Siobhan O'Farrell

The ACCESS coupled model: description, control climate and evaluation

4OASIS3.2–5 coupling framework. The primary goal of the ACCESS-CM development is to provide the Australian climate community with a new generation fully coupled climate model for climate research, and to participate in phase five of the Coupled Model Inter-comparison Project (CMIP5). This paper describes the ACCESS-CM framework and components, and presents the control climates from two versions of the ACCESS-CM, ACCESS1.0 and ACCESS1.3, together with some fields from the 20 th century historical experiments, as part of model evaluation. While sharing the same ocean sea-ice model (except different setups for a few parameters), ACCESS1.0 and ACCESS1.3 differ from each other in their atmospheric and land surface components: the former is configured with the UK Met Office HadGEM2 (r1.1) atmospheric physics and the Met Office Surface Exchange Scheme land surface model version 2, and the latter with atmospheric physics similar to the UK Met Office Global Atmosphere 1.0 includ ing modifications performed at CAWCR and the CSIRO Community Atmosphere Biosphere Land Exchange land surface model version 1.8. The global average annual mean surface air temperature across the 500-year preindustrial control integrations show a warming drift of 0.35 °C in ACCESS1.0 and 0.04 °C in ACCESS1.3. The overall skills of ACCESS-CM in simulating a set of key climatic fields both globally and over Australia significantly surpass those from the preceding CSIRO Mk3.5 model delivered to the previous coupled model inter-comparison. However, ACCESS-CM, like other CMIP5 models, has deficiencies in various as pects, and these are also discussed.

Global warming in a coupled climate model including oceanic eddy-induced advection

The Gent and McWilliams (GM) parameterization for large-scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of a global coupled ocean-atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on model behavior under constant atmospheric CO2 and on the model response to increasing CO2. The control (constant CO2) runs show substantial differences in the oceanic stratification and extent of convection, similar to differences found previously using uncoupled ocean models. The transient (increasing CO2) runs show moderate differences in the rate of oceanic heat sequestration (less in the GM case), as expected based on passive tracer uptake studies. However, the surface warming is weaker in the GM case, especially over the Southern Ocean, which is contrary to some recent supposition. Reasons for the reduced warming in the GM case are discussed.

Global comparison of the regional rainfall results of enhanced greenhouse coupled and mixed layer ocean experiments: Implications for climate change scenario development

The extent of agreement amongst current global climate models (GCMs) on the global pattern of rainfall change simulated under enhanced greenhouse conditions is assessed. We consider the results of five experiments which use a simple mixed layer ocean formulation and five which use a fully dynamic ocean model (‘coupled experiments’). For many regions of the northern hemisphere there is strong agreement amongst both mixed layer and coupled experiments on the sign of simulated rainfall change. However, in the southern hemisphere there are large, and apparently systematic, differences between the coupled and mixed layer experiments. In particular, whereas the mixed layer experiments agree on simulated rainfall increase in summer in the tropics and subtropics of the Australian sector, the coupled experiments agree (although more weakly) on rainfall decreases. These differences appear to relate to the much reduced warming simulated by the coupled experiments in the high latitudes of the southern hemisphere. However, recent oceanographie evidence suggests that this suppressed warming may be considerably overestimated. We conclude therefore that despite the in-principle advantages of coupled models, it may be too soon to base some regionally specific climate change scenarios solely on the results of coupled experiments.

Energy and water transport in climates simulated by a general circulation model that includes dynamic sea ice

We analyze energy and water transport in present, doubled CO 2 , and tripled CO 2 climates simulated by the Mark 2 CSIRO nine-level general circulation model with a mixed layer ocean. The model differs from the Mark 1 version by the inclusion of dynamic sea ice, a semi-Lagrangian water vapor transport, and an enhanced land-surface scheme, and it includes prescribed ocean heat transport. We describe a 30-year climatology of the 1xCO 2 simulation, emphasizing the sea ice and the mean meridional energy and water transport. The ice depths, concentrations, and velocities are moderately realistic in both hemispheres. Poleward energy transport is inferred (calculated indirectly from vertical energy fluxes) for both the atmosphere and ocean, although the oceanic flux is much weaker than observational estimates for the southern hemisphere. Atmospheric water transport is also poleward outside the tropics and compares well with observations. Energy transport within the ice layer has been evaluated by both direct and indirect methods. As it is largely due to the latent heat of ice formation, it is closely proportional to the water transport by ice. The meridional transports by ice of both energy and water are relatively important at high latitudes. The divergence of the ice energy transport corresponds to a significant component of the surface energy budget, reaching ±10 W m -2 or more at some polar locations. The equilibrated doubled CO 2 global mean surface warming of the Mark 2 mixed layer model is 4.3°C. The reduction from the Mark 1 result (4.8°C) follows largely from a 40% reduction of the warming over high-latitude oceans. This is attributed to the presence of dynamically induced leads in the ice cover. The equilibrated warming for 3 x CO 2 is 6.8°C. The model atmosphere transports less heat poleward in the doubled CO 2 climate, largely as a response to increased solar radiation absorbed at high latitudes. This behavior contrasts with the change at CO 2 doubling in a transient simulation by the Mark 2 model coupled to a full ocean model, in which heat is taken up in the midlatitudes, particularly by the Southern Ocean, and supplied by a net top-of-atmosphere radiative imbalance distributed over all latitudes (global mean, 1.8 W m -2 ). The atmospheric water transport is enhanced by 10-20% in the warmer climates at most latitudes.

