The synergistic effect of the preceding winter mid-latitude North Atlantic and summer tropical eastern Indian Ocean SST on summer extreme heat events in northern China

Summer extreme heat events happen frequently in northern China during recent decades, which have serious impacts on the society and ecosystem. The present study reveals that there is a synergistic effect of the preceding winter positive mid-latitude North Atlantic SST anomaly (pMNA SSTA) and summer negative tropical eastern Indian Ocean SST anomaly (nTEI SSTA) on strengthening the summer extreme heat events in northern China. The extreme heat events are stronger and more frequent when the two factors cooccur, and the probability of a strengthened extreme heat events is higher, which indicates a synergistic effect of the two factors. The preceding winter pMNA SSTA and summer nTEI SSTA exert their synergistic effect through a series of coupled oceanic-land-atmospheric bridges. The preceding winter pMNA SSTA could lead to an anomalous anticyclone over central Asia via the eastward propagating Rossby wave, which decreases snowfall and the subsequent snow cover there. The negative snow cover anomaly may persist into spring and induce a local anomalous anticyclone in spring via the snow-hydrological effect, which decreases the precipitation over the southern flank of the anomalous anticyclone. The decreased soil moisture persists into summer and induces the eastward propagating Rossby wave, and favors the increase of atmosphere thickness over northern China. The summer nTEI SSTA can also induce the anomalous anticyclone over northern China via the northeastward Rossby wave propagation. Thus, the two factors exhibit evident synergistic effect on the atmospheric circulation anomaly over northern China. The anomalous anticyclone corresponds to the increased atmosphere thickness, which favors the increase of air temperature in northern China and strengthening of extreme heat events. Therefore, the preceding winter pMNA SSTA and summer nTEI SSTA have significant synergistic effect on strengthening the summer extreme heat events in northern China.