The Tropospheric Circulation 



653 



Fig. 307. Upwelling as induced by a wind parallel to the coast illustrated by the stream lines 



in the vertical plane perpendicular to the coast. In the numerical example D^= 162 m and D^ 



= 162 km; the width of the coastal wind belt is about 340 km. 



Structure with two vortices in the upper layers, one of which is situated close to 

 the coast and the other near the outer boundary of the wind belt. The upwelling due to 

 a longshore wind (Fig. 307) is far more effective in lowering the temperature of the 

 coastal region than that induced by an off-shore wind, since the former one brings a 

 larger amount of colder water to the surface from deeper levels than the latter. This 

 theory put forward by Hidaka deals only with the stationary case; no attention is 

 paid to the water stratification which as shown by observations plays a decisive role 

 for the processes involved before a steady state is reached. 



The process of upwelling is shown by observations to be variable with time. If 

 the duration of the wind is as short as a few hours, the off-shore component of surface 

 water transport will not be very large since drift currents will not fully develop. If the 

 winds are more or less steady for several hours up to as much as a day, the drift 

 currents may develop but they will not be followed by considerable upwelling because 

 of oscillations of the thermoline. However, the process will be different if the wind 

 continues for several days up to a week. If the wind continues for a longer time- 

 interval than about a week, the surface currents will reach a steady state with an inter- 

 mediate stage for a wind lasting a few days up to a week during which the geostrophic 

 equilibrium is approached. This latter section of the process has been dealt with 

 theoretically by Yoshida (1955) using the conditions in Californian waters as a guide. 

 In his model the .v-axis is directed eastwards, the >'-axis directed northwards and repre- 

 sents the coast line. The r-axis is chosen positive downwards with z = being placed 

 along a mean sea level. The conditions were taken as constant in a north-south 



