Decadal drought variability over North America: Mechanisms and predictability - PDF

The physical mechanisms and potential predictability of North American drought on decadal timescales are reviewed in a simple and straightforward manner amenable to a wide audience. During decadal droughts, the tropical oceans, most notably cold states of the Pacific but also warm states of the Atlantic, provide forcing that continually nudges the atmosphere toward circulation anomalies that favor high pressure over southern North America and dry conditions. However, even in these regions, and even more so in the northwest and northeast, the oceans exert less than dominant control and actual drought onset, evolution and termination can deviate due, presumably, to potent internal atmosphere variability. The ocean influence, however, justifies efforts to determine if the driving sea surface temperature anomalies in the tropical Pacific and Atlantic are predictable beyond the seasonal to interannual timescale. Evidence to date, based on initialized predictions with coupled models, is tantalizingly suggestive that useful predictability on these timescales may exist within the atmosphere-ocean system although relevance to North American decadal drought has not yet been demonstrated. These recent advances in drought science and prediction warrant continued research aimed at developing useful long term predictions of drought that can guide adaptation and minimize the associated widespread social and economic disruptions.

REFERENCE

Seager, R. and M. Ting, 2017: Decadal drought variability over North America: Mechanisms and predictability. Curr. Clim. Change Rep.,  1-9, doi:10.1007/s40641-017-0062-1.

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On the role of tropical ocean forcing of the persistent North American west coast ridge of winter 2013/14a - PDF

The causes of the high pressure ridge at the North American west coast during winter 2013/14, the driest winter of the recent California drought, are examined. The ridge was part of an atmosphere–ocean state that included anomalies, defined relative to a 1979–2014 mean, of circulation across the Northern Hemisphere, warm sea surface temperatures (SSTs) in the tropical western and northeastern Pacific and the south Indian Ocean, and cool SSTs in the central tropical Pacific. The SST anomalies differ sufficiently between datasets that, when used to force atmosphere models, the simulated circulation anomalies vary notably in realism. Recognizing uncertainty in the SST field, the authors use idealized tropical SST anomaly experiments to identify an optimal combination of SST anomalies that forces a circulation response that best matches observations. The optimal SST pattern resembles that observed but the associated circulation pattern is much weaker than observed, suggesting an important but limited role for ocean forcing. Analysis of the equilibrium and transient upper-troposphere vorticity balance indicates that the SST-forced component of the ridge arose as a summed effect of Rossby waves forced by SST anomalies across the tropical Indo-Pacific oceans and drives upper-troposphere convergence and subsidence at the west coast. The ridge, in observations and model, is associated with northward and southward diversion of storms. The results suggest that tropical Indo-Pacific ocean SSTs helped force the west coast ridge and drought of winter 2013/14.

REFERENCE

Seager, R. and N. Henderson, 2016: On the role of tropical ocean forcing of the persistent North American west coast ridge of winter 2013/14a. J. Climate, 29: 8027 - 8049, DOI: 10.1175/JCLI-D-16-0145.1.

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Western boundary currents and climate change - PDF

A recent paper in Journal of Geophysical Research-Oceans connects recent changes in atmospheric circulation to poleward movement and intensification of western boundary currents. Causes and characteristics of past and future trends in surface wind stress and western boundary currents are discussed here.

REFERENCE

Seager, R. and I.R. Simpson, 2016: Western boundary currents and climate change. J. Geo. Res.: Oceans, 121: doi:10.1002/2016JC012156.

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Medieval megadroughts in the Four Corners region:
Characterization and causes - PDF

Analyses of tree rings across North America make very clear that between about 800A.D. and 1400 A.D. the American West experienced one severe multidecadal drought after another, with only brief respite, that amounted to a more arid climate during the Medieval period than in subsequent centuries or now (Cook et al., 2004). One of these ’megadroughts’, at the end of the 13th Century is the ’Great Drouth’ originally identified by Douglass (1929, 1935) and which has been linked to a population decrease in the Puebloan societies of the Four Corners region or even an abandonment of well developed settlements (e.g. Benson et al. (2007)). Here we attempt to characterize these droughts in space and time and offer ideas as to what caused them.

