Abstract: Based on ERA-40 monthly mean reanalysis data over the period 1958-2002, this paper studies the relationship between the spring Arctic Oscillation (AO) and the summer surface air temperature (SAT) over the Yangtze River valley by adopting the year-to-year increment approach. The winter ENSO signal is filtered out in advance. Results show that the AO in May is significantly and positively correlated with the SAT over the Yangtze River valley in August. The possible physical mechanism for this relationship is investigated by means of regression analysis. The air-ocean interaction over the middle and low latitudes might play an important role. Following a stronger positive AO in May, an anomalous subsidence flow appears over the region 10°-15°N and the Equator. At the same time, Hadley circulation over the western Pacific becomes weaker, leading to an anomalous anticyclonic divergent flow in the lower troposphere. Accordingly, significant easterly anomalies appear over the equatorial western Pacific. Since the easterly anomalies are located at the local maximum center of the climatological mean sea surface temperature (SST), the warm SSTs pile up to the west via advection, inducing the above-normal SST over the equatorial western Pacific. The positive SST anomalies persist from May to August. This can trigger an anomalous cyclonic circulation to the northwest through a Gill-like atmospheric response (Gill, 1980), which is conducive to the maintenance of the western Pacific subtropical high over the Yangtze River valley, resulting in the positive SAT anomalies over the region, and vice versa.