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David J. Karoly

Abstract

Composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the Southern Hemisphere (SH) circulation features associated with El Niño-Southern Oscillation (ENSO) events. The period of analyses available (1972–83) has limited the composites to include only three ENSO events. The reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period (1950–79) of SH rawinsonde station data.

In the SH winter, a weak equivalent-barotropic wavetrain pattern of anomalies extends over Australia and the South Pacific Ocean to South America. This wavetrain pattern is quite variable in amplitude and location between ENSO events, although it is more stable over the subtropical Pacific. In the SH summer, the circulation anomalies are more zonally symmetric, with increased height at low and high latitudes and decreased height in middle latitudes. The circulation anomalies in the SH summer are more stable than in winter, with similar patterns of anomalies in the subtropics and middle latitudes in all events.

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David J. Karoly and Karl Braganza

Abstract

Variations of Australian-average mean temperature and diurnal temperature range over the twentieth century are investigated. The observed interannual variability of both is simulated reasonably well by a number of climate models, but they do not simulate the observed relationship between the two. Comparison of the observed warming and reduction in diurnal temperature range with climate model simulations shows that Australian temperature changes over the twentieth century were very unlikely to be due to natural climate variations alone. It is likely that there has been a significant contribution to the observed warming during the second half of the century from increasing atmospheric greenhouse gases and sulfate aerosols.

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David J. Karoly and Qigang Wu

Abstract

Trends in surface temperature over the last 100, 50, and 30 yr at individual grid boxes in a 5° latitude–longitude grid are compared with model estimates of the natural internal variability of these trends and with the model response to increasing greenhouse gases and sulfate aerosols. Three different climate models are used to provide estimates of the internal variability of trends, one of which appears to overestimate the observed variability of surface temperature at interannual and 5-yr time scales. Significant warming trends are found at a large fraction of the individual grid boxes over the globe, a much larger fraction than can be explained by internal climate variations. The observed warming trends over the last 50 and 30 yr are consistent with the modeled response to increasing greenhouse gases and sulfate aerosols in most of the models. However, in some regions, the observed century-scale trends are significantly larger than the modeled response to increasing greenhouse gases and sulfate aerosols in the atmosphere. Warming trends consistent with the response to anthropogenic forcing are detected at scales on the order of 500 km in many regions of the globe.

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Melissa S. Bukovsky and David J. Karoly

Abstract

In this study, the Weather Research and Forecasting (WRF) model is employed as a nested regional climate model to dynamically downscale output from the National Center for Atmospheric Research’s (NCAR’s) Community Climate System Model (CCSM) version 3 and the National Centers for Environmental Prediction (NCEP)–NCAR global reanalysis (NNRP). The latter is used for verification of late-twentieth-century climate simulations from the WRF.

This analysis finds that the WRF is able to produce precipitation that is more realistic than that from its driving systems (the CCSM and NNRP). It also diagnoses potential issues with and differences between all of the simulations completed. Specifically, the magnitude of heavy 6-h average precipitation events, the frequency distribution, and the diurnal cycle of precipitation over the central United States are greatly improved. Projections from the WRF for late-twenty-first-century precipitation show decreases in average May–August (MJJA) precipitation, but increases in the intensity of both heavy precipitation events and rain in general when it does fall. A decrease in the number of 6-h periods with rainfall accounts for the overall decrease in average precipitation. The WRF also shows an increase in the frequency of very heavy to extreme 6-h average events, but a decrease in the frequency of all events lighter than those over the central United States. Overall, projections from this study suggest an increase in the frequency of both floods and droughts during the warm season in the central United States.

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Sophie C. Lewis and David J. Karoly

Abstract

Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperature changes. Observational records indicate that DTR has decreased over the last 50 yr because of differential changes in minimum and maximum temperatures. However, modeled changes in DTR in previous climate model simulations of this period are smaller than those observed, primarily because of an overestimate of changes in maximum temperatures. This present study examines DTR trends using the latest generation of global climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and utilizes the novel CMIP5 detection and attribution experimental design of variously forced historical simulations (natural-only, greenhouse gas–only, and all anthropogenic and natural forcings). Comparison of observed and modeled changes in DTR over the period of 1951–2005 again reveals that global DTR trends are lower in model simulations than observed across the 27-member multimodel ensemble analyzed here. Modeled DTR trends are similar for both experiments incorporating all forcings and for the historical experiment with greenhouse gases only, while no DTR trend is discernible in the naturally forced historical experiment. The persistent underestimate of DTR changes in this latest multimodel evaluation appears to be related to ubiquitous model deficiencies in cloud cover and land surface processes that impact the accurate simulation of regional minimum or maximum temperatures changes observed during this period. Different model processes are likely responsible for subdued simulated DTR trends over the various analyzed regions.

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Ailie J. E. Gallant and David J. Karoly

Abstract

Changes in the area of Australia experiencing concurrent temperature and rainfall extremes are investigated through the use of two combined indices. The indices describe variations between the fraction of land area experiencing extreme cold and dry or hot and wet conditions. There is a high level of agreement between the variations and trends of the indices from 1957 to 2008 when computed using (i) a spatially complete gridded dataset without rigorous quality control checks and (ii) spatially incomplete high-quality station datasets with rigorous quality control checks. Australian extremes are examined starting from 1911, which is the first time a broad-scale assessment of Australian temperature extremes has been performed prior to 1957. Over the whole country, the results show an increase in the extent of hot and wet extremes and a decrease in the extent of cold and dry extremes annually and during all seasons from 1911 to 2008 at a rate of between 1% and 2% decade−1. These trends mostly stem from changes in tropical regions during summer and spring. There are relationships between the extent of extreme maximum temperatures, precipitation, and soil moisture on interannual and decadal time scales that are similar to the relationships exhibited by variations of the means. However, the trends from 1911 to 2008 and from 1957 to 2008 are not consistent with these relationships, providing evidence that the processes causing the interannual variations and those causing the longer-term trends are different.

