The (Pacific Decadal Oscillation) PDO signature is shown in the bottom two globe projections. The left global projection corresponds to a positive PDO, the right to a negative PDO. Compare both of these to the equipotential tidal surface formed by the 18.6 year Nodal tides.
The Equatorial Pacific Ocean moves up and down by +/- 7 to 8 mm every 18.6 years, in anti-phase to the North Pacific Ocean. It is possible that slow upward and downward movement of the ocean surface may be responsible for periodic up whelming of cool deep-ocean water on bi-decadal (18.6 year) and ~ 55.8 (= 3 x 18.6) year times scales.