On the representation of major stratospheric warmings in reanalyses

Abstract

Abstract. Major sudden stratospheric warmings (SSWs) represent one of the most abrupt phenomena of the boreal wintertime stratospheric variability, and constitute the clearest example of coupling between the stratosphere and the troposphere. A good representation of SSWs in climate models is required to reduce their biases and uncertainties in future projections of stratospheric variability. The ability of models to reproduce these phenomena is usually assessed with just one reanalysis. However, the number of reanalyses has increased in the last decade and their own biases may affect the model evaluation. Here we compare the representation of the main aspects of SSWs across reanalyses. The examination of their main characteristics in the pre- and post-satellite periods reveals that reanalyses behave very similarly in both periods. However, discrepancies are larger in the pre-satellite period compared to afterwards, particularly for the NCEP-NCAR reanalysis. All datasets reproduce similarly the specific features of wavenumber-1 and wavenumber-2 SSWs. A good agreement among reanalyses is also found for triggering mechanisms, tropospheric precursors, and surface response. In particular, differences in blocking precursor activity of SSWs across reanalyses are much smaller than between blocking definitions.

The Whole Atmosphere Community Climate Model Version 6 (WACCM6)

“…The ensemble mean frequency (black bars) is close to observations. While Ayarzagüena et al () have noted that there are differences in the timing and frequency distribution of SSWs in “observations” (different reanalyses), this does not affect the conclusions here. Results are the same with NCEP reanalyses over the satellite era.…”

Uncertainty in the response of sudden stratospheric warmings and stratosphere-troposphere coupling to quadrupled CO2 concentrations in CMIP6 models

“…In fact, we have specifically restricted the analysis period to 1958-2014 to perform a rigorous quantitative comparison with JRA-55. This reanalysis shows a very good performance in representing SSWs (Ayarzagüena et al, 2019) and is the most modern reanalyses of the three that extend longer than the satellite era and assimilate more than surface data (ERA-40, NCEP/NCAR reanalysis, and JRA-55).…”

“…Indeed, 6 of the 11 models exhibit a statistically stronger negative SLP anomaly in that area under abrupt4xCO 2 forcing than in the piControl runs. This could be related to some changes in the tropospheric precursors of SSWs because these anomalies have been identified as the remainder of the deepening of the Aleutian low preceding SSWs in observations (Ayarzagüena et al, 2019;. Nevertheless, more work is required to understand all the details.…”

Uncertainty in the Response of Sudden Stratospheric Warmings and Stratosphere‐Troposphere Coupling to Quadrupled CO2 Concentrations in CMIP6 Models

“…In fact, we have specifically restricted the analysis period to 1958–2014 to perform a rigorous quantitative comparison with JRA‐55. This reanalysis shows a very good performance in representing SSWs (Ayarzagüena et al, ) and is the most modern reanalyses of the three that extend longer than the satellite era and assimilate more than surface data (ERA‐40, NCEP/NCAR reanalysis, and JRA‐55).…”

“…Indeed, 6 of the 11 models exhibit a statistically stronger negative SLP anomaly in that area under abrupt4xCO 2 forcing than in the piControl runs. This could be related to some changes in the tropospheric precursors of SSWs because these anomalies have been identified as the remainder of the deepening of the Aleutian low preceding SSWs in observations (Ayarzagüena et al, ; Charlton & Polvani, ). Nevertheless, more work is required to understand all the details.…”