Seasonal prediction of the boreal winter stratosphere

Alice Portal,Daniela I V Domeisen,Froila M Palmeiro,Javier García-Serrano,Paolo Ruggieri,Silvio Gualdi

doi:10.1007/s00382-021-05787-9

Abstract

The predictability of the Northern Hemisphere stratosphere and its underlying dynamics are investigated in five state-of-the-art seasonal prediction systems from the Copernicus Climate Change Service (C3S) multi-model database. Special attention is devoted to the connection between the stratospheric polar vortex (SPV) and lower-stratosphere wave activity (LSWA). We find that in winter (December to February) dynamical forecasts initialised on the first of November are considerably more skilful than empirical forecasts based on October anomalies. Moreover, the coupling of the SPV with mid-latitude LSWA (i.e., meridional eddy heat flux) is generally well reproduced by the forecast systems, allowing for the identification of a robust link between the predictability of wave activity above the tropopause and the SPV skill. Our results highlight the importance of November-to-February LSWA, in particular in the Eurasian sector, for forecasts of the winter stratosphere. Finally, the role of potential sources of seasonal stratospheric predictability is considered: we find that the C3S multi-model overestimates the stratospheric response to El Niño–Southern Oscillation (ENSO) and underestimates the influence of the Quasi–Biennial Oscillation (QBO).

Highlights

The dynamical evolution of the stratosphere is driven by different processes
In the mean flow at 10 hPa we identify three latitudinal regions: the tropics, found to be highly predictable due to the initialisation of the Quasi–Biennial Oscillation (QBO) phase; the subtropics, characterised by a low interannual variability that is not predicted by C3S hindcasts; the extratropics, where the stratospheric polar vortex is variably predicted among the forecast systems
In this work we have assessed the variability and prediction skill of the winter stratosphere in the C3S seasonal prediction systems initialised in November

Summary

Introduction

The dynamical evolution of the stratosphere is driven by different processes. Radiative absorption by ozone causes differential warming in the vertical due to the increased absorption at higher levels, while a meridional gradient is generatedThis paper is a contribution to the MEDSCOPE special issue on the drivers of variability and sources of predictability for the European and Mediterranean regions at subseasonal to multiannual time scales. The dynamical evolution of the stratosphere is driven by different processes. Radiative absorption by ozone causes differential warming in the vertical due to the increased absorption at higher levels, while a meridional gradient is generated. This paper is a contribution to the MEDSCOPE special issue on the drivers of variability and sources of predictability for the European and Mediterranean regions at subseasonal to multiannual time scales. MEDSCOPE is an ERA4CS project co-funded by JPI Climate. The special issue was coordinated by Silvio Gualdi and Lauriane Batté. In the Northern Hemisphere, strong anomalies in the stratospheric circulation are known to propagate towards lower levels and to influence mid-latitude surface weather and climate (see Kidston et al 2015, for a review)

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