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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.

The late spring/early summer flooding that occurred in the American Midwest between May and June 2008 resulted from a combination of large‐scale atmospheric circulation patterns that supported a steady influx of moisture into the area. A low pressure system centered over the central‐western United States steered a strong jet and associated storms along its eastern edge from the west to southwest and an anomalously strong Great Plains Low Level Jet brought continuous warm and moist air into the area from the Gulf of Mexico into the area. We examine and quantify here the impact these circulation patterns had on the hydroclimatology of the Midwest highlighting the magnitude, frequency, geographic distribution, and temporal evolution of precipitation that ultimately magnified the flooding. Historical precipitation records were used to assess the regional rainfall characteristics at various geographic and time scales. Five distinct hydroclimatic characteristics contributed to the definition of the 2008 flood including persistent high surface soil moisture conditions prior to flooding exasperated by anomalously high rainfall, extreme rainfall totals covering extensive areas, increased frequency of shorter‐term, smaller‐magnitude events, persistent multiday heavy precipitation events, and extreme flood‐producing rain storms. The major flooding lasted for approximately 24 days and most greatly impacted the state of Iowa, southern Wisconsin, and central Indiana. Its occurrence during the May–June period makes the event especially unusual for this region.

Associated synoptic patterns of the Pacific Decadal Oscillation (PDO) significantly impact relationships between surface air temperature (SAT) and the Arctic Oscillation (AO) across the United States in winter. Using historical data (1900 to 2002) and composite analyses, this study demonstrates that without the influence of the PDO, winter SATs are most significantly altered during various phases of the AO throughout the Ohio Valley region and in the South. More specifically, extreme phases of the PDO significantly alter SAT responses to positive phases of the AO west of the Cascades, in the Ohio Valley region, and along the northeast coast of the US. SAT regimes related to negative phases of the AO are significantly modified by the different phases of the PDO across the northern Great Plains, and throughout the West. When the AO and PDO are both negative, winters are typically significantly cooler throughout the upper Midwest, the Great Plains and in the Northwest, in comparison to winters when PDO has been neutral. When the 2 indices are out of phase, winters are cooler (warmer) than during neutral PDO years west of the Cascades and warmer (cooler) east of the mountain range during the AO + /PDO -(AO -/PDO + ) conditions. Extreme phases of the PDO modify the north-south structure of the mean sea level pressure (SLP) field over the Northern Hemisphere, these changes being consistent with the modifications observed in the SAT patterns across the study area.

A growing amount of evidence points to a notable linkage between the changing Arctic cryosphere and weather in the middle latitudes of the Northern Hemisphere. Recent studies propose a series of mechanisms that make plausible the connection between Arctic amplification/sea ice decline and extreme weather. Using composite analyses, this study examines associations between the frequency of occurrence of boreal summer daily extreme surface air temperatures across North America and simultaneous mean Arctic sea ice concentration (SIC) conditions during the period 1979−2013. Four distinct regions show coherent relationships including large sections of the eastern USA, Canada and the Canadian Arctic, central North America, southeast USA, and the west coast from southern Canada to Alaska. Across the eastern USA and Canada, as well as in western North America, the connections are principally shaped by low ice conditions with an expected decline in the incidence of cool nights/days and an increase in the incidence of warm nights/days. The ice−temperature relationships observed in the other regions are mostly shaped by high ice conditions. Synoptic analyses indicate the associations to be reflected in mean summer surface air temperature (SAT) and surface anomaly flows, as well as in the 500 and 200 hPa geopotential height flow and mean zonal wind anomaly patterns. Areas with the greatest atmospheric flow modifications have been generally associated with regions that display most notable extreme temperature frequency modifications.

Arctic sea ice has been shrinking at unprecedented rates over the past three decades. These cryospheric changes have coincided with greater incidence of global extreme weather conditions, including increased severity and frequency of summer heatwaves and extreme rainfall events. Recent studies identify potential physical mechanisms related to Rossby wave and resonance theories that may attribute the observed changes in extreme summer weather patterns to Arctic sea ice decline. This study explores the linkages between summer Arctic sea ice variability and hydroclimate of the north‐central United States (US) during the 1979 to 2013 period. Since 1979, summers with low sea ice conditions have coincided with significant increases in mean, minimum, maximum, and dew point air temperatures. Also apparent are increases in seasonal precipitation, the number of wet days, heavy (>95th percentile) precipitation days, and accumulated precipitation over the region. These moisture changes coincide with atmospheric patterns typically observed during anomalously wet summers, known to prompt flooding across the Upper Mississippi River Valley (UMRV) region. Low sea ice summers have coincided with (1) enhanced southerly air flow and increased activity of the Great Plains Low Level Jet (GPLLJ) over the study area, (2) increased occurrence of moist tropical air masses over the UMRV region, and (3) amplified 500 hPa flow over the Pacific‐North American region with a ridge situated over the central‐eastern portions of the North American continent emanating from Greenland and the central Arctic basin. The results suggest summer Arctic sea ice variability has been associated with recent hydroclimate anomalies of the north‐central United States and the UMRV region and add to our growing knowledge of the connections between a changing Arctic environment and concurrent mid‐latitude climate variability.

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