Falko K. Fye

Paleoclimatic Analogs to Twentieth-Century Moisture Regimes Across the United States

Abstract Instrumental Palmer Drought Severity Indexes (PDSI) averaged over the western United States and Great Plains document three major decadal moisture regimes during the twentieth century: the early twentieth-century pluvial, the Dust Bowl drought, and the 1950s drought. Tree-ring reconstructed PDSI for the contiguous Unites States replicates these three twentieth-century moisture regimes, and have been used to search for possible analogs over the past 500 yr. The early twentieth-century wet regime from 1905 to 1917 appears to have been the wettest episode across the West since A.D.1500, but similar pluvials occurred in the nineteenth, seventeenth, and sixteenth centuries. The Dust Bowl drought (1929–40) was most severe over the northern Plains to the northern Rockies. No close analogs are found for the full severity and geographical focus of the Dust Bowl drought over the past 500 yr. The 1950s drought (1946–56) was concentrated over the Southwest and was replicated by some 12 droughts of similar sp...

Cool- and Warm-Season Precipitation Reconstructions over Western New Mexico

Precipitation over the southwestern United States exhibits distinctive seasonality, and contrasting ocean‐ atmospheric dynamics are involved in the interannual variability of cool- and warm-season totals. Tree-ring chronologies based on annual-ring widths of conifers in the southwestern United States are well correlated with accumulated precipitation and have previously been used to reconstruct cool-season and annual precipitation totals. However, annual-ring-width chronologies cannot typically be used to derive a specific record of summer monsoon-season precipitation. Some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring, and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool- and warm-season precipitation and their associated large-scale ocean‐atmospheric dynamics. Two 2139-yr-long reconstructions of cool- (November‐May) and early-warm season (July) precipitation have been developed from ancient conifers and relict wood at El Malpais National Monument, New Mexico. Both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large-scale SST and 500-mb height fields using instrumental precipitation data from New Mexico. Above^ ^

Early 21st‐Century Drought in Mexico

Prolonged drought conditions have persisted over western North America since at least 1999, affecting snowpack, stream discharge, reservoir levels, and wildfire activity [Mote et al., 2005; Westerling et al., 2006; MacDonald et al., 2008]. Instrumental precipitation, temperature, and Palmer Drought Severity Indices (PDSI) indicate that severe and sustained drought began in 1994 in Mexico, where it has continued with only limited relief for the past 15 years. This late twentieth-and early 21st-century Mexican drought (referred to below as the [early 21st-century drought]) has equaled some aspects of the 1950s drought, which is the most severe drought evident in the instrumental climate record for Mexico (1900–2008). Large-scale changes in ocean-atmospheric circulation have contributed to the lower than normal precipitation that has led to the current drought [Seager, 2007], but global warming and the sharp regional warming across Mexico, which appears to have been aggravated by land cover changes [Englehart and Douglas, 2005], may have added an anthropogenic component to the early 21st-century drought.

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Niño/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Niña and warm Atlantic SSTs prevailed, and the 1980–1990s when El Niño and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Niño and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.

Twentieth-Century Sea Surface Temperature Patterns in the Pacific during Decadal Moisture Regimes over the United States*

Abstract Three great moisture anomalies were observed during the twentieth century over the western United States: a pluvial from 1905 to 1917, the Dust Bowl drought (1929–40), and the Southwestern drought of 1946–56. A composite analysis of the concurrent Pacific sea surface temperature (SST) field is used to infer the atmospheric circulation that may have been associated with these objectively defined decadal dry and wet periods. The early-twentieth-century pluvial occurred during a 13-yr SST regime with unusually cold water in the northern and northwestern North Pacific and in the eastern North Pacific. This pattern would favor a “Pineapple Express–like” mean storm track into the west. Warm ENSO-like conditions also observed during the pluvial would have favored an enhanced subtropical jet stream into the southwestern United States. The 11-yr Dust Bowl drought occurred during a poorly defined Pacific SST regime, although unusually cold water was present in the far western North Pacific. Weak warm SST c...

