Search Results

You are looking at 21 - 30 of 30 items for

  • Author or Editor: Tim Cowan x
  • Refine by Access: All Content x
Clear All Modify Search
Matt Hawcroft
,
Sally Lavender
,
Dan Copsey
,
Sean Milton
,
José Rodríguez
,
Warren Tennant
,
Stuart Webster
, and
Tim Cowan

Abstract

From late January to early February 2019, a quasi-stationary monsoon depression situated over northeast Australia caused devastating floods. During the first week of February, when the event had its greatest impact in northwest Queensland, record-breaking precipitation accumulations were observed in several locations, accompanied by strong winds, substantial cold maximum temperature anomalies, and related wind chill. In spite of the extreme nature of the event, the monthly rainfall outlook for February issued by Australia’s Bureau of Meteorology on 31 January provided no indication of the event. In this study, we evaluate the dynamics of the event and assess how predictable it was across a suite of ensemble model forecasts using the Met Office numerical weather prediction (NWP) system, focusing on a 1-week lead time. In doing so, we demonstrate the skill of the NWP system in predicting the possibility of such an extreme event occurring. We further evaluate the benefits derived from running the ensemble prediction system at higher resolution than used operationally at the Met Office and with a fully coupled dynamical ocean. We show that the primary forecast errors are generated locally, with key sources of these errors including atmosphere–ocean coupling and a known bias associated with the behavior of the convection scheme around the coast. We note that a relatively low-resolution ensemble approach requires limited computing resources, yet has the capacity in this event to provide useful information to decision-makers with over a week’s notice, beyond the duration of many operational deterministic forecasts.

Open access
Hanh Nguyen
,
Matthew C. Wheeler
,
Jason A. Otkin
,
Thong Nguyen-Huy
, and
Tim Cowan

Abstract

This study describes flash drought (FD) inferred from the evaporative stress index (ESI) over Australia and its relationship to vegetation. During 1975–2020, FD occurrence ranges from less than 1 per decade in the central arid regions to 10 per decade toward the coasts. Although FD can occur in any season, its occurrence is more frequent in summer in the north, winter in the southern interior and southwest, and across a range of months in the far southeast and Tasmania. With a view toward real-time monitoring, FD “declaration” is defined as the date when the ESI declines to at least −1, i.e., drought conditions, after at least 2 weeks of rapid decline. Composite analysis shows that evaporative demand begins to increase about 5–6 weeks before declaration with an increase in solar radiation, while evapotranspiration initially increases with evaporative demand but then decreases in response to the soil moisture depletion. Solar radiation increases simultaneously with precipitation deficit, both reaching their peak around declaration. FD intensity peaks with soil moisture depletion, 2–3 weeks after declaration. The composite wind speed only shows a modest increase around declaration. The composite FD ends 4 weeks after rapid decreases in solar radiation and increases in precipitation. Satellite-derived vegetation health composites show pronounced decline in the nonforested regions, peaking about 4–8 weeks after FD declaration, followed by a recovery period lasting about 12 weeks after flash drought ends. The forest-dominated regions, however, are little impacted. Modeled pasture growth data show reduced values for up to 3 months after the declaration month covering the main agricultural areas of Australia.

Significance Statement

Flash drought describes a fast intensification or rapid development of drought conditions with potential severe impacts on agriculture and ecosystems. This study describes the climatology and typical evolution of flash drought over Australia for the period 1975–2020. An objective definition of flash drought, using high-resolution observational-based datasets, is proposed and its spatiotemporal variability is provided, as well as its relationship with vegetation health and pasture growth. This constitutes a guideline for understanding flash drought in Australia and its impacts on vegetation.

