Drought and vegetation change in the central Rocky Mountains: Potential climatic mechanisms associated with the mega drought at 4200 cal yr BP

Abstract. Droughts are a naturally re-occurring phenomena that result in economic and societal losses. Yet, the most historic droughts that occurred in the 1930s and 1950s in the Great Plains and western United States were both shorter in duration and less severe than megadroughts that have plagued the region in the past. Roughly 4200 years ago, a ∼150-year long megadrought occurred in the central Rocky Mountains, as indicated by sedimentary pollen evidence documenting a brief and unique change in vegetation composition from Long Lake, southeastern Wyoming. Neighbouring the central Rocky Mountains, several dune fields reactivated in the western Great Plains around this time period illustrating a severe regional drought. While sedimentary pollen provides evidence of past drought, paleoecological evidence does not provide context for the climate mechanisms that may have caused the drought. Thus, a modern climate analogue technique was applied to the sedimentary pollen and regional dune reactivation evidence identified from the region to provide a conceptual framework for exploring possible mechanisms responsible for the observed ecological changes. The modern climate analogues of 2002/2012 illustrate that warm and dry conditions persisted through the growing season and were associated with anomalously higher-than-normal geopotential heights centred over the Great Plains. In the spring, higher-than-normal heights suppressed moisture transport via the low-level jet from the Gulf of Mexico creating a more southwesterly component of flow. In the summer, higher-than-normal heights persisted over the northern Great Plains resulting in a wind shift with an easterly component of flow, drawing in dry continental air into the study region. In both cases, lower-than-normal moisture in the atmosphere (via 850 mbar specific humidity) inhibited uplift and potential precipitation. Thus, if the present scenario existed during the 4.2 ka drought, the associated climatic responses are consistent with local and regional proxy data suggesting regional drought conditions in the central Rocky Mountains and western Great Plains.

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Middle Cenozoic uplift and concomitant drying in the central Rocky Mountains and adjacent Great Plains

Geology ◽

10.1130/g35444.1 ◽

2014 ◽

Vol 42 (6) ◽

pp. 547-550 ◽

Cited By ~ 23

Author(s):

Majie Fan ◽

Paul Heller ◽

Sarah D. Allen ◽

Brian G. Hough

Keyword(s):

Great Plains ◽

Rocky Mountains ◽

Central Rocky Mountains

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West-East Stratigraphic Cross Section of Cretaceous Rocks, Central Rocky Mountains to East-Central Great Plains, Utah, Colorado, and Kansas

AAPG Bulletin ◽

10.1306/20b241d2-170d-11d7-8645000102c1865d ◽

1991 ◽

Vol 75 ◽

Author(s):

FOUCH, T. D., U.S. Geological Surve

Keyword(s):

Cross Section ◽

Great Plains ◽

Rocky Mountains ◽

East Central ◽

Central Rocky Mountains

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A 10,300 14C yr Record of Climate and Vegetation Change from Haiti

Quaternary Research ◽

10.1006/qres.1999.2062 ◽

1999 ◽

Vol 52 (2) ◽

pp. 159-170 ◽

Cited By ~ 80

Author(s):

Antonia Higuera-Gundy ◽

Mark Brenner ◽

David A. Hodell ◽

Jason H. Curtis ◽

Barbara W. Leyden ◽

...

Keyword(s):

Vegetation Change ◽

Human Impacts ◽

Regional Climate ◽

Regional Climate Change ◽

Early Holocene ◽

Pollen Record ◽

Caribbean Islands ◽

Moist Forest ◽

Dry Conditions ◽

American Tropics

Pleistocene and Holocene vegetation dynamics in the American tropics are inferred largely from pollen in continental lake sediments. Maritime influences may have moderated climate and vegetation changes on Caribbean islands. Stable isotope (δ18O) study of a 7.6-m core from Lake Miragoane, Haiti, provided a high-resolution record of changing evaporation/precipitation (E/P) since ∼10,300 14C yr B.P. The Miragoane pollen record documents climate influences and human impacts on vegetation in Hispaniola. The δ18O and pollen data near the base of the core indicate cool, dry conditions before ∼10,000 14C yr B.P. Lake Miragoane filled with water in the early Holocene as E/P declined and the freshwater aquifer rose. Despite increasing early Holocene moisture, shrubby, xeric vegetation persisted. Forest expanded ∼7000 14C yr B.P. in response to greater effective moisture and warming. The middle Holocene (∼7000–3200 14C yr B.P.) was characterized by high lake levels and greatest relative abundance of pollen from moist forest taxa. Climatic drying that began ∼3200 14C yr B.P. may have driven some mesophilic animal species to extinction. The pollen record of the last millennium reflects pre-Columbian (Taino) and European deforestation. Long-term, Holocene vegetation trends in southern Haiti are comparable to trends from continental, lowland circum-Caribbean sites, suggesting a common response to regional climate change.

