A new approach to the East Asian winter monsoon—seasonal cycle and seasonal scale variability under global warming

East Asia experiences strong winter monsoon characterized by cold temperatures and strong northwesterly winds along the continental boundary. The lower tropospheric temperature exhibits sinusoidal variation during winter every year on top of annual variation of temperature. Cyclostationary Empirical Orthogonal Function (CSEOF) analysis and the multiple regression method in CSEOF space are used to decompose physically distinct components of the East Asian winter monsoon (EAWM). The 120 days of winter (November 17-March 16) per year over 62 years (1948-2010) are analyzed using the NCEP daily reanalysis dataset. The first CSEOF mode of 850 hPa temperatures depicts the seasonal evolution of the EAWM. The contrast in heat capacity between the continent and the northwestern Pacific results in a differential heating in the lower troposphere. Its temporal evolution drives the strengthening and weakening of the Siberian High and the Aleutian Low. The latter, in turn, produces intricate intraseasonal circulation change resulting in regional changes in thermal advection and surface heat fluxes. The second CSEOF mode explains the interannual variability of the EAWM. It describes overall winter warming/cooling and its amplitude variation on interannual time scales. When East Asia is warmer (EAWM is weak) during one winter, sea level pressure is lower in the northern region of East Asia, and slightly higher over the Northwestern Pacific. The northwesterly along the continental boundary is reduced thereby increasing thermal advection over Korea, Japan and the neighborhood. As a result, turbulent heat flux is significantly reduced over the western (120°E-160°E, 35°N-60°N) part of the North Pacific. The corresponding PC time series shows a warming trend, most likely due to global warming, in the midst of stronger interannual variability.