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The article presents a study exploring causes of drought in northern High Plains in 2017. Investigated is the potential impact of sea surface temperature (SST) anomalies and atmospheric changes on the occurrence of precipitation deficits. Discussed are probability density functions (PDFs) of drought indications.

The factors impacting western U.S. winter precipitation during the 2015/16 El Niño are investigated using the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), data, and simulations with the Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model forced with specified sea surface temperatures (SSTs). Results reveal that the simulated response to the tropical Pacific SST associated with the 2015/16 El Niño was to produce wetter than normal conditions over much of the North American west coast including California—a result at odds with the negative precipitation anomalies observed over much of the southwestern United States. It is shown that two factors acted to partly counter the canonical ENSO response in that region. First, a potentially predictable but modest response to the unusually strong and persistent warm SST in the northeastern Pacific decreased precipitation in the southwestern United States by increasing sea level pressure, driving anticyclonic circulation and atmospheric descent, and reducing moisture transport into that region. Second, large-scale unforced (by SST) components of atmospheric variability (consisting of the leading modes of unpredictable intraensemble variability) resembling the positive phase of the North Atlantic Oscillation and Arctic Oscillation are found to be an important contributor to the drying over the western United States. While a statistical reconstruction of the precipitation from our simulations that account for internal atmospheric variability does much to close the gap between the ensemble-mean and observed precipitation in the southwestern United States, some differences remain, indicating that model error is also playing a role. [ABSTRACT FROM AUTHOR]

A series of stationary wave model (SWM) experiments are performed in which the boreal summer atmosphere is forced, over a number of locations in the continental United States, with an idealized diabatic heating anomaly that mimics the atmospheric heating associated with a dry land surface. For localized heating within a large portion of the continental interior, regardless of the specific location of this heating, the spatial pattern of the forced atmospheric circulation anomaly (in terms of 250-hPa eddy streamfunction) is largely the same: a high anomaly forms over west-central North America and a low anomaly forms to the east. In supplemental atmospheric general circulation model (AGCM) experiments, similar results are found; imposing soil moisture dryness in the AGCM in different locations within the U.S. interior tends to produce the aforementioned pattern, along with an associated near-surface warming and precipitation deficit in the center of the continent. The SWM-based and AGCM-based patterns generally agree with composites generated using reanalysis and precipitation gauge data. The AGCM experiments also suggest that dry anomalies imposed in the lower Mississippi River valley have remote surface impacts of particularly large spatial extent, and a region along the eastern half of the U.S.-Canadian border is particularly sensitive to dry anomalies in a number of remote areas. Overall, the SWM and AGCM experiments support the idea of a positive feedback loop operating over the continent: dry surface conditions in many interior locations lead to changes in atmospheric circulation that act to enhance further the overall dryness of the continental interior. [ABSTRACT FROM AUTHOR]

Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forced impacts on precipitation on interannual time scales. Decadal changes in SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s 'climate shifts' in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land-atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought. [ABSTRACT FROM AUTHOR]

The dominant pattern of SST variability in the Pacific during its cold phase produces pronounced precipitation deficits over the continental United States throughout the annual cycle. This study investigates the observed physical and dynamical processes through which the cold Pacific pattern affects U.S. precipitation, particularly the causes for the peak dry impacts in fall, as well as the nature of the differences between the summer and fall responses. Results show that the peak precipitation deficit over the United States during fall is primarily due to reduced atmospheric moisture transport from the Gulf of Mexico into the central and eastern United States and secondarily a reduction in local evaporation from land-atmosphere feedback. The former is associated with a strong and systematic low-level northeasterly flow anomaly over the southeastern United States that counteracts the northwest branch of the climatological North Atlantic subtropical high. The above northeasterly anomaly is maintained by both diabatic heating anomalies in the nearby intra-American seas and diabatic cooling anomalies in the tropical Pacific. In contrast, the modest summertime precipitation deficit over the central United States is mainly an intensification of the local dry anomaly in the preceding spring from local land-atmosphere feedback; the rather weak and disorganized atmospheric circulation anomalies over and to the south of the United States make little contribution. An evaluation of the NASA Seasonal-to-Interannual Prediction Project (NSIPP-1) AGCM simulations shows it to be deficient in simulating the warm season tropical convection responses over the intra-American seas to the cold Pacific pattern and thereby the precipitation responses over the United States, a problem that appears to be common to many AGCMs. [ABSTRACT FROM AUTHOR]