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John T. Allen, David J. Karoly, and Kevin J. Walsh

Abstract

The influence of a warming climate on the occurrence of severe thunderstorm environments in Australia was explored using two global climate models: Commonwealth Scientific and Industrial Research Organisation Mark, version 3.6 (CSIRO Mk3.6), and the Cubic-Conformal Atmospheric Model (CCAM). These models have previously been evaluated and found to be capable of reproducing a useful climatology for the twentieth-century period (1980–2000). Analyzing the changes between the historical period and high warming climate scenarios for the period 2079–99 has allowed estimation of the potential convective future for the continent. Based on these simulations, significant increases to the frequency of severe thunderstorm environments will likely occur for northern and eastern Australia in a warmed climate. This change is a response to increasing convective available potential energy from higher continental moisture, particularly in proximity to warm sea surface temperatures. Despite decreases to the frequency of environments with high vertical wind shear, it appears unlikely that this will offset increases to thermodynamic energy. The change is most pronounced during the peak of the convective season, increasing its length and the frequency of severe thunderstorm environments therein, particularly over the eastern parts of the continent. The implications of this potential increase are significant, with the overall frequency of potential severe thunderstorm days per year likely to rise over the major population centers of the east coast by 14% for Brisbane, 22% for Melbourne, and 30% for Sydney. The limitations of this approach are then discussed in the context of ways to increase the confidence of predictions of future severe convection.

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Wenju Cai, Peter H. Whetton, and David J. Karoly

Abstract

Recent results from greenhouse warming experiments, most of which follow the Intergovernmental Panel on Climate Change (IPCC) IS92a scenario, have shown that under increasing atmospheric CO2 concentration, the Antarctic Oscillation (AAO) exhibits a positive trend. However, its response during the subsequent CO2 stabilization period has not been explored. In this study, it is shown that the upward trend of the AAO reverses during such a stabilization period. This evolution of an upward trend and a subsequent reversal is present in each ensemble of three greenhouse simulations using three versions of the CSIRO Mark 2 coupled climate model. The evolution is shown to be linked with that of surface temperature, which also displays a corresponding trend and reversal, incorporating the well-known feature of interhemispheric warming asymmetry with smaller warming in the Southern Hemisphere (smaller as latitude increases) than that in the Northern Hemisphere during the transient period, and vice versa during the stabilization period. These results indicate that once CO2 concentration stabilizes, reversal of the AAO trend established during the transient period is likely to be a robust feature, as it is underpinned by the likelihood that latitudinal warming differences will reduce or disappear. The implication is that climatic impacts associated with the AAO trend during the transient period may be reversible if CO2 stabilization is achieved.

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Ailie J. E. Gallant, David J. Karoly, and Karin L. Gleason

Abstract

The utility of a combined modified climate extremes index (mCEI) is presented for monitoring coherent trends in multiple types of climate extremes across large regions. Its usefulness lies in its ability to distill complex spatiotemporal fields into a simple, flexible nonparametric index.

Two versions of the mCEI are computed that incorporate changes in several annual- or daily-scale temperature-related and moisture-related extremes. Applying data from the contiguous United States, Europe, and Australia detects consistent and statistically significant increases in the spatial prevalence of climate extremes from 1950 to 2012. All three continental-scale regions show increasingly widespread warm annual- and daily-scale minimum and maximum temperature extremes, a decreasing spatial extent of cool annual- and daily-scale minimum and maximum temperature extremes, and increasing areas where the proportion of annual total precipitation falls on heavy-rain days. There were no statistically significant trends toward more widespread, annual-scale drought or moisture surplus in any region.

The dependence of annual extremes on the frequency of daily-scale extremes is highlighted by the strong covariations between annual- and daily-scale extremes in all regions. By the nature of construction of the combined indices, the differences in the trends of the mCEI and daily-scale mCEI (dmCEI) suggest that extremes in more areas are changing primarily because of a shift of temperature and daily rainfall distributions toward warm extremes and heavy-rainfall extremes.

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John T. Allen, David J. Karoly, and Kevin J. Walsh

Abstract

The influence of a warming climate on the occurrence of severe thunderstorms over Australia is, as yet, poorly understood. Based on methods used in the development of a climatology of observed severe thunderstorm environments over the continent, two climate models [Commonwealth Scientific and Industrial Research Organisation Mark, version 3.6 (CSIRO Mk3.6) and the Cubic-Conformal Atmospheric Model (CCAM)] have been used to produce simulated climatologies of ingredients and environments favorable to severe thunderstorms for the late twentieth century (1980–2000). A novel evaluation of these model climatologies against data from both the ECMWF Interim Re-Analysis (ERA-Interim) and reports of severe thunderstorms from observers is used to analyze the capability of the models to represent convective environments in the current climate. This evaluation examines the representation of thunderstorm-favorable environments in terms of their frequency, seasonal cycle, and spatial distribution, while presenting a framework for future evaluations of climate model convective parameters. Both models showed the capability to explain at least 75% of the spatial variance in both vertical wind shear and convective available potential energy (CAPE). CSIRO Mk3.6 struggled to either represent the diurnal cycle over a large portion of the continent or resolve the annual cycle, while in contrast CCAM showed a tendency to underestimate CAPE and 0–6-km bulk magnitude vertical wind shear (S06). While spatial resolution likely contributes to rendering of features such as coastal moisture and significant topography, the distribution of severe thunderstorm environments is found to have greater sensitivity to model biases. This highlights the need for a consistent approach to evaluating convective parameters and severe thunderstorm environments in present-day climate: an example of which is presented here.

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