BEERS ET AL.: PLANKTON AND UPWELLING OFF PERU 



Figure 4. — Dynamic topography (in dynamic meters) 

 of the sea surface referenced to the 300 db surface. 

 Patch 1 was observed 13-21 June 1969; Patch 2, 18-21 

 June 1969. Data from a detailed analysis of stations 

 designated by a square ( H ) are given in Tables 1 and 2. 



In order to obtain information on the turbu- 

 lent activity of the water in the vicinity of each 

 patch, the Vasaila frequency, N, an indicator of 

 static instability, and the Richardson number, 

 Ri (Phillips, 1966), an indicator of dynamic in- 

 stability, were calculated at various depths for 

 the three stations in Patch 1 and the two current 

 meter stations associated with Patch 2 (Table 1 

 and 2). The Vasaila frequency, based on the 

 vertical density gradient, is often used as an in- 

 dication of the vertical stability in a water col- 

 umn assuming static conditions, i.e., no vertical 

 shear. Small positive values for N imply the 

 possibility of weak vertical mixing and negative 

 values indicate the probable overturn of the 

 water layer being studied. For the purpose of 

 comparison it was assumed a Vasaila number 

 less than 1 X 10"^ see"' signified the start of 

 static instability. Since the effects of a pos- 

 sible vertical shear are not considered, this 

 measure will give an inaccurate estimate of the 



likelihood of significant turbulence in the pres- 

 ence of nonuniform horizontal currents. The 

 Richardson number, Ri, is generally used to esti- 

 mate whether turbulent mixing is an important 

 factor for consideration. A Richardson number 

 less than 0.25 is considered indicative of dynamic 

 instability and the development of turbulent 

 mixing. Owing to a lack of current meter data 

 for Patch 1 it was necessary to estimate the 

 shear used for determination of Ri from geo- 

 strophic currents. 



In Patch 1 conditions of static instability are 

 specifically indicated at a depth of 30 to 50 m 

 for Station 54 (Table 1). The water columns 

 at the other two stations associated with this 

 patch were weakly stable. Dynamic instability 

 was also indicated between 30 and 50 m at Sta- 

 tion 54. In addition, the upper 10 m of Station 

 59 showed dynamic instability. Although static 

 stability was greater at Station 78 than at the 

 other two stations, the water column was dy- 

 namically unstable from the surface to 50 m 

 because of the larger vertical shear present. 



Conditions in Patch 2 (Table 2) contrasted 

 with those in Patch 1. Vertical shear was gen- 

 erally much less than in Patch 1. This, how- 

 ever, may be pai'tly attributable to diflferences 

 arising between direct measurements of currents 

 (Patch 2) and estimates of currents based on 

 the horizontal distribution of mass (Patch 1). 

 Compared with Patch 1, the water column in 

 Patch 2 was more stable. The Richardson num- 

 bers from the west side of the front on Patch 2 

 are all very high and indicate conditions that 

 are not favorable for vertical turbulent mixing. 

 In Patch 2 the meridional component of Ri is 

 smaller and suggests that, while the water col- 

 umn is not dynamically unstable in the upper 

 50 m, turbulent mixing might not subside as 

 quickly as if the water column was highly strat- 

 ified. 



Direct current measurements to the east and 

 west of the front (Patch 2) were made to 200 m 

 and 500 m, respectively. If water motion within 

 the patch at 200 m, however, was similar to the 

 measurements at that depth on the west side 

 of the front, the change in current velocity would 

 be to increase the velocity in the eastward di- 

 rection by about 4 cm/sec and thereby increase 



865 



