Modeling study of the destructive interference between the tropical Indian Ocean and eastern Pacific in their forcing in the southern winter extratropical stratosphere during ENSO

By conducting sensitivity experiments with the Whole Atmosphere Community Climate Model, this study reveals the interference of the sea surface temperature (SST) forcing in the southern winter stratosphere between the tropical Indian Ocean (TIO) and tropical Pacific Ocean (TPO) during ENSO. Similar to the variance pattern of zonal-mean zonal winds with two maximum centers on both flanks of the southern polar night jet, the southern circumpolar zonal wind responses to TPO and TIO SST forcings also exhibit a dipole pattern, indicating their meridional shifting effect on the polar jet. However, a warm TPO SST favors an equatorward shift of the polar jet, while a warm TIO SST induces a poleward shift. This has resulted in an observed weaker ENSO signal in the southern winter stratosphere than that purely forced by the ENSO TPO SST. Further diagnosis indicates that the interference between the TIO and TPO is dynamically associated with the destructive interference of the wave train forced by the TIO with the Pacific-South America-like wave train pattern forced by the TPO. As a result, the southern extratropical wavenumber-1 response to the TPO is nearly out of phase with that to the TIO throughout the troposphere and stratosphere. While upward propagation of planetary waves is significantly suppressed in midlatitudes but enhanced in high latitudes when warm SST anomalies are confined over the TPO, the opposite pattern is observed when warm anomalies are confined over the TIO. Furthermore, the significant destructive interference between the TIO and TPO in their forcings in the southern winter stratosphere is also confirmed in reanalysis data.