The synergistic effect of the summer NAO and northwest Pacific SST on extreme heat events in the central–eastern China

The synergistic effect of the summer positive North Atlantic Oscillation (pNAO) and positive northwest Pacific (pNWP) sea surface temperature anomaly (SSTA) on the interannual variability of the extreme heat events in the central–eastern China (CEC) is investigated in this study. The two factors act synergistically in strengthening the extreme heat events in the CEC, and the CEC is likely to experience a hotter summer when both the summer pNAO and pNWP SSTA occur. The two factors influence the extreme heat events in the CEC via a series of atmospheric bridges. The pNWP SSTA increases the strength of pNAO via the eastward propagating Rossby wave from the western Pacific. The enhanced pNAO induces a stronger eastward Rossby wave propagation across the Eurasian continent, and leads to a strong anomalous anticyclone over the CEC. The significantly increased atmosphere thickness increases the air temperature of the layer, and favors the extreme heat events in the CEC. Besides, the anomalous southerlies on the west side of the anomalous anticyclone are also favorable for the increase in air temperature. Furthermore, the pNWP SSTA also has adjacent effect on the atmospheric circulation over the CEC, which could lead to a positive geopotential height anomaly. Therefore, the summer pNAO and pNWP SSTA act synergistically in influencing the atmospheric circulation over the CEC, and thereby significantly increase the extreme heat events in the CEC. Compared with the cooccurrence of the summer pNAO and pNWP SSTA, the magnitude of the extreme heat events decreases greatly when only one factor happens, which highlights the synergistic effect of the two factors.