Browsing by Subject "climate variability"

Now showing 1 - 3 of 3
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    An analysis of atmospheric and oceanic variability on North American regional climate under historical and future conditions
    (2019-08) Serakos, Allison
    Large-scale climate patterns affect many areas of our global environment either directly or indirectly. They provide the background conditions for regional climate patterns and weather events that in turn affect many areas of our society, such as agriculture, water supply, energy demand, and natural ecosystem development. Many scientists have analyzed how these large-scale modes of climate variability influence the climate response over North America on an individual basis, however, it is still unclear how the combination of modes of variability affect North American climate. In this study, I analyze the four leading modes of climate variability that influence North American climate patterns: the El Niño – Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), the Pacific – North American (PNA) Pattern, and the Northern Annular Mode (NAM). Through an observational analysis, I show which climate pattern has the largest influence on a particular region of North America as well as the most common combinations of patterns to influence the continent. The oceanic patterns, ENSO and AMO, have the largest influence on the background conditions of regional climate due to their slow-varying nature, while the atmospheric patterns, PNA and NAM, have the largest influence on wintertime temperature and precipitation anomalies. The North American climate response of the combined PNA and NAM patterns is seasonally dependent, where in the boreal winter they have the largest influence when in opposition and in the summer when in the same phase. This is due in part to their common link with the Aleutian Low in the North Pacific. Furthermore, the AMO has a large influence on summer climate due to its weak seasonality and the lack of strength of the other regional climate patterns. Moreover, this study also presents an analysis on the North American climate influence of the ENSO and AMO using the NCAR CESM1 global climate model. The model results for the ENSO response are corroborated well with observations, however, the AMO response in the model is weak, which might be a result of the internal metrics of the model or a weak AMO forcing. The largest North American climate response occurs when the combined ENSO and AMO patterns are in opposition. The ENSO and AMO response is also simulated under RCP8.5 end-of-century conditions to analyze the pattern response under global warming. An eastward shift in the ENSO teleconnection occurs under future warming with the largest shift occurring under +ENSO conditions. An eastward shift occurs under –ENSO conditions only when in combination with a +AMO in the winter, suggesting the AMO having an influence on the ENSO teleconnection. Additionally, the shift in the ENSO response highlights the nonlinear nature of the ENSO teleconnection and an element of future climate change over North America.
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    Assessing Historical Trends in Snowpack Variability Across the Northern Rocky Mountains Using Remote Sensing and Dendrochronology Approaches
    (2013-06) Crawford, Christopher
    Mountain snowpack across the western United States is declining because of warming spring temperatures during the modern period. Earlier snowmelt has been documented for numerous localities throughout the American West using ground-based snow-water-equivalent measurements and gauged streamflow. This research uses historical satellite imagery, tree-ring records, and instrumental climate observations from the northern Rocky Mountain (NRM) region to evaluate the climatic controls on mountain snowpack spatiotemporal variability, assess historical spring snowmelt trends, and contextualize modern climatic change with pre-instrumental climate variability. A suite of methodological approaches is employed to develop and calibrate satellite, tree-ring, and instrumental climate records using time-series analysis techniques. Together, these NRM region climate records suggest that precipitation in the form of mountain snowpack extent varies on interannual to decadal timescales. Of more importance, spring mountain snowpack appears to be decreasing in areal extent during the 20th and early 21st centuries driven largely by modern spring warming.
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    Climate modifies response of non-native and native species richness to nutrient enrichment
    (2016) Flores-Moreno, Habacuc; Reich, Peter B; Lind, Eric M; Sullivan, Lauren L; Seabloom, Eric W; Yahdjian, Laura; Macdougall, Andrew S; Reichmann, Lara G; Alberti, Juan; Báez, Selene; Bakker, Jonathan D; Cadotte, Marc W; Caldeira, Maria C; Chaneton, Enrique J; D'Antonio, Carla M; Fay, Philip A; Firn, Jennifer; Hagenah, Nicole; Harpole, W Stanley; Iribarne, Oscar; Kirkman, Kevin P; Knops, Johannes M H; La Pierre, Kimberly J; Laungani, Ramesh; Leakey, Andrew D B; Mcculley, Rebecca L; Moore, Joslin L; Pascual, Jesus; Borer, Elizabeth T
    Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change.