Abstract
Long climate simulations with the Met Office Hadley Centre General Circulation Model show weak El Niño-Southern Oscillation (ENSO) amplitude asymmetry between El Niño and La Niña phases compared with observations. This lack of asymmetry is explored through the framework of a perturbed parameter experiment. Two key hypotheses for the lack of asymmetry are tested. First, the possibility that westerly wind burst activity is biased is explored. It is found that the observed difference in wind burst activity during El Niño and La Niña tends to be underestimated by the model. Secondly, the warming due to subsurface non-linear advection is examined. While the model exhibits non-linear dynamic warming during both La Niña and El Niño, and thus a contribution to ENSO asymmetry, it is shown to be consistently underestimated in comparison with ocean reanalyses. The non-linear zonal advection term contributes most to the deficiency and the simulation of the anomalous zonal currents may be playing a key role in its underestimation. Compared with the ocean reanalyses, the anomalous zonal currents associated with ENSO are too weak in the vicinity of the equatorial undercurrent and the surface wind driven zonal currents extend too deep.
Highlights
El Niño-Southern Oscillation (ENSO) is the strongest natural interannual climate signal in the tropics, with oscillations between warm El Niño and cold La Niña phases occurring every few years
Assessment of ENSO characteristics for HadGEM-GC3.1 at MM resolution shows that this model performs favorably with observations across a range of performance metrics (Williams et al, 2018) and in historical simulations, the parameter experiment (PPE) shows a range of plausible ENSO variability (Yamazaki et al, 2021)
Because skewness in the more commonly used Niño3 region (150-90◦W, 5◦N-5◦S) is weak in the PPE, we focus on a region in the far east Pacific (FEP) (110-80◦W, 5◦N-5◦S) that maximizes skewness in the observations (Su et al, 2010; Santoso et al, 2017)
Summary
El Niño-Southern Oscillation (ENSO) is the strongest natural interannual climate signal in the tropics, with oscillations between warm El Niño and cold La Niña phases occurring every few years. The distribution of sea surface temperature (SST) variability in the Pacific is not Gaussian, in that the strongest El Niño events are stronger than the strongest La Niña events in the east Pacific (e.g., Burgers and Stephenson, 1999). This amplitude asymmetry in SSTs is associated with asymmetry of temperatures in the ocean subsurface (Jin et al, 2003; Zhang et al, 2009; Hayashi and Jin, 2017).
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