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Klocker A.,University of Tasmania | Klocker A.,Massachusetts Institute of Technology | McDougall T.J.,Center for Australian Climate and Weather Research
Journal of Physical Oceanography | Year: 2010

Recent work on the global overturning circulation and its energetics assumes that processes caused by nonlinearities of the equation of state of seawater are negligible. Nonlinear processes such as cabbeling and thermobaricity cause diapycnal motion as a consequence of isopycnal mixing. The nonlinear equation of state also causes the helical nature of neutral trajectories; as a consequence of this helical nature, it is not possible to define a continuous "density" surface that aligns with neutral tangent planes. The result is an additional diapycnal advection, which needs to be accounted for in water-mass analysis. In this paper, the authors take advantage of new techniques in constructing very accurate continuous density surfaces to more precisely estimate isopycnal and diapycnal processes caused by the nonlinear equation of state. They then quantify the diapycnal advection due to each of these nonlinear processes and show that they lead in total to a significant downward diapycnal advection, particularly in the Southern Ocean. The nonlinear processes are therefore another source of dense water formation in addition to high-latitude convection. To maintain the abyssal stratification in the global ocean, these dense water masses have to be brought back toward surface layers, and this can occur by either diabatic or adiabatic processes. Including these nonlinear processes into the advection-diffusion balance, the authors show that observed diapycnal diffusivities are unlikely to be able to support the amount of dense water produced in the global ocean, thus placing more importance on the adiabatic way of bringing the deep waters back to the surface. © 2010 American Meteorological Society.

Bari M.A.,Extended Hydrological Prediction | Amirthanathan G.E.,Extended Hydrological Prediction | Timbal B.,Center for Australian Climate and Weather Research
Modelling for Environment's Sake: Proceedings of the 5th Biennial Conference of the International Environmental Modelling and Software Society, iEMSs 2010 | Year: 2010

The population of the Perth-Bunbury region in Western Australia is predicted to increase to 3.1 million by 2050. Water supply is a key issue, as below-average rainfall since the mid-1970s has led to about 40% decline in the streamflow. General Circulation Models (GCMs) project a further decrease in rainfall leading to diminished water resources in the future and posing a threat to water supply and the environment. In this experimental study we assess the impact of climate change at Serpentine Reservoir using data from eleven GCMs which contributed to the latest Intergovernmental Panel on Climate Change (IPCC) assessment report. Data from two emission scenarios (A2 and B1) were used and downscaled, using a state-of-the-art statistical downscaling model, to a 5 km resolution compatible with catchment modelling. The LUCICAT rainfall-runoff model was calibrated for the Serpentine catchment and then changes in runoff were projected using the downscaled rainfall data. Land use and potential evaporation were not changed for the future rainfall-runoff modelling. Nearly all GCMs projected reductions in rainfall by mid (2046-2065) and late (2081-2100) 21st century compared to 1981-2000 period. There was a significant variation in projected rainfall reductions between different GCMs and emission scenarios. Under the A2 climate scenario, there could be a further 14-24% reduction in rainfall, and this would result in a 49-69% reduction in reservoir inflow by the mid to end of the 21st century. Rainfall reduction under B1 scenario would be around 12% and corresponded to streamflow reduction of about 45-46%.

Soldatenko S.,Center for Australian Climate and Weather Research | Steinle P.,Center for Australian Climate and Weather Research | Tingwell C.,Center for Australian Climate and Weather Research | Chichkine D.,University of Waterloo
Advances in Meteorology | Year: 2015

Variational data assimilation (VDA) remains one of the key issues arising in many fields of geosciences including the numerical weather prediction. While the theory of VDA is well established, there are a number of issues with practical implementation that require additional consideration and study. However, the exploration of VDA requires considerable computational resources. For simple enough low-order models, the computational cost is minor and therefore models of this class are used as simple test instruments to emulate more complex systems. In this paper, the sensitivity with respect to variations in the parameters of one of the main components of VDA, the nonlinear forecasting model, is considered. For chaotic atmospheric dynamics, conventional methods of sensitivity analysis provide uninformative results since the envelopes of sensitivity functions grow with time and sensitivity functions themselves demonstrate the oscillating behaviour. The use of sensitivity analysis method, developed on the basis of the theory of shadowing pseudoorbits in dynamical systems, allows us to calculate sensitivity functions correctly. Sensitivity estimates for a simple coupled dynamical system are calculated and presented in the paper. To estimate the influence of model parameter uncertainties on the forecast, the relative error in the energy norm is applied. © 2015 Sergei Soldatenko et al.

O'Kane T.J.,CSIRO | O'Kane T.J.,Center for Australian Climate and Weather Research | Matear R.J.,CSIRO | Matear R.J.,Center for Australian Climate and Weather Research | And 4 more authors.
Journal of Geophysical Research C: Oceans | Year: 2014

Ocean storm tracks have previously been associated with the midlatitude western boundary currents (WBCs) and the Antarctic Circumpolar Current (ACC). Here we identify and examine large-scale baroclinically unstable waves occurring within waveguides associated with potential density gradients in the subtropical regions of the Southern Hemisphere (SH) oceans where the trade winds and westerlies meet and at depths associated with mode water formation. In contrast to the Northern Hemisphere subtropics, the SH pathways are more extensive allowing large-scale coherent disturbances to communicate information westward from the midlatitudes to the subtropics (South Pacific Ocean) and from the subtropics to the tropics (Indian Ocean). Particular consideration is given to the subtropical South Pacific Ocean as this is a region where resonant interactions between large-scale Rossby waves and significant topographic features have been reported to occur. Using an ocean general circulation model and a simple potential energy transfer diagnostic, we identify the relevant nonlinearly modified structures comparing their propagation characteristics to planetary Rossby waves calculated using a shallow water model. Although at first appearance baroclinic disturbances resemble planetary Rossby waves, we show they are inherently nonlinear, multiscale and are amplified where topography occurs. The location of the disturbances coincides with regions of high variability in sea surface height observed in satellite altimetry and their speeds closely match the large-scale coherent westward propagating structures described in the observational literature. Our study provides evidence that, in addition to the midlatitude WBCs and the ACC, significant ocean storm tracks are also manifest in the SH subtropics. Key Points Characterization of global oceanic large-scale baroclinic instabilities Comparison of wave-like baroclinic disturbances and planetary Rossby waves Identification of decadal modes of variability in the subtropical oceans © 2014. American Geophysical Union. All Rights Reserved.

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