This article was published on 06 August 2022.
👤 Alberto Alberello, Luke G. Bennetts, Miguel Onorato, Marcello Vichi, Keith MacHutchon, Clare Eayrs, Butteur Ntamba Ntamba, Alvise Benetazzo, Filippo Bergamasco, Filippo Nelli, Rohinee Pattani, Hans Clarke, Ippolita Tersigni & Alessandro Toffoli
Abstract
The marginal ice zone is the dynamic interface between the open ocean and consolidated inner pack ice. Surface gravity waves regulate marginal ice zone extent and properties, and, hence, atmosphere-ocean fluxes and ice advance/retreat. Over the past decade, seminal experimental campaigns have generated much needed measurements of wave evolution in the marginal ice zone, which, notwithstanding the prominent knowledge gaps that remain, are underpinning major advances in understanding the region’s role in the climate system. Here, we report three-dimensional imaging of waves from a moving vessel and simultaneous imaging of floe sizes, with the potential to enhance the marginal ice zone database substantially. The images give the direction–frequency wave spectrum, which we combine with concurrent measurements of wind speeds and reanalysis products to reveal the complex multi-component wind-plus-swell nature of a cyclone-driven wave field, and quantify evolution of large-amplitude waves in sea ice.
Introduction
Improved observational capabilities are needed to understand the often paradoxical and baffling regional and inter-annual variabilities of Antarctic sea ice. Autonomous platforms that operate in harsh polar environments, such as autonomous underwater vehicles and drones, are pushing the boundaries for in-situ observations, generating data for essential calibration and validation of satellite remote sensing, and measuring properties beyond the capabilities of contemporary satellites. The marginal ice zone (MIZ), which is characterised by dynamic interactions between large-amplitude surface waves and relatively small and thin ice floes, is difficult for satellites to capture and a major target for improved observations. Wave evolution and ice properties in the MIZ are intimately coupled, and, hence, there is demand for a technology capable of simultaneously monitoring both wave activity and ice cover properties, which can capture data during storms when wave–ice interactions are most intense.
Historical in-situ measurements of waves in the MIZ show the ice cover attenuates wave energy exponentially over distance at a frequency-dependent rate that induces a downshift of the peak frequency, as well as modifying the directional wave spectrum. The attenuation rate has become a research focus, as it informs predictions of the width of the ice-covered region impacted by waves and, hence, the MIZ extent. Major advances in measuring wave attenuation in the MIZ have been made over the past decade, including dedicated campaigns in the Arctic and Antarctic. State-of-the-art in-situ measurements mostly come from arrays of wave buoys, where the buoys can be traditional open water buoys deployed between floes in regions of low ice concentration (usually close to the ice edge) or bespoke buoys deployed on ice floes large enough to support the buoys (usually away from the ice edge) but small enough that the floes follow the waves. Attenuation rates are generally calculated by applying an exponential decay ansatz to measurements provided by neighbouring buoys, in terms of the significant wave height or a more detailed analysis in which the ansatz is applied to each component of the one-dimensional (frequency) wave spectrum, under the assumption of a stationary wave field.
The recent surge in measurements (including remote sensing) has generated a new understanding of wave attenuation in the MIZ, particularly on how the wave attenuation rate depends on frequency. Certain theoretical models reproduce the observed frequency dependence, but the dominant sources of attenuation are still hotly debated and empirical models often rely upon. Further, the measurements have revealed a large range of attenuation rates, even at comparable frequencies, which is attributed to dependence on ice cover properties, such as ice thickness, areal ice concentration and floe sizes, as well as momentum transfer from winds over the ice cover. Satellite and model-hindcast data have been used to derive empirical relationships between measured attenuation rates and ice concentration, ice thickness and winds. In contrast, floe sizes in the MIZ are below satellite resolutions and have only recently been integrated into large-scale models, so that coincident floe size data have been limited to visual observations during deployment. Overall, data on ice properties are too sparse or unreliable to validate theoretical models.
Stereo-imaging techniques are emerging as a tool for in-situ monitoring of waves and ice properties in the MIZ. In principle, the images can be used from a moving vessel, as in open waters, to reconstruct the sea surface elevation in time and space, thus enabling analysis of wave dynamics in two-dimensional physical space, the frequency–direction spectral domain, and wave statistics. Airborne synthetic aperture radar (SAR) is an alternative method to measure frequency–direction wave spectra and has been applied over 60–80km long transects of the MIZ. However, stereo-imaging, being an in-situ technique, can be used to measure sea-ice geometrical properties simultaneously and can be combined with co-located meteorological measurements, e.g., wind velocities. Further, in contrast to SAR, stereo-imaging resolves wind sea components of the wave spectrum (short wavelength systems under the influence of local winds), as well as swell (long wavelength systems no longer under the effect of winds).
To date, the use of stereo-imaging techniques in the MIZ has been limited in scope. Campbell et al. use a camera system on a fixed platform on the edge of a lake to quantify incoming and reflected energy fluxes of relatively small waves (<0.3m) in pancake and brash ice. Smith & Thomson use camera images from a moored vessel in the Arctic MIZ during calm conditions (significant wave heights typically around 1m) to calculate bulk wave properties and pancake floe velocities. Alberello et al. use an autonomous stereo-camera system on a vessel moving through the winter Antarctic MIZ during a cyclone to measure pancake floe shapes and sizes.
In this article, we demonstrate the potential to monitor the evolution of the frequency–direction wave spectrum from the images captured by Alberello et al. combined with automated image reconstruction software. We report the extreme sea state created by the cyclone over a >40km transect into the Antarctic MIZ, and validate a subset of the results with co-located buoy measurements. The sea state deep into the MIZ during the cyclone is shown to be more complex than previously thought, and partitioning of the two-dimensional spectra is required to analyse wave evolution of the cyclone-driven wind sea. Further, evidence is given of momentum transfer from winds through 100% ice concentration, based on comparing attenuation of the significant wave height over distance with an empirical model, which is considered to be a benchmark due to the large size of the underlying data set and that the measurements were made in the Antarctic MIZ during the sea-ice growth period.
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