Sources of Variability in Gulf of Maine Circulation, and the Observations Needed to Model it

Variability in the circulation of coastal oceans must ultimately be driven by changes in the meteorological conditions which force currents in the coastal ocean, and by variability in the waters entering the coastal ocean from elsewhere. If a coastal ocean is to be accurately understood and modeled, the external forcing which drive the largest portions of the circulation variability must be observed adequately. Thus some method of comparing the relative importance of various sources of circulation variability must be developed, so that there is confidence that the most important sources of variability are included in any analysis or modeling. This is done for variability in the time-integrated transport across a section in a coastal ocean. The relative importance of various sources of circulation variability in the Gulf of Maine are then quantified, with an emphasis on variability on timescales longer than tidal or weather-band timescales. In order to concentrate on interannual changes, the seasonal cycle is not included in estimates of variability. It is found that the variability forced by fluctuations in the winds and the volume of water entering from the Scotian Shelf to the Gulf of Maine produce roughly comparable amounts of circulation variability. However, changes in the density structure of the GoM produce changes in time-integrated transport that are an order of magnitude larger, at least in the central Gulf of Maine. The changes in the large-scale density gradients are governed by mixing processes in the Gulf and by changes in the water masses entering the Gulf of Maine from the Scotian Shelf and the Northeast Channel. Unless the heat, freshwater and volume transport of the waters entering the Gulf of Maine are routinely observed, numerical models will fail to capture much of the variability in the circulation of the Gulf. An analysis is given of the minimal set of observations needed to allow numerical models of the Gulf of Maine to adequately resolve the true variability in the circulation.