Is Absence of Sea Ice a Causal Factor in Recent Antarctic Ice Shelf Break-ups?
Investigators: Ted Scambos (CIRES/NSIDC), and Robert Massom (Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre, Australia)
Background: MODIS satellite images from February and March of this year indicate the southwestern part of the Wilkins Ice Shelf is currently undergoing (as of this writing) an abrupt Larsen B-style break-up event (Figure 1). The Wilkins is situated on the southwestern flank of the Antarctic Peninsula (at 70.25°S, 73.0°W). The event marks the latest in a series of similar ice shelf disintegrations (Larsen A Ice Shelf in 1995; a partial event on the Wilkins in 1998; and a major event on Larsen B Ice Shelf in 2002; see Scambos et al., 2003). These events have become iconic indicators of rapid climate change in the world’s ice. To date, the break-ups have been linked to intense summer surface melt in the years prior to break-up, associated with a regional air temperature warming trend of 0.5ºC per decade (Vaughan et al., 2003). A hydro-fracture model for the break-ups has been proposed (Scambos et al, 2000). Additional factors may come from ‘toppling block’ forces within the fractured ice shelf (MacAyeal et al., 2003), and natural weakening by rift formation (e.g., Glasser and Scambos, 2008). However, an additional common factor, hitherto unstudied, is a complete absence sea ice cover at the ice shelf fronts, preceding the events for several weeks.
Objective: In our proposed study, we address these questions:
We shall address these questions through analysis of satellite imagery, meteorological data (station and reanalysis), available in situ observations, ocean wave model data (e.g, NCEP/NOAA Global Wavewatch III), ICESat data, high-resolution (2m) satellite imagery (already acquired but embargoed as of this writing), and a state-of-the-art wave – ice shelf – sea ice interaction model (Williams and Squire, 2007). The latter simulates how ice-coupled waves traveling beneath a uniform, floating sea-ice sheet propagate into a second sheet of different thickness (i.e. an ice shelf 200-300 m thick) by way of an arbitrarily defined transition region e.g. fast ice. The model will be fine-tuned, and its output compared, with observations in the period leading up to break-up events. The impact of the presence or absence of the protective pack/fast ice “buffer” on wave energy entering an ice sheet will be investigated by turning it on and off in the model. This will provide insight into the possible effect of anomalous exposure of the ice shelf to enhanced destructive ocean wave/swell energy associated with a negative sea ice extent anomaly. In the case of the current Wilkins break-up, wave energy may have reached the ice front from a severe storm to the northwest in late February. We shall further test the hypothesis that long-period ocean swell from far-remote storms – energy propagating vast distances in the ocean - may have contributed to or even triggered the abrupt ice shelf calving. The analysis will also consider the timing, extent, and severity of melt on the Wilkins ice shelf surface, and melt day trends over the past 20 years.
The work will compare the present Wilkins break-up with similar past break-up events, and also those of the Larsen A and B Ice Shelves in 1995 and 2002 respectively. Further south, Larsen C has thinned and continued warming could lead to its breakup within the next decade. Can we forecast break-ups based on sea ice and climate anomalies?
Importance: Ice shelves are sensitive climatic indicators, and it is important to fully understand the processes responsible for their recent rapid and abrupt demise. The shelf breakup events around the Antarctic Peninsula over the past two decades have removed features that appear to have been stable over the previous several centuries to millennia - with major physical and ecological ramifications (Domack et al., 2005). While ice shelf disintegration does not contribute directly to sea-level rise, the resultant acceleration of outlet-glacier ice discharge into the Southern Ocean following their removal does (e.g., Rignot et al., 2004). It follows that changes in sea ice extent may impact sea level rise via its (hypothesized) role in ice shelf disintegration. Further, the event underlines the complexity of an Antarctic system undergoing rapid change, and highlights the need to understand the links between different components of the cryosphere. The Antarctic Peninsula may well be a model for a future, warmer Antarctica – with future changes occurring at a greater scale, and speed, than was previously considered possible.
Collaborators: Funding is required to support a month-long collaborative visit by Dr Rob Massom (a sea ice geophysicist with the Australian Antarctic Division and Antarctic Climate & Ecosystems CRC, Australia) to conduct the research. Publication of the results will follow. Much of Dr Massom’s recent work has focused on the impact of anomalous atmospheric circulation patterns on Antarctic sea ice, and ecology (e.g, Massom et al., 2008). The proposed work also involves strong international and inter-disciplinary collaboration: University of Chicago (Prof. Doug MacAyeal), the Australian Bureau of Meteorology (Dr Neil Adams), Lamont-Doherty Earth Observatory (Dr Sharon Stammerjohn), the British Antarctic Survey (Drs David Vaughan and John Turner), and University of Otago (New Zealand – Prof. Vernon Squire, who is a world authority on wave-ice shelf-sea ice interaction).
Why it is innovative: The Wilkins Ice Shelf break-up event of February-March 2008 presents a unique opportunity. An unusually good series of remote sensing data sets have been collected; and unprecedented resources monitoring climate, ocean, and ice physical parameters are available for this event. Our hypothesis that open ocean is a key precursor, and that wave action and/or solar heating may be necessary to initiate the runaway calving, can be examined in greater detail than in any previous similar event. The proposed work is closely aligned to the International Polar Year via “IPY in the Antarctic Peninsula - Ice and Climate” (IPY project 107, http://classic.ipy.org/development/eoi/proposal-details.php?id=107). Our proposal seeks to determine previously-unstudied links between Antarctic sea ice distribution and ice shelf disintegration, driven by anomalous patterns in atmospheric circulation.