Characteristics of Summer Stationary Waves in the Northern Hemisphere

Abstract

Summer stationary waves in the Northern Hemisphere are separated by a midlatitude transition zone into the subtropical monsoon regime with a vertical phase reversal and the subarctic regime with a vertically uniform structure. The dynamics and maintenance mechanism of the subtropical stationary waves have been investigated in the context of monsoon circulation. Depicted in terms of streamfunction with 40-yr ECMWF Re-Analysis (ERA-40), the dynamic characteristics of stationary waves in the transition zone and the subarctic region are thus the focus of this study. The dynamics and maintenance mechanism of these waves were explored with the streamfunction budget and the velocity potential maintenance equations. Stationary waves across the transition region consist of anticyclonic shear zones over the North Pacific and North Atlantic and a cyclonic shear zone in east Eurasia. These transition elements are linked to subtropical oceanic anticyclones and continental thermal lows. At high latitudes, a three-wave structure emerges with a weak central Eurasian trough aligned with two deep oceanic troughs. A longitudinal phase change occurs across the transition zone, but the direction of the east?west circulation associated with the transitional anticyclonic (cyclonic) zone is the same as that of the subtropical trough (high). This phase change is caused by the dynamics transition from the Sverdrup regime to the Rossby regime because of the increasing importance of relative vorticity advection. At high latitudes, relative vorticity advection becomes the dominant dynamic process in the upper atmosphere, but is negligible in the lower troposphere. This subarctic dynamic regime results in the vertically uniform structure of stationary waves. These waves are maintained by in situ diabatic heating (cooling) ahead of three subarctic troughs (ridges). Thus, the structure of the east?west circulation of subarctic stationary waves is opposite to that of subtropical stationary waves. These findings not only disclose more detailed structure and dynamics of summer stationary waves, but also provide a more complete basis to validate summer climate simulations and to search for the cause of interannual variation in summer climate.