Tropical Pacific Forcing of North American Medieval Megadroughts: Testing the Concept with an Atmosphere Model Forced by Coral-Reconstructed SSTs*

Abstract

The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from a.d. 1320 to a.d. 1462 and show overall colder conditions as well as extended multidecadal La Niña–like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856–2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified “megadroughts,” a.d. 1360–1400 and a.d. 1430–60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

Support for global climate reorganization during the “Medieval Climate Anomaly”

“…We explore this idea using a synthesis of globally distributed proxy records and results from new coupled model experiments examining the response to a warmer Indian-West Pacific Ocean. The model results are also discussed in the context of previous experiments bearing on the role of tropical SSTs in driving recent and past climate change (e.g., Rodwell et al 1999;Branstator 2000;Hoerling et al 2001;Bader and Latif 2003;Giannini et al 2003;Hurrell et al 2004;Hoerling et al 2004;Deser and Phillips 2006;Seager et al 2008), and others examining the climate response to changes in irradiance and high latitude temperatures (e.g., Clement et al 1996;Shindell et al 2001;Meehl et al 2003;Sun et al 2004;Mann et al 2005;Pierce et al 2006;Ammann et al 2007;Timmermann et al 2007a, b, Lu et al 2007). …”

“…Letter indices correspond to time series in Fig. 3; numbers correspond to entries in Table 3 Seager et al (2007Seager et al ( , 2008 Kenya; Halfman et al 1994;Verschuren 2004) and from southern Oman (Fleitmann et al 2003; Fig 3L). (Thompson et al 1995;Wang et al 2007; Fig.…”

“…The general idea of a tendency for stronger cool-season North Atlantic westerlies during the MCA, as occurs with a more positive North Atlantic Oscillation (NAO), has persisted and been discussed as a possible driver to explain changes in diverse regional proxy records (e.g., Keigwin 1996;Proctor et al 2000;Mangini et al 2005;Lund et al 2006;Esper et al 2007;Sicre et al 2008a;Massé et al 2008;Wanamaker et al 2008;Trouet et al 2009). Similarly, evidence for cool-season aridity and related climate changes in the western US during medieval times is now available from a wide range of proxy and archeological records (e.g., Muhs 1985;Mehringer and Wigand 1990;Swetnam 1993;Stine 1994;Hughes and Funkhouser 1998;Kennett and Kennett 2000;Cook et al 2004;Jones and Schwaitalla 2008;see Woodhouse 2004; for reviews), indicating a northward shift in the boreal winter storm track across the eastern North Pacific and western North America and a contraction and/or westward shift in the Aleutian Low (Stine 1994;MacDonald and Case 2005;Seager et al 2007aSeager et al , 2008.…”

“…Given the well established associations between winter North Pacific circulation patterns, cool season precipitation over the western US, and interannual-to-interdecadal SST variability in the tropical Pacific (Bjerknes 1969;Lau 1985;Schonher and Nicholson 1989;Mantua et al 1997;Rajagopalan et al 2000), the proxy evidence is consistent with the idea that MCA aridity in the American West (and contraction of the Aleutian Low) was a response, at least in part, to cooler central and eastern tropical Pacific SSTs. This idea has been explored by consideration of available proxy data and model results Seager et al 2007aSeager et al , 2008. One important result from some of these latter studies has been the suggestion that while many of these MCA-LIA climate anomalies can be explained by changes in tropical Pacific SSTs alone, others suggest an important role for SST changes elsewhere.…”

Decadal predictability of soil water, vegetation, and wildfire frequency over North America

“…US and leading modes of sea surface temperature (SST) anomalies, such as the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO) (Seager et al 2005(Seager et al , 2008Meehl and Hu 2006;Sheffield and Wood 2008;Schubert et al 2009;Famiglietti et al 2011;Dai 2011;Hu et al 2013), and ENSO (Kanamitsu et al 2003;Barlow et al 2001;Kumar 2002, 2004;McCabe et al 2008;Dai 2013). To determine the relation (not the causality) between SST and simulated decadal hydroclimate variability in the CESM CTL run, we calculated the correlations and regressions of the SST and sea level pressure (SLP) anomaly fields with the leading principal Correlation coefficients are calculated using annual mean data for the first EOF mode of annual precipitation, 10-year running mean data for the low-frequency precipitation, and 10-year running mean data leading with 5-year for the total water storage.…”