Abstract: The relationship between mid-high latitude blocking in North hemisphere (NH) summer and the decline of summer Arctic sea ice is analyzed by using NCEP/NCAR daily 500 hPa geopotential height reanalysis data. Results show that there are high blocking frequencies over Bering Strait, Okhotsk Sea, Eurasia and Greenland, which are extremely similar to the areas with reduction of Arctic sea ice in the summer in recent 30 years, as observed from satellite data. The difference of surface temperature, meridional heat transport and temperature advection in 850 hPa anomaly fields between days with and without blocking are calculated to show that mid-high latitude blocking have a significant impact on the reduction of sea ice. The increase of blocking frequency in NH mid-high latitudes will increase the surface air temperature, poleward heat transport and temperature advection so as to accelerate the melting of Arctic sea ice. The thermodynamic effects caused by blocking over Okhostk Sea and Eurasia can significantly result in the reduction of Arctic the sea ice.

Key words: Arctic sea ice, Arctic amplification, blocking