Southern hemisphere circulation patterns and associated anomalies for austral summer 2015-16 are reviewed, with an emphasis on the tropical Pacific as well as Australian rainfall and temperatures. Following the peak of El Niño in November 2015, summer 2015-16 featured continued near-record El Niño conditions in the tropical Pacific but saw the emergence of cooler subsurface waters in the equatorial Pacific. A moderate Madden Julian Oscillation (MJO) pulse and positive Southern Annular Mode (SAM) ontributed to average to above average rainfall across much of Australia, while the Maritime Continent and parts of far northern Australia saw continued below average rainfall.Sea surface temperatures during summer 2015-16 were the warmest on record for the southern hemisphere oceans, with very warm ocean temperatures in the Indian Ocean and Australian region, including the warmest summer sea surface temperatures on record around Tasmania. Air temperatures were also warmer than normal across Australia throughout the season, with a significant heatwave in southeast Australia during December.
{"title":"Seasonal climate summary southern hemisphere (summer 2015-16): strong El Niño peaks and begins to weaken","authors":"Acacia S. Pepler","doi":"10.1071/es16023","DOIUrl":"https://doi.org/10.1071/es16023","url":null,"abstract":"Southern hemisphere circulation patterns and associated anomalies for austral summer 2015-16 are reviewed, with an emphasis on the tropical Pacific as well as Australian rainfall and temperatures. Following the peak of El Niño in November 2015, summer 2015-16 featured continued near-record El Niño conditions in the tropical Pacific but saw the emergence of cooler subsurface waters in the equatorial Pacific. A moderate Madden Julian Oscillation (MJO) pulse and positive Southern Annular Mode (SAM) ontributed to average to above average rainfall across much of Australia, while the Maritime Continent and parts of far northern Australia saw continued below average rainfall.Sea surface temperatures during summer 2015-16 were the warmest on record for the southern hemisphere oceans, with very warm ocean temperatures in the Indian Ocean and Australian region, including the warmest summer sea surface temperatures on record around Tasmania. Air temperatures were also warmer than normal across Australia throughout the season, with a significant heatwave in southeast Australia during December.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"3 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138530437","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Anthropogenic heat release is a key component of the urban heat island. However, it is often excluded from studies of the urban heat island because reliable estimates are not available. This omission is important because anthropogenic heat can contribute up to 4ºC to the urban heat island, and increases heat stress to urban residents. The exclusion of anthropogenic heat means the urban heat island effect on temperatures may be under-estimated. Here we estimate anthropogenic heat for four Australian capital cities (Brisbane, Sydney, Melbourne and Adelaide) to inform the management of the urban heat island in a changing climate. Anthropogenic heat release was calculated using 2011 population census data and an inventory of hourly traffic volume, building electricity and gas use. Melbourne had the highest annual daily average anthropogenic heat emissions, which reached 376 W/m2in the city centre during the daytime, while Brisbane’s emissions were 261 W/m2 and Sydney’s were 256W/m2. Adelaide had the lowest emissions, with a daily average of 39 W/m2 in the city centre. Emissions varied within and among the four cities and decreased rapidly with distance from the city centre, to 2 at 20 km from the city in Brisbane, and 15 km in Adelaide. The highest emissions were found in the city centres during working hours. The peak emissions reached in the centre of Melbourne are similar to the peak emissions in London and Tokyo, where anthropogenic heat is a large component of the urban heat island. This indicates that anthropogenic heat could be an important contributor to the urban heat island in Australian capital cities, and needs to be considered in climate adaptation studies. This is an important problem because climate change, combined with an ageing population and urban growth, could double the deaths from heatwaves in Australian cities over the next 40 years.
