Atmospheric dynamics controlling ENSO growth rate and period in a series of idealised worlds simulations

A recent study of a series of simulations with idealised geometries of the tropical ocean basins and land found remarkable changes in ENSO and tropical basin interactions that suggest that atmospheric dynamics are largely controlling ENSO dynamics. Here we will discuss shallow water atmosphere (Gill-type) model results to explore how simplified atmospheric dynamics can control growth rate (Bjerknes feedback) and period of ENSO. We find that for single tropical ocean basins larger than the Pacific the Bjerknes feedback becomes weaker due to the zonal length of the SST forcing and at the same time the meridional winds become stronger. This result suggest that basins larger than the Pacific will have weaker ENSO variability due to the atmospheric dynamics controlling the wind stress. Interactions with heat sources in remote tropical ocean basins have the ability to strongly enhance the Bjerknes feedback leading to stronger control on ENDO dynamics than the basin size. The changes in the winds stress also affect the period of ENSO by altering the wind stress curl, which for larger ocean basins gets closer to the equator and thereby increasing the Rossby wave speed supporting the finding that larger basins have short ENSO periods.