The response of the marine atmospheric boundary layer (MABL) to a sharp, small-scale sea-surface temperature (SST) front was quantified for the Ushant tidal front in the Iroise Sea by a hectometric, atmosphere-only simulation (Redelsperger et al. 2019): under weak winds crossing the front, the coupling between wind stress and SST was dominated by stability-induced changes in turbulent mixing rather than by the pressure-adjustment mechanism. What remains open is to establish this mechanism attribution and its regime structure under realistic two-way air–sea coupling: how ocean-surface currents, wave-state-dependent turbulent fluxes, and high-frequency coupling among atmosphere, ocean and wave models modify the relative roles of turbulent mixing, pressure adjustment and Coriolis effects for a narrow, advected front; over which parameter ranges (wind speed, front width, boundary-layer depth, frontal SST amplitude, cross-front wind direction and season) each mechanism dominates in realistic settings; how the single-event conclusions generalize across other frontal amplitudes, widths, wind directions and seasons; and what the front-induced MABL modifications mean for downstream mesoscale weather and for air–sea feedbacks in the upper ocean.

Background

Sea-surface temperature (SST) fronts — narrow ribbons of strong horizontal temperature gradient at the ocean surface — leave a clear imprint on the overlying marine atmospheric boundary layer (MABL, the turbulent lowest kilometre of the atmosphere over the sea): surface wind speed, wind stress and turbulent heat fluxes increase over warm water and decrease over cold water. Two canonical mechanisms compete to explain this coupling. The vertical (downward) momentum-mixing mechanism attributes the wind change to the change of MABL stratification across the front: over warm water the boundary layer is destabilized, turbulent mixing strengthens, and more momentum is transported down to the surface (and vice versa over cold water). The pressure-adjustment mechanism attributes it to the horizontal pressure gradient that the SST gradient imprints on the MABL, driving a secondary circulation toward the warm side of the front, analogous to a sea breeze. Which mechanism dominates is known to depend on wind speed, atmospheric stability, and the amplitude, sharpness and width of the front.

The Ushant front in the Iroise Sea off Brittany (northwestern France) is a sharp tidal front of barotropic origin (maintained by tidal stirring of the water column against the seasonal stratification, rather than by buoyancy forcing alone), typically 3–5 °C over roughly 20 km, present from spring to autumn and displaced by several kilometres per day by the tidal currents. Redelsperger et al. (2019) simulated one representative weak-wind event (2 September 2011; near-surface wind of about 3 m/s crossing the front from the warm to the cold side at roughly 45°) with the Meso-NH atmospheric model (a research mesoscale model) in a two-way nested configuration reaching 100 m horizontal resolution (a 'hectometric' simulation), forced by a high-resolution, time-evolving SST field from the MARS 3D coastal-ocean model and by Météo-France operational analyses at the outer boundary. Contrary to most previously published work, they found the stress–SST coupling to be largely controlled by the change of atmospheric stability across the front rather than by the change of surface wind speed, and the MABL momentum budget to be governed by turbulent mixing acting on a shorter length scale than both the pressure-adjustment mechanism and the Coriolis term (the deflecting effect of Earth's rotation, whose strength is measured by the Coriolis parameter ff) — a consequence of the conjunction of weak wind and a narrow front, which makes horizontal advection relatively strong. Their simulation was nevertheless atmosphere-only: the SST was prescribed from an ocean run, and ocean-surface currents, surface-wave effects on the turbulent fluxes, and any feedback of the atmospheric response on the ocean and wave state were absent.

Four linked issues therefore remain open, and together define the present problem: the effect of including surface currents, wave-modulated flux parameterizations and two-way, high-frequency coupling among atmosphere, ocean (MARS 3D) and wave models (e.g., WaveWatch III, a spectral model that predicts the evolution of the ocean-wave spectrum) on the mechanism attribution for narrow, advected fronts; the generalization of the single-event, weak-wind conclusions to other frontal amplitudes, widths, wind directions and seasons; the mapping, in realistic settings, of the parametric boundaries — in wind speed VV, front width LfL_f, MABL depth hh, frontal SST amplitude, cross-front wind direction and season, with the Coriolis parameter ff fixed by the site and the turbulent diffusivities diagnosed from the flow — separating regimes where pressure adjustment or Coriolis effects overtake turbulent mixing; and the quantification of the impacts of the front-induced MABL modifications on downstream mesoscale weather (cloud, fog, precipitation) and on air–sea feedbacks in the upper ocean.

Problem Statement

How do two-way air–sea coupling and the frontal parameter regime control the response of the marine atmospheric boundary layer to a sharp, small-scale SST front such as the Ushant front in the Iroise Sea? The problem is to (1) determine how two-way, high-frequency coupling among atmosphere, ocean and wave models, at hectometric atmospheric resolution and including ocean-surface currents and wave-state-dependent turbulent-flux parameterizations, modifies the MABL response and the relative contributions of stability-induced turbulent mixing, pressure adjustment, and the Coriolis term for the narrow, advected front, relative to the atmosphere-only result of Redelsperger et al. (2019); (2) establish the parametric boundaries — over the independent controls wind speed VV, front width LfL_f, MABL depth hh, frontal SST amplitude, cross-front wind direction and season, with the turbulent diffusivities diagnosed from the flow rather than prescribed and the Coriolis parameter ff fixed by the site — separating the regimes in which turbulent mixing, pressure adjustment, or Coriolis effects dominate (the dominant mechanism being the one acting on the shortest length scale in the MABL momentum budget), in realistic settings, on the basis of a bounded set of coupled experiments spanning these conditions rather than exhaustive coverage of the parameter space, and thereby assess how far the single-event, weak-wind conclusions generalize across other frontal amplitudes, widths, wind directions, and seasons; and (3) quantify the impacts of these front-induced MABL modifications on downstream mesoscale weather (cloud, fog, precipitation) and on air–sea feedbacks in the upper ocean (mixing, frontal and eddy evolution), so that the significance of such sub-grid-scale air–sea interaction for weather forecasting and for ocean and climate model parameterizations can be judged.

References

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