A Comparison of the Transition of Equatorial Waves between Two Types of ENSO Events in a Multilevel Model

The differences in the transitions of equatorial mixed Rossby-gravity (MRG) waves to off-equatorial tropical depression (TD)-type disturbances during ENSO events are investigated with a global baroclinic anomaly model. The model reproduces reasonably the perturbation evolution within realistic three-dimensional summer mean states corresponding to El Nino (EN) and La Nina (LN) years. Based on wave structure and energetics diagnosis, the results indicate that, following the longitudinal shift of the favorable environmental fields, the wave characteristics are altered accordingly. In the presence of a circulation-convection feedback, the wave train exhibits more rapid growth, a more eastern location of transition, and a more northward-shifting component during EN years than during LN years. The convective heating acts as a leading energy source to supply the wave growth and the increase in eddy kinetic energy is directly attributed to barotropic conversion in the monsoon region. Sensitivity experiments show that the dynamic effect alone fails to capture the observed wave behaviors although the damped modes also experience a scale contraction and a slight northward migration. The near-surface thermodynamic fields related to sea surface temperature (SST) and low-level specific humidity can play a crucial role in the scale contraction and the propagation characteristics for tropical synoptic waves. The heating feedback scheme combining the actions of SST and low-level moisture can amplify and accelerate the modification of wave characteristics initiated by the dynamic effect, producing a tighter wave structure and steering the wave train toward the warmer and moister ocean.