Forest ecosystems in the north central and northeastern U.S. account for about 23% of the total forested area in the U.S and 47% of the nation’s hardwood inventory. These forest ecosystems provide a wide variety of economic, social, biological, and physical benefits for the region and the entire world. The health and sustainability of forest ecosystems in the north central and northeastern U.S. and world-wide is dependent on the atmospheric environments in which these forest ecosystems exist. As part of the USDA Forest Service’s Northern Global Change Research Program (NGCRP) and its effort to examine how multiple interacting stresses are affecting or are likely to affect forest ecosystems at multiple spatial and temporal scales, research has been conducted on identifying the relevant spatial and temporal patterns of climate, climate variability, and atmospheric disturbances over the north central and northeastern U.S. that impact the region’s forested ecosystems. This particular paper provides an overview of research conducted on short-term (daily to monthly) precipitation variability in the region during the different seasons and the implications for forest health and sustainability in the region.
Daily precipitation data from the National Climate Data Center’s Summary of the Day data base for about 1200 observation sites throughout the NGCRP region for the period 1950-1993 have been used in the analyses. The precipitation data at each observation site were segmented into four seasonal data sets, with each data set covering a three-month period. A power spectrum analysis using the fast Fourier transform and data windowing was performed on each station-season precipitation data set to estimate the power spectral density at periods ranging from 2-32 days. The average maximum power values in the 2-4 day, 4-8 day, 8-16 day, and 16-32 day periods for each observation site and season were then determined to provide an indication of the relative significance of winter, spring, summer, and fall precipitation events with these defined periods across the NGCRP region. Similar analyses were performed on precipitation events for each season during past El Nino episodes to assess potential changes in short-term precipitation variability over the region during these episodes.
Results from the analyses indicate that spatial variations in the significance of 2-4 day, 4-8 day, 8-16 day, and 16-32 day precipitation variability modes exist across the NGCRP region, and that the spatial variations are seasonally dependent. During the winter months, the 16-32 day precipitation variability mode is most significant over the western sections of the NGCRP region (Minnesota, Wisconsin, Iowa, and Missouri), while the shorter 2-4 day and 4-8 day variability modes characterize the Appalachian and Atlantic coastal states during the winter. During the summer months, the significance of the 2-4 day variability mode over the central and northern Atlantic coastal states is reduced, and is enhanced over much of the rest of the NGCRP region. Short-term precipitation variability changes during El Nino episodes were found to be statistically significant over portions of the Ohio River Valley during the winter months.