Alternating Oceanographic States in the Gulf of Maine: Variable Water Mass and Nutrient Fluxes

Analyses of mooring data from the past 13 years show that the oceanography of shelf and slope waters in the Gulf of Maine region is both highly variable, evolving over time, and exhibits episodically alternating water properties that last months to years. The Gulf is fed by surface and deep water inflows: Relatively fresh and cold Scotian Shelf Water (SSW) enters at the surface, and two types of relatively warm and salty Slope Water (Labrador Slope Water, LSW, and Warm Slope Water, WSW) enter at depth and along the bottom from beyond the shelf edge. These Slope Water masses are the major source of dissolved inorganic nutrients to the Gulf. Variable fluxes of these three water masses alter the Gulf’s water properties (temperature and salinity), the total flux of nutrients to the Gulf, and the relative proportions of nitrate and silicate in the interior Gulf, thereby producing variability in water temperatures and plankton community structure. Unlike decades past, inflows of Slope Waters of either type (LSW versus WSW) no longer appear to be correlated with the North Atlantic Oscillation, which had been shown earlier to influence the relative proportions of LSW and WSW that enter the Gulf. In recent times we are seeing episodes of alternating dominance of SSW or Slope Water that last months to several years. Cold water episodes are characterized by greater influxes of colder, fresher, less dense, low-nitrate and high-silicate Scotian Shelf Water, with a concomitant reduced inflow of deep, nutrient-rich Slope Water of either type. These cold water episodes alternate with warm water episodes of reduced SSW inflows and greater Slope Water influxes, principally WSW with its high nitrate load. We have been in a warm-water oceanographic state for much of the last five or six years. The anomalous 2012 warming event is placed against this backdrop of hydrographic changes and water mass fluxes to the Gulf.