The Efficiency of Upward Wave Propagation Near the Tropopause

Recent work has highlighted that not all periods with anomalous heat flux at 100hPa were preceded by anomalous heat flux in the troposphere (Birner and Alberts 2017; White et al 2019; Camara et al 2019), and the goal of this work is to understand the factors that govern the efficiency of upward wave propagation near the tropopause. The index of refraction of Matsuno (1970) has been used to offer guidance on the direction of wave propagation within the stratosphere. Specifically, waves are preferentially refracted towards regions with a more positive index of refraction and ducted away from regions in which the index of refraction is more negative. However, the index of refraction was derived under the assumption that buoyancy frequency is constant at all height levels, which is clearly not true near the tropopause. This assumption allowed Matsuno to ignore certain height dependent buoyancy frequency terms, and here we explore the impact of these terms near the tropopause.

Using the dataset of the European Center for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) we defined 'transmitting' composites consisting of more efficient upward propagation events between 300hPa and 100hPa. Similarly, periods of less efficient upward propagation events between 300hPa and 100hPa are composited as 'decaying' events. We computed the index of refraction profile using a median, percentage of negative days and the trimmed mean (Wilks 2011), and also consider the terms neglected by Matsuno. We find that  the index of refraction can account for the difference between the decaying and transmitting composite.