Publication Date:
2023-08-23
Description:
The decadal variability of El Niño / Southern Oscillation (ENSO) is investigated in the preindustrial control run of the GFDL-ESM2M fully coupled climate model. Overall, the climate model has quite a realistic representation of relevant ENSO properties: the probability distribution of Niño3.4 sea surface temperature (SST) anomalies is positively skewed, the highest equatorial Pacific SST variability is observed in boreal winter with the corresponding decrease in variability during spring, and the decadal climate variability shows a shift of the ENSO spatial pattern. Nevertheless, compared to the ERA-20C reanalysis product, the model shows problems most climate models have: the anomalous cold equatorial Pacific SST with the largest bias located on the eastern side, strong easterly winds over the western equatorial region, the rising branch of the Walker Circulation located too far west and the too strong subsidence regime east of the date line. Two main periods of about 60 years with high and low ENSO amplitudes are observed, ranging between l.5° C and 0.7°C. Here it is shown, that the High and Low epochs have remarkably different mean states, which can explain the differences in simulated ENSO amplitudes. The High epoch is characterized by a weaker zonal equatorial SST gradient and a warmer Niño3 SST. The less intense Walker Circulation reduces the subsidence branch, and the negative shortwave (SW) feedback during El Niño events is extended over the Niño3 domain. The stronger convective response over the eastern equatorial Pacific enhances the SST variability, increasing considerably during boreal winter, and the strong non-linearities in atmospheric feedbacks are kicked forming strong East Pacific-like (EP) El Niño events. Hence, the ENSO asymmetry is remarkably incremented. During the Low epoch, the zonal equatorial SST gradient is increased with cooler Niño3 SST. The Walker Circulation is intensified and the subsidence branch over the Niño3 region is strengthened. The Niño3 domain also coincides with the reduction of the negative SW feedback during El Niño events, as well as the incapability of the atmospheric regime to turn into a convective state, when SST anomalies are turned positive. In addition, the Niño3.4 SST variability and the wind feedback are considerably decreased during boreal winter. There are indications that the reduced SST variability of the Low epoch is caused by the too strong subsidence branch over the Niño3 region, which restricts the seasonal southward migration of the Intertropical Convergence Zone (ITCZ), and hampers the evolution of strong EP El Niño events. However, the convective response is maintained over the western equatorial Pacific, outside of the strongest mean subsidence region, as shown by the highest negative SW feedback. Therefore, during this time period the frequency of Central Pacific-like (CP) El Niño events is increased, shifting the ENSO spatial pattern, and reducing SST variability in lack of strong EP El Niños. Correspondingly, the non-linearities between the positive and negative phases of ENSO are reduced, diminishing the ENSO asymmetry. In summary, these results show how important the mean state is for the ENSO amplitude and asymmetry.
Keywords:
Course of study: MSc Climate Physics
Type:
Thesis
,
NonPeerReviewed
Format:
text
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