{"title":"Large seasonal and diurnal anthropogenic heat flux across four Australian cities","authors":"S. Chapman, J.E.M. Watson, C.A. McAlpine","doi":"10.1071/es16022","DOIUrl":"https://doi.org/10.1071/es16022","url":null,"abstract":"Anthropogenic heat release is a key component of the urban heat island. However, it is often excluded from studies of the urban heat island because reliable estimates are not available. This omission is important because anthropogenic heat can contribute up to 4ºC to the urban heat island, and increases heat stress to urban residents. The exclusion of anthropogenic heat means the urban heat island effect on temperatures may be under-estimated. Here we estimate anthropogenic heat for four Australian capital cities (Brisbane, Sydney, Melbourne and Adelaide) to inform the management of the urban heat island in a changing climate. Anthropogenic heat release was calculated using 2011 population census data and an inventory of hourly traffic volume, building electricity and gas use. Melbourne had the highest annual daily average anthropogenic heat emissions, which reached 376 W/m2in the city centre during the daytime, while Brisbane’s emissions were 261 W/m2 and Sydney’s were 256W/m2. Adelaide had the lowest emissions, with a daily average of 39 W/m2 in the city centre. Emissions varied within and among the four cities and decreased rapidly with distance from the city centre, to 2 at 20 km from the city in Brisbane, and 15 km in Adelaide. The highest emissions were found in the city centres during working hours. The peak emissions reached in the centre of Melbourne are similar to the peak emissions in London and Tokyo, where anthropogenic heat is a large component of the urban heat island. This indicates that anthropogenic heat could be an important contributor to the urban heat island in Australian capital cities, and needs to be considered in climate adaptation studies. This is an important problem because climate change, combined with an ageing population and urban growth, could double the deaths from heatwaves in Australian cities over the next 40 years.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"167 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138530426","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Motivated by the important impacts of extreme rainfall, this study extends the CSIRO and BoM (2015) analyses and projections of 20-year means and daily extremes to rainfall on the monthly timescale. Frequency distributions for monthly rainfall rates simulated by 40 CMIP5 models for the 1986-2005 period are compared with those from the AWAP 0.25° gridded observational data. Distributions spatially-averaged over Australian regions provide a signature of seasonal rainfall. Composites of months in the top and lowest deciles for each grid point and each of the four seasons are then evaluated, along with the frequency of rainfall rates exceeding thresholds ranging from 0.5 mm d-1 to 8 mm d-1.The simulated changes by 2080-2099 under the RCP8.5 scenario for the various rainfall statistics are assessed. Maps of the ensemble mean of changes of the lowest and top deciles, as a percentage of the 1986-2005 base, partly reflect the tendency for increased mean rain in summer and autumn, with decreases in winter and spring. There is also a change in the frequency distribution, with the top decile rainfall tending to increase and the lowest decile to decrease. Bar graphs are used to represent the range of change across the models, for each of four seasons and four regions. In most cases the bars for each statistic cover both declines and increases, but there is again a shift towards the positive in the progression from lowest decile to top decile. The changes are consistent with a broadening of the distribution of monthly amounts. Model spatial resolution is not a major influence on the changes. These projections for monthly rainfall statistics should be applicable to a range of climate impacts.
由于极端降雨的重要影响，本研究扩展了CSIRO和BoM(2015)在月时间尺度上对20年平均值和日极端降雨的分析和预测。将1986-2005年40个CMIP5模式模拟的月降雨率的频率分布与AWAP 0.25°格点观测数据进行了比较。澳大利亚各地区的空间平均分布提供了季节性降雨的特征。然后评估每个网格点和每个季节的最高和最低十分位数的月份组合，以及降雨量超过0.5 mm d-1至8 mm d-1阈值的频率。评估了在RCP8.5情景下2080-2099年各种降水统计的模拟变化。最低和最高十分位数的总体平均变化率图，作为1986-2005年基数的百分比，部分反映了夏季和秋季平均降雨量增加的趋势，而冬季和春季则减少。频率分布也有变化，最高十分位数降水有增加的趋势，最低十分位数降水有减少的趋势。柱状图用于表示四个季节和四个地区的各个模型的变化范围。在大多数情况下，每个统计数据的柱状图都涵盖了下降和增加，但从最低的十分位数到最高的十分位数的过程中，再次向正方向转变。这些变化与每月数额分配的扩大是一致的。模式空间分辨率不是影响变化的主要因素。这些对月降雨量统计的预估应适用于一系列气候影响。
