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1
van Rensch, Peter, e Wenju Cai. "Indo-Pacific–Induced Wave Trains during Austral Autumn and Their Effect on Australian Rainfall". Journal of Climate 27, n. 9 (23 aprile 2014): 3208–21. http://dx.doi.org/10.1175/jcli-d-13-00611.1.
Testo completoAbstract (sommario):
Abstract During austral winter and spring, the El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), individually or in combination, induce equivalent-barotropic Rossby wave trains, affecting midlatitude Australian rainfall. In autumn, ENSO is at its annual minimum, and the IOD has usually not developed. However, there is still a strong equivalent-barotropic Rossby wave train associated with tropical Indian Ocean sea surface temperature (SST) variability, with a pressure anomaly to the south of Australia. This wave train is similar in position, but opposite in sign, to the IOD-induced wave train in winter and spring and has little effect on Australian rainfall. This study shows that the SST in the southeastern tropical Indian Ocean (SETIO) displays a high variance during austral autumn, with a strong influence on southeast and eastern Australian rainfall. However, this influence is slightly weaker than that associated with SST to the north of Australia, which shares fluctuations with SST in the SETIO region. The SST north of Australia is coherent with a convective dipole in the tropical Pacific Ocean, which is the source of a wave train to the east of Australia influencing rainfall in eastern Australia. ENSO Modoki is a contributor to the convective dipole and as a result it exerts a weak influence on eastern Australian rainfall through the connecting north Australian SST relationship. Thus, SST to the north of Australia acts as the main agent for delivering the impact of tropical Indo-Pacific variability to eastern Australia.
2
Cai, Wenju, Peter van Rensch, Tim Cowan e Harry H. Hendon. "Teleconnection Pathways of ENSO and the IOD and the Mechanisms for Impacts on Australian Rainfall". Journal of Climate 24, n. 15 (1 agosto 2011): 3910–23. http://dx.doi.org/10.1175/2011jcli4129.1.
Testo completoAbstract (sommario):
Abstract Impacts of El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) on Australian rainfall are diagnosed from the perspective of tropical and extratropical teleconnections triggered by tropical sea surface temperature (SST) variations. The tropical teleconnection is understood as the equatorially trapped, deep baroclinic response to the diabatic (convective) heating anomalies induced by the tropical SST anomalies. These diabatic heating anomalies also excite equivalent barotropic Rossby wave trains that propagate into the extratropics. The main direct tropical teleconnection during ENSO is the Southern Oscillation (SO), whose impact on Australian rainfall is argued to be mainly confined to near-tropical portions of eastern Australia. Rainfall is suppressed during El Niño because near-tropical eastern Australia directly experiences subsidence and higher surface pressure associated with the western pole of the SO. Impacts on extratropical Australian rainfall during El Niño are argued to stem primarily from the Rossby wave trains forced by convective variations in the Indian Ocean, for which the IOD is a primary source of variability. These equivalent-barotropic Rossby wave trains emanating from the Indian Ocean induce changes to the midlatitude westerlies across southern Australia, thereby affecting rainfall through changes in mean-state baroclinicity, west–east steering, and possibly orographic effects. Although the IOD does not mature until austral spring, its impact on Australian rainfall during winter is also ascribed to this mechanism. Because ENSO is largely unrelated to the IOD during austral winter, there is limited impact of ENSO on rainfall across southern latitudes of Australia in winter. A strong impact of ENSO on southern Australia rainfall in spring is ascribed to the strong covariation of ENSO and the IOD in this season. Implications of this pathway from the tropical Indian Ocean for impacts of both the IOD and ENSO on southern Australian climate are discussed with regard to the ability to make seasonal climate predictions and with regard to the role of trends in tropical SST for driving trends in Australian climate.
3
Rotstayn, L. D., M. A. Collier, R. M. Mitchell, Y. Qin, S. K. Campbell e S. M. Dravitzki. "Simulated enhancement of ENSO-related rainfall variability due to Australian dust". Atmospheric Chemistry and Physics 11, n. 13 (12 luglio 2011): 6575–92. http://dx.doi.org/10.5194/acp-11-6575-2011.
Testo completoAbstract (sommario):
Abstract. Australian dust emissions are highly episodic, and this may increase the importance of Australian dust as a climate feedback agent. We compare two 160-year coupled atmosphere-ocean simulations of modern-day climate using the CSIRO Mark 3.6 global climate model (GCM). The first run (DUST) includes an interactive treatment of mineral dust and its direct radiative effects. The second run (NODUST) is otherwise identical, but has the Australian dust source set to zero. We focus on the austral spring season, when the correlation between rainfall and the El Niño Southern Oscillation (ENSO) is strongest over Australia. The ENSO-rainfall relationship over eastern Australia is stronger in the DUST run: dry (El Niño) years tend to be drier, and wet (La Niña) years wetter. The amplification of ENSO-related rainfall variability over eastern Australia represents an improvement relative to observations. The effect is driven by ENSO-related anomalies in radiative forcing by Australian dust over the south-west Pacific Ocean; these anomalies increase (decrease) surface evaporation in La Niña (El Niño) years. Some of this moisture is advected towards eastern Australia, where increased (decreased) moisture convergence in La Niña (El Niño) years increases the amplitude of ENSO-related rainfall variability. The modulation of surface evaporation by dust over the south-west Pacific occurs via surface radiative forcing and dust-induced stabilisation of the boundary layer. The results suggest that (1) a realistic treatment of Australian dust may be necessary for accurate simulation of the ENSO-rainfall relationship over Australia, and (2) radiative feedbacks involving dust may be important for understanding natural rainfall variability over Australia.
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Rotstayn, L. D., M. A. Collier, R. M. Mitchell, Y. Qin e S. K. Campbell. "Simulated enhancement of ENSO-related rainfall variability due to Australian dust". Atmospheric Chemistry and Physics Discussions 11, n. 1 (19 gennaio 2011): 1595–639. http://dx.doi.org/10.5194/acpd-11-1595-2011.
Testo completoAbstract (sommario):
Abstract. Average dust emissions from Australia are small compared to those from the major sources in the Northern Hemisphere. However, they are highly episodic, and this may increase the importance of Australian dust as a climate feedback agent. We compare two 160-year coupled atmosphere-ocean simulations of modern-day climate using the CSIRO Mark 3.6 global climate model (GCM). The first run (DUST) includes an interactive treatment of mineral dust and its direct radiative effects. The second run (NODUST) is otherwise identical, but has the Australian dust source set to zero. We focus on the austral spring season, when the correlation between rainfall and the El Niño Southern Oscillation (ENSO) is strongest over Australia. We find that the ENSO-rainfall relationship over eastern Australia is stronger in the DUST run: dry (El Niño) years tend to be drier, and wet (La Niña) years wetter. The ENSO-rainfall relationship is also weaker over north-western Australia in the DUST run. The amplification of ENSO-related rainfall variability over eastern Australia and the weaker ENSO-rainfall relationship over the north-west both represent an improvement relative to observations. The suggested mechanism over eastern Australia involves stabilisation of the surface layer due to enhanced atmospheric heating and surface cooling in El Niño years, and enhanced ascent and moisture convergence driven by atmospheric heating in La Niña years. The results suggest that (1) a realistic treatment of Australian dust may be necessary for accurate simulation of the ENSO-rainfall relationship over Australia, and (2) radiative feedbacks involving dust may be important for understanding natural rainfall variability over Australia.
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Rathore, Saurabh, Nathaniel L. Bindoff, Caroline C. Ummenhofer, Helen E. Phillips, Ming Feng e Mayank Mishra. "Improving Australian Rainfall Prediction Using Sea Surface Salinity". Journal of Climate 34, n. 7 (aprile 2021): 2473–90. http://dx.doi.org/10.1175/jcli-d-20-0625.1.
Testo completoAbstract (sommario):
AbstractThis study uses sea surface salinity (SSS) as an additional precursor for improving the prediction of summer [December–February (DJF)] rainfall over northeastern Australia. From a singular value decomposition between SSS of prior seasons and DJF rainfall, we note that SSS of the Indo-Pacific warm pool region [SSSP (150°E–165°W and 10°S–10°N) and SSSI (50°–95°E and 10°S–10°N)] covaries with Australian rainfall, particularly in the northeast region. Composite analysis that is based on high or low SSS events in the SSSP and SSSI regions is performed to understand the physical links between the SSS and the atmospheric moisture originating from the regions of anomalously high or low, respectively, SSS and precipitation over Australia. The composites show the signature of co-occurring La Niña and negative Indian Ocean dipole with anomalously wet conditions over Australia and conversely show the signature of co-occurring El Niño and positive Indian Ocean dipole with anomalously dry conditions there. During the high SSS events of the SSSP and SSSI regions, the convergence of incoming moisture flux results in anomalously wet conditions over Australia with a positive soil moisture anomaly. Conversely, during the low SSS events of the SSSP and SSSI regions, the divergence of incoming moisture flux results in anomalously dry conditions over Australia with a negative soil moisture anomaly. We show from the random-forest regression analysis that the local soil moisture, El Niño–Southern Oscillation (ENSO), and SSSP are the most important precursors for the northeast Australian rainfall whereas for the Brisbane region ENSO, SSSP, and the Indian Ocean dipole are the most important. The prediction of Australian rainfall using random-forest regression shows an improvement by including SSS from the prior season. This evidence suggests that sustained observations of SSS can improve the monitoring of the Australian regional hydrological cycle.
