Research ArticleOCEANOGRAPHY

Submesoscale-selective compensation of fronts in a salinity-stratified ocean

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Science Advances  28 Feb 2018:
Vol. 4, no. 2, e1701504
DOI: 10.1126/sciadv.1701504
  • Fig. 1 Schematic drawing of the mechanism by which cooling creates density-compensated submesoscale fronts and cold SST filaments.

    In a salinity-stratified upper ocean, submesoscale instabilities of salinity fronts cause restratification and shallowing of the SML. Cooling is concentrated within shallow (most stratified) SMLs, creating a larger drop in SST along salinity-stratified fronts, at scales of submesoscale restratification. SST and SSS become correlated (salty warm and fresh cold, compensating their effects on density) at length scales ~O(1) km.

  • Fig. 2 Winter-monsoon sea surface conditions in the northern Indian Ocean (west, AS; east, BoB).

    (A) Surface salinity (November to December 2013, Aquarius satellite data). Thin lines mark the ship’s track from 10 to 26 November (blue) and from 28 November to 13 December (white). Thick segments correspond to sections shown in Fig. 3. The dashed line marks the limits of international waters. (B) Nighttime SST (11 to 18 December, MODIS). (C) Net heat flux and surface winds (November to December 2013, NCEP reanalysis). Colored track show net daily heat flux measured during the surveys. (D) Chlorophyll a (Chl a) (25 November to 26 December, MODIS); track of the cyclonic storm Madi (6 to 13 December, Joint Typhoon Warning Center) shown in black. Dates of satellite data composites are chosen to provide a cloud-free view during the ship survey period.

  • Fig. 3 Surface-layer salinity and temperature structure in the BoB observed between 29 November and 3 December 2013.

    (A) 1200 km south-to-north transect (marked on map) of surface salinity (red) and temperature (blue) from a shipboard TSG. Shaded boxes show a 100- and 10-km section in detail. The ranges of vertical T and S axes are proportionally scaled by the thermal expansion and haline contraction coefficients α and β, such that graphically equal displacements have equal effect on density. Compensating correlation is seen at scales below O(10) km. (B) Salinity and (C) temperature section composed of UCTD profiles (numbered, dashed vertical lines) taken during the 100-km section plotted above. Density (in black contours) and a few profiles of stratification (log N2 indicated by white bar graphs) are overlaid. Note the cooler surface waters confined above warmer subsurface layers by stable salinity stratification.

  • Fig. 4 Scale dependence of T-S compensation.

    (A) Probability distributions of R (density ratio) for different horizontal gradient scales, 100, 10, and 1 km, as estimated from the longest ship section (leg 2, section A in previous figures). Black lines show the distribution of R for all gradients, whereas purple bars denote the distribution of R for only the largest gradients (selected as the 95th percentile of individual αΔT or βΔS magnitudes shown in fig. S2). Density compensation of fronts is greater at 10-km scales than 1000-km scales and greater at 1-km scales than 10-km scales (R = 1 and Tu = π/4 denote perfect compensation). (B) Median of R distribution as a function of horizontal length scale for each of the six ship transects. Pairs of αΔT and βΔS are computed from a wavelet decomposition of the individual ship transects mapped in Fig. 2A. Lines (N) and (S) are from the earlier cruise, and lines (a) to (d) are from the later cruise, both during November to December 2013. All transects are consistent in showing that greater density compensation is favored at smaller scales. (C) Schematic demonstrating the relationship between density ratio R = αΔT/βΔS and Turner angle Tu.

  • Fig. 5 Numerical simulation of a submesoscale salinity front forced by surface cooling, developing O(1)-km scale T-S compensation.

