Internannual Variability of Boundary Fluxes and Water Mass Properties in the Gulf of Maine/Georges Bank System During GLOBEC

Peter C. Smith, David Mountain, Bob Houghton, Jim Bisagni, Dick Limeburner

The primary advective inputs of mass, heat and freshwater to the Gulf of Maine occur along the eastern side of Northeast Channel (NEC) and around the southwest tip of Nova Scotia off Cape Sable. Monthly mean observations of current, temperature and salinity at these sites (NECE,C2 respectively) over three GLOBEC field years(October 1993-September 1996) reveal some familiar features of the mean inflow and annual cycle (e.g. surface inflow at C2 peaks in early winter; deep inflow at NECE peaks in late summer) and some new insights including:

• a. a strong surface inflow at NECE peaking in spring (March-April), suggesting processes governing shallow and deep seasonal inflows are different,
• b. a distinct annual cycle in NECE cross channel current, favoring "crossover" towards Georges Bank in winter, and
• c. the absence of a significant annual salinity cycle at any depth at NECE, suggesting that salinity is controlled by offshore processes.

The anomaly time series from the moorings show three distinct low-frequency events:

1. a period of enhanced warm, salty NECE inflow at the beginning of the record (October 1993-September 1994), followed by
2. a sharp transition to enhanced cold, fresh C2 inflow during the winter of 1994/1995, and
3. a second period of cold, fresh C2 inflow during the first 9 months of 1996.

   During these events, salinity and temperature anomalies were generally similar, but the mass fluxes were out of phase, i.e. enhanced NECE inflow was associated with reduced C2 inflow and vice versa. Hydrographic sections and frontal charts based on satellite imagery suggest that the presence of three warm core rings off the mouth of NEC during the first period was responsible for the warm, saline inflow anomalies. The ranges of temperature (±2) and salinity (±0.6) anomalies at the mooring sites are comparable to those associated with decadal variability at the Scotian Shelf (Petrie & Drinkwater,1993).

   Hydrographic observations from the interior of the Gulf and on the Bank during 1991-1997 reveal variations similar to those on the inflow boundaries, but lagged by 1-3 months due to advection Surface layer (0-30m) anomalies with respect to the MARMAP climatology (1977-1987) suggest that the entire period was relatively fresh, especially during the early 1990s and following the cold, fresh C2 inflow events in 1995 and 1996. Recent measurements in the Bay of Fundy (Prince 5) indicate that salinities are now recovering to normal levels. T-S analyses of deep waters (150-200m) in the interior basins show mixtures of predominantly Warm Slope and Scotian Shelf surface waters, in contrast to observations from the cold 1960's when a much larger Labrador Slope Water component was present.

   On Georges Bank, broad scale survey results show a continuous decrease in surface layer (0-30m) salinities from 1995 through 1997, reaching down to levels near 32.0. Furthermore, oxygen isotope analysis indicated that the fresh Bank waters in 1996 and 1997 represent variable mixtures of Scotian Shelf and Slope Waters that are virtually devoid of Maine Coastal Water. The largest volumes of Scotian Shelf Water are found in 1997, consistent with the increased fresh water inflow off Cape Sable in mid-1996.

   In addition to advective fluxes, interannual differences of surface heat flux induce variations in heat content and stratification on the Bank. On the southern flank, a one-dimensional potential energy model provides monthly estimates of stratification in three depth ranges for the period 1985-1992, based on observations of solar insolation, wind and tidal mixing, and verified by hydrographic measurements. Internannual variations are absent from the shallow (<60m) zone and maximum in the deepest (80-100m), where anomalously high stratification occurs in 1992, when surface salinities were low on the Bank. Estimates for 1995 indicate normal levels of seasonal stratification in the deep zone, but low salinities in the 1996 and 1997 may lead to enhanced stratification in those years (to be determined).

   Finally, there is intermittent evidence in historical hydrographic records for direct "crossovers" of surface waters from the western Scotian Shelf (Browns Bank, BB) to the eastern end of Georges Bank(GB). Climatological seasonal cycles of near-surface salinities from these two areas show contrasts (GB-BB) of 0.7-1.1 during December-May, falling to 0.0-0.3 in August-October. Using an observed GB anomaly <0.5 with respect to the annual cycle as a "crossover" index reveals a distinct 3-5 year periodicity in these events, some of which have been well-documented (e.g. February 1978, March 1979). As expected, observed "crossovers" are most prevalent in the January-June and nearly absent in July-October.

   References:

   Petrie, B. and K. Drinkwater. 1993. Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945-1990. J. Geophys. Res., 98(11), 20,079-20,089.