North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought

Abstract

Unlike the major droughts of the twentieth century that are readily identified in the instrumental record, similar events in the nineteenth century have to be identified using a combination of proxy data, historical accounts and a sparse collection of early instrumental records. In the USA, three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century: 1856-1865, 1870-1877 and 1890-1896. Each of these events is shown to coincide with the existence of an anomalously cool, La Nifia-like tropical Pacific. To examine the physical mechanisms behind these droughts two ensembles of simulations with an atmosphere general circulation model (AGCM) were generated: the first forces an AGCM with the observed history of Sea Surface Temperatures (SSTs) everywhere from 1856 to 2001 (the GOGA experiment), the second forces the AGCM only with tropical Pacific SSTs, being coupled to a two-layer entraining mixed layer (ML) ocean elsewhere (the POGA-ML experiment). Owing to a sparsity of instrumental precipitation data at this time, proxy evidence from tree rings is used as verification. A comparison of modelled soil moisture with tree-ring reconstructions of the Palmer Drought Severity Index (PDSI), a proxy for soil moisture, from the North American Drought Atlas is made. Both the POGA-ML and GOGA ensemble means capture the three multi-year droughts of the mid to late nineteenth century, indicating that the droughts were SST forced. The similarity of the POGA-ML and GOGA simulations implies that the component of each drought signal that is forced by the SST is driven ultimately by the La Nifia-like tropical Pacific. The global atmosphere-ocean context of each of the mid to late-nineteenth century droughts reveals a zonally and hemispherically symmetric pattern consistent with forcing from the tropics. In addition, Rossby wave propagation from the cooler equatorial Pacific amplifies dry conditions over the USA. Finally, using published coral data for the last millennium to reconstruct a NINO 3.4 history, the modern-day relationship between NINO 3.4 and North American drought is applied to recreate two of the severest Mediaeval ‘drought epochs’ in the western USA. The large-scale spatial similarity to the Drought Atlas data demonstrates the potential link between a colder eastern equatorial Pacific and the persistent North American droughts of the Mediaeval period.

The role of linear wave refraction in the transient eddy–mean flow response to tropical Pacific SST anomalies

“…The midlatitude response is characterized by a southward shift and zonal extension of the Pacific jet and storm track into the southwestern USA during El Niño (EN), and a northward deflection of the jet and storm track during La Niña (LN; Hoerling and Ting, 1994;Trenberth and Hurrell, 1994;Straus and Shukla, 1997;Compo and Sardeshmukh, 2004;Orlanski, 2005;Eichler and Higgins, 2006), with corresponding changes in precipitation systems (Schubert et al, 2004b,a;Seager et al, 2005bSeager et al, , 2008Herweijer et al, 2006;Cook et al, 2007;Seager, 2007). ENSO also affects the Atlantic and Europe, but the response is not as robust as in the Pacific (e.g.…”

The global footprint of persistent extra‐tropical drought in the instrumental era

“…In North America, the most severe multi-year droughts of the last 150 years were the 'Civil War' drought (1856-1865, the 1870s and 1890s droughts, the infamous Dust Bowl drought of the 1930s, the late 1940s-mid 1950s southwestern drought and the present-day drought that has gripped the West since 1998 (Woodhouse and Overpeck, 1998;Cole et al, 2002;Fye et al, 2003;Herweijer et al, 2006;Seager et al, 2005a). In the southern part of South America, in a semi-arid area that encompasses the Andean foothills, the Sierra Cordoba and the Pampas in Argentina, along with Uruguay and southern Brazil, extended dry spells have also been noted in the 1930s and 1950s (Mechoso and Iribarren, 1992;Scian and Donnari, 1997;Robertson and Mechoso, 1998;Compagnucci et al, 2002), and inferred from tree-ring data in the 1860s and 1870s (Villalba et al, 1998).…”

A 1,200-year perspective of 21st century drought in southwestern North America

“…Associations between SSTs and Southwestern drought during this period have been explored with paleoclimatic data and modeling (4,(33)(34)(35) and although the paleoclimatic data that document Pacific Ocean conditions during the medieval period are not in total agreement, most show temperatures in the eastern Pacific indicative of cool El Niño/Southern Oscillation, or La Niña-type conditions (22). More unequivocal evidence exists for a warm North Atlantic (36).…”