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Kingsford, R. T., P. S. Wong, L. W. Braithwaite e M. T. Maher. "Waterbird abundance in eastern Australia, 1983 - 92". Wildlife Research 26, n. 3 (1999): 351. http://dx.doi.org/10.1071/wr96062.
Testo completoAbstract (sommario):
We examined the relationships between abundance of 16 species of waterbirds and the rainfall in eastern Australia, the rainfall west of this region, the annual Southern Oscillation index (SOI), the wetland area, and hunting in eastern Australia for the period 1983–92. Data were collected during aerial surveys of eastern Australia. For most explanatory variables, lags of up to five years before aerial surveys were also investigated during these analyses. The analyses covered all nine game species (plumed whistling-duck, Australian shelduck, Australian wood duck, pink-eared duck, grey teal, chestnut teal, Pacific black duck, Australasian shoveler, hardhead) and seven non-game species (Australian pelican, white-faced heron, yellow-billed spoonbill, freckled duck, black swan, black-winged stilt, red-necked avocet). Regression models were developed for all species apart from Australian pelicans. Rainfall and climate indices generally were most correlated with the species’ abundance. Bonferroni adjustments to significance levels meant that there were significant variables in regression models for seven of the 16 species. Abundance indices for plumed whistling-duck, chestnut teal, hardhead, black swan and black-winged stilt were related to the climate variables (rainfall, SOI) and wetland area, whereas abundance of pink-eared duck and red-necked avocets were negatively related. Abundance of chestnut teal was positively related to numbers of hunting licences sold. The results are equivocal about the role of hunting in determining waterfowl abundance, which is probably a reflection of few data points and numbers of variables included. In general, abundance indices of waterbirds appear to have decreased between 1983 and 1992, which may correspond to other factors not modelled
7
Lim, Eun-Pa, Harry H. Hendon, Debra Hudson, Guomin Wang e Oscar Alves. "Dynamical Forecast of Inter–El Niño Variations of Tropical SST and Australian Spring Rainfall". Monthly Weather Review 137, n. 11 (1 novembre 2009): 3796–810. http://dx.doi.org/10.1175/2009mwr2904.1.
Testo completoAbstract (sommario):
Abstract The relationship between variations of Indo-Pacific sea surface temperatures (SSTs) and Australian springtime rainfall over the last 30 years is investigated with a focus on predictability of inter–El Niño variations of SST and associated rainfall anomalies. Based on observed data, the leading empirical orthogonal function (EOF) of Indo-Pacific SST represents mature El Niño conditions, while the second and fourth modes depict major east–west shifts of individual El Niño events. These higher-order EOFs of SST explain more rainfall variance in Australia, especially in the southeast, than does the El Niño mode. Furthermore, intense springtime droughts tend to be associated with peak warming in the central Pacific, as captured by EOFs 2 and 4, together with warming in the eastern Pacific as depicted by EOF1. The ability to predict these inter–El Niño variations of SST and Australian rainfall is assessed with the Australian Bureau of Meteorology dynamical coupled model seasonal forecast system, the Predictive Ocean and Atmospheric Model for Australia (POAMA). A 10-member ensemble of 9-month hindcasts was generated for the period 1980–2006. For the September–November season, the leading 2 EOFs of SST are predictable with lead times of 3–6 months, while SST EOF4 is predictable out to a lead time of 1 month. The teleconnection between the leading EOFs of SST and Australian rainfall is also well depicted in the model. Based on this ability to predict major east–west variations of El Niño and the teleconnection to Australian rainfall, springtime rainfall over eastern Australia, and major drought events are predictable up to a season in advance.
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Hendon, Harry H., David W. J. Thompson e Matthew C. Wheeler. "Australian Rainfall and Surface Temperature Variations Associated with the Southern Hemisphere Annular Mode". Journal of Climate 20, n. 11 (1 giugno 2007): 2452–67. http://dx.doi.org/10.1175/jcli4134.1.
Testo completoAbstract (sommario):
Abstract Daily variations in Australian rainfall and surface temperature associated with the Southern Hemisphere annular mode (SAM) are documented using observations for the period 1979–2005. The high index polarity of the SAM is characterized by a poleward contraction of the midlatitude westerlies. During winter, the high index polarity of the SAM is associated with decreased daily rainfall over southeast and southwest Australia, but during summer it is associated with increased daily rainfall on the southern east coast of Australia and decreased rainfall in western Tasmania. Variations in the SAM explain up to ∼15% of the weekly rainfall variance in these regions, which is comparable to the variance accounted for by the El Niño–Southern Oscillation, especially during winter. The most widespread temperature anomalies associated with the SAM occur during the spring and summer seasons, when the high index polarity of the SAM is associated with anomalously low maximum temperature over most of central/eastern subtropical Australia. The regions of decreased maximum temperature are also associated with increased rainfall. Implications for recent trends in Australian rainfall and temperature are discussed.
9
Pook, M. J., J. S. Risbey, P. C. McIntosh, C. C. Ummenhofer, A. G. Marshall e G. A. Meyers. "The Seasonal Cycle of Blocking and Associated Physical Mechanisms in the Australian Region and Relationship with Rainfall". Monthly Weather Review 141, n. 12 (25 novembre 2013): 4534–53. http://dx.doi.org/10.1175/mwr-d-13-00040.1.
Testo completoAbstract (sommario):
Abstract The seasonal cycle of blocking in the Australian region is shown to be associated with major seasonal temperature changes over continental Antarctica (approximately 15°–35°C) and Australia (about 8°–17°C) and with minor changes over the surrounding oceans (below 5°C). These changes are superimposed on a favorable background state for blocking in the region resulting from a conjunction of physical influences. These include the geographical configuration and topography of the Australian and Antarctic continents and the positive west to east gradient of sea surface temperature in the Indo-Australian sector of the Southern Ocean. Blocking is represented by a blocking index (BI) developed by the Australian Bureau of Meteorology. The BI has a marked seasonal cycle that reflects seasonal changes in the strength of the westerly winds in the midtroposphere at selected latitudes. Significant correlations between the BI at Australian longitudes and rainfall have been demonstrated in southern and central Australia for the austral autumn, winter, and spring. Patchy positive correlations are evident in the south during summer but significant negative correlations are apparent in the central tropical north. By decomposing the rainfall into its contributions from identifiable synoptic types during the April–October growing season, it is shown that the high correlation between blocking and rainfall in southern Australia is explained by the component of rainfall associated with cutoff lows. These systems form the cyclonic components of blocking dipoles. In contrast, there is no significant correlation between the BI and rainfall from Southern Ocean fronts.
10
Stephens, D. J., e T. J. Lyons. "Rainfall-yield relationships across the Australian wheatbelt". Australian Journal of Agricultural Research 49, n. 2 (1998): 211. http://dx.doi.org/10.1071/a96139.
Testo completoAbstract (sommario):
A network of rainfall stations was selected across the Australian wheatbelt and monthly rainfall regressed with wheat yields from the surrounding shires for the period 1976-87. Yields were found to be strongly related to fluctuations in total rainfall amount and the seasonal distribution of rainfall through the year. These temporal relationships vary spatially and appear to be regulated by the water-holding capacity of regional soils. Sixteen agrometeorological zones were defined with similar rainfall-yield relationships. In all these, autumn rains that permit an early sowing, and finishing rains after July, are most important for higher yields. As the rainfall distribution becomes more winter-dominant, both crop yield variability and the usefulness of high winter rainfall decreases. Heavy rainfall in the month after sowing can have a negative effect in southern Australia, as plants are more prone to suffer potential yield losses from a wet soil profile. Waterlogging has a large negative effect in the south-west of Western Australia, such that the rainfall distribution can be more important than the rainfall amount. Rainfall-yield correlations are generally more positive in drier regions, and are enhanced by persistent rainfall anomalies between April and November during El Niño Southern Oscillation years.
11
Freund, Mandy, Benjamin J. Henley, David J. Karoly, Kathryn J. Allen e Patrick J. Baker. "Multi-century cool- and warm-season rainfall reconstructions for Australia's major climatic regions". Climate of the Past 13, n. 12 (30 novembre 2017): 1751–70. http://dx.doi.org/10.5194/cp-13-1751-2017.
