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Ariaan Purich
,
Tim Cowan
,
Seung-Ki Min
, and
Wenju Cai

Abstract

In recent decades, Southern Hemisphere midlatitude regions such as southern Africa, southeastern Australia, and southern Chile have experienced a reduction in austral autumn precipitation; the cause of which is poorly understood. This study focuses on the ability of global climate models that form part of the Coupled Model Intercomparison Project phase 5 to simulate these trends, their relationship with extratropical and subtropical processes, and implications for future precipitation changes. Models underestimate both the historical autumn poleward expansion of the subtropical dry zone and the positive southern annular mode (SAM) trend. The multimodel ensemble (MME) is also unable to capture the spatial pattern of observed precipitation trends across semiarid midlatitude regions. However, in temperate regions that are located farther poleward such as southern Chile, the MME simulates observed precipitation declines. The MME shows a strong consensus in twenty-first-century declines in autumn precipitation across southern Chile in both the medium–low and high representative concentration pathway (RCP) scenarios and across southern Africa in the high RCP scenario, but little change across southeastern Australia. Projecting a strong positive SAM trend and continued subtropical dry-zone expansion, the models converge on large SAM and dry-zone-expansion-induced precipitation declines across southern midlatitudes. In these regions, the strength of future precipitation trends is proportional to the strength of modeled trends in these phenomena, suggesting that unabated greenhouse gas–induced climate change will have a large impact on austral autumn precipitation in such midlatitude regions.

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Wenju Cai
,
Peter van Rensch
,
Tim Cowan
, and
Harry H. Hendon

Abstract

Recent research has shown that the climatic impact from El Niño–Southern Oscillation (ENSO) on middle latitudes west of the western Pacific (e.g., southeast Australia) during austral spring (September–November) is conducted via the tropical Indian Ocean (TIO). However, it is not clear whether this impact pathway is symmetric about the positive and negative phases of ENSO and the Indian Ocean dipole (IOD). It is shown that a strong asymmetry does exist. For ENSO, only the impact from El Niño is conducted through the TIO pathway; the impact from La Niña is delivered through the Pacific–South America pattern. For the IOD, a greater convection anomaly and wave train response occurs during positive IOD (pIOD) events than during negative IOD (nIOD) events. This “impact asymmetry” is consistent with the positive skewness of the IOD, principally due to a negative skewness of sea surface temperature (SST) anomalies in the east IOD (IODE) pole. In the IODE region, convection anomalies are more sensitive to a per unit change of cold SST anomalies than to the same unit change of warm SST anomalies. This study shows that the IOD skewness occurs despite the greater damping, rather than due to a breakdown of this damping as suggested by previous studies. This IOD impact asymmetry provides an explanation for much of the reduction in spring rainfall over southeast Australia during the 2000s. Key to this rainfall reduction is the increased occurrences of pIOD events, more so than the lack of nIOD events.

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Wenju Cai
,
Ariaan Purich
,
Tim Cowan
,
Peter van Rensch
, and
Evan Weller

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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Tim Cowan
,
Peter van Rensch
,
Ariaan Purich
, and
Wenju Cai

Abstract

Relationships of the Indian Ocean dipole (IOD), El Niño–Southern Oscillation (ENSO), and the southern annular mode (SAM) with atmospheric blocking are investigated using a linear framework over the austral autumn–spring (cool) seasons for southeast Australia (SEA). Positive blocking events occurring at 130°–140°E increase the likelihood of cutoff low pressure systems developing that generate significant rainfall totals across SEA. In mid to late austral autumn (April–May), blocking is coherent with negative IOD events. During this season, a negative IOD event and blocking are associated with warm sea surface temperature anomalies in the eastern tropical Indian Ocean and a blocking high pressure cell south of Australia. An anomalous cyclonic pressure center over southern Australia directs tropical moisture flux anomalies to the region. Despite this, only a small portion of a negative IOD's impact on SEA rainfall comes through blocking events. During austral winter, ENSO is coherent with blocking; anomalous tropical moisture fluxes from the western Pacific during a La Niña merge with anomalous cyclonic flows centered over SEA, delivering enhanced rainfall via cutoff lows. The low pressure cell constitutes a center of the Southern Oscillation associated with ENSO. This ENSO-blocking coherence is considerably weaker in austral spring, whereby circulation anomalies associated with blocking resemble a SAM-like pattern. As such, a large portion of the SAM's impact on SEA spring rainfall occurs in conjunction with blocking events. The relative importance of associations between the dominant climate modes and blocking in generating the drought-breaking cool season precipitation in 2010 across SEA is discussed.

