A simple GCM study on the relationship between ENSO and the southern annular mode

This study investigates the relationship between El Ni~no-Southern Oscillation (ENSO) and southern annular mode (SAM) events with an idealized general circulation model. A series of model calculations are performed to examine why positive (negative) intraseasonal SAM events are observed to occur much more frequently during La Ni~na (El Ni~no). Sevendifferent model runs are performed: a control run, three El Ni~no runs (the first with a zonally symmetric heating field, the second with a zonally asymmetric heating/cooling field, and the third that combines both fields), and three La Ni~na runs (with heating fields of opposite sign).The model runs with the zonally symmetric and combined heating fields are found to yield the same relationship between the phase of ENSO and the preferred phase for SAM events as is observed in the atmosphere. In contrast, the zonally asymmetric model runs are found to have the opposite SAM-ENSO phase preference characteristics. Since a reduced midlatitude meridional potential vorticity gradient has been linked to a greaterfrequency of positive-phase SAM events, and vice versa for negative SAM events, the meridional potential vorticity gradient in the various model runs was compared. The results suggest that the phase preference of SAM events during ENSO arises from the impact of the zonal-mean heating on the midlatitude meridional potential vorticity gradient.