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Dowdy A.J.,The Center for Australian Weather and Climate Research
Atmospheric Science Letters | Year: 2014

Tropical cyclone (TC) observations are used to examine changes in the TC climatology of the Australian region. The ability to investigate long-term changes in TC numbers improves when the El Niño-Southern Oscillation (ENSO) is considered. Removing variability in TC numbers associated with ENSO shows a significant decreasing trend in TC numbers at the 93-98% confidence level. Additionally, there is some indication of a temporal change in the relationship between ENSO and TC numbers, with ENSO accounting for about 35-50% of the variance in TC numbers during the first half of the study period, but only 10% during the second half. © 2014 Royal Meteorological Society.

Whan K.,Australian National University | Whan K.,University of New South Wales | Timbal B.,The Center for Australian Weather and Climate Research | Lindesay J.,Australian National University
International Journal of Climatology | Year: 2014

The intensity and position of the sub-tropical ridge (STR) have strong relationships with rainfall variability in southern Australia. The combined effect of intensity and position in March-April-May (MAM) and June-July-August (JJA) is the focus of this research. Linear statistics were used first: area-averaged and Australia-wide spatial correlations of STR intensity and position with precipitation in south-west eastern Australia reveal that STR intensity has a much stronger and more widespread relationship with precipitation in both seasons. Over time, these relationships vary in magnitude and spatial extent with the sign of the correlation changing between two 50-year epochs. These nonlinearities were investigated further using classification trees. Area-averaged precipitation data (terciles) for south-west eastern Australia was classified on the basis of STR intensity and position. In both seasons the classification trees identify STR intensity as the primary partition defining the dry group, supporting the linear analysis. In the transition season of MAM, the time of year when the mean position of the STR is more southerly, STR position is important in distinguishing between a 'winter-like' and a 'summer-like' wet groups, providing STR intensity is low. Vector wind analyses were computed to explain the composite seasonal precipitation anomaly results in terms of different circulation patterns associated with these two wet groups. The frequency of wet and dry cases in each group was examined with changes evident over the recent years. The research confirms that STR intensity is more important than STR position in explaining inter-annual rainfall variability across southern Australia but also demonstrates the additional role of STR position in MAM. These results explain the low correlation between rainfall and STR position and why this relationship has evolved during the 20th century as the mean location of the STR has shifted south in MAM. © 2013 Royal Meteorological Society.

Uslu B.,National Oceanic and Atmospheric Administration | Uslu B.,University of Washington | Power W.,Institute of Geological & Nuclear Sciences | Greenslade D.,The Center for Australian Weather and Climate Research | And 2 more authors.
Pure and Applied Geophysics | Year: 2011

On 15 July 2009, a Mw 7. 8 earthquake occurred off the New Zealand coast, which by serendipitous coincidence occurred while the International Tsunami Symposium was in session in Novosibirsk, Russia. The earthquake generated a tsunami that propagated across the Tasman Sea and was detected in New Zealand, Australia and as far away as the US West coast. Small boats close to the epicenter were placed in jeopardy, but no significant damage was observed despite a measured run-up height of 2. 3 m in one of the Sounds in close proximity to the source (Wilson in GNS Science Report 46:62 2009). Peak-to-trough tsunami heights of 55 cm were measured at Southport, Tasmania and a height of 1 m was measured in Jackson Bay, New Zealand. The International Tsunami Symposium provided an ideal venue for illustration of the value of immediate real-time assessment and provided an opportunity to further validate the real time forecasting capabilities with the scientific community in attendance. A number of agencies with responsibility for tsunami forecast and/or warning, such as the NOAA Center for Tsunami Research, the Pacific Tsunami Warning Center, GNS Science in New Zealand, the Australian Bureau of Meteorology and the European Commission Joint Research Centre were all represented at the meeting and were able to demonstrate the use of state of the art numerical models to assess the tsunami potential and provide warning as appropriate. © 2011 Springer Basel AG.

Asseng S.,University of Florida | McIntosh P.C.,CSIRO | Wang G.,The Center for Australian Weather and Climate Research | Khimashia N.,CSIRO
European Journal of Agronomy | Year: 2012

Achievable grain yields can vary widely between seasons in rain-fed agriculture. Adjusting N fertiliser inputs according to achievable grain yields could reduce over-fertilisation in low-yielding seasons and allow increasing gross margins in potential high-yielding seasons. Seasonal rainfall forecasts from the coupled ocean-atmosphere global circulation model POAMA were skill tested and employed for N fertiliser decision making in the Western Australian wheat-belt. The POAMA seasonal rainfall forecast showed significant skill in forecasting rainfall season types in southern regions of the Western Australian wheat-belt. This skill resulted in about A$50ha -1 of additional benefits when used in N management decisions in wheat cropping. However, such a forecast should not be used without considering other systems knowledge available to farmers. Combining a forecast with systems information such as initial soil water conditions can be crucial in obtaining value from a forecast. Another important factor to consider is the risk behaviour of farmers, where the gross margin from additional fertiliser is expected to exceed the cost by a factor of two or more. Finally, variations in fertiliser cost and wheat prices are critical in determining the benefits from using a forecast system for management decisions in agriculture. Using a forecast for only the wet season-type can further increase a forecast value because the additional gains in wet seasons are often higher than the savings from reduced fertiliser in dry seasons. It is expected that skilful seasonal forecasting systems will become increasingly valuable in regions where rainfall is decreasing because they help to capture benefits in the declining number of potentially high-yielding seasons and minimise the losses in the increasing number of low-yielding seasons. © 2012.

Sandery P.A.,The Center for Australian Weather and Climate Research | O'Kane T.J.,The Center for Australian Weather and Climate Research | O'Kane T.J.,CSIRO
Quarterly Journal of the Royal Meteorological Society | Year: 2014

A coupled ocean-atmosphere dynamical ensemble prediction system is used to study coupled initialization and bred cyclic modes in the case of Tropical Cyclone (TC) Yasi. Ocean initial perturbations are constructed to identify the fastest-growing nonlinear modes in the ocean response to the TC. The ensemble provides a characterization of how initial and evolving dynamical ocean perturbations influence the coupled system through surface fluxes under extreme conditions. Results show how sea-surface temperature perturbations project into atmospheric perturbations of pressure and moisture content within the storm environment. By calculating the local bred vector dimension for ocean-surface velocity, we show that a low-dimensional subspace forms along the track of TC Yasi. The iterative approach to coupled initialization used in this study generates cyclic modes that are embedded on to the dynamics of regions critical to the coupled ocean-atmosphere TC dynamics. The ensemble mean forecasted sea-surface temperature and sea-surface height associated with the ocean response is in better agreement with observations, despite the biases the coupled model inherits from its component models. Both model and observations reveal a twin cold core structure in the ocean wake of TC Yasi. © 2013 Royal Meteorological Society.

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