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Richard Seager and Martin Hoerling

observed and idealized sea surface temperatures (SSTs). These were used to test hypotheses of oceanic forcing of drought-inducing atmospheric circulation anomalies. Links between North American precipitation variability and the El Niño–Southern Oscillation (ENSO), with, in its El Niño phase, a tendency to increased winter precipitation across southern North America, had begun to be noticed in the 1970s and early 1980s (see Rasmusson and Wallace 1983 ) and explained in terms of Rossby wave propagation

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Siegfried D. Schubert, Ronald E. Stewart, Hailan Wang, Mathew Barlow, Ernesto H. Berbery, Wenju Cai, Martin P. Hoerling, Krishna K. Kanikicharla, Randal D. Koster, Bradfield Lyon, Annarita Mariotti, Carlos R. Mechoso, Omar V. Müller, Belen Rodriguez-Fonseca, Richard Seager, Sonia I. Seneviratne, Lixia Zhang, and Tianjun Zhou

drought. Many drought research studies have focused on particular regions or selected events, whereas others have examined the global distribution of droughts, their forcing factors, and their predictability. Efforts in operational environments now routinely assess current and future drought conditions over a variety of temporal and spatial scales. This broad range of activities, as well as many drought impact studies, suggests a need to document our collective understanding of and capabilities to

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Lixia Zhang and Tianjun Zhou

.8. The SPI over north China also witnesses large variation, especially in spring and summer ( Figs. 4d,f ). Because both the features and the forcing mechanisms of the droughts in northwest China and the East Asian monsoon region are different, we review studies of droughts in these two regions separately in this section, with a focus on the year-by-year changes. Fig . 4. (left) The distribution of the precipitation standard deviation (mm day −1 ) during 1950–2009 in (a) DJF, (c) MAM, (e), JJA, and

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Belen Rodríguez-Fonseca, Elsa Mohino, Carlos R. Mechoso, Cyril Caminade, Michela Biasutti, Marco Gaetani, J. Garcia-Serrano, Edward K. Vizy, Kerry Cook, Yongkang Xue, Irene Polo, Teresa Losada, Leonard Druyan, Bernard Fontaine, Juergen Bader, Francisco J. Doblas-Reyes, Lisa Goddard, Serge Janicot, Alberto Arribas, William Lau, Andrew Colman, M. Vellinga, David P. Rowell, Fred Kucharski, and Aurore Voldoire

) and aerosol–radiative forcings ( Kim et al. 2010 ). These effects can potentially interact with each other. For example, the variability of land surface conditions can affect the circulation over the ocean, which in turn can modify the SSTs and indirectly affect conditions over land ( Ma et al. 2013 ). The existence of significant impacts on WAM rainfall of slowly varying climate subcomponents indicates the potential for useful long-range forecasts ( Vellinga et al. 2013 ; Gaetani and Mohino 2013

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Bradfield Lyon

1. Introduction Drought is an inescapable feature of the Greater Horn of Africa’s generally semiarid climate and with roughly 75% of the labor force in the region involved in smallholder, rain-fed agriculture ( Salami et al. 2010 ), its impacts on crops and human populations can be acute. The severe drought of 2010/11 in Kenya, Somalia, and southeastern Ethiopia, for example, affected some ten million people and was a contributing factor to more than 250 000 fatalities in Somalia alone ( UN

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Mathew Barlow, Benjamin Zaitchik, Shlomit Paz, Emily Black, Jason Evans, and Andrew Hoell

addition to directly affecting northern and southeastern Pakistan, appears to also remotely influence precipitation over the northern part of the domain via an influence on temperature and stability ( Schiemann et al. 2007 ). It has also been suggested that the Indian monsoon may remotely force subsidence over the region ( Rodwell and Hoskins 1996 , 2001 ), with orographic interaction playing an important role ( Rodwell and Hoskins 2001 ; Tyrlis et al. 2013 ; Simpson et al. 2015 ), and observational

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Siegfried D. Schubert, Hailan Wang, Randal D. Koster, Max J. Suarez, and Pavel Ya. Groisman

of the westerlies (inhibiting the inflow of moisture from the west) contributing to the drought conditions. More recently, Nakamura et al. (1997) contrasted Pacific and Atlantic blocking events and found that incoming wave activity associated with a quasi-stationary Rossby wave train is of primary importance in the development of blocking over Europe, while the forcing from synoptic-scale transients is key for the development over the North Pacific. In addition to blocking, a number of other

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Wenju Cai, Ariaan Purich, Tim Cowan, Peter van Rensch, and Evan Weller

/seasonal deficits during the WWII Drought. This suggests that, with the exception of the southeast Australia autumn and SWWA winter deficits, the severe conditions observed during the Millennium Drought can be induced by natural variability alone, as they have been observed previously during a period with little climate change forcing. b. Observed rainfall trends congruent with trends in climate indices Given the impact of the three main climate modes on rainfall, we examine whether trends in their indices

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Omar V. Müller, Ernesto Hugo Berbery, Domingo Alcaraz-Segura, and Michael B. Ek

convective schemes, the decision to activate or not activate deep convection in BMJ is not based entirely upon cloud layer moisture. The reference profiles for the BMJ scheme are calculated by lifting parcels from the boundary layer, so their time dependence responds directly to the land surface forcing and the time dependent response of the mixed layer. For these reasons, alterations to conditions within the planetary boundary layer have an effect on the simulations (K. Betts and Z. Janjić 2013

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