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Richard Seager and Naomi Henderson

Abstract

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.

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Richard Seager and Naomi Henderson

Abstract

The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flow–humidity covariances be archived at high vertical resolution.

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Xiaoqiong Li, Mingfang Ting, Cuihua Li, and Naomi Henderson

Abstract

Changes of the Asian summer monsoon in response to anthropogenic forcing are examined using observations and phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel, multirealization ensemble. In the twentieth century, CMIP5 models indicate a predominantly drying Asian monsoon, while in the twenty-first century under the representative concentration pathway 8.5 (RCP8.5) scenario, monsoon rainfall enhances across the entire Asian domain. The thermodynamic and dynamic mechanisms causing the changes are evaluated using specific humidity and winds, as well as the moisture budget. The drying trend in the CMIP5 historical simulations and the wetting trend in the RCP8.5 projections can be explained by the relative importance of dynamic and thermodynamic contributions to the total mean moisture convergence. While the thermodynamic mechanism dominates in the future, the historical rainfall changes are dominated by the changes in circulation. The relative contributions of aerosols and greenhouse gases (GHGs) on the historical monsoon change are further examined using CMIP5 single-forcing simulations. Rainfall reduces under aerosol forcing and increases under GHG forcing. Aerosol forcing dominates over the greenhouse effect during the historical period, leading to the general drying trend in the all-forcing simulations. While the thermodynamic change of mean moisture convergence in the all-forcing case is dominated by the GHG forcing, the dynamic change of mean moisture convergence in the all-forcing case is dominated by the aerosol forcing.

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Mingfang Ting, Richard Seager, Cuihua Li, Haibo Liu, and Naomi Henderson

Abstract

The net surface water budget, precipitation minus evaporation (PE), shows a clear seasonal cycle in the U.S. Southwest with a net gain of surface water (positive PE) in the cold half of the year (October–March) and a net loss of water (negative PE) in the warm half (April–September), with June and July being the driest months of the year. There is a significant shift of the summer drying toward earlier in the year under a CO2 warming scenario, resulting in substantial spring drying (March–May) of the U.S. Southwest from the near-term future to the end of the current century, with gradually increasing magnitude. While the spring drying has been identified in previous studies, its mechanism has not been fully addressed. Using moisture budget analysis, it was found that the drying is mainly due to decreased mean moisture convergence, partially compensated by the increase in transient eddy moisture flux convergence. The decreased mean moisture convergence is further separated into components as a result of changes in circulation (dynamic changes) and changes in atmospheric moisture content (thermodynamic changes). The drying is found to be dominated by the thermodynamic-driven changes in column-averaged moisture convergence, mainly due to increased dry zonal advection caused by the climatological land–ocean thermal contrast, rather than by the well-known “dry get drier” mechanism. Furthermore, the enhanced dry advection in the warming climate is dominated by the robust zonal mean atmospheric warming, leading to equally robust spring drying in the southwestern United States.

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David Chapman, Mark A. Cane, Naomi Henderson, Dong Eun Lee, and Chen Chen

Abstract

The authors investigate a sea surface temperature anomaly (SSTA)-only vector autoregressive (VAR) model for prediction of El Niño–Southern Oscillation (ENSO). VAR generalizes the linear inverse method (LIM) framework to incorporate an extended state vector including many months of recent prior SSTA in addition to the present state. An SSTA-only VAR model implicitly captures subsurface forcing observable in the LIM residual as red noise. Optimal skill is achieved using a state vector of order 14–17 months in an exhaustive 120-yr cross-validated hindcast assessment. It is found that VAR outperforms LIM, increasing forecast skill by 3 months, in a 30-yr retrospective forecast experiment.

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Richard Seager, Martin Hoerling, Siegfried Schubert, Hailan Wang, Bradfield Lyon, Arun Kumar, Jennifer Nakamura, and Naomi Henderson

Abstract

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.

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Richard Seager, Haibo Liu, Naomi Henderson, Isla Simpson, Colin Kelley, Tiffany Shaw, Yochanan Kushnir, and Mingfang Ting

Abstract

The hydrological cycle in the Mediterranean region, as well as its change over the coming decades, is investigated using the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) and phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations and projections of the coming decades. The Mediterranean land regions have positive precipitation minus evaporation, PE, in winter and negative PE in summer. According to ERA-Interim, positive PE over land in winter is sustained by transient eddy moisture convergence and opposed by mean flow moisture divergence. Dry mean flow advection is important for opposing the transient eddy moisture flux convergence in the winter half year and the mass divergent mean flow is a prime cause of negative PE in the summer half year. These features are well reproduced in the CMIP5 ensemble. The models predict reduced PE over the Mediterranean region in the future year-round. For both land and sea, a common cause of drying is increased mean flow moisture divergence. Changes in transient eddy moisture fluxes largely act diffusively and cause drying over the sea and moistening over many land areas to the north in winter and drying over western land areas and moistening over the eastern sea in summer. Increased mean flow moisture divergence is caused by both the increase in atmospheric humidity in a region of mean flow divergence and strengthening of the mass divergence. Increased mass divergence is related to increased high pressure over the central Mediterranean in winter and over the Atlantic and northern Europe in summer, which favors subsidence and low-level divergence over the Mediterranean region.

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Richard Seager, Lisa Goddard, Jennifer Nakamura, Naomi Henderson, and Dong Eun Lee

Abstract

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 Niña 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 Niña 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.

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Richard Seager, Naomi Henderson, Mark A. Cane, Haibo Liu, and Jennifer Nakamura

Abstract

The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific–North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo–west Pacific, an arrangement of tropical oceans and Pacific–North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case.

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Richard Seager, Allison Hooks, A. Park Williams, Benjamin Cook, Jennifer Nakamura, and Naomi Henderson

Abstract

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.

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