Penny Maher (University of Utrecht)

The tropics are expanding poleward at about 1 degree per decade in observations. The Hadley cell edge is one common measure of tropical expansion. One alternative method is the trend in the subtropical jet position. Unlike the Hadley cell edge, the observed trends in the subtropical jet position are much smaller and often not significant. This makes us question if current methods adequately identify the subtropical jet position. The “tropopause gradient method” is introduced to locate the subtropical jet – using the peak gradient in potential temperature along the dynamic tropopause.

Using the tropopause gradient method we find the subtropical jet trends are smaller than the 1 degree per decade trends, providing further evidence that the subtropical jet trends are smaller than trends in the Hadley cell edge. We find the subtropical jet trends depend on the reanalysis product used which we attribute to inconsistent lower stratosphere temperature trends. The subtropical jet position and trends are also sensitive to data sampling – the subtropical jet is located further poleward when daily data is used compared to monthly data.

There is no robust change in the subtropical jet position in either hemisphere. The small subtropical jet trends identified in reanalysis are not the result of natural variability obscuring the signal, as trends of 0.3 degrees per decade can reliably be detected. Further investigation is needed to account for the small subtropical jet latitude trends compared to trends in the Hadley cell edge.

Brief Biography: Penny completed a Bachelor in Physics at the University of Wollongong and a PhD in Climate Science at the University of New South Wales in Australia. Penny is currently a post-doc scientist at the University of Exeter, UK. She is part of the ParaCon project that aims to significantly improve the representation of convection across model scales from 1-100km. In this position she is working with Geoff Vallis and John Thuburn, with support from the Met Office. She is also involved in a project to building a reduced complexity version of the Met Office Unified Model.