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1. Introduction
Multiyear to decadal hydrologic cycles (DHCs), including extremely dry and wet spells, affect crop production; pasture and range conditions; urban and rural water systems; infrastructure; electricity generation; river navigation; recreation; livestock production and health; ecological integrity; and, ultimately, regional and national economies (see, e.g., Mehta et al. 2013a). The recognition by decision-makers of increasing societal consequences of floods and droughts, some clearly attributable to DHCs, has prompted legislation at local, state, and national levels, encouraging improvements in water management and water use efficiency.
Floods cause loss of life and property in coastal regions and low-lying lands and often lead to public health hazards that threaten the lives of survivors. The effects of multiyear to decadal floods and droughts, however, are particularly dramatic in the agriculture sector, resulting in billions of dollars in crop losses around the world annually. Droughts affect more people worldwide than any other natural hazard (Wilhite 2000). Shortages of water for drinking and irrigation, consequent reductions in food production, and other stresses due to drought can contribute to social and political strife, civil wars, and international conflicts (Gleick 1993; Cooley et al. 2013). In addition to the need for information in times of immediate flood and drought emergency, planning for water, food, energy, and urban infrastructure development would benefit greatly, were it available, from reliable information on prospects for wet and dry periods extending one, two, or more decades into the future. But, skillful forecasts of this kind will require that the causes of DHCs and their impacts be better understood.
A substantial body of research has emerged in the last two decades focused on understanding causes and mechanisms of natural decadal climate variability (DCV; e.g., Meehl et al. 2009; Murphy et al. 2010) and its influences on terrestrial hydrology. Several DCV phenomena have been identified: these include the Pacific decadal oscillation (PDO; Mantua et al. 1997), the tropical Atlantic sea surface temperature (SST) gradient variability (TAG; Houghton and Tourre 1992; Mehta and Delworth 1995; Chang et al. 1997; Mehta 1998; Enfield et al. 1999), the west Pacific warm pool SST variability (WPWP; Wang and Mehta 2008), and decadal variability of interannual El Niño-La Niña events (Balmaseda et al. 1995; Kestin et al. 1998; Power et al. 1999; An and...