604 



The Tropospheric Circulation 



the similarity with the vertical velocity field shown in Fig. 269 is remarkable. It should 

 be noted that the vertical velocity component does not vanish at the lower boundary of 

 the upper mixed layer. The current does not follow the inclined surface of this boun- 

 dary unless the divergence of mass transport in the upper mixed layer is zero. This does 

 therefore never correspond to the conditions shown in Fig. 269. 



Also in the Indian Ocean conditions are similar with the same much weaker develop- 

 ment of the phenomenon in its eastern parts (see Pt. I, p. 172, Fig. 75). Since the 

 thermal equator remains here always south of the equator the tropospheric circula- 

 tion is again rather asymmetrical and, as in the Pacific, the southern hemispheric 

 branch is the stronger one. However, while the conditions in this branch are almost 

 unchanged throughout the total year, complications must appear in the Northern 

 Hemisphere due to the seasonal changes in the current system of the sea surface. 

 During the summer months the strong south-west monsoon current extends down to 

 the lower layers of the troposphere and the subtropical undercurrents are suppressed. 

 The available sections do not show the nature of this change. Probably the highly 

 saline water masses of the southern hemispheric lower currents extend into the Nor- 

 thern Hemisphere and partly enter the influence region of the wind drifts of the south- 

 west monsoon. 



The Equatorial Undercurrent. Cromwell, Montgomery and Stroup (1954) 

 discovered an Equatorial Undercurrent in the Central Pacific in a zone between the 

 equator and latitude 1° N. and at a depth of 50-150 m. It is found as a narrow east- 

 ward current both in the lower part of the top layer at the equator and in the upper 

 part of the thermocline in this zone, where the South Equatorial Current extends into 

 the Northern Hemisphere. Its position in the vertical and horizontal circulation of this 

 area is sketched in Fig. 279. Fofonoff and Montgomery (1955) have shown that the 

 Equatorial Undercurrent agrees with a simple application of the vorticity equation 



"seasuRFace 



' eOUATOftua. W«0£RCU«W£hT 



Fig. 279. Meridional section showing idealized currents in the surface layer (about 100 m 



deep within about 3° of equator, reader looking west). The flux components in the plane 



of the section are indicated by broken arrows. Zonal components of velocity at the top 



and the bottom of the layer are indicated by diagonal arrows drawn in perspective. 



