SECT. 3] DYNAMICS OF OCEAN CtTRRENTS 343 



(v) Stress-driven barotropic transport. For the divergence of Ekman transport 

 to be balanced by the divergence of barotropic transport, the barotropic flow 

 must cross contours of fjh at a rate proportional to the divergence of Ekman 

 transport and inversely proportional to the rate of change oi ffh normal to its 

 contours. Thus, in a region where the Ekman transport is divergent, the baro- 

 tropic flow will tend to converge towards a ridge and flow away from a trough 

 provided changes in depth are sufficient to overcome the variation of the 

 Coriolis parameter. This class of flow is encountered in theoretical studies of 

 the homogeneous ocean model. 



(vi) Thermohaline transports. The divergence of a non-zonal baroclinic flow 

 can be balanced by a barotropic flow crossing contours off/h at a rate sufficient 

 to reduce the total divergence to zero. In the general case, the barotropic 

 transport will not be equal and directed in the opposite direction to the baro- 

 clinic transport so that a net circulation will result. If the bottom is level, this 

 class reduces to a purely internal mode such that the sum of the transports is 

 zero. Flows of this class can be set up by thermohaline processes such as those 

 considered by Stommel and Arons (1960a). 



(vii) Mixed transport. If the Ekman transport is such that its divergence 

 cannot be balanced solely by baroclinic flow, a mixed transport results in 

 which both baroclinic and barotropic flows are present. This class can be 

 separated into classes (iv) and (v) by splitting the Ekman transport into two 

 parts such that the divergence of each part balances separately that of the 

 baroclinic and barotropic flow. The ratio between baroclinic and barotropic 

 transports cannot be determined from the momentum equations alone. 



(viii) Internal baroclinic flow. A purely baroclinic flow can exist such that 

 the divergence over a portion of the depth is balanced over the remainder of 

 the depth. The baroclinic transport is zero. From the point of view of the 

 integrated equations, this class represents the trivial case as all transport 

 components vanish. However, it is possible for this class of flow to exist in a 

 three-layer, or more complex, ocean model. 



Transports of classes (iv) and (v) can be generalized by replacing the diverg- 

 ence of Ekman transport by an arbitrary mass source, Q, at the surface of the 

 ocean. We could then include circulation set up by precipitation and evapora- 

 tion in these classes. 



In all eight classes, the flow can alter the distribution of temperature and 

 salinity by convection giving rise to changes of density and, consequently, 

 interactions with the baroclinic mode. The interactions do not enter directly 

 into the reduced system of integrated equations, but arise from the non-linear 

 acceleration terms and from the conservation equations for heat and salt. For 

 the transports described above to exist in the steady state, there must be 

 implicit mechanisms that hold the density constant against the convective 

 influence of the flows. Conversely, we can regard the conservation of heat and 

 salt as additional restrictions which will further reduce the types of flow 

 allowed in each of the eight classes given here. 



