bluuebunny/crossref_metadata_2025_split huggingface dataset:
Toward Transient Subgrid-Scale Gravity Wave Representation in Atmospheric Models. Part II: Wave Intermittency Simulated with Convective Sources
Authors: Kim Young-Ha , Bölöni Gergely , Borchert Sebastian , Chun Hye-Yeong , Achatz Ulrich 
Published: August 2020

AbstractIn a companion paper, the Multiscale Gravity Wave Model (MS-GWaM) has been introduced and its application to a global model as a transient subgrid-scale parameterization has been described. This paper focuses on the examination of intermittency of gravity waves (GWs) modeled by MS-GWaM. To introduce the variability and intermittency in wave sources, convective GW sources are formulated, using diabatic heating diagnosed by the convection parameterization, and they are coupled to MS-GWaM in addition to a flow-independent source in the extratropics accounting for GWs due neither to convection nor to orography. The probability density function (PDF) and Gini index for GW pseudomomentum fluxes are assessed to investigate the intermittency. Both are similar to those from observations in the lower stratosphere. The intermittency of GWs over tropical convection is quite high and is found not to change much in the vertical direction. In the extratropics, where nonconvective GWs dominate, the intermittency is lower than that in the tropics in the stratosphere and comparable to that in the mesosphere, exhibiting a gradual increase with altitude. The PDFs in these latitudes seem to be close to the lognormal distributions. Effects of transient GW–mean-flow interactions on the simulated GW intermittency are assessed by performing additional simulations using the steady-state assumption in the GW parameterization. The intermittency of parameterized GWs over tropical convection is found to be overestimated by the assumption, whereas in the extratropics it is largely underrepresented. Explanation and discussion of these effects are included.
ENSO Precipitation Anomalies along the Equatorial Pacific: Moist Static Energy Framework Diagnostics
Authors: Annamalai H. 
Published: August 2020

AbstractWith the recognition that equatorial Pacific precipitation anomalies are fundamental to global teleconnections during ENSO winters, the present research applies vertically integrated moist static energy (MSE) budget analysis to historical simulations of CMIP5 models. Process-based assessment is carried out to understand if the models capture the differing processes that account for regional precipitation anomalies along the equatorial Pacific and to isolate a few leading processes that account for the diversified precipitation response to similar SST forcing and vice versa. To assess SST biases in CMIP5, analysis is also carried out in AMIP5 solutions. Diagnostics reveal that models have limitations in representing the “sign” of MSE sources and sinks and, even if they do, compensating errors dominate the budget. The diverse response in precipitation depends on model parameterizations that determine anomalous net radiative flux divergence in the column, free troposphere moisture, and MSE export out of the column, although these processes are not independent. Diagnostics derived from AMIP5 solutions support the findings from CMIP5. The implication is that biases in representing any one of these processes are expected to imprint on others, acknowledging the tight connections among moisture, convection, and radiation. CMIP5 models have limitations in representing the basic states in SST and precipitation over the Niño-3.4 region, and the different convective regimes over the equatorial central and eastern Pacific regions with implications for ENSO. Study limitations are that MSE sources/sinks depend on parameterizations and their interactions, making it difficult to isolate one particular process for attribution. Budgets estimated from monthly anomalies do not capture contributions from high-frequency variability that are vital in closing the budgets.
How Do Volcanic Eruptions Influence Decadal Megadroughts over Eastern China?
Authors: Ning Liang , Chen Kefan , Liu Jian , Liu Zhengyu , Yan Mi , Sun Weiyi , Jin Chunhan , Shi Zhengguo 
Published: August 2020

AbstractThe influence and mechanism of volcanic eruptions on decadal megadroughts over eastern China during the last millennium were investigated using a control (CTRL) and five volcanic eruption sensitivity experiments (VOLC) from the Community Earth System Model (CESM) Last Millennium Ensemble (LME) archive. The decadal megadroughts associated with the failures of the East Asian summer monsoon (EASM) are associated with a meridional tripole of sea surface temperature anomalies (SSTAs) in the western Pacific from the equator to high latitudes, suggestive of a decadal-scale internal mode of variability that emerges from empirical orthogonal function (EOF) analysis. Composite analyses further showed that, on interannual time scales, within a decade after an eruption the megadrought was first enhanced but then weakened, due to the change from an El Niño state to a La Niña state. The impacts of volcanic eruptions on the magnitudes of megadroughts are superposed on internal variability. Therefore, the evolution of decadal megadroughts coinciding with strong volcanic eruptions demonstrate that the impacts of internal variability and external forcing can combine to influence hydroclimate.
Characterizing the Seasonal Cycle of the Northern Australian Rainy Season
Authors: Uehling John , Misra Vasubandhu 
Published: August 2020

