Impact of stratospheric ozone on the subseasonal prediction skill in the Southern Hemisphere spring

The Antarctic polar vortex and the associated ozone change have been recognized as a key factor that influences both local and large-scale circulations in the Southern Hemisphere (SH) extratropics. Their downward impacts are also evident in the subseasonal-to-seasonal (S2S) and long-term climate predictions especially in austral spring and summer. However, most operational S2S models, including the Global Seasonal Forecasting System version 5 (GloSea5), use climatological ozone and ignore time-varying ozone associated with polar vortex variability. This study explores the possible impact of stratospheric ozone on SH S2S prediction skill by conducting the two sets of reforecast experiments with the GloSea5. The reforecasts are initialized on 1st September of every year for the period of 2004-2018 with either climatological or observed ozone from the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) data. It turns out that the reforecasts with observed ozone have an improved prediction skill at 5- and 6-week lead forecasts than those with climatological ozone. The surface prediction skills also increase over the southern Australia and New Zealand. These results suggest that more realistic stratospheric ozone forcing could improve the SH prediction skill on subseasonal-to-seasonal timescale.