Sensitivity of an intermediate ocean-atmosphere coupled model of the tropical Pacific to its oceanic vertical structure

An intermediate ocean-atmosphere coupled model (ICM) of the tropical Pacific is used to investigate the sensitivity of the coupled system to the oceanic vertical structure. The model consists of a three-baroclinic-mode tropical Pacific Ocean and a Gill tropical atmosphere. The mixed layer is similar to the Zebiak and Cane (ZC) coupled atmosphere-ocean model but uses a different parameterization for subsurface temperature derived from XBT data. A 100-yr simulation is made, and the long-term variability of the ICM is briefly discussed and compared with the the observations and the ZC model. The ICM reproduces regular ENSO events with a spectrum peak centered at the period 3.5 yr. The presence of higher-order vertical modes in the oceanic component of the model allows for reduced off-equatorial variability of zonal current and thermocline anomalies and for finer meridional scale for SST anomaly variability in comparison with a simulation with one baroclinic mode only. The nature of the processes sustaining the oscillations in the ICM is then investigated based on the coupled instabilities and "delayed oscillator" theories. It is shown that the oceanic vertical structure is responsible for the ENSO period in the model. During Fl Nino events, high-order modes tend to generate warm anomalies in the central Pacific and set an equilibrium between the atmosphere and vertical advection that leads to an unstable Kelvin mode propagating slowly eastward. Conversely the gravest modes favor the free propagation of Rossby waves and enhance zonal advection over vertical advection, leading to unstable Rossby modes. The cold phase is a fast westward propagation as in the ZC model, which is not very sensitive to the vertical structure of the ocean. Rossby wave reflection at the western meridional boundary is important in the model far the shifts from a warm (cold) phase to a cold (warm) phase consistently with the delayed oscillator theory. The efficiency of the shift is, however, dependent on the vertical structure of the reflected Kelvin waves. For instance, a second baroclinic Kelvin wave is more efficient in initiating the warming than a single first baroclinic Kelvin wave. Some aspects of the spatial and temporal variability of the density field are included in the model through the formulation of thermocline displacement anomalies. Sensitivity tests based on realistic density profiles support that the observed modulation of occurrence of the El Nino events may have its source from tropical-extratropical water mass exchanges.