Search Results

You are looking at 11 - 20 of 23 items for

  • Author or Editor: Neville Nicholls x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Neville Nicholls

No Abstract available.

Full access
Neville Nicholls

Abstract

The absence of an upward trend in normalized building damage in Australian bushfires may reflect reduced vulnerability (due to improved weather forecasts and other factors) offsetting increases in the frequency or intensity of bushfires.

Full access
Neville Nicholls
and
Alex Kariko

Abstract

The number, average length, and average intensity of rain events at five stations located in eastern Australia have been calculated for each year from 1910 to 1988, using daily rainfall totals. A rain event has been defined as a period of consecutive days on which rainfall has been recorded on each day. Inter-relationships between the rain-event variables (at each station and between stations), along with their relationships with annual rainfall and the El Niño-Southern Oscillation, have been investigated. Trends in the time series of the rain-event variables have also been examined. Annual rainfall variations are found to be primarily caused by variations in intensity. Fluctuations in the three rain-event variables are essentially independent of each other. This is due, in some cases, to inter-relationships at interdecadal time scales offsetting relationships of the opposite sense at shorter time scales. The large-scale geographical nature of east Australian rainfall fluctuations mainly reflects interstation correlations in the number of events. The El Niño-Southern Oscillation affects rainfall mainly by influencing the number and intensity of rain events. Twentieth century increases in east Australian rainfall have been due, primarily, to increased numbers of events. Intensity of rain events has generally declined, offsetting some of the increase in rainfall expected from more frequent events.

Full access
Andrew Solow
and
Neville Nicholls

Abstract

A statistical model of the relationship between tropical cyclone frequency in the Australian region and an index of the strength and phase of the southern oscillation is developed for the period 1910–88. The modeling is nonstandard because the cyclone record is incomplete early in this period. The fitted model indicates that the mean annual number of cyclones during a major cold event is twice that during a major warm event.

Full access
Roger Stone
,
Graeme Hammer
, and
Neville Nicholls

Abstract

A forecast method capable of estimating date of last frost and number of frosts per season in northeastern Australia some months in advance is described. Forecast “skill” is achieved using either Southern Oscillation index (SOI) patterns (phases) during the previous austral autumn or a linear discriminant approach and the SOI. When applying these systems, it is possible to provide significantly different probability distributions of day of last frost and number of frosts, depending on the SOI patterns observed during the previous season. An analysis of the time series of frost frequency and date of last frost suggests an apparent warming trend in the data, resulting in a trend toward earlier dates of last frost and fewer numbers of frosts at many of the locations analyzed. The beneficial implications of the proposed frost forecasting system for enterprises such as winter agriculture in the region are believed to be significant.

Full access
Scott Power
,
Brian Sadler
, and
Neville Nicholls

Water flow into dams that supply Perth in Western Australia (WA) has fallen by 50% since the mid-1970s, and this has severely tested water managers. Climate change scenarios available since the 1980s have suggested that global warming will reduce rainfall over southern Australia, including Perth. Water managers recognize the uncertainties associated with the projections, including the significant differences that exist between the timing and magnitude of the observed changes and modeled projections. The information has, nevertheless, influenced their decision making.

To understand why, we need to consider the broader environment in which the water managers operate. One key factor is that the imposition of severe water restrictions can lead to significant economic loss and increased unemployment. Prolonged restrictions can therefore create strong debate in the wider community. In recognition of this, state government policy requires that water managers ensure that the chance of having severe restrictions is kept low. Severe restrictions have not been imposed since 1979, but moderate restrictions are more common, and were imposed as recently as 2002. Scrutiny of water management can become intense even after moderate restrictions are imposed, and at these times it is unacceptable to many people if a known risk—even if very uncertain—is perceived to have been ignored in earlier planning. Climate science has established regional drying driven by global warming as a risk, and so global warming has to be addressed in planning. Water managers also need climate science to reassure the public that the restrictions imposed were necessary because of unprecedented changes in rainfall, not because of poor management.

In recent years much of the influence that climate science has had on water managers can be attributed to the Indian Ocean Climate Initiative (IOCI). IOCI is a research partnership between the Western Australia Water Corporation, other state government agencies, and two national meteorological research organizations. Water managers saw their participation in IOCI as one strand of a broader risk management plan. They did not have the luxury of deferring important decision making for certainty that climate science might never bring, but were very interested in what climate science might provide “now.”

The participation of water managers in IOCI enabled them to influence research planning to better meet their needs. Water managers did not just want predictions or technical explanations of an individual scientist's latest work. They wanted reliable and balanced advice on broader issues, explanations, clarification, realistic expectations, and an appreciation of uncertainty. They wanted climate information related to water management issues in a form relevant to the region. “Localized” information is more suitable for inclusion in their decision making, and of more use to them for both informing, and stimulating discussion within, the wider community.

Full access
Jennifer L. Catto
,
Neville Nicholls
, and
Christian Jakob

Abstract

Interannual variations in the sea surface temperature (SST) to the north of Australia are strongly linked to variations in Australian climate, including winter rainfall and tropical cyclone numbers. The north Australian SSTs are also closely linked to ENSO and tropical Pacific SSTs, with the relationship exhibiting a strong seasonal cycle. Credible predictions of Australian climate change therefore depend on climate models being able to represent ENSO and its connection to north Australian SSTs, the topic of this study.

