School of Earth Sciences - Research Publications

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    A transport model study of the breakup of the Antarctic ozone hole in November 2000
    Grainger, Simon ; Karoly, David J. (American Geophysical Union, 2003)
    A 3-D off-line transport model is used to examine the breakup of the Antarctic ozone hole in late November and early December 2000. The use of a transport model enables an analysis of the vortex breakup that is not possible from the use of ozonesonde observations alone. By initializing ozone mixing ratio on 1 September 2000, and using parameterized ozone production and loss rates, the evolution of the Antarctic ozone hole is simulated. The model simulation shows that during late November and early December 2000, the Antarctic ozone hole splits into two sections, with low-ozone air subsequently transported over New Zealand and south-eastern Australia. Modeled ozone values agree well with ozonesonde profiles, confirming the role of horizontal transport in the dilution of mid-latitude ozone.
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    Precipitation Simulations Using WRF as a Nested Regional Climate Model
    Bukovsky, MS ; Karoly, DJ (AMER METEOROLOGICAL SOC, 2009-10)
    Abstract This note examines the sensitivity of simulated U.S. warm-season precipitation in the Weather Research and Forecasting model (WRF), used as a nested regional climate model, to variations in model setup. Numerous options have been tested and a few of the more interesting and unexpected sensitivities are documented here. Specifically, the impacts of changes in convective and land surface parameterizations, nest feedbacks, sea surface temperature, and WRF version on mean precipitation are evaluated in 4-month-long simulations. Running the model over an entire season has brought to light some issues that are not otherwise apparent in shorter, weather forecast–type simulations, emphasizing the need for careful scrutiny of output from any model simulation. After substantial testing, a reasonable model setup was found that produced a definite improvement in the climatological characteristics of precipitation over that from the National Centers for Environmental Prediction–National Center for Atmospheric Research global reanalysis, the dataset used for WRF initial and boundary conditions in this analysis.
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    Decadal prediction: can it be skilful?
    Meehl, Gerald A. ; Goddard, Lisa ; MURPHY, JAMES ; Stouffer, Ronald J. ; Boer, George ; Danabasoglu, Gokhan ; Dixon, Keith ; Giorgetta, Marco A. ; Greene, Arthur M. ; Hawkins, Ed ; Hegerl, Gabriele ; KAROLY, DAVID ; Keenlyside, Noel ; Kimoto, Masahide ; Kirtman, Ben ; Navarra, Antonio ; Pulwarty, Roger ; Smith, Doug ; Stammer, Detlef ; Stockdale, Timothy (American Meteorological Society, 2009)
    A new field of study, “decadal prediction,” is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer-term climate change projections, and focuses on time-evolving regional climate conditions over the next 10–30 yr. Numerous assessments of climate information user needs have identified this time scale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal time scale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated Coupled Model Intercomparison Model, phase 5 (CMIP5) experiments, some of which will be assessed for the IPCC Fifth Assessment Report (AR5). These experiments will likely guide work in this emerging field over the next 5 yr.
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    Transport out of the Antarctic polar vortex from a three-dimensional transport model
    Li, SH ; Cordero, EC ; Karoly, DJ (AMER GEOPHYSICAL UNION, 2002-06)
    A three‐dimensional chemical transport model is utilized to study the transport out of the Antarctic polar vortex during the southern hemisphere spring. On average, over five consecutive years between 1993 and 1997, horizontal transport out of the vortex into the midlatitude stratosphere is smaller than vertical transport into the troposphere. However, there is significant interannual variability in the magnitude of mass exchange, which is related to year‐to‐year fluctuations in planetary wave activity. In 1994 the net loss of the vortex tracer mass in September is similar to that in October. However, the relative mass flux entering the midlatitude stratosphere and the troposphere differ between the two months. The ratio of horizontal transport out of the vortex to vertical transport into the troposphere is about 3:7 in September and 5:5 in October, indicating the higher permeability of the vortex in October compared to September. The September mass flux into the troposphere is larger than in October, consistent with the fact that stronger diabatic cooling occurs in September than October over Antarctica. The estimated ozone change at southern midlatitudes due to the intrusion of ozone‐depleted air from high latitudes during September–October 1994 is about −0.44% per decade, which could contribute up to 10% of observed ozone decline at southern midlatitudes in spring. This amount is an underestimate of the dilution effect from high latitudes during the spring season, as it does not include the vortex breakup in late spring.
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    Diurnal temperature range as an index of global climate change during the twentieth century
    Braganza, Karl ; Karoly, David J. ; Arblaster, J. M. (American Geophysical Union, 2004)
    The usefulness of global-average diurnal temperature range (DTR) as an index of climate change and variability is evaluated using observations and climate model simulations representing unforced climate variability and anthropogenic climate change. On decadal timescales, modelled and observed intrinsic variability of DTR compare well and are independent of variations in global mean temperature. Observed reductions in DTR over the last century are large and unlikely to be due to natural variability alone. Comparison of observed and anthropogenic-forced model changes in DTR over the last 50 years show much less reduction in DTR in the model simulations due to greater warming of maximum temperatures in the models than observed. This difference is likely attributed to increases in cloud cover that are observed over the same period and are absent in model simulations.
