Quantifying the Role of Rossby Wave Breaking in Mediterranean Precipitation Events between the Early and Late 21st Century within the CESM Large Ensemble 2.

Highly amplified Rossby waves are subject to breaking when their propagation speed slows in the vicinity of the jet stream. Known as Rossby wave breaking (RWB), these events frequently occur in the highly-sheared exit regions of the North Pacific and North Atlantic jets. They can generate substantial tropospheric impacts including surface temperature anomalies and precipitation extremes. RWB is frequently observed across the Mediterranean region, which has a semi-arid climate that is vulnerable to changes in the frequency and intensity of precipitation events. Previous studies have shown a projected drying of the Mediterranean in the future, but the role of RWB events in this drying has yet to be quantified. RWB events (classified as anticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB)) are identified on the Northern Hemisphere dynamic tropopause (DT) using the Community Earth System Large Ensemble v2 (CESM-LENS2) during December, January, and February (DJF) 1999-2014 (historical period) and DJF 2085-2100 (future period). Up to 15-25% (40%) of Mediterranean precipitation (extreme) days are found to be associated with AWB in the historical period. Future changes in precipitation and drought associated with RWB for this region are explored by investigating: the role of thermodynamic impacts of a warming climate, shifts in RWB regions in response to changes in the location and speed of the jet, and differences in the tropospheric structures of RWB events. By quantifying the role of RWB in generating Mediterranean precipitation now and into the future, we hope to further our understanding of projected future changes in precipitation in this region.