A Midlatitude–ENSO Teleconnection Mechanism via Baroclinically Unstable Long Rossby Waves

The possibility of generating decadal ENSO variability via an ocean teleconnection to the midlatitude Pacific is studied. This is done by analyzing the sensitivity of the equatorial stratification to midlatitude processes using an ocean general circulation model, the adjoint method, and a quasigeostrophic normal-mode stability analysis. It is found that, on timescales of 2–15 yr, the equatorial Pacific is most sensitive to midlatitude planetary Rossby waves traveling from the midlatitudes toward the western boundary and then to the equator. Those waves that propagate through baroclinically unstable parts of the subtropical gyre are amplified by the baroclinic instability and therefore dominate the midlatitude signal arriving at the equator. This result implies that decadal variability in the midlatitude Pacific would be efficiently transmitted to the equatorial Pacific from specific areas of the midlatitude Pacific that are baroclinically unstable, such as the near-equatorial edges of the subtropical gyres (158N and 128S). The Rossby waves that propagate via the baroclinically unstable areas are of the advective mode type, which follow the gyre circulation to some degree and arrive from as far as 258N and 308S in the east Pacific. It is shown that the baroclinic instability amplifying these waves involves critical layers due to the vertical shear of the subtropical gyre circulation, at depths of 150–200 m.