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Wenju Cai
,
Peter van Rensch
,
Tim Cowan
, and
Arnold Sullivan

Abstract

An asymmetry, and its multidecadal variability, in a rainfall teleconnection with the El Niño–Southern Oscillation (ENSO) are described. Further, the breakdown of this relationship since 1980 is offered as a cause for a rainfall reduction in an ENSO-affected region, southeast Queensland (SEQ). There, austral summer rainfall has been declining since around the 1980s, but the associated process is not understood.

It is demonstrated that the rainfall reduction is not simulated by the majority of current climate models forced with anthropogenic forcing factors. Examination shows that ENSO is a rainfall-generating mechanism for the region because of an asymmetry in its impact: the La Niña–rainfall relationship is statistically significant, as SEQ summer rainfall increases with La Niña amplitude; by contrast, the El Niño–induced rainfall reductions do not have a statistically significant relationship with El Niño amplitude. Since 1980, this asymmetry no longer operates, and La Niña events no longer induce a rainfall increase, leading to the observed SEQ rainfall reduction. A similar asymmetric rainfall teleconnection with ENSO Modoki exists and shares the same temporal evolutions.

This breakdown is caused by an eastward shift in the Walker circulation and the convection center near Australia’s east coast, in association with a post-1980 positive phase of the interdecadal Pacific oscillation (IPO). Such a breakdown occurred before 1950, indicating that multidecadal variability alone could potentially be responsible for the recent SEQ rainfall decline. An aggregation of outputs from climate models to distill the impact of climate change suggests that the asymmetry and the breakdown may not be generated by climate change, although most models do not perform well in simulating the ENSO–rainfall teleconnection over the SEQ region.

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Andrew G. Marshall
,
Matthew C. Wheeler
, and
Tim Cowan

Abstract

We assess seasonally varying impacts of the Madden–Julian oscillation (MJO) on Australian maximum and minimum temperature anomalies and extremes, and their modulation by El Niño–Southern Oscillation (ENSO), for the period June 1974–May 2022. Our composite-based approach uses observed temperatures from the Australian Gridded Climate Data, and 850-hPa wind data from the NCEP–NCAR reanalysis, to show how relationships to temperature and circulation evolve over the eight-phase life cycle of the MJO, which we derive from the real-time multivariate MJO index. The MJO has significant impacts on Australian temperatures and winds in all parts of the country at various times throughout the year, and to varying degrees. Two of the most pronounced impacts are 1) daytime warming across southeastern Australia in MJO phase 3 during spring associated with a strong anomalous anticyclone and 2) nighttime cooling over Queensland in MJO phase 7 during winter associated with anomalous advection of cool dry continental air. La Niña acts to significantly lessen both of these impacts, while El Niño enhances both the phase 3 warming over southern Australia in spring and the phase 7 overnight cooling over southern Queensland in winter. We show how the MJO can combine with El Niño and La Niña to have strong compounding influences, thus highlighting the importance of understanding interactions between multiple modes of climate variability and how they relate to Australian temperatures and extremes.

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Tim Cowan
,
Matthew C. Wheeler
, and
Andrew G. Marshall

Abstract

This study first re-examines the impact of the Madden–Julian oscillation (MJO) on weekly rainfall probabilities and wind anomalies across Australia, motivated by the need for a contemporary understanding of the MJO’s influence on Australian rainfall, whether this has changed from a previous assessment published in 2009. With an extra 15 years of observations, we show that the strong impact of MJO phases 5 and 6 on northern Australia’s austral summer rainfall has weakened by around 5% over Australia’s Top End. In addition, austral spring has seen a weakening of the suppressed rainfall teleconnection with MJO phases 2 and 3 over southeast Australia. The weakened relationships make it a little harder to use the MJO to explain rainfall variations over northern Australia in summer and southeast Australia in spring in the current climate. The study’s second motivation is to further document the combined influence of El Niño–Southern Oscillation (ENSO) and the MJO on rainfall. In summer during El Niño, as compared with La Niña or neutral ENSO conditions, there are stronger reductions in rainfall probabilities over northern Australia associated with the dry MJO phases 8, 1, and 2, but the significantly increased rainfall probabilities in MJO phases 5 and 6 remain much the same. Indeed, the MJO dominates over ENSO in its influence on weekly rainfall probabilities in the north in summer. In contrast, ENSO tends to dominate across subtropical and southern Australia in spring. The updated probability maps are an important resource for estimating the intraseasonal influence of the MJO and ENSO on Australian rainfall.

