Investigating ocean currents top-down and bottom-up

Investigating ocean currents top-down and bottom-up

Top image: Eli Anne Ersdal with one of the moorings before deployment. Photo: UNIS.

This autumn six moorings were placed around Svalbard to measure ocean bottom pressures at the sea floor. The measurement will give us the weight of the water column above the sea floor and we can calculate ocean currents between the moorings. In addition, using satellite data to measure current strengths at the ocean surface, more reliable data will be produced on the warm water transport to the Arctic Ocean.

7 October 2014
Text: Eli Anne Ersdal, UNIS PhD candidate in physical oceanography

The climate in Norway and Svalbard is relatively mild, considering the high latitude of these locations. The warmest surface water in the world is found in the Gulf of Mexico, and great wind systems sets up an ocean current across the Atlantic Ocean. On branch of this circulation spins off and continues northwards along the coast of Norway, west of Spitsbergen and into the Arctic Ocean.

Every year the UNIS students in the Polar Ocean Climate course (AGF-214) leave for a ten day scientific cruise with the vessel Håkon Mosby. The main purpose of this survey is to monitor the water masses and ocean circulation within the fjords on the shelf area on the west coast. The extension of the Gulf Stream flows along the shelf and is in this region called the West Spitsbergen Current (WSC).

Traditionally oceanographic data are collected though field survey where you use a research vessel and sail out at sea to measure parameters as temperature, salinity and current. This gives us a snap shot of the hydrography and current picture.

Students with CTD on RV Håkon Mosby

AGF-214 students taking water samples from the CTD onboard RV Håkon Mosby. Photo: Ragnheid Skogseth/UNIS

Satellites measure ocean current strength
UNIS is now leading a project called “Remote Sensing of Ocean Circulation and Environmental Mass Change” (REOCIRC) that works on developing a method for using satellite measurement to estimate the strength of the ocean currents. The satellites pass over the polar region several times a day, which results in better data coverage in both time and space.

The satellite uses an altimeter to measure the distance down to the sea surface, and this is what we use to calculate the ocean current. After adjusting these measurements we end up with the tilt of the sea surface. If you travel a distance of 100 km at sea, you might also have moved 10-30 cm in the vertical direction depending on weather the sea surface tilts slightly upwards or downwards.

The tilt of the sea surface represents a horizontal pressure change. The ocean works more or less like the atmosphere and it is the pressure difference between high- and low pressures that generates currents. A steep sea surface tilt represents a big pressure difference, which again generates a strong current. To validate the satellite data we have developed a new mooring system where the main task is the measure ocean bottom pressure.

Mooring set for deployment

The mooring is set for deployment. Photo: Eli Anne Ersdal/UNIS

Moorings measure bottom pressure
During this autumn’s cruise six of these mooring were deployed. The ocean bottom pressures sensors are placed at the edge of the current so the pressure difference across the current are measured.

There are two main branches of interest, and the most important one is the WSC which carries warm and saline waters west of Svalbard and into the Arctic Ocean. The second is the Coastal Current (CC) which transports fresh and cold Arctic Water northwards on the shelf adjacent to the coast.

The CC is monitored along the West Spitsbergen Shelf (WSS), starting form Sørkapp and until the northwestern corner of Spitsbergen. This gives us the possibility to estimate the influence of the freshwater input from all the fjords along the west coast. The remaining moorings monitor the WSC on the Yermak Plateau.

These moorings will be retrieved in a year’s time to download the data they have recorded over a full year. These data will then be compared to satellite data and current meter measurement in order to develop oceanography from space concept.

REOCIRC is a basic natural science research project that will apply state of the art instrumentation and theory to study remote sensed ocean circulation and heat fluxes towards the Arctic Ocean and the corresponding environmental mass changes in response to the estimated heat fluxes. The poorly monitored oceanic heat flux may fundamentally affect the future evolution of the Arctic sea ice cover.

By building upon experimental and theoretical strengths in oceanography, and remote sensing and earth observation methods, combinations and collaborations in these approaches will be addressed in an innovative ways.

Project leader is Professor Frank Nilsen, Head of the AGF department at UNIS.

Analysing data onboard RV Håkon Mosby

AGF-214 students analyse the data collected onboard RV Håkon Mosby. Photo: Ragnheid Skogseth/UNIS

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