Representation of the ENSO Combination Mode and its Asymmetric SST Response in Different Resolutions of HadGEM3

摘要

Previous studies have revealed a combination mode (C-mode) occurring in the Indo-Pacific region,arising from nonlinear interactions between ENSO and the western Pacific warm pool annual cycle.This paper evaluates the simulation of this C-mode and its asymmetric SST response in HadGEM3 and its resolution sensitivity using three sets of simulations at horizontal resolutions of N96,N216 and N512.The results show that HadGEM3 can capture well the spatial pattern of the C-mode associated surface wind anomalies,as well as the asymmetric response of SST in the tropical Pacific,but it strongly overestimates the explained variability of the C-mode compared to the ENSO mode.The model with the three resolutions is able to reproduce the distinct spectral peaks of the C-mode at the near annual combination frequencies,but the performance in simulating the longer periods is not satisfactory,presumably due to the unrealistic simulation of the ENSO mode.Increasing the horizontal resolution can improve the consistency between atmospheric and oceanic representations of the C-mode,but not necessarily enhance the accuracy of C-mode simulation compared with observation.