Probable Causes of the Abnormal Ridge Accompanying the 2013-14 California Drought: ENSO Precursor and Anthropogenic Warming Footprint

The 2013–2014 California drought was initiated by an anomalous high-amplitude ridge system. The anomalous ridge was investigated using reanalysis data and the Community Earth System Model (CESM). It was found that the ridge emerged from continual sources of Rossby wave energy in the western North Pacific starting in late summer and subsequently intensified into winter. The ridge generated a surge of wave energy downwind and deepened further the trough over the northeast U.S., forming a dipole. The dipole and associated circulation pattern is not linked directly with either El Niño–Southern Oscillation (ENSO) or Pacific Decadal Oscillation; instead, it is correlated with a type of ENSO precursor. The connection between the dipole and ENSO precursor has become stronger since the 1970s, and this is attributed to increased greenhouse gas loading as simulated by the CESM. Therefore, there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013–2014 and the associated drought

Modern Climatology - Full Text

Climatology, the study of climate, is no longer regarded as a single discipline that treats climate as something that fluctuates only within the unchanging boundaries described by historical statistics. The field has recognized that climate is something that changes continually under the influence of physical and biological forces and so, cannot be understood in isolation but rather, is one that includes diverse scientific disciplines that play their role in understanding a highly complex coupled “whole system” that is the Earth’s climate. The modern era of climatology is echoed in this book. On the one hand it offers a broad synoptic perspective but also considers the regional standpoint as it is this that affects what people need from climatology, albeit water resource managers or engineers etc. Aspects on the topic of climate change – what is often considered a contradiction in terms – is also addressed. It is all too evident these days that what recent work in climatology has revealed carries profound implications for economic and social policy; it is with these in mind that the final chapters consider acumens as to the application of what has been learned to date. This book is divided into four sections that cover sub-disciplines in climatology. The first section contains four chapters that pertain to synoptic climatology, i.e., the study of weather disturbances including hurricanes, monsoon depressions, synoptic waves, and severe thunderstorms; these weather systems directly impact humanity. The second section on regional climatology has four chapters that describe the climate features within physiographically defined areas. The third section is on climate change which involves both past (paleoclimate) and future climate: The first two chapters cover certain facets of paleoclimate while the third is centered towards the signals (observed or otherwise) of climate change. The fourth and final section broaches the sub-discipline that is often referred to as applied climatology; this represents the important goal of all studies in climatology–one that affects modes of living. Here, three chapters are devoted towards the application of climatological research that might have useful application for operational purposes in industrial, manufacturing, agricultural, technological and environmental affairs. Please click here to explore the components of this work.https://digitalcommons.usu.edu/modern_climatology/1014/thumbnail.jp

Observed change in Sahel rainfall, circulations, African easterly waves, and Atlantic hurricanes since 1979

Here, we examine the dynamic properties associated with the recent increase in the Sahel rainfall using an ensemble of five global reanalysis datasets (1979–2010). The rainfall that has been observed to be increasing over the Sahel is accounted for by enhancements in both the tropical easterly jet and the African easterly jet, both of which are known to induce wet anomalies. Moreover, positional shifts in the African easterly jet and African easterly waves (AEWs) accompanied the northward migration of the Sahel rainband. Change in the African easterly jet and AEWs are coupled to a northward shift and amplification of convective activity; this signals an increased potential for the occurrence of flash floods along the northern Sahel. In addition, the result from a wave tracking analysis suggests that the change in AEWs is closely linked to increased activity of intense hurricanes in the North Atlantic. The synoptic concurrence of AEWs in driving the dynamics of the Sahel greening and the increase in tropical cyclogeneses over the North Atlantic is an important aspect in the evaluation of climate model projections

Large-Scale Control of the Arabian Sea Monsoon Inversion in August

The summer monsoon inversion in the Arabian Sea is characterized by a large amount of low clouds and August as the peak season. Atmospheric stratification associated with the monsoon inversion has been considered a local system influenced by the advancement of the India–Pakistan monsoon. Empirical and numerical evidence from this study suggests that the Arabian Sea monsoon inversion is linked to a broader-scale monsoon evolution across the African Sahel, South Asia, and East Asia–Western North Pacific (WNP), rather than being a mere byproduct of the India–Pakistan monsoon progression. In August, the upper-tropospheric anticyclone in South Asia extends sideways corresponding with the enhanced precipitation in the subtropical WNP, equatorial Indian Ocean, and African Sahel while the middle part of this anticyclone weakens over the Arabian Sea. The increased heating in the adjacent monsoon systems creates a suppression effect on the Arabian Sea, suggesting an apparent competition among the Africa–Asia–WNP monsoon subsystems. The peak Sahel rainfall in August, together with enhanced heating in the equatorial Indian Ocean, produces a critical effect on strengthening the Arabian Sea thermal inversion. By contrast, the WNP monsoon onset which signifies the eastward expansion of the subtropical Asian monsoon heating might play a secondary or opposite role in the Arabian Sea monsoon inversion

Examination of the Climate Factors That Reduced Wheat Yield in Northwest India during the 2000s

