The formation, evolution and interannual variability of Maine Intermediate Water in Jordan Basin

Twelve years of moored observations in Jordan Basin reveal the mean seasonal character of its water and provide an insight into the nature of its interannual evolution. Jordan Basin’s deepest external connection is across the 180m deep Truxton Swell. Below this depth, Jordan Basin’s exchange with the other basins is restricted and the deep water mass is governed by the dynamics of supply and in-situ mixing. This deep water layer, Maine Bottom Water is supplied seasonally and episodically by inflows from the Slope Sea through the Northeast Channel and Georges Basin, and is mixed en-route over more than 200km of varying bathymetry, in the Gulf of Maine’s strong tides. The interannual variability in the character of this water, dominates over seasonal and higher frequency variability and occurs through a series of long term step changes lasting from several months to more than a year. The step changes observed strongly correlate with step changes observed in the bottom of the Northeast Channel, whose source waters are formed external to the Gulf of Maine.

Sandwiched between the warm and salty deep water layer and the surface waters of the Gulf of Maine, is the distinctive mid-depth layer, Maine Intermediate Water (MIW). Formed by convective overturning and wind driven mixing in winter, it is characterised spring through fall by a mid depth temperature minimum. This relatively cool mid depth layer is vitally important to the GoM. It is a critical low temperature habitat and operates as a heat sink to the surface and deep layers, as a control on the temperatures of both. 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. While salinity increases with depth throughout the water column, the MIW layer is colder than the deep layer more than 90 percent of the time and, except during its winter formation, is colder than the surface layer. Years with relatively thin and warm MIW layers exacerbate the effect of atmospheric heating in the surface layer whereas those years with relatively cool thick MIW layers draw down surface temperatures.