This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5-6 and 6-9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6-9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5-6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6-9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation. [ABSTRACT FROM AUTHOR]

This study examines the cause of the extreme snowstorm activity along the U.S. East Coast during the winter of 2009/10 with a focus on the role of sea surface temperature (SST) anomalies. The study employs the Goddard Earth Observing System, version 5 (GEOS-5) atmospheric general circulation model (AGCM) run at high resolution and forced with specified observed or idealized SST. Comparisons are made with the winter of 1999/2000, a period that is characterized by SST anomalies that are largely of opposite sign. When forced with observed SSTs, the AGCM response consists of a band of enhanced storminess extending from the central subtropical North Pacific, across the southern United States, across the North Atlantic, and across southern Eurasia, with reduced storminess to the north of these regions. Positive precipitation and cold temperature anomalies occur over the eastern United States, reflecting a propensity for enhanced snowstorm activity. Additional idealized SST experiments show that the anomalies over the United States are, to a large extent, driven by the ENSO-related Pacific SST. The North Atlantic SSTs contribute to the cooler temperatures along the East Coast of the United States, while the Indian Ocean SSTs act primarily to warm the central part of the country. It is further shown that the observed upper-tropospheric height anomalies have a large noise (unforced) component over the Northern Hemisphere, represented over the North Atlantic by a North Atlantic Oscillation (NAO)-like structure. The signal-to-noise ratios of the temperature and precipitation fields nevertheless indicate a potential for predicting the unusual storm activity along the U.S. East Coast several months in advance. [ABSTRACT FROM AUTHOR]

This study investigates the structure of the African easterly jet, focusing on instability processes on a seasonal and subseasonal scale, with the goal of identifying features that could provide increased predictability of Atlantic tropical cyclogenesis. The Modern-Era Retrospective Analysis for Research and Applications (MERRA) is used as the main investigating tool. MERRA is compared with other reanalyses datasets from major operational centers around the world and was found to describe very effectively the circulation over the African monsoon region. In particular, a comparison with precipitation datasets from the Global Precipitation Climatology Project shows that MERRA realistically reproduces seasonal precipitation over that region. The verification of the generalized Kuo barotropic instability condition computed from seasonal means is found to have the interesting property of defining well the location where observed tropical storms are detected. This property does not appear to be an artifact of MERRA and is present also in the other adopted reanalysis datasets. Therefore, the fact that the areas where the mean flow is unstable seems to provide a more favorable environment for wave intensification, could be another factor to include-in addition to sea surface temperature, vertical shear, precipitation, the role of Saharan air, and others-among large-scale forcings affecting development and tropical cyclone frequency. In addition, two prominent modes of variability are found based on a spectral analysis that uses the Hilbert-Huang transform: a 2.5-6-day mode that corresponds well to the African easterly waves and also a 6-9-day mode that seems to be associated with tropical-extratropical interaction. [ABSTRACT FROM AUTHOR]

This study examines the nature of boreal summer subseasonal atmospheric variability based on the new NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) for the period 1979-2010. An analysis of the June, July, and August subseasonal 250-hPa meridional υυ-wind anomalies shows distinct Rossby wave-like structures that appear to be guided by the mean jets. On monthly subseasonal time scales, the leading waves [the first 10 rotated empirical orthogonal functions (REOFs) of the 250-hPa υυ wind] explain about 50%% of the Northern Hemisphere υυ-wind variability and account for more than 30%% (60%%) of the precipitation (surface temperature) variability over a number of regions of the northern middle and high latitudes, including the U.S. northern Great Plains, parts of Canada, Europe, and Russia. The first REOF in particular consists of a Rossby wave that extends across northern Eurasia where it is a dominant contributor to monthly surface temperature and precipitation variability and played an important role in the 2003 European and 2010 Russian heat waves. While primarily subseasonal in nature, the Rossby waves can at times have a substantial seasonal mean component. This is exemplified by REOF 4, which played a major role in the development of the most intense anomalies of the U.S. 1988 drought (during June) and the 1993 flooding (during July), though differed in the latter event by also making an important contribution to the seasonal mean anomalies. A stationary wave model (SWM) is used to reproduce some of the basic features of the observed waves and provide insight into the nature of the forcing. In particular, the responses to a set of idealized forcing functions are used to map the optimal forcing patterns of the leading waves. Also, experiments to reproduce the observed waves with the SWM using MERRA-based estimates of the forcing indicate that the wave forcing is dominated by submonthly vorticity transients. [ABSTRACT FROM AUTHOR]

