Mode Water Variability in a Model of the Subtropical Gyre: Response to Anomalous Forcing

Abstract

The response of mode water formation to typical atmospheric forcing anomalies is studied as a possible mechanism for generating the observed interannual to decadal variability in mode water. An isopycnal model of the North Atlantic subtropical gyre, coupled to a mixed layer model, is used for this purpose. Geometry and forcing are idealized. The control run shows that mode water is a well-ventilated water mass. Formation rates up to 200 m yr⁻¹ are found at the outcrop of the mode water layer. In a series of experiments the sensitivity to the position of the anomalous forcing and to the timescale of the forcing is examined. The anomalous forcing has a dipole pattern that mimics the spatial structure of the North Atlantic oscillation. Anomalous cooling induces a positive thickness anomaly in the mode water layer at the center of the gyre and a negative anomaly at the eastern side of the gyre. The response to anomalous heat flux forcing appears to be sensitive to the position of the forcing anomaly with respect to the formation region of mode water. The formation and attenuation of the positive thickness anomaly turns out to be mainly controlled by entrainment and detrainment from the mixed layer. In the model, it takes five years to attenuate the thickness anomaly. Enhanced wind forcing generates westward propagating thickness anomalies. Adjustment takes place by long baroclinic waves. The center of the gyre, where dominant mode water variability is observed, appears to be relatively unaffected by anomalous wind forcing. It is concluded that variability in mode water formation of the observed amplitude and timescale can be generated in the model by heat loss variations of the observed amplitude. The response to a series of heat loss events is determined by a storage mechanism by which consecutive cold winters, despite interrupting warm winters, can induce prolonged thickness anomalies in the mode water layer.