Two different propagation patterns of spatiotemporally contiguous heatwaves in China

Heatwaves detrimentally affect human health and ecosystems. While previous studies focused on either temporal changes or spatial extents of heatwaves, their spatiotemporal contiguity and propagation patterns are unclear. Here, we investigate the climatology, long-term trends, and interannual variations of spatiotemporally contiguous heatwaves across China during 1961 to 2018. Two distinct propagation patterns are identified by introducing a spatiotemporally contiguous events tracking (SCET) method and the k-means clustering. Type 1 contiguous heatwaves mostly generate over eastern China and move southwestward at shorter distances, while Type 2 heatwaves generally initiate over western China and move southeastward at longer distances, with stronger intensity, longer lifetime, and larger coverage. Since the 1960s, both types exhibit significant increases in frequency, intensity, coverage, lifetime, and traveling distances, with relatively larger magnitudes for Type 2, but significant decreases in moving speed only appear for Type 1. On interannual timescale, preceding winter El Niño prolongs the duration and slows down the moving speed of Type 1 through the western North Pacific subtropical high. Type 2 contiguous heatwaves are intensified by the negative spring North Atlantic Oscillation via the upper-atmospheric Rossby wave train.