Antarctic Circumnavigation Expedition
ACE was the first project of the Swiss Polar Institute. It was designed to enhance international relations and collaboration between countries, as well as to spark the interest of a new generation of young scientists and explorers in polar research. From December 2016 to March 2017, scientific teams from all over the world have boarded the Russian research vessel Akademik Treshnikov for an unprecedented expedition around Antarctica. From biology to climatology to oceanography, researchers have been working on a number of interrelated fields for the future of this Continent.
The map of the expedition
The Akademik Treshnikov in the Antarctic waters
We participate to the expedition with a project entitled “Metocean properties of the Sub-Antarctic and Antarctic waters: winds, waves, currents, ice, and their interactions” (Project 17).
Waves in the Southern Ocean are the biggest on the planet. They exert extreme stresses on the coastline of the Sub-Antarctic Islands, which affects coastal morphology and the delicate natural environment that the coastline offers. In Antarctic waters, the sea ice cover reflects a large proportion of the wave energy, creating a complicated sea state close to the ice edge. The remaining proportion of the wave energy penetrates deep into the ice-covered ocean and breaks the ice into relatively small floes. Then, the waves herd the floes and cause them to collide and raft.
There is a lack of field data in the Sub-Antarctic and Antarctic Oceans. Thus, wave models are not well calibrated and perform poorly in these regions. Uncertainties relate to the difficulties to model the strong interactions between waves and currents (the Antarctic Circumpolar and tidal currents) and between waves and ice (reflected waves modify the incident field and ice floes affect transmission into the ice-covered ocean). Drawbacks in wave modelling undermine our understanding and ability to protect this delicate ocean and coastal environment.
By installing a Wave and Surface Current Monitoring System (WaMoS II, a marine X-Band radar) on the vessel and using the meteo-station on-board, this project created a large database of winds, waves and surface currents. The instrumentation operated in any weather and visibility conditions, and at night, monitoring the ocean continuously over the entire Circumnavigation. Concurrent satellite observations of sea ice was conducted onshore over the duration of the cruise to complement the database.
Records are used for (1) assessing the wave environment in the Southern Oceans, (2) study waves-in-ice, and (3) for calibrating/validating wave models (in particular, WAVEWATCH III, the world leading wave model).
Wave observation during the Antarctic Circumnavigation Expedition
Pancake ice floe size distribution during the winter expansion of the Antarctic marginal ice zone
The floe size distribution affects ice concentration and volume close to the ice edge, in the marginal ice zone, where ocean waves regulate floe sizes and floes are generally the smallest, meaning they are prone to melting in warmer seasons. At present the only field data available to validate and improve the models are empirical distributions derived for pack ice spanning several orders of magnitude
The size distribution of pancake ice floes is calculated from images acquired during the 2017 S.A. Agulhas II voyage to the Antarctic marginal ice zone during the winter expansion season.
Acquired camera image and floe detection.
Shape of the floes (left), area distribution (centre) and number distribution (left).
Results show that 50 % of the sea ice area is made up by floes with diameters 2.3–4 m. The floe size distribution shows two distinct slopes on either side of the 2.3–4 m range. Growth of pancakes from frazil forms the distribution of small floes (D < 2.3 m), and welding of pancakes forms the distribution of large floes (D > 4 m).
Alberello, A., Onorato, M., Bennetts, L., Vichi, M., Eayrs, C., MacHutchon, K. and Toffoli, A., 2019. Brief communication: Pancake ice floe size distribution during the winter expansion of the Antarctic marginal ice zone. The Cryosphere, 13, 41-48
Drift of pancake ice floes in the winter Antarctic marginal ice zone during polar cyclones
During the Antarctic winter, most of the sea ice mass budget is comprised of small pancake ice floes, which form rapidly in wavy conditions, and create a dynamic environment. However, there are only a handful of in-situ observations of pancake ice drift, particularly during the intense polar cyclones that frequently reshape the ice cover, which are needed to generate better understanding and modelling of pancake ice response to winds, waves and currents.
We report a set of pancake ice drift and wave-in-ice measurements over nine days in which four polar cyclones impacted the region, from buoys deployed on pancake floes 100 km in from the edge, and we develop a drift model.
The data shows how the cyclones affect ice drift, and contains the fastest measured ice speed in the Southern Ocean (0.75 m/s). The instantaneous drift speed closely correlates with the wind speed, and the ice also displays a 13 h period rotational motion that we reproduce in the model with forcing from ocean currents. We show that pancake ice is in free drift, despite sea ice covering the entire ocean surface in the measurement region, and that the model predicts drift accurately over two days with calibration of only two parameters.
Measured buoy drift and model predictions.
Alberello, A., Bennetts, L., Heil, P., Eayrs, C., Vichi, M., MacHutchon, K., Onorato, M. and Toffoli, A., 2019. Drift of pancake ice floes in the winter Antarctic marginal ice zone during polar cyclones. Submitted