Modeling of Tropical Forcing of Persistent Droughts and Pluvials over Western North America: 1856–2000*

Abstract

The causes of persistent droughts and wet periods, or pluvials, over western North America are examined in model simulations of the period from 1856 to 2000. The simulations used either (i) global sea surface temperature data as a lower boundary condition or (ii) observed data in just the tropical Pacific and computed the surface ocean temperature elsewhere with a simple ocean model. With both arrangements, the model was able to simulate many aspects of the low-frequency (periods greater than 6 yr) variations of precipitation over the Great Plains and in the American Southwest including much of the nineteenth-century variability, the droughts of the 1930s (the “Dust Bowl”) and 1950s, and the very wet period in the 1990s. Results indicate that the persistent droughts and pluvials were ultimately forced by persistent variations of tropical Pacific surface ocean temperatures. It is argued that ocean temperature variations outside of the tropical Pacific, but forced from the tropical Pacific, act to strengthen the droughts and pluvials. The persistent precipitation variations are part of a pattern of global variations that have a strong hemispherically and zonally symmetric component, which is akin to interannual variability, and that can be explained in terms of interactions between tropical ocean temperature variations, the subtropical jets, transient eddies, and the eddy-driven mean meridional circulation. Rossby wave propagation poleward and eastward from the tropical Pacific heating anomalies disrupts the zonal symmetry, intensifying droughts and pluvials over North America. Both mechanisms of tropical driving of extratropical precipitation variations work in summer as well as winter and can explain the year-round nature of the precipitation variations. In addition, land–atmosphere interactions over North America appear important by (i) translating winter precipitation variations into summer evaporation and, hence, precipitation anomalies and (ii) shifting the northward flow of moisture around the North Atlantic subtropical anticyclone eastward from the Plains and Southwest to the eastern seaboard and western Atlantic Ocean.

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).…”

“…Much recent attention has been focussed on understanding the nature, extent and forcing of drought in the western United States (Hoerling and Kumar, 2003;Fye et al, 2004;Schubert et al, 2004a;Herweijer et al, 2005;Seager et al, 2005a). Cole et al (2002) and Fye et al (2004) noted that persistently cold tropical Pacific sea surface temperature (SST) anomalies have co-occurred with the major North American droughts since the mid-nineteenth century.…”

“…It is well known that on interannual timescales, La Niña winters are characterized by reduced precipitation over much of the northern subtropics and mid-latitudes, particularly over western North America (Trenberth and Branstator, 1992;Trenberth and Guillemot, 1996;Cole et al, 2002;Seager et al, 2005b). Recently, it has also been demonstrated that over longer timescales, a persistently cool tropical Pacific can provide the steady atmospheric forcing necessary for North American drought: each persistent North American drought since the 1850s has been simulated in an ensemble of climate model simulations forced by the observed history of tropical Pacific SSTs alone (Seager et al, 2005a;Herweijer et al, 2005;Huang et al, 2005). In the extratropics, alongside zonal asymmetries produced by Rossby wave propagation from the cooler equatorial Pacific, a zonally and hemispherically symmetric component to the forcing is observed.…”

“…In the extratropics, alongside zonal asymmetries produced by Rossby wave propagation from the cooler equatorial Pacific, a zonally and hemispherically symmetric component to the forcing is observed. This pattern is a lower frequency realization of the subtropical jet -transient eddy -mean meridional circulation interaction mechanism (the Tropical Modulation of Mid-latitude Eddies, TMME, mechanism) that works throughout the year to promote eddydriven descent in mid-latitudes when the tropical Pacific is cool (Seager et al, 2003(Seager et al, , 2005aRobinson, 2005). The existence of this mechanism suggests that drought over North America should appear as part of a larger, global pattern of mid-latitude drought.…”

Dangerous Atmospheric Drought in European Russia Under Recent Summer Warming

“…Исследования процессов, инициирующих засуху, выявили ряд факторов, которые могут оказывать влияние на возникновение засухи: в их числе круп-номасштабные атмосферные механизмы, ассоциирующиеся с режимами кли-матической изменчивости и аномалиями температуры поверхности океана (ТПО) (Schubert et al, 2004;Seager et al, 2005). Модельные оценки показали, что принятие в расчет Атлантического долгопериодного колебания (АДК) увеличивает в 1.5 -2 раза (в зависимости от амплитуды воздействия) вероят-ность аномально жарких режимов в июле на юге ЕЧР (Семенов и др., 2014).…”