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Characteristics and Variations of Low-Level Jets and Environmental Factors Associated with Summer Precipitation Extremes over the Great Plains

Journal of Climate ◽

10.1175/jcli-d-18-0553.1 ◽

2019 ◽

Vol 32 (16) ◽

pp. 5123-5144 ◽

Cited By ~ 1

Author(s):

Derek Hodges ◽

Zhaoxia Pu

Keyword(s):

Moisture Content ◽

Great Plains ◽

Large Scale ◽

Summer Precipitation ◽

Reanalysis Data ◽

Precipitation Extremes ◽

Structural Variations ◽

Low Level ◽

Low Level Jets ◽

Regional Reanalysis

Abstract Low-level jets (LLJs) are associated with 10%–45% of the summer precipitation in the U.S. Great Plains region (GPR). This study uses the NCEP North American Regional Reanalysis data product (1979–2017) to characterize the association between LLJs and precipitation extremes (anomalously wet versus dry) during the summer months (June–August) over the GPR. It is found that the number, distribution, and direction of LLJs are not clearly associated with the precipitation anomalies. The characteristics and structural variations of the LLJs and their large-scale and mesoscale environment are then examined to identify the links between LLJs and precipitation extremes. Results show that dry and wet summers vary by synoptic anomaly patterns. During dry summers the anomalous ridging results in a warmer and drier environment, primarily through subsidence, which inhibits precipitation near LLJs. In contrast, during wet summers, a reduction in subsidence occurs, resulting in stronger lift and a cooler and moister environment, which leads to enhanced precipitation near LLJs. The LLJ speed, orientation, and spatial properties vary according to the synoptic anomaly patterns. LLJs do not drive precipitation extremes, but instead, they respond to them. Specifically, the LLJ exit region is characterized by stronger baroclinity and higher moisture content during the wet years. The higher moisture content allows for ascending air parcels to reach saturation more quickly, while the stronger baroclinity increases the warm advection associated with the LLJ. This, in turn, leads to faster rising motion and is therefore closely associated with the location and intensity of the LLJ associated precipitation.

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Vegetation and climate change during the Medieval Climate Anomaly and the Little Ice Age on the southern Cape coast of South Africa: Pollen evidence from Bo Langvlei

The Holocene ◽

10.1177/0959683620950444 ◽

2020 ◽

Vol 30 (12) ◽

pp. 1716-1727

Author(s):

Nadia du Plessis ◽

Brian M Chase ◽

Lynne J Quick ◽

Torsten Haberzettl ◽

Thomas Kasper ◽

...

Keyword(s):

Little Ice Age ◽

Vegetation Change ◽

Storm Track ◽

Ice Age ◽

Pollen Record ◽

Western Cape ◽

Medieval Climate Anomaly ◽

Cape Coast ◽

Climate Anomaly ◽

Dry Conditions

This paper presents continuous, high resolution fossil pollen and microcharcoal records from Bo Langvlei, a lake in the Wilderness Embayment on South Africa’s southern Cape coast. Spanning the past ~1300 years and encompassing the Medieval Climate Anomaly (MCA; c. AD 950–1250) and the Little Ice Age (LIA; c. AD 1300–1850), these records provide a rare southern African perspective on past temperature, moisture and vegetation change during these much debated periods of the recent geological past. Considered together with other records from the Wilderness Embayment, we conclude that conditions in the region during the MCA chronozone were – in the context of the last 1300 years – likely relatively dry (reduced levels of Afrotemperate forest pollen) and perhaps slightly cooler (increased percentages of Stoebe-type pollen) than present. The most significant phase of forest expansion, and more humid conditions, occurred during the transition between the MCA and the most prominent cooling phase of the LIA. The LIA is clearly identified at this locality as a period of cool, dry conditions between c. AD 1600 and 1850. The mechanisms driving the changes observed in the Bo Langvlei pollen record appear to be generally linked to changes in temperature, and changes in the influence of tropical circulation systems. During warmer periods, moisture availability was higher at Bo Langvlei, and rainfall was perhaps less seasonal. During colder periods, precipitation resulting from tropical disturbances was more restricted, resulting in drier conditions. While increased precipitation has been reported during the LIA from Verlorenvlei in the Western Cape as a result of an equatorward displacement of the westerly storm-track at this time, the opposing response at Bo Langvlei suggests that any increased influence of westerlies was insufficient to compensate for the concurrent reduction in tropical/local rainfall in the region.

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Expression of the “4.2 ka event” drought in the southern Rocky Mountains, USA

10.5194/cp-2021-149 ◽

2021 ◽

Author(s):

David Thomas Liefert ◽

Bryan Nolan Shuman

Keyword(s):

Great Plains ◽

North American ◽

Rocky Mountains ◽

Winter Precipitation ◽

Mountain Lake ◽

Pollen Record ◽

Southern Rocky Mountains ◽

The North ◽

Climate Event ◽

Western Great Plains

Abstract. The use of the climatic anomaly known as the “4.2 ka event” as the stratigraphic division between the mid- and late Holocene has prompted debate over its impact, geographic pattern, and significance. The anomaly has primarily been described as abrupt drying, but evidence of hydroclimate change at ca. 4 ka is inconsistent among sites globally, and few sites in North America document a major drought. Climate records from the southern Rocky Mountains demonstrate the challenge with diagnosing the extent and severity of the anomaly. Dune-field chronologies and a pollen record in southeast Wyoming reveal several centuries of low moisture at around 4.2 ka and prominent low stands in lakes in Colorado suggest the drought was unique amid Holocene variability, but detailed carbonate oxygen isotope (δ18Ocarb) records from Colorado do not record it. We find new evidence from δ18Ocarb in a small mountain lake in southeast Wyoming of an abrupt reduction in effective moisture or snowpack from approximately 4.2–4 ka that coincides in time with the other evidence from the southern Rocky Mountains and the western Great Plains of regional drying at around 4.2 ka. We find that the δ18Ocarb in our record may reflect cool-season inputs into the lake, which do not appear to track the strong enrichment of heavy oxygen by evaporation during summer months today. The modern relationship differs from some widely applied conceptual models of lake-isotope systems and may indicate reduced winter precipitation rather than enhanced evaporation at ca. 4.2 ka. Inconsistencies among the North American records, particularly in δ18Ocarb trends, thus show that site-specific factors can prevent identification of the patterns of multi-century drought. However, the prominence of the drought at ca. 4 ka among a growing number of sites in the North American interior suggests it was a regionally substantial climate event amid other Holocene variability.

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