Basic atmospheric dynamics control on ENSO and tropical basin interactions

A recent study of a series of simulations with idealized worlds of tropical ocean basins and land configurations found remarkable changes in El Niño Southern Oscillation (ENSO) and tropical basin interactions, suggesting that atmospheric processes play a dominant role in regulating ENSO dynamics. Here we will discuss results from shallow water atmosphere (Matsuno-Gill-type) model to explore how simplified atmospheric dynamics can control growth rate (Bjerknes feedback) and period of ENSO. We find that equatorial heat sources have an optimal zonal length of ~ 40° longitude at which the equatorial zonal wind response (Bjerknes feedback) is the strongest, while the meridional wind convergence towards the equator monotonically increases with zonal length of the heat source. This suggests that basins larger than the Pacific can have weaker ENSO variability due to the atmospheric dynamics controlling the wind stress. The meridional width of the heat source enhances the Bjerknes feedback, reduces meridional wind convergence and shifts the maximum wind curl further away from the equator, thereby decreasing the oceanic Rossby wave speeds. These changes in the wind patterns force the ENSO pattern of larger basins to be more equatorially confined and shorten the ENSO period. More importantly, interactions with heat sources in remote tropical ocean basins can strongly enhance the Bjerknes feedback leading to a stronger control on ENSO dynamics than the basin size. Out-of-phase remote heat source can further control the period of ENSO. The results suggest that basic atmospheric dynamics strongly control both the growth rate and period of ENSO.