Abstract
AbstractInertial oscillations are a ubiquitous feature of the surface ocean. Here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea. Observations of temperature and current structure, from a mooring in the Western Irish Sea, reveal episodes of strong currents (>0.3 m s−1) lasting several days, resulting in enhanced shear across the thermocline. While the episodes of strong currents are coincident with windy periods, the variance in the shear is not directly related to the wind stress. The shear varies on a subinertial time scale with the formation of shear maxima lasting several hours occurring at the local inertial period of 14.85 h. These shear maxima coincide with the orientation of the surface current being at an angle of approximately 90° to the right of the wind direction. Observations of the water column structure during windy periods reveal deepening of the surface mixed layer in a series of steps which coincide with a period of enhanced shear. During the periods of enhanced shear gradient, Richardson number estimates indicate Ri−1 ≥ 4 at the base of the surface mixed layer, implying the deepening as a result of shear instability. A one‐dimensional vertical exchange model successfully reproduces the magnitude and phase of the shear spikes as well as the step‐like deepening. The observations and model results therefore identify the role of wind shear alignment as a key entrainment mechanism driving surface mixed layer deepening in a shallow, seasonally stratified sea.
Highlights
The surface of the ocean acts as a critical interface within the Earth system, linking the ocean and the atmosphere
We investigate the role of the wind in deepening the surface mixed layer in a seasonally stratified tidally swept shallow sea
The largest shears are observed across the thermocline at these times, with shear taking the form of significant enhancements lasting several hours which are separated from one another by approximately one local inertial period (14.85 h)
Summary
The surface of the ocean acts as a critical interface within the Earth system, linking the ocean and the atmosphere. Oscillating currents at, or close to, the local inertial frequency have been widely observed as energetic features in the ocean since they were first reported by Helland-Hansen and Ekman [] and appear an almost ubiquitous feature of oceanic motion [e.g., Pollard, 1980; D’Asaro, 1985; Jordi and Wang, 2008; Hunter et al, 2007; Sobarzo et al, 2007] except where frictional damping is high [Simpson et al, 2002] In consequence, they have been implicated as a major source of mechanical energy which drives diapycnal mixing in the ocean interior [D’Asaro, 1985; Zhang et al, 2014] and deepens the surface mixed layer of the ocean through driving entrainment [Pollard et al, 1972]. Diapcynal mixing and SML deepening provide a key bio geochemical pathway supplying limiting nutrients to the euphoric zone [Sharples et al, 2001; Rippeth et al, 2009] and sustain the primary production once the spring bloom has removed limiting nutrients from the surface layer [Sharples et al, 2013]
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