A quasi-stationary surface high pressure stays over the cold sea surface of Okhotsk for several days or even a week occasionally from late April to August. The cool, wet surface northeasterlies from the high cause unusually low temperatures and lack of sunshine throughout the eastern coast of Japan. Abnormally cool summers in Japan are characterized by the frequent occurrence of the Okhotsk High in July and August. It is known that the occurrence is associated with the blocking aloft. In the present study we examine a typical time evolution of the strongest Okhotsk Highs observed around [55N, 145E] over the recent 27 years. Composites were made of the 8-day lowpass-filtered fields of dynamical variables, including amplitude and barotropic feedback of the high-frequency migratory transients, based on the 15 strongest events of the surface Okhotsk High in July. The composites were made relative to the peak time of each of the events. The same compositing was repeated for the 15 strongest events in May.
In the May composites, a north-south dipole of strong circulation anomalies is evident over the central North Pacific 6 days before the peak. The anticyclonic anomalies associated with the dipole amplify as they gradually retrograde until they are anchored over the Sea of Okhotsk 2 days before the peak. The activity of baroclinic waves is enhanced upstream of those anticyclonic anomalies and suppressed to the south, which exerts such feedback as to strongly reinforce those anomalies.
In contrast, no precursor can be found over the north Pacific in the July composites. Rather, strong anticyclonic anomalies are observed over Europe 6 days before the peak. The westerlies aloft split into two branches over Siberia and the north Pacific, one along the Arctic coast and the other in the midlatitudes. A quasi-stationary wavetrain that emanates from the ridge over Europe eastward along the Arctic jet reaches the Far-Eastern Siberia and anchors the anticyclonic anomalies there. Since the wavetrain cannot propagate across the weak westerlies between those branches, the wave activity is deposited into the anticyclonic anomalies, resulting in a rapid development the blocking and a surface high over the Sea of Okhotsk. The feedback from the high-frequency transients only plays a secondary role.
These distinctive differences in the blocking formation over the Far East between the two periods can be attributed to marked changes in the jet structure and stormtrack activity from May to July. In May, strong feedback forcing associated with enhanced stormtrack activity right after its spring maximum (following the midwinter minimum) acting upon somewhat relaxed PV gradients leads effectively to blocking formation. In July, stormtrack activity is strongly suppressed in the presence of the weakened and split westerlies, but local breaking of a stationary Rossby wave packet is enough for blocking formation under the very relaxed background PV gradients.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics