• Why can’t we simulate the central Pacific type of ENSO very well?

    With the more frequent occurrence of the central Pacific (CP) type of El Ni?o since the turn of the 21st century, the existing theories of ENSO (El Ni?o–Southern Oscillation) mechanisms, as well as our ability to predict ENSO, face severe challenges. Hence, since the concept of CP El Ni?o was put forward, it has drawn broad attention among scientific researchers. However, the physical mechanisms of CP ENSO are still not fully understood, and simulating it tends to yield grave defects, even in state-of-the-art general circulation models (GCMs). Owing to the complexity of the physical processes and parameters included in GCMs, it is hard to find ways to directly improve the situation.

    Dr. Xianghui FANG, from Fudan University, and Prof. Fei ZHENG, from the Institute of Atmospheric Physics, used a simple linear air–sea coupled model (GMODEL), which can accurately depict the strength distribution of the thermocline (TH) and zonal advective (ZA) feedbacks in the equatorial Pacific, to investigate the two types of ENSO. Their findings, published in Advances in Atmospheric Sciences (Fang and Zheng, 2018) show that the model can reproduce the main characteristics of CP ENSO if the TH feedback is switched off and the ZA feedback is retained as the only positive feedback, confirming the dominant role played by ZA feedback in the development of CP ENSO. Further experiments indicated that, through a simple nonlinear control approach, many ENSO characteristics, including the existence of both CP and EP El Ni?o and the asymmetries between El Ni?o and La Ni?a, can be successfully captured using the simple linear air–sea coupled model.

     

    Schematic diagrams of the (a) EP (eastern Pacific) and (b) CP (central Pacific) types of El Ni?o (also referred to as El Ni?o Modoki). The maximum sea surface temperature anomalies are in the EP during EP El Ni?o, whereas they are concentrated in the CP during CP El Ni?o [figure adapted from Ashok et al. (2009)]

    “These analyses indicate that an accurate depiction of the climatological sea surface temperature distribution and the related ZA feedback, which are the subject of severe biases in GCMs, is very important in simulating a realistic CP El Ni?o,” argues Dr. Xianghui FANG.

     

    References

    Fang, X. H., and F. Zheng, 2018: Simulating eastern- and central-Pacific type ENSO using a simple coupled model. Adv. Atmos. Sci., 35(6), https://doi.org/10.1007/s00376-017-7209-9

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