Stationary Wave Activity Associated with the East Asian Winter Monsoon Pathway

Abstract

The pathway of the East Asian winter mon- soon (EAWM) that usually leads to the out-of-phase pat- tern of surface air temperature between northern and southern East Asia is an important feature in the variabil- ity of the EAWM besides its strength. Using the European Centre for Medium-Range Weather Forecasts 40-year (ERA40) reanalysis dataset, this study investigates the pathway-related stationary wave activity to explore the mechanism of the interannual variations in the EAWM pathway. It reveals that when the southern pathway of the EAWM is strong, the phase of the climatological station- ary wave tends to be shifted westward significantly in both the horizontal and vertical directions by an anoma- lous wavenumber 2 pattern at mid-latitudes, whereas the changes are relatively small in the subtropics. The hori- zontal changes in the stationary wave phase facilitate a north-south-oriented East Asian trough in the middle tro- posphere that eventually produces the strong southern pathway of the EAWM. The vertical changes in the sta- tionary wave, in contrast, feature a westward-tilted phase line with height over the North Pacific, indicating en- hanced upward propagation of waves into the strato- sphere. This result suggests that the phase of stationary waves at mid-latitudes dominate the interannual variations in the EAWM pathway. Moreover, it supports our previ- ous interpretation of the possible role of the North Pacific sea surface temperature (SST) in the EAWM pathway variability. It also implies that the excitation of anomalous mid-latitude stationary waves may be the key in the re- sponse of the EAWM pathway to the North Pacific SST.  Keywords: East Asian winter monsoon, pathway, sta- tionary wave, phase Citation: Wang, L., 2014: Stationary wave activity asso- ciated with the East Asian winter monsoon pathway, At- mos. Oceanic Sci. Lett., 7, 7-10, doi:10.3878/j.issn.1674- 2834.13.0043.