Ekman Veering, Internal Waves, and Turbulence Observed under Arctic Sea Ice

Abstract

The ice-ocean system is investigated on inertial to monthly time scales using winter 2009-10 observations from the first ice-tethered profiler (ITP) equipped with a velocity sensor (ITP-V). Fluctuations in surface winds, ice velocity, and ocean velocity at 7-m depth were correlated. Observed ocean velocity was primarily directed to the right of the ice velocity and spiraled clockwise while decaying with depth through the mixed layer. Inertial and tidal motions of the ice and in the underlying ocean were observed throughout the record. Just below the ice-ocean interface, direct estimates of the turbulent vertical heat, salt, and momentum fluxes and the turbulent dissipation rate were obtained. Periods of elevated internal wave activity were associated with changes to the turbulent heat and salt fluxes as well as stratification primarily within the mixed layer. Turbulent heat and salt fluxes were correlated particularly when the mixed layer was closest to the freezing temperature. Momentum flux is adequately related to velocity shear using a constant ice-ocean drag coefficient, mixing length based on the planetary and geometric scales, or Rossby similarity theory. Ekman viscosity described velocity shear over the mixed layer. The ice-ocean drag coefficient was elevated for certain directions of the ice-ocean shear, implying an ice topography that was characterized by linear ridges. Mixing length was best estimated using the wavenumber of the beginning of the inertial subrange or a variable drag coefficient. Analyses of this and future ITP-V datasets will advance understanding of ice-ocean interactions and their parameterizations in numerical models.

An analytical model for wind-driven Arctic summer sea ice drift

“…At face value our model may not appear to be applicable to these data because the measurements were made in the Beaufort Sea in winter, when the SIC is close to 100 % and internal stress is likely to be dynamically significant (Leppäranta, 2005). However, the analysis of Cole et al (2014) suggests that the ice floe velocity was in fact close to a free drift regime and that the vertical buoyancy flux in the IOBL was small compared to previous winter observations (see, e.g., McPhee, 2008). Consequently, our model largely captures the dependence of the ice speed and turning angle on the surface wind speed.…”

“…Thus the shear becomes dominated by the Ekman spiral, over which the shear turns by 45 • . This is consistent with Rossby similarity theory (McPhee, 2008(McPhee, , 2012 In this study, we employ the canonical value of K * o = 0.028 (McPhee, 1994(McPhee, , 2008, and we use C io = 0.0071 based on the estimate of Cole et al (2014) from the ice-tethered profiler with a velocity sensor (ITP-V) data. This combination of K * o and C io produces a θ IOBL of around 15 • (red dot in Fig.…”

“…4, we evaluate our model against recent observations from an ice-tethered profiler (Cole et al, 2014), focusing on the angles between the wind and ice velocities and between the ice and ocean velocities. At face value our model may not appear to be applicable to these data because the measurements were made in the Beaufort Sea in winter, when the SIC is close to 100 % and internal stress is likely to be dynamically significant (Leppäranta, 2005).…”

“…The effect of the mean geostrophic upper-ocean currents on the average circulation of sea ice pack is known to be as important as the mean wind field (Thorndike and Colony, 1982). However, the role of the winds becomes increasingly important over shorter timescales: on timescales from days to months, surface wind variability explains more than 70 % of the sea ice motion (Thorndike and Colony, 1982) and is well correlated with the surface ocean velocity (Cole et al, 2014). The synoptic eddy surface winds result in a primary mode of upper-ocean velocity variability with a period of 2-5 days over the icecovered Arctic Ocean (Plueddemann et al, 1998).…”

The winter pack-ice zone provides a sheltered but food-poor habitat for larval Antarctic krill

“…57 ) and mapped from an original horizontal resolution of 25 km onto the horizontal Cartesian grid (1/4°) of the ocean current data prior to use in the advection model. Because sea-ice cover prevents the estimation of altimetry-derived ocean currents, under-ice ocean currents were estimated from the sea-ice motion data, scaling the magnitude of the velocity components by 0.43 and applying an offset of − 35° to the sea-ice motion direction 58,59 . This simplified, fixed relationship gives an indication of the main direction of flow beneath the ice at the regional scale that we are interested in for this study.…”