Modulation of Variability and Projected Change in Winter Precipitation by Extratropical Cyclone Activity over Mid-latitude Regions

Because most of the mid-latitude winter precipitation is due to a passage of extratropical cyclones, the relationship between extratropical cyclones and precipitation has significant implications for the mid-latitude climate. Therefore, quantifying the relationship between extratropical cyclone activity (denoted by pp hereafter) and precipitation at various different mid-latitude locations allows comparisons that can yield significant insights. Specifically, comparing the degree to which extratropical cyclones modulate winter precipitation in fourteen different mid-latitude regions yields insights into the relationship between pp and precipitation. Furthermore, comparing the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Community Earth System Model (CESM) projections from both historical and future runs can help explain the causes of inter-model variability and the impacts of such variability on imitating the observed relationships between pp and precipitation in each region.

In this study, the National Center for Environmental Prediction (NCEP) observed sea level pressure and precipitation data between the years of 1979 and 2015 was utilized to explore the relationship between extratropical cyclones and precipitation in the mid-latitude regions. In each region, the future (2061 – 2099) CMIP5 and CESM precipitation projections were compared to a ‘prediction' based on the relationship between pp and precipitation in the historical (1961 – 2000) time period.

The results of this study indicate a strong modulation of precipitation by extratropical cyclones in many mid-latitude regions, but not all. In regions of more significant modulation, the climate models imitate the relationship between pp and precipitation more closely. Further results suggest that CMIP5 and CESM ‘predicted' precipitation values are often different from their actual projections. These results suggest that inter-model variability may have a strong effect on future precipitation projections.