资源环境科技发展态势分析平台

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.

Plain Language Summary The northeastern United States received 53% more annual extreme precipitation after 1996 than in the years from 1901-1995. In this study, we explore the causes of the extreme precipitation increase by examining a subset of heavy precipitation days that account for the vast majority of the overall change in extreme precipitation across the 1996 shift. The months of September and October contributed the most to the overall extreme precipitation increase, due primarily to an increased frequency of extreme events caused by tropical cyclones. Extreme events caused by fronts and extratropical cyclones also contributed substantially to the overall extreme precipitation increase, especially during the months of June, July, and March. Increased extreme precipitation from tropical cyclones is associated with warmer Atlantic sea surface temperatures and more water vapor in the atmosphere. Enhanced extreme precipitation from fronts and extratropical cyclones is coincident with a wavier jet stream and potentially connected to weaker west-to-east steering winds. By describing how, when, and why extreme precipitation has increased over the Northeast, this study can help a variety of stakeholders, including towns, emergency responders, and utilities, adapt to the current regime of extreme precipitation, as well as inform future work attributing this extreme precipitation shift.