Pioneer work on Kelvin waves

Pioneer work on Kelvin waves

Top image: Master student Ragnhild Lundmark Daae in Kongsfjorden on a scientific cruise in 2005. Photo: private. 

UNIS master student Ragnhild Lundmark Daae has investigated the internal Kelvin waves in Kongsfjorden. The data shows that the topography of the fjord modifies the internal Kelvin waves, something which has not been studied in connection with Kelvin waves in Kongsfjorden before.

7 July 2008
Text: Eva Therese Jenssen

In a new master thesis UNIS and UiB-student Ragnhild Lundmark Daae has investigated the internal Kelvin waves’ behavior in Kongsfjorden. The data analyzed from 2003 and the winter season 2004/05 indicate that the topography of the fjord play an important role in how the Kelvin waves behave.

Pioneer scientific work
Daae presented her master thesis in physical oceanography last week at UNIS. The thesis, titled “Long periodic vorticity waves in Kongsfjorden”, investigate how Kelvin waves are created and how they behave in Kongsfjorden.

The Kelvin wave follows the coastal line. In the northern hemisphere the Kelvin wave flows along the coast with the shoreline on the right side of the wave. In the southern hemisphere it flows along the coast on the left side of the shoreline.

The research Daae has been conducting is characterized as pioneer work by her UNIS advisor, associate professor Frank Nilsen.

– There has not been much scientific work on this wave earlier, Daae says.

Important biological function
The Kelvin wave gives information about the inflow of water from the continental shelf into the Kongsfjorden.

The water the wave brings with it from the shelf can either be warm Atlantic water or colder, Arctic water, depending on the composition of the shelf water.

The Kelvin wave can transport zooplankton into the fjord system, which can support larger populations of higher trophic levels, such as fish and birds. Thus, this wave mechanism can be quite important to the biological system in the fjord.

Daae examined data from two moorings located along the south-eastern coast of Kongsfjorden, along the Brøggerhalvøya. She used two data sets, one from 2003 and one from the winter season 2004/05. One mooring was UNIS property which was stationary in both time periods. The other mooring was placed by the Scottish Association of Marine Science (SAMS).

Corresponding rotation
In 2003 this mooring was located 2.4 km from the UNIS mooring into the fjord. In 2004/05 the SAMS mooring was located 6.4 km further into the fjord from the UNIS mooring. The data collected was from 94 m depth at all three moorings.

Daae has investigated the flow’s strength and direction and employed different analytical methods to see if there is correspondence between the current vectors at the two moorings, especially concerning the rotation of the wave.

– The rotation analysis tells us about the differences and similarities of the rotation observed at the UNIS mooring and the SAMS mooring. The data shows that the two moorings were corresponding in all the plots we did in the 35-48 hours time interval, Daae explains.

Different behavior due to topography
The Kelvin wave forms after a storm or heavy low pressure system has passed outside the coastline on the continental shelf. According to current theory, the Kelvin wave will go straight into a fjord and then straight out of the fjord system.

However, Daae’s analysis shows that the Kelvin wave behaves differently in Kongsfjorden due to the topography of the coastal shelf. In Kongsfjorden the coastline has a steep slope and this affects the behavior of the Kelvin wave when it enters the fjord system.

The wave will, instead of just flowing in and out, be pushed further out into the fjord because of the slope, and thus creating a vorticity wave further into the fjord, and not along the coast line. Thus, the Kelvin wave theory must be modified to incorporate such changes in wave behavior due to topographic conditions.

Daae’s master thesis thus presents pioneer work within the Kelvin wave theory, as her research shows that topographic conditions along the shore line influences the behavior of the Kelvin wave.

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