a importantly, the amount of mode water formation is roughly proportional to the strength of cooling (Fig. 4.60). In addition, it also depends on the strength of the Ekman pumping. Ekman pumping plays the role of preconditioning the dome-shaped thermocline in the basin interior. Since stratification in the deep ocean is weaker than that on the shallow density layers, strong Ekman pumping can create a dome with weaker stratification in the middle. As a result, the same amount of cooling generates more mode water; however, the effect due to Ekman pumping is not dominant. On the contrary, the strength of cooling is the dominating factor in subpolar mode water formation.

Perturbations due to climate variability Using this model we can also study the variability of the circulation induced by anomalous forcing. Within the framework of the ideal-fluid thermocline, a major difference between the subtropical and subpolar gyres is that potential vorticity in the ventilated thermocline changes with the alterations in the upper boundary conditions, but potential vorticity in the unventilated thermocline is specified a priori. As a result, the potential vorticity function in the subpolar basin does not change with the upper boundary forcing conditions.

For example, in the subtropical basin, when the outcrop lines are non-zonal, anomalous Ekman pumping can create perturbations in the form of the second baroclinic mode, which propagate along characteristics. The existence of such perturbations is due to changes in the potential vorticity function in the ventilated thermocline. However, in this ideal-fluid thermocline model, potential vorticity in the thermocline in a subpolar basin is specified a priori; thus, anomalous Ekman pumping can only change the flow field to the west of the perturbation source.

In order to demonstrate this point, we show the results from an experiment in which the subpolar gyre is driven by the same Ekman pumping velocity plus a small perturbation, i.e., the total Ekman pumping is in the form:

where 0q = 55°, Xo = 40°, AX = A0 = 5° (Fig. 4.61).

Under such anomalous Ekman pumping velocity, perturbations propagate westward from the source of Ekman pumping anomaly (Fig. 4.62). Perturbations induced by the anomalous Ekman pumping are in the form of the barotropic mode, with no baroclinic structure. This is a dramatic difference from the rich baroclinic structure induced by surface forcing conditions, as in the case of the subtropical basin discussed in Section 4.9.

Ekman pumping (10-4 cm/s)

Fig. 4.61 Ekman upwelling velocity (in 10-6 m/s), including a patch of strong upwelling in the middle of the basin.

Ekman pumping (10-4 cm/s)

Fig. 4.61 Ekman upwelling velocity (in 10-6 m/s), including a patch of strong upwelling in the middle of the basin.

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