{"title":"Extreme monthly rainfall over Australia in a changing climate","authors":"Ian G. Watterson, Zhi-Weng Chua, Pandora K. Hope","doi":"10.1071/es16025","DOIUrl":"https://doi.org/10.1071/es16025","url":null,"abstract":"Motivated by the important impacts of extreme rainfall, this study extends the CSIRO and BoM (2015) analyses and projections of 20-year means and daily extremes to rainfall on the monthly timescale. Frequency distributions for monthly rainfall rates simulated by 40 CMIP5 models for the 1986-2005 period are compared with those from the AWAP 0.25° gridded observational data. Distributions spatially-averaged over Australian regions provide a signature of seasonal rainfall. Composites of months in the top and lowest deciles for each grid point and each of the four seasons are then evaluated, along with the frequency of rainfall rates exceeding thresholds ranging from 0.5 mm d-1 to 8 mm d-1.The simulated changes by 2080-2099 under the RCP8.5 scenario for the various rainfall statistics are assessed. Maps of the ensemble mean of changes of the lowest and top deciles, as a percentage of the 1986-2005 base, partly reflect the tendency for increased mean rain in summer and autumn, with decreases in winter and spring. There is also a change in the frequency distribution, with the top decile rainfall tending to increase and the lowest decile to decrease. Bar graphs are used to represent the range of change across the models, for each of four seasons and four regions. In most cases the bars for each statistic cover both declines and increases, but there is again a shift towards the positive in the progression from lowest decile to top decile. The changes are consistent with a broadening of the distribution of monthly amounts. Model spatial resolution is not a major influence on the changes. These projections for monthly rainfall statistics should be applicable to a range of climate impacts.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"22 6","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138509289","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Lam P. Hoang, Michael J. Reeder, Gareth. J. Berry, Juliane Schwendike
Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie pole-ward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.
{"title":"Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics","authors":"Lam P. Hoang, Michael J. Reeder, Gareth. J. Berry, Juliane Schwendike","doi":"10.1071/es16026","DOIUrl":"https://doi.org/10.1071/es16026","url":null,"abstract":"Extreme rainfall in the tropics is frequently linked with coherent synoptic-scale potential vorticity (PV) disturbances. Here, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima with a focus on those that occur during summer over the African and Australian tropics. These two regions are chosen for comparison because of their geographical and climatological similarities. In particular, in both regions oceans lie equatorward and extensive deserts lie pole-ward, a juxtaposition that produces a reversal in the mean north-south temperature gradient and, through thermal wind, a low level easterly jet.In general, in the lower troposphere there are more coherent PV maxima in the tropics in the summer hemisphere than the winter hemisphere. These coherent PV maxima generally move with the background flow in the lower troposphere. The largest meridional flux of coherent PV maxima lies along eastern Australia with about half of the coherent PV maxima generated through the filamentaton and eventual isolation of midlatitude PV. In contrast, in the north African tropics, coherent PV maxima are generated mostly in the tropics and move westward through the west African monsoon region.Composites based on the extreme rainfall days for two regions are broadly similar with large, statistically significant PV maxima to the east of the maximum positive rainfall anomalies. The vertical structures of the PV fields in the two regions reveal a cyclonic PV maximum in the mid-troposphere collocated with the maximum of diabatic heating. The composite horizontal wind structures in the Australian tropics show structures similar to mesoscale convective systems (MCSs), whereas in the African tropics, they are similar to easterly waves.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"22 4","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138509290","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Here we examine winds associated with extreme rainfall and major flooding in coastal catchments and more broadly over southeastern Australia. Both radio-sonde and re-analysis data are examined. In every case (i) atmospheric moisture content is high and (ii) the low-level winds are onshore, and in almost every case (iii) the wind-direction turns anti-cyclonically with increasing height up to 500 hPa. Data from Brisbane extending back more than 50 years is consistent with this behavior: winds turn anti-cyclonically with increasing height on days with heavy rainfall, whereas winds turn cyclonically with increasing height on days with light or no rainfall. In the coastal zone, extreme rainfall rarely occurs without (i), (ii) and (iii). In eastern Australia beyond the coastal zone, conditions (i) and (iii) are also associated with extreme rainfall. We found very few cases where such conditions were not associated with extreme rainfall in this broader region. This study extends previous work by showing that the link between turning winds and rainfall exists in both the tropics and subtropics, and the link applies in cases of extreme rainfall and associated major flooding.
