In a number of fundamental respects, the decadal oscillation resembles the North Pacific oscillation reported in other modeling (Latif and Barnett, 1994, 1996; Yukimoto et al., 1996, 1998; Knutson and Manabe, 1998) and observational (Latif et al., 1997; Nakamura et al., 1997; Nakamura and Yamagata, 1998) studies. It is generated by air-sea interaction confined within the subtropics, and latent heat-flux feedback is necessary for its existence.
The decadal oscillation affects equatorial SST by altering the amount of surface water that is drained from the northern tropics, which is the driving force of the North Pacific STC. As such, this mechanism is similar to, but fundamentally different from, one recently proposed by (Gu and Philander, 1997). In both studies, decadal SST variability in the tropics is generated by heat-transport anomalies associated with variations in the North Paciflc STC; however, in our model these anomalies are generated by transport changes in the upper branch of the STC, whereas in (Gu and Philander, 1997) they are caused by temperature changes in the lower branch induced by midlatitude surface heat-flux anomalies.
The simplicity of the coupled model is an advantage, enabling us to identify readily key dynamical processes. It is also a limitation in that other significant processes, such as the background annual cycle and realistic mixed-layer physics, are neglected. Additionally, because one part of the model atmosphere is based upon empirical correlations, the system allows only a limited exploration of atmospheric dynamics. We are currently extending the model to overcome these shortcomings.
ACKNOWLEDGMENTS
This research was supported by the GOALS program through NOAA grant NA76GP0353. Richard Kleeman also thanks the hospitality and support of the International Research Institute for Climate Prediction at the Lamont Dougherty Environmental Laboratory, where he carried out much of this research. We thank Yochanan Kushnir and Yves Tourre for stimulating discussions, and Kevin Kohler for his invaluable programming assistance.
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