G. Sutherland

Observations of wave dispersion and attenuation in landfast ice

Observations of wave propagation in landfast ice were obtained in Tempelfjorden, Svalbard during March 2015. Wave motion was measured near the ice edge using inertial motion units and consisted of a combination of swell from the North Atlantic and wind-generated waves. The waves were observed to be unidirectional in the ice with comparable magnitudes in the vertical and horizontal displacements. The dispersion relation was calculated from the measured phase difference between two adjacent sensors separated by a distance of approximately 60 m. Deviations from the gravity wave dispersion relation were observed during the growth phase of the waves and were consistent with the presence of flexural waves. This period of wave growth was accompanied by significant wave attenuation in the high frequency portion of the wave spectrum which persisted for 3–5 h.

Enhanced Turbulence Associated with the Diurnal Jet in the Ocean Surface Boundary Layer

AbstractDetailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate e below the depth of the diurnal jet, there was an observed increase in e within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of e. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diu...

Parameterizing air‐sea gas transfer velocity with dissipation

Author(s): Esters, L; Landwehr, S; Sutherland, G; Bell, TG; Christensen, KH; Saltzman, ES; Miller, SD; Ward, B | Abstract:

The relationship between ocean surface turbulence and air-sea gas transfer velocity: An in-situ evaluation

Although the air-sea gas transfer velocity k is usually parameterized with wind speed, the so-called small-eddy model suggests a relationship between k and ocean surface dissipation of turbulent kinetic energy . Laboratory and field measurements of k and have shown that this model holds in various ecosystems. Here, field observations are presented supporting the theoretical model in the open ocean. These observations are based on measurements from the Air-Sea Interaction Profiler and eddy covariance CO2 and DMS air-sea flux data collected during the Knorr11 cruise. We show that the model results can be improved when applying a variable Schmidt number exponent compared to a commonly used constant value of 1/2. Scaling to the viscous sublayer allows us to investigate the model at different depths and to expand its applicability for more extensive data sets.

A Method to Estimate Reflection and Directional Spread Using Rotary Spectra from Accelerometers on Large Ice Floes

AbstractThe directional wave spectra in sea ice are an important aspect of wave evolution and can provide insights into the dominant components of wave dissipation, that is, dissipation due to scattering or dissipation due to viscous processes under the ice. A robust method for the measurement of directional wave spectra parameters in sea ice from a three-axis accelerometer—or a heave, pitch, and roll sensor—is proposed. The method takes advantage of certain aspects of sea ice and makes use of rotary spectra techniques to provide model-free estimates for the mean wave direction, directional spread, and reflection coefficient. The method is ideally suited for large ice floes—that is, where the ice floe length scale is much greater than the wavelength—but a framework is provided to expand the parameter space where the method may be effective.