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The figure is from Carrillo's Ph.D. dissertation (2014), which represents the summer spatiotemporal pattern of the 50-100-year low-frequency drought mode. In the North American monsoon (NAM) region, precipitation is highly related to large-scale variations of the climate system: the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Variability (PDV). In this research, the first monsoon-sensitive network of tree-ring chronologies analyzes the NAM variability during the past four centuries. The Multi-Taper-Method Singular Value Decomposition (MTM-SVD) emphasizes the detection of spatiotemporal pattern in the low-frequency regime. MTM-SVD determined the dominant modes of variability on earlywood (EW) and latewood adjusted (LWadj) chronologies (for winter and summer).
Here, tree-ring chronologies characterize the dominant climate modes of NAM variability. In the high-frequency regime, the monsoon tree-ring network reproduces the interannual variability of precipitation in the cool and warm seasons, similar to the period of the instrumental record. For instance, the first two dominant LWadj modes capture the continental-scale precipitation pattern associated with ENSO-PDV and the Circumglobal Teleconnection (CGT). In the low-frequency regime, EW and LWadj reveal a low-frequency variability in the 50-100 year band, which is beyond the scope of the instrumental record to resolve. Considering the dual influences of cool and warm seasons, megadrought would appear to derive from two conditions: (1) La Niña-like and low PDV that would tend to decrease winter precipitation, and (2) the CGT pattern that would tend to decrease monsoon precipitation in the intermountain west (while increasing precipitation in the Southern Great Plains). |
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