Testo completoAbstract (sommario):
Abstract. Australian seasonal rainfall is strongly affected by large-scale ocean–atmosphere climate influences. In this study, we exploit the links between these precipitation influences, regional rainfall variations, and palaeoclimate proxies in the region to reconstruct Australian regional rainfall between four and eight centuries into the past. We use an extensive network of palaeoclimate records from the Southern Hemisphere to reconstruct cool (April–September) and warm (October–March) season rainfall in eight natural resource management (NRM) regions spanning the Australian continent. Our bi-seasonal rainfall reconstruction aligns well with independent early documentary sources and existing reconstructions. Critically, this reconstruction allows us, for the first time, to place recent observations at a bi-seasonal temporal resolution into a pre-instrumental context, across the entire continent of Australia. We find that recent 30- and 50-year trends towards wetter conditions in tropical northern Australia are highly unusual in the multi-century context of our reconstruction. Recent cool-season drying trends in parts of southern Australia are very unusual, although not unprecedented, across the multi-century context. We also use our reconstruction to investigate the spatial and temporal extent of historical drought events. Our reconstruction reveals that the spatial extent and duration of the Millennium Drought (1997–2009) appears either very much below average or unprecedented in southern Australia over at least the last 400 years. Our reconstruction identifies a number of severe droughts over the past several centuries that vary widely in their spatial footprint, highlighting the high degree of diversity in historical droughts across the Australian continent. We document distinct characteristics of major droughts in terms of their spatial extent, duration, intensity, and seasonality. Compared to the three largest droughts in the instrumental period (Federation Drought, 1895–1903; World War II Drought, 1939–1945; and the Millennium Drought, 1997–2005), we find that the historically documented Settlement Drought (1790–1793), Sturt's Drought (1809–1830) and the Goyder Line Drought (1861–1866) actually had more regionalised patterns and reduced spatial extents. This seasonal rainfall reconstruction provides a new opportunity to understand Australian rainfall variability by contextualising severe droughts and recent trends in Australia.
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Frederiksen, Jorgen S., e Stacey L. Osbrough. "Tipping Points and Changes in Australian Climate and Extremes". Climate 10, n. 5 (19 maggio 2022): 73. http://dx.doi.org/10.3390/cli10050073.
Testo completoAbstract (sommario):
Systematic changes, since the beginning of the 20th century, in average and extreme Australian rainfall and temperatures indicate that Southern Australian climate has undergone regime transitions into a drier and warmer state. South-west Western Australia (SWWA) experienced the most dramatic drying trend with average streamflow into Perth dams, in the last decade, just 20% of that before the 1960s and extreme, decile 10, rainfall reduced to near zero. In south-eastern Australia (SEA) systematic decreases in average and extreme cool season rainfall became evident in the late 1990s with a halving of the area experiencing average decile 10 rainfall in the early 21st century compared with that for the 20th century. The shift in annual surface temperatures over SWWA and SEA, and indeed for Australia as a whole, has occurred primarily over the last 20 years with the percentage area experiencing extreme maximum temperatures in decile 10 increasing to an average of more than 45% since the start of the 21st century compared with less than 3% for the 20th century mean. Average maximum temperatures have also increased by circa 1 °C for SWWA and SEA over the last 20 years. The climate changes in rainfall an d temperatures are associated with atmospheric circulation shifts.
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Taschetto, Andréa S., e Matthew H. England. "El Niño Modoki Impacts on Australian Rainfall". Journal of Climate 22, n. 11 (1 giugno 2009): 3167–74. http://dx.doi.org/10.1175/2008jcli2589.1.
Testo completoAbstract (sommario):
Abstract This study investigates interseasonal and interevent variations in the impact of El Niño on Australian rainfall using available observations from the postsatellite era. Of particular interest is the difference in impact between classical El Niño events wherein peak sea surface temperature (SST) anomalies appear in the eastern Pacific and the recently termed El Niño “Modoki” events that are characterized by distinct warm SST anomalies in the central Pacific and weaker cold anomalies in the west and east of the basin. A clear interseasonal and interevent difference is apparent, with the maximum rainfall response for Modoki events occurring in austral autumn compared to austral spring for classical El Niños. Most interestingly, the Modoki and non-Modoki El Niño events exhibit a marked difference in rainfall impact over Australia: while classical El Niños are associated with a significant reduction in rainfall over northeastern and southeastern Australia, Modoki events appear to drive a large-scale decrease in rainfall over northwestern and northern Australia. In addition, rainfall variations during March–April–May are more sensitive to the Modoki SST anomaly pattern than the conventional El Niño anomalies to the east.
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Frederiksen, Carsten S., Simon Grainger e Xiaogu Zheng. "Potential predictability of Australian seasonal rainfall". Journal of Southern Hemisphere Earth Systems Science 68, n. 1 (2018): 65. http://dx.doi.org/10.1071/es18005.
Testo completoAbstract (sommario):
The potential predictability of Australian seasonal mean rainfall at 800 stations is estimated using an analysis of variance method for the period 1957-2015 and for all twelve three-month seasons. The method estimates the contribution of the slow, potentially predictable, signal of the rainfall to the total inter-annual variance, after removing the climate noise due to intra-seasonal and weather variability.The results show that there are stations, in all seasons, where the potential predictability is relatively high, and can be greater than half of the total inter-annual variance. Largest potential predictability, coherent over eastern Australia, occurs during the transition to spring, and in spring seasons. Large and coherent potential predictability also occurs during the autumn seasons over Queensland and south-eastern Australia. For summer and the northern wet seasons, the potential predictability is larger over the northeast coastal stations, in the southeast, central east and central west of the continent. During winter, relatively large and coherent potential predictability occurs over the southeast, the central east, and in an implied northwest-southeast band across the continent. Patterns of seasonal forecast skill from the coupled Predictive Ocean Atmosphere Model for Australia are shown to be highly consistent with our estimates of the potential predictability.Factors that may influence the potential predictability are briefly discussed in the light of previous studies that have considered the relationships between the slow, potentially predictable, components of rainfall and the atmospheric and oceanic circulations. Prominent among these are the El Niño-Southern Oscillation, the Southern Annular mode and the meridional Indian Ocean Dipole.
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Taschetto, Andréa S., Alex Sen Gupta, Caroline C. Ummenhofer e Matthew H. England. "Can Australian Multiyear Droughts and Wet Spells Be Generated in the Absence of Oceanic Variability?" Journal of Climate 29, n. 17 (19 agosto 2016): 6201–21. http://dx.doi.org/10.1175/jcli-d-15-0694.1.
Testo completoAbstract (sommario):
Abstract Anomalous conditions in the tropical oceans, such as those related to El Niño–Southern Oscillation and the Indian Ocean dipole, have been previously blamed for extended droughts and wet periods in Australia. Yet the extent to which Australian wet and dry spells can be driven by internal atmospheric variability remains unclear. Natural variability experiments are examined to determine whether prolonged extreme wet and dry periods can arise from internal atmospheric and land variability alone. Results reveal that this is indeed the case; however, these dry and wet events are found to be less severe than in simulations incorporating coupled oceanic variability. Overall, ocean feedback processes increase the magnitude of Australian rainfall variability by about 30% and give rise to more spatially coherent rainfall impacts. Over mainland Australia, ocean interactions lead to more frequent extreme events, particularly during the rainy season. Over Tasmania, in contrast, ocean–atmosphere coupling increases mean rainfall throughout the year. While ocean variability makes Australian rainfall anomalies more severe, droughts and wet spells of duration longer than three years are equally likely to occur in both atmospheric- and ocean-driven simulations. Moreover, they are essentially indistinguishable from what one expects from a Gaussian white noise distribution. Internal atmosphere–land-driven megadroughts and megapluvials that last as long as ocean-driven events are also identified in the simulations. This suggests that oceanic variability may be less important than previously assumed for the long-term persistence of Australian rainfall anomalies. This poses a challenge to accurate prediction of long-term dry and wet spells for Australia.
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Lim, Eun-Pa, e Harry H. Hendon. "Understanding the Contrast of Australian Springtime Rainfall of 1997 and 2002 in the Frame of Two Flavors of El Niño". Journal of Climate 28, n. 7 (27 marzo 2015): 2804–22. http://dx.doi.org/10.1175/jcli-d-14-00582.1.
Testo completoAbstract (sommario):
Abstract This study investigates the causes and predictability of the different springtime rainfall responses over Australia for El Niño in 1997 and 2002. The rainfall deficit over Australia is generally assumed to be linearly related to the strength of El Niño. However, Australia received near-normal springtime rainfall during the record strong El Niño in 1997, whereas it suffered from severe drought, especially in the east, during the weak El Niño of 2002. Statistical reconstruction of the rainfall anomalies and forecasts produced from the Australian Bureau of Meteorology’s dynamical seasonal forecast system [Predictive Ocean and Atmosphere Model for Australia (POAMA)] demonstrated that the eastward and westward shifts of the maximum SST warming of El Niño contributed to the near-normal and dry responses of Australian spring rainfall in 1997 and 2002, respectively. Hence, the contrasting rainfall responses were largely predictable. However, the dry conditions in 2002 were significantly amplified by the occurrence of the record strength negative phase of the southern annular mode (SAM), which could only be predicted with the use of realistic atmospheric initial conditions in the atmosphere–ocean coupled configuration of POAMA. Therefore, predictability of the severity of the 2002 drought over Australia was strongly constrained by the predictability of the SAM, despite the high predictability of the drier than normal condition of 2002 spring that stems from the anomalous central Pacific warming of 2002 El Niño.
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Brown, Josephine R., Christian Jakob e John M. Haynes. "An Evaluation of Rainfall Frequency and Intensity over the Australian Region in a Global Climate Model". Journal of Climate 23, n. 24 (15 dicembre 2010): 6504–25. http://dx.doi.org/10.1175/2010jcli3571.1.
