Declines in Artic sea ice cover and thickness have been observed continuously during the modern satellite era, but appear to have accelerated in the past decade. These more extreme ice-loss events first appeared in 2007 and set a record in 2012, declining to just 54 per cent of the 1981–2010 average.
The sea ice loss has important impacts on both atmospheric and oceanic systems. Included are more energetic wave events in the Arctic marginal ice zones (MIZs). In turn, more energetic swells can quickly break up an ice shield, opening up MIZs to additional wave growth and more solar heating of the upper ocean.
This is called the ice–albedo feedback effect. The result is accelerated ice melt that intensifies the advent of a seasonally ice-free Arctic.
Other wave-related impacts include: changes to the momentum, energy, mass, heat and gas exchange across the air–sea interface; enhanced coastal erosion rates due to the enhanced wave action on thawing and vulnerable shorelines; and safety threats to increasing offshore activities and Arctic shipping.
The scale of adverse effects has resulted in a push to better characterise the wave climate in the Arctic Ocean.
The subject has become of interest to the US Navy, which is facing unpredictable sailing conditions in the Arctic Ocean, especially as sea ice retreat is occurring faster than simulated by most current climate models.
That means that the wave–ice interaction is not clearly understood and is oversimplified in the present third-generation wave models.
To rectify this problem the US Office of Naval Research has funded the University of Melbourne’s Professor of Ocean Engineering Alexander Babanin to develop more comprehensive models of the wind–wave climate and to identify trends for the entire Arctic basin. Dr Qingxiang Liu is playing an important research role.
Key to Professor Babanin’s work are satellite-based, altimeter measurements that provide high-quality ocean surface wave measurements and wind observations.
The project used 20 years’ worth of altimeter measurements, across three satellite missions (ERS-2, Envisat, and CryoSat-2). The focus was on the Arctic summer (August–September), the season of greatest open ocean area and wave fetch (the horizontal distance over which wave-generating winds blow).
To capture climatic variability, the Arctic Ocean was subdivided into eight regions: the Beaufort Sea, Chukchi Sea, East Siberian Sea, Laptev Sea, Kara Sea, Barents Sea, Greenland Sea and Baffin Bay.
The project successfully developed ways to remove error from altimeter measurements and has resulted in a calibrated and validated multi-platform altimeter database. Additional sea ice data was used from the US National Snow and Ice Data Center archive.
The analysis of altimeter measurements identified regionally pertinent trends for wave–ice interactions in the Arctic Ocean in relation to wave height and wind speed. Among the impacts identified are trends that contribute to sea ice loss.
Dr Liu explains that wave height was found to be clearly increasing in two regions: the Laptev Sea and the western Beaufort–Chukchi Seas. Extreme storm winds are also increasing in these regions and the effective fetch is becoming larger because of the ice retreat.
In the Beaufort–Chukchi Seas, the increasing wave energy seems to be preferentially directed at the Alaskan coast.
In the Laptev Sea, however, the prevalent waves propagate northward to the ice cover. This has important implications: “This wave energy arriving at the ice edge in the Laptev Sea is becoming increasingly important in terms of fracturing ice floes and pack,” Dr Liu says.
The trend for more energetic waves contributing to ice melt is not universal, however.
In the Barents and Kara Seas, for example, winds and waves initially increased between 1996 and 2006 and later decreased. That leaves a role for large-scale atmospheric circulations (such as the Arctic Oscillation and Arctic dipole anomaly) to impact on wind and wave variation in the Atlantic sector. While such a decrease is counterintuitive, it is predicted by climate models that are fully coupled with waves and ice.
“A comprehensive coupled air–wave–ice–ocean model framework is necessary to quantify the complex ice–wave feedback,” Professor Babanin says. “So a long-term goal is to improve wave modeling using new physics that are adapted and validated for the Beaufort and Chukchi Seas.”
Importantly, the new modeling capability is proving suitable for operational wave forecasts. That means that altimeter climatology and the wave models can be used to study current and future wind–wave and ice trends, with benefits extending to Antarctic seas.