First, the observational datasets of the Met Office Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) and the NOAA Extended Reconstructed Sea Surface Temperature (ERSST) are used to document the links between the Niño-3.4 index and a north Australian SST index, and the temporal evolution of north Australian SSTs during ENSO events. During austral autumn, the correlation between Niño-3.4 SST and north Australian SST is positive, while in austral spring it is strongly negative. During El Niño events, the north Australian SST anomalies become negative in the austral spring preceding the development of the positive Niño-3.4 SST anomalies.

The coupled models participating in the Coupled Model Intercomparison Project phase 3 (CMIP3) are evaluated in terms of this temporal evolution of Niño-3.4 SST and the relationship to north Australian SST for the twentieth-century simulations. Some of the models perform very well, while some do not capture the seasonal cycle of correlations at all. The way in which these relationships may change in the future is examined using the A2 emissions scenario in those models that do a reasonable job of capturing the present-day observed relationship, and very little change is found.

Full access
Jennifer L. Catto
,
Neville Nicholls
, and
Christian Jakob

Abstract

Aspects of the climate of Australia are linked to interannual variability of the sea surface temperatures (SSTs) to the north of the country. SST anomalies in this region have been shown to exhibit strong, seasonally varying links to ENSO and tropical Pacific SSTs.

Previously, the models participating in phase 3 of the Coupled Model Intercomparison Project (CMIP3) have been evaluated and found to vary in their abilities to represent both the seasonal cycle of correlations between the Niño-3.4 and north Australian SSTs and the evolution of SSTs during composite El Niño and La Niña events. In this study, the new suite of models participating in the CMIP5 is evaluated using the same method. In the multimodel mean, the representation of the links is slightly improved, but generally the models do not capture the strength of the negative correlations during the second half of the year. The models also still struggle to capture the SST evolution in the north Australian region during El Niño and La Niña events.

Full access
Lisa V. Alexander
,
Petteri Uotila
,
Neville Nicholls
, and
Amanda Lynch

Abstract

A high-quality daily dataset of in situ mean sea level pressure was collated for Australia for the period from 1907 to 2006. This dataset was used to assess changes in daily synoptic pressure patterns over Australia in winter using the method of self-organizing maps (SOMs). Twenty patterns derived from the in situ pressure observations were mapped to patterns derived from ERA-40 data to create daily synoptic pressure fields for the past century. Changes in the frequencies of these patterns were analyzed. The patterns that have been decreasing in frequency were generally those most strongly linked to variations in the southern annular mode (SAM) index, while patterns that have increased in frequency were more strongly correlated with variations in the positive phase of El Niño–Southern Oscillation. In general, there has been a reduction in the rain-bearing systems affecting southern Australia since the beginning of the twentieth century. Over the past century, reductions in the frequencies of synoptic patterns with a marked trough to the south of the country were shown to be linked to significant reductions in severe storms in southeast Australia and decreases in rainfall at four major Australian cities: Sydney, Melbourne, Adelaide, and Perth. Of these, Perth showed the most sustained decline in both the mean and extremes of rainfall linked to changes in the large-scale weather systems affecting Australia over the past century. The results suggest a century-long decline in the frequency of low pressure systems reaching southern Australia, consistent with the southward movement of Southern Hemisphere storm tracks. While most of these trends were not significant, associated changes in rainfall and storminess appear to have had significant impacts in the region.

Full access
John L. McBride
,
Malcolm R. Haylock
, and
Neville Nicholls

Abstract

Various earlier studies have demonstrated that rainfall in the Maritime Continent–Indonesia region is strongly related to the El Niño–Southern Oscillation (ENSO) during the dry half of the year but has a very weak association with ENSO during the summer–wet season months. This relationship is investigated over a wider domain through the use of outgoing longwave radiation (OLR) data as a proxy for rainfall.

Consistent with the hypothesis of Haylock and McBride, it is found that the large-scale structure of the low-order empirical orthogonal functions (EOFs) of OLR have a strong resemblance to the patterns of correlation between OLR and the Southern Oscillation index (SOI). This supports the hypothesis that the predictable component of rainfall is determined by the component that is spatially coherent, as quantified through EOF analysis.

As was found earlier with rainfall, the region of largest correlation between interannual OLR anomalies and the SOI lies in the winter hemisphere. The predictable component of OLR (or rainfall) remains in the region of the Maritime Continent throughout the year and thus does not accompany the minimum OLR (maximum rainfall) during its annual interhemispheric progression as the major monsoon heat source.

The sign of the OLR–SOI relationship is such that the Maritime Continent has increased rainfall during a La Niña or cold event. Patterns of correlation between sea surface temperature and the SOI show the existence of a region to the east of the Maritime Continent whereby sea surface temperature anomalies are positive during these (La Niña) conditions. This is in the sense of a direct relationship, that is, positive sea surface temperature anomalies corresponding to increased rainfall.

The annual cycle of the sea surface temperature structure of ENSO is represented by the first EOF of the interannual sea surface temperature series for each separate calendar month. The region of the sea surface temperature anomaly giving the direct relationship with Maritime Continent rainfall is part of the “boomerang-shaped” pattern that lies between and has the opposite sign from the anomalies in the eastern-central Pacific and the Indian Oceans. Besides being a fundamental component of the large-scale sea surface temperature structure of ENSO, the boomerang pattern goes through an annual cycle such that it has maximum amplitude in the winter hemisphere. This suggests that interannual variations of Maritime Continent rainfall are in direct response to upstream sea surface temperature anomalies in the ENSO boomerang pattern.

Full access