AbstractIn this paper we introduce an objective definition for the onset and the retreat of the northern Australian rainy season that overlaps significantly with the Australian monsoon season. We define onset and retreat dates of the northern Australian rainy season as being the first and the last day of the year when the daily rain rate exceeds and falls below the climatological annual mean rain rate, respectively. However, our definition of onset/demise is not as restrictive as the traditional monsoon season that seeks the arrival of the westerlies and the equatorward retreat of the trough at its onset and demise, respectively. As defined in this paper, the length of the rainy season is longer than the monsoon season and includes the pre- and post-monsoon rainfall. It is noted that an early or later onset date of the northern Australian rainy season is associated with a longer or shorter, wetter or drier, and colder or warmer season, respectively. Similar relationship is also observed with demise date variations, which are, however, weaker than the onset date variations. Furthermore, we find that the relationship of the northern Australian seasonal rainfall variations with ENSO variability becomes stronger when we account for variations in the length of the rainy season compared to the fixed (December–February) monsoon season length. We also find a significant linear trend over the time period of the analysis from 1901 to 2015 toward an increasing length of the northern Australian rainy season that influences the corresponding rising trend of seasonal rainfall anomalies.
Coupled Climate Model Simulation of Tropical–Extratropical Cloud Bands over Southern Africa
Authors: James Rachel , Hart Neil C. G. , Munday Callum , Reason Chris J. C. , Washington Richard 
Published: August 2020

AbstractThere are increasing efforts to use climate model output for adaptation planning, but meanwhile there is often limited understanding of how models represent regional climate. Here we analyze the simulation in global coupled climate models of a key rainfall-generating mechanism over southern Africa: tropical temperate troughs (TTTs). An image-processing algorithm is applied to outgoing longwave radiation data from satellites and models to create TTT event sets. All models investigated produce TTTs with similar circulation features to observed. However, there are large differences among models in the number, intensity, and preferred longitude of events. Five groups of models are identified. The first group generates too few TTTs, and relatively dry conditions over southern Africa compared to other models. A second group generates more TTTs and wet biases. The contrast between these two groups suggests that the number of TTTs could explain intermodel variations in climatological rainfall. However, there is a third group of models that simulate up to 92% more TTTs than observed, but do not have large rainfall biases, as each TTT event is relatively weak. Finally, there are a further two groups that concentrate TTTs over the subcontinent or the ocean, respectively. These distinctions between models are associated with the amount of convective activity in the Congo Basin, the magnitude of moisture fluxes into southern Africa, and the degree of zonal asymmetry in upper-level westerly flow. Model development focused on tropical convection and the representation of orography is needed for improved simulation of TTTs, and therefore southern African rainfall.
The Impact of Anthropogenic Forcing and Natural Processes on Past, Present, and Future Rainfall over Victoria, Australia
Authors: Rauniyar Surendra P. , Power Scott B. 
Published: August 2020

AbstractCool-season (April to October) rainfall dominates the annual average rainfall over Victoria, Australia, and is important for agriculture and replenishing reservoirs. Rainfall during the cool season has been unusually low since the beginning of the Millennium Drought in 1997 (~12% below the twentieth-century average). In this study, 24 CMIP5 climate models are used to estimate 1) the extent to which this drying is driven by external forcing and 2) future rainfall, taking both external forcing and internal natural climate variability into account. All models have preindustrial, historical, and twenty-first-century (RCP2.6, RCP4.5, and RCP8.5) simulations. It is found that rainfall in the past two decades is below the preindustrial average in two-thirds or more of model simulations. However, the magnitude of the multimodel median externally forced drying is equivalent to only 20% of the observed drying (interquartile range of 40% to −4%), suggesting that the drying is dominated by internally generated rainfall variability. Underestimation of internal variability of rainfall by the models, however, increases the uncertainties in these estimates. According to models the anthropogenically forced drying becomes dominant from 2010 to 2029, when drying is evident in over 90% of the model simulations. For the 2018–37 period, it is found that there is only a ~12% chance that internal rainfall variability could completely offset the anthropogenically forced drying. By the late twenty-first century, the anthropogenically forced drying under RCP8.5 is so large that internal variability appears too small to be able to offset it. Confidence in the projections is lowered because models have difficulty in simulating the magnitude of the observed decline in rainfall.
The Boreal Winter El Niño Precipitation Response over North America: Insights into Why January Is More Difficult to Predict Than February
Authors: Lim Young-Kwon , Schubert Siegfried D. , Chang Yehui , Wang Hailan 
Published: August 2020

