Discussion of Regional decadal scale hydrography changes in North Atlantic and the biological response

Charles Hannah(leader), Peter Wiebe (Rapporteur)


The purpose of this discussion was to highlight some of the recent analyses that have been done to characterize the changes in hydrography that have taken place over the past 50 or so years and possible biological responses.

The discussion was initiated by a presentation by Charles Hannah which detailed the long-term changes that have occurred on Georges Bank and the adjacent Slope Water. The cold 60's were contrasted with the warm 70's. During the 60's, there is a marked increase in the contribution of the Labrador Current Water into the circulation of the Northwest Atlantic region (Grand Banks, Gulf of St. Lawrence, Scotian Shelf, Gulf of Maine, Georges Bank, and Slope Water). A circulation model was used to examine the nature of the flow fields that these varying hydrographic conditions would give rise to. The region considered extended from Halifax to Long Island. In the cold years when the water was cold and relatively fresh, there was an increased net transport and higher velocities along the shelf/Slope Water frontal region on the southern flank of Georges Bank. In the warm years, when the water was warmer and saltier, there was a substantially reduced current flow on the periphery of the Bank. In contrast, there was no decadal scale change in the stratification on the Bank. There is still an outstanding question about the importance of local heat flux versus advective heat flux controlling the changes in temperature structure on the Bank.

A larger context for thinking about the climate changes impacting the region was provided by Peter Wiebe who talked about the variation in ocean zooplankton occurs over a wide range of temporal and spatial scales: from weeks to millennia and from millimeters to scale of the ocean basins. Dramatic changes in the climate of the Northern Hemisphere have taken place since the height of the last ice age. Then, extensive ice cover of both land and sea altered sea level and modified the ocean basin circulation patterns. Biogeographic range compression of North Atlantic zooplankton at the glacial maximum 18000 yrs BP is indicated by fossil and living foraminiferal and coccolithophorid assemblages. A significantly reduced area of continental shelf habitat also occurred. The end of the Younger Dryas at ~ 10,000 BP and the beginning of the Holocene (last 10000 years) marked the beginning of the current interglacial period with more modest variations in climate change. However, the glacial/interglacial cycle of events are now seen as amplified versions of cyclical changes of 1000 to 2000 yrs that are evident in both Greenland ice cores and deep sea sediments throughout the current inter-glacial and back well into the past glacial period. These are reflected in the relative proportions of polar/subpolar and temperate/subtropical foraminferal species. More recently the Medieval Warm Period (~1400 yrs. BP) and the little ice age (~400 yrs BP) are evident in the North Atlantic foraminiferal fossil record. Climatic changes during the past half-century (of much smaller amplitude) have been linked to biological transformations, or regime shifts, in plankton and fish communities. At smaller scales of variation, decadal variation in zooplankton biomass and indicator species' abundances may result from long-term climatic trends. For example, variation in biomass and species' abundances correlate significantly with climatic events, such as the North Atlantic Oscillation in the Northeast Atlantic and possibly the Northwest Atlantic, and advective events on the Scotian Shelf may be linked to three Great Salinity Anomalies of past half century.

The recent changes in hydrography and fluorescence for Georges Bank and the Gulf of Maine based on the GLOBEC data set were described by David Mountain. On top of the Bank, the strongest signal is seen in the salinity data. Throughout the GLOBEC years, there has been a monotonic decrease in salinity from 1995 to 1998. This trend is part of a larger trend of increasingly negative salinity anomalies that began in the mid 1970's. Chlorophyll fluorescence also shows a monotonic trend of decreasing values from 1995 to 1998. But when the fluorescence values are converted to chlorophyll using calibration regressions, based on CTD/Rosette water sample chlorophyll extractions, the trend reverses to some extent. Higher chlorophyll's are seen in 1998 and 1997

D. Mountain also presented a graph showing the pattern of total spring zooplankton biomass on Georges Bank for the period 1970 to 1990. Biomass varied by a factor of 3 or 4 over two large cyclical periods and appeared inversely correlated with winter NAO index after the values were detrended. He cautioned that plots of other biomass data from adjacent regions or different time periods for the Bank did not show as good a relationship.

Water sources for Georges Bank and the changes seen during the GLOBEC years were discussed by Bob Houghton. The origin of freshwater sources for the Gulf of Maine is the Gulf of St. Lawrence consisting of St. Lawrence River Water and Labrador Shelf Water in the Gulf, that enters via the Strait of Belle Isle. These mix with the underlying Slope Water in the Gulf and along the Scotian Shelf becoming, in the Northeast Channel, a 2-end member mixing curve whose freshwater end member consist of 5.6% St. Lawrence River Water and 94.6% Labrador Shelf Water by volume or by freshwater, relative to 34.8 PSU, 48% and 52% respectively. These proportions remained constant for the years 1995-97 even though the salinity of surface waters varied due to relative mixing with more saline water at depth.

The water on the central Georges Bank consists of a mixture of water entering the Northeast Channel with Gulf of Maine River outflow. There is considerable inter annual variation in the Maine River Water proportion ranging from 3.3% in 1982 to 0.23% in 1996. There is no correlation between Georges Bank salinity, which decreased monotonically from 1995 to 1998, and the proportion of Maine River Water on the Bank (Fig. 1). The inter annual fluctuations of salinity on Georges Bank (Fig. 2) and the surrounding flanks are driven by variations in upstream Scotian Shelf sources and mixing and not changes in local river outflow.

D. Mountain presented a temperature-salinity diagram which showed that the low salinity water in Northeast Channel (or was it on Georges Bank???) for the period 1995-97 was of a different origin than that from 1998. The 1998 water has similar properties to the 1960's.