{"title":"A vertical wind structure that leads to extreme rainfall and major flooding in southeast Australia","authors":"Jeff Callaghan, Scott B. Power","doi":"10.1071/es16024","DOIUrl":"https://doi.org/10.1071/es16024","url":null,"abstract":"Here we examine winds associated with extreme rainfall and major flooding in coastal catchments and more broadly over southeastern Australia. Both radio-sonde and re-analysis data are examined. In every case (i) atmospheric moisture content is high and (ii) the low-level winds are onshore, and in almost every case (iii) the wind-direction turns anti-cyclonically with increasing height up to 500 hPa. Data from Brisbane extending back more than 50 years is consistent with this behavior: winds turn anti-cyclonically with increasing height on days with heavy rainfall, whereas winds turn cyclonically with increasing height on days with light or no rainfall. In the coastal zone, extreme rainfall rarely occurs without (i), (ii) and (iii). In eastern Australia beyond the coastal zone, conditions (i) and (iii) are also associated with extreme rainfall. We found very few cases where such conditions were not associated with extreme rainfall in this broader region. This study extends previous work by showing that the link between turning winds and rainfall exists in both the tropics and subtropics, and the link applies in cases of extreme rainfall and associated major flooding.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"49 11","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138509298","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Alejandro Di Luca, Jason P. Evans, Acacia S. Pepler, Lisa V. Alexander, Daniel Argüeso
Due to their large influence on both severe weather and water security along the east coast of Australia, it is increasingly important to understand how East Coast Lows (ECLs) may change over coming decades. Changes in ECLs may occur for a number of reasons including changes in the general atmospheric circulation (e.g. poleward shift of storm tracks) and/or changes in local conditions (e.g. changes in sea surface temperatures). Numerical climate models are the best available tool for studying these changes however, in order to assess future projections, climate model simulations need to be evaluated on how well they represent the historical climatology of ECLs. In this paper, we evaluate the performance of a 15-member ensemble of regional climate model (RCM) simulations to reproduce the climatology of cyclones obtained using three high-resolution reanalysis datasets (ERA-Interim, NASA-MERRA and JRA55). The performance of the RCM ensemble is also compared to results obtained from the global datasets that are used to drive the RCM ensemble (four general circulation model simulations and a low resolution reanalysis), to identify whether they offer additional value beyond the driving data. An existing cyclone detection and tracking algorithm is applied to derive a number of ECL characteristics and assess results at a variety of spatial scales. The RCM ensemble offers substantial improvement on the coarse-resolution driving data for most ECL characteristics, with results typically falling within the range of observational uncertainty, instilling confidence for studies of future projections. The study clearly highlights the need to use an ensemble of simulations to obtain reliable projections and a range of possible future changes.
{"title":"Evaluating the representation of Australian East Coast Lows in a regional climate model ensemble","authors":"Alejandro Di Luca, Jason P. Evans, Acacia S. Pepler, Lisa V. Alexander, Daniel Argüeso","doi":"10.1071/es16011","DOIUrl":"https://doi.org/10.1071/es16011","url":null,"abstract":"Due to their large influence on both severe weather and water security along the east coast of Australia, it is increasingly important to understand how East Coast Lows (ECLs) may change over coming decades. Changes in ECLs may occur for a number of reasons including changes in the general atmospheric circulation (e.g. poleward shift of storm tracks) and/or changes in local conditions (e.g. changes in sea surface temperatures). Numerical climate models are the best available tool for studying these changes however, in order to assess future projections, climate model simulations need to be evaluated on how well they represent the historical climatology of ECLs. In this paper, we evaluate the performance of a 15-member ensemble of regional climate model (RCM) simulations to reproduce the climatology of cyclones obtained using three high-resolution reanalysis datasets (ERA-Interim, NASA-MERRA and JRA55). The performance of the RCM ensemble is also compared to results obtained from the global datasets that are used to drive the RCM ensemble (four general circulation model simulations and a low resolution reanalysis), to identify whether they offer additional value beyond the driving data. An existing cyclone detection and tracking algorithm is applied to derive a number of ECL characteristics and assess results at a variety of spatial scales. The RCM ensemble offers substantial improvement on the coarse-resolution driving data for most ECL characteristics, with results typically falling within the range of observational uncertainty, instilling confidence for studies of future projections. The study clearly highlights the need to use an ensemble of simulations to obtain reliable projections and a range of possible future changes.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"22 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138543070","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study extends recent projections of monthly and daily precipitation over Australia by analysing the full frequency distribution of daily rain amounts and making projections of the new statistics wet-day fraction and top percentile of rain. Simulations from an ensemble of 33 CMIP5 models are used, together with six simulations from the downscaling model CCAM, with the data analysed on the model grids. Consistent with its higher resolution (0.5°), CCAM provides a more skilful simulation for the extreme grid point rainfall than most CMIP5 models. CCAM compares well with AWAP gridded data for wet-day fraction, while there is a wide range of CMIP5 results. In the future climate of 2080–2099 under the RCP8.5 scenario, changes in mean rainfall of both signs occur within the CMIP5 ensemble for most regions and seasons, although mean winter rainfall in southern Australia declines 5 to 30 per cent in most models and in CCAM. CCAM simulates increases in summer, and also more wet days, in contrast to CMIP5. Aside from the north in winter, the changes from CMIP5 become increasingly positive, on stepping from mean to top percentile to twenty-year extreme rainfall, a contrast of typically 25 per cent. There is much less contrast between these statistics from CCAM. The distributions of rain amounts shed light on these different projections. Averaged over Australia and four seasons, CCAM produces a broader distribution than the CMIP5 ensemble mean. However much of the future increase is in the 2 to 8 mm daily range, whereas CMIP5 distributions tend to shift towards amounts in the range 30 mm to 200 mm. Further assessment of such distributions in both these and newer versions of CCAM, ACCESS and other GCMs is recommended.