This study assesses the regional-scale summer precipitation produced by the dynamical downscaling of analyzed large-scale fields. The main goal of this study is to investigate how much the regional model adds smaller scale precipitation information that the large-scale fields do not resolve. The modeling region for this study covers the southeastern United States (Florida, Georgia, Alabama, South Carolina, and North Carolina) where the summer climate is subtropical in nature, with a heavy influence of regional-scale convection. The coarse resolution (2.5° latitude/longitude) large-scale atmospheric variables from the National Center for Environmental Prediction (NCEP)/DOE reanalysis (R2) are downscaled using the NCEP/Environmental Climate Prediction Center regional spectral model (RSM) to produce precipitation at 20 km resolution for 16 summer seasons (1990-2005). The RSM produces realistic details in the regional summer precipitation at 20 km resolution. Compared to R2, the RSM-produced monthly precipitation shows better agreement with observations. There is a reduced wet bias and a more realistic spatial pattern of the precipitation climatology compared with the interpolated R2 values. The root mean square errors of the monthly R2 precipitation are reduced over 93% (1,697) of all the grid points in the five states (1,821). The temporal correlation also improves over 92% (1,675) of all grid points such that the domain-averaged correlation increases from 0.38 (R2) to 0.55 (RSM). The RSM accurately reproduces the first two observed eigenmodes, compared with the R2 product for which the second mode is not properly reproduced. The spatial patterns for wet versus dry summer years are also successfully simulated in RSM. For shorter time scales, the RSM resolves heavy rainfall events and their frequency better than R2. Correlation and categorical classification (above/near/below average) for the monthly frequency of heavy precipitation days is also significantly improved by the RSM. [ABSTRACT FROM AUTHOR]

The Modern-Era Retrospective Analysis for Research and Applications (MERRA) was undertaken by NASA''s Global Modeling and Assimilation Office with two primary objectives: to place observations from NASA''s Earth Observing System satellites into a climate context and to improve upon the hydrologic cycle represented in earlier generations of reanalyses. Focusing on the satellite era, from 1979 to the present, MERRA has achieved its goals with significant improvements in precipitation and water vapor climatology. Here, a brief overview of the system and some aspects of its performance, including quality assessment diagnostics from innovation and residual statistics, is given. By comparing MERRA with other updated reanalyses [the interim version of the next ECMWF Re-Analysis (ERA-Interim) and the Climate Forecast System Reanalysis (CFSR)], advances made in this new generation of reanalyses, as well as remaining deficiencies, are identified. Although there is little difference between the new reanalyses in many aspects of climate variability, substantial differences remain in poorly constrained quantities such as precipitation and surface fluxes. These differences, due to variations both in the models and in the analysis techniques, are an important measure of the uncertainty in reanalysis products. It is also found that all reanalyses are still quite sensitive to observing system changes. Dealing with this sensitivity remains the most pressing challenge for the next generation of reanalyses. Production has now caught up to the current period and MERRA is being continued as a near-real-time climate analysis. The output is available online through the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC). [ABSTRACT FROM AUTHOR]

