Atmospheric circulation features in the ACCESS model simulations for CMIP5: historical simulation and future projections

Abstract

We evaluate the performance of two versions of the ACCESS model (1.0 and 1.3) in simulating both the historical (1979−2008) and projected (2071−2100) atmospheric circulations during, principally, the austral winter under two CMIP5 emission scenarios (RCP4.5 and RCP8.5). The model biases are estimated relative to two recent reanalysis datasets, while the projected circulation changes are assessed against the simulated historical circulations. Overall, both ACCESS models display model biases comparable in magnitude to other CMIP5 model biases. The most significant biases include the upper-tropospheric cold and polar warm biases, a westerly wind bias in the tropical upper troposphere and easterly wind biases in the southern and northern mid-latitudes, a narrower than observed Hadley circulation cell, a stronger Walker circulation cell, and drying (moistening) near the outer edges of the ascending (descending) branch of the Hadley cell. The projected circulation changes for the late 21st century in ACCESS simulations are largely similar to those found in the previous generation climate models including upper-tropospheric and polar warmings, a stronger subtropical jet, a poleward shifted mid-latitude jet, a deeper Hadley cell with its descending branch expanding poleward, and a weakened Walker circulation. However, our analysis also reveals a moderate intensification of the projected Hadley cell in the RCP4.5, but not the RCP8.5, simulations. Most of the projected changes are similar in ACCESS1.0 and ACCESS1.3, except that the Walker circulation change in ACCESS1.3 is essentially an eastward shift of its ascending branch to the east of the dateline, while that in ACCESS1.0 is an in-place weakening of the circulation.