Goal 2: Quantify the role of air-sea interaction and surface forcing in the dynamics and vertical velocity of submesoscale variability

How does this goal help address the hypothesis?

Extensive model and observational studies highlight a variety of ways by which air-sea 11
interaction can induce horizontal divergence of surface currents and thus force vertical velocities. The wind stress can be modified by oceanic SST and velocity gradients (Dewar and Flierl, 1987; Fairall et al., 1996; Chelton et al., 2004; O’Neill et al. 2005) while the resulting convergence of the ocean Ekman layer can be modified by surface vorticity (Stern, 1965; McGillicuddy et al., 2007; 2008). At submesoscales, these effects increase in magnitude and other phenomena also appear. Frontal structure is strongly affected by the relative direction of the wind with ‘downfront’ Ekman transports sharpening the fronts and inducing vertical exchange while ‘upfront’ transports slump the front and stratify the upper ocean (Thomas, 2005). Surface waves, the ultimate mediators of air-sea momentum exchange, can be strongly modulated at fronts, suggesting that even the basic formulation of air-sea exchange in terms of simple bulk coefficients may break down at sufficiently small scales. Thus, an understanding of submesoscale structure and vertical velocity requires that air-sea interaction parameters be measured simultaneously with the other submesoscale measurements.

What needs to be done to achieve this goal and why?

As a scatterometer, DopplerScatt will measure backscatter intensity and Doppler shift at various look angles, and thus derive surface wind vectors, on the same scales as surface velocity, providing an unprecedented new opportunity to examine submesoscale modulation of wind stress. Interpretation of these data requires additional measurements. First, local measurement of standard bulk flux parameters (SST, wind, air temperature, humidity, solar radiation and downwelling infrared radiation). These measurements will be made on the research vessel using standard instrumentation and on at least two Wave Gliders. These platforms will repeatedly survey across oceanic submesoscale features thereby measuring their corresponding variations of atmospheric boundary layer parameters for comparison with DopplerScatt data. Second, directional surface wave fields will be measured from an airplane-mounted Lidar (Melville et al. 2016) using the methods described in Lenain and Melville (2017). The observations will resolve surface wave wavenumber spectra for wavelengths from 100’s of m to 10’s of cm, and thus span both the equilibrium and saturation ranges important for wave breaking and momentum transfer. The same aircraft will measure wave breaking rates using a well-established technique of measuring foam distribution via IR and visual imaging (Deike et al., 2017). Additional surface wave spectral measurements will be made from the Wave Gliders with a minimum resolution of a few meters’ wavelength.