Interannual variability of boundary fluxes and water mass properties in the Gulf of Maine and on Georges Bank: 1993–1997

Abstract

Analysis of three years (October 1993–September 1996) of monthly mean current, temperature and salinity observations from moorings in the major Gulf of Maine (GOM) inflows off southwest Nova Scotia (C2) and in Northeast Channel (NECE) reveals some new features of the annual NECE cycles, including (1) a peak in near-surface (<75 m) inflow in spring (vs. late summer at depth), suggesting that dynamic control of shallow and deep layers may be different, (2) a maximum near-surface cross-channel flow toward Georges Bank (GB) in late winter, suggesting a climatological tendency for Scotian Shelf “cross-overs” in that season, and (3) the absence of a significant salinity cycle over most of the water column. Deviations from the annual cycle indicate that the first part of the observation period was characterized by enhanced warm, saline, deep (>75 m) NECE inflow, followed by later episodes of enhanced cold, fresh inflow at C2 and shallow NECE. Generally, the flow rates at C2 and deep NECE were out of phase, with increased inflow at C2 (deep NECE) associated with reduced inflow at deep NECE (C2) and cooler, fresher (warmer,saltier) conditions at both sites. Freshwater transport anomalies to the Gulf are maximum in the surface layers and largely negative(positive) over the first(last) half of the measurements. The timing of these freshwater inflow variations is consistent with observed fluctuations in hydrographic measurements in the GOM and GB, which reached peak salinities in late 1994, then declined through 1995–1996. Oxygen isotope analysis suggests that almost all of the fresh water present on the central cap of GB in 1996 and early 1997 is of northern (Scotian Shelf) origin as opposed to 1994 and 1995 when Maine River Waters contributed 38 and 26%, respectively, to the freshwater (relative to 34.8) on the cap. A simple box model driven by observed changes in the boundary fluxes indicates that over the last half of the measurement period (April 1995–September 1996), the volumetric flow rate through the GOM increased by 10 5 m 3 s −1 (roughly 17% of the total transport, 5.83×10 5 m 3 s −1), and that increased freshwater fluxes in the surface layers at C2 and NECE produced a net decrease of 0.73 in the salinity of the outflow waters. Average volumetric transports at C2(NECE) were roughly twice(half) those observed in the late 1970s, but the total is consistent with climtological estimates. The net change in the freshwater fluxes exceeds the total climatological mean estimate. Examination of possible local and remote sources confirms that the origin of the 1996–1997 freshwater anomaly is in the northern Labrador Sea/Baffin Bay and results from exceptionally cold winters in the early 1990s. Analysis of a similar event in the early 1980s suggests their occurrence is part of a quasi-decadal climate signal which follows the North Atlantic Oscillation (NAO).