REFERENCE

Seager, R., and E.R. Cook, 2015: Medieval megadroughts in the Four Corners region: Characterization and causes. Presentation: The Society for American Archaelogy, Austin, Texas, April 2007.

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Causes of the 2011 to 2014 California drought - PDF

The causes of the California drought during November–April winters of 2011/12–2013/14 are analyzed using observations and ensemble simulations with seven atmosphere models forced by observed SSTs. Historically, dry California winters are most commonly associated with a ridge off the west coast but no obvious SST forcing. Wet winters are most commonly associated with a trough off the west coast and an El Niño event. These attributes of dry and wet winters are captured by many of the seven models. According to the models, SST forcing can explain up to a third of California winter precipitation variance. SST forcing was key to sustaining a high pressure ridge over the west coast and suppressing precipitation during the three winters. In 2011/12 this was a response to a La Niña event, whereas in 2012/13 and 2013/14 it appears related to a warm west–cool east tropical Pacific SST pattern. All models contain a mode of variability linking such tropical Pacific SST anomalies to a wave train with a ridge off the North American west coast. This mode explains less variance than ENSO and Pacific decadal variability, and its importance in 2012/13 and 2013/14 was unusual. The models from phase 5 of CMIP (CMIP5) project rising greenhouse gases to cause changes in California all-winter precipitation that are very small compared to recent drought anomalies. However, a long-term warming trend likely contributed to surface moisture deficits during the drought. As such, the precipitation deficit during the drought was dominated by natural variability, a conclusion framed by discussion of differences between observed and modeled tropical SST trends.

REFERENCE

Seager, R, M. Hoerling, S. Schubert, H. Wang, B. Lyon, A. Kumar, J. Nakamura, N. Henderson, 2015: Causes of the 2011 to 2014 California drought. J. Climate, 28, DOI: 10.1175/JCLI-D-14-00860.1, 6997-7024.

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Climatology, Variability and Trends in U.S. Vapor Pressure Deficit, an Important Fire-Related Meteorological Quality - PDF

Unlike the commonly used relative humidity, vapor pressure deficit (VPD) is an absolute measure of the difference between the water vapor content of the air and its saturation value and an accurate metric of the ability of the atmosphere to extract moisture from the land surface. VPD has been shown to be closely related to variability in burned forest areas in the western United States. Here, the climatology, variability, and trends in VPD across the United States are presented. VPD reaches its climatological maximum in summer in the interior southwest United States because of both high temperatures and low vapor pressure under the influence of the northerly, subsiding eastern flank of the Pacific subtropical anticyclone. Maxima of variance of VPD are identified in the Southwest and southern plains in spring and summer and are to a large extent driven by temperature variance, but vapor pressure variance is also important in the Southwest. La Niña–induced circulation anomalies cause subsiding, northerly flow that drives down actual vapor pressure and increases saturation vapor pressure from fall through spring. High spring and summer VPDs can also be caused by reduced precipitation in preceding months, as measured by Bowen ratio anomalies. Case studies of 2002 (the Rodeo–Chediski and Hayman fires, which occurred in Arizona and Colorado, respectively) and 2007 (the Murphy Complex fire, which occurred in Idaho and Nevada) show very high VPDs caused by antecedent surface drying and subsidence warming and drying of the atmosphere. VPD has increased in the southwest United States since 1961, driven by warming and a drop in actual vapor pressure, but has decreased in the northern plains and Midwest, driven by an increase in actual vapor pressure.

REFERENCE

Seager, R., A. Hooks, A.P. Williams, B. Cook, J. Nakamua and N. Henderson, 2015: Climatology, Variability and Trends in U.S. Vapor Pressure Deficit, an Important Fire-Related Meteorological Quality. J. App. Meteor. Climat., 54, 1121-1141, DOI: 10.1175/JAMC-D-14-0321.1.

