On the asymmetry of forecast errors in the Northern winter stratosphere between vortex weakening and strengthening conditions

Abstract

This study investigates the asymmetry of forecast errors in the Northern winter stratosphere between vortex weakening and strengthening conditions. Previous studies suggest that the stratospheric forecast errors for medium-range time scales of about two weeks are larger for weakening conditions of the polar vortex than for strengthening conditions even though they have anomalies of similar magnitudes. We explore the asymmetry by comparing the one-month hindcast data of the Japan Meteorological Agency to the Japanese 55-year Reanalysis data. We define the vortex weakening and strengthening conditions of anomalies of similar magnitudes using an empirical orthogonal function analysis of polar stratospheric temperatures. Results indicate that the larger forecast errors in the stratosphere for the vortex weakening conditions originate from those of the planetary wave forcing in the upper troposphere. In particular, it is more difficult to forecast planetary wave amplification that leads to the vortex weakening conditions than forecasting wave attenuation that leads to the vortex strengthening conditions. Examining forecast errors between major stratospheric sudden warming events (MSSWs) and vortex intensification events (VIs) defined by the zonal mean zonal wind in the high latitude stratosphere, we also show that it is more difficult to forecast the MSSWs than the VIs and further discuss the cause for the difference.