AbstractThis study examines the within-season monthly variation of the El Niño response over North America during December–March using the NASA/GEOS model. In agreement with previous studies, the skill of 1-month-lead GEOS coupled model forecasts of precipitation over North America is largest (smallest) for February (January), with similar results in uncoupled mode. A key finding is that the relatively poor January skill is the result of the model placing the main circulation anomaly over the northeast Pacific slightly to the west of the observed, resulting in precipitation anomalies that lie off the coast instead of over land as observed. In contrast, during February the observed circulation anomaly over the northeast Pacific shifts westward, lining up with the predicted anomaly, which is essentially unchanged from January, resulting in both the observed and predicted precipitation anomalies remaining off the coast. Furthermore, the largest precipitation anomalies occur along the southern tier of states associated with an eastward extended jet—something that the models capture reasonably well. Simulations with a stationary wave model indicate that the placement of January El Niño response to the west of the observed over the northeast Pacific is the result of biases in the January climatological stationary waves, rather than errors in the tropical Pacific El Niño heating anomalies in January. Furthermore, evidence is provided that the relatively poor simulation of the observed January climatology, characterized by a strengthened North Pacific jet and enhanced ridge over western North America, can be traced back to biases in the January climatology heating over the Tibet region and the tropical western Pacific.
Warming Patterns Affect El Niño Diversity in CMIP5 and CMIP6 Models
Authors: Freund Mandy B. , Brown Josephine R. , Henley Benjamin J. , Karoly David J. , Brown Jaclyn N. 
Published: August 2020

AbstractGiven the consequences and global significance of El Niño–Southern Oscillation (ENSO) events it is essential to understand the representation of El Niño diversity in climate models for the present day and the future. In recent decades, El Niño events have occurred more frequently in the central Pacific (CP). Eastern Pacific (EP) El Niño events have increased in intensity. However, the processes and future implications of these observed changes in El Niño are not well understood. Here, the frequency and intensity of El Niño events are assessed in models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), and results are compared to extended instrumental and multicentury paleoclimate records. Future changes of El Niño are stronger for CP events than for EP events and differ between models. Models with a projected La Niña–like mean-state warming pattern show a tendency toward more EP but fewer CP events compared to models with an El Niño–like warming pattern. Among the models with more El Niño–like warming, differences in future El Niño can be partially explained by Pacific decadal variability (PDV). During positive PDV phases, more El Niño events occur, so future frequency changes are mainly determined by projected changes during positive PDV phases. Similarly, the intensity of El Niño is strongest during positive PDV phases. Future changes to El Niño may thus depend on both mean-state warming and decadal-scale natural variability.
Freeze–Thaw Changes of Seasonally Frozen Ground on the Tibetan Plateau from 1960 to 2014
Authors: Luo Siqiong , Wang Jingyuan , Pomeroy John W. , Lyu Shihua 
Published: August 2020

AbstractThe freeze–thaw changes of seasonally frozen ground (SFG) are an important indicator of climate change. Based on observed daily freeze depth of SFG from meteorological stations on the Tibetan Plateau (TP) from 1960 to 2014, the spatial–temporal characteristics and trends in SFG were analyzed, and the relationships between them and climatic and geographical factors were explored. Freeze–thaw changes of SFG on a regional scale were assessed by multiple regression functions. Results showed multiyear mean maximum freeze depth, freeze–thaw duration, freeze start date, and thaw end date that demonstrate obvious distribution characteristics of climatic zones. A decreasing trend in maximum freeze depth and freeze–thaw duration occurred on the TP from 1960 to 2014. The freeze start date has been later, and the thaw end date has been significantly earlier. The freeze–thaw changes of SFG significantly affected by soil hydrothermal conditions on the TP could be assessed by elevation and latitude or by air temperature and precipitation, due to their high correlations. The regional average of maximum freeze depth and freeze–thaw duration caused by climatic and geographical factors were larger than those averaged using meteorological station data because most stations are located at lower altitudes. Maximum freeze depth and freeze–thaw duration have decreased sharply since 2000 on the entire TP. Warming and wetting conditions of the soil resulted in a significant decrease in maximum freeze depth and freeze–thaw duration in the most area of the TP, while drying soil results in a slight increase of them in the southeast of the TP.
Uncertainties in Ocean Latent Heat Flux Variations over Recent Decades in Satellite-Based Estimates and Reduced Observation Reanalyses
Authors: Robertson Franklin R. , Roberts Jason B. , Bosilovich Michael G. , Bentamy Abderrahim , Clayson Carol Anne , Fennig Karsten , Schröder Marc , Tomita Hiroyuki , Compo Gilbert P. , Gutenstein Marloes , Hersbach Hans , Kobayashi Chiaki , Ricciardulli Lucrezia , Sardeshmukh Prashant , Slivinski Laura C. 
Published: August 2020

