The Climatological Influence of Eurasian winter surface conditions on the summer circulation in the Asian continent and Indo-Pacific in the CFSv2 Seasonal Reforecasts

This study evaluates the possible connection between winter conditions in Eurasia and summer circulation in the Asian continent and Indo-Pacific region using the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2). We have conducted 30-year ensemble reforecasts (1979-2008) initialized with observation-based ocean, land, atmosphere and sea ice states from the beginning of each month in January to May using CFSv2. All reforecasts are integrated to the end of September. Our results demonstrate that differences between reforecasts initialized in February (FIR) and May (MIR) depicts enhanced cooling in FIR from surface to mid troposphere over the Eurasia and northern Pacific Ocean, which leads to enhanced lower geopotential height at 200hPa (H200) during June to July mean (JJ) over the northern central Eurasia. Because of the geostrophic balance between wind and geopotential height gradient, the westerly jet at 200hPa in FIR is enhanced during JJ between 30^o to 45^oN over the Asian continent and Northern Pacific. The excessive snow cover that lingers into spring in FIR is primarily due to a slow melting rate, which may be due to a positive snow-albedo feedback. These errors are associated with the bias in the surface energy budget, mainly in the upward shortwave radiation and surface sensible heat flux, and may be an important cause of the intense cold bias at the surface to mid-troposphere in Eurasia, which is related to significant biases in geopotential heights between the mid-troposphere to 200hPa and in turn contributes to an upper level large-scale 200hPa circulation bias over the Asian monsoon region during May to September. The low geopotential heights in the upper troposphere over Eurasia cause the westerly jet stream to increase in the reforecasts during May to September over the Asian continent and Indo-Pacific. In contrast, reforecasts initialized in May have less bias in geopotential heights at 200hPa and 200hPa-winds over the Asian continent and Indo-Pacific region during the summer monsoon season. The results of this study demonstrate the importance of the parameterization of snow, SWE and surface albedo for improving seasonal prediction over continental areas in coupled models.