Synoptic-scale atmospheric circulation anomalies associated with summertime daily precipitation extremes in the middle–lower reaches of the Yangtze River Basin

Abstract

The mechanisms of short-lived precipitation extremes in boreal summer over the middle–lower reaches of the Yangtze River Basin (MLYRB) during 1961–2014 are explored using gridded observational and reanalysis datasets. Daily precipitation extremes are defined by the 75th and 95th percentiles and identified for selected regions in the MLYRB. Moisture budget analysis is utilized to quantify the major factors responsible for the variability of extreme precipitation events. Then the atmospheric variables are composited according to these extreme events, to illustrate the temporal evolution of the corresponding synoptic-scale structures. The results show that moisture flux convergence plays a dominant role in the variations of extreme rainfall for both percentile events, and the contribution of evaporation is not evident. Moreover, the dynamic component associated with changes in atmospheric circulation makes a larger contribution than the nonlinear component, followed by the thermodynamic component, owing to changes in specific humidity. The moisture transport pathways increase with increasing magnitude of extreme rainfall intensity. Circulation field anomalies move into the MLYRB during the onset of precipitation extremes for both percentile cases, with stronger anomalies occurring for the 95th percentile cases. Furthermore, the water vapor fluxes transported from the Northwest Pacific are pronounced during the 95th percentile events. A cross-section of specific humidity and vertical gradient of equivalent potential temperature further indicates that dynamic properties play a crucial role in the development of extreme precipitation events linked with quasi-stationary frontal activities across the MLYRB during the past few decades. Composites of Eady growth rate and outgoing longwave radiation anomalies reveal that extratropical cyclones passing through the MLYRB produce a large amount of extreme rainfall by synoptic disturbance.