The Physical Nature Of The Two Modes Of ENSO

The air-sea interaction in the tropical Pacific is the key element of the globalclimate system, in which the El Ni o-Southern Oscillation (ENSO) behaviors as amajor contributor. ENSO has been long investigated for its diversity of oscillations inthe scientific community. In this thesis, instability analysis is performed to alinearized modified Zebiak-Cane (ZC) model, which is improved by the author, todetect the most unstable modes under continuous varying basic states and physicalparameters. The main results, in one hand, support the earlier conclusions on thistopic, and, in the other hand, are enlightened by the following unique findings.1. The defects of the ZC model in simulating the ENSO winds anomalies are longblamed for the unreasonable strong winds variation over the tropical eastern Pacific. Itis the failure of simulating the right pattern of atmospheric heating that should beresponsible for such defects. In order to reproduce the atmospheric heating whichshares a similar pattern as the observed precipitation anomalies, the parameterizationof the vertical momentum mixing and the parameter of convective instability areadopted to improve the atmospheric component of ZC model. And it is shown that theENSO winds are greatly improved, in particular over the tropical eastern Pacific,which is represented by wind anomalies with broader horizontal scales and strongeramplitude than those from the original model. In total, the ENSO wind anomaliesfrom the modified model resemble the observation more.2. The improved atmospheric model is coupled with the original ocean model to setup a new ZC model, and after it is linearized, the ENSO modes are investigated. It is found that the most unstable modes are the quasi-quadrennial (QQ) and quasi-biennial(QB) modes, whose peak SST anomalies are centered in the tropical eastern Pacificand central Pacific, respectively. Due to the enhancement of the wind anomalies, theboth modes are stronger than those in an earlier and similar study. The two modes caneither coexsist or exsist independently in some typical mean states. The results alsoshow that the QQ (QB) mode increases (decreases) as the increasement of thethermocline feedback, and the QQ mode is less sensitive to the strength of the air-seacoupling and thermal damping, as well as the oceanic wave speed. In general, the QQ(QB) mode depends more on the oceanic dynamics in the sub-surface (surface) layer.3. The heat budget analysis of the QQ and QB modes confirms the above-mentionedresults. For the QQ mode, the thermocline feedback (the vertical advection of theanomalous subsurface temperature by the mean upwelling) contributes to thedevelopment and phase transition of the SST anomalies. However, For the QB mode,it is the zonal advective feedback (the zonal advection of the mean SST gradient bythe anomalous zonal currents).4. Taking the spatial-temporal pattern of the QQ and QB modes and theircontrolling mechanism, we conclude that the QQ mode in this mode corresponses tothe conventional El Ni o and the QB mode the new type of El Ni o.