The Asian-Pacific Oscillation and Its Impact on Climate

The teleconnection of summer Northern Hemisphere (especially the Asian-Pacific region) and its impact on regional climate have always been the hot issues which atmospheric scientists focus their attention on. A series of research work has been done for the Asian-Pacific Oscillation (APO) recently and reveals the characteristics of APO systematically .Also, it researches the relationship between the APO and atmospheric circulation and monsoon rainfall, as well as cyclone activities. There is also discussion about impact factors and mechanisms of the APO.
The APO is defined as a zonal seesaw of the tropospheric temperature in the mid-latitudes of the Asian-Pacific region. When the troposphere is cooling in the mid-latitudes of the Asian continent, it is warming in the mid-latitudes of the central and eastern North Pacific as well as in North America and the North Atlantic Ocean, and vice versa. It reflects an out-of-phase relationship in variability of the eddy temperature between Asia and the North Pacific and is associated with the out-of-phase relationship in atmospheric heating. This teleconnection pattern over the extratropical Northern Hemisphere is revealed through the empirical orthogonal function analysis of summer upper tropospheric eddy temperature.
The summer APO index shows multiple-time-scale variability. It has a quasi-5-year period and shows a decadal variation, with a period of 90 and 10~13 years. The values of APO were low from the 1880s to the mid-1910s and high from the 1920s to the 1940s and tended to a high-index polarity before 1975 and afterward to a low-index polarity. With higher APO index conditions, in the upper troposphere, the summer South Asian high and the North Pacific trough are stronger, while the westerly jet stream over the extratropics of Asia and the easterly jet stream over South Asia strengthen. Also, the Asian low and the North Pacific subtropical high are stronger in the lower troposphere. The anomalous southerlies prevail at the mid-latitudes of East Asia and the anomalous westerlies prevail over South China and South Asia, meaning stronger moisture transport toward Asia. Corresponding to a higher-APO index, the Mei-yu front is more northward, with more precipitation in northern China and southern India, while precipitation decreases from the valley of the Yangtze River to southern Japan and near the Philippines. Thus, the APO index might be used to indicate the variability of the Asian monsoon and rainfall.
The APO intensity has a close relationship with the tropical cyclone (TC) activities over the western North Pacific (WNP) and the coastal waters of eastern China (CWEC) during summer. When the APO is stronger, the atmospheric circulation over the CWEC is manifested by a low-level anomalous cyclonic circulation, a decreasing vertical shear of zonal wind between the high and low levels of the troposphere, and the strengthened convection. These features are favorable for the maintenance and development of the TC. The APO also modulates the steering current that affects the movement of the TC. Corresponding to a stronger APO, the easterly wind south of the high strengthens, which favors the TC to move northwestwards or westwards at more northern latitudes, leading to an increase of the number of the TC that enters into the CWEC. Thus corresponding to a higher APO index, the TC activities over the WNP are located in a more westward and northward position and the TC number over the CWEC greatly increases.
The formation of the APO is associated with the zonal vertical circulation caused by a difference in the solar radiative heating between the Asian continent and the North Pacific. There are significant links between APO and solar radiation at the periods of 250, 120~160, 60~70 and 15 years. The numerical simulations (both the NCAR CAM3 and the CCSM3) further reveal that the summer Asian land (including the Tibetan Plateau) heating enhances the local tropospheric temperature and upward motion, and decreases the tropospheric temperature over the central and eastern Pacific. This leads to the formation of the APO. APO is closely related to the dominant pattern of summertime Pacific sea surface temperature (SST) that is characterized by an out-ofphase relationship between the tropics and the extratropics, with a significantly positive correlation between APO and SST in the extratropical North Pacific and a significantly negative correlation in the tropical eastern Pacific. However, sensitivity experiments show that the anomalous atmospheric circulation and the anomalous centers of the extratropical tropospheric temperature over the Asian-Pacific region triggered by the Pacific SST anomalies are not consistent with those observations. That is, the Pacific decadal oscillation and EL Nino/La Nina over the tropical eastern Pacific do not exert strong influences on the APO. Conversely, the Asian land elevated heating seems to play a more important role in generating the climate anomalies over the Asian-Pacific region.
In general, this paper summarizes the latest research developments of the Asian-Pacific Oscillation, which will help to understand the activity patterns of APO and its climate effect systematically and to further pursue some research into APO and its application in climate diagnosis and prediction. Also, it provides a new way to explore interactions between the Asian and Pacific atmospheric circulations.