Recent studies demonstrate that ocean–atmosphere forcing by persistent sea surface temperature (SST) anomalies is a primary driver of seasonal-to-interannual hydroclimatic variability, including drought events. Other studies, however, conclude that although SST anomalies influence the timing of drought events, their duration and magnitude over continental regions is largely governed by land–atmosphere feedbacks. Here the authors evaluate the direct influence of SST anomalies on the stochastic characteristics of precipitation and drought in two ensembles of AGCM simulations forced with observed (interannually varying) monthly SST and their climatological annual cycle, respectively. Results demonstrate that ocean–atmosphere forcing contributes to the magnitude and persistence of simulated seasonal precipitation anomalies throughout the tropics but over few mid- and high-latitude regions. Significant autocorrelation of simulated seasonal anomalies over oceans is directly forced by persistent SST anomalies; over land, SST anomalies are shown to enhance autocorrelation associated with land–atmosphere feedbacks. SST anomalies are shown to have no significant influence on simulated drought frequency, duration, or magnitude over most midlatitude land regions. Results suggest that severe and sustained drought events may occur in the absence of persistent SST forcing and support recent conclusions that ocean–atmosphere forcing primarily influences the timing of drought events, while duration and magnitude are governed by other mechanisms such as land–atmosphere feedbacks. Further analysis is needed to assess the potential model dependence of results and to quantify the relative contribution of land–atmosphere feedbacks to the long-term stochastic characteristics of precipitation and drought. [ABSTRACT FROM AUTHOR]

The mechanisms of summertime diurnal precipitation in the US Great Plains were examined with the two-dimensional (2D) Goddard Cumulus Ensemble (GCE) cloud-resolving model (CRM). The model was constrained by the observed large-scale background state and surface flux derived from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program’s Intensive Observing Period (IOP) data at the Southern Great Plains (SGP). The model, when continuously-forced by realistic surface flux and large-scale advection, simulates reasonably well the temporal evolution of the observed rainfall episodes, particularly for the strongly forced precipitation events. However, the model exhibits a deficiency for the weakly forced events driven by diurnal convection. Additional tests were run with the GCE model in order to discriminate between the mechanisms that determine daytime and nighttime convection. In these tests, the model was constrained with the same repeating diurnal variation in the large-scale advection and/or surface flux. The results indicate that it is primarily the surface heat and moisture flux that is responsible for the development of deep convection in the afternoon, whereas the large-scale upward motion and associated moisture advection play an important role in preconditioning nocturnal convection. In the nighttime, high clouds are continuously built up through their interaction and feedback with long-wave radiation, eventually initiating deep convection from the boundary layer. Without these upper-level destabilization processes, the model tends to produce only daytime convection in response to boundary layer heating. This study suggests that the correct simulation of the diurnal variation in precipitation requires that the free-atmospheric destabilization mechanisms resolved in the CRM simulation must be adequately parameterized in current general circulation models (GCMs) many of which are overly sensitive to the parameterized boundary layer heating. [ABSTRACT FROM AUTHOR]

Model deficiencies limit a subseasonal or seasonal forecast system’s ability to produce accurate predictions. In this paper, an approach for transforming the output of a forecast system into a revised forecast is presented; it is designed to correct for some of the deficiencies in the system (particularly those associated with the spatial correlation structures of the forecasted fields) and thereby increase forecast skill. The approach, based on the joint consideration of the correlation structures present in the observational record and the inherent potential predictability of the model, is tested on a preexisting subseasonal forecast experiment. It is shown to produce modest but significant increases in the accuracy of forecasted precipitation and near-surface air temperature at monthly time scales. [ABSTRACT FROM AUTHOR]

This study examines the predictability of seasonal mean Great Plains precipitation using an ensemble of century-long atmospheric general circulation model (AGCM) simulations forced with observed sea surface temperatures (SSTs). The results show that the predictability (intraensemble spread) of the precipitation response to SST forcing varies on interannual and longer time scales. In particular, this study finds that pluvial conditions are more predictable (have less intraensemble spread) than drought conditions. This rather unexpected result is examined in the context of the physical mechanisms that impact precipitation in the Great Plains. These mechanisms include El Niño–Southern Oscillation’s impact on the planetary waves and hence the Pacific storm track (primarily during the cold season), the role of Atlantic SSTs in forcing changes in the Bermuda high and low-level moisture flux into the continent (primarily during the warm season), and soil moisture feedbacks (primarily during the warm season). It is found that the changes in predictability are primarily driven by changes in the strength of the land–atmosphere coupling, such that under dry conditions a given change in soil moisture produces a larger change in evaporation and hence precipitation than the same change in soil moisture would produce under wet soil conditions. The above changes in predictability are associated with a negatively skewed distribution in the seasonal mean precipitation during the warm season—a result that is not inconsistent with the observations. [ABSTRACT FROM AUTHOR]

