How the annual cycle affects the extratropical response to ENSO

The relationship between tropical remote forcing and seasonality in the extratropics is examined with a set of numerical experiments that use prescribed sea surface temperature (SST) in the tropical Pacific and a simple thermodynamic slab mixed-layer model outside the prescribed region coupled to an atmospheric general circulation model. The numerical experiments use an idealized El Nino-Southern Oscillation (ENSO) evolution where the peak phase (with respect to the annual cycle) can be arbitrarily shifted. In this case, we shift the phase of ENSO by 6 months. An ENSO composite analysis indicates that the extratropical remote response is phase locked with the local season, not ENSO. Pacific basin zonal mean cross section shows that the tropical atmosphere continuously responds to the prescribed SST forcing, but the atmospheric bridge connecting to the extratropics occurs in specific seasons.