Evolution and dynamics of summertime blocking over the Far East and the associated surface Okhotsk high

Time evolution and dynamics of an upper-level blocking ridge over the Far East and the associated surface high over the Sea of Okhotsk that give rise to abnormal summertime coolness over eastern Japan are investigated through a composite analysis applied to distinct surface high events. The formation mechanism and vertical structure of the blocking differ fundamentally between May and July, attributable to seasonal changes in the jet structure, storm-track activity over the Far East and the land-sea thermal contrast within the Okhotsk region. In May, forcing from migratory eddies is strong enough for precursory anticyclonic anomalies retrograding slowly over the North Pacific to develop into a blocking ridge. The ridge remains equivalent barotropic under the weak land-sea thermal contrast in the Okhotsk region. In contrast, the eddy forcing is no longer the primary factor for the blocking formation in July, in the presence of the weakened and split westerlies. Rather, the propagation of a stationary Rossby wave packet that has emanated from precursory anticyclonic anomalies over northern Europe stagnates over the Far East, and the subsequent local breaking of the packet leads to the blocking formation. In a particular case in July 1993, a wave packet involved in blocking formation could be traced back as far upstream as the east coast of North America over the two previous weeks. Acting on the strong thermal contrast in July between the cool sea surface and a warm land mass to the west, the anomalous surface easterlies induced by the blocking ridge can build up a cold surface anticyclone through cold advection, rendering the blocking anomalies distinctly baroclinic.