Are Simulated Megadroughts in the North American Southwest Forced?*

Abstract

Multidecadal drought periods in the North American Southwest (258-42.58N, 1258-1058W), so-called megadroughts, are a prominent feature of the paleoclimate record over the last millennium (LM). Six forced transient simulations of the LM along with corresponding historical and 500-yr preindustrial control runs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are analyzed to determine if atmosphere-ocean general circulation models (AOGCMs) are able to simulate droughts that are similar in persistence and severity to the megadroughts in the proxy-derived North American Drought Atlas. Megadroughts are found in each of the AOGCM simulations of the LM, although there are intermodel differences in the number, persistence, and severity of these features. Despite these differences, a common feature of the simulated megadroughts is that they are not forced by changes in the exogenous forcing conditions. Furthermore, only the Community Climate System Model (CCSM), version 4, simulation contains megadroughts that are consistently forced by cooler conditions in the tropical Pacific Ocean. These La Niña-like mean states are not accompanied by changes to the interannual variability of the El Niño-Southern Oscillation system and result from internal multidecadal variability of the tropical Pacific mean state, of which the CCSM has the largest magnitude of the analyzed simulations. Critically, the CCSM is also found to have a realistic teleconnection between the tropical Pacific and North America that is stationary on multidecadal time scales. Generally, models with some combination of a realistic and stationary teleconnection and large multidecadal variability in the tropical Pacific are found to have the highest incidence of megadroughts driven by the tropical Pacific boundary conditions.

Multi-proxy reconstructions of May–September precipitation field in China over the past 500 years

“…This suggests that MJJAS mean precipitation anomalies over the past 380 years in China may be largely controlled by the internal variability rather than by www.clim-past.net/13/1919/2017/ external forcing during the interval (AD 1470-1849). Similar conclusions were derived from the comparison of reconstructed and simulated hydroclimatic variables over the past millennium in North America (Coats et al, 2015) and in East Africa (Klein et al, 2016).…”

The PMIP4 contribution to CMIP6 – Part 3: The last millennium, scientific objective, and experimental design for the PMIP4 <i>past1000</i> simulations

“…Such comparative analyses emphasise the methods appropriate for data-model compar- isons that target hydroclimate in order to understand climate change at regional scales and the mechanisms of climate variability at decadal to centennial timescales (e.g. Coats et al, 2015b). PMIP has provided to CMIP6 a comprehensive list of output variables that includes all necessary variables for analyses of atmospheric, oceanic, and land surface processes (see Sect.…”

Winter‐to‐summer precipitation phasing in southwestern North America: A multicentury perspective from paleoclimatic model‐data comparisons

“…For comparison of these hydroclimate reconstructions to the observed sea surface temperature record we employ the Kaplan extended SST v2 product, which is a 5°latitude × 5°longitude gridded SST field for the period 1856 to present [Kaplan et al, 1998]. This is a well-validated SST data set that has been used for other paleoclimate modeldata comparisons [Coats et al, 2013a[Coats et al, , 2015a[Coats et al, , 2015b.…”

“…We specifically apply the growing paradigm of paleoclimate model-data comparisons [e.g., Anchukaitis et al, 2012;Ault et al, 2013Ault et al, , 2014Coats et al, 2013aCoats et al, , 2015aCoats et al, , 2015bFernández-Donado et al, 2012;Phipps et al, 2013;Schmidt et al, 2013] to characterize winter-to-summer precipitation phasing in the Southwest. We use independent tree ring-based reconstructions of NAM and winter precipitation variability [Griffin et al, 2013] in the NAM2 region (113.25°W-107.75°W, 30°N-35.25°N [Gochis et al, 2009]) for the period 1539-2008 Common Era (C.E.)…”