Tilt Meter Buoys Jeremy Wilkinson

The Arctic is warming faster than any other region of the globe. Over the past few decades this warming has been accompanied by a reduction of perennial ice

within the Arctic Basin; a decrease in the extent of sea ice of about 15% as well as a decline by some 40% in the thickness of summer sea ice. Moreover, accelerated change is predicted including a temperature rise of more than 4ºC over the next 50 years and the disappearance of summer sea ice by 2040.

The disappearance of summer sea ice in the Arctic is a climatic event that has not been seen before. If predictions prove right, and later the century the Arctic does indeed become ice free, then this change will have enormous consequences on both the local and global environment, as well as the associated socio-economic impacts affecting human beings, human health and human activities.

The Arctic Ocean represents one of the most serious challenges for the monitoring and measurement of the physical environment. One of the hardest parameters to obtain on a synoptic scale is the measurement of sea ice thickness. This can only be achieved with satellite-mounted sensors; however there are at present no sensors that can measure the thickness of sea ice directly. The only satellite-borne technique that shows promise is radar and laser altimetry, which measures the height of the sea ice above the ocean’s surface, this is known as freeboard. However this technique uses a number of broad assumptions to change ice freeboard to ice thickness, and has not yet been fully validated in comparative experiments. Other satellite-based techniques using SAR or passive microwave involve inference of ice thickness from other measured parameters. Airborne techniques (laser altimetry for freeboard;

electromagnetic sounding for thickness) are expensive for obtaining data over large areas, while through-ice techniques (hole drilling, surface sounding) are purely local. At present the only way to map the sea ice thickness over large regions is with upward looking sonars mounted on nuclear powered submarines.

Due to military operations most parts of the Arctic Ocean have now been mapped at various times by under-ice sonar.

It is from the sonar profiling of the sea ice during these missions that the main information on sea ice thinning over the past decades has come. However with the end of the Cold war the deployment of British and US submarines in the Arctic has become more sporadic and their operations have been severely reduced in scope. The number of submarines obtaining ice thickness data from the Arctic has diminished to the point where we are no longer acquiring enough

data to show us what spatial and temporal trends are occurring.

Until satellite sensors are able to obtain accurate ice thickness data we need another method to obtain continuous, synoptic, and long-term monitoring of ice thickness.

Recently developed theory suggests that the propagation of flexural-gravity waves in ice have a spectral peak at a frequency which is a function of ice thickness. In other words, if we measure the oscillation spectrum on the ice

Testing of the tiltmeter bouy

surface, we can derive information on ice thickness. In fact this technique has the potential to measure and monitor the evolution of the modal multiyear ice thickness along the whole wave propagation path, from the open ocean to the measurement site.

Flexural gravity waves originate as open ocean swell in the Greenland Sea, but evolve as they cross they pass through sea ice into a spectrum where the peak energy is concentrated at longer periods, usually around 30 seconds. These tiny oscillations can be detected in the central Arctic by very sensitive instruments such as tiltmeters and strainmeters. For decades sea-ice researchers have used different methods to measure the propagation of waves, originating from ocean swell, through sea ice. Most of these instruments were delicate to transport, maintain and labour intensive to install. Furthermore they required constant attention to ensure that the sensors were always in range, and due to the relatively high recording frequency,

data was recorded internally. This in turn demanded that the instrument be revisited for data recovery. Recently scientists from the Scottish Association for Marine Science in partnership with the University of Cambridge developed an autonomous system to measure and transmit information on the propagation of flexural gravity waves in sea ice.

During our participation in the APLIS/SEDNA ice camp we were able to deploy 2 of these systems in the Beaufort Sea region of the Arctic Ocean (D10 and D14).

A further 3 were deployed as part of the EU funded DAMOCLES programme;

one at the North Pole (D11); one east

of the North Pole (D9); and one between Greenland and the North Pole (D12).

This enabled good coverage of the entire Arctic Ocean with respect to gravity wave propagation. The following shows the location of the buoys at the start of the experiment. Details are summarised in table 3.2.

Buoy Table 3.2. Table showing the deployment details for each buoy. Also included is the distance from the ice edge to each buoy. The table is arranged with respect to distance to the ice edge i.e.

buoy closest to the ice edge at the top of the table.