Restricted access
Tim Cowan
,
Matthew C. Wheeler
,
S. Sharmila
,
Sugata Narsey
, and
Catherine de Burgh-Day

Abstract

Rainfall bursts are relatively short-lived events that typically occur over consecutive days, up to a week. Northern Australian industries like sugar farming and beef are highly sensitive to burst activity, yet little is known about the multiweek prediction of bursts. This study evaluates summer (December–March) bursts over northern Australia in observations and multiweek hindcasts from the Bureau of Meteorology’s multiweek to seasonal system, the Australian Community Climate and Earth-System Simulator, Seasonal version 1 (ACCESS-S1). The main objective is to test ACCESS-S1’s skill to confidently predict tropical burst activity, defined as rainfall accumulation exceeding a threshold amount over three days, for the purpose of producing a practical, user-friendly burst forecast product. The ensemble hindcasts, made up of 11 members for the period 1990–2012, display good predictive skill out to lead week 2 in the far northern regions, despite overestimating the total number of summer burst days and the proportion of total summer rainfall from bursts. Coinciding with a predicted strong Madden–Julian oscillation (MJO), the skill in burst event prediction can be extended out to four weeks over the far northern coast in December; however, this improvement is not apparent in other months or over the far northeast, which shows generally better forecast skill with a predicted weak MJO. The ability of ACCESS-S1 to skillfully forecast bursts out to 2–3 weeks suggests the bureau’s recent prototype development of a burst potential forecast product would be of great interest to northern Australia’s livestock and crop producers, who rely on accurate multiweek rainfall forecasts for managing business decisions.

Open access
Ariaan Purich
,
Tim Cowan
,
Wenju Cai
,
Peter van Rensch
,
Petteri Uotila
,
Alexandre Pezza
,
Ghyslaine Boschat
, and
Sarah Perkins

Abstract

Atmospheric and oceanic conditions associated with southern Australian heat waves are examined using phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. Accompanying work analyzing modeled heat wave statistics for Australia finds substantial increases in the frequency, duration, and temperature of heat waves by the end of the twenty-first century. This study assesses the ability of CMIP5 models to simulate the synoptic and oceanic conditions associated with southern Australian heat waves, and examines how the classical atmospheric setup associated with heat waves is projected to change in response to mean-state warming. To achieve this, near-surface temperature, mean sea level pressure, and sea surface temperature (SST) from the historical and high-emission simulations are analyzed. CMIP5 models are found to represent the synoptic setup associated with heat waves well, despite showing greater variation in simulating SST anomalies. The models project a weakening of the pressure couplet associated with future southern Australian heat waves, suggesting that even a non-classical synoptic setup is able to generate more frequent heat waves in a warmer world. A future poleward shift and strengthening of heat wave–inducing anticyclones is confirmed using a tracking scheme applied to model projections. Model consensus implies that while anticyclones associated with the hottest future southern Australian heat waves will be more intense and originate farther poleward, a greater proportion of heat waves occur in association with a weaker synoptic setup that, when combined with warmer mean-state temperatures, gives rise to more future heat waves.

Full access
Hanna Heidemann
,
Joachim Ribbe
,
Tim Cowan
,
Benjamin J. Henley
,
Christa Pudmenzky
,
Roger Stone
, and
David H. Cobon

Abstract

Monsoonal rainfall in northern Australia (AUMR) varies substantially on interannual, decadal, and longer time scales, profoundly impacting natural systems and agricultural communities. Some of this variability arises in response to sea surface temperature (SST) variability in the Indo-Pacific linked to both El Niño–Southern Oscillation (ENSO) and the interdecadal Pacific oscillation (IPO). Here we use observations to investigate unresolved issues regarding the influence of the IPO and ENSO on AUMR. Specifically, we show that during negative IPO phases, central Pacific (CP) El Niño events are associated with below-average rainfall over northeast Australia, an anomalous anticyclonic pattern to the northwest of Australia, and eastward moisture advection toward the date line. In contrast, CP La Niña events (distinct from eastern Pacific La Niña events) during negative IPO phases drive significantly wet conditions over much of northern Australia, a strengthened Walker circulation, and large-scale moisture flux convergence. During positive IPO phases, the impact of CP El Niño and CP La Niña events on AUMR is weaker. The influence of central Pacific SSTs on AUMR has been stronger during the recent (post-1999) negative IPO phase. The extent to which this strengthening is associated with climate change or merely natural internal variability is not known.

