Inter-annual Variability of Winter Asia-Pacific Oscillation and Its Relationship with the East Asian Climate Anomalies

Using 1948-2011 NCEP/NCAR monthly reanalysis data and 1951-2010 precipitation data at 160 meteorological stations of China, the empirical orthogonal function (EOF), correlation analysis and composite analysis methods, the winter (December, January and February) atmospheric teleconnection over the Asian-Pacific and its association with East Asian winter monsoon and precipitation are examined. Results show an Asian-Pacific Oscillation (APO) teleconnection in upper-tropospheric temperature disturbance between the Asian-western Pacific and the central-eastern Pacific region during winter, which is similar to the APO phenomenon during summer. When the mid-upper troposphere temperature in mid-lower latitudes of East Asia is warmer, it is colder in mid-lower latitudes of the central and eastern North Pacific, and vice versa. The APO reflects the variability of the zonal thermal contrast between Asia and North Pacific. Compared to summer, the Asian anomalous center of DJF APO is southward and eastward in position. DJF APO index does not show a significant linear trend. The power spectrum analysis shows that the APO index has the varying periods of 2-6 years. Corresponding to a higher APO index, geopotential height disturbance in mid-upper tropospheres is lower in mid-higher latitudes of East Asia and it is higher in mid-higher latitudes of central and eastern North Pacific. Also, there is a "seesaw" in lower latitudes between the Asian-western Pacific region and the central-eastern Pacific, but with a reversed phase comparing to that in higher latitudes. This anomalous feature in geopotential height in the upper troposphere indicates a westward long-wave trough in East Asia and a northward high pressure in the tropics of East Asia, with a deep anti-cyclonic anomaly over southern China. Wind anomalies of northeasterly prevail over southern China in the lower troposphere, with divergence anomalies of water vapor and downward motion anomalies in southern China. Accordingly, the local precipitation decreases. However, these relationships between DJF APO and atmospheric circulation are different from those during summer. Moreover, the APO index is highly correlated with ENSO as well.