Increased Interannual Variability in the Dipole Mode of Extreme High-Temperature Events over East China during Summer after the Early 1990s and Associated Mechanisms

The dominant mode of the interannual variability in the frequency of extreme high-temperature events (FEHE) during summer over eastern China showed a dipole mode with reversed anomalies of FEHE over northeastern and southern China. This study found that the interannual variability of this dipole mode underwent an interdecadal increase after the early 1990s. The anomalous atmospheric circulation responsible for the FEHE dipole mode was associated with the air-sea interaction over the western tropical Pacific and North Atlantic. Due to the weakened correlation between the SST in the tropical Pacific and in the Indian Ocean after the early 1990s, a meridional atmospheric wave train induced by the anomalous SST around the Maritime Continent (MCSST) was intensified during 1994-2013, which was also contributed by the increased interannual variability of MCSST. However, under the influence of the anomalous SST in the Indian Ocean concurrent with the anomalous MCSST, the meridional wave train was weakened and contributed less to the dipole mode during 1972-93. In addition, the dipole mode was associated with the atmospheric wave trains at middle to high latitudes, which were different during the two periods and related to different air-sea interaction in the North Atlantic. The interannual variability of the dipole mode induced by the associated SST anomalies in the North Atlantic during 1994-2013 was significantly larger than that during 1972-93. Therefore, the interannual variability of the dipole mode was increased after the early 1990s.