Thermodynamical control over the magnitude of El Niņo warming

The magnitude of El Niņo warming, defined as the SST anomaly in the central and eastern equatorial Pacific, is a key parameter in determining its climatic effect worldwide. The physics that controls the magnitude of El Niņo warming, however, is not well understood. Recent studies of the heat budget of El Niņos have suggested that the existence of El Niņo is fundamentally related to the poleward heat removal from the tropical Pacific. In light of accumulated theoretical and observational evidence, we put forward the hypothesis that the magnitude of El Niņo warming is proportional to the temperature difference between the tropical maximum SST Tw (the SST of the tropical western Pacific warm-pool) and the characteristic temperature of the thermocline water Tc (or the characteristic temperature of the equatorial undercurrent that feeds the equatorial upwelling). We hypothesize that the observed variability in the magnitude of El Niņo warming on the decadal and longer time scales may be accounted for at least in part by variations in this thermal contrast on these time-scales.

We plan to test this hypothesis and elucidate the involved mechanisms through the use of two coupled models: a simple coupled model for the tropical Pacific region and an improved version of the NCAR climate system model (CSM). The simple coupled model represents the lowest order approximation of the tropical coupled system and serves as a conceptual tool for interpreting results from the latter more complex model. The NCAR CSM model is a coupled GCM and is comprehensive in the inclusion of physical processes. Coupled experiments will be performed to ascertain and understand the relationship between the amplitude of ENSO and the value of Tw-Tc. The ocean models will be forced with observed SST and wind to obtain variability in Tc and thereby variability in Tw-Tc over the last 50-100 years. Coupled experiments with specially designed thermal forcing upon the ocean will be conducted to quantify the extent to which the observed variability in the amplitude of ENSO on decadal and longer time scales can be attributed to the variability in the thermal contrast between Tw and Tc on these time-scales. Experiments addressing directly the response of the value of Tw-Tc and the response of ENSO amplitude to global warming will be conducted. In connection with the use of these two models, we also exploit the data from the NCEP reanalysis and existing outputs from NCAR and GFDL models.

The research will advance our understanding of the constitution of the thermal forcing of ENSO and thereby our understanding of the origin of the variability in the magnitude of El Niņo warming on the decadal and longer time-scales. In particular, it will shed light on the question of whether El Niņo will become more energetic in response to global warming and thereby help to address the societal concern about whether the exceptionally strong El Niņo of 1997-98 is due to global warming.