Prolonged episodes of persistently dry or wet conditions are common features of most proxy-based reconstructions of past hydroclimatic variability. These so-called “Joseph” events might be due to external forcings that push sea-surface temperatures into warm or cold states, and thereby increase the likelihood of widespread megadroughts or megapluvials. Alternatively, internal ocean-atmosphere variability alone might be able to produce long-lasting and spatially extensive wet or dry intervals, even in the absence of any exotic external influences. In this study, we use a simple statistical emulator to establish benchmarks for decadal or multidecadal patterns in the western United States. We constructed a linear inverse model that included three key aspects of the El Niño-Southern Oscillation (monthly sea-surface temperatures, zonal surface wind stress, and sea-surface height), and restricted the spatial domain of each field to include only the tropical Pacific. By also including western United States hydroclimate information in the LIM, we are able to test whether ENSO variability and stochastic weather ‘noise’ could be sufficient to create low-frequency coherence within proxy networks. More broadly, if simulated drought patterns generated by the LIM are able to match the frequency, intensity, or spatial extent of droughts reconstructed by proxies, that implies that neither exotic forcings nor climate variability outside the tropical Pacific are required to produce widespread megadroughts in this region. If prolonged departures from the mean are indeed emergent but unremarkable features of western North America’s hydroclimate, we might be able to estimate their future occurrence as a linear combination of changes in the mean state and the linear dynamics that have governed their behavior in the past.
S. St. George, T. Ault, C. Carrillo, S. Coats, J. Mankin, J. Smerdon, What to expect when you’re expecting decadal variability in hydroclimatic proxies, PAGES 5th Open Science Meeting, Zaragoza, Spain, May 9-13, 2017.