Mechanisms of the Extreme Precipitation Jump in the Northeastern United States after 1996

Extreme precipitation over the Northeastern United States, defined as the top 1% of wet days by precipitation amount, increased abruptly by 53%, after 1996 compared to 1901–1995. Past studies have established some consensus on the large-scale atmospheric features associated with this change; however, the climatological mechanisms leading to the 1996 shift to a more extreme precipitation period remain unclear. This study explores the large-scale circulation patterns responsible for the abrupt increase in extreme precipitation after 1996 by addressing the following questions: How have different meteorological causes (e.g., tropical cyclones, extratropical cyclones, and fronts) contributed to these extreme precipitation events? What are the relationships between changes in two climate indices (i.e., NAO, AMO) and extreme precipitation increase?

To address these questions, we first identified extreme precipitation events using station observations for the years before and after the 1996 jump that overlap with ERA-Interim reanalysis (i.e. 1979–1995 vs. 1996–2016). We then attributed each event to a meteorological cause using weather maps and hurricane track data when appropriate. Next, we analyzed the large-scale meteorological fields on extreme precipitation event days in both periods and the relationships between the events, large-scale meteorological fields, and two climate modes using ERA-Interim reanalysis and ocean observations. We find that September and October are the months mainly responsible for the jump in annual extreme precipitation, and tropical cyclones are the primary driver of the abrupt increase in the season, followed by fronts and then extratropical cyclones. We further explore the large-scale atmospheric drivers of these events using a series of composite maps of influential meteorological variables.