Testo completoAbstract (sommario):
Abstract Observed regional rainfall characteristics can be analyzed by examining both the frequency and intensity of different categories of rainfall. A complementary approach is to consider rainfall characteristics associated with regional synoptic regimes. These two approaches are combined here to examine daily rainfall characteristics over the Australian region, providing a target for model simulations. Using gridded daily rainfall data for the period 1997–2007, rainfall at each grid point and averaged over several sites is decomposed into the frequency of rainfall events and the intensity of rainfall associated with each event. Daily sea level pressure is classified using a self-organizing map, and rainfall on corresponding days is assigned to the resulting synoptic regimes. This technique is then used to evaluate rainfall in the new Australian Community Climate and Earth-System Simulator (ACCESS) global climate model and separate the influence of large-scale circulation errors and errors due to the representation of subgrid-scale physical processes. The model exhibits similar biases to many other global climate models, simulating too frequent light rainfall and heavy rainfall of insufficient intensity. These errors are associated with particular synoptic regimes over different sectors of the Australian continent and surrounding oceans. The model simulates only weak convective rainfall over land during the summer monsoon, and heavy rainfall associated with frontal systems over southern Australia is also not simulated. As the model captures the structure and frequency of synoptic patterns, but not the associated rainfall intensity or frequency, it is likely that the source of the rainfall errors lies in model physical parameterizations rather than large-scale dynamics.
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Wheeler, Matthew C., Harry H. Hendon, Sam Cleland, Holger Meinke e Alexis Donald. "Impacts of the Madden–Julian Oscillation on Australian Rainfall and Circulation". Journal of Climate 22, n. 6 (15 marzo 2009): 1482–98. http://dx.doi.org/10.1175/2008jcli2595.1.
Testo completoAbstract (sommario):
Abstract Impacts of the Madden–Julian oscillation (MJO) on Australian rainfall and circulation are examined during all four seasons. The authors examine circulation anomalies and a number of different rainfall metrics, each composited contemporaneously for eight MJO phases derived from the real-time multivariate MJO index. Multiple rainfall metrics are examined to allow for greater relevance of the information for applications. The greatest rainfall impact of the MJO occurs in northern Australia in (austral) summer, although in every season rainfall impacts of various magnitude are found in most locations, associated with corresponding circulation anomalies. In northern Australia in all seasons except winter, the rainfall impact is explained by the direct influence of the MJO’s tropical convective anomalies, while in winter a weaker and more localized signal in northern Australia appears to result from the modulation of the trade winds as they impinge upon the eastern coasts, especially in the northeast. In extratropical Australia, on the other hand, the occurrence of enhanced (suppressed) rainfall appears to result from induced upward (downward) motion within remotely forced extratropical lows (highs), and from anomalous low-level northerly (southerly) winds that transport moisture from the tropics. Induction of extratropical rainfall anomalies by remotely forced lows and highs appears to operate mostly in winter, whereas anomalous meridional moisture transport appears to operate mainly in the summer, autumn, and to some extent in the spring.
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Nicholls, Neville, Wasyl Drosdowsky e Beth Lavery. "Australian rainfall variability and change". Weather 52, n. 3 (marzo 1997): 66–72. http://dx.doi.org/10.1002/j.1477-8696.1997.tb06274.x.
Testo completo
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J., Arblaster, Meehl G. e Moore A. "Interdecadal modulation of Australian rainfall". Climate Dynamics 18, n. 6 (1 febbraio 2002): 519–31. http://dx.doi.org/10.1007/s00382-001-0191-y.
Testo completo
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Tozer, C. R., A. S. Kiem e D. C. Verdon-Kidd. "On the uncertainties associated with using gridded rainfall data as a proxy for observed". Hydrology and Earth System Sciences Discussions 8, n. 5 (15 settembre 2011): 8399–433. http://dx.doi.org/10.5194/hessd-8-8399-2011.
Testo completoAbstract (sommario):
Abstract. Gridded rainfall datasets are used in many hydrological and climatological studies, in Australia and elsewhere, including for hydroclimatic forecasting, climate attribution studies and climate model performance assessments. The attraction of the spatial coverage provided by gridded data is clear, particularly in Australia where the spatial and temporal resolution of the rainfall gauge network is sparse. However, the question that must be asked is whether it is suitable to use gridded data as a proxy for observed point data, given that gridded data is inherently "smoothed" and may not necessarily capture the temporal and spatial variability of Australian rainfall which leads to hydroclimatic extremes (i.e. droughts, floods)? This study investigates this question through a statistical analysis of three monthly gridded Australian rainfall datasets – the Bureau of Meteorology (BOM) dataset, the Australian Water Availability Project (AWAP) and the SILO dataset. To demonstrate the hydrological implications of using gridded data as a proxy for gauged data, a rainfall-runoff model is applied to one catchment in South Australia (SA) initially using gridded data as the source of rainfall input and then gauged rainfall data. The results indicate a markedly different runoff response associated with each of the different sources of rainfall data. It should be noted that this study does not seek to identify which gridded dataset is the "best" for Australia, as each gridded data source has its pros and cons, as does gauged or point data. Rather the intention is to quantify differences between various gridded data sources and how they compare with gauged data so that these differences can be considered and accounted for in studies that utilise these gridded datasets. Ultimately, if key decisions are going to be based on the outputs of models that use gridded data, an estimate (or at least an understanding) of the uncertainties relating to the assumptions made in the development of gridded data and how that gridded data compares with reality should be made.
22
Ummenhofer, Caroline C., Alexander Sen Gupta, Andréa S. Taschetto e Matthew H. England. "Modulation of Australian Precipitation by Meridional Gradients in East Indian Ocean Sea Surface Temperature". Journal of Climate 22, n. 21 (1 novembre 2009): 5597–610. http://dx.doi.org/10.1175/2009jcli3021.1.
Testo completoAbstract (sommario):
Abstract This study explores the impact of meridional sea surface temperature (SST) gradients across the eastern Indian Ocean on interannual variations in Australian precipitation. Atmospheric general circulation model (AGCM) experiments are conducted in which the sign and magnitude of eastern Indian Ocean SST gradients are perturbed. This results in significant rainfall changes for western and southeastern Australia. A reduction (increase) in the meridional SST gradient drives a corresponding response in the atmospheric thickness gradients and results in anomalous dry (wet) conditions over Australia. During simulated wet years, this seems to be due to westerly anomalies in the thermal wind over Australia and anomalous onshore moisture advection, with a suggestion that the opposite occurs during dry conditions. Thus, an asymmetry is seen in the magnitude of the forced circulation and precipitation response between the dry and wet simulations. To assess the relative contribution of the SST anomalies making up the meridional gradient, the SST pattern is decomposed into its constituent “poles,” that is, the eastern tropical pole off the northwest shelf of Australia versus the southern pole in the central subtropical Indian Ocean. Overall, the simulated Australian rainfall response is linear with regard to the sign and magnitude of the eastern Indian Ocean SST gradient. The tropical eastern pole has a larger impact on the atmospheric circulation and Australian precipitation changes relative to the southern subtropical pole. However, there is clear evidence of the importance of the southern pole in enhancing the Australian rainfall response, when occurring in conjunction with but of opposite sign to the eastern tropical pole. The observed relationship between the meridional SST gradient in the eastern Indian Ocean and rainfall over western and southeastern Australia is also analyzed for the period 1970–2005. The observed relationship is found to be consistent with the AGCM results.
23
Shi, Ge, Wenju Cai, Tim Cowan, Joachim Ribbe, Leon Rotstayn e Martin Dix. "Variability and Trend of North West Australia Rainfall: Observations and Coupled Climate Modeling". Journal of Climate 21, n. 12 (15 giugno 2008): 2938–59. http://dx.doi.org/10.1175/2007jcli1908.1.
Testo completoAbstract (sommario):
Abstract Since 1950, there has been an increase in rainfall over North West Australia (NWA), occurring mainly during the Southern Hemisphere (SH) summer season. A recent study using twentieth-century multimember ensemble simulations in a global climate model forced with and without increasing anthropogenic aerosols suggests that the rainfall increase is attributable to increasing Northern Hemisphere aerosols. The present study investigates the dynamics of the observed trend toward increased rainfall and compares the observed trend with that generated in the model forced with increasing aerosols. It is found that the observed positive trend in rainfall is projected onto two modes of variability. The first mode is associated with an anomalously low mean sea level pressure (MSLP) off NWA instigated by the enhanced sea surface temperature (SST) gradients toward the coast. The associated cyclonic flows bring high-moisture air to northern Australia, leading to an increase in rainfall. The second mode is associated with an anomalously high MSLP over much of the Australian continent; the anticyclonic circulation pattern, over northern Australia, determines that when rainfall is anomalously high, west of 130°E, rainfall is anomalously low east of this longitude. The sum of the upward trends in these two modes compares well to the observed increasing trend pattern. The modeled rainfall trend, however, is generated by a different process. The model suffers from an equatorial cold-tongue bias: the tongue of anomalies associated with El Niño–Southern Oscillation extends too far west into the eastern Indian Ocean. Consequently, there is an unrealistic relationship in the SH summer between Australian rainfall and eastern Indian Ocean SST: the rise in SST is associated with increasing rainfall over NWA. In the presence of increasing aerosols, a significant SST increase occurs in the eastern tropical Indian Ocean. As a result, the modeled rainfall increase in the presence of aerosol forcing is accounted for by these unrealistic relationships. It is not clear whether, in a model without such defects, the observed trend can be generated by increasing aerosols. Thus, the impact of aerosols on Australian rainfall remains an open question.
