Coherent Structure Of Mid-summer Interdecadal Climate Change Over East Asia: Observations And Numerical Simulations

The coherent interdecadal climate change over East Asia in mid-summer at the beginning of the 21 st century is thoroughly examined in this study by virtue of the interdecadal differences and time series analyses using station-observed and reanalysis data. The July and August(JA) mean upper-tropospheric temperature over East Asia shows a significant increasing trend, contrary to the decreasing trend in the late 1970 s. The largest warming center is over northern China(between 30°N–45°N and 85°E–120°E) around 300 hPa. Together with the temperature rising, the geo-potential height rises above the warming center and drops below, which connects closely to a correspondingly significant decadal shift of the general circulation over East Asia. In the upper-level of the atmosphere, an anomalous anti-cyclone dominates, and the 200–hPa westerly jet strengthens due to the increasing of pole-ward geopotential height gradient. In the lower-troposphere, the anomalous southerly wind increases around Yangtze River Valley and the East Asian summer monsoon intensifies. The integrated circulation changes seriously impact summer precipitation over East Asia. The so-called “southern flood and northern drought”(SFND) pattern since the 1970 s over eastern China has changed. As the cooling center in the 1970 s moves southward, the dry belt moves southward as well. A wet belt dominates the Huaihe River Valley after the temperature trend reversal at 2005 while southern China experiences a dry condition.The coherent interdecadal climate change in East Asia presents an objective metric to assess the model performance on reveal the East Asian summer monsoon variability. Based on the result of the observation, we evaluated the simulation of the coherent circulation structures correspond to the changes of mid-summer(July–August) rainfall over eastern China(30°N–40°N, 110°E–120°E) in NCAR CAM5(National Center of Atmospheric Research Community Atmospheric Model version 5) model. Forced by historical sea surface temperatures(SSTs), the NCAR CAM5 reasonably reproduces coherent changes of temperature and large-scale circulations, corresponding to the change in rainfall under different resolutions. Results show that when the rainfall decreases over eastern China, the model reproduces a remarkable warm center in the upper troposphere with an anomalous anticyclone appears above, and an increase in anomalous westerlies to its north. An anomalous anticyclone also occurs in the lower troposphere, along with anomalous southerlies to its east which indicates strengthening of the East Asian summer monsoon. Both the circulation changes in the upper and lower troposphere favor a decrease in precipitation over central eastern China. Although there are some deficiencies in the NCAR CAM5 simulations in terms of the changes in magnitude and location of the rainfall centers, in general, the model reasonably reproduces the configuration of the large-scale circulation patterns and surface rainfall. This further confirms that the climate variations across East Asia most likely to arise from a regional response to global climate change. The well-simulated configuration by NCAR CAM5 also indicates the reliability of the model and its potential to reveal the mechanisms of interdecadal changes of the East Asian monsoon system.To further investigate the possible mechanisms of interdecadal change of East Asian climate, the relationship between the coherent circulation structure change over East Asian and the PDO(Pacific Decadal Oscillation) pattern are examined by a series of simulations using the NCAR CAM5. These simulations are forced separately by anomalous PDO positive and negative patterns as well as the eastern tropical and subtropical SST anomalous patterns. Results shows that the PDO negative pattern can produce the East Asian three-dimensional circulation change structure characterized by the upper-tropospheric temperature rise. The coherent westerly jet shift and low level anomalous southerlies are also reproduced. PDO positive pattern reproduced opposite phase of climate change circulation structure in East Asia with differences in magnitude and positions. Both the eastern tropical and subtropical PDO pattern can reproduce the coherent change of the three-dimensional circulation structure of East Asia except that the magnitude is weaker than north Pacific simulation.