Evidence low-frequency climate variability mode in the North American Monsoon System as diagnosed by a WRF dynamically downscaled 20th-century reanalysis

Abstract:

Our previous work considered North American monsoon climate variability with a monsoon-sensitive network of tree-ring chronologies in the Southwest. We were able to identify a very low-frequency centennial scale climate signal responsible for megadroughts of the past four centuries. Here we continue to investigate very low-frequency climate variability within the context of a twentieth-century atmospheric reanalysis that has been dynamically downscaled with the Weather Research and Forecasting (WRF) model. We apply several different matrix methods techniques to these data to identify dominant and statistical significant spatiotemporal cool and warm season precipitation patterns. Our main question of interest is to ascertain whether the major western U.S. droughts in the last 140 years are driven by very low-frequency climate variability on the decadal to centennial time scale. We present evidence that the answer to this question is affirmative and that major droughts are tied to coherent changes in atmospheric circulation at a planetary scale in this very-low frequency regime. Use of a regional atmospheric model is necessary to resolve the spatial variability in precipitation patterns that are associated with the droughts. The downscaled 20^th century reanalysis product is thus suitable to be applied to a hydrologic model to estimate impacts of historical dry and wet periods in the 20^th century on water resources.