NOTES AND CORRESPONDENCE Relationship between Springtime Surface Temperature and Early Summer Blocking Activity over Siberia

Influence of springtime surface condition over Siberia upon subsequent summertime blocking activity over northeastern Siberia, and the Okhotsk sea is examined. It is found that when the surface temperature over Siberia is high in April (the high years), blocking events over northeastern Siberia, and the Okhotsk sea in May and June, occur more frequently than climatology. In the high years, a jet stream over the north of Siberia is formed in the upper troposphere, corresponding to the enhanced meridional temperature gradient, and the polar frontal jet is observed from April to the beginning of July over East Asia. During this period, the meridional gradient of potential vorticity is more gradual, between this high-latitude jet stream and a subtropical jet, consistent with the more frequent occurrences of blocking. Associated with the enhancement of blocking activity, is the intensification of the surface Okhotsk high and cold anomaly to its south over the northern part of Japan during the early summer season of the high years. The finding of this study suggests monitoring the springtime surface condition over northern Siberia as one of the promising predictors for the early-summer climate over East Asia.