Winter streamgage data at 554 locations in the U.S. Geological Survey's Hydro‐Climatic Data Network are used to evaluate interactions between the Atlantic multidecadal oscillation (AMO), the Pacific/North American (PNA) teleconnection pattern, and warm and cold equatorial Pacific Niño 3.4 events in producing Mississippi River basin discharge variations. The AMO is linked to statistically significant stream flow differences over the upper Mississippi Valley, as well as low (high) winter stream flow from 1930–1959 (1965–1994). Its influence in the lower Mississippi Valley is however not as apparent because of 4 consecutive wet winters, 1949–1952, that disrupt the AMO warm/dry period. Persistent PNA zonal flow, and a La Niña event, occur during these winters and they are closely linked to high Tennessee and Ohio River valley stream levels. PNA‐influenced stream flow on these rivers has a greater long‐term winter impact on the lower Mississippi basin than does flow from the upper Mississippi.

No abstract

A synoptic climatological weather classification scheme incorporating both surface and upper-air data is developed for the central United States based on an automated two-step cluster analysis. It employs daily NCEP-NCAR reanalysis data over all seasons of 57 yr in creating synoptic types from surface and upper-air (925, 850, 700, and 500 hPa) temperature and humidity data as well as sea level pressure, geopotential heights, and winds aloft. The cluster analysis creates 10 synoptic types exhibiting distinct seasonal preferences, with three each that occur primarily in summer and winter, and four that occur primarily in winter and the transition seasons, particularly spring. The typing scheme generates synoptic patterns largely characterized by distinctive surface circulations, baroclinic vertical structure, and thermal advection. Interannual variations occur in the frequencies of the synoptic types, some of which are out of phase with each other. The annual frequencies of two winter synoptic types, associated respectively with strong zonal and meridional flow, are highly correlated (r k 0.63-0.73) to the phase of the Pacific-North American teleconnection pattern, while Niño-3.4 equatorial Pacific sea surface temperatures are linked to a synoptic type producing low pressure around the Gulf Coast.

Numerous studies have suggested streamflow discharge in the conterminous U.S. has been increasing, particularly in the east starting in the latter half of the 20th century. Northern Hemisphere (NH) hydroclimatic variability has been connected to shifts in large‐scale atmospheric teleconnection patterns. This study ascertained the spatial and temporal influences of the extreme phases of the North Atlantic Oscillation (NAO) on interannual streamflow variability across the eastern U.S. during the summer season. We also assessed the presence of any delayed streamflow responses to NAO configurations during previous spring (March‐April‐May) and winter (December‐January‐February) seasons across the study area. Methods of inquiry included the use of various statistical and compositing analyses applied to records of mean daily summer streamflow obtained from the Hydro‐Climatic Data Network between 1950 and 2010. Results of this study suggest that summer streamflow across eastern U.S. may be related to and thus potentially predicted from the NAO up to three seasons in advance, information of significant hydrologic consequences for the area. Depending upon the season and geographic location, the relationships may be both linear and nonlinear in nature and may be masked by the presence of temporal trends in the index. Summer streamflow across the eastern U.S., in general, displays greatest response to the negative phase of the NAO during the previous winter and the concurrent summer seasons. Geographically, the greatest potential for predictability exists across the Northeast where cohesive responses to NAO exist in all three seasons, winter, spring, and summer.

This paper identifies and documents the major large-scale atmospheric circulation characteristics that preceded and facilitated the devastating floods across the Midwestern United States along the upper Mississippi, Missouri, and Wabash River basins in the late spring/early summer of 2008. These circulation features were also placed in the context of the 1993 floods that occurred within a similar temporal and spatial domain. This process included investigating the relationship between various atmospheric conditions and the timing of flood-producing rainfall events at monthly and sub-monthly timescales. Unlike in 1993, the 2008 flood event took place 1 month earlier in the year (May), lasted over a much shorter time period, and extended eastwards into Indiana. The comparison confirmed much previous work regarding the factors associated with Midwest flooding during the early warm-season and revealed the possible influence of the state of the North Atlantic Ocean and the North Atlantic oscillation (NAO) on warm-season flooding across the Midwest. Both the 2008 and 1993 events were preceded by an NAO positive phase; the pattern switched phase about 2 months before the onset of the flooding and remained negative for the duration of the floods. While the major atmospheric conditions known to be associated with major flooding across the Midwest were present in 2008 (e.g. a strong Great Plains low-level jet), the level of their development, persistence, and geographic orientation/position was very distinct from those observed in 1993. These conditions were also embedded in different state of the remote hemispheric circulation over the Pacific North American region. Together, these variations contributed to the 2008 Midwest floods that occurred earlier in the warm season, were shorter-lived, and had an impact on a slightly different geographic area.