Two types of North American droughts related to different atmospheric circulation patterns

Angela-Maria Burgdorf,Jörg Franke,Stefan Brönnimann

doi:10.5194/cp-15-2053-2019

Angela-Maria Burgdorf, Jörg Franke + Show 1 more

Open Access

https://doi.org/10.5194/cp-15-2053-2019

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Abstract

Abstract. Proxy-based studies suggest that the southwestern USA is affected by two types of summer drought, often termed Dust Bowl-type droughts and 1950s-type droughts. The spatial drought patterns of the two types are distinct. It has been suggested that they are related to different circulation characteristics, but a lack of observation-based data has precluded further studies. In this paper, we analyze multi-annual summer droughts in North America back to 1600 in tree-ring-based drought reconstructions and in a global, monthly three-dimensional reconstruction of the atmosphere. Using cluster analysis of drought indices, we confirm the two main drought types and find a similar catalog of events as previous studies. These two main types of droughts are then analyzed with respect to 2 m temperatures (T2m), sea-level pressure (SLP), and 500 hPa geopotential height (GPH) in boreal summer. 1950s-type droughts are related to a stronger wave train over the Pacific–North American sector than Dust Bowl-type droughts, whereas the latter show the imprint of a poleward-shifted jet and establishment of a Great Plains ridge. The 500 hPa GPH patterns of the two types differ significantly not only over the contiguous United States and Canada but also over the extratropical North Atlantic and the Pacific. Dust Bowl-type droughts are associated with positive GPH anomalies, while 1950s-type droughts exhibit strong negative GPH anomalies. In comparison with 1950s-type droughts, the Dust Bowl-type droughts are characterized by higher sea-surface temperatures (SSTs) in the northern North Atlantic. Results suggest that atmospheric circulation and SST characteristics not only over the Pacific but also over the extratropical North Atlantic affect the spatial pattern of North American droughts.

Highlights

Since the turn of the 21st century, prolonged drought events have afflicted large parts of North America, predominantly the southwestern United States (Seager, 2007; Weiss et al, 2009; Cayan et al, 2010; Seager and Vecchi, 2010; Seager and Hoerling, 2014)
We find four drought periods that are not classified by Fye et al (2003) whereas they find two droughts that our analysis does not capture
Out of the 11 (12) drought periods, three are assigned to opposite clusters in our study compared to Fye et al (2003): we found the 1772–1776 and the 1869– 1874 droughts to be Dust Bowl-type droughts while Fye et al (2003) depict them as 1950s-type droughts

Summary

Introduction

Since the turn of the 21st century, prolonged drought events have afflicted large parts of North America, predominantly the southwestern United States (Seager, 2007; Weiss et al, 2009; Cayan et al, 2010; Seager and Vecchi, 2010; Seager and Hoerling, 2014). Drought conditions are a regular feature of the climate in the western United States and have repeatedly affected the region in the past (e.g., Cook et al, 2007; Seager et al, 2009). Severe droughts in the paleoclimate record include decadal to multidecadal droughts during medieval times (∼ AD 900 to ∼ AD 1300), characterized by persistent aridity and increased temperatures over western North America (Woodhouse and Overpeck, 1998; Cook et al, 2004; Woodhouse et al, 2010)

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