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Atmosphere and Ocean Origins of North American Droughts - PDF
The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

REFERENCE

Seager, R. and M. Hoerling, 2014: Atmosphere and Ocean Origins of North American Droughts. J. Climate, 27: 4581-4606.

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Dynamical causes of the 2010/11 Texas-Northern Mexico drought - PDF

The causes of the Texas–northern Mexico drought during 2010–11 are shown, using observations, reanalyses, and model simulations, to arise from a combination of ocean forcing and internal atmospheric variability. The drought began in fall 2010 and winter 2010/11 as a La Nina event developed in the tropical Pacific Ocean. ~ Climate models forced by observed sea surface temperatures (SSTs) produced dry conditions in fall 2010 through spring 2011 associated with transient eddy moisture flux divergence related to a northward shift of the Pacific–North American storm track, typical of La Nina events. In contrast the observed drought was not ~ associated with such a clear shift of the transient eddy fields and instead was significantly influenced by internal atmospheric variability including the negative North Atlantic Oscillation of winter 2010/11, which created mean flow moisture divergence and drying over the southern Plains and southeast United States. The models suggest that drought continuation into summer 2011 was not strongly SST forced. Mean flow circulation and moisture divergence anomalies were responsible for the summer 2011 drought, arising from either internal atmospheric variability or a response to dry summer soils not captured by the models. The summer of 2011 was one of the two driest and hottest summers over recent decades but it does not represent a clear outlier to the strong inverse relation between summer precipitation and temperature in the region. Seasonal forecasts at 3.5-month lead time did predict onset of the drought in fall and winter 2010/11 but not intensification into summer 2011, demonstrating the current, and likely inherent, inability to predict important aspects of North American droughts.

REFERENCE

Seager, R., L. Goddard, J. Nakamura, N. Henderson and D. Lee, 2014: Dynamical causes of the 2010/11 Texas-northern Mexico drought. J. Hydrometeorology, 15, 39-68, doi: 10.1175/JHM-D-13-024.1.

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Dynamical and thermodynamical causes of large-scale changes in the hydrological cycle over North America in response to global warming - PDF

The mechanisms of model-projected atmospheric moisture budget change across North America are examined in simulations conducted with 22 models from phase 5 of the Coupled Model Intercomparison Project. Modern-day model budgets are validated against the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis. In the winter half year transient eddies converge moisture across the continent while the mean flow wets the west from central California northward and dries the southwest. In the summer half year there is widespread mean flow moisture divergence across the west and convergence over the Great Plains that is offset by transient eddy divergence. In the winter half year the models project drying for the southwest and wetting to the north. Changes in the mean flow moisture convergence are largely responsible across the west but intensified transient eddy moisture convergence wets the northeast. In the summer half year widespread declines in precipitation minus evaporation (P 2 E) are supported by mean flow moisture divergence across the west and transient eddy divergence in the Great Plains. The changes in mean flow convergence are related to increases in specific humidity but also depend on changes in the mean flow including increased low-level divergence in the U.S. Southwest and a zonally varying wave that wets the North American west and east coasts in winter and dries the U.S. Southwest. Increased transient eddy fluxes occur even as low-level eddy activity weakens and arise from strengthened humidity gradients. A full explanation of North American hydroclimate changes will require explanation of mean and transient circulation changes and the coupling between the moisture and circulation fields.

REFERENCE

Seager, R., D. Neelin, I. Simpson, H. Liu, N. Henderson, T. Shaw, Y. Kushnir, M. Ting and B. Cook, 2014: Dynamical and thermodynamical causes of large-scale changes in the hydrological cycle over North America in response to global warming. J. Climate, 27: 7921-7948, doi: 10.1175/JCLI-D-14-00153.1.