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Robert J. Allan
and
Malcolm R. Haylock

Abstract

A study of atmospheric and oceanic circulation features in the wider Australian region is undertaken in an attempt to establish the cause(s) of the observed decrease in austral winter (JJA) rainfall over the southwestern portion of Western Australia. Basic regional analyses reveal that long-term mean sea level pressure (MSLP) at Perth, Western Australia, is negatively correlated with southwestern Australian rainfall in JJA over the period 1876–1989. This significant MSLP-rainfall relationship is also observed when using smoothed data series, which resolve a decadal-multidecadal signal embedded within a long-term fluctuation. The latter is punctuated by a downward (upward) rainfall (MSLP) trend over the last 50–60 years that is most pronounced since the mid-1960s.

Such relationships are examined further using Southern Hemisphere gridded MSLP, sea surface temperature (SST), and cloudiness data in the Australian sector during the period 1911–1989. On decadal to multidecadal time frames (MSLP bandpass-filtered in the 7–20-year band), MSLP is out of phase between the Australian continent and the high latitudes of the Southern Ocean. Alternations in the sign of MSLP anomalies over these regions are observed during wet and dry extremes in southwestern Australian JJA rainfall. This is suggestive of changes in the longwave pattern, and thus a propensity for modulation of frontal activity, in the Australian region. Some coherent variations are also seen in SST and cloudiness fields, with perhaps an indication that cloud patterns are more organized across the southwest in wet winters.

The long-term MSLP analyses (low-pass-filtered MSLP to remove frequencies less than 25 years) indicate the influence of a different forcing on the regional climate system. The dominant pattern that emerges is of MSLP anomalies that are out of phase between the south-southwest of Australia and to the southeast of New Zealand. Since 1911, the above configuration of anomalies across southern latitudes has evolved slowly from a negative/positive to a positive/negative alignment. During this period, simultaneous correlations between Darwin MSLP and southwestern Australian JJA rainfall have also changed, from coherent and significant to insignificant. This suggests that low-frequency fluctuations in the El Niño-Southern Oscillation (ENSO) phenomenon may have played a major role in this process.

It would appear that southwestern Australian JJA rainfall patterns are modulated by a long-term MSLP signal with a pronounced trend in recent decades that is punctuated by decadal-multidecadal MSLP pulses. These MSLP-rainfall relationships are associated with circulation fluctuations in Australian longitudes that may be linked to low-frequency characteristics of the ENSO phenomenon. However, wider Southern Hemisphere data have not yet been analyzed to test this hypothesis further. Interestingly, MSLP fields resolved in this study bear little resemblance to 2×CO2 MSLP simulations of enhanced greenhouse conditions in any of the low- or high-resolution general circulation model (GCM) intercomparisons over the Australian region.

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Robert J. Allan
,
Janette A. Lindesay
, and
Chris J. C. Reason

Abstract

Several independent historical studies of global atmospheric and oceanic parameters have identified low-frequency fluctuations in the global climate system. Much of this research has focused on Europe, the Atlantic Ocean, and North America. However, recent interest has begun to encompass decadal to multidecadal variability across the Indo-Pacific region. Such variability has been detected in sea surface temperature (SST), mean sea level pressure (MSLP), and surface wind fields over both the landmasses and the oceans.

Around the Indian Ocean basin, the broad periods before and after the 1940s show important differences in features such as Indian southwest monsoonal rainfall and circulation patterns, relationships between austral summer rainfall in southern Africa and the El Ni˜o–Southern Oscillation phenomenon, and Australasian MSLP. Very little is known about this variability, particularly during the austral summer. In an effort to isolate such fluctuations and work toward understanding the physical dynamics operating on such timescales, SST, MSLP, atmospheric circulation, vertical motion, and cloudiness anomalies are constructed and analyzed for austral summer (JFM) conditions over the Indian Ocean region during four 21-yr epochs since 1900.

