The role of linear interference in the Annular Mode response to Tropical forcing

Recent observational and modelling studies have shown that eastern tropical Pacific Ocean (TPO) warming associated with the El Nino-Southern Oscillation (ENSO) is linked to the negative phase of the wintertime NAM. The TPO-NAM link involves a Rossby wave teleconnection from the tropics to the extratropics and to the polar stratosphere. For a TPO warming, the Rossby wave teleconnection is associated with an increase in stratospheric wave driving that in turn induces a negative NAM anomaly in the stratosphere and troposphere. Previous work further suggests that tropical Indian Ocean (TIO) warming is associated with a positive NAM anomaly, which is of opposite sign to the TPO case. The TIO case is, however, difficult to interpret because the TPO and TIO warmings are not independent. To better understand the dynamics of tropical influences on the NAM, the current study investigates the NAM response to imposed TPO and TIO warming in general circulation models (GCMs). The NAM responses to the two warmings have opposite sign, and can be of surprisingly similar amplitude even though the TIO forcing is relatively weak. It is shown that the sign and strength of the NAM response is often simply related to the phasing, and hence the linear interference, between the Rossby wave response and the climatological stationary wave. Results from various configurations of the NCAR CCSM and the GFDL GCMs will be presented and the utility of the linear interference diagnostic in understanding model differences will be discussed.