Drivers and Surface Signal of Interannual Variability of Boreal Stratospheric Final Warmings

Abstract

Springtime stratospheric final warming (SFW) variability has been suggested to be linked to the tropospheric circulation, particularly over the North Atlantic sector. These findings, however, are based on reanalysis data that cover a rather short period of time (1979 to present). The present work aims to improve the understanding of drivers, trends and surface impact of dynamical variability of boreal SFWs using chemistry‐climate models. We use multidecadal integrations of the fully coupled chemistry‐climate models Community Earth System Model version 1 (Whole Atmosphere Community Climate Model) and ECHAM/Modular Earth Submodel System Atmospheric Chemistry‐O. Four sensitivity experiments are analyzed to assess the impact of external factors; namely, the quasi‐biennial oscillation, sea surface temperature (SST) variability, and anthropogenic emissions. SFWs are classified into two types with respect to their vertical development; that is, events which occur first in the midstratosphere (10‐hPa first SFWs) or first in the upper stratosphere (1‐hPa first SFWs). Our results confirm previous reanalysis results regarding the differences in the time evolution of stratospheric conditions and near‐surface circulation between 10 and 1‐hPa first SFWs. Additionally, a tripolar SST pattern is, for the first time, identified over the North Atlantic in spring months related to the SFW variability. Our analysis of the influence of remote modulators on SFWs revealed that the occurrence of major warmings in the previous winter favors the occurrence of 10‐hPa first SFWs later on. We further found that quasi‐biennial oscillation and SST variability significantly affect the ratio between 1‐hPa first and 10‐hPa first SFWs. Finally, our results suggest that ozone recovery may impact the timing of the occurrence of 1‐hPa first SFWs.

Statistical Characteristics of Major Sudden Stratospheric Warming Events in CESM1-WACCM: A Comparison with the JRA55 and NCEP/NCAR Reanalyses

“…Provided that the average length of an SSW event is limited and there will be repetition of zonal wind reversal between westerlies and easterlies, no new SSW events will be defined again within 20 consecutive days after a former SSW event is identified already [8,9]. In addition, stratospheric final warming is excluded as the zonal mean zonal winds change from westerlies to easterlies and stay unchanged in the direction since it marks the annual cycle of the stratospheric circulation from wintertime to summertime [54].…”

Parallel Comparison of Major Sudden Stratospheric Warming Events in CESM1-WACCM and CESM2-WACCM

“…Once a major SSW is defined, there will not be any new SSW events defined repeatedly within 20 consecutive days afterward [4,33,43]. Furthermore, stratospheric final warming is excluded as the zonal mean zonal wind reversal from westerlies to easterlies marks the annual cycle of the stratospheric circulation from wintertime to summertime [46].…”

Vertical Structure of the Arctic Spring Transition in the Middle Atmosphere

“…thus decrease the zonal wind below, while for 1 hPa-first events the PWs break around 1 hPa (Hardiman et al, 2011;Thiélemont et al, 2019).…”

“…However, they also found that winters without a SSW are followed by 1 hPa-first SFW which we cannot confirm with our analysis. One possible explanation for this discrepancy may be that Thiélemont et al (2019) used model data (CESM/WACCM) that include significantly less 10 hPa-first SFW events compared to ERA-Interim. However, we also note, that for single cases the 10 hPa-first and 1 hPa-first classification might be different especially for the no negative NAM spring transition where the average onset days at 10 and 0.7 hPa are close together.…”