The extremely wet spring of 2022 in Southwest China was driven by La Nina and Tibetan Plateau warming

In the spring of 2022, an excessive amount of rainfall fell in Southwest China (SWC) under the background of frequent droughts in history. This extreme event occurred in the decaying phase of a La Nina event, and thus, presumably La Nina played a role in this extreme event. Based on observational diagnoses and model forecasts, the atmospheric circulation anomalies, contributions of remote forcing, and the predictability of this event were examined in this work. It is suggested that La Nina and the Tibetan Plateau upper-tropospheric warming are two major factors leading to the extreme event. In addition to the recognized impact of La Nina, the upper-tropospheric warming over the Tibetan Plateau modulates the Asian atmospheric circulation by inducing a northwest-southeast wave pattern extending from the Ural Mountains to the Indochina Peninsula via the western Tibetan Plateau. The meridional heat contrast associated with the Tibetan Plateau warming favors upward motion and excessive rainfall in SWC. The statistical connection between the SWC spring rainfall anomaly and the northwest-southeast wave pattern is confirmed by a climate model forecast. The model captured the wet pattern in SWC in spring 2022 in short (1-3 months) lead real-time predictions though there are biases in the area and severity. That may be due to that the model did not well capture the atmospheric circulation anomalies at the middle and high latitudes associated with the Tibetan Plateau upper-tropospheric warming. These results indicate that such an event is predictable to some extent if both the ENSO evolution and heat condition over the Tibetan Plateau can be well predicted.