While a connection between the El Niño–Southern Oscillation (ENSO) and the Northern Hemisphere wintertime stratospheric polar vortex appears robust in observational studies focusing on the period before 1979 and in many modeling studies, this connection is not evident over the past few decades. In this study, the factors that have led to the disappearance of the ENSO-vortex relationship are assessed by comparing this relationship in observational data and in operational subseasonal forecasting models over the past few decades. For reforecasts initialized in December, the models simulate a significantly weaker vortex during El Niño than La Niña (LN) as occurred before 1979, but no such effect was observed to have occurred. The apparent cause of this is the eastern European and western Siberian height anomalies present during ENSO. The observed LN events were associated with persistent ridging over eastern Europe as compared to El Niño. Although the Subseasonal-to-Seasonal models are initialized with this ridge, the ridge quickly dissipates. As ridging over this region enhances wave flux entering the stratosphere, the net effect is no robust stratospheric response to ENSO in the observations despite a North Pacific teleconnection that would, in isolation, lead to less wave flux for LN. The anomalies in the eastern European sector in response to ENSO likely reflect unforced internal atmospheric variability.
Bibliographical noteFunding Information:
C. I. G., C. S., and I. P. W. are supported by the Israel Science Foundation (Grant 1558/14) and by a European Research Council starting Grant under the European Unions Horizon 2020 research and innovation program (Grant agreement 677756). D. D. is supported by the Swiss National Science Foundation through Grant PP00P2_170523. S.-W. Son's work is partly supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT (2017R1E1A1A01074889). We thank the three anonymous reviewers for their helpful comments. Correspondence is to be addressed to C. I. G. This work is based on S2S data. S2S is a joint initiative of the World Weather Research Programme (WWRP) and the World Climate Research Programme (WCRP). The original S2S database is hosted at ECMWF as an extension of the TIGGE database, and can be downloaded from the ECMWF server (http://apps.ecmwf.int/datasets/data/s2s/levtype=sfc/type=cf/).
©2019. The Authors.
- Arctic vortex
- El Niño
- subseasonal forecasting