Dynamical Processes Related to the Appearance of the Okhotsk High during Early Midsummer

Abstract

The authors identified an upper-level pressure anomaly pattern corresponding to the interannual variability of the Okhotsk high in midsummer (late July and early August) as a predominant anomaly pattern in the Northern Hemisphere, by using objectively analyzed data. According to the results of empirical orthogonal function (EOF) analyses and composite analyses, a positive pressure anomaly appeared near the tropopause over eastern Siberia in years with strong Okhotsk highs. Examination of the heat budget in the lower troposphere revealed that a negative surface temperature anomaly observed in northern Japan was brought by the advection of the climatological temperature gradient from the anomalous wind associated with the upper-level anticyclonic anomaly. It was also demonstrated that the anomaly field over Siberia does not accompany predominant vorticity forcing or Rossby wave propagation from the west with a specific phase. However, positive kinetic energy conversion from the climatological basic field to the anomaly field is estimated. The energy conversion contributes to maintaining the anomaly pattern. By the numerical experiments using a linear barotropic model, it is suggested that the upper-level anomaly pattern related to the anomalous Okhotsk high appears through the interaction with the climatological basic field, even though the external forcings are homogeneously distributed.