Frequency anomalies and characteristics of extratropical cyclones during extremely wet, dry, windy and calm seasons in the extratropics

Abstract. Extreme meteorological seasons are highly relevant because of their severe impacts on many socioeconomic sectors. However, a global statistical characterisation of observed extreme seasons is challenging, because at any specific location only very few such seasons occurred during the limited period with available reanalysis data sets. This study therefore uses 1050 years of present-day (1990–1999) climate simulations of the Community Earth System Model Large Ensemble (CESM-LE) to systematically identify extremely wet, dry, windy and calm seasons in the Northern Hemisphere (NH) and Southern Hemisphere (SH) extratropics during winter and summer, and to quantify the role of extratropical cyclones for their occurrence. Extreme seasons are defined as spatially coherent regions of extreme seasonal mean precipitation or near-surface wind. Compared to the climatology, extremely wet seasons are associated with positive anomalies in cyclone frequency in large parts of the extratropics. In the SH storm track and at the downstream ends of the NH storm tracks, cyclones contributing to wet winters are also anomalously intense and typically originate unusually far to the west and south, while in the subtropical North Atlantic and over the eastern Mediterranean they are on average more stationary than climatologically. During wet summers, many continental regions are not associated with strong anomalies in any of the cyclone characteristics (e.g., most of North America, the coastal regions around the Mediterranean Sea and southern Asia), which points to the importance of other processes like convection, orographic ascent or, over southern Asia, monsoon precipitation. Windy seasons are often associated with anomalously few, but particularly intense cyclones, especially during winter. Positive anomalies in both cyclone frequency and intensity are found in the southern North Atlantic during winter, which suggests that windy winters in this region occur during southward shifts in the position of the main storm track. The patterns of dry and calm seasons mainly contrast with those of wet and windy seasons, i.e., they are often characterised by particularly few or weak cyclones or a combination thereof. In all four types of extreme seasons, there is remarkably large spatial and seasonal variability in the cyclone properties, especially over the continents. In addition to the systematic analysis based on the climate model, two past extreme seasons have been studied with ERA5 reanalyses over the United Kingdom, the wet winter 2013/14 associated with anomalously many and intense cyclones and the windy winter 1988/89 associated with anomalously few, but intense cyclones. The results are consistent with those from the climate model for this region, suggesting that the model captures the cyclone properties reasonably well during extreme seasons. Overall, it can be concluded that (i) anomalies in the seasonal frequency and/or intensity distribution of extratropical cyclones are crucial for the occurrence of many extreme seasons in the extratropics, and (ii) this link shows substantial geographical and seasonal variability.