24
Wang, Guomin, e Harry H. Hendon. "Sensitivity of Australian Rainfall to Inter–El Niño Variations". Journal of Climate 20, n. 16 (15 agosto 2007): 4211–26. http://dx.doi.org/10.1175/jcli4228.1.
Testo completoAbstract (sommario):
Abstract Australia typically experiences drought during El Niño, especially across the eastern two-thirds of the continent during austral spring (September–November). There have, however, been some interesting departures from this paradigm. For instance, the near-record-strength El Niño of 1997 was associated with near-normal rainfall. In contrast, eastern Australia experienced near-record drought during the modest El Niño of 2002. This stark contrast raises the issue of how the magnitude of the drought is related to the character and magnitude of El Niño, for instance as measured by the broadscale sea surface temperature (SST) anomaly in the equatorial eastern Pacific. Internal (unpredictable) atmospheric noise is one plausible explanation for this contrasting behavior during these El Niño events. Here, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Niño and, in particular, the greatest sensitivity is to the SST variations on the eastern edge of the Pacific warm pool rather than in the eastern Pacific where El Niño variations are typically largest. Positive SST anomalies maximized near the date line in 2002, but in 1997 maximum anomalies were shifted well into the eastern Pacific, where their influence on Australian rainfall appears to be less. These findings provide a plausible physical basis for the view that forecasting the strength of El Niño is not sufficient to accurately predict rainfall variations across Australia during El Niño.
25
Hendon, Harry H., Eun-Pa Lim e Guo Liu. "The Role of Air–Sea Interaction for Prediction of Australian Summer Monsoon Rainfall". Journal of Climate 25, n. 4 (8 febbraio 2012): 1278–90. http://dx.doi.org/10.1175/jcli-d-11-00125.1.
Testo completoAbstract (sommario):
Abstract Forecast skill for seasonal mean rainfall across northern Australia is lower during the summer monsoon than in the premonsoon transition season based on 25 years of hindcasts using the Predictive Ocean Atmosphere Model for Australia (POAMA) coupled model seasonal forecast system. The authors argue that this partly reflects an intrinsic property of the monsoonal system, whereby seasonally varying air–sea interaction in the seas around northern Australia promotes predictability in the premonsoon season and demotes predictability after monsoon onset. Trade easterlies during the premonsoon season support a positive feedback between surface winds, SST, and rainfall, which results in stronger and more persistent SST anomalies to the north of Australia that compliment the remote forcing of Australian rainfall from El Niño in the Pacific. After onset of the Australian summer monsoon, this local feedback is not supported in the monsoonal westerly regime, resulting in weaker SST anomalies to the north of Australia and with lower persistence than in the premonsoon season. Importantly, the seasonality of this air–sea interaction is captured in the POAMA forecast model. Furthermore, analysis of perfect model forecasts and forecasts generated by prescribing observed SST results in largely the same conclusion (i.e., significantly lower actual and potential forecast skill during the monsoon), thereby supporting the notion that air–sea interaction contributes to intrinsically lower predictability of rainfall during the monsoon.
26
Tihema, Tamika. "Seasonal climate summary for the southern hemisphere (summer 2017–18): an exceptionally warm season for Australia – a short-lived and weak La Niña". Journal of Southern Hemisphere Earth Systems Science 69, n. 1 (2019): 351. http://dx.doi.org/10.1071/es19018.
Testo completoAbstract (sommario):
This is a summary of the southern hemisphere atmospheric circulation patterns and meteorological indices for summer 2017–18; an account of seasonal rainfall and temperature for the Australian region is also provided. A short-lived and weak La Niña developed but decayed by the end of February 2018. Sea-surface temperatures were exceptionally warm in the Tasman Sea from late 2017 to early 2018. It was an exceptionally warm summer for Australia, and the third-warmest mean temperature on record for the nation. Summer rainfall was close to the long-term average for Australia, with aboveaverage rainfall in west and below-average rainfall in the east.
27
Tozer, C. R., A. S. Kiem e D. C. Verdon-Kidd. "On the uncertainties associated with using gridded rainfall data as a proxy for observed". Hydrology and Earth System Sciences 16, n. 5 (23 maggio 2012): 1481–99. http://dx.doi.org/10.5194/hess-16-1481-2012.
Testo completoAbstract (sommario):
Abstract. Gridded rainfall datasets are used in many hydrological and climatological studies, in Australia and elsewhere, including for hydroclimatic forecasting, climate attribution studies and climate model performance assessments. The attraction of the spatial coverage provided by gridded data is clear, particularly in Australia where the spatial and temporal resolution of the rainfall gauge network is sparse. However, the question that must be asked is whether it is suitable to use gridded data as a proxy for observed point data, given that gridded data is inherently "smoothed" and may not necessarily capture the temporal and spatial variability of Australian rainfall which leads to hydroclimatic extremes (i.e. droughts, floods). This study investigates this question through a statistical analysis of three monthly gridded Australian rainfall datasets – the Bureau of Meteorology (BOM) dataset, the Australian Water Availability Project (AWAP) and the SILO dataset. The results of the monthly, seasonal and annual comparisons show that not only are the three gridded datasets different relative to each other, there are also marked differences between the gridded rainfall data and the rainfall observed at gauges within the corresponding grids – particularly for extremely wet or extremely dry conditions. Also important is that the differences observed appear to be non-systematic. To demonstrate the hydrological implications of using gridded data as a proxy for gauged data, a rainfall-runoff model is applied to one catchment in South Australia initially using gauged data as the source of rainfall input and then gridded rainfall data. The results indicate a markedly different runoff response associated with each of the different sources of rainfall data. It should be noted that this study does not seek to identify which gridded dataset is the "best" for Australia, as each gridded data source has its pros and cons, as does gauged data. Rather, the intention is to quantify differences between various gridded data sources and how they compare with gauged data so that these differences can be considered and accounted for in studies that utilise these gridded datasets. Ultimately, if key decisions are going to be based on the outputs of models that use gridded data, an estimate (or at least an understanding) of the uncertainties relating to the assumptions made in the development of gridded data and how that gridded data compares with reality should be made.
28
Pepler, A., B. Timbal, C. Rakich e A. Coutts-Smith. "Indian Ocean Dipole Overrides ENSO’s Influence on Cool Season Rainfall across the Eastern Seaboard of Australia". Journal of Climate 27, n. 10 (9 maggio 2014): 3816–26. http://dx.doi.org/10.1175/jcli-d-13-00554.1.
Testo completoAbstract (sommario):
Abstract The strong relationship between eastern Australian winter–spring rainfall and tropical modes of variability such as the El Niño–Southern Oscillation (ENSO) does not extend to the heavily populated coastal strip east of the Great Dividing Range in southeast Australia, where correlations between rainfall and Niño-3.4 are insignificant during June–October. The Indian Ocean dipole (IOD) is found to have a strong influence on zonal wind flow during the winter and spring months, with positive IOD increasing both onshore winds and rainfall over the coastal strip, while decreasing rainfall elsewhere in southeast Australia. The IOD thus opposes the influence of ENSO over the coastal strip, and this is shown to be the primary cause of the breakdown of the ENSO–rainfall relationship in this region.
29
Frederiksen, Carsten, Simon Grainger e Xiaogu Zheng. "Potential predictability of Australian seasonal rainfall". Journal of Southern Hemisphere Earth System Science 68, n. 1 (31 dicembre 2018): 65–100. http://dx.doi.org/10.22499/3.6801.005.
Testo completo
30
Simmonds, Ian, e Pandora Hope. "Persistence Characteristics of Australian Rainfall Anomalies". International Journal of Climatology 17, n. 6 (maggio 1997): 597–613. http://dx.doi.org/10.1002/(sici)1097-0088(199705)17:6<597::aid-joc173>3.0.co;2-v.
Testo completo
31
FitzGerald, RD, ML Curll e EW Heap. "Wheat for fodder and grain on the Northern Tablelands of New South Wales". Australian Journal of Experimental Agriculture 35, n. 1 (1995): 93. http://dx.doi.org/10.1071/ea9950093.
Testo completoAbstract (sommario):
Thirty varieties of wheat originating from Australia, UK, USA, Ukraine, and France were evaluated over 3 years as dual-purpose wheats for the high rainfall environment of the Northern Tablelands of New South Wales (mean annual rainfall 851 mm). Mean grain yields (1.9-4.3 t/ha) compared favourably with record yields in the traditional Australian wheatbelt, but were much poorer than average yields of 6.5 t/ha reported for UK crops. A 6-week delay in sowing time halved grain yield in 1983; cutting in spring reduced yield by 40% in 1986. Grazing during winter did not significantly reduce yields. Results indicate that the development of wheat varieties adapted to the higher rainfall tablelands and suited to Australian marketing requirements might help to provide a useful alternative enterprise for tableland livestock producers.
32
Suppiah, R. "The Australian summer monsoon: a review". Progress in Physical Geography: Earth and Environment 16, n. 3 (settembre 1992): 283–318. http://dx.doi.org/10.1177/030913339201600302.
