Labrador slope water entering the Gulf of Maine—response to the North Atlantic Oscillation

The percent of Labrador Slope Water (%LSW) in the waters entering the Gulf of Maine at depth (150–200 m) through the Northeast Channel is estimated from temperature/salinity measurements for the period 1964–2008. Comparison of the annual average %LSW with the North Atlantic Oscillation (NAO) lagged two years shows that both series have very similar, but opposite, long term trends. High %LSW in the 1960s decreased to low values in the 1990s as the NAO increased from low to high values over the same period. A regression of the residuals of the two series from their long term trends also is significant at the 5% level, indicating that the %LSW at the Northeast Channel has a persistent relationship on a year-to-year basis with the NAO over the full range of variation in the NAO. The response of the %LSW to the NAO, however, appeared substantially reduced after about 1990. Changes in the potential nutrient flux into the Gulf of Maine/Georges Bank system associated with changes in the %LSW cannot account for changes in chlorophyll a concentrations estimated for three multi-year stanzas during the sampling period.