Seaglider track and
SST

 

The iAOOS Seaglider Project

 

The seaglider project is is a part of the project entitled "Integrated Arctic Observing System" that aims at developing a modern observational system for the Arctic Ocean and adjacent seas. This project, which is funded by The Norwegian Research Council, is a part of the International Polar Year. The Seaglider Project is administrated by the Norwegian Meteorological Institute, with Cecilie Mauritzen as project leader and advisor. Responcible scientist is Frode Høydalsvik. The Seaglider was developed by scientists and engineers from Applied Physics Laboratory (APL) and School of Oceanography at the University of Washington. The iAOOS Seaglider Project is carried out in collaboration with Craig Lee and Jason Gobat at Integrative Observational Platforms (IOP), APL.

The glider currently operating is Seaglider 140 (SG140), deployed on September 30, 2009 by Dr. Jason Gobat and Prof. Kjell Arild Orvik (University of Bergen), who also organized the deployment cruise with G.O. Sars. Seaglider 140 is, as the former glider, Seaglider 160 (deployed by Angela Wood (APL) and Prof. Orvik), gliding along the Svinøy section, where Prof. Kjell Arild and his team at the University of Bergen have several moorings.

The first glider, Seaglider 17 (SG017) finished its zonal (66° N) transects between the shelf break and 1° W, in October 2008. The SG017 data clearly has demonstrated the Seaglider's capability of being used to estimate volume currents in the Norwegian Atlantic Current. Our experiment has shown, however, that it is more difficult to get good estimates in the viscinity of the shelf break, due to the strong currents here. At the same time, volume flux estimates from direct measurements in the western branch of the Norwegian Currents at the Svinøy have been less reliable than the estimates from the western branch (Orvik et al, 2001).

It was therefore decided to move the glider to the Svinøy section. Operationally, this is advantagous, because of the short distance between the coast and the shelf (the glider is designed to operate in deep water, and works best if launched at 500 m depth or more). The combination of moorings and continuous glider operation is expected to yield very valuable scientific results.

 

About the Seaglider

The Seaglider is an autonomous underwater vehicle (AUV), and unlike conventional AUVs, it does not use propulsion to move in the water. Rather, it changes its buoyancy by inflating or deflating a bladder. By using its wings, and by moving a battery pack, it can change its roll and pitch, and glides in the water, in a similar way to the sailplanes. The Seaglider has two battery packs. The light-weight battery pack is used to operate the computer motherboard, the satellite phone, and the scientific instruments. The heavier battery pack is used to operate the pump inflating the bladder, and the engines changing pitch and roll. The glider is about 1.8 meters long with its antenna, weights about 52 kilograms, and has a volume of about 52 liters. Typically, the glider is used for measurements in the deep sea, diving from the surface to a maximum depth of about 1000 meters, using a little less than 8 hours per dive, moving about 4 kilometers over ground. The diving depth and its velocity depends on the ocean currents currents, and how "hard" the glider is driven (the intensity of pumping). The Seaglider currently operating can reach a maximum horizontal velocity of a little less than 40 cm/s. The glider stores its measurements taken at the programmed sample intervals in its flash memory. After a dive, it sends the data and its GPS position via the Iridium Satellite to the computer base station at Applied Physics Laboratory at the University of Washington. The glider can then receive new instructions about which target (position) to aim for, how deep it should go, and how frequently the data sampling should be.

 

Data material

Very valuable data from the glider transects is now being compared to, and analyzed in conjunction with historical and present OWSM data. Strength and weaknesses of the ship measurements versus the glider data are being assessed, both from a practical/logistic point of view, and from a scientific perspective. Among others, the glider data provides information about the velocity, direction, and structure of the Norwegian Atlantic Current, as opposed to the oceanographic time series from the weather ship. Computer algorithms to calculate volume transports based on glider data have been developed, and are now being assessed.

The Norwegian North-Atlantic Current can be divided into two branches in the area where the Seaglider is operating. The eastern branch follows the isobaths north/northeastwards along the continental slope. It extends from the shelf break and 30-50 km away, in the direction of the Vøring Plateau. The current in this branch is deep, and does not vary very much with depth (it is a so-called "barotropic current"). The western branch follows the isobaths at the skirts of the Vøring Plateau, and is associated with the front between the warm and saline Atlantic Water on the east side and colder and fresher water on the west side. This branch is also considered to be 30-50 kilometers wide, but the current here is different from that of the inner branch, being more shallow and depth-dependent; it is "baroclinic", see Nilsen and Nilsen (2007).

However, our measurements from the zonal transect at 66° N clearly shows that - at least at the time of measuring - the "baroclinic" boundary current is not that baroclinic after all. There are important deep water currents, which have significant contribution to the currents further up in the Atlantic layer. These deep water currents have a typical magnitude of 10 cm/s. Interestingly, we find that even if the velocities of the cores are dominating, the contribution to the total volume transport from the "drift" outside the cores is important. The glider measures important depth-averaged currents over large parts of the section. These currents contribute significantely to the total volume transport.

At wintertime, the finger print of the Norwegian Atlantic current is easily seen from a satellite image showing seasurface temperature (see the uppermost figure to the left), as the Atlantic water reaches all the way up to the surface. It is interesting to see - almost in realtime - how the glider navigates through the Norwegian Atlantic Current and waters of Arctic orginin, and how it is affected by the amnient current field. We have allready seen examples of how warm rings (warm, Atlantic Water being surrounded by colder water of Arctic origin), evident from the satellite image, also leaves its finger print in the depth averaged current, which the glider measures indirectly. In theory, a warm ring of sufficient size, should exhibit an anti-cyclonic (clockwise) circulation compared to the adjacent water. This is just what we have observed here.

In spring, we observe how the summer thermocline is establishing, and how the fresh coastal water is extending westward.

 

Responsible for this page: Frode Høydalsvik

Seaglider track and
SST (zoomed)

Example 3D Seaglider track

Recent Seaglider
salinity and temperature profiles