Wave Riding

The meteorology team on Oden is primarily interested in the ways that Arctic clouds influence sea ice, and vice versa. An important component of this is the sea state, the height and length of all the sea surface waves, in or close to the edge of the sea ice.

Waves influence the turbulent transfer of momentum and gases, such as CO2 and methane, between the air and water. The sea state is dependent on the wind speed, the distance the wind has blown over open water (termed the fetch) and on the presence of swell (longer period, more regular wave motion, typically created in a storm some distance away).

The presence of sea ice affects waves both by limiting the fetch, and by dampening the waves themselves. Waves in turn can break up the edge of the ice pack, and swell waves can travel into and through sea ice areas. All these interactions are quite poorly understood due to the difficulty of making good measurements in and around sea ice.

On Oden we are measuring the sea state using a Waverider buoy, deployed from the ship. The buoy floats on the sea surface, moving with the waves as they pass by. It uses instruments such as accelerometers to measure both the vertical wave motion (the heave) and the horizontal direction in which the waves are moving.

The waverider crew

The waverider crew

Due to the presence of sea ice, which might trap the buoy, and for operational reasons, we can’t let the buoy free drift and so we are deploying it tethered, on a rope 200m long (including a snatch-chain between two floats to prevent Oden pulling on the buoy). We deploy it during stations, when the ship has stopped, and is putting instrumentation down to the sea floor to measure the sediments and/or the water properties. The stations usually last between 30 minutes and 3 hours.

buoy, waves, and ice

buoy, waves, and ice

Deploying the buoy is straightforward – it is lowered over the side and allowed to drift away from the ship – actually it is the ship that drifts away from the buoy as the wind pushes on Oden like a large sail, and carries her away from the relatively stationary buoy. We then monitor the buoy while it is deployed to make sure it isn’t hit by any passing sea ice and that it doesn’t interfere with any of the other instruments and cables being lowered into the water nearby. To recover the buoy, the rope is simply pulled in – a good workout when time is tight and there is a lot of wave motion!

View from the waverider alongside Oden

View from the waverider alongside Oden

 

buoy & gull

buoy & gull

As well as our own science, we are also using the buoy to help calibrate Synthetic Aperture Radar (SAR) measurements made onboard satellites. SARs measure wave height and are typically tuned for open ocean conditions. Near-ice measurements such as those we are making help to better calibrate the sensors for Arctic regions. Coordinating a buoy deployment at the same time as when a satellite is overhead, and when winds and waves are high, isn’t always easy. When conditions are calmer, we at least get the opportunity for nice photos of Oden from the water.

The Oden from the waverider buoy

The Oden from the waverider buoy

 

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