On the value of reanalyses prior to 1979 for dynamical studies of stratosphere–troposphere coupling

Peter Hitchcock

doi:10.5194/acp-19-2749-2019

Abstract

Abstract. Studies of stratosphere–troposphere coupling, particularly those seeking to understand the dynamical processes underlying the coupling following extreme events such as major stratospheric warmings, suffer significantly from the relatively small number of such events in the “satellite” era (1979 to present). This limited sampling of a highly variable dynamical system means that composite averages tend to have large uncertainties. Including years during which radiosonde observations of the stratosphere were of sufficiently high quality substantially extends this record, reducing this sampling uncertainty by up to 20 %. Moreover, many open questions in this field involve aspects of tropospheric dynamics likely to be better constrained by “conventional” (i.e. radiosonde and surface-based) observations. Based on an intercomparison of reanalyses, a quantitative case is made that for many purposes the improved sampling obtained by including this period outweighs the reduced precision of the reanalyses in the Northern Hemisphere. Studies of stratosphere–troposphere coupling should therefore consider the use of this period when using reanalysis data. These results also support continued attention on this period from centres producing reanalyses.

Highlights

One of the central challenges to the detailed study of the large-scale coupling between the stratosphere and the troposphere is the relatively limited record of high-quality, global observations
The variability of DJF zonal winds is large in the Arctic stratospheric polar vortex, and to a lesser extent in the region of the quasibiennial oscillation (QBO) and on the flanks of the tropospheric jets
The analysis has considered both the radiosonde and satellite eras to be to some extent uniform in time in their properties, yet the observational record evolved during these periods as well

Summary

Introduction

One of the central challenges to the detailed study of the large-scale coupling between the stratosphere and the troposphere is the relatively limited record of high-quality, global observations. Despite the availability of nearly four decades of global satellite-based observations, the length of the observational record remains a fundamental limitation to this statistical approach. This is demonstrated explicitly here, as well as by another closely related contribution (Gerber and Martineau, 2018) to the Stratospheretroposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP; Fujiwara et al, 2017). The coupling between the stratosphere and the troposphere remains a significant source of uncertainty in projected climate changes over the coming century (Manzini et al, 2014; Simpson et al, 2018), as well as an important source of skill in seasonal forecasting (Sigmond et al, 2013). Accounting for sampling error in such large-scale dynamical phenomena is a major concern for many other dynamical questions, including identifying regional signals of climate change and teleconnection patterns (e.g. Deser et al, 2017)

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