Mechanisms of Abrupt Extreme Precipitation Change Over the Northeastern United States

In 1996, the northeastern United States experienced an abrupt increase in extreme precipitation, but the causal mechanisms driving this increase remain poorly understood. We find that 89% of the 1996–2016 increase relative to 1979–1995 is explained by only 273 unique extreme events affecting >5 stations and occurring in the months of February, March, June, July, September, and October. We use daily weather maps to classify the 273 extreme precipitation events by meteorological cause (tropical cyclones, fronts, and extratropical cyclones) and use reanalysis data to determine large‐scale changes in the atmosphere and ocean associated with increased extreme precipitation for each classification. Results show that tropical cyclones account for almost half (48%) of the post‐1996 extreme precipitation increase, while fronts and extratropical cyclones are responsible for 25% and 15% of the increase, respectively. The remaining 11% is from extreme events in the other 6 months of the year and extreme events that affected <5 stations. The increase in extreme precipitation from tropical cyclones after 1996 is associated with a shift to the Atlantic Multidecadal Oscillation warm phase, higher total column water vapor, and potentially weakened steering winds. September and October tropical cyclones caused significantly more extreme precipitation during the current Atlantic Multidecadal Oscillation warm phase (1996–present) than during the last warm phase (1928–1962), despite the same number of northeast tropical cyclones in both periods. Increased extreme precipitation from fronts is associated with a wavier (higher amplitude) jet stream, which likely facilitates the development of more frequent fronts through the advection of cool northern air into the American Midwest.