Light in a Dark Arctic Ocean

Solar radiation is the main heat source to the ocean. In ice-covered oceans, part of the solar energy can penetrate the sea ice to heat the water. Light attenuation through sea ice is a main topic of Arctic study. Usually, optical observations are conducted in summer, as there is sun available then.

However, the angle of sunlight projected onto the ice surface is declining with the maximum solar height about 30 degrees in summer. The sunlight in ice is dispersive and light paths that the sunlight travels are greater than the ice thickness. So the attenuation measured with certain ice thickness is related to the solar height. The attenuation coefficient should be corrected by different solar height. People need to know the attenuation when the light is vertically projected on the ice, as then the light path is equal to the ice thickness. Unluckily, there is no such an opportunity from natural light in the Arctic.

A group from the Ocean University of China has prepared an artificial lamp for use on the ice. The lamp is made of 10 fluorescent lamps with the similar light spectrum of sun. By putting the lamp on the ice surface and shining downward, an underwater instrument, PRR-800 with 18 channels, records the intensity of the light, which is attenuated by sea ice and snow. The experiment tests both cases with snow and without snow in different ice thicknesses and different locations. The power of the lamp is about 500 W/m2, little bit brighter than sunlight in summer. In the dark Arctic Ocean, the light must be a marvelous spectacle to the oceanic inhabitants.

It is expected that the experiment results will give the attenuation coefficient with the light path equal to ice thickness, which shows the nature of sea ice attenuation for lights of different wavelengths. Using this result, we can better estimate the light penetrating through sea ice with different solar heights and different ice thicknesses, which will be benefit to calculate the thermal budget through an ice sheet in any season. Detected light under ice can illustrate the attenuation not only by ice thickness, but also by the particles and brine in the ice. The data can be used in studies of ice physics and physical oceanography, and has potential connection with sea ice ecology.

Marched to the ice, the equipment is more than 100 kg, including a lamp, an ice auger, a power generator, two optical instruments, a computer, and a series of cables and connectors. Through a hole with diameter of 20 cm, the underwater instrument is deployed with a cable. Then, rotating the connector, the instrument is turned 90 degrees to the bottom of ice. Up to now, we have successfully obtained data in six locations with the maximum ice thickness of 110 cm.