The ACCESS coupled model: documentation of core CMIP5 simulations and initial results

Martin Dix1, Peter Vohralik2, Daohua Bi1, Harun Rashid1, Simon Marsland1, Siobhan O’Farrell1, Petteri Uotila1, Tony Hirst1, Eva Kowalczyk1, Arnold Sullivan1, Hailin Yan1, Charmaine Franklin1, Zhian Sun3, Ian Watterson1, Mark Collier1, Julie Noonan1, Leon Rotstayn1, Lauren Stevens1, Peter Uhe1 and Kamal Puri3 1Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology, CSIRO Marine and Atmospheric Research, Australia 2CSIRO Materials Science and Engineering, Australia 3CAWCR/Bureau of Meteorology, Australia

Evaluation of ACCESS climate model ocean diagnostics in CMIP5 simulations

Global and regional diagnostics are used to evaluate the ocean performance of the Australian Community Climate and Earth System Simulator coupled model (ACCESS-CM) contributions to the Climate Model Intercomparison Project phase 5 (CMIP5). Two versions of ACCESS-CM have been submitted to CMIP; namely CSIRO-BOM ACCESS1.0 and CSIRO-BOM ACCESS1.3. Results from six of the core CMIP5 experiments (piControl, historical, rcp45, rcp85, 1pctCO2, and abrupt4xCO2) are evaluated for each of the two ACCESS-CM model versions. Overall, both model versions exhibit a reasonable and stable representation of key diagnostics of ocean climate performance in the pre-industrial control simulations, including a meridional overturning circulation with North Atlantic Deep Water maxima in the range 22–24 Sv, and a poleward heat transport maximum of around 1.5 PW. For the projected climate change scenarios considered the ACCESS-CM results are in reasonable agreement with responses found in other CMIP models, with the familiar ocean warming, and reduction in strength of meridional overturning and poleward heat transport. Drifts in the control simulations of both global ocean salinity and global sea-level are opposite in sign for ACCESS1.0 and ACCESS1.3, suggesting problems exist in the closure of the hydrological cycle. The simulation of ocean climate change over the historical period shows a weak response compared to observations, which manifests as a late response of ocean warming and sea level rise starting around 1990 in the model, compared to the mid 1960s in observations. Further historical simulations are underway to ascertain if this late response in ACCESS is a robust model feature, or just low frequency variability. If the weak response over the historical period proves robust, the likely cause is a too strong cooling from atmospheric aerosols. Broadening the set of experiments to further investigate the relative warming response of the ACCESS-CM to greenhouse gases compared to the cooling response to aerosols is underway, and preliminary results do suggest that the cooling due to aerosols is strong in the historical simulations.

ACCESS-OM: the Ocean and Sea ice Core of the ACCESS Coupled Model

Daohua Bi1, Simon J. Marsland1, Petteri Uotila1, Siobhan O’Farrell1, Russell Fiedler2, Arnold Sullivan1, Stephen M. Griffies3, Xiaobing Zhou4, and Anthony C. Hirst1 1 CAWCR/CSIRO Marine and Atmospheric Research, Aspendale, Australia 2 CAWCR/CSIRO Marine and Atmospheric Research, Hobart, Australia 3 NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA 4 CAWCR/Bureau of Meteorology, Melbourne, Australia

The sea-ice performance of the Australian climate models participating in the CMIP5

The sea-ice performance of the Australian climate models participating in the CMIP5 experiment, ACCESS1.0, ACCESS1.3 and CSIRO-Mk3.6, is assessed. Comparison with model output from five other international climate modelling centres and observational data are also included in the assessment process. The assessment takes into account modelled climatologies and interannual variability of the sea ice extent, concentration, thickness and transport. The ACCESS models give good simulations of the global sea-ice, within the scatter of models studied, and is one of the top performing models for sea-ice metrics. CSIRO-Mk3.6 has too extensive sea-ice and the sea-ice model would likely have performed significantly better after adjustment of the model parameters. As a consequence, ACCESS and CSIRO-Mk3.6 show opposite hemispheric climate sensitivities in terms of sea ice. The ACCESS models generally capture the observed decline of the Arctic sea ice over the period of 1981–2011, but not the small increase in the Antarctic sea ice, although the simulated changes over this period are generally smaller in the Antarctic than in the Arctic. In the Arctic, the sea-ice reductions in the ACCESS models occur in the Laptev and Kara Seas rather than in the Chukchi and Beaufort Seas as observed.

Use of Passive Tracers as a Diagnostic Tool in Coupled Model Simulations—Northern Hemisphere

Abstract This study focuses on the uptake of a passive idealized tracer in the Northern Hemisphere oceans from two coupled ocean–atmosphere simulations: a standard horizontal diffusion case and the second case including the Gent and McWilliams (GM) eddy mixing parameterization. The results are compared with tracer uptake in stand-alone synchronous and asynchronous ocean simulations for the same cases. The GM set of integrations shows tracer penetration reduced from the standard set in all water mass formation regions. There is a strong similarity in the tracer distributions in the stand-alone ocean simulations in both the standard and GM cases. Changes in the velocity fields between the stand-alone ocean and coupled simulations explain many of the differences in the modeled tracer concentrations. There is a particular focus in the study on the dynamics of modeled water mass formation for North Pacific Intermediate Water, Labrador Sea Water, northeast Atlantic mode water, and North Atlantic Deep Water. The...