A Precipitation Seesaw Mode between Northeast Asia and Siberia in Summer Caused by Rossby Waves over the Eurasian Continent

There is a dominant seesaw pattern in summertime precipitation between northeast Asia and Siberia. Examined here is a mechanism of the seesaw mode, focusing on quasi-stationary Rossby waves propagating on two upper-tropospheric waveguides along the Asian jet and over northern Eurasia. Empirical orthogonal function (EOF) analysis to the low-frequency (10-day low-pass filtered) variation during summer reveals wave-propagation patterns on both of the waveguides. The time evolution of the two composite fields, constructed using the obtained EOF modes for each phase of the seesaw mode, was investigated by the wave-activity flux. In both composite fields, eastward-propagating Rossby waves originating from the North Atlantic–European sector branch off around eastern Europe onto the two waveguides, which become out of phase toward the east because of the difference in longitudinal wavelengths along them. This results in opposite phase anomalies of meridional winds over northeast Asia and Siberia. Budget analyses of vorticity, heat, and water vapor on a day with significant wave patterns along the two waveguides revealed the main balances; the zonal advection of vorticity anomalies around the tropopause was partly balanced by an anomalous stretching effect over these two regions, and the adiabatic heating anomalies in the midtroposphere associated with the stretching effect around the tropopause were balanced with diabatic heating anomalies, explained by condensation heating related to the precipitation anomalies. These events occur frequently in July, when the climatological precipitation is the largest, particularly over northeast Asia.