Open access
Nora L. S. Fahrenbach
,
Massimo A. Bollasina
,
BjØrn H. Samset
,
Tim Cowan
, and
Annica M. L. Ekman

Abstract

Observations show a significant increase in Australian summer monsoon (AUSM) rainfall since the mid-twentieth century. Yet the drivers of this trend, including the role of anthropogenic aerosols, remain uncertain. We addressed this knowledge gap using historical simulations from a suite of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the CESM2 Large Ensemble, and idealized single-forcing simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results suggest that Asian anthropogenic aerosol emissions played a key role in the observed increase in AUSM rainfall from 1930 to 2014, alongside the influence of internal variability. Sulfate aerosol emissions over Asia led to regional surface cooling and strengthening of the climatological Siberian high over eastern China, which altered the meridional temperature and sea level pressure gradients across the Indian Ocean. This caused an intensification and southward shift of the Australian monsoonal westerlies (and the local Hadley cell) and resulted in a precipitation increase over northern Australia. Conversely, the influence of increased greenhouse gas concentrations on AUSM rainfall was minimal due to the compensation between thermodynamically induced wettening and transient eddy-induced drying trends. At a larger scale, aerosol and greenhouse gas forcing played a key role in the climate response over the Indo-Pacific sector and eastern equatorial Pacific, respectively (coined the “tropical Pacific east–west divide”). These findings contribute to an improved understanding of the drivers of the multidecadal trend in AUSM rainfall and highlight the need to reduce uncertainties in future projections under different aerosol emission trajectories, which is particularly important for northern Australia’s agriculture.

Significance Statement

Australian summer monsoon (AUSM) rainfall plays a vital role in sustaining northern Australia’s unique biodiversity and extensive agricultural industry. While observations show a significant increase in AUSM rainfall since the mid-twentieth century, the causes remain uncertain. We find that anthropogenic aerosol emissions from Asia played a key role in driving this multidecadal AUSM rainfall trend by inducing dynamic adjustments over the Indo-Pacific sector. These findings highlight the need to consider different aerosol emission trajectories when assessing future projections of AUSM rainfall.

Open access
Pandora Hope
,
Mei Zhao
,
S. Abhik
,
Gen Tolhurst
,
Roseanna C. McKay
,
Surendra P. Rauniyar
,
Lynette Bettio
,
Avijeet Ramchurn
,
Eun-Pa Lim
,
Acacia S. Pepler
,
Tim Cowan
, and
Andrew B. Watkins
Open access
Bo Christiansen
,
Carmen Alvarez-Castro
,
Nikolaos Christidis
,
Andrew Ciavarella
,
Ioana Colfescu
,
Tim Cowan
,
Jonathan Eden
,
Mathias Hauser
,
Nils Hempelmann
,
Katharina Klehmet
,
Fraser Lott
,
Cathy Nangini
,
Geert Jan van Oldenborgh
,
René Orth
,
Peter Stott
,
Simon Tett
,
Robert Vautard
,
Laura Wilcox
, and
Pascal Yiou

Abstract

An attribution study has been performed to investigate the degree to which the unusually cold European winter of 2009/10 was modified by anthropogenic climate change. Two different methods have been included for the attribution: one based on large HadGEM3-A ensembles and one based on a statistical surrogate method. Both methods are evaluated by comparing simulated winter temperature means, trends, standard deviations, skewness, return periods, and 5% quantiles with observations. While the surrogate method performs well, HadGEM3-A in general underestimates the trend in winter by a factor of ⅔. It has a mean cold bias dominated by the mountainous regions and also underestimates the cold 5% quantile in many regions of Europe. Both methods show that the probability of experiencing a winter as cold as 2009/10 has been reduced by approximately a factor of 2 because of anthropogenic changes. The method based on HadGEM3-A ensembles gives somewhat larger changes than the surrogate method because of differences in the definition of the unperturbed climate. The results are based on two diagnostics: the coldest day in winter and the largest continuous area with temperatures colder than twice the local standard deviation. The results are not sensitive to the choice of bias correction except in the mountainous regions. Previous results regarding the behavior of the measures of the changed probability have been extended. The counterintuitive behavior for heavy-tailed distributions is found to hold for a range of measures and for events that become more rare in a changed climate.