Testo completoAbstract (sommario):
The Australian summer monsoon influences the climate of the Australian tropics during the period from December to March. During this period, interannual and intraseasonal variations of rainfall associated with global-scale circulation anomalies strongly effect human life and economic activities in this region. Any changes in the global-scale circulation patterns in relation to changes in the heat balance components under enhanced greenhouse condition could alter monsoonal circulation characteristics and thus could bring serious impacts to human life in the monsoon-dominated region. To provide a basis for looking at changes in monsoonal characteristics under enhanced greenhouse condition, the current understanding of the Australian summer monsoonal circulation characteristics is reviewed here. Detailed information is given on the formation and the steady development of the Pilbara heat low over the northwestern part of Australia and the importance of the location of the monsoon shear line, active and break cycles of the monsoon, influence of South China Sea cold surges on monsoon activity, 40-50 oscillation in monsoon rainfall and winds and their link to El Nino/ Southern Oscillation (ENSO) phenomenon, the influence of the ENSO phenomenon on rainfall on interannual time scales and the link between monsoonal activity and tropical cyclones. The problems related to the above mentioned topics and their research priorities are highlighted.
33
Lim, Eun-Pa, Harry H. Hendon, David L. T. Anderson, Andrew Charles e Oscar Alves. "Dynamical, Statistical–Dynamical, and Multimodel Ensemble Forecasts of Australian Spring Season Rainfall". Monthly Weather Review 139, n. 3 (1 marzo 2011): 958–75. http://dx.doi.org/10.1175/2010mwr3399.1.
Testo completoAbstract (sommario):
Abstract The prediction skill of the Australian Bureau of Meteorology dynamical seasonal forecast model Predictive Ocean Atmosphere Model for Australia (POAMA) is assessed for probabilistic forecasts of spring season rainfall in Australia and the feasibility of increasing forecast skill through statistical postprocessing is examined. Two statistical postprocessing techniques are explored: calibrating POAMA prediction of rainfall anomaly against observations and using dynamically predicted mean sea level pressure to infer regional rainfall anomaly over Australia (referred to as “bridging”). A “homogeneous” multimodel ensemble prediction method (HMME) is also introduced that consists of the combination of POAMA’s direct prediction of rainfall anomaly together with the two statistically postprocessed predictions. Using hindcasts for the period 1981–2006, the direct forecasts from POAMA exhibit skill relative to a climatological forecast over broad areas of eastern and southern Australia, where El Niño and the Indian Ocean dipole (whose behavior POAMA can skillfully predict at short lead times) are known to exert a strong influence in austral spring. The calibrated and bridged forecasts, while potentially offering improvement over the direct forecasts because of POAMA’s ability to predict the main drivers of springtime rainfall (e.g., El Niño and the Southern Oscillation), show only limited areas of improvement, mainly because strict cross-validation limits the ability to capitalize on relatively modest predictive signals with short record lengths. However, when POAMA and the two statistical–dynamical rainfall forecasts are combined in the HMME, higher deterministic and probabilistic skill is achieved over any of the single models, which suggests the HMME is another useful method to calibrate dynamical model forecasts.
34
Langford, Sally, e Harry H. Hendon. "Improving Reliability of Coupled Model Forecasts of Australian Seasonal Rainfall". Monthly Weather Review 141, n. 2 (1 febbraio 2013): 728–41. http://dx.doi.org/10.1175/mwr-d-11-00333.1.
Testo completoAbstract (sommario):
Abstract Seasonal rainfall predictions for Australia from the Predictive Ocean Atmosphere Model for Australia (POAMA), version P15b, coupled model seasonal forecast system, which has been run operationally at the Australian Bureau of Meteorology since 2002, are overconfident (too low spread) and only moderately reliable even when forecast accuracy is highest in the austral spring season. The lack of reliability is a major impediment to operational uptake of the coupled model forecasts. Considerable progress has been made to reduce reliability errors with the new version of POAMA2, which makes use of a larger ensemble from three different versions of the model. Although POAMA2 can be considered to be multimodel, its individual models and forecasts are similar as a result of using the same perturbed initial conditions and the same model lineage. Reliability of the POAMA2 forecasts, although improved, remains relatively low. Hence, the authors explore the additional benefit that can be attained using more independent models available in the European Union Ensemble-Based Predictions of Climate Changes and their Impacts (ENSEMBLES) project. Although forecast skill and reliability of seasonal predictions of Australian rainfall are similar for POAMA2 and the ENSEMBLES models, forming a multimodel ensemble using POAMA2 and the ENSEMBLES models is shown to markedly improve reliability of Australian seasonal rainfall forecasts. The benefit of including POAMA2 into this multimodel ensemble is due to the additional information and skill of the independent model, and not just due to an increase in the number of ensemble members. The increased reliability, as well as improved accuracy, of regional rainfall forecasts from this multimodel ensemble system suggests it could be a useful operational prediction system.
35
Telcik, Nicola, e Charitha Pattiaratchi. "Influence of Northwest Cloudbands on Southwest Australian Rainfall". Journal of Climatology 2014 (8 aprile 2014): 1–11. http://dx.doi.org/10.1155/2014/671394.
Testo completoAbstract (sommario):
Northwest cloudbands are tropical-extratropical feature that crosses the Australian continent originating from Australia’s northwest coast and develops in a NW-SE orientation. In paper, atmospheric and oceanic reanalysis data (NCEP) and Reynolds reconstructed sea surface temperature data were used to examine northwest cloudband activity across the Australian mainland. An index that reflected the monthly, seasonal, and interannual activity of northwest cloudbands between 1950 and 1999 was then created. Outgoing longwave radiation, total cloud cover, and latent heat flux data were used to determine the number of days when a mature northwest cloudband covered part of the Australian continent between April and October. Regional indices were created for site-specific investigations, especially of cloudband-related rainfall. High and low cloudband activity can affect the distribution of cloudbands and their related rainfall. In low cloudband activity seasons, cloudbands were mostly limited to the south and west Australian coasts. In high cloudband activity seasons, cloudbands penetrated farther inland, which increased the inland rainfall. A case study of the southwest Australian region demonstrated that, in a below average rainfall year, cloudband-related rainfall was limited to the coast. In an above average rainfall year, cloudband-related rainfall occurred further inland.
36
Pepler, Acacia S. "Seasonal climate summary southern hemisphere (summer 2015-16): strong El Niño peaks and begins to weaken". Journal of Southern Hemisphere Earth Systems Science 66, n. 4 (2016): 361. http://dx.doi.org/10.1071/es16023.
Testo completoAbstract (sommario):
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.
37
Smith, I. N., L. Wilson e R. Suppiah. "Characteristics of the Northern Australian Rainy Season". Journal of Climate 21, n. 17 (1 settembre 2008): 4298–311. http://dx.doi.org/10.1175/2008jcli2109.1.
Testo completoAbstract (sommario):
Abstract A trend of increasing rainfall over much of north and northwest Australia over recent decades has contrasted with decreases over much of the rest of the continent. The increases have occurred during the summer months when the rainy season is dominated by the Australian monsoon but is also affected by other events such as tropical cyclones, Madden–Julian oscillations, and sporadic thunderstorms. The problem of diagnosing these trends is considered in terms of changes in the timing of the rainy season. While numerous definitions for rainy/monsoon season onset exist, most are designed to be useful in a predictive sense and can be limited in their application to diagnostic studies, particularly when they involve predetermined threshold amounts. Here the authors define indices, based on daily rainfall observations, that provide relatively simple, robust descriptions of each rainy season at any location. These are calculated using gridded daily rainfall data throughout the northern Australian tropics and also for selected stations. The results indicate that the trends in summer rainfall totals over the period from 1950 to 2005 appear to be mainly the result of similar trends in average intensity. Furthermore, the links between the September–October average Southern Oscillation index indicate that ENSO events affect season duration rather than average intensity. Because duration and average intensity are derived as independent features of each season, it is argued that the trends in rainfall totals are largely unrelated to trends in ENSO and most likely reflect the influence of other factors. Finally, diagnosing these features of the rainy season provides a basis for assessing the confidence one can attach to different climate model projections of changes to rainfall.
38
Berry, Gareth J., Michael J. Reeder e Christian Jakob. "Coherent Synoptic Disturbances in the Australian Monsoon". Journal of Climate 25, n. 24 (15 dicembre 2012): 8409–21. http://dx.doi.org/10.1175/jcli-d-12-00143.1.
Testo completoAbstract (sommario):
Abstract Coherent synoptic-scale weather systems within the Australian monsoon are identified and tracked in the isentropic potential vorticity (PV) field from the ECMWF Interim Reanalysis (ERA-Interim) dataset during the Southern Hemisphere summer. The resulting dataset is then used to compile statistics and synoptic composites of Australian monsoon disturbances. On average, a synoptic system is found in the region every 2.5 days. However, the time interval between consecutive events is highly variable, meaning that the synoptic activity in the Australian monsoon is not well represented by commonly employed spectral techniques. The analysis reveals that most synoptic systems originate within the Australian monsoon, but at the 315-K level (approximately 700 hPa) a significant proportion of systems are first detected near the east coast of the continent where extratropical Rossby waves are observed to frequently break. The average Australian monsoon weather system propagates from east to west at approximately 6 m s−1 and has a characteristic length scale of 2000 km. Synoptic composite structures show some resemblance to African easterly waves; they move along a midtropospheric (approximately 700 hPa) easterly wind maximum and have peak meridional winds at this level. Composite rainfall shows that rainfall is significantly enhanced ahead (west) of the synoptic PV maximum and suppressed behind. It is estimated that in some parts of northwestern Australia 40%–50% of the summertime rainfall occurs with a tracked monsoon disturbance in the vicinity.
