Dissipation of wind waves by pancake and frazil ice in the autumn Beaufort Sea

Abstract

A model for wind‐generated surface gravity waves, WAVEWATCH III®, is used to analyze and interpret buoy measurements of wave spectra. The model is applied to a hindcast of a wave event in sea ice in the western Arctic, 11–14 October 2015, for which extensive buoy and ship‐borne measurements were made during a research cruise. The model, which uses a viscoelastic parameterization to represent the impact of sea ice on the waves, is found to have good skill—after calibration of the effective viscosity—for prediction of total energy, but over‐predicts dissipation of high frequency energy by the sea ice. This shortcoming motivates detailed analysis of the apparent dissipation rate. A new inversion method is applied to yield, for each buoy spectrum, the inferred dissipation rate as a function of wave frequency. For 102 of the measured wave spectra, visual observations of the sea ice were available from buoy‐mounted cameras, and ice categories (primarily for varying forms of pancake and frazil ice) are assigned to each based on the photographs. When comparing the inversion‐derived dissipation profiles against the independently derived ice categories, there is remarkable correspondence, with clear sorting of dissipation profiles into groups of similar ice type. These profiles are largely monotonic: they do not exhibit the “roll‐over” that has been found at high frequencies in some previous observational studies.

On the Increasing Importance of Air-Sea Exchanges in a Thawing Arctic: A Review

“…SHEBA was particularly notable since surface turbulent flux data using both the EC [10] and bulk transfer [88] methods for a full annual cycle over sea ice were obtained. Data collected has increased our understanding of the seasonal cycle of surface turbulent and radiative fluxes [10,100] and the influence of these fluxes on sea ice growth, melt, and the transitions between these states [101][102][103][104]. Other field campaigns in the Pacific sector of the Arctic include the SeaState campaign [105,106], which measured meteorological and EC method turbulent fluxes over the Chukchi and Beaufort Seas in October and November 2015 during sea ice freeze-up conditions.…”

“…Both of these campaigns provided data that increased our understanding of the interactions between the atmospheric circulation and surface turbulent and radiative fluxes in the sea ice freeze-up and melt seasons. Other campaigns have featured buoy measurements of surface turbulent fluxes, such as in the Beaufort Sea [104,106]. A campaign slated for 2019-2020, the multidisciplinary drifting observatory for the study of the Arctic climate (MOSAiC; mosaicobservatory.org), modeled after the SHEBA campaign, will provide year-round measurements in the Eurasian/N.…”

“…Recently, increased wave action in the Arctic Ocean has been observed and attributed to the increased ice-free water distance or fetch over which the winds blow [50,[236][237][238]. Changes in wave action and wave breaking directly modulate air-sea exchange [231,239,240] such that increased wave heights and lengths increase surface turbulent fluxes, sea spray, and can potentially enhance sea ice break-up [104,237]. Indirectly, wave activity can influence surface turbulent fluxes by affecting ocean mixed-layer properties.…”

“…Given less sea ice in the future, the ocean mixed-layer response to the atmospheric forcing by the wind may be amplified influencing stratification and sea ice growth and melt. Emerging research argues that complex interactions and feedbacks between waves, surface turbulent fluxes, and ocean mixed-layer processes will be consequential to future Arctic climate change [104,237]. In ice-free conditions, for example, a larger fetch increases wave action and deepens the mixed layer allowing the distribution of heat over a large mass of water.…”

“…A range of atmospheric and oceanic forcing mechanisms, including both high and low frequency variability, influence surface turbulent fluxes: atmospheric variability, sea ice extent variability, ocean heat storage, and mixed layer processes [39,104,152,231,246]. This section describes the magnitude of typical surface turbulent flux inter-annual variability, outlines the general factors, and highlights the importance of the episodic nature of air-sea energy exchange.…”