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Ge Liu, Ping Zhao, Junming Chen, and Song Yang


The authors explore the preceding factors of summertime Asian–Pacific Oscillation (APO) using observations and output from the NCEP Climate Forecast System version 2 (CFSv2). Results show that the winter and spring sea surface temperatures (SSTs) in the tropical central-eastern Pacific (TCEP) and the spring sea level pressure (SLP) over the north Indian Ocean (NIO) are significantly correlated with summer APO. The preceding TCEP SST anomaly tends to exert a delayed impact on summer APO through the following process. The previous winter TCEP SST anomaly persists until spring and results in SLP anomaly over the NIO in spring. The latter induces a vertical motion anomaly over the western Tibetan Plateau, which alters spring rainfall and underlying soil moisture in situ, further modulating local surface air temperature during the following summer and hence the summer APO. The CFSv2 has high skills in predicting the winter and spring TCEP SST and the spring NIO SLP and successfully captures the observed relationships of TCEP SST and NIO SLP with summer APO. This result explains why the CFSv2 is capable of predicting the summer APO teleconnection by several months in advance.

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Junming Chen, Ping Zhao, Song Yang, Ge Liu, and Xiuji Zhou


The Asian–Pacific Oscillation (APO) is a dominant teleconnection pattern linking the climate anomalies over Asia, the North Pacific, and other regions including North America. The National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) successfully simulates many summer-mean features of the upper-tropospheric temperature, the South Asian high, the westerly and easterly jet streams, and the regional monsoons over Asia and Africa. It also well simulates the interannual variability of the APO and associated anomalies in atmospheric circulation, precipitation, surface air temperature (SAT), and sea surface temperature (SST). Associated with a positive APO are a strengthened South Asian high; a weakened extratropical upper-tropospheric westerly jet stream over North America; strengthened subtropical anticyclones over the Northern Hemisphere oceans; and strengthened monsoons over North Africa, India, and East Asia. Meanwhile, increased precipitation is found over tropical North Africa, South Asia, northern China, and tropical South America; decreased precipitation is seen over subtropical North Africa, the Middle East, central Asia, southern China, Japan, and extratropical North America. Low SAT occurs in North Africa, India, and tropical South America and high SAT appears in extratropical Eurasia and North America. SST increases in the extratropical Pacific and the North Atlantic but decreases in the tropical Pacific. The summer APO and many of the associated climate anomalies can be predicted by the NCEP CFSv2 by up to 5 months in advance. However, the CFSv2 skill of predicting the SAT in the East Asian monsoon region is low.

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Ge Song, Bohua Huang, Rongcai Ren, and Zeng-Zhen Hu


In this article, the interannual variability of upper-ocean temperature in the equatorial Indian Ocean (IO) and its basinwide connections are investigated using 58-yr (1958–2015) comprehensive monthly mean ocean reanalysis data. Three leading modes of an empirical orthogonal function (EOF) analysis dominate the variability of upper-ocean temperature in the equatorial IO over a wide range of time scales. A coherent interannual band within the first two EOF modes identifies an oscillation between the zonally tilting thermocline across the equatorial IO in its peak phases and basinwide displacement of the equatorial thermocline in its transitional phases. Consistent with the recharge oscillation paradigm, this oscillation is inherent in the equatorial IO with a quasi-periodicity around 15 months, in which the wind-induced off-equatorial Rossby waves near 5°–10°S provide the phase-transition mechanism. This intrinsic IO oscillation provides the biennial component in the observed IOD variations. The third leading mode shows a nonlinear long-term trend of the upper-ocean temperature, including the near-surface warming along the equatorial Indian Ocean, accompanied by cooling trend in the lower thermocline originating farther south. Such vertical contrary trends may lead to an enhanced stratification in the equatorial IO.

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