Decadal variations of the relationship between the summer North Atlantic Oscillation and middle East Asian air temperature

摘要

The relationship between the summer (July–September) North Atlantic Oscillation (SNAO) and simultaneous East Asian air temperature is investigated. The results show that the SNAO is related to middle East Asian summer air temperature, however this linkage varies with time on decadal timescale: a strong connection appears after the late 1970s but a weak connection before the late 1970s. Further analysis indicates that this instable relationship may have resulted from the shift of the SNAO mode around the late 1970s. In the period of 1979–2003, the centers of the SNAO mode are located more eastward. A positive-phase (negative-phase) SNAO produces a strong lower-level divergence (convergence) over the Asian jet entrance region, in turn stimulates a strong upper-level convergence (divergence) via the Ekman pumping. Such a convergence (divergence) then excites a zonally oriented quasi-stationary barotropical Rossby wave train along the Asian upper-level jet. Thus the SNAO signal is transported eastward to East Asia, resulting in an anomalous summer air temperature over middle East Asia. However, in the period of 1951–1975, the centers of the SNAO mode are located more westward. The associated upper-level divergence/convergence is away from the Asian jet entrance region, and the SNAO signal cannot be transported eastward to East Asia. Hence the connection is broken.