Chapter XVII 



Effect of Wind on the Mass Field and 

 on the Density Current 



Under stationary conditions all the forces acting must be in equilibrium and the mass 

 distribution must be adapted to this equilibrium if it is to be maintained. In this case 

 it is not possible to distinguish between cause and effect; there is usually a mutual 

 adjustment between the internal field of force and the current present. If there is a change 

 in the field of force then there must also be a subsequent change in the current; 

 conversely if there is a change in the current there must be a rearrangement of the 

 field of force until equilibrium is again restored. These circumstances should be kept 

 in mind for an understanding of the way in which wind influences density currents. 



1. A Limited and Stratified sea 



Conditions in a limited trough-like sea shall be considered first. Work in this 

 direction has been done by Palmen (1926, 1930 a, b and with Laurila as co-worker, 

 1938) for the Gulf of Finland and the Gulf of Bothnia, principally in particular 

 cases which are only able to give some insight into the mechanism of the processes 

 which occur. The influence on the water stratification occurs as follows : 



We assume at first no wind at all over a barotropic sea ; the isosteric surfaces and 

 especially the transition layer between the top layer and the deep water will then 

 follow level surfaces (Niveauflachen). If a wind starts, the surface waters are forced 

 to move first in the direction of the wind, but the Coriolis force will soon produce a 

 deflection to the right (Northern Hemisphere) and a piling-up of the water along the 

 sea coasts. In an elongated ocean bay the final result will be a current predominantly 

 occurring along its longer axis. In addition to the wind-generated current in the top 

 layer a gradient (Stau) current is then added in the deeper layers due to the piling up of 

 water which will flow approximately in the opposite direction. Thus a vertical circula- 

 tion in a longitudinal direction is set up and an equilibrium state is present in which the 

 transport due to the surface current is exactly balanced by that of the deep current. 

 This quasi-stationary state of the current is fixed at each level by an equilibrium 

 between the gradient force, the Coriolis force and the frictional force. Since a stronger 

 current is only possible along the longitudinal axis of the bay it follows that the direc- 

 tion of the gradient force usually does not coincide with the direction of the current 

 itself but the deviation will not be great. In addition to the principal gradient in a 

 longitudinal direction in the layers above and below the level of current reversal (layer 

 of no motion) there will also occur smaller components of the pressure force acting 

 at right angles to the direction of the current. These will be largest at the surface and 



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