546 



Ejfect of Wind on the Mass Field and on the Density Current 



type the difference in water level was 4-4 cm and for the west type this difference is 

 3-4 cm. The absolute velocity of the wind-generated surface current will thus be for 

 the east type 6-9 cm/sec towards the west and for the west type 5-3 cm/sec towards 

 the east. Current measurements give 7-5 and 6-0 cm/sec, which is in good agreement. 

 The relative changes in velocity with depth can be calculated by ordinary methods 

 (equation XV.20) from the mass field and can then be converted to absolute velocities 

 using the surface velocities given above. Table 148 containing these values shows clearly 

 the division of the current structure into two layers; at the middle of the Gulf of 

 Finland the current reversal is at a depth of approximately 27 m. It changes in a 

 corresponding way towards the Finnish and Estonian coasts. The calculated values 

 are a little too large, since friction has been neglected, but otherwise are in satisfactory 

 agreement with observed values. In some special cases for a strong wind and steeper 

 inclination of the isosteres in the transverse section, the velocities are much greater 

 (for instance, 7 October 1936; surface velocity 23-5 cm/ sec) and the layer of no motion 

 occurs at greater depth (about 35 m) in full agreement with the observed values. 



Table 146. Current stratification for different wind directions in the Gulf of Finland 

 (according to Palmen) (positive sign towards west; negative sign towards east) 



When the wind is in a direction other than directly east or west only the eastern or 

 western component will have any effect. The inclination of the isosteres in the trans- 

 verse section will therefore be correspondingly less and the number of solenoids will 

 thus be reduced and must therefore show a dependence on the wind direction. 



The rearrangement of stratification caused by the wind in an elongated oceanic 

 region will thus proceed in the following way: 



(1) A steady wind with a component along the longitudinal axis of the sea will 

 originate a vertical circulation; this will be made up of a drift current in the top 

 layer and a corresponding gradient current in the deep water. 



(2) This current system will produce a vertical transverse circulation which in turn 

 will give rise to an inclination of the density transition layer and of the isosteric 

 surfaces, that is, the longitudinal circulation produced by wind will give rise to a 

 solenoid field at right angles to this circulation. The strength of this field will be a 

 function of the wind influence. When an equilibrium state is reached this cross 

 circulation will vanish. 



(3) A transverse slope in the physical sea level will develop at the same time and its 

 intensity will also be dependent on the wind. 



(4) From the solenoid field and the transverse slope of the sea surface the current 

 structure in a transverse section can be calculated. In a steady equilibrium state the 

 slope of the internal boundary surface in a two-layered sea will be greater than that 