Significance Statement

Accompanying forecasts of multiweek rainfall, the Australian Bureau of Meteorology provide average condition maps showing the long-term relationship between the Madden–Julian oscillation (MJO) and Australian weekly rainfall. Motivated by discussions with northern Australian beef producers, we updated the maps using high-resolution data and found that in the austral summer, the association between the MJO and northern rainfall has weakened in the past 15 years. Despite this, the MJO still dominates over El Niño–Southern Oscillation (ENSO) as a driver of changes in week-to-week rainfall over northern Australia in summer, although ENSO dominates farther south in spring. This study gives users an improved understanding of what to expect in terms of upcoming weekly weather when interpreting rainfall and MJO predictions.

Open access
Tim Cowan
,
Ariaan Purich
,
Sarah Perkins
,
Alexandre Pezza
,
Ghyslaine Boschat
, and
Katherine Sadler

Abstract

Extremes such as summer heat waves and winter warm spells have a significant impact on the climate of Australia, with many regions experiencing an increase in the frequency and duration of these events since the mid-twentieth century. With the availability of Coupled Model Intercomparison Project phase 5 (CMIP5) climate models, projected changes in heat waves and warm spells are investigated across Australia for two future emission scenarios. For the historical period encompassing the late twentieth century (1950–2005) an ensemble mean of 15 models is able to broadly capture the observed spatial distribution in the frequency and duration of summer heat waves, despite overestimating these metrics along coastal regions. The models achieve a better comparison to observations in their simulation of the temperature anomaly of the hottest heat waves. By the end of the twenty-first century, the model ensemble mean projects the largest increase in summer heat wave frequency and duration to occur across northern tropical regions, while projecting an increase of ~3°C in the maximum temperature of the hottest southern Australian heat waves. Model consensus suggests that future winter warm spells will increase in frequency and duration at a greater rate than summer heat waves, and that the hottest events will become increasingly hotter for both seasons by century’s end. Even when referenced to a warming mean state, increases in the temperature of the hottest events are projected for southern Australia. Results also suggest that following a strong mitigation pathway in the future is more effective in reducing the frequency and duration of heat waves and warm spells in the southern regions compared to the northern tropical regions.

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Ge Shi
,
Wenju Cai
,
Tim Cowan
,
Joachim Ribbe
,
Leon Rotstayn
, and
Martin Dix

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.

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Tim Cowan
,
Gabriele C. Hegerl
,
Ioana Colfescu
,
Massimo Bollasina
,
Ariaan Purich
, and
Ghyslaine Boschat

Abstract

Record-breaking summer heat waves were experienced across the contiguous United States during the decade-long “Dust Bowl” drought in the 1930s. Using high-quality daily temperature observations, the Dust Bowl heat wave characteristics are assessed with metrics that describe variations in heat wave activity and intensity. Despite the sparser station coverage in the early record, there is robust evidence for the emergence of exceptional heat waves across the central Great Plains, the most extreme of which were preconditioned by anomalously dry springs. This is consistent with the entire twentieth-century record: summer heat waves over the Great Plains develop on average ~15–20 days earlier after anomalously dry springs, compared to summers following wet springs. Heat waves following dry springs are also significantly longer and hotter, indicative of the importance of land surface feedbacks in heat wave intensification. A distinctive anomalous continental-wide circulation pattern accompanied exceptional heat waves in the Great Plains, including those of the Dust Bowl decade. An anomalous broad surface pressure ridge straddling an upper-level blocking anticyclone over the western United States forced substantial subsidence and adiabatic warming over the Great Plains, and triggered anomalous southward warm advection over southern regions. This prolonged and amplified the heat waves over the central United States, which in turn gradually spread westward following heat wave emergence. The results imply that exceptional heat waves are preconditioned, triggered, and strengthened across the Great Plains through a combination of spring drought, upper-level continental-wide anticyclonic flow, and warm advection from the north.

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