Southern hemisphere climate change

The change in the position of the jet in recent decades is a response to the concomitant forcing from decreasing stratos pheric ozone and increasing greenhouse gases, with models unable to reproduce the shift without a representation of stra tospheric ozone depletion. Recent changes in the final warming date of the Southern Hemisphere polar vortex have also be en found to be strongly determined by decreases in stratospheric ozone concentrations, with final warming dates observed to be later in the 1990s compared to the 1980s.

The stratosphere and troposphere are coupled, so anomalies in the stratospheric polar vortex have an impact down to the surface. This coupling predominantly occurs in the late spring, or summer, when the final warming of the polar vortex st rongly influences both the stratospheric and tropospheric circulation. The downward propogation of the final warming sig nature is thought to begin around 1hPa. As such, the representation of changes in final warming date may be sensitive to the position of the model top, which is often located near or below 1 hPa in models.

With the CMIP5 set of models including a substantial number of 'high-top' models, which explicitly resolve the stratosph ere, we can ask the question: 'How does an improved representation of the lower stratosphere change our understanding of past tropospheric climate and future climate projections?' My interests focus on four specific questions:

• Do 'high-top' models better represent past climatology and trends than those with a 'low-top'?
• What are the anticipated future changes in Austral jet position and final warming date?
• What are the mechanisms for changes in jet position, and how do they relate to the differences between high- and low -top models?
• What are the drivers of change in final warming date?