Ian Watterson

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.

Tropical cyclone frequencies inferred from Gray's yearly genesis parameter: Validation of GCM tropical climates

The Gray Yearly Genesis Parameter (YGP) is an empirical diagnostic tool used to infer regions in which necessary (but not sufficient) conditions exist for tropical cyclone development. This parameter is used here as a measure of the implied tropical cyclone frequency and area of occurrence in climate simulations generated by a General Circulation Model (GCM). In a simulation of the current climate, the CSIRO9 GCM shows reasonable agreement with the original Gray climatology. The YGP for a doubled CO2 simulation is presented.

Objective Assessment of Extratropical Weather Systems in Simulated Climates

An automated weather system identification and tracking scheme is used to appraise the skill of the CSIRO9 GCM in replicating contemporary extratropical cyclone and anticyclone behavior, and to assess possible changes as a result of doubled CO2. Cyclones are identified as centers of cyclonic vorticity rather than pressure minima, which can vanish if the background pressure gradient increases. Comparison with an observational dataset from ECMWF revealed that the GCM control simulation realistically reproduced the present-day storm track locations, but with slightly fewer and generally weaker systems overall. These errors are consistent with the coarser resolution of the GCM and its underestimation of the strength and baroclinicity of the polar vortex in both hemispheres. Comparison between 1 and 2 3 CO2 GCM simulations revealed increases in both 500-hPa geopotential height and 1000‐500-hPa thickness for doubled CO2. As in other studies, these changes are largest near the poles, resulting in weaker westerlies and reduced tropospheric baroclinicity. Decreases of 10%‐15% in both cyclone and anticyclone activity consistent with these circulation changes are found. However, there is some evidence of increased winter cyclone activity near the downstream end of the principal storm tracks. There is also a general reduction in the number and strength of intense storms, despite generally lower central pressures, which arise from global-scale decreases in sea level pressure in the doubled CO 2 atmosphere rather than from greater storm vigor. This underscores the need for GCM projections of midlatitude ‘‘storminess’’ to employ more realistic measures of storm activity and intensity.

Modes of Interannual Variability of the Southern Hemisphere Circulation Simulated by the CSIRO Climate Model

This study examines the capability of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) climate model in simulating the observed modes of interannual variability of the Southern Hemisphere circulation. Modes of variability in the 500-hPa geopotential height (Z500) field of the following three experiments are examined: 1) a coupled experiment, in which the atmosphere and the ocean are fully coupled, producing El Nino-Southern Oscillation (ENSO) cycles and allowing full air-sea interactions; 2) a mixed layer experiment, in which the atmosphere is coupled to an ocean mixed layer heat equation allowing limited air-sea interactions; and 3) a climatology experiment, in which the atmosphere is forced by an observed SST climatology with a fixed annual cycle, allowing no air-sea interactions. It is found that the observed modes are reasonably simulated in all three experiments, although the amplitude of the model modes is generally smaller than that of the observed. These modes include the high-latitude mode (i.e., the Antarctic Oscillation), the Pacific-South American (PSA) mode, and the wavenumber-3 mode. It is also found that the response of the mid- to high-latitude atmosphere circulation to the model ENSO forcing projects mainly onto the PSA mode. Many features of the PSA mode are similar to those associated with the Pacific-North American mode in the Northern Hemisphere. In response to these Z500 modes, the ocean produces coherent modes of variability, but the oceanic feedback effect appears to be weak. The amplitude of anomalies associated with each mode of the Z500 field in the three experiments shows little difference, suggesting that these Z500 modes can be generated by atmospheric internal dynamics alone, and that the ocean dynamics, air-sea interactions, and ENSO forcing are not essential.

Seasonal and Interannual Variability of Tropical Cyclogenesis: Diagnostics from Large-Scale Fields

Abstract Grays seasonal genesis parameter (SGP) is reassessed as a diagnostic quantity for both climatological and single-season tropical cyclogenesis. The SGP applied to global analyses from recent years is able to locate the regions of genesis activity during 1967–86. The SGP based on the climatology of a simulation by the CSIR09 atmospheric model using prescribed ocean temperatures for 1979–88 has similar skill. The SGP applied to single-season means is then assessed as a diagnostic for interannual variation of cyclogenesis. Increased cyclogenesis in the central Pacific during the 1982/83 El Nino coincides with increased SGP. CSIRO9 simulated similar variations in the SGP. Moderate correlations are found between the time series of the observed and inferred simulated cyclogenesis numbers in the central Pacific, eastern North Pacific, and North Atlantic regions during 1979–88. However, elsewhere the correlations were poor.

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.

The Structure and Predictability of the “High-Latitude Mode” in the CSIRO9 General Circulation Model

Abstract The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction. A simple model of the zonal wind index with stochastic...

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.

Tropical Cyclone-like Vortices in a Limited Area Model: Comparison with Observed Climatology

Abstract A climate simulation of a limited area model implemented over the Australian region is analyzed for the presence of low pressure systems that have some of the physical characteristics of tropical cyclones. The model is run at a horizontal resolution of 125 km and is nested within a GCM simulation of 10 Januarys. The model simulates those variables that are believed to be important for tropical cyclone formation reasonably well, as evaluated using Gray’s Seasonal Genesis Parameter. Objective criteria are used to detect tropical cyclone-like vortices (TCLVs) in the model. The composite structure of the simulated storms and the life cycle of a typical TCLV are described. Like tropical cyclones, the simulated TCLVs have warm cores, low-level wind maxima, and their tracks and regions of occurrence are similar to those observed for tropical cyclones. In general, the TCLVs simulated by the limited area model are weaker than observed, as determined by a measure of the area-averaged low-level tangential w...

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