Atmosphere circulation over the Far East of Russia in 2013, during extreme flood in the Amur basin

Atmospheric processes over the Far East of Russia and the North-West Pacific are investigated for understanding the reasons of the rainfall over the Amur basin in the summer of 2013, extreme by its duration and intensity, that has caused catastrophic flood in the middle and lower flow of the Amur River. On the base of detailed analysis of the air circulation, the reasons of such rainy weather are analyzed, and recommendations for its prediction are proposed. The circulation is described quantitatively by several parameters, as average atmospheric pressure at the sea level, geopotential height, air temperature, indices of integrated air transfer over certain areas, and indices of cyclonic activity over the Amur basin and tropical zone. Prevailing pressure systems are revealed, which are responsible for formation of precipitation fields over the Amur basin. In particular, the long and intense rains over the Amur basin in May-August of 2013 were caused by cyclones propagating along the stable atmospheric frontal zone located in this area. They were blocked by the baric ridge over the Okhotsk Sea, so the deep and stable atmosphere depression (Far-Eastern Low) was formed over the middle and lower flow of the Amur in July-August of 2013 with the negative pressure anomaly > 1.7σ (standard deviation), where the cyclones deepened below than usually (on average in 5 hPa below). This depression was maintained by intensive meridional air transfer in the troposphere over East Asia that transported heat and moisture from the tropical zone. This transfer was reasoned by huge reserves of moisture, latent and sensible heat over southeastern Asia that was formed by abnormally strong equatorial trade-winds and unusually frequent tropical cyclones, mostly going by eastern trajectories because of strong western spur of the Hawaiian High that blocked their ways. All these patterns were presumably conditioned by strengthening of the Hawaiian High. As the result of these large-scale circumstances, the air mass with tropical properties was moved to temperate latitudes, and cyclones converted them to heavy rains just following to their usual paths across the Amur basin.