Barotropic Transport Research Articles

Barotropic response to cooling

Imposed horizontal density differences in a nonrotating fluid generate vertical circulation which has vanishing vertically integrated transports. When the system is rotating, geostrophic velocities can balance the density differences and the vertically integrated transports need not vanish locally. In a two‐layer fluid, finite amplitude disturbances lead to barotropic flows that have the same direction as the velocity in the layer that thickens as a result of the disturbance. Specific calculations are carried out for the geostrophic adjustment model in situations that approximate those in which 18° water is formed south of the Gulf Stream. The upper layer transport that results from sudden cooling (as simulated by density differences that are initially unbalanced geostrophically) is in the same direction as the Gulf Stream transport and comparable to it in magnitude. A lower level transport of the same magnitude flows in the opposite direction with a maximum value about an internal radius of deformation to the right of that of the upper layer. The barotropic transport is about 1/5 as large and flows downstream in the Gulf Stream and upstream to the right of the Gulf Stream.

A wind-driven zonal channel with stratification and bottom topography

The wind-driven flow in a zonal channel on a rotating Earth is analysed in the presence of stratification and bottom topography. The velocity field related to the surface input of wind stress, heat and precipitation. The barotropic transport in the channel is reduced by the presence of bottom topography. Narrow jets and counter currents are formed over a zonal ridge in the baroclinic state.

On the Estimation of Absolute Geostrophic Volume Transport Applied to the Antarctic Circumpolar Current

An objective method of estimating the geostrophic barotropic volume transport across an oceanographic section is developed and applied to the transport of the Antarctic Circumpolar Current through the Drake Passage. The total geostrophic transport through a passage can be broken up into a baroclinic component, calculable from hydrographic data, and a barotropic component which is calculated objectively using direct current meter measurements at some fixed level. The influence of measurement noise is incorporated into the calculation of the rms error in the objective estimate of barotropic transport which depends on the number of current meters at the fixed level, the “correlation length” of the through passage velocity component, the noise variance and noise scale. For measurement noise variance not exceeding 10% it is shown that an accurate estimate (rms error ≤20%) of the barotropic transport can be made when the spacing of the current meters is less than or equal to the correlation length. Using six long-term (35 weeks) current meter measurements at 2700 m in the Drake Passage, it is found that the baroclinic geostrophic transport of 100 Sv relative to 2700 m (Nowlin et al., 1977) should be corrected by an average of 27 Sv which is the contribution from the geostrophic barotropic transport. The total range of the 35 weekly estimates is 220 Sv and the rms error of the estimates due to insufficient spatial coverage is ±14 Sv. Two weekly estimates of the barotropic transport were made from 12 current meters all al approximately 2700 m across the passage, and varied considerably from the corresponding estimates made with only six current meters. The rms error for 12 current meters is only ±4.5 Sv assuming the rms through passage speed to be 3 cm s−1.

The effect of bathymetry on the steady baroclinic ocean circulation

A general solution is derived for the steady wind- and thermohaline-driven barotropic velocity field in a variable-depth ocean under the mild assumption that the bathymetry does not penetrate up to the depth of appreciable horizontal density gradients. Bi-linear depth depencies give a simple form to the resulting integral and since they can be used to represent typical ocean bathymetries, they permit analytical solutions to be obtained for a wide class of driving functions. In particular the case of a western continental shelf, numerically solved by Holland (1973) , is considered and the results are shown to be in agreement with this. The physical basis for the steady baroclinic circulation in an ocean of variable depth is made clear by vorticity arguments and, in particular, the “bottom torque” can be related directly to the stretching term in the barotropic vorticity balance. The resulting stream-line pattern depends also upon the coupling of the barotropic transport to the Ekman and baroclinic transports at the boundaries through the no-flux boundary condition.

Two-layer Exchange in an Estuary Basin, with Special Reference to the Baltic Sea

Stationary and transient states of a two-layer fjord-type estuary are discussed analytically. The forcing functions are the outer salinity S0, the fresh-water supply qf, and a meteorologically forced barotropic transport qm. Forced nonlinear, time-dependent cases have been studied numerically. Some associated laboratory experiments are described. The main results obtained are as follows: (i) A single steady state exists; this is approached in an exponential-like way. (ii) The total mixing through the interface must vary with depth (decrease for increasing interface depth) to allow a stable steady state. (iii) The static stability increases with increasing fresh-water supply, up to a critical value where the two-layer model breaks down. (iv) An added oscillatory component in S0 increases and in qf decreases the esturay salinity and the static stability. The effect of an oscillatory qm may go in either direction. (v) The statistical steady state is sensitive to certain high-order statistical features of the forcing functions. It is suggested that changes in such statistical features, rather than changes in mean forcing conditions, may explain observed physical-chemical secular variations in the Baltic, in particular the drop of oxygen concentrations of the deep water.