NAO influence on sub‐decadal moisture variability over central North America

[1] A strong statistically significant spectral peak with a frequency of 7-8 years is computed for tree-ring reconstructed summer PDSI averaged at continental and regional scales over North America for the period 1645-1990. A similar, though non-significant spectral peak is computed for instrumental summer PDSI averaged over North America for the shorter period from 1900-1990. The winter NAO index (1781-2002) has a very strong spectral signature at this same 7-8 year sub-decadal frequency and is coherent with summer PDSI across a broad sector of the central U.S. Composite analyses confirm a PDSI response to extremes of the NAO over the central U.S., with drought prevalent during negative extremes, and wetness during positive extremes. The winter NAO index leads summer PDSI and a contingency table analysis indicates that extrema in the winter NAO index may have modest forecast value for the following spring-summer moisture regimes over the central U.S.

Interannual to decadal climate and streamflow variability estimated from tree rings

Publisher Summary This chapter discusses selected tree-ring research in the continental United States, with a particular focus on hydroclimatic variability, environmental and human impacts, and recent developments with new multicentury tree-ring chronologies in the eastern and western United States. The tree-ring community is actively pursuing the development of long, climate-sensitive tree-ring chronologies in the United States, including novel species and habitat types. The emerging continent-wide network of moisture-sensitive tree-ring chronologies will be exceptionally valuable for documenting the geographical impact and recurrence of great drought extremes. The hemispheric footprint of these decadal moisture regimes may also be used to constrain the concurrent large-scale ocean-atmospheric circulation. The dendroclimatic community focuses primarily on expanding the geographic scope and time depth of this outstanding network. To maximize the social and scientific value of this unparalleled array of natural environmental proxies, procedures and secure funding need to be instituted for the timely updating of selected high-quality chronologies across North America to monitor climatic variability and change as they occur.

Diatom-inferred wind activity at Lac du Sommet, southern Québec, Canada: A multiproxy paleoclimate reconstruction based on diatoms, chironomids and pollen for the past 9500 years

Paleo surface wind for southern Quebec was inferred quantitatively for the past 9500 years from diatom assemblages archived in the sediment of the shallow mountain Lac du Sommet using biweekly sediment trap samples along an elevation gradient in the study area. The wind reconstruction was compared with diatom-inferred dissolved organic carbon concentration, chironomid-inferred mean August air temperature, pollen, grain size and loss-on-ignition. Increased lake circulation, interpreted as indicating stronger surface winds, was inferred from diatoms around 8700, 4500, 3000 to 2000 cal. yr BP and during the past 250 years. Increased lake stratification was diatom-inferred from 7500 to 5000 cal. yr BP and between 1500 and 500 cal. yr BP. Diatom-inferred paleowinds were congruent with the regional fire history. In general, diatom production was significantly correlated with diatom-inferred lake circulation. Diatom-inferred lake circulation and diatom production were not correlated with the pollen assemblage changes, diatom-inferred dissolved organic carbon and chironomid-inferred August air temperature, which were highly intercorrelated. After the disappearance of the meltwater in the St Lawrence River valley, the chironomids reflected a warming trend that lasted until about 5000 cal. yr BP, trees replaced shrubs and diatom-inferred dissolved organic carbon increased from 4 mg/l to 6 mg/l. Diatom-inferred lake circulation exhibited periodicities of 200 and 900 years, whereas chironomid-inferred August air temperatures indicated a distinct (significant) 200 year periodicity.

Decadal drought effects on endangered woodpecker habitat

The critically endangered ivory-billed woodpecker (Campephilus principalis) apparently has been rediscovered in old-growth bald cypress (Taxodium distichum, Figure 1) and swamp tupelo (Nyssa aquatica) forests of Bayou DeView, located seven kilometers northwest of Brinkley, Ark. [Fitzpatrick et al., 2005]. The evaluation of the impact of drought on forest history and wildlife population levels is critical to the conservation of the ivory-billed woodpecker and other similarly endangered species. Tree ring chronologies have been developed from old-growth forests at Bayou DeView to aid in this assessment. This article also describes a conceptual model that has proven useful for the discovery of other noncommercial old-growth cypress-tupelo remnants in the Southeast. These relict cypress-tupelo stands may be candidates for conservation, restoration, and perhaps the eventual reintroduction of the ivory bill and other increasingly rare species native to this ecosystem.