RRS Discovery Cruise 242, 07 Sep-06 Oct 1999. Atlantic - Norwegian Exchanges

This report describes RRS Discovery cruise 242 from 07 September to 06 October 1999. The cruise title is Atlantic - Norwegian Exchanges. There are two distinct parts to this experiment with closely related objectives. The first is to measure the pathways and flux of warm, upper ocean water northward through the Iceland Basin and Rockall Trough to high latitudes. The second is to measure the returning flux of cold, deep water that flows through the Faroe Bank Channel into the North Atlantic. A full depth hydrographic section was occupied between Scotland and Iceland, repeating the frequently occupied Rockall Trough section (occupied 36 times between March 1975 and January 1996) and it's recent extension from Rockall to Iceland (occupied in 1997, 1998 and now in 1999). A second section was occupied from southeast Iceland to Lousy Bank (occupied in 1962,1990 and 1996). These two sections comprised CTD/LADCP stations with discrete vertical samples for salinity, oxygen, silicate, nitrate and phosphate. Horizontal station spacing was ~30 km in the Iceland Basin but much closer over steep bathymetry and in the Rockall Trough. Ancilliary measurements of transmittence and reversing temperature and salinity were also made. Shipbourne observations were made throughout the cruise and comprised ADCP, navigation, meteorology, waves, echosounding and surface temperature and salinity. These two sections were designed to measure the pathways of the northward flow through the Iceland Basin and Rockall Trough. Differences to earlier occupations will show the time variability of these flows. In the Faroe Bank Channel and on the Iceland Ridge eight sections were occupied (some repeats) to examine the cold outflow into the North Atlantic. These sections were made in the Faroe Bank Channel and downstream of the sill, at a horizontal separation of between 15 km and 40 km. Five of these were standard CTD/LADCP sections with chemistry observations. Three sections were also occupied using the BRIDGET deep tow vehicle. This vehicle carried a CTD and 12 bottle rosette for water samples as well as some auxiliary sensors. The key novelty was the mounting of self-contained downward and upward looking ADCP's. BRIDGET was towed at 100 m off bottom giving cross-stream measurements at high resolution of the velocity structure of the overflow. These sections were taken in and just downstream from the Faroe Bank Channel sill. We will examine the initial adjustment of the overflow and with contemporaneous observations made in the Faroe Shetland (Fisheries Research Services, Aberdeen) the role of hydraulic control at the Faroe Bank Channel sill. The two sections in the Iceland Basin cross the overflow 360 km and 660 km downstream from the source defining the far field location and properties of the overflow. A short section of XBT observations was made along the Wyville-Thomson Ridge to measure the temperature at two saddle points where it is occasionally observed that cold Faroe Bank Channel water passes into the Rockall Trough.