Retrospective decadal and interannual hydrographic and circulation cariability in the Scotia-Maine region

C.G. Hannah, J.W. Loder, B. Petrie, J.A. Shore, E.A. Gonzalez and S. Oakey

We report on progress towards the goal of our retrospective studies (U.S. GLOBEC and GLOBEC Canada): developing quantitative representations of the spatial structure of, and indices for, the decadal and interannual variability in hydrography and circulation in the Scotian Shelf and Gulf of Maine region. We start with a brief review of the spatial structure of the hydrography during the cold period in the 1960's and contrast the spatial patterns with the warmer 1970's. In short the 1960's had increased equatorward advection of cold water along the shelf edge. This resulted in near-bottom temperature reductions on the shelf of as much as 4° C. As well there were reduced levels of inorganic nutrients and increased levels of dissolved oxygen.

Then we present preliminary results which suggest that the colder than normal shelf-waters during the early to mid 1960's was the result of several wintertime pulses of cold water of Labrador current origin rather than a persistent decrease in the temperature at the shelf-edge. This event-dominated scenario is consistent with the recent observations (1997/8) of a cold pulse of water which reduced the temperature at 200 m in Georges Basin by 2° C and to the lowest levels since the mid 1960's.

We close with the results of a model based study of the interannual variability in drift and retention for the Browns Bank haddock spawning area for the years 1975-90. The circulation fields were based on the seasonal-mean circulation and flow components derived from monthly wind and sea level data. The circulation variability forcing can reinforce either drift to the Bay of Fundy or retention in the vicinity of the bank, or introduce additional loss pathways. Comparison with biologically-based indices for haddock survival and retention indicates that circulation variability alone is not the dominant influence on the retention of larval haddock. The spatial and temporal structure of the circulation means that factors such as the timing and location of peak spawning are important.