Regional Variation of Transient Precipitation and Rainless-day Frequency Across a Subcontinental Hydroclimate Gradient

Abstract

Extreme daily and transient precipitation events have been on the rise in the continental United States. These changes have the potential to disrupt human and natural systems. Tree rings can reconstruct annual estimates of past climate, but have limitations in reconstructing extreme precipitation events. We analyzed instrumental records to evaluate patterns in daily, weekly, and seasonal precipitation in four regions spanning a climatic gradient in the eastern United States. Relationships between tree-ring reconstructions of hydroclimate and precipitation events were analyzed to characterize extreme years over the last 1000 years. From 1944–2013, the Hudson Valley and Ohio Valley regions have experienced wetter summers as well as an increase in the frequency of daily rainfall. Coinciding with these increases, half or more of the extreme wet years in these two regions have occurred in the last 20 years. Significant differences in the structure of weekly growing-season precipitation between extreme wet and dry years were found in late May and late June in the Ohio Valley and early June in the Mississippi Valley, with negligible differences in the northern and southern ends of our gradient. We also found dry-spell duration was significantly different between extreme wet and dry years in all regions except for the northern end of our study gradient. In contrast, dry-spell frequency was significantly different between extreme wet and dry years in all regions except for the southern end of our gradient. Reconstructed Palmer Drought Severity Index (PDSI) was significantly and positively correlated with total summer rainfall and significantly and negatively correlated with rainless-day frequency in all regions, with stronger correlations during extreme years. Working in the region with the strongest relations, we reconstructed summer precipitation and summer rainless days in the Hudson Valley back to 1525 CE and 1625 CE, respectively. From these reconstructions, we infer that the 20th century is characterized by more extreme summer precipitation totals and a decline in rainless days with 75.8% of the last 33 years having fewer dry days than the 377-year mean. The forecasted changes toward longer, more intense droughts over the next century are not yet realized in our study regions. However, should these shifts occur, human and natural systems will likely undergo abrupt change in response to alterations in hydrology, ecological disturbance, and terrestrial productivity, with the Northeast potentially being most vulnerable.