Open access
Sally L. Lavender
,
Tim Cowan
,
Matthew Hawcroft
,
Matthew C. Wheeler
,
Chelsea Jarvis
,
David Cobon
,
Hanh Nguyen
,
Debra Hudson
,
S. Sharmila
,
Andrew G. Marshall
,
Catherine de Burgh-Day
,
Sean Milton
,
Alison Stirling
,
Oscar Alves
, and
Harry H. Hendon

Abstract

Since 2017, the Northern Australia Climate Program (NACP) has assisted the pastoral grazing industry to better manage drought risk and climate variability. The NACP funding is sourced from the beef cattle industry, government, and academia, representing the program’s broad range of aims and target beneficiaries. The program funds scientists in the United Kingdom and Australia, in addition to extension advisers called “Climate Mates” across a region that supports 15 million head of cattle. Many Climate Mates are employed in the cattle sector and have existing relationships in their communities and capacity to meaningfully engage with the program’s intended beneficiaries—red meat producers. The NACP is a prime example of a successful end-to-end program, integrating climate model improvements (research) with tailored forecast products (development), through to direct stakeholder engagement (extension), on-ground application of technologies (adoption), and improvement in industry and community resilience (impact). The climate information needs of stakeholders also feed back to the research and development components, ensuring the scientific research directly addresses end-user requirements. For any scientific research program, ensuring that research output has measurable real-world impact represents a key challenge. This is more difficult in cases where the scientific research is several steps away from the customer’s needs. This paper gives an overview of the NACP and research highlights, discussing how the end-to-end framework could be adapted and applied in other regions and industries. It seeks to provide a roadmap for other groups to follow to produce more targeted research with identifiable real-world benefits.

Free access
Daniela I. V. Domeisen
,
Christopher J. White
,
Hilla Afargan-Gerstman
,
Ángel G. Muñoz
,
Matthew A. Janiga
,
Frédéric Vitart
,
C. Ole Wulff
,
Salomé Antoine
,
Constantin Ardilouze
,
Lauriane Batté
,
Hannah C. Bloomfield
,
David J. Brayshaw
,
Suzana J. Camargo
,
Andrew Charlton-Pérez
,
Dan Collins
,
Tim Cowan
,
Maria del Mar Chaves
,
Laura Ferranti
,
Rosario Gómez
,
Paula L. M. González
,
Carmen González Romero
,
Johnna M. Infanti
,
Stelios Karozis
,
Hera Kim
,
Erik W. Kolstad
,
Emerson LaJoie
,
Llorenç Lledó
,
Linus Magnusson
,
Piero Malguzzi
,
Andrea Manrique-Suñén
,
Daniele Mastrangelo
,
Stefano Materia
,
Hanoi Medina
,
Lluís Palma
,
Luis E. Pineda
,
Athanasios Sfetsos
,
Seok-Woo Son
,
Albert Soret
,
Sarah Strazzo
, and
Di Tian

Abstract

Extreme weather events have devastating impacts on human health, economic activities, ecosystems, and infrastructure. It is therefore crucial to anticipate extremes and their impacts to allow for preparedness and emergency measures. There is indeed potential for probabilistic subseasonal prediction on time scales of several weeks for many extreme events. Here we provide an overview of subseasonal predictability for case studies of some of the most prominent extreme events across the globe using the ECMWF S2S prediction system: heatwaves, cold spells, heavy precipitation events, and tropical and extratropical cyclones. The considered heatwaves exhibit predictability on time scales of 3–4 weeks, while this time scale is 2–3 weeks for cold spells. Precipitation extremes are the least predictable among the considered case studies. ­Tropical cyclones, on the other hand, can exhibit probabilistic predictability on time scales of up to 3 weeks, which in the presented cases was aided by remote precursors such as the Madden–Julian oscillation. For extratropical cyclones, lead times are found to be shorter. These case studies clearly illustrate the potential for event-dependent advance warnings for a wide range of extreme events. The subseasonal predictability of extreme events demonstrated here allows for an extension of warning horizons, provides advance information to impact modelers, and informs communities and stakeholders affected by the impacts of extreme weather events.

Full access