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Paul A. Knapp

Lewis and Clark's entry into to the American West in search of an inland Northwest Passage is considered among the greatest expeditions in American history. The Corps of Discovery were also lucky as their travels west of the 100th meridian occurred during a “window” of generally favorable climatic conditions. Use of reconstructed summer Palmer drought severity index (PDSI) values from 1700–1978 indicate that drought frequency at locations along the Lewis and Clark trail ranged from 4 to 12 yr and that the probability of encountering a drought either on the outbound or return trip approached 50% at some locations. Exact date comparisons of meteorological conditions during periods of extended encampment (i.e., 1–5 months) between 1804–06 with long-term records of nearby weather stations indicate that the Corps of Discovery avoided drought and traveled during a cooler and/or substantially wetter period than historical averages. Examination of reconstructed Southern Oscillation index (SOI) and Pacific decadal oscillation (PDO) values suggest wetter conditions prevailed in 1804–06 because of the co-occurrence of La Nina conditions during a cold PDO phase. Although the Corps of Discovery suffered hardships because of the wetter conditions, they avoided the more serious consequences of severe droughts that occurred in 1800 and 1808. Drought conditions along the semiarid and arid portions of the trail would have reduced forage yield for the game that were their principal source of food and increased their chances for starvation. Additionally, lower streamflow conditions along their principal navigation routes would have required greater effort and time to haul their supplies to the Continental Divide, perhaps delaying their expedition by a year.

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Paul A. Knapp and Peter T. Soulé

Abstract

In mid-autumn 2002, an exceptional 5-day cold spell affected much of the interior Pacific Northwest, with minimum temperatures averaging 13°C below long-term means (1953–2002). On 31 October, minimum temperature records occurred at 98 of the 106 recording stations, with records lowered in some locations by 9°C. Calculation of recurrence intervals of minimum temperatures shows that 50% of the stations experienced a >500-yr event. The synoptic conditions responsible were the development of a pronounced high pressure ridge over western Canada and an intense low pressure area centered in the Intermountain West that promoted strong northeasterly winds. The cold spell occurred near the end of the growing season for an ecologically critical and dominant tree species of the interior Pacific Northwest—western juniper—and followed an extended period of severe drought. In spring 2003, it became apparent that the cold had caused high rates of tree mortality and canopy dieback in a species that is remarkable for its longevity and resistance to climatic stress. The cold event altered western juniper dominance in some areas, and this alteration may have long-term impacts on water budgets, fire intensities and frequencies, animal species interrelationships, and interspecific competition among plant species.

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Peter T. Soulé and Paul A. Knapp

Abstract

Climatic singularities offer a degree of orderliness to notable meteorological events that are typically characterized by significant temporal variability. Significant deviations from normal daily maximum temperatures that occur following the passage of a strong midlatitude cyclone in mid- to late August in the northern Rocky Mountains of the United States are recognized in the local culture as the “August Singularity.” Daily standardized maximum temperature anomalies for August–October were examined for eight climate stations in Montana and Idaho as indicators of major midlatitude storms. The data indicate that a single-day negative maximum temperature singularity exists for 13 August. Further, a 3-day singularity event exists for 24–26 August. It is concluded that the concept of an August Singularity in the northern Rockies is valid, because the high frequency of recorded negative maximum temperature anomalies suggests that there are specific time intervals during late summer that are more likely to experience a major midlatitude storm. The principal benefit to society for the August Singularity is that the reduced temperatures help in the efforts to control wildfires that are common this time of year in the northern Rockies.

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Paul A. Knapp, Peter T. Soulé, and Henri D. Grissino-Mayer

Abstract

The occurrence of moderate and severe sustained droughts in the interior Pacific Northwest (PNW) from 1733 to 1980 was mapped using 18 western juniper (Juniperus occidentalis var. occidentalis Hook.) tree-ring chronologies. The frequency and duration of both moderate and severe sustained droughts are substantially greater in the northwest region of the interior PNW. Thus, this area is identified as a drought core region. These droughts are chiefly attributed to the presence of a Pacific blocking high off the NW coast that is associated with significant reductions in cool season precipitation. Specifically, the impacted northwest region lies within a transition zone between the fluxes of marine airflow during the cool season months and interior air during the warm season months. The waxing and waning of the boundaries of this transition zone particularly affect western juniper trees growing in this region. During years in which a blocking high is present, marine airflow is substantially reduced, exposing the trees in the transition zone to substantially drier springtime conditions that limit soil moisture and reduce radial growth. Although the most severe and persistent droughts were concentrated in the northwest region, four large-scale droughts also impacted the entire interior PNW during the study period. These droughts occurred principally during PDO warm phases, suggesting a regional-scale linkage to this climatic oscillation.

