In the northern hemisphere, the process of upwelling occurs when the 

 predominant winds blow north along the coast. The wind-driven current, 

 because of Coriolis force, is then to the right of the direction of the wind 

 and consequently offshore. Water that is moved offshore is replaced by 

 vertical movement of water from below. The probable source of marine water is 

 MIW. The dominant wind direction of the Maine coast in summer is from the 

 southwest, which is appropriate for establishing upwelling. 



Upwelling regions have the most productive fisheries in the world. Upwelling 

 appears to occur along the entire eastern part of the characterization area 

 (regions 5 and 6; Yentsch et al. 1976, and Graham 1970b). Localized upwelling 

 may occur in the deeper channel areas of Casco Bay (region 1; Hulbert and 

 Corwin 1970), southeast of Monhegan Island in the summer (region 2; Yentsch 

 and Glover 1977) and near Matinicus Island (region 4; Bertrand 1977). 



Currents are a combination of tidal and nontidal currents. Tidal currents, 

 the response to the gravitational attraction of the earth-moon-sun system, are 

 oscillatory currents, producing no net displacement of water over a tidal 

 cycle in offshore waters. At maximum ebb and flood, tidal current velocities 

 are usually the strongest currents in coastal waters of the Gulf of Maine. 

 Tidal-current prediction is done from long-term records of the velocity 

 fields . 



Nontidal cuirents, comprised of three types of circulation: wind-driven, 

 geostrophic, and thermohaline (principally in estuaries); are a set of 

 responses to meteorological and oceanographical conditions. These different 

 types of circulation are interrelated and changes in one will affect the 

 others. Unlike tidal currents, nontidal currents exhibit marked short-term 

 variability in response to changing conditions and result in net displacements 

 or transports of water. Storm surges are a general set of nontidal currents 

 in response to wind events. The rate of river discharge or changes in 

 offshore hydrography also influence the coastal water currents. 



Water movements are significant because water transports material such as 

 plankton, nutrients, or pollutants. Moving water supplies food, carries away 

 wastes and distributes larval organisms. Nontidal currents determine the 

 general transport of material but tidal currents greatly increase the total 

 distance and area traveled. 



Tables of tidal current predictions are published annually by the U.S. 

 Department of Commerce as an aid to navigators. The following general 

 description of Maine's tidal currents is taken from the U.S. Department of 

 Commerce NOAA-NOS Atlantic Coast Pilot (1976): 



Along the coast of Maine eastward of Portland (region 

 1) the flood sets eastward and has greater velocity 

 than the ebb which sets westward. 



With easterly or southeasterly winds the currents 

 have a tendency to set toward the shore. 



At Portland Lighted Horn Buoy P the tidal current is 

 weak, being on the average less than 0.3 knot at time 

 of strength, setting 335° on the flood and 140° on 



4-22 