In this study the authors examine the impact of El Niño–Southern Oscillation (ENSO) on precipitation events over the continental United States using 49 winters (1949/50–1997/98) of daily precipitation observations and NCEP–NCAR reanalyses. The results are compared with those from an ensemble of nine atmospheric general circulation model (AGCM) simulations forced with observed SST for the same time period. Empirical orthogonal functions (EOFs) of the daily precipitation fields together with compositing techniques are used to identify and characterize the weather systems that dominate the winter precipitation variability. The time series of the principal components (PCs) associated with the leading EOFs are analyzed using generalized extreme value (GEV) distributions to quantify the impact of ENSO on the intensity of extreme precipitation events. The six leading EOFs of the observations are associated with major winter storm systems and account for more than 50% of the daily precipitation variability along the West Coast and over much of the eastern part of the country. Two of the leading EOFs (designated GC for Gulf Coast and EC for East Coast) together represent cyclones that develop in the Gulf of Mexico and occasionally move and/or redevelop along the East Coast producing large amounts of precipitation over much of the southern and eastern United States. Three of the leading EOFs represent storms that hit different sections of the West Coast (designated SW for Southwest coast, WC for the central West Coast, and NW for northwest coast), while another represents storms that affect the Midwest (designated by MW). The winter maxima of several of the leading PCs are significantly impacted by ENSO such that extreme GC, EC, and SW storms that occur on average only once every 20 years (20-yr storms) would occur on average in half that time under sustained El Niño conditions. In contrast, under La Niña conditions, 20-yr GC and EC storms would occur on average about once in 30 years, while there is little impact of La Niña on the intensity of the SW storms. The leading EOFs from the model simulations and their connections to ENSO are for the most part quite realistic. The model, in particular, does very well in simulating the impact of ENSO on the intensity of EC and GC storms. The main model discrepancies are the lack of SW storms and an overall underestimate of the daily precipitation variance. [ABSTRACT FROM AUTHOR]

Summer simulations over the contiguous United States and Mexico with the Regional Atmospheric Modeling System (RAMS) dynamically downscaling the NCEP–NCAR Reanalysis I for the period 1950–2002 (described in Part I of the study) are evaluated with respect to the three dominant modes of global SST. Two of these modes are associated with the statistically significant, naturally occurring interannual and interdecadal variability in the Pacific. The remaining mode corresponds to the recent warming of tropical sea surface temperatures. Time-evolving teleconnections associated with Pacific SSTs delay or accelerate the evolution of the North American monsoon. At the period of maximum teleconnectivity in late June and early July, there is an opposite relationship between precipitation in the core monsoon region and the central United States. Use of a regional climate model (RCM) is essential to capture this variability because of its representation of the diurnal cycle of convective rainfall. The RCM also captures the observed long-term changes in Mexican summer rainfall and suggests that these changes are due in part to the recent increase in eastern Pacific SST off the Mexican coast. To establish the physical linkage to remote SST forcing, additional RAMS seasonal weather prediction mode simulations were performed and these results are briefly discussed. In order for RCMs to be successful in a seasonal weather prediction mode for the summer season, it is required that the GCM provide a reasonable representation of the teleconnections and have a climatology that is comparable to a global atmospheric reanalysis. [ABSTRACT FROM AUTHOR]

