Ice and ocean velocity in the Arctic marginal ice zone: Ice roughness and momentum transfer

Sylvia T Cole,William J Shaw,Macarena Ortiz,John M Toole,Alek A Petty,Jeremy P Wilkinson,Mary-Louise Timmermans,Hans Graber,Shawn G Gallaher,Ted Maksym,Christian Haas,Sinéad L Farrell,Luc Rainville,Jackie A Richter-Menge,Timothy P Stanton,Byongjun Hwang,Ratnaksha Lele,Jody W Deming,James Thomson

doi:10.1525/elementa.241

Abstract

The interplay between sea ice concentration, sea ice roughness, ocean stratification, and momentum transfer to the ice and ocean is subject to seasonal and decadal variations that are crucial to understanding the present and future air-ice-ocean system in the Arctic. In this study, continuous observations in the Canada Basin from March through December 2014 were used to investigate spatial differences and temporal changes in under-ice roughness and momentum transfer as the ice cover evolved seasonally. Observations of wind, ice, and ocean properties from four clusters of drifting instrument systems were complemented by direct drill-hole measurements and instrumented overhead flights by NASA operation IceBridge in March, as well as satellite remote sensing imagery about the instrument clusters. Spatially, directly estimated ice-ocean drag coefficients varied by a factor of three with rougher ice associated with smaller multi-year ice floe sizes embedded within the first-year-ice/multi-year-ice conglomerate. Temporal differences in the ice-ocean drag coefficient of 20–30% were observed prior to the mixed layer shoaling in summer and were associated with ice concentrations falling below 100%. The ice-ocean drag coefficient parameterization was found to be invalid in September with low ice concentrations and small ice floe sizes. Maximum momentum transfer to the ice occurred for moderate ice concentrations, and transfer to the ocean for the lowest ice concentrations and shallowest stratification. Wind work and ocean work on the ice were the dominant terms in the kinetic energy budget of the ice throughout the melt season, consistent with free drift conditions. Overall, ice topography, ice concentration, and the shallow summer mixed layer all influenced mixed layer currents and the transfer of momentum within the air-ice-ocean system. The observed changes in momentum transfer show that care must be taken to determine appropriate parameterizations of momentum transfer, and imply that the future Arctic system could become increasingly seasonal.

Highlights

The canonical view of subinertial ocean currents immediately beneath sea ice consists of a logarithmic boundary layer, within which the stress is independent of depth, and an Ekman layer, where the influence of the Earth’s rotation becomes important (Figure 1; McPhee, 2008)
Cole et al: Ice and ocean velocity in the Arctic marginal ice zone of these layers are affected directly by the stratification in the upper tens of meters and the intensity of upper ocean turbulence, which is influenced by several factors including the ice speed, roughness and concentration
The results presented here consist of a characterization of the air-ice-ocean system, the seasonal cycle of the ice-ocean drag coefficient and other indicators of momentum transfer (including upper ice surface topography), an investigation of underice roughness and Rossby similarity, and the kinetic energy budget of the ice

Summary

Introduction

The canonical view of subinertial ocean currents immediately beneath sea ice consists of a logarithmic boundary layer, within which the stress is independent of depth, and an Ekman layer, where the influence of the Earth’s rotation becomes important (Figure 1; McPhee, 2008). Cole et al: Ice and ocean velocity in the Arctic marginal ice zone of these layers (e.g., layer thicknesses, current speed variations with depth, rate of turning, etc.) are affected directly by the stratification in the upper tens of meters and the intensity of upper ocean turbulence, which is influenced by several factors including the ice speed, roughness and concentration All of these parameters are changing on decadal scales in the Arctic; ice speeds are increasing, the ice cover is becoming thinner and younger, ice concentrations less than 100% extend over a larger fraction of the year, and stratification is increasing (Kwok et al, 2009; Comiso, 2012; Cavalieri and Parkinson, 2012; McPhee, 2012; Kwok et al, 2013; Peralta-Ferriz and Woodgate, 2015). Untangling the effects of changes in ice speed, ice roughness, ice concentration and stratification on the upper ocean currents is necessary for understanding and modeling the air-ice-ocean system on daily to decadal timescales

Methods

Results

Discussion

Conclusion