Altimetric observation of wave attenuation through the Antarctic marginal ice zone using ICESat-2

Abstract

Abstract. The Antarctic marginal ice zone (MIZ) is a highly dynamic region where sea ice interacts with ocean surface waves generated in ice-free areas of the Southern Ocean. Improved large-scale (satellite-based) estimates of MIZ extent and variability are crucial for understanding atmosphere–ice–ocean interactions and biological processes and detection of change therein. Legacy methods for defining the MIZ are typically based on sea ice concentration thresholds and do not directly relate to the fundamental physical processes driving MIZ variability. To address this, new techniques have been developed to measure the spatial extent of significant wave height attenuation in sea ice from variations in Ice, Cloud and land Elevation Satellite-2 (ICESat-2) surface heights. The poleward wave penetration limit (boundary) is defined as the location where significant wave height attenuation equals the estimated error in significant wave height. Extensive automated and manual acceptance/rejection criteria are employed to ensure confidence in along-track wave penetration width estimates due to significant cloud contamination of ICESat-2 data or where wave attenuation is not observed. Analysis of 304 ICESat-2 tracks retrieved from four months of 2019 (February, May, September and December) reveals that sea-ice-concentration-derived MIZ width estimates are far narrower (by a factor of ∼ 7 on average) than those from the new technique presented here. These results suggest that indirect methods of MIZ estimation based on sea ice concentration are insufficient for representing physical processes that define the MIZ. Improved large-scale measurements of wave attenuation in the MIZ will play an important role in increasing our understanding of this complex sea ice zone.

Three-dimensional imaging of waves and floes in the marginal ice zone during a cyclone

“…Autonomous platforms that operate in harsh polar environments, such as autonomous underwater vehicles 3 and drones 4 , 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 5 , 6 and a major target for improved observations 7 , 8 . Wave evolution and ice properties in the MIZ are intimately coupled 9 – 11 , 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.…”

Floes, the marginal ice zone and coupled wave-sea-ice feedbacks

“…The influence of ocean surface waves is also used to define the MIZ [10]. Waves can be observed propagating long distances within the sea ice [8,9,11–15]. A companion review in this special issue discusses the theory and observation of waves in the MIZ [16].…”

“…A companion review in this special issue discusses the theory and observation of waves in the MIZ [16]. Recently, new satellite platforms have allowed the observation of waves in sea ice at high resolution and at global scales, making it feasible to quantify global MIZ extent [13,15,17]. Yet as sea ice is an efficient damper of high-frequency wave energy, waves in sea ice are often long-period swell waves, episodically generated by storms or mesoscale weather events [12,18–20].…”

“…At any instant, the area of ice-covered regions with energetic ocean surface waves can be much smaller than that ‘influenced’ by waves more generally. In global measurements with the ICESat-2 altimeter, the MIZ was defined as those regions where waves were identified at least 7.5% of the time [6]—and of 4402 individual observations of sea ice heights in the Southern Ocean, just 304 (7.0%) were identified visually as having waves in them [15]. The MIZ is dynamic, and ICESat-2 overflights inconsistently sample the constantly varying sea ice surface.…”

Marginal ice zone dynamics: history, definitions and research perspectives

“…Methods for extracting waves height and wave attenuation from SAR imagery have been developed and used with Seasat [33], with the European Remote Sensing Satellite 2 (ERS2) [34] and more recently with the higher resolution wave mode imagery of Sentinel-1 [35,36] from which the directional wave spectrum can be obtained as well as information about wave attenuation. As demonstrated recently, wave parameters can also be obtained from ICESat2 altimeter data, a satellite mission launched in 2018 [37,38].…”