Thermohaline circulation cannot be dissociated from geostrophic forces acting under the influence of baroclinic quasi-stationary waves. It is demonstrated in the way the gyral Rossby waves move towards the poles after leaving the gyre.
In the North Atlantic the modulated current associated with the second antinode moves towards the northeast beyond 40°W to form the North Atlantic Drift. In the southern hemisphere the modulated currents associated with the three subtropical gyres merge with the Antarctic Circumpolar Current. Particularly powerful is the Agulhas south of Africa, whose maximum speed reaches 0.10 m/s to both periods (0.5 and 8 years), comparable to the Gulf Stream or Kuroshio. The drift to south of the modulated circumpolar current hints active thermohaline circulation, more diffuse than in the North Atlantic. Whether in the North Atlantic or along the Antarctic, the thermohaline circulation behaves as an overflow as a result of geostrophic forces arising from gyral Rossby waves at mid-latitudes.
But the poleward drift of the gyral Rossby wave cannot occur in the North Pacific that is virtually closed by the Bering Strait. Thus, it drifts eastward along the parallel 35-40°N, forming the North Pacific Drift, as shown in the sea surface temperature anomaly for the 8-year period: associated with the antinode of the standing wave outside the gyre, it drifts to latitude 150°W, while the thermal anomaly associated with the previous half-cycle, in phase opposition, extends along the north-west coast of north America. Primarily contributing to the climate of north-west America, it then drifts southwest around the subtropical gyre, merging with long-period thermal anomalies. In the absence of outlet, the modulated current is partially diverted to the southern hemisphere via modulated currents associated with the fourth meridional mode Rossby wave equatorially-trapped, i.e. the North Equatorial Counter-Current and the South Equatorial Current, the latter supplying the western boundary currents flowing northward and southward.