    (A) Surface- and depth-dependent salinity (left) and temperature (right) from our process study numerical ocean model, or PSOM, configured in a periodic channel. The model domain extends 10 times deeper than what is shown. Top: Model initial condition at t = 0 days. Middle: t = 8 days, comprising 7 days of model integration without any heat flux and 1 day of cooling by a constant and uniform surface heat flux of −50 W/m2. Bottom: t = 14 days of which the last 7 days are with cooling. (B) Distributions of surface density ratio R, initially (top), after 1 day of cooling (middle), and after 1 week of cooling (bottom). Filtering for horizontal gradient scales of order 10 km (pale green) and 1 km (pink). Lines show distributions for ratios of all gradients, whereas shaded bars show distributions for ratios of only the largest T and S gradients, as in Fig. 4. (C) Temperature change (of a cross-channel section) after 1 week of surface cooling. Isopycnals (black contours) and stratification (yellow bars). (D) SSS (red) and SST (blue) across the section initially (dashed line) and after the front evolves and is cooled for 1 week (solid line).

  • Fig. 6 Climatological estimate of the global prevalence of cooling-induced T-S compensation.

    (A) Possible occurrence of cooling-induced compensation where both the SML is cooled and density gradients are salinity-dominated. The estimate, cast as a likelihood between 0 and 1, is based on a co-occurrence of the necessary factors calculated from a climatology of surface heat flux and horizontal density ratios (normalized factors designated q and r), with greater weight placed on larger (negative) fluxes and stronger salinity control (smaller density ratio). The maximum (Γ = qr) of the 12 months for each pixel is plotted in (A), and darker shades indicate more favorable conditions. (B) Month of the maximum Γ (1 = January, 12 = December), generally corresponding to local winter months.

  • Fig. 7 Idealized SSS submesoscale front, where the shallow restratification is due to the slumping of the lateral salinity gradient.

    Temperature is shaded in color, and contours of density are in black.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/e1701504/DC1

    section S1. Satellite data

    section S2. Compensation between salinity and temperature for fronts of varying strength

    section S3. Density compensation at a salinity front illustrated in a one-dimensional framework

    section S4. Model sensitivity tests

    section S5. Vertical density compensation: Weakening the density stratification by cooling

    table S1. Parameter configuration space of sensitivity tests.

    fig. S1. Similar to Fig. 2 but for winter 2015.

    fig. S2. Scatterplots of density variability at a wavelength of 0.6 km due to salinity and temperature gradients along the longest straight section from the cruise (leg 2, section A).

    fig. S3. Illustration of active compensation and creation of spice variance with a series of one-dimensional column models.

    fig. S4. Simulations assessing the model’s sensitivity to the value of horizontal diffusivity, set at 0.2, 1.0, and 5.0 m2/s.

    fig. S5. Tests assessing the sensitivity of the simulation to the horizontal grid resolution, set at 0.25 and 0.5 km, for the same domain (48 × 96 km) and the same value of horizontal diffusivity (1.0 m2/s).

    fig. S6. Tests assessing model evolution with no surface cooling, as well as with surface cooling during model spin-up.

    fig. S7. Schematic showing variants of simple salinity fronts and stratification.

  • Supplementary Materials

    This PDF file includes:

    • section S1. Satellite data
    • section S2. Compensation between salinity and temperature for fronts of varying strength
    • section S3. Density compensation at a salinity front illustrated in a onedimensional framework
    • section S4. Model sensitivity tests
    • section S5. Vertical density compensation: Weakening the density stratification by cooling
    • table S1. Parameter configuration space of sensitivity tests.
    • fig. S1. Similar to Fig. 2 but for winter 2015.
    • fig. S2. Scatterplots of density variability at a wavelength of 0.6 km due to salinity and temperature gradients along the longest straight section from the cruise (leg 2, section A).
    • fig. S3. Illustration of active compensation and creation of spice variance with a series of one-dimensional column models.
    • fig. S4. Simulations assessing the model’s sensitivity to the value of horizontal diffusivity, set at 0.2, 1.0, and 5.0 m2/s.
    • fig. S5. Tests assessing the sensitivity of the simulation to the horizontal grid resolution, set at 0.25 and 0.5 km, for the same domain (48 × 96 km) and the same value of horizontal diffusivity (1.0 m2/s).
    • fig. S6. Tests assessing model evolution with no surface cooling, as well as with surface cooling during model spin-up.
    • fig. S7. Schematic showing variants of simple salinity fronts and stratification .

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