资源环境科技发展态势分析平台

This study evaluates the possible influence of the winter surface conditions in Eurasia on the summer circulation over the Asian continent and Indo-Pacific region. We have analysed multi-seasonal ensemble reforecasts for 30 years (1979-2008) using the National Centers for Environmental Prediction Climate Forecast System version 2 initialized at the beginning of each month from January to May. It is found that the reforecasts initialized in winter (e.g., February) overestimate the snow cover fraction, depth and water equivalent, as well as surface albedo in the excessively snow-covered portion of Eurasia from March to June. These biases are generated and perpetuated by a snow-albedo feedback, leading to excessive upwards shortwave radiation reflected from the overly snow-covered surface and an intense cold bias from the surface to mid-troposphere. Originating over land, the cold bias is extended eastwards over the northwestern North Pacific by the advection of prevailing westerly winds. The cold air temperature in the broad mid-latitude Asian-Pacific region causes significantly lower geopotential heights at pressure levels in the middle and upper troposphere and thus increases the upper-level westerly winds on its southern flank over the Asian continent and Indo-Pacific. A slower than observed snow melting rate helps the winter cold bias persists well into the summer season in these runs. As a result, compared with the reforecasts initialized in spring (e.g., May), winter-initialized reforecasts feature lower geopotential heights in the upper troposphere over Eurasia and a stronger subtropical jet over the Asian continent and the North Pacific from May to September, especially in early summer. The CFSv2 reforecasts in both sets of cases have too little total cloud fraction over Eurasia during June-August, leading to enhanced downwards shortwave radiation.