Impact of the initial stratospheric polar vortex state on East Asian spring rainfall prediction in seasonal forecast models

The possible impact of the stratospheric polar vortex on East Asian spring rainfall and the representation of this effect in seasonal forecast models are assessed. A strong stratospheric polar vortex is typically associated with dry conditions across Southeastern China and wet conditions across Korea and Japan in spring. Seasonal forecast models have a decent skill of forecasting the stratospheric polar vortex strength in March. However, they show a wide spread in the predictability of East Asian spring rainfall, possibly due to the underrepresentation of the linkage between the stratospheric polar vortex and East Asian rainfall. The dry impact of strong polar vortex on Southeastern China is forecasted, but the wet impact on Korea and Japan is missing. Tropospheric positive height anomalies over Lake Baikal extend farther equatorward in East Asia during a strong vortex, corresponding to an anomalous anticyclone and less rainfall across Southeastern China. In contrast, the observed anomalous cyclonic shear at 200 hPa and the observed anomalous cyclone at 850 hPa in spring are not realistically forecasted over Northeast Asia, explaining the low skill of producing the wet anomalies in Korea and Japan by models. All models can forecast the wet pattern in East Asia during the El Niño spring. The observed southwesterly anomalies along the coastal China, Korea, and Japan following El Niño are also forecasted by models. Compared with the remote impact of El Niño, the downward impact of the stratosphere on East Asian spring rainfall is still a challenge for most models.