AbstractFour state-of-the-art satellite-based estimates of ocean surface latent heat fluxes (LHFs) extending over three decades are analyzed, focusing on the interannual variability and trends of near-global averages and regional patterns. Detailed intercomparisons are made with other datasets including 1) reduced observation reanalyses (RedObs) whose exclusion of satellite data renders them an important independent diagnostic tool; 2) a moisture budget residual LHF estimate using reanalysis moisture transport, atmospheric storage, and satellite precipitation; 3) the ECMWF Reanalysis 5 (ERA5); 4) Remote Sensing Systems (RSS) single-sensor passive microwave and scatterometer wind speed retrievals; and 5) several sea surface temperature (SST) datasets. Large disparities remain in near-global satellite LHF trends and their regional expression over the 1990–2010 period, during which time the interdecadal Pacific oscillation changed sign. The budget residual diagnostics support the smaller RedObs LHF trends. The satellites, ERA5, and RedObs are reasonably consistent in identifying contributions by the 10-m wind speed variations to the LHF trend patterns. However, contributions by the near-surface vertical humidity gradient from satellites and ERA5 trend upward in time with respect to the RedObs ensemble and show less agreement in trend patterns. Problems with wind speed retrievals from Special Sensor Microwave Imager/Sounder satellite sensors, excessive upward trends in trends in Optimal Interpolation Sea Surface Temperature (OISST AVHRR-Only) data used in most satellite LHF estimates, and uncertainties associated with poor satellite coverage before the mid-1990s are noted. Possibly erroneous trends are also identified in ERA5 LHF associated with the onset of scatterometer wind data assimilation in the early 1990s.
Observed Temperature Changes in the Troposphere and Stratosphere from 1979 to 2018
Authors: Steiner A. K. , Ladstädter F. , Randel W. J. , Maycock A. C. , Fu Q. , Claud C. , Gleisner H. , Haimberger L. , Ho S.-P. , Keckhut P. , Leblanc T. , Mears C. , Polvani L. M. , Santer B. D. , Schmidt T. , Sofieva V. , Wing R. , Zou C.-Z. 
Published: August 2020

AbstractTemperature observations of the upper-air atmosphere are now available for more than 40 years from both ground- and satellite-based observing systems. Recent years have seen substantial improvements in reducing long-standing discrepancies among datasets through major reprocessing efforts. The advent of radio occultation (RO) observations in 2001 has led to further improvements in vertically resolved temperature measurements, enabling a detailed analysis of upper-troposphere/lower-stratosphere trends. This paper presents the current state of atmospheric temperature trends from the latest available observational records. We analyze observations from merged operational satellite measurements, radiosondes, lidars, and RO, spanning a vertical range from the lower troposphere to the upper stratosphere. The focus is on assessing climate trends and on identifying the degree of consistency among the observational systems. The results show a robust cooling of the stratosphere of about 1–3 K, and a robust warming of the troposphere of about 0.6–0.8 K over the last four decades (1979–2018). Consistent results are found between the satellite-based layer-average temperatures and vertically resolved radiosonde records. The overall latitude–altitude trend patterns are consistent between RO and radiosonde records. Significant warming of the troposphere is evident in the RO measurements available after 2001, with trends of 0.25–0.35 K per decade. Amplified warming in the tropical upper-troposphere compared to surface trends for 2002–18 is found based on RO and radiosonde records, in approximate agreement with moist adiabatic lapse rate theory. The consistency of trend results from the latest upper-air datasets will help to improve understanding of climate changes and their drivers.
Dramatic Weakening of the Tropical Easterly Jet Projected by CMIP6 Models
Authors: Huang Sihua , Wang Bin , Wen Zhiping 
Published: August 2020

AbstractThe upper-level tropical easterly jet (TEJ) is a crucial component of the summer monsoon system and tropical general circulation. The simulation and projection of the TEJ, however, have not been assessed. Here we evaluate models’ fidelity and assess the future change of the TEJ by utilizing 16 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Most of the models can reproduce the TEJ reasonably well in terms of climatology, seasonal evolution, and interannual variability. Nevertheless, underestimation of the TEJ’s intensity and extent is identified, with the maximum bias occurring in the jet centers over the tropical Indian Ocean (IO) and the tropical eastern Pacific (EP). Under the shared socioeconomic pathway 5–8.5, the multimodel ensemble projects a remarkable reduction in the central TEJ intensity by about 18% over the IO and 77% over the EP toward the end of the twenty-first century. The mean intensity of TEJ will weaken by about 11%, and the extent will reduce by 6%, suggesting a significantly weakened upper-level monsoon circulation in the future climate. The projected El Niño–like warming pattern over the tropical Pacific may play a critical role in the future weakening of the TEJ via inducing suppressed rainfall over the tropical eastern IO and Central America. The model uncertainties in the projected TEJ changes may arise from the uncertainties in the models’ projected tropical EP warming. The sensitivity of future projections to model selection is also examined. Results show that the selection of models based on different physical considerations does not yield a significantly different projection.