as a function of depth in the strongly turbulent area. b) Scatter plot between lateral gradients of buoyancy, |bx|, and Richardson number, Ri, in the strongly turbulent area. Ri<2 coincide with strong buoyancy gradients (|bx|>2.5 x 10-7s-2), highlighting the ageostrophic character of the dynamical regime encountered by the seal and the expected strong frontogenesis processes at play.

Extended Data Fig. 3 Map of finite size Lyapunov exponents.

Map of FSLE over the entire domain on 13 November 2014. FSLE are greatly enhanced in the strongly turbulent region (black rectangle and in red in Fig. 1) compared to the rest of the domain.

Extended Data Fig. 4 Finite size Lyapunov exponents and horizontal gradient of buoyancy, vertical velocities and vertical heat transport at 300 m.

Times series of a) Horizontal gradients of buoyancy at 300 m sampled by the seal (in black) and FSLE derived from satellite altimetry along the seal’s track (in blue). b) Vertical velocities at 300 m derived from the seal and satellite data by solving the omega equation (see main text and Methods). c) Vertical heat transport (see Methods). The areas described in the main text and in Fig. 1 are highlighted by the colored rectangles.

Extended Data Fig. 5 Daily averaged vertical velocities and vertical heat transport from the high-resolution numerical simulation.

Daily averaged vertical section from the high-resolution numerical simulation for November 22, 2011 at 52°S of a) Vertical velocities b) Vertical heat transport. Isopycnals are shown by the black lines. Enhanced vertical velocities and heat transport with a width of 5-10 km are found in the ocean interior and, in particular, below the mixed layer, similar to the observation presented in the main text.

Extended Data Fig. 6 Averaged vertical heat transport from the high-resolution numerical simulation.

2-D (x,y) view of 10-day averaged vertical heat transport (VHT) at a) 50 m and b) 200 m. Isotherms are shown in black. Domain averaged values are respectively 92 and 197 W/m2. VHT is enhanced at depth and follows the isotherms on the eddy edges, and its averaged value is directed upward (positive value), all of which is consistent with the observational results presented in the main text.

Extended Data Fig. 7 Domain averaged vertical heat transport from the high-resolution numerical simulation.

Monthly averaged vertical heat transport (<VHT>) as a function of depth over the entire domain from the high-resolution numerical simulation. VHT is directed upwards (positive values) and its magnitude is similar - although even higher - than what is derived from the observational data presented in the main text.

Extended Data Fig. 8 Distance between two dives and angle between the seal’s trajectory and the fronts.

a) Histogram of the distance between two dives. Median distance between two dives is 700 m (dotted line) and the shortest distance is 100 m. b) Histogram of the angle between the seal’s trajectory and the stretching FSLE it encounters for FSLE>0.15 day-1. Oblique crossings are most frequent and a normalization is implemented to correct for it (see Methods).

Supplementary information

Supplementary Information

Supplementary Discussion Figs. 1 and 2.

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Siegelman, L., Klein, P., Rivière, P. et al. Enhanced upward heat transport at deep submesoscale ocean fronts. Nat. Geosci. 13, 50–55 (2020). https://doi.org/10.1038/s41561-019-0489-1

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