{"title":"The distribution of daily rainfall in Australia and simulated future changes","authors":"Ian G. Watterson, Tony Rafter","doi":"10.1071/es17010","DOIUrl":"https://doi.org/10.1071/es17010","url":null,"abstract":"This study extends recent projections of monthly and daily precipitation over Australia by analysing the full frequency distribution of daily rain amounts and making projections of the new statistics wet-day fraction and top percentile of rain. Simulations from an ensemble of 33 CMIP5 models are used, together with six simulations from the downscaling model CCAM, with the data analysed on the model grids. Consistent with its higher resolution (0.5°), CCAM provides a more skilful simulation for the extreme grid point rainfall than most CMIP5 models. CCAM compares well with AWAP gridded data for wet-day fraction, while there is a wide range of CMIP5 results. In the future climate of 2080–2099 under the RCP8.5 scenario, changes in mean rainfall of both signs occur within the CMIP5 ensemble for most regions and seasons, although mean winter rainfall in southern Australia declines 5 to 30 per cent in most models and in CCAM. CCAM simulates increases in summer, and also more wet days, in contrast to CMIP5. Aside from the north in winter, the changes from CMIP5 become increasingly positive, on stepping from mean to top percentile to twenty-year extreme rainfall, a contrast of typically 25 per cent. There is much less contrast between these statistics from CCAM. The distributions of rain amounts shed light on these different projections. Averaged over Australia and four seasons, CCAM produces a broader distribution than the CMIP5 ensemble mean. However much of the future increase is in the 2 to 8 mm daily range, whereas CMIP5 distributions tend to shift towards amounts in the range 30 mm to 200 mm. Further assessment of such distributions in both these and newer versions of CCAM, ACCESS and other GCMs is recommended.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"9 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138530434","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We assessed the ability of the Bureau of Meteorology’s ACCESS-S1 dynamical forecast system to simulate and predict high rainfall extremes for each season over Australia, especially focusing on the role of the Madden-Julian Oscillation (MJO). Using retrospective forecasts for the period 1990–2012, we show that ACCESS-S1 simulated the observed modulation of extreme weekly mean rainfall by each phase of the MJO reasonably well; however the simulated changes in probabilities tended to be weaker than those observed, especially across the far north during the austral summer season. The ability of the model to (i) simulate the observed modulation of extreme rainfall and (ii) predict the MJO to a lead time of four weeks, translated to enhanced forecast skill for predicting the occurrence of extreme weekly mean rainfall across much of Australia at times when the MJO was strong, compared to when the MJO was weak, during the austral spring and summer seasons in weeks 2 and 3. However, skill reduced across the central far north during the summer when the MJO was strong, suggesting the model is not good at depicting the MJO’s convective phases as it protrudes southward over northern Australia. During autumn and winter, there was little indication of changes in forecast skill, depending on the strength of the MJO. The results of this study will be useful for regional applications when the MJO is forecast to be strong during spring and summer, particularly where the swing in probability of extreme rainfall is large for specific phases of the MJO.