39
Narsey, Sugata, Michael J. Reeder, Christian Jakob e Duncan Ackerley. "An Evaluation of Northern Australian Wet Season Rainfall Bursts in CMIP5 Models". Journal of Climate 31, n. 19 (ottobre 2018): 7789–802. http://dx.doi.org/10.1175/jcli-d-17-0637.1.
Testo completoAbstract (sommario):
The simulation of northern Australian wet season rainfall bursts by coupled climate models is evaluated. Individual models produce vastly different amounts of precipitation over the north of Australia during the wet season, and this is found to be related to the number of bursts they produce. The seasonal cycle of bursts is found to be poor in most of the models evaluated. It is known that northern Australian wet season bursts are often associated with midlatitude Rossby wave packets and their surface signature as they are refracted toward the tropics. The relationship between midlatitude waves and the initiation of wet season bursts is simulated well by the models evaluated. Another well-documented influence on the initiation of northern Australian wet season bursts is the Madden–Julian oscillation (MJO). No model adequately simulated the tropical outgoing longwave radiation temporal–spatial patterns seen in the reanalysis-derived OLR. This result suggests that the connection between the MJO and the initiation of northern Australian wet season bursts in models is poor.
40
Grose, Michael R., James S. Risbey, Aurel F. Moise, Stacey Osbrough, Craig Heady, Louise Wilson e Tim Erwin. "Constraints on Southern Australian Rainfall Change Based on Atmospheric Circulation in CMIP5 Simulations". Journal of Climate 30, n. 1 (gennaio 2017): 225–42. http://dx.doi.org/10.1175/jcli-d-16-0142.1.
Testo completoAbstract (sommario):
Atmospheric circulation change is likely to be the dominant driver of multidecadal rainfall trends in the midlatitudes with climate change this century. This study examines circulation features relevant to southern Australian rainfall in January and July and explores emergent constraints suggested by the intermodel spread and their impact on the resulting rainfall projection in the CMIP5 ensemble. The authors find relationships between models’ bias and projected change for four features in July, each with suggestions for constraining forced change. The features are the strength of the subtropical jet over Australia, the frequency of blocked days in eastern Australia, the longitude of the peak blocking frequency east of Australia, and the latitude of the storm track within the polar front branch of the split jet. Rejecting models where the bias suggests either the direction or magnitude of change in the features is implausible produces a constraint on the projected rainfall reduction for southern Australia. For RCP8.5 by the end of the century the constrained projections are for a reduction of at least 5% in July (with models showing increase or little change being rejected). Rejecting these models in the January projections, with the assumption the bias affects the entire simulation, leads to a rejection of wet and dry outliers.
41
Schepen, Andrew, Q. J. Wang e David Robertson. "Evidence for Using Lagged Climate Indices to Forecast Australian Seasonal Rainfall". Journal of Climate 25, n. 4 (8 febbraio 2012): 1230–46. http://dx.doi.org/10.1175/jcli-d-11-00156.1.
Testo completoAbstract (sommario):
Abstract Lagged oceanic and atmospheric climate indices are potentially useful predictors of seasonal rainfall totals. A rigorous Bayesian joint probability modeling approach is applied to find the cross-validation predictive densities of gridded Australian seasonal rainfall totals using lagged climate indices as predictors over the period of 1950–2009. The evidence supporting the use of each climate index as a predictor of seasonal rainfall is quantified by the pseudo-Bayes factor based on cross-validation predictive densities. The evidence strongly supports the use of climate indices from the Pacific region with weaker, but positive, evidence for the use of climate indices from the Indian region and the extratropical region. The spatial structure and seasonal variation of the evidence for each climate index is mapped and compared. Spatially, the strongest supporting evidence is found for forecasting in northern and eastern Australia. Seasonally, the strongest evidence is found from August–October to November–January and the weakest evidence is found from March–May to May–July. In some regions and seasons, there is little evidence supporting the use of climate indices for forecasting seasonal rainfall. Climate indices derived from sea surface temperature anomalies in the Pacific region show stronger persistence in the relationship with Australian seasonal rainfall totals than climate indices derived from sea surface temperature anomalies in the Indian region. Climate indices derived from atmospheric variables are also strongly supported, provided they represent the large-scale circulation. Many climate indices are found to show similar supporting evidence for forecasting Australian seasonal rainfall, leading to the prospect of combining climate indices in multiple predictor models and/or model averaging.
42
Whitehead, Peter J., Jeremy Russell-Smith e Cameron Yates. "Fire patterns in north Australian savannas: extending the reach of incentives for savanna fire emissions abatement". Rangeland Journal 36, n. 4 (2014): 371. http://dx.doi.org/10.1071/rj13129.
Testo completoAbstract (sommario):
Anthropogenic fires in Australia’s fire-prone savannas produce up to 3% of the nation’s accountable greenhouse gas (GHG) emissions. Incentives to improve fire management have been created by a nationally accredited savanna burning emissions abatement methodology applying to 483 000 km2 of relatively high-rainfall (>1000 mm p.a.) regions. Drawing on 15 years of fire mapping, this paper assesses appropriate biophysical boundaries for a savanna burning methodology extended to cover lower-rainfall regions. We examine a large random sample of points with at least 300 mm of annual rainfall, to show that: (a) relative fire frequencies (percentage of years with fire) decline from 33.3% in higher-rainfall regions (>1000 mm) to straddling ~10% in the range 300–700 mm; (b) there are no marked discontinuities in fire frequency or fire seasonality down the rainfall gradient; (c) at all annual rainfalls, fire frequency is higher when rainfall is more strongly seasonal (very low rainfall in the driest quarter); (d) below 500 mm fire regimes are particularly variable and a large proportion of sampled sites had no fire over the study period; (e) fire is more likely to occur later in the fire season (generating relatively higher emissions) in the 600–700-mm annual rainfall band than in other parts of the rainfall gradient; (f) woodland savannas are most common above and predominantly grassland systems are more common below ~600-mm annual rainfall. We propose that development of a complementary lower-rainfall savanna burning methodology apply to regions between 600 and 1000-mm annual rainfall and ≤15 mm of rainfall in the driest quarter, adding an area more than 1.5 times the existing methodology’s coverage. Given greater variability in biophysical influences on fire regimes and observed levels of fire frequency within this lower-rainfall domain, we suggest that criteria for determining baseline (pre-project) periods require estimates of mean annual emissions equivalent in precision to the project on which the higher-rainfall methodology was based.
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Pepler, Acacia, Linden Ashcroft e Blair Trewin. "The relationship between the subtropical ridge and Australian temperatures". Journal of Southern Hemisphere Earth Systems Science 68, n. 1 (2018): 201. http://dx.doi.org/10.1071/es18011.
Testo completoAbstract (sommario):
The intensity and latitude of the subtropical ridge over eastern Australia is strongly associated with southeast Australian rainfall, particularly during the cool months of the year. We show that the subtropical ridge also exerts a strong influence on temperatures across much of Australia, with warmer daytime temperatures and more warm extremes across southern Australia when the subtropical ridge is stronger than average, which is largely independent of the relationship between the subtropical ridge and rainfall. A strong subtropical ridge is also linked to warmer than average minimum temperatures over southern Australia throughout much of the year, except from May to August when a strong ridge is associated with cooler mean minimum temperatures and an increased frequency of cool nights. This relationship, and the observed strengthening of the subtropical ridge during autumn and winter in recent decades, can partially explain the weaker warming trends in minimum temperatures in southeast Australia compared to elsewhere in the country over the period 1960-2016.
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Cai, Wenju, Ariaan Purich, Tim Cowan, Peter van Rensch e Evan Weller. "Did Climate Change–Induced Rainfall Trends Contribute to the Australian Millennium Drought?" Journal of Climate 27, n. 9 (23 aprile 2014): 3145–68. http://dx.doi.org/10.1175/jcli-d-13-00322.1.
Testo completoAbstract (sommario):
Abstract The Australian decade-long “Millennium Drought” broke in the summer of 2010/11 and was considered the most severe drought since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean dipole (IOD), affecting southern Australia in winter and spring; the southern annular mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño–Southern Oscillation, affecting northern and eastern Australia in most seasons and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry zone, has possible implications for recent rainfall declines in autumn. Using observations and simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they severely underestimate the observed poleward expansion of the subtropical dry zone and associated impacts. Thus, although climate models generally suggest that Australia’s Millennium Drought was mostly due to multidecadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.
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Newby, M., S. W. Franks e C. J. White. "Estimating urban flood risk – uncertainty in design criteria". Proceedings of the International Association of Hydrological Sciences 370 (11 giugno 2015): 3–7. http://dx.doi.org/10.5194/piahs-370-3-2015.
