Tropical climate variability in ACCESS-CM2 coupled climate model

Harun A Rashid¹

1. CSIRO Oceans and Atmosphere, CSIRO, Aspendale, VIC, Australia

Climate models are important for understanding the dynamics of the atmosphere and ocean and for predicting and projecting the future climate variability and change. Recently, version 2 of the Australian Community Climate and Earth System Simulator (ACCESS-CM2) has been developed to participate in the Climate Model Inter-comparison Project phase 6 (CMIP6). Climate simulations from ACCESS-CM2 are now being submitted to the CMIP6 archive to be analysed for the sixth Assessment Report (AR6). In this presentation, I examine the tropical interannual climate variability simulated by ACCESS-CM2. In particular, I investigate to what extent ACCESS-CM2 simulates the observed features of the important tropical variability modes, such as El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD). I also compare the ACCESS-CM2 simulated ENSO and IOD features with those simulated by the UK Met Office HadGEM3 GC3.1 model. The latter model shares the same atmosphere model with ACCESS-CM2, but has different ocean and land surface models. It is of interest to learn to what extent a different ocean model can influence the simulated ENSO and IOD modes.   

We find that ACCESS-CM2 simulates all the major features of observed mean climate and of ENSO and IOD. However, there are systematic errors of varying degree in both the simulated mean climate and interannual variability modes. For example, the ENSO-driven sea-surface temperature (SST) variability in the equatorial eastern Pacific is almost biennial in ACCESS-CM2 simulations, in contrast to the observed 3–7-year variability. Also, the simulated zonal wind stress variability in the equatorial Pacific is notably weaker than the observed. We’ll discuss the possible causes of these and other systematic model biases.

Future changes of the Pacific Equatorial Undercurrent: a Lagrangian investigation

Annette Stellema^{1, 2}, Alex Sen Gupta^{1, 2}, Andréa S Taschetto^{1, 2}, Ming Feng^{3, 4}

1. Climate Change Research Centre, University of New South Wales, Sydney, NSW, Australia
2. ARC Centre of Excellence for Climate Extremes, Sydney, NSW, Australia
3. Indian Ocean Marine Research Centre, CSIRO Oceans and Atmosphere, Crawley, WA, Australia
4. Centre for Southern Hemisphere Oceans Research, CSIRO Oceans and Atmosphere, Hobart, TAS, Australia

The tropical Pacific Ocean is a climatically import region as it is the atmosphere's largest source of oceanic CO2 and generates 20-30% of global primary productivity that support many local fisheries. The nutrient and oxygen-rich Equatorial Undercurrent regulates the tropical Pacific’s high primary productivity, oxygen minimum zones and the cycling of carbon. As such future changes to tropical Pacific circulation have the potential to impact local ecosystems, commercial fisheries and the global uptake of carbon. While studies report conflicting projected changes of the tropical Pacific oxygen minimum zones, it is suggested that changes depend on a balance between the compensating effects of Equatorial Undercurrent transport and biological activity. Studies project robust changes of the Equatorial Undercurrent and its low latitude western boundary sources, however, the changes to the Equatorial Undercurrent water sources, pathways and evolution requires further examination. Here, we investigate how and why the Equatorial Undercurrent sources and pathways are projected to change in a future of increasing greenhouse emissions. We discuss sensitivity experiments, hindcasts and future changes of Equatorial Undercurrent water transport using Lagrangian particles. The particles are advected in the eddy-resolving Ocean Forecasting Australia Model version 3 (OFAM3), which is a downscaling run under the RCP8.5 emission scenario.