{"title":"Influence of the Madden-Julian Oscillation on multiweek prediction of Australian rainfall extremes using the ACCESS-S1 prediction system","authors":"A. Marshall, H. Hendon, D. Hudson","doi":"10.1071/es21001","DOIUrl":"https://doi.org/10.1071/es21001","url":null,"abstract":"We assessed the ability of the Bureau of Meteorology’s ACCESS-S1 dynamical forecast system to simulate and predict high rainfall extremes for each season over Australia, especially focusing on the role of the Madden-Julian Oscillation (MJO). Using retrospective forecasts for the period 1990–2012, we show that ACCESS-S1 simulated the observed modulation of extreme weekly mean rainfall by each phase of the MJO reasonably well; however the simulated changes in probabilities tended to be weaker than those observed, especially across the far north during the austral summer season. The ability of the model to (i) simulate the observed modulation of extreme rainfall and (ii) predict the MJO to a lead time of four weeks, translated to enhanced forecast skill for predicting the occurrence of extreme weekly mean rainfall across much of Australia at times when the MJO was strong, compared to when the MJO was weak, during the austral spring and summer seasons in weeks 2 and 3. However, skill reduced across the central far north during the summer when the MJO was strong, suggesting the model is not good at depicting the MJO’s convective phases as it protrudes southward over northern Australia. During autumn and winter, there was little indication of changes in forecast skill, depending on the strength of the MJO. The results of this study will be useful for regional applications when the MJO is forecast to be strong during spring and summer, particularly where the swing in probability of extreme rainfall is large for specific phases of the MJO.","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"73 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91069917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Earlier papers have addressed floods from warm-air advection (WAA) in southeast Australia and around the globe, and extreme rainfall in US hurricanes and Australian tropical cyclones (TCs). This is the first paper to address the WAA phenomena in causing monsoon and TC floods and in TC-like systems which develop over the interior of northern Australia. The inland events help explain Australia’s worst tropical flooding disaster in 1916. A disastrous series of floods during late January and early February 2019 caused widespread damage in tropical north Queensland both in inland regions and along the coast. This occurred when some large-scale climate influences, including the sea surface temperatures suggested conditions would not lead to major flooding. Therefore, it is important to focus on the weather systems to understand the processes that resulted in the extreme rainfall responsible for the flooding. The structure of weather systems in most areas involved a pattern in which the winds turned in an anticyclonic sense as they ascended from the low to middle levels of the atmosphere (often referred to as WAA) which was maintained over large areas for 11 days. HYSPLIT air parcel trajectory observations were employed to confirm these ascent analyses. Examination of a period during which the heaviest rain was reported and compared with climatology showed a much stronger monsoon circulation, widespread WAA through tropical Queensland where normally its descending equivalent of cold-air advection is found, and higher mean sea level pressures along the south Queensland coast. The monsoon low was located between strong deep monsoon westerlies to the north and strong deep easterlies to the south which ensured its slow movement. This non-TC event produced heavy inland rainfall. Extreme inland rainfall is rare in this region. Dare et al. (2012), using data from 1969/70 to 2009/10, showed that over north Queensland non-TC events produced a large percentage of the total rainfall. The vertical structure associated with one of the earlier events that occurred in 2008 had sufficient data to detect strong and widespread WAA overlying an onshore moist tropical airstream. This appears to have played a crucial role in such extreme rainfall extending well inland and perhaps gives insight to the cause of a 1916 flooding disaster at Clermont which claimed around 70 lives. Several other events over the inland Tropics with strong WAA also help explain the 1916 disaster.