The results of this research suggest that SSTs were cooler at midlatitudes and warmer in the subtropical southern Indian Ocean in the periods 1900–20 and 1921–41, compared with the 1942–62 and 1963–83 epochs. The most pronounced changes are found along the Agulhas Current outflow zone across the midlatitudes of the southwest Indian Ocean, with indications of coherent SST fluctuations in the northwest regions of the basin and in the northwest Pacific Ocean. Changes in surface wind anomalies are also observed. During 1900–20 and 1921–41, an anomalous atmospheric cyclonic feature is seen over the southern Indian Ocean, while in the later 1963–83 period, a distinct anticyclonic anomaly is evident in this region. This change suggests that the semi-permanent anticyclone in the mean flow field of the atmosphere over the southern Indian Ocean in JFM was weaker in the first 42 yr of this century. Concurrent variations are found in the trade wind regime over the western equatorial Pacific. Velocity potential field anomalin derived from the surface winds, show a strengthening of tropical-subtropical convergence over time. These observations, together with those of cloudiness and MSLP and a brief examination of near-global MSLP correlations and SST data back to 1879, point to a consistent fluctuation in ocean–atmosphere forcings during this century. Independent ocean general circulation model simulations involving modulations to global wind stresses or the Indonesian throughflow suggest that the sub-tropical/midlatitude southern Indian Ocean, and particularly the Agulhas outflow zone, is sensitive to low-frequency changes in wind and/or thermohaline forcing. Such long-term fluctuations in the mean state of the climate system may have ramifications for interannual variability and features such as the El Niño–Southem Oscillation phenomenon.

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John A. Church
,
Neil J. White
,
Allan J. Clarke
,
Howard J. Freeland
, and
Robert L. Smith

Abstract

The Australian Coastal Experiment (ACE) was designed to test coastal-trapped wave (CTW) theory and the generation of coastal-trapped waves by the wind. For the ACE dataset, we use CTW theory to attempt to hindcast the observed alogshelf currents and coastal sea levels at locations remote from the upstream (in the CTW sense) boundary of the ACE region. Local (in the ACE region) wind forcing is responsible for only about a quarter of the CTW energy flux at Stanwell Park (the center of the ACE region), and the remainder enters the ACE region from the south and propagates northward through the ACE region. Including the second-mode CTW improves the correlation between the hindcast and the observed near-bottom currents on the upper slope at Stanwell Park, but the use of the third-mode CTW cannot be justified. A linear bottom drag coefficient of r = 2.5 × 10−4 m s−1 works better than a larger drag coefficient, and simplifying the CTW equations by assuming the modes are uncoupled does not detract from the quality of the hindcasts. The hindcast and observed coastal sea levels are correlated at greater 2 than the 99% significance level. For the nearshore locations at Stanwell Park, the hindcast and observed alongshelf currents are correlated at greater than the 99% significance level, and the CTW model can account for about 40% of the observed variance. On the shelf at Stanwell Park, we find the hindcasts agree with the observations only if direct wind forcing within the ACE region and the correct (nonzero) upstream boundary conditions are included. However, even after attempting to remove the effects of the eddies and the East Australian Current, the CTW model is not useful for predicting the currents on the slope at Stanwell Park and on the shelf and slope at Newcastle (the northern boundary of the ACE region). The currents at these locations are dominated by the effect of the East Australian Current and its eddies.

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John A. Church
,
Allan J. Clarke
,
Neil J. White
,
Howard J. Freeland
, and
Robert L. Smith

Abstract

No abstract available.

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Richard J. Murnane
,
Michael Crowe
,
Allan Eustis
,
Susan Howard
,
Judy Koepsell
,
Robert Leffler
, and
Robert Livezey

The National Oceanic and Atmospheric Administration (NOAA), the Risk Prediction Initiative, and the Weather Risk Management Association jointly sponsored a workshop that examined a variety of issues related to climate forecast and data products produced by NOAA. Workshop participants included individuals from a variety of private sector firms active in the weather risk market. The workshop provided an opportunity for a direct and technical exchange with NOAA service providers regarding climate data and forecast products and their scientific basis, interpretation, performance, planned improvements, access, and use. In addition, the workshop examined, in light of current NOAA services, the weather risk industry's requirements for climate data and forecasts. Discussions from the workshop are summarized, and a list of actions developed at the workshop that will enhance for all users the use, accessibility, and value of NOAA climate data and forecast products is provided.

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Robert J. Allan
,
Karen Beck
, and
William M. Mitchell

Abstract

A preliminary study is made of simultaneous and lagged seasonal cross correlations between northern Australian sea level and district rainfall anomalies. Zero-lagged patterns show strong links with the El Niño/Southern Oscillation (ENSO) phenomenon over northern and eastern Australia during austral winter-spring seasons. These findings are similar to, but more robust than, those reported for simultaneous cross correlations between Australian district rainfall and either the southern oscillation index, Darwin mean sea level pressure, or northern Australian sea surface temperatures. This appears to be a consequence of the strong integrating response of northern Australian sea levels, particularly to inverse barometric and wind forcings on seasonal time scales.

