5:15 PM - 7:15 PM
[AAS06-P05] Connection Between Atlantic Blocking Events Characteristics and Sudden Stratospheric Warming Using ERA5 Reanalysis Data
Keywords:Sudden stratospheric warming, Atlantic blocking events, Eddy heat flux anomalies
The occurrence of sudden stratospheric warmings (SSWs) is a vital feature of the extratropical stratosphere. SSWs sometimes take place in response to tropospheric forcing, i.e., intensification of planetary waves. Blocking events have been hypothesized as one candidate for such intensification. Previous studies showed that blocking events in specific areas such as the North Atlantic, North America, and Europe tend to weaken the stratospheric polar vortex. However, only a small proportion of blocking events in such regions trigger SSWs. It remains unclear how SSW-triggering blocking events are different from their non-SSW counterparts.
This study investigated the connection between Atlantic blocking events and SSWs by using the ERA5 reanalysis data. Blocking events over the Atlantic region are known to weaken the stratospheric jet. The data period for the present study is 54 years long from 1970/1971 to 2023/2024 (NDJF). We followed Barriopedro et al. (2006) for identifying blocking events and Charlton and Polvani (2006) for defining SSW. Furthermore, we focused on heat flux anomalies (HFa) by waves (deviation from the zonal mean) in 45°-75°N, 200 hPa as an index for the propagation of planetary waves to the stratosphere. We examine HFa by decomposing it into the interference term between the climatological and anomaly fields, and the nonlinear anomaly term.
First, we examine time evolutions of HFa, and the zonal mean zonal wind at 60°N, 10hPa using the blocking onset as lag=0 [day]. The zonal wind weakens a few days after the onset for blocking events that accompany HFa increases around the onset.
Next, we perform a composite analysis of the blocking events based on the intensity of HFa for lag=0 to 11. It shows that when the blocking events exhibit large HFa, it is mainly contributed by a combination of the defined Atlantic blocking events with an intensified trough to their east, which tilts westward with height. The westward tilt is associated with a confinement of the blocking events (positive phase part of geopotential height waves) below 50 hPa.
Furthermore, we compared SSW-triggering blocking events to their non-SSW counterparts. The former exhibits significantly larger