Cluster occurrence of extreme rainfall events over Indo-Pacific and their associated diverse Rossby wave patterns

Clustering extreme rainfall events are successive occurrences over multiple regions. As climate continues to warm up, cluster occurrence is becoming a prevailing feature of extreme weather events and leading to significant socioeconomic impacts. Understanding the associated atmospheric teleconnection patterns and their underlying mechanisms can help quantify their risk, i.e., the probability of occurrence and severity of cluster extremes in the future. In this study, we identified over 400 events of clustering extreme rainfall events over South Asia, East Asia, and North America in the past 42 years. Diagnostic analyses of these events reveal the diversity of teleconnection that paved the road to the events. Three Rossby wave patterns: (1) circum-Pacific Rossby wave, (2) cross-Pacific Rossby wave, and (3) Pacific anticyclone Rossby wave breaking, are the major synoptic-scale dynamics responsible for clustering rainfall events. Specifically, the circum-Pacific Rossby wave dominates in autumn and early winter, while the cross-Pacific Rossby wave pattern prevails during the Indian summer monsoon season. The occurrence frequency of the anticyclone Rossby wave breaking does not show significant seasonal differences.

The key driving mechanisms behind these wave patterns are: 1) The poleward propagation of the circum-Pacific wave can be excited by the heating anomaly originating in the tropics. 2) The mid-latitude cross-Pacific Rossby wave is a portion of the circum-global teleconnection pattern. This recurrent Rossby wave connects Asia and North America, influenced by the Indian summer monsoon. 3) Pacific anticyclone Rossby wave breaking is a quasi-stationary synoptic wave pattern causing persistent extreme weather. The frequency of this pattern increases significantly during La Niña years with a relatively weak subtropical jet. The single or synergistic effects of these three patterns cause the cluster occurrence of extreme rainfall. Findings from this work offer a better understanding of rainfall teleconnection and tropic/midlatitude interaction.