The importance of ENSO Nonlinearities in Tropical Pacific Response to External Forcing.

Abstract:

The interactions between the timescales of tropical Pacific variability –seasonal to interannual to multidecadal- remain an open question. Paleoclimate records from the mid-Holocene indicate that a weakening of the eastern Pacific annual cycle is associated with a significant reduction in ENSO activity. In addition, we show here that last-millennium proxies indicate a negative correlation between ENSO activity and the zonal tropical Pacific SST gradient at decadal timescales, which supports the hypothesis that residual nonlinear heating from large ENSO events warms the cold tongue and thus decreases the zonal SST gradient beyond interannual scales (ENSO rectification hypothesis). On the other hand, the response of the seasonal cycle, ENSO, and the zonal SST gradient to global warming is inconclusive: While climate models project a weakening of the zonal SST gradient, the magnitude of this weakening as well as the sign and magnitude of the changes in ENSO and in the eastern Pacific annual cycle ranges significantly among models. Then the question becomes: do models exaggerate or mute tropical Pacific warming and annual cycle changes based on what kind of El Niño they simulate? What is the dominant balance of feedbacks that creates ENSO nonlinearities, and does it affect model projections of multidecadal tropical Pacific climate? Here, we diagnose the role of nonlinearities in ENSO response to external forcing using low-order models, which are derived from GCM simulations and encompass nonlinearities and seasonal dependences. We find that the relative balance between the Bjerknes feedback, atmospheric damping, and nonlinear advection feedbacks in the eastern Pacific not only influences the projected changes in ENSO variance and skewness, but also affects the response of the cold tongue at multidecadal timescales. Hence, we find that climate models with weak ENSO nonlinearities may overestimate the warming of the cold tongue, in the absence of strong thermodynamic damping and ENSO rectification processes that could balance the effects of radiative forcing.