Such relationships are explored further in lagged cross correlations. As observed in other studies of seasonal persistence associated with ENSO and anti-ENSO phases, rainfall precursors extend in time from one to two seasons. However. significant seasonal cross correlations are found at longer intervals, reflecting the marked sea level response to the often observed anti-ENSO to ENSO sequence of the phenomenon. In particular, austral spring sea levels in northern Australia are negatively correlated with the following austral winter rainfall over parts of northern and south-southeastern Australia. At present an operational evaluation of precursos must await the installation of new telemetering tide gauges in northern Australian ports.

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Robert J. Allan
,
Neville Nicholls
,
Phil D. Jones
, and
Ian J. Butterworth

Abstract

An extension of the Tahiti minus Darwin Southern Oscillation Index (SOI) from 1882 back to 1876 is reported following the recovery of early Darwin mean sea-level pressure data spanning the period 1865–81. As a result, we are able to compare, for the first time, the major 1877–78 and 1982–83 ENSO events on the basis of this commonly used index. Early Darwin and Jakarta data are also examined in terms of a measure of the Australian response to documented El Niño and/or ENSO events in 1866, 1868, 1871, 1873, 1874 and 1875.

The SOI during the 1877–78 ENSO event has a similar temporal response to that in 1982–83, but the index is slightly weaker than in the recent event. Examination of documentary evidence confirms the severity of the drought conditions that affected the Australian continent during the 1877–78 ENSO, and shows that this response is in line with the wider Indo-Pacific impacts reported in the literature. Earlier El Niño phases in 1868 and 1873 are not resolved distinctly in either the Darwin or Jakarta pressure data. This appears to illustrate that El Niño event histories do not always indicate wider ENSO influences in the Indo-Pacific basin, particularly during weak to moderate phases.

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Ranjini Swaminathan
,
Robert J. Parker
,
Colin G. Jones
,
Richard P. Allan
,
Tristan Quaife
,
Douglas I. Kelley
,
Lee de Mora
, and
Jeremy Walton

Abstract

A key goal of the 2015 Paris Climate Agreement is to keep global mean temperature change at 2°C and if possible under 1.5°C by the end of the century. To investigate the likelihood of achieving this target, we calculate the year of exceedance of a given global warming threshold (GWT) temperature across 32 CMIP6 models for Shared Socioeconomic Pathway (SSP) and radiative forcing combinations included in the Tier 1 ScenarioMIP simulations. Threshold exceedance year calculations reveal that a majority of CMIP6 models project warming beyond 2°C by the end of the century under every scenario or pathway apart from the lowest emission scenarios considered, SSP1–1.9 and SSP1–2.6, which is largely a function of the ScenarioMIP experiment design. The U.K. Earth System Model (UKESM1) ScenarioMIP projections are analyzed in detail to assess the regional and seasonal variations in climate at different warming levels. The warming signal emerging by midcentury is identified as significant and distinct from internal climate variability in all scenarios considered and includes warming summers in the Mediterranean, drying in the Amazon, and heavier Indian monsoons. Arctic sea ice depletion results in prominent amplification of warming and tropical warming patterns emerge that are distinct from interannual variability. Climate changes projected for a 2°C warmer world are in almost all cases exacerbated with further global warming (e.g., to a 4°C warmer world).

Open access
Reinder A. Feddes
,
Holger Hoff
,
Michael Bruen
,
Todd Dawson
,
Patricia de Rosnay
,
Paul Dirmeyer
,
Robert B. Jackson
,
Pavel Kabat
,
Axel Kleidon
,
Allan Lilly
, and
Andrew J. Pitman

From 30 September to 2 October 1999 a workshop was held in Gif-sur-Yvette, France, with the central objective to develop a research strategy for the next 3–5 years, aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data.

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Sandrine Bony
,
Robert Colman
,
Vladimir M. Kattsov
,
Richard P. Allan
,
Christopher S. Bretherton
,
Jean-Louis Dufresne
,
Alex Hall
,
Stephane Hallegatte
,
Marika M. Holland
,
William Ingram
,
David A. Randall
,
Brian J. Soden
,
George Tselioudis
, and
Mark J. Webb

Abstract

Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs’ range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.

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