Testo completoAbstract (sommario):
Abstract. The design of urban stormwater infrastructure is generally performed assuming that climate is static. For engineering practitioners, stormwater infrastructure is designed using a peak flow method, such as the Rational Method as outlined in the Australian Rainfall and Runoff (AR&R) guidelines and estimates of design rainfall intensities. Changes to Australian rainfall intensity design criteria have been made through updated releases of the AR&R77, AR&R87 and the recent 2013 AR&R Intensity Frequency Distributions (IFDs). The primary focus of this study is to compare the three IFD sets from 51 locations Australia wide. Since the release of the AR&R77 IFDs, the duration and number of locations for rainfall data has increased and techniques for data analysis have changed. Updated terminology coinciding with the 2013 IFD release has also resulted in a practical change to the design rainfall. For example, infrastructure that is designed for a 1 : 5 year ARI correlates with an 18.13% AEP, however for practical purposes, hydraulic guidelines have been updated with the more intuitive 20% AEP. The evaluation of design rainfall variation across Australia has indicated that the changes are dependent upon location, recurrence interval and rainfall duration. The changes to design rainfall IFDs are due to the application of differing data analysis techniques, the length and number of data sets and the change in terminology from ARI to AEP. Such changes mean that developed infrastructure has been designed to a range of different design criteria indicating the likely inadequacy of earlier developments to the current estimates of flood risk. In many cases, the under-design of infrastructure is greater than the expected impact of increased rainfall intensity under climate change scenarios.
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Sharples, Jason J., Michael F. Hutchinson e Damian R. Jellett. "On the Horizontal Scale of Elevation Dependence of Australian Monthly Precipitation". Journal of Applied Meteorology 44, n. 12 (1 dicembre 2005): 1850–65. http://dx.doi.org/10.1175/jam2289.1.
Testo completoAbstract (sommario):
Abstract Determination of the scale of the interaction between precipitation and topography is important for the accurate interpolation of rainfall in mountainous areas and also provides insight into the physical processes involved. In this paper, trivariate thin-plate smoothing splines are used to investigate the scale of interaction between monthly precipitation and topography by interpolating monthly rainfall over three subregions of the Australian continent, incorporating different climatic conditions and rainfall types. The interpolations are based upon elevations derived from digital elevation models (DEMs) of various resolutions. All of the DEMs are local averages of version 2.0 of the 9-s-resolution DEM of Australia. The results suggest that the optimal scale of the interaction between precipitation and topography, as it pertains to the elevation-dependent interpolation of monthly precipitation in Australia, is between 5 and 10 km. This is in agreement with results of similar studies that addressed daily precipitation over Switzerland.
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Verdon-Kidd, D. C., e A. S. Kiem. "Non–stationarity in annual maxima rainfall across Australia – implications for Intensity–Frequency–Duration (IFD) relationships". Hydrology and Earth System Sciences Discussions 12, n. 3 (30 marzo 2015): 3449–75. http://dx.doi.org/10.5194/hessd-12-3449-2015.
Testo completoAbstract (sommario):
Abstract. Rainfall Intensity–Frequency–Duration (IFD) relationships are commonly required for the design and planning of water supply and management systems around the world. Currently IFD information is based on the "stationary climate assumption" – that weather at any point in time will vary randomly and that the underlying climate statistics (including both averages and extremes) will remain constant irrespective of the period of record. However, the validity of this assumption has been questioned over the last 15 years, particularly in Australia, following an improved understanding of the significant impact of climate variability and change occurring on interannual to multidecadal timescales. This paper provides evidence of non-stationarity in annual maxima rainfall timeseries using 96 daily rainfall stations and 66 sub-daily rainfall stations across Australia. Further, the effect of non-stationarity on the resulting IFD estimates are explored for three long-term sub-daily rainfall records (Brisbane, Sydney and Melbourne) utilising insights into multidecadal climate variability. It is demonstrated that IFD relationships may under- or over-estimate the design rainfall depending on the length and time period spanned by the rainfall data used to develop the IFD information. It is recommended that non-stationarity in annual maxima rainfall be explicitly considered and appropriately treated in the ongoing revisions of Engineers Australia's guide to estimating and utilising IFD information, "Australian Rainfall and Runoff", and that clear guidance needs to be provided on how to deal with the issue of non-stationarity of extreme events (irrespective of whether that non-stationarity is due to natural or anthropogenic climate change). The findings of our study also have important implications for other regions of the world that exhibit considerable hydroclimatic variability and where IFD information is based on relatively short data sets.
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Li, Xiao-Feng, Jingjing Yu e Yun Li. "Recent Summer Rainfall Increase and Surface Cooling over Northern Australia since the Late 1970s: A Response to Warming in the Tropical Western Pacific". Journal of Climate 26, n. 18 (9 settembre 2013): 7221–39. http://dx.doi.org/10.1175/jcli-d-12-00786.1.
Testo completoAbstract (sommario):
Abstract Rainfall over northern Australia (NA) in austral summer is the largest water source of Australia. Previous studies have suggested a strong zonal-dipole trend pattern in austral summer rainfall since 1950, with rainfall increasing in northwest Australia (NWA) but decreasing in northeast Australia (NEA). The dynamics of rainfall increase in NWA was linked to sea surface temperature (SST) in the south Indian Ocean and the rainfall decrease in NEA was associated with SST in the northeast Indian Ocean. This study reports that, in contrast to a zonal-dipole trend pattern, a dominant wetting pattern over NA has recently been observed in the post-1979 satellite era. The recent NA rainfall increase also manifests as the first leading mode of summer rainfall variability over the Australian continent. Further investigation reveals that SST in the tropical western Pacific (TWP) has replaced the SST in the south and northeast Indian Ocean as the controlling factor responsible for the recent NA rainfall increase. Direct thermal forcing by increasing TWP SST gives rise to an anomalous Gill-type cyclone centered around NA, leading to anomalously high rainfall. As such, the increasing SST in the TWP induces over 50% of the observed rainfall wetting trend over NA. The increased rainfall in turn induces land surface cooling in NA. This mechanism can be confirmed with results obtained from sensitivity experiments of a numerical spectral atmospheric general circulation model. Thus, increasing SST in the TWP has contributed much of the recent summer rainfall increase and consequently the surface cooling over NA.
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Feng, Juan, Jianping Li e Yun Li. "A Monsoon-Like Southwest Australian Circulation and Its Relation with Rainfall in Southwest Western Australia". Journal of Climate 23, n. 6 (15 marzo 2010): 1334–53. http://dx.doi.org/10.1175/2009jcli2837.1.
Testo completoAbstract (sommario):
Abstract Using the NCEP–NCAR reanalysis, the 40-yr ECMWF Re-Analysis (ERA-40), and precipitation data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Australian Bureau of Meteorology, the variability and circulation features influencing southwest Western Australia (SWWA) winter rainfall are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is called the southwest Australian circulation (SWAC) because of its several distinct features characterizing a monsoonal circulation: the seasonal reversal of winds, alternate wet and dry seasons, and an evident land–sea thermal contrast. The seasonal march of the SWAC in extended winter (May–October) is demonstrated by pentad data. An index based on the dynamics’ normalized seasonality was introduced to describe the behavior and variation of the winter SWAC. It is found that the winter rainfall over SWWA has a significant positive correlation with the SWAC index in both early (May–July) and late (August–October) winter. In weaker winter SWAC years, there is an anticyclonic anomaly over the southern Indian Ocean resulting in weaker westerlies and northerlies, which are not favorable for more rainfall over SWWA, and the opposite combination is true in the stronger winter SWAC years. The SWAC explains not only a large portion of the interannual variability of SWWA rainfall in both early and late winter but also the long-term drying trend over SWWA in early winter. The well-coupled SWAC–SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulations such as the southern annular mode (SAM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and ENSO Modoki (EM). The result offers qualified support for the argument that the monsoon-like circulation may contribute to the rainfall decline in early winter over SWWA. The external forcing of the SWAC is also explored in this study.
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Lee, S. Y., e T. Y. Koh. "Teleconnection between Australian winter temperature and Indian summer monsoon rainfall". Atmospheric Chemistry and Physics 12, n. 2 (16 gennaio 2012): 669–81. http://dx.doi.org/10.5194/acp-12-669-2012.
Testo completoAbstract (sommario):
Abstract. The pattern of evaporative sources and the direction of the large-scale circulation over the Indian Ocean during the boreal summer raises the question of whether atmospheric conditions in Australia could influence conditions over the Indian subcontinent, despite the long passage of air over the Indian Ocean. The authors propose that such an influence is sometimes possible when there is unusually low temperature over inland Australia during the austral winter, through the mechanism where such a temperature extreme enhances evaporation rate over the eastern tropical Indian Ocean and hence enhances rainfall over two regions in western India after 13–19 days. Results from trajectory calculations indicate that such an influence is mechanistically feasible, with air of Australian origin contributing 0.5–1.5% of the climatological net precipitation for monsoon seasonal rainfall over western India. Statistics performed on reanalysis, satellite and in situ data are consistent with such a mechanism. Since extreme winter temperature in Australia is often associated with cold-air outbreaks, the described mechanism may be an example of how southern hemispheric mid-latitude weather can influence northern hemispheric monsoon rainfall. Further study is recommended through modelling and comparison with various known causes of atmospheric variability to confirm the existence of such a mechanism and determine the extent of its influence during specific low temperature episodes.