How Momentum Coupling Affects SST Variance and Large-Scale Pacific Climate Variability in CESM

The contribution of buoyancy (thermal + freshwater fluxes) versus momentum (wind driven) coupling to SST variance in climate models is a longstanding question. Addressing this question has proven difficult because a gap in the model hierarchy exists between the fully coupled (momentum 1 buoyancy 1 ocean dynamics) and slab-mixed layer ocean coupled (thermal with no ocean dynamics) versions. The missing piece is a thermally coupled configuration that permits anomalous ocean heat transport convergence decoupled from the anomalous wind stress. A mechanically decoupled model configuration is provided to fill this gap and diagnose the impact of momentum coupling on SST variance in NCAR CESM. A major finding is that subtropical SST variance increases when momentum coupling is disengaged. An "opposing flux hypothesis" may explain why the subtropics (midlatitudes) experience increased (reduced) variance without momentum coupling. In a subtropical easterly wind regime, Ekman fluxes (Q(ek)') oppose thermal fluxes (Q(th)'), such that when the air and sea are mechanically decoupled (Q(ek)' = 0), Q(ek)' + Q(th)' variance increases. As a result, SST variance increases. In a midlatitude westerly regime where Q(ek)' and Q(th)' typically reinforce each other, SST variance is reduced. Changes in mean surface winds with climate change could impact the Q(ek)' and Q(th)' covariance relationships. A by-product of mechanically decoupling the model is the absence of ENSO variability. The Pacific decadal oscillation operates without momentumcoupling or tropical forcing, although the pattern is modified with enhanced (reduced) variability in the subtropics (midlatitudes). Results show that Ekman fluxes are an important component to tropical, subtropical, and midlatitude SST variance.