The Relation Between Two Types of Frequent Wave Breaking Over Southeastern Siberia and Different Conditions of Unseasonable Midsummer Weather Over Japan

ABSTRACT Possible different impacts of anticyclonic and cyclonic types of Rossby wave breaking (RWB) over southeastern Siberia on the anomalous circulation and associated unseasonable midsummer weather over Japan have not been investigated before. In this study, we have examined anticyclonic and cyclonic RWB and their different effects in July, when Japan experiences a relatively large interannual variability of the surface temperature. Regression analyses against the interannual variabilities of RWB occurrence frequencies show that the frequent anticyclonic reversals in July are accompanied by Rossby‐wave‐packet propagation along the enhanced waveguide associated with the polar‐front jet. The associated negative height and sea‐level‐pressure anomalies that extend from Japan to the east provide suitable conditions for anomalous northeasterly cold advection in the lower level of the atmosphere and cold summer temperatures over Japan. A dynamically induced anomalous ascent that is linked to the enhanced and southward‐deflected subtropical jet is associated with anomalous upward motions to the south‐to‐southeast of Japan. Enhanced eastward moisture fluxes and increasing precipitable water are also seen over this area. The frequent cyclonic RWB in July is accompanied by Rossby‐wave‐packet propagation along the enhanced waveguide associated with the subtropical jet in addition to that associated with the polar‐front jet. The associated negative height anomalies over northeastern China and the negative sea‐level‐pressure anomalies from China to Japan lead to anomalously warm advection over western Japan and anomalously cold advection, which is weak and limited, near northern Japan. A dynamically induced anomalous ascent that is linked to the southward meandering of the subtropical jet is associated with anomalous upward motions from eastern China to Japan. Enhanced moisture fluxes and increasing precipitable water are seen over this area. These differences imply that the morphologies of RWB cause the different patterns of temperature and precipitation anomalies associated with unseasonable midsummer weather over Japan.