Climate variability inferred from a continuously stratified model of the ideal-fluid thermocline

Climate variability in the subtropical gyre interior induced by anomalous s urface thermal forcing, Ekman pumping, mixed layer depth variability, and a nomalous subpolar water formation is examined, using a continuously stratif ied model of the ideal-fluid thermocline. Cooling (heating) induces a negat ive (positive) potential vorticity perturbation in the ventilated thermocli ne, and the associated density perturbations propagate downstream in the fo rm of second and higher baroclinic modes. The second baroclinic mode resemb les the traditional second baroclinic mode because it has a thermal structu re with cooling (warming) in the upper thermocline and warming (cooling) in the lower thermocline. Anomalous Ekman pumping can also induce density perturbations that propagat e westward in the form of the first baroclinic mode. In addition, if the ou tcrop lines are nonzonal, there are density perturbations that propagate do wnstream in the form of the second or third baroclinic modes. Perturbations in the sea surface elevation are mostly confined to the region of anomalou s forcing. On the other hand, when the low potential vorticity anomaly in t he subpolar mode water spreads into the subtropical basin, both the unventi lated and ventilated thermocline move downward. Consequently, temperature a t a given depth seems to increase.