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Jason T. Ortegren, Paul A. Knapp, Justin T. Maxwell, William P. Tyminski, and Peter T. Soulé

Abstract

From the 344 state climate divisions in the conterminous United States, nine distinct regions of warm-season drought variability are identified using principal component analysis. The drought metric used is the Palmer hydrological drought index for the period 1895–2008. The focus of this paper is multidecadal drought variability in the Southeast (SEUS) and eastern Gulf South (EGS) regions of the United States, areas in which the low-frequency forcing mechanisms of warm-season drought are still poorly understood. Low-frequency drought variability in the SEUS and EGS is associated with smoothed indexed time series of major ocean–atmosphere circulation features, including two indices of spatiotemporal variability in the North Atlantic subtropical anticyclone (Bermuda high). Long-term warm-season drought conditions are significantly out of phase between the two regions. Multidecadal regimes of above- and below-average moisture in the SEUS and EGS are closely associated with slow variability in sea surface temperatures in the North Atlantic Ocean and with the summer mean position and mean strength of the Bermuda high. Multivariate linear regression indicates that 82%–92% of the low-frequency variability in warm-season moisture is explained by two of the three leading principal components of low-frequency variability in the climate indices. The findings are important for water resource managers and water-intensive industries in the SEUS and EGS. The associations identified in the paper are valuable for enhanced drought preparedness and forecasting in the study area and potentially for global models of coupled ocean–atmosphere variability.

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Justin T. Maxwell, Jason T. Ortegren, Paul A. Knapp, and Peter T. Soulé

Abstract

Precipitation from land-falling tropical cyclones (TCs) has a significant hydroclimatic influence in the southeastern United States, particularly during drought years. The frequency with which TCs ended drought conditions was examined for southeastern coastal states from Texas to North Carolina during 1895–2011. The region was divided into the Gulf Coast states (GCS) and the southeastern Atlantic coast states (ACS). The spatiotemporal patterns of tropical cyclone drought busters (TCDBs) were analyzed. Larger-scale ocean–atmosphere influences on TCDBs were examined using chi-squared analysis. The ACS experienced TCDBs more frequently and farther inland compared to the GCS. The number of TCDBs has significantly increased with time in the ACS. TCDBs numbers in the GCS did not exhibit significant increases, but the area alleviated of drought conditions increased significantly in the last 117 years. The dominant larger-scale ocean–atmosphere forcing of TCDBs was a combination of a warm Atlantic Ocean [positive Atlantic multidecadal oscillation index (AMO+)] and weak westerlies [negative North Atlantic Oscillation index (NAO−)]. AMO+ leads to an increase in the number of TCs throughout the North Atlantic basin, and NAO− increases the likelihood of TC landfall by controlling the steering of TCs toward the southeastern United States.

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Joshua C. Bregy, Justin T. Maxwell, Scott M. Robeson, Jason T. Ortegren, Peter T. Soulé, and Paul A. Knapp

Abstract

Tropical cyclones (TCs) are an important source of precipitation for much of the eastern United States. However, our understanding of the spatiotemporal variability of tropical cyclone precipitation (TCP) and the connections to large-scale atmospheric circulation is limited by irregularly distributed rain gauges and short records of satellite measurements. To address this, we developed a new gridded (0.25° × 0.25°) publicly available dataset of TCP (1948–2015; Tropical Cyclone Precipitation Dataset, or TCPDat) using TC tracks to identify TCP within an existing gridded precipitation dataset. TCPDat was used to characterize total June–November TCP and percentage contribution to total June–November precipitation. TCP totals and contributions had maxima on the Louisiana, North Carolina, and Texas coasts, substantially decreasing farther inland at rates of approximately 6.2–6.7 mm km−1. Few statistically significant trends were discovered in either TCP totals or percentage contribution. TCP is positively related to an index of the position and strength of the western flank of the North Atlantic subtropical high (NASH), with the strongest correlations concentrated in the southeastern United States. Weaker inverse correlations between TCP and El Niño–Southern Oscillation are seen throughout the study site. Ultimately, spatial variations of TCP are more closely linked to variations in the NASH flank position or strength than to the ENSO index. The TCP dataset developed in this study is an important step in understanding hurricane–climate interactions and the impacts of TCs on communities, water resources, and ecosystems in the eastern United States.

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