Tropical-extratropical interactions in an intermediate coupled model of the Pacific basin

The modulation of El Nino and the Southern Oscillation (ENSO) on decadal timescales by transport variations in the North Pacific Subtropical Cell (STC), a shallow meridional overturning cell which contributes to equatorial upwelling, forced by off-equatorial winds is explored in a hybrid coupled ocean-atmosphere model. It is shown that not only do the extratropics modulate tropical variability, this modulation feeds back to the extratropics through atmospheric teleconnections, altering the amplitude and frequency of the extratropical oscillations.

The hybrid coupled model of the Pacific basin used in this study is designed so that tropical SST anomalies feedback to the extratropics through atmospheric teleconnections while extratropical anomalies feedback to the tropics through oceanic teleconnections. This model has two distinct oscillations: an ENSO-like interannual oscillation and and an extratropical oscillation with a timescale of approximately 12 years.

Model runs with the extratropics both coupled to the tropics and independent of the tropics demonstrate that the North Pacific STC creates an oceanic bridge by which self-sustained oscillations in the North Pacific can modulate ENSO on decadal timescales. The instability and transition mechanisms which sustain this extratropical oscillation and modify the STC are presented and discussed.