A New Way to Measure Stratosphere-Troposphere Coupling in Observations and IPCC Models

Abstract:

This project explores new intercomparisons between reanalysis data and CMIP5 models and introduces a new way to quantify stratosphere-troposphere (S-T) coupling, through the movement of mass in and out of the polar cap at different levels, as measured by mean polar cap pressure at fixed heights. The aims of this project are to investigate how mass movement is related to variations in stratospheric circulations, such as the wintertime polar vortex; assess how it is represented in climate models; establish whether different model parameterisations produce different S-T coupling outcomes; and identify any projected changes as a result of climate change. The results of this investigation will be beneficial for furthering our understanding of S-T coupling processes, and improving medium-range forecasting skill through use of stratospheric data. This work forms part of a three-and-a-half year PhD project with potential links to the ongoing SPARC Reanalysis Intercomparison Project (S-RIP). This work uses data from ERA-interim reanalysis and climate models of varying complexity. The data has been zonally averaged, and geopotential height values from the data are interpolated across pressure levels to produce pressure values at a set of defined heights. From a composite of deseasonalised data of the polar cap (65-90 degrees), it was discovered that a maximal stratospheric anomaly of increased pressure occurs between 12 and 20 km, with a similar structure in both the northern and southern hemispheres. Concurrently, this influx of mass seems to induce polewards air flow in the troposphere, creating an area of stored atmospheric mass in the column of air above the pole. This phenomenon is present in the wintertime during the period of ERA-interim, and is recreated with good accuracy in CMIP5 models; a similar shape and magnitude are clearly reproduced, and models with a high top (> 50 km) perform best. These occurrences follow a pattern which is similar to the Northern and Southern Annular Modes, and can be quantified by a comparative index.