Interannual and decadal variability in an intermediate coupled model of the Pacific region

Processes that cause decadal variability in an intermediate coupled ocean-atmosphere model of the Pacific basin, both at northern midlatitudes and in the Tropics, are studied. The model's ocean component is a variable-temperature 3 1/2-layer system. Its atmospheric component consists of two basic parts: an empirical model, constructed from patterns obtained by the singular value decomposition (SVD) statistical technique that determines wind stress anomalies from model sea surface temperature (SST), and a simple representation of the planetary boundary layer to calculate the surface heat flux anomalies. A third part specifies stochastic wind stress forcing from observed variability. In addition, the model is specifically designed to separate tropical and extratropical interactions, such that the Tropics can force the extratropics through the atmosphere but the extratropics can only feed back to the Tropics through the ocean. Solutions develop two types of oscillations: an ENSO-like interannual mode and a decadal mode. As in many models of ENSO, the interannual mode is driven by positive, ocean-atmosphere feedbacks near the equator, and time-delayed negative feedback is provided by off-equatorial Rossby waves. For parameter choices that amplify midlatitude coupling by 30% (Φ_o = 1.3), a self-sustained decadal oscillation develops in the North Pacific without any tropical interactions. Diagnostic analyses show that it is maintained by ocean-to-atmosphere feedbacks in the northwest and subtropical northeast Pacific, and by atmospheric teleconnections from those regions to the northeast ocean. For weaker coupling (Φ_o = 1.2), the decadal mode is damped. In this case, the mode can be sustained by atmospheric teleconnections from the Tropics associated with the interannual mode, but not by extratropical stochastic forcing. Although including stochastic forcing does generate variability at decadal time-scales, a distinct decadal spectral peak only exists when the decadal mode is active. Decadal variability is carried to the equator by variations in the transport, rather than temperature, of the North Pacific subtropical cell. These variations modulate near-equatorial SST by altering the amount of cool, thermocline water that upwells in the eastern equatorial Pacific, which in turn feeds back to the interannual mode.