This study examines the sensitivity of the North American warm season diurnal cycle of precipitation to changes in horizontal resolution in three atmospheric general circulation models, with a primary focus on how the parameterized moist processes respond to improved resolution of topography and associated local/regional circulations on the diurnal time scale. It is found that increasing resolution (from approximately 2° to ½° in latitude–longitude) has a mixed impact on the simulated diurnal cycle of precipitation. Higher resolution generally improves the initiation and downslope propagation of moist convection over the Rockies and the adjacent Great Plains. The propagating signals, however, do not extend beyond the slope region, thereby likely contributing to a dry bias in the Great Plains. Similar improvements in the propagating signals are also found in the diurnal cycle over the North American monsoon region as the models begin to resolve the Gulf of California and the surrounding steep terrain. In general, the phase of the diurnal cycle of precipitation improves with increasing resolution, though not always monotonically. Nevertheless, large errors in both the phase and amplitude of the diurnal cycle in precipitation remain even at the highest resolution considered here. These errors tend to be associated with unrealistically strong coupling of the convection to the surface heating and suggest that improved simulations of the diurnal cycle of precipitation require further improvements in the parameterizations of moist convection processes. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO) is known to have a substantial impact on the variability of the Asian–Australian summer monsoons. An important, but not well understood, aspect of the MJO–monsoon connection is the meridional propagation of bands of enhanced or reduced precipitation that are especially pronounced during the northern summer. In this study, the nature of the seasonality of the MJO is examined, with a focus on the meridional propagation, using both observations and simulations with an atmospheric general circulation model (AGCM). A key result is that the AGCM, when forced with idealized eastward propagating equatorial dipole heating anomalies, reproduces the salient features of the observed seasonality in the precipitation and wind fields associated with the MJO, including meridional propagation into the Indian and Australian summer monsoon regions. An analysis of the simulations and observations shows that the off-equatorial precipitation anomalies are initiated by surface frictional convergence/divergence associated with the Rossby wave response to the leading pole of the equatorial heating dipole. The off-equatorial precipitation anomalies develop further by interacting with the trailing pole of the equatorial dipole heating to produce a northwest–southeast (or southwest–northeast) oriented line of surface convergence/divergence that propagates to the east. Since the prescribed heating does not vary by season, the seasonal asymmetry in the response must be the result of the seasonal changes in the background state. In particular, the results suggest that seasonal changes in both the vertical wind shear and static stability play a role. [ABSTRACT FROM AUTHOR]

In this study, numerical simulations of the twentieth-century climate are evaluated, focusing on the changes in the intensity of the global water cycle. A new model diagnostic of atmospheric water vapor cycling rate is developed and employed that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values with more variability. Climate simulations were performed using SSTs of the early (1902–21) and late (1979–98) twentieth century along with the appropriate CO2 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-yr climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the simulated hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend, while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America, experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These model diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time. [ABSTRACT FROM AUTHOR]

In the past 50 years, sea surface temperatures (SSTs) in the tropical latitudes have trended toward a warmer ocean state. As a response, tropical land surface temperatures, as well as tropical tropospheric temperatures (as manifested in the variations in the 200-mb tropical heights), have also trended upward. Analysis of trends in the tropical precipitation fields, however, remains problematic because of the scarcity of the observed data over the tropical oceans. Using both observed data and data from atmospheric general circulation model simulations, trends in tropical precipitation over the ocean and land are analyzed. The analysis reveals that in the tropical latitudes over land, the precipitation trend differs from the trend in the surface temperature. Oceanic precipitation has an increasing trend that is consistent with increasing SSTs, whereas over the tropical land regions precipitation decreases. In contrast, land temperatures increase in phase with the trend in SSTs. It is suggested that the combination of increasing surface temperature and decreasing precipitation could produce considerably greater societal consequences compared with the traditionally argued scenario in which both temperature and precipitation increase in response to increasing SSTs. [ABSTRACT FROM AUTHOR]

This study examines intraseasonal (20-70 day) variability in the South Asian monsoon region during 1997/ 98 in ensembles of 10 simulations with 10 different atmospheric general circulation models. The 10 ensemble members for each model are forced with the same observed weekly sea surface temperature (SST) but differ from each other in that they are started from different initial atmospheric conditions. The results show considerable differences between the models in the simulated 20-70-day variability, ranging from much weaker to much stronger than the observed. A key result is that the models do produce, to varying degrees, a response to the imposed weekly SST The forced variability tends to be largest in the Indian and western Pacific Oceans where, for some models, it accounts for more than a quarter of the 20-70-day intraseasonal variability in the upper-level velocity potential during these two years. A case study of a strong observed Madden-Julian oscillation (MJO) event shows that the models produce an ensemble mean eastward-propagating signal in the tropical precipitation field over the Indian Ocean and western Pacific, similar to that found in the observations. The associated forced 200-mb velocity potential anomalies are strongly phase locked with the precipitation anomalies, propagating slowly to the east (about 5 m s[sup -1]) with a local zonal wavenumber-2 pattern that is generally consistent with the developing observed MJO. The simulated and observed events are, however, approximately in quadrature, with the simulated response leading by 5-10 days. The phase lag occurs because, in the observations, the positive SST anomalies develop upstream of the main convective center in the subsidence region of the MJO, while in the simulations, the forced component is in phase with the SST. For all the models examined here, the intraseasonal variability is dominated by the free (intraensemble) component. The results of the case study presented here show that... [ABSTRACT FROM AUTHOR]

