Stratospheric Polar Vortex Prolongs the Persistence of Northern Annular Mode During Winter: The Role of Refractive Index

Abstract

Previous studies have demonstrated significant modulation of stratospheric polar vortex (SPV) on the northern annular mode (NAM) during winter, whereas the specific processes are not fully understood. This study investigates how the SPV prolongs the persistence of the NAM on the subseasonal timescales during winter. Our results suggest that the quasi-geostrophic refractive index (RI) at the high latitudes of mid-to-upper troposphere (HL-MTUT) may serve as a critical mediator in this process. Specifically, the preestablished SPV anomaly can persistently alter the RI at HL-MTUT through directly modifying the vertical curvature of zonal wind in this region. The RI anomaly at HL-MTUT, which reflects changes in the waveguide properties of zonal flow, can affect the meridional propagation of tropospheric eddies and is a key factor in the eddy-zonal flow interactions associated with the NAM. Acting as a mediator, the HL-MTUT RI will transmit the persistent anomalous signal of the SPV, which has a longer timescale compared to tropospheric variability, into the intrinsic dynamic processes of NAM. Consequently, the SPV will prolong the persistence of NAM. The above mechanism is further supported by a comparison of active-SPV and inactive-SPV years: In active-SPV years, a greater persistence of the NAM is accompanied by a correspondingly greater persistence in both the vertical curvature of zonal wind and the RI at HL-MTUT.