Evolution of the Arctic Marginal Ice Zone

Ce message est une copie de celui mis sur le forum 'climat: atmosphere/glace/ocean', mais il me semble etre interessant pour ce forum.

Evolution of the Arctic Marginal Ice Zone

Objective: To monitor the wave spectrum of the Arctic Ocean & examine the role of significant local wave generation on breakup and retreat of Arctic sea ice.

Rationale

A significant feature of the recent (2007–2012) decrease in ice extent has been the retreat of the ice edge away from the coast and continental shelves. At the time of minimum ice extent, the ice edge has been located above the deep ocean, exposing large areas of
previously ice-covered waters. The Beaufort Sea and Canada Basin north of Alaska and Yukon have experienced the fastest decline and greatest loss in arctic summer ice.

The summer ice retreat and opening of the Beaufort Sea are creating conditions that favour the formation of a marginal ice zone (MIZ). The seasonal evolution of the MIZ is a complex interplay between atmospheric, sea ice, and oceanographic processes, with potentially strong feedbacks between them. As sea ice transitions from nearly continuous ice floes during winter conditions, through a wave-influenced MIZ and finally to open water, the changing state of the ice fundamentally changes the coupling of energy and momentum between the atmosphere, ice, and ocean. The influence of wind, waves, and passing storms creates a highly variable distribution of floe sizes near the ice edge, both spatially and temporally. This dynamically-forced breakup can enhance thermodynamic melt through increased solar absorption in newly formed open water, melt of broken ice and brash, and wave-induced melt and upwelling of warmer water from below.

Because the sea ice cover moderates atmosphere–ocean fluxes and the ocean affects the ice cover through fracturing, divergence, and melting, the ice–ocean system is strongly coupled within the MIZ. Highly variable ice and ocean conditions are a source of large perturbations that can trigger feedbacks leading to rapid summertime retreats of the sea ice cover.

Surface wave induced deformations are responsible for fracturing the ice cover and reducing the size of floes across the MIZ. Small broken-up ice floes are more mobile than large, compacted floes of the pack interior. This mobility is a significant characteristic of the MIZ. Floes at the seaward edge of the MIZ are vulnerable to being swept out to the open ocean. Small floes within the MIZ readily respond to divergent oceanic or atmospheric forcing compared to the ice pack, decreasing ice concentration inside the MIZ during divergent forcing events. As well has having expected marked effects on the drifting pack, the newly-available mechanical energy also affects fast ice, and mid-season breakouts -previously unknown along the northern Alaska coast – are now becoming more common, often stranding indigenous hunters who have not adapted to the changing regime, exposing fragile coastlines to wave erosion and complicating access from shore to facilities.

Despite the growing importance of wave-induced failure of fast- or drifting ice, and considerable progress on the theoretical side, very little data exist, primarily due to previous technological limitations: it has not previously been feasible to deploy suitable instruments for long periods required if the environment is to be characterized prior to breakup. The lack of data feeds forward into large-scale modelling capabilities – no global models currently even crudely parameterize wave-related effects, since there is no data available to test any such parameterization.

We are therefore developing an array of innovative wave-measuring buoys to address this data shortfall. Buoys will transmit continual
time-series data and are designed to survive the hostile environment of the MIZ. They will revolutionise the study of waves in the Arctic with an unprecedented amount of year-round field data. The instruments are being developed under both French national funding (ANR Blanc, 2011-2016) and as part of a consortium funded by the U.S. Office of Naval Research (ONR) Marginal Ice Zone
Departmental Research Initiative (also 2011-2016).

The instruments will be used to:
· Provide a year-round data on wave energies in the Arctic Ocean
· Examine whether the observed wave energies are sufficiently high to
break the floes, as we expect, through sophisticated modelling techniques
· Examine whether floe failure has taken place when predicted by the
models, hence driving model improvements
· Hence map zones of wave-induced fracture and determine their importance
to the mass balance of Arctic sea ice in summer

The retrieved wave data will be related to (a) distance from the ice edge (from passive microwave satellite data); (b) ice thickness; (c) thermal history of the ice (from co-located ice mass balance buoys).

We are deploying 5 prototype buoys this summer (2013), from the icebreakers Louis St. Laurent, Aaron and Polarstern. This will be followed by a further 25 buoys in Spring 2014, deployed in an array of 5x5 clusters, spread over 400km of expected ice retreat in the Beaufort Sea. These 25 buoys will be co-located with other fixed and mobile platforms to untangle the complex processes occurring in the MIZ. A further 10 buoys will be deployed in Autumn 2014 in the Central Arctic.

French researchers involved: Martin Doble (LOV), Jerome Weiss (LGGE), David Marsan (ISTERRE), Eric Larose (ISTERRE).

Projects are carried out in collaboration with multiple institutions in the UK (U.Cambridge, British Antarctic Survey), U.S. (APL/UW, WHOI, NPS, NRL, Yale) and Norway (NERSC)