Observations show that the North Pacific Subtropical Cell (STC) has been spinning down since the 1970s, reducing the amount of cold water that upwells in the tropics and hence increasing equatorial SST. This observed decadal STC variability may be due to decadal variations in the subtropical wind stress or a two-way coupling between the tropics and extratropics, such that, the decadal modulation of tropical SST by STC transport variations transmits back to the midlatitudes via atmospheric teleconnections.

In order to explore these issues more fully, a coupled hybrid model has been developed that is composed of a 4.5 layer reduced gravity Pacific ocean model coupled to the NCAR AGCM CAM2. This coupled model explicitly includes atmospheric dynamics, realistically simulates seasonal variability of the ocean mixed layer and interannual ENSO variability. The model is anomaly-coupled so that feedback loops can be isolated and studied.

In this talk we present results, based on model runs with the extratropics both coupled to the tropical Pacific Ocean and independent of the tropical Pacific Ocean, that identify which feedbacks and feedback loops cause the STC transport to be modulated on decadal timescales.