{"title":"Weather systems and extreme rainfall generation in the 2019 north Queensland floods compared with historical north Queensland record floods","authors":"J. Callaghan","doi":"10.1071/ES20005","DOIUrl":"https://doi.org/10.1071/ES20005","url":null,"abstract":"\u0000Earlier papers have addressed floods from warm-air advection (WAA) in southeast Australia and around the globe, and extreme rainfall in US hurricanes and Australian tropical cyclones (TCs). This is the first paper to address the WAA phenomena in causing monsoon and TC floods and in TC-like systems which develop over the interior of northern Australia. The inland events help explain Australia’s worst tropical flooding disaster in 1916. A disastrous series of floods during late January and early February 2019 caused widespread damage in tropical north Queensland both in inland regions and along the coast. This occurred when some large-scale climate influences, including the sea surface temperatures suggested conditions would not lead to major flooding. Therefore, it is important to focus on the weather systems to understand the processes that resulted in the extreme rainfall responsible for the flooding. The structure of weather systems in most areas involved a pattern in which the winds turned in an anticyclonic sense as they ascended from the low to middle levels of the atmosphere (often referred to as WAA) which was maintained over large areas for 11 days. HYSPLIT air parcel trajectory observations were employed to confirm these ascent analyses. Examination of a period during which the heaviest rain was reported and compared with climatology showed a much stronger monsoon circulation, widespread WAA through tropical Queensland where normally its descending equivalent of cold-air advection is found, and higher mean sea level pressures along the south Queensland coast. The monsoon low was located between strong deep monsoon westerlies to the north and strong deep easterlies to the south which ensured its slow movement. This non-TC event produced heavy inland rainfall. Extreme inland rainfall is rare in this region. Dare et al. (2012), using data from 1969/70 to 2009/10, showed that over north Queensland non-TC events produced a large percentage of the total rainfall. The vertical structure associated with one of the earlier events that occurred in 2008 had sufficient data to detect strong and widespread WAA overlying an onshore moist tropical airstream. This appears to have played a crucial role in such extreme rainfall extending well inland and perhaps gives insight to the cause of a 1916 flooding disaster at Clermont which claimed around 70 lives. Several other events over the inland Tropics with strong WAA also help explain the 1916 disaster.\u0000","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"8 1","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-03-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75204885","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
East coast lows (ECLs) are low pressure systems that occur near the east coast of Australia. But not all lows cause the same level of impact, and a small proportion of ECLs are responsible for more than half of all days with widespread rainfall above 50mm in this region. In this study, we combine analyses of cyclones at both the surface and 500hPa levels to assess the locations of cyclones responsible for widespread heavy rainfall on the east coast. We found that the majority of days with widespread totals above 100mm on the east coast occur when a low at 500hPa over inland southeast Australia coincides with a surface low located more directly over the east coast. Such events occur on about 15 days per year but are responsible for more than 50% of days with widespread heavy rainfall on the eastern seaboard of Australia. We also found that extreme rainfall was most likely when both the surface and upper cyclones were very strong, when measured using the maximum Laplacian of pressure/height. The seasonal frequency of cyclones at the surface and 500hPa were found to be only weakly correlated with each other and often had opposing relationships (albeit weak in magnitude) with both global climate drivers and indices of local circulation variability. Trends in cyclone frequency were weak over the period 1979–2019, but there was a small decline in the frequency of deep cyclone days, which was statistically significant in some parts of the southeast. Understanding which ECLs are associated with heavy rainfall will help us to better identify how future climate change will influence ECL impacts.
{"title":"Intense east coast lows and associated rainfall in eastern Australia","authors":"A. Pepler, A. Dowdy","doi":"10.1071/ES20013","DOIUrl":"https://doi.org/10.1071/ES20013","url":null,"abstract":"\u0000East coast lows (ECLs) are low pressure systems that occur near the east coast of Australia. But not all lows cause the same level of impact, and a small proportion of ECLs are responsible for more than half of all days with widespread rainfall above 50mm in this region. In this study, we combine analyses of cyclones at both the surface and 500hPa levels to assess the locations of cyclones responsible for widespread heavy rainfall on the east coast. We found that the majority of days with widespread totals above 100mm on the east coast occur when a low at 500hPa over inland southeast Australia coincides with a surface low located more directly over the east coast. Such events occur on about 15 days per year but are responsible for more than 50% of days with widespread heavy rainfall on the eastern seaboard of Australia. We also found that extreme rainfall was most likely when both the surface and upper cyclones were very strong, when measured using the maximum Laplacian of pressure/height. The seasonal frequency of cyclones at the surface and 500hPa were found to be only weakly correlated with each other and often had opposing relationships (albeit weak in magnitude) with both global climate drivers and indices of local circulation variability. Trends in cyclone frequency were weak over the period 1979–2019, but there was a small decline in the frequency of deep cyclone days, which was statistically significant in some parts of the southeast. Understanding which ECLs are associated with heavy rainfall will help us to better identify how future climate change will influence ECL impacts.\u0000","PeriodicalId":55419,"journal":{"name":"Journal of Southern Hemisphere Earth Systems Science","volume":"19 5","pages":""},"PeriodicalIF":3.6,"publicationDate":"2021-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72457919","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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