Blueprints for Medieval hydroclimate

Abstract

According to tree ring and other records, a series of severe droughts that lasted for decades afflicted western North America during the Medieval period resulting in a more arid climate than in subsequent centuries. A review of proxy evidence from around the world indicates that North American megadroughts were part of a global pattern of Medieval hydroclimate that was distinct from that of today. In particular, the Medieval hydroclimate was wet in northern South America, dry in mid-latitude South America, dry in eastern Africa but with strong Nile River floods and a strong Indian monsoon. This pattern is similar to that accompanying persistent North American droughts in the instrumental era. This pattern is compared to that associated with familiar climate phenomena. The best fit comes from a persistently La Nin˜a-like tropical Pacific and the warm phase of the so-called Atlantic Multidecadal Oscillation. A positive North Atlantic Oscillation (NAO) also helps to explain the Medieval hydroclimate pattern. Limited sea surface temperature reconstructions support the contention that the tropical Pacific was cold and the subtropical North Atlantic was warm, ideal conditions for North American drought. Tentative modeling results indicate that a multi-century La Nin˜a-like state could have arisen as a coupled atmosphere-ocean response to high irradiance and weak volcanism during the Medieval period and that this could in turn have induced a persistently positive NAO state. A La Nin˜a-like state could also induce a strengthening of the North Atlantic meridional overturning circulation, and hence warming of the North Atlantic Ocean, by (i) the ocean response to the positive NAO and by shifting the southern mid-latitude westerlies poleward which (ii) will increase the salt flux from the Indian Ocean into the South Atlantic and (iii) drive stronger Southern Ocean upwelling. r

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

“…3.2.1). This pattern incorporates a weakened Aleutian low and stronger NAO, features long suggested by others for the MCA on the basis of regional proxy records (e.g., Lamb 1965;LaMarche 1974;Stine 1994;Keigwin 1996;Cobb et al 2003;Mangini et al 2005;MacDonald and Case 2005;Seager et al 2007a;Trouet et al 2009). Interestingly, the main aspects of the extra-tropical NH circulation changes can be reproduced by shifting the climatological pattern slightly poleward, and bear some resemblance to those resulting from expansion of the Hadley Cell in greenhouse warming experiments (e.g.…”

“…3S by the treering derived drought-area index of Cook et al 2004) is quite similar to those in Europe, the North Atlantic and the tropical Pacific [e.g., compare Fig. 3S with B, G, H (the shift in the Morocco record is notably later than the others) and P; see also , Seager et al 2007a, with the most widespread aridity in western US occurring between 900 and 1350 AD. Such a decrease in precipitation over the western US implies a northward shift in the storm track over the region and over the eastern North Pacific during the MCA, with a weaker (or contracted) Aleutian Low and a stronger Northeast Pacific High off the west coast of the US, as indicated by some reconstructions (MacDonald and Case 2005;.…”

“…3.3), our simulations demonstrate that modest warming of the IndianPacific warm pool can elicit a clear response in global circulation, hydroclimatic and broad-scale SST patterns. Further, they provide a plausible dynamically consistent context within which to view the patterns of MCA-LIA climate change inferred from proxy records-including some features not explained by a cooler eastern/central tropical Pacific alone Seager et al 2007a). …”

“…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.…”