The Formation and Evolution of Maine Intermediate Water in Jordan Basin, Gulf of Maine

Twelve years of moored observations in Jordan Basin, at UMOOS buoy M01, from 2003 to 2015, provide insight into the nature of the interannual formation and evolution of Maine Intermediate Water. Deep and bottom water in Jordan Basin are supplied by seasonally and episodically varying inflows of Warm Slope Water and Labrador Slope Water, which enter the Gulf of Maine through the Northeast Channel. The interannual variability in the water properties of Jordan Basin deep water ,at depths of 200 m and 250 m dominates over seasonal and higher frequency variability and can be described as step changes lasting from months to more than a year. Sandwiched between the warm and salty deep layer and the summer surface waters of the Gulf of Maine is the distinctive mid-depth layer, Maine Intermediate Water (MIW, Hopkins & Garfield, 1979). MIW is formed by convective overturning in winter, which is then capped by the seasonal freshening and warming of surface water. MIW is characterized from spring through fall by a mid-depth temperature minimum, a feature that plays an important role in the Gulf of Maine’s unique dynamics. MIW is an important low temperature habitat for species such as Calanus Finmarchicus, and is fundamental to the physical dynamics of Jordan Basin, affecting vertical and horizontal stratification, vertical current shear and basin circulation. For more than 90% of the year, MIW is colder than the waters below it, and except duration its winter formation in late fall / early winter, it is also colder than the water above it. Because of its lower temperature, it has the potential to act as a heat sink for the surface and deep layers, drawing down their temperatures through diffusion and turbulent mixing. It is through the MIW layer that the deep nutrient rich slope water is mixed into the surface layer. Jordan Basin also exports MIW to Wilkinson Basin (Taylor and Mountain, 2009), and out of the Gulf of Maine to the New York Bight (Fairbanks, R.G., 1982), potentially affecting the cold pool there. The character and spatial extent of Maine Intermediate Water are controlled by three factors: inter-annual change in the supply of deep slope water; variability in surface processes; and the timing and nature of advection events. The UMOOS Buoy M time series reveals the complex character of MIW, its seasonal formation and its interannual variability in temperature, salinity, thickness and formation depth. Long term (annual and multi-year) salinity and temperature anomalies observed in the intermediate water layer of Jordan Basin are influenced by the deep layer’s almost constant upward export of salt and heat. Advection events and their effect on stratification of the water column in the months prior to and during MIW formation are important determinants of MIW formation depth. This thesis explores the role of advection and its timing in MIW formation and evolution and discuss the relationship between stratification preceding, during and following MIW formation on the depth and character of MIW.

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