In India, a significant reduction of wheat yield would cause a widespread impact on food security for 1.35 billion people. The two highest wheat producing states, Punjab and Haryana in northern India, experienced a prolonged period of anomalously low wheat yield during 2002–2010. The extent of climate variability and change in influencing this prolonged reduction in wheat yield was examined. Daily air temperature (Tmax and Tave) was used to calculate the number of days above optimum temperature and growing degree days (GDD) anomaly. Two drought indices, the standard precipitation and evapotranspiration index and the radiation-based precipitation index, were used to describe the drought conditions. Groundwater variability was assessed via satellite-based approximation. The analysis results indicate that the wheat yield loss corresponds to the increase in the number of days with a temperature above 35 °C during the maturity stage (March). Reduction in monsoon rainfall led to a depletion of groundwater and reduced surface water for irrigation in the wheat growing season (November–March). Higher temperatures, coupled with water shortage and irregular irrigation, also appear to impact the yield reduction. In hindsight, improving the agronomic practices to minimize crop water usage could be an adaptation strategy to maintain the desired wheat yield in the face of climate-induced drought and precipitation anomaly

Gridded Snow Water Equivalent Reconstruction for Utah Using Forest Inventory and Analysis Tree-Ring Data

Snowpack observations in the Intermountain West are sparse and short, making them difficult for use in depicting past variability and extremes. This study presents a reconstruction of April 1 snow water equivalent (SWE) for the period of 1850–1989 using increment cores collected by the U.S. Forest Service, Interior West Forest Inventory and Analysis program (FIA). In the state of Utah, SWE was reconstructed for 38 snow course locations using a combination of standardized tree-ring indices derived from both FIA increment cores and publicly available tree-ring chronologies. These individual reconstructions were then interpolated to a 4-km grid using an objective analysis with elevation correction to create an SWE product. The results showed a significant correlation with observed SWE as well as good correspondence to regional tree-ring-based drought reconstructions. Diagnostic analysis showed statewide coherent climate variability on inter-annual and inter-decadal time-scales, with added geographical details that would not be possible using courser pre-instrumental proxy datasets. This SWE reconstruction provides water resource managers and forecasters with better spatial resolution to examine past variability in snowpack, which will be important as future hydroclimatic variability is amplified by climate change

A Combined Dynamical and Statistical Downscaling Technique to Reduce Biases in Climate Projections: An Example for Winter Precipitation and Snowpack in the Western United States

Large biases associated with climate projections are problematic when it comes to their regional application in the assessment of water resources and ecosystems. Here, we demonstrate a method that can reduce systematic biases in regional climate projections. The global and regional climate models employed to demonstrate the technique are the Community Climate System Model (CCSM)and the Weather Research and Forecasting (WRF) model. The method first utilized a statistical regression technique and a global reanalysis dataset to correct biases in the CCSM-simulated variables (e.g., temperature, geopotential height, specific humidity, and winds) that are subsequently used to drive the WRF model. The WRF simulations were conducted for the western United States and were driven with (a) global reanalysis, (b) original CCSM, and (c) bias-corrected CCSM data. The bias-corrected CCSM data led to a more realistic regional climate simulation of precipitation and associated atmospheric dynamics, as well as snow water equivalent (SWE), in comparison to the original CCSM-driven WRF simulation. Since most climate applications rely on existing global model output as the forcing data (i.e., they cannot re-run or change the global model), which often contain large biases, this method provides an effective and economical tool to reduce biases in regional climate downscaling simulations of water resource variables

Investigating Forest Inventory and Analysis-Collected Tree-Ring Data from Utah as a Proxy for Historical Climate

Increment cores collected as part of the periodic inventory in the Intermountain West were examined for their potential to represent growth and be a proxy for climate (precipitation) over a large region (Utah). Standardized and crossdated time-series created from pinyon pine (n=249) and Douglas-fir (n=274) increment cores displayed spatiotemporal patterns in growth differences both between species and by region within Utah. However, the between-species interrelationship of growth was strong over much of the state and indicated both species respond similarly to climate variations. Indeed, pinyon pine and Douglas-fir exhibited a significant and spatially coherent response to instrumental precipitation data. Previous water year (5-month lag) exhibited the strongest relationship to tree-ring increment for both species. Results suggest increment cores collected by Forest Inventory and Analysis are excellent proxies for historical precipitation

Evaluations of NAM forecasts on midtropospheric perturbation-induced convective storms over the U.S. northern plains

In the U.S. northern plains, summer progressive convective storms that occur in weakly forced environ- ments are often coupled with short-wave perturbations that are embedded in the midlevel northwesterly flow. These midtropospheric perturbations (MPs) are capable of inducing propagating convection that contributes to a majority of the rainfall over the northern plains during July and August. There is a possibility that the difficulties of numerical weather prediction models in forecasting summer convective rainfall over the northern plains are partly attributed to their deficiency in forecasting MPs. The present study tests this possibility through examining operational forecasts by the North American Mesoscale (NAM) model during the summers of 2005 and 2006. Forecasted MPs exhibit slower propagation speeds and weaker relative vorticity than the observations leading to systematic position errors. Underpredicted vorticity magnitudes weaken horizontal vorticity ad- vection that influences the vorticity tendency throughout the MP life cycle and, in turn, slows the propagation speed of MPs. Moreover, biases of weak ambient flow speed and vortex stretching contribute to the mag- nitude and propagation speed errors of MPs. Skill scores of precipitation forecasts associated with MPs are low, but can be considerably improved after removing the MP position error that displaces the rainfall pat- tern. The NAM also tends to underpredict precipitation amounts. A modified water vapor budget analysis reveals that the NAM insufficiently generates atmospheric humidity over the central United States. The shortage of moisture in the forecast reduces the water vapor flux convergence that is part of the precipitation process. The precipitation bias may feed back to affect the MP growththrough the bias in heating, thus further slowing the perturbation