Stephen Jeffrey

Declining Aerosols in CMIP5 Projections: Effects on Atmospheric Temperature Structure and Midlatitude Jets

The effects of declining anthropogenic aerosols in representative concentration pathway 4.5 (RCP4.5) are assessed in four models from phase 5 the Coupled Model Intercomparison Project (CMIP5), with a focus on annual, zonal-mean atmospheric temperature structure and zonal winds. For each model, the effect of declining aerosols is diagnosed from the difference between a projection forced by RCP4.5 for 2006‐2100 and another that has identical forcing, except that anthropogenic aerosols are fixed at early twenty-first-century levels. The response to declining aerosols is interpreted in terms of the meridional structure of aerosol radiative forcing, which peaks near 408N and vanishes at the South Pole. Increasing greenhouse gases cause amplified warming in the tropical upper troposphere and strengthening midlatitude jets in both hemispheres. However, for declining aerosols the vertically averaged tropospheric temperature response peaks near 408N, rather than in the tropics. This implies that for declining aerosols the tropospheric meridional temperature gradient generally increases in the Southern Hemisphere (SH), but in the Northern Hemisphere (NH) it decreases in the tropics and subtropics. Consistent with thermal wind balance, the NH jet then strengthens on its poleward side and weakens on its equatorward side, whereas the SH jet strengthens more than the NH jet. The asymmetric response of the jets is thus consistent with the meridional structure of aerosol radiative forcing and the associated tropospheric warming: in the NH thelatitudeofmaximumwarmingis roughlycollocatedwith thejet, whereasin theSH warmingisstrongestin the tropics and weakest at high latitudes.

Anthropogenic effects on the subtropical jet in the Southern Hemisphere: aerosols versus long-lived greenhouse gases

We use single-forcing historical simulations with a coupled atmosphere?ocean global climate model to compare the effects of anthropogenic aerosols (AAs) and increasing long-lived greenhouse gases (LLGHGs) on simulated winter circulation in the Southern Hemisphere (SH). Our primary focus is on the subtropical jet, which is an important source of baroclinic instability, especially in the Australasian region, where the speed of the jet is largest. For the period 1950 to 2005, our simulations suggest that AAs weaken the jet, whereas increasing LLGHGs strengthen the jet. The different responses are explained in terms of thermal wind balance: increasing LLGHGs preferentially warm the tropical mid-troposphere and upper troposphere, whereas AAs have a similar effect of opposite sign. In the mid-troposphere, the warming (cooling) effect of LLGHGs (AAs) is maximal between 20S and 30S; this coincides with the descending branch of the Hadley circulation, which may advect temperature changes from the tropical upper troposphere to the subtropics of the SH. It follows that LLGHGs (AAs) increase (decrease) the mid-tropospheric temperature gradient between low latitudes and the SH mid-latitudes. The strongest effects are seen at longitudes where the southward branches of the Hadley cell in the upper troposphere are strongest, notably at those that correspond to Asia and the western Pacific warm pool.

Ocean circulation response to anthropogenic-aerosol and greenhouse gas forcing in the CSIRO-Mk3.6 coupled climate model

We use the CSIRO-Mk3.6 coupled climate model to examine the impact of anthropogenic aerosols (AAs) and long-lived greenhouse gases (GHGs) on aspects of the global ocean circulation. Focusing on the second half of the twentieth century, we compare multiple ten member ensembles of historical climate change, which are forced by different combinations of forcing agents; these different simulations enable us to separately diagnose the effects of changes in AAs and GHGs. We also compare ten member 21st century ensembles driven by Representative Concentration Pathways 4.5 (RCP4.5) and 8.5 (RCP8.5). To a large degree the pattern of change in the oceans due to the impact of AAs is similar to the effect of increasing GHGs, but of opposite sign. The Atlantic Meridional Overturning Circulation (and associated North Atlantic Deep Water formation) strengthens in response to historical changes in AAs but weakens in response to increasing GHGs. Similarly, the Indonesian Throughflow strengthens in response to AAs, and weakens in response to increasing GHGs. The Drake Passage Transport, however, shows a small weakening (strengthening) due to historical changes in AAs (GHGs) in the ensemble mean. The change in the Drake Passage Transport is much clearer in the 21st century, in which it increases strongly in response to increasing GHGs and decreasing AAs in both RCP4.5 and RCP8.5. The results suggests that without the influence of AAs, changes in ocean circulation would have already followed a path much more like one dominated by increasing GHGs. Considering that the AA levels are expected to decrease during the next few decades, the effects of increasing GHGs on ocean circulation will be amplified accordingly.

The relative performance of Australian CMIP5 models based on rainfall and ENSO metrics

We assess the performance of 30 CMIP5 and two CMIP3 models using metrics based on an all-Australia average rainfall and NINO3.4 sea surface temperatures (SSTs). The assessment provides an insight into the relative performance of the models at simulating long-term average monthly mean values, interannual variability and the seasonal cycles. It also includes a measure of the ability to capture observed rainfall-NINO3.4 SST correlations. In general, the rainfall features are reasonably simulated and there is relatively little difference amongst the models but the NINO3.4 SST features appear more difficult to simulate as evidenced by the greater range in metric scores. We find little evidence of consistency in the sense that a relatively good metric score for one feature does not imply a relatively good score for another related (but independent) feature. The assessment indicates that more recent models perform slightly better than their predecessors, especially with regard to the NINO3.4 metrics. We also focus on the ability of models to reproduce the observed seasonal cycle of rainfall-SST correlations since this is a direct indicator of a models potential utility for seasonal forecasting over Australia. This indicates some relatively good models (CNRM, HadGEM2-ESM, MPI-ESM-LR and MPI-ESM-MR) and some relatively poor models (CSIRO-Mk3.5, FGOALS, GISS-E2-HP1 and INMCM4). We find that the ACCESS1.3 and CSIRO-Mk3.6 models rank as near median performers on this metric and represent improvements over their predecessors (ACCESS1.0, CSIRO-Mk3.0 and CSIRO-Mk3.5).

Simulated and projected summer rainfall in tropical Australia: links to atmospheric circulation using the CSIRO-Mk3.6 climate model

Simulations of mean climate characteristics and atmospheric circulation patterns in the tropical region of Australia during the austral summer (December to February) are assessed by comparing against observations. An examination of the observed climatologies of mean sea level pressure, winds at lower and upper levels and rainfall with simulated climatologies show that the model captures the spatial structures of observed patterns fairly well. However, there are some discrepancies in the magnitudes between observed and modelled parameters. The model can reasonably reproduce the observed link between tropical Australian rainfall variability and the atmospheric circulation patterns. Changes in circulation patterns and rainfall are investigated for Representative Concentration Pathways (RCPs) 4.5 and 8.5. Spatial patterns of changes in circulation parameters and rainfall are very similar for both RCP 4.5 and RCP 8.5, but the magnitudes are larger for the RCP 8.5. Under anthropogenic climate change conditions, the CSIRO-Mk3.6 climate model simulates an atmospheric circulation pattern reflecting weaker monsoon conditions in the Australian region, and hence, reduced rainfall over tropical Australia. A slightly increased pressure over northwest Australia and slightly decreased pressure over north Asia is simulated. Winds at lower and upper tropospheric levels indicate opposing anomalies and reduced rainfall over a broader region that encompasses northern Australia, parts of Indonesia and around the Philippines. However, an increase in rainfall is simulated for the region east of Papua New Guinea.