[DLS] Richard Seager (Columbia University)

“Tropical ocean dynamics drive the observed strengthening of the zonal SST gradient and models get it”

Over the past decades of increased radiative forcing the zonal SST gradient across the tropical Pacific Ocean has strengthened. Waters have actually got colder in the central equatorial Pacific.  This long-term gradient strengthening is not captured realistically by any runs of any models participating in the CMIP6 and the Large Ensemble experiments and, hence, is outside the range of the combined effect of modeled forced responses plus internal variability.  The centrality of the tropical Pacific in the global climate system means that model simulations and projections of phenomena as varied as drought in southwest North America and East Africa, tropical cyclone risk in the Pacific and Atlantic basins and even climate sensitivity, are wrong. Here, using analysis of observations and simple modeling, we argue that the observed gradient strengthening arises from processes internal to the tropical Pacific Ocean.  Key is subsurface cooling and thermocline shoaling explained in terms of wind-driven dynamical adjustment and changes in shear-driven mixing.  This forced Pacific Climate Change (PCC) pattern is distinct in ocean dynamics from the Pacific Decadal Oscillation with the latter oscillating in time while the PCC has steadily emerged over time.  No coupled climate models reproduce the PCC pattern and even those that strengthen the gradient somewhat do so for the wrong reasons.  Surprisingly, ocean models forced by the observed history of surface fluxes of heat and momentum spuriously warm the eastern equatorial Pacific much like the coupled models they are incorporated into. Even more surprising, ECMWF’s ocean reanalysis system ORAS5 can only reproduce cold tongue cooling via a “deus ex machina” surface heat flux adjustment.  These results collectively suggest that the inability of models to simulate observed tropical Pacific changes arises, at least in part, from their inability to correctly represent the ocean processes that communicate subsurface temperature changes with the mixed layer and SST.

EAPS Department Lecture Series —

Weekly talks aimed to bring together the entire EAPS community, given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. Runs concurrently with class 12.S501.

Contact: eapsinfo@mit.edu