Numerous studies suggest that local feedback of surface evaporation on precipitation, known recycling, is a significant source of water for precipitation. Quantitative results on the exact amount of recycling have been difficult to obtain in view of the inherent limitations of diagnostic recycling calculations. The current study describes a calculation of the amount of local and remote geographic sources of surface evaporation for precipitation, based on the implementation of three-dimensional constituent tracers of regional water vapor sources [termed “water vapor tracers” (WVTs)] in a general circulation model. The major limitation on the accuracy of the recycling estimates is the veracity of the numerically simulated hydrological cycle, though it is noted that this approach also can be implemented within the context of a data assimilation system. In the WVT approach, each tracer is associated with an evaporative source region for a prognostic three-dimensional variable that represents a partial amount of the total atmospheric water vapor. The physical processes that act on a WVT are determined in proportion to those that act on the model's prognostic water vapor. In this way, the local and remote sources of water for precipitation can be predicted within the model simulation and validated against the model's prognostic water vapor. As a demonstration of the method, the regional hydrologic cycles for North America and India are evaluated for six summers (June, July, and August) of model simulation. More than 50% of the precipitation in the midwestern United States came from continental regional sources, and the local source was the largest of the regional tracers (14%). The Gulf of Mexico and Atlantic regions contributed 18% of the water for midwestern precipitation, but further analysis suggests that the greater region of the tropical Atlantic Ocean may also contribute significantly. In most North American continental regions, the local source... [ABSTRACT FROM AUTHOR]

Focuses on a study which examined precipitation recycling over the central United States (U.S.) using the Goddard Earth Observing System Data Assimilation System (GEOS-1 DAS) of the U.S. National Aeronautics and Space Administration. Information on the GEOS-1 DAS and the bulk diagnostic recycling model; Regional hydrological behavior in the U.S.; Issues in diagnostic recycling.

Subseasonal variations in warm-season (May--August) precipitation over the central and eastern United States are shown to be strongly linked to variations in the moisture entering the continent from the Gulf of Mexico within a longitudinally confined "channel" (referred to here as the Texas corridor or TC). These variations reflect the development of low-level southerly wind maxima (or jets) on a number of different timescales in association with distinct subcontinental and larger-scale phenomena. On the diurnal timescale, the TC moisture flux variations are tied to the development of the Great Plains low-level jet. The composite nighttime anomalies are characterized by a strong southerly moisture flux covering northeast Mexico and the southern Great Plains, and enhanced boundary layer convergence and precipitation over much of the upper Great Plains. The strongest jets tend to be associated with an anomalous surface low over the Great Plains, reflecting a predilection for periods when midlatitude weather systems are positioned to produce enhanced southerly flow over this region. On subsynoptic (2--4 days) timescales the TC moisture flux variations are associated with the development and evolution of a warm-season lee cyclone. These systems, which are most prevalent during the early part of the warm season (May and June), form over the central Great Plains in association with an upper-level shortwave and enhanced upper-tropospheric cross-mountain westerly flow. A low-level southerly wind maximum or jet develops underneath and perpendicular to the advancing edge of enhanced midtropospheric westerlies. The clash of anomalous southerly moisture flux and a deep intrusion of anomalous northerly low-level winds results in enhanced precipitation eventually stretching from Texas to the Great Lakes. On synoptic (4--8 days) timescales the TC moisture flux variations are associated with the propagation and intensification of a warm-season mid-latitude... [ABSTRACT FROM AUTHOR]