Ocean Climate of the NW Atlantic during the 1960s and 70s and Consequences for Gadoid Populations
The first Backward Facing Workshop (BF-I), held at the Bedford Institute of Oceanography, Dartmouth, Nova Scotia, 8-10 March 1995, focused on recent conditions of excessive cold from West Greenland to the Middle Atlantic Bight. BF-I identified the early 1880s as a target for retrospective data analysis and interpretation and included a study of the tilefish kill event of 1882 and possible physical mechanisms associated with it. Additional details are in the ICES CM 1995/A:7 report, and retrospective modeling papers by Bob Marsh et al. (1998) are in press in the journal Fisheries Oceanography. The second workshop (BF-II), was held at the Institute of Marine Research, Bergen, Norway, 21-23 March 1996, and focused on the incidence, causes and ecosystem effects of extreme cold in the marine environment of the Barents Sea and Baltic during the early 1940s; details are available in the ICES CM 1996/A:9 report.
The third backward facing workshop (BF-III) was held in Woods Hole, on 4-6 May 1998, co-convened by Cisco Werner, Steve Murawski and Keith Brander.
The Terms of Reference for the workshop were:
The 1960's decade was the coolest period on record in most areas from the Laurentian Channel to the Middle Atlantic Bight. Subsurface freshening of water masses co-occurred with the cooling period, and historical nutrient and oxygen data indicate lower nitrate and higher dissolved oxygen concentrations in the region during the cold 1960's, especially at depth. The cooling of the shelf waters in the 1960s was largely due to the presence of cold, fresher Labrador Slope waters offshore that subsequently penetrated onto the shelf, with largest temperature and salinity changes observed along the shelf edge from the Scotian Shelf to the Middle Atlantic Bight. Rapid warming of the water column followed during the late 1960's and 1970's when the cold Labrador Water was replaced by warm Slope Water.
The cause of the presence of cold slope waters during the 1960s along the shelf edge off the Scotian Shelf, and adjacent areas to the south, is believed to be increased transport of the Labrador Current. In the deep 100-300m layers, the Labrador Current transport near the Tail of the Grand Banks has been estimated to be 1.6 Sv during the 1960s, and approximately 1.2 Sv higher than it was in the 1950s and 1970s. Past studies suggest an inverse relationship between the strength of the Labrador Current's southwestward transport and the North Atlantic Oscillation (NAO) index. During the 1960s, the NAO was at its lowest since the 1880s.
Given the strong contrast in ocean climate between the 1960s and '70s it is perhaps disappointing that obvious biological consequences are not apparent. Survival rates on Georges Bank and Browns Bank correlate quite well for both cod and haddock stocks, suggesting there may be processes affected by common physical forcings, but more work is needed to define these processes. Several data sets were identified for future study and distribution (e.g., from MARMAP, CPR, SSIP, etc.) and a follow-up workshop was recommended (perhaps in Woods Hole as early as the Fall of 1999) with the aim of continuing the synthesis of factors influencing cod and haddock recruitment, and to provide a forum for the integration of new information from studies currently underway.
Reports of the BF-III workshop (ICES CM 1998/C:9, 89pp) are available from http://www.ices.dk/reports/occ/wkbfac98.doc (286Kbytes) in Microsoft Word format and http://www.ices.dk/reports/occ/wkbfac98.zip (7,390Kbytes) in zip (compressed) format.