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    Comparison of glacier-inferred temperatures with observations and climate model simulations
    Ren, Diandong ; KAROLY, DAVID (American Geophysical Union, 2006)
    A reconstructed temperature history for different regions of the globe was prepared by Oerlemans (2005) from length changes of 169 glaciers. In this study, we compare the glacier-inferred temperature variations over 1900–1990 with observed and climate model simulated temperatures for the global average and for five regions: Southern Hemisphere, northwest America, Atlantic, Asia, and the Alps. There is reasonable agreement between the glacier-inferred temperature trends and the observed temperature trends for the globe and in four of the five regions (except northwest America, NWAme). The trends for the globe and in these regions are significantly different from zero, cannot be explained by natural variability (again except for NWAme), and are consistent with the model-simulated response to anthropogenic forcing in all regions. Hence, it is likely that the glacier length reductions are outside the range of natural variations and due in part to regional warming associated with increasing concentrations of greenhouse gases in the atmosphere.
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    Implications of changes in the atmospheric circulation on the detection of regional surface air temperature
    Wu, Qigang ; Karoly, David J. (American Geophysical Union, 2007)
    Previous studies have shown that observed significant warming trends in surface air temperature (SAT) consistent with the response to anthropogenic forcing are detected at scales on the order of 500 km in many regions of the globe. However, regional SAT trends project strongly on the dominant natural atmospheric circulation modes, such as the Arctic Oscillation (AO) and the hemispheric Pacific-North America (PNA)-like patterns. The warming associated with the changes of atmospheric circulation is not well simulated in current coupled climate models. In this study, we explore the influence of the exclusion of warming related to changes of the atmospheric circulation on the detection of a regional response to combined anthropogenic and natural forcings. We compare observed SAT trends over the second half of the 20th century with those simulated in response to natural and anthropogenic climate forcings in a suite of six current coupled general circulation models. Control runs from these models are used to provide estimates of the internal variability of trends. We find that the detection of the regional response to combined anthropogenic and natural forcing is robust to the exclusion of warming related to changes of the atmospheric circulation considered here.
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    Atypical influence of the 2007 La Nina on rainfall and temperature in southeastern Australia
    Gallant, Ailie J. E. ; Karoly, David J. (American Geophysical Union, 2009)
    Interannual climate variations associated with El Niño – Southern Oscillation (ENSO) dominate rainfall and temperature variability in southeastern Australia's Murray-Darling Basin (MDB), an important region for agricultural productivity. Following a decade-long dry period, a La Niña during 2007 failed to provide above average rainfall and cool temperatures in the southern half of the MDB, typical of most La Niña events for the region. Instead, annual (winter half-year) rainfall was 17% (35%) below average and maximum temperatures 0.91°C (1.26°C) above average. Based on the past variability between La Niña events, the combined probability of such anomalies is less than 2%. It is likely that these anomalies contain some contribution from a positive Indian Ocean Dipole (IOD). However, the IOD and other large-scale circulation features are unlikely to explain the atypical conditions that occurred in the southern MDB during the 2007 La Niña.
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    A climate reconstruction of Sydney Cove, New South Wales, using weather journal and documentary data, 1788-1791
    Gergis, Joëlle ; Karoly, David J. ; Allan, Rob J. (Australian Meteorological and Oceanographic Society, 2009)
    This study presents the first analysis of the weather conditions experienced at Sydney Cove, New South Wales, during the earliest period of the European settlement of Australia. A climate analysis is presented for January 1788 to December 1791 using daily temperature and barometric pressure observations recorded by William Dawes in Sydney Cove and a temperature record kept by William Bradley on board the HMS Sirius anchored in Port Jackson (Sydney Harbour) in the early months of the First Fleet’s arrival in Australia. Remarkably, the records appear comparable with modern day measurements taken from Sydney Observatory Hill, displaying similar daily variability, a distinct seasonal cycle and considerable inter-annual variability. To assess the reliability of these early weather data, they were cross-verified with other data sources, including anecdotal observations recorded in First Fleet documentary records and independent palaeoclimate reconstructions. Some biases in the temperature record, likely associated with the location of the thermometer, have been identified. Although the 1788–1791 period experienced a marked La Niña to El Niño fluctuation according to palaeoclimatic data, the cool and warm intervals in Sydney over this period cannot be conclusively linked to El Niño– Southern Oscillation (ENSO) conditions. This study demonstrates that there are excellent opportunities to expand our description of pre-20th century climate variability in Australia while contributing culturally significant material to the emerging field of Australian environmental history.
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    Gravity waves observed in temperature, wind and ozone data over Macquarie Island
    Chane-Ming, Fabrice ; Guest, Fiona ; Karoly, David J. (Australian Meteorological and Oceanographic Society, 2003)
    Characteristics of inertia-gravity waves are analysed in high-resolution vertical profiles of temperature, winds and ozone collected at Macquarie Island (54°S, 159°E) during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) observation programme in 1994. Two particular techniques are outlined to identify gravity-wave modes. The first is based on the continuous wavelet transform and seeks altitudes where the atmospheric fluctuations satisfy the gravity-wave polarisation relations in the temperature and wind soundings. The second analyses the phase and amplitude relationship of small-scale wavelike signatures seen in temperature and ozone profiles. The wavelet method identified gravity-wave modes with vertical wavelengths of 1-10 km, horizontal wavelengths of 50-1000 km and intrinsic frequencies of 1-2 f. Both methods reveal the presence of dominant modes with vertical wavelength <4 km in the upper troposphere and lower stratosphere over Macquarie Island. The energy activity of observed modes agrees well with the seasonal cycle of the upper level jet at 10 km height. These techniques together with classical spectral methods are applied to the case study of 25 October 1994 for which three quasi-monochromatic coherent modes with upwardly propagating wave energy are detected in the lower stratosphere.