Investigating tropical versus extratropical influences on the Southern Hemisphere tropical edge in the Unified Model

Since the late 1970s, observations have shown a widening of the tropical Hadley cell (HC) circulation. State-of-the-art climate models reproduce the general trend along with a projected continuous expansion. Discrepancies in expansion rates of observation- and model-based studies have been attributed to differences in applied methods, natural variability and model shortcomings. Furthermore, the driving influence of tropical or extratropical processes on these changes is not well understood. All of this highlights the dynamical mechanisms and the region of origin controlling the tropical width are still insufficiently understood. Here we examine the influence of systematic model biases of the atmosphere-only Unified Model (UM) onto the simulation of the Southern Hemisphere (SH) tropical edge. We utilize nudged experiments with prescribed sea surface temperatures, where potential temperature and horizontal winds are relaxed back to ERA-Interim reanalysis for a 20-year period in selected regions. Correcting model biases in the tropics and extratropics separately allows us to dissect the dominant remote impacts of present model errors onto the SH tropical edge simulation. The experiments are applied to established tropical width metrics ranging from near-surface to upper-level metrics capturing the poleward flank of the HC. We find both regions work remotely to reduce errors in the UM fields and location of the tropical edge. Surprisingly, correcting the extratropical biases, south of 45 degrees S, more consistently improves the tropical width across the metrics and seasons than nudging the tropics (10 degrees N-10 degrees S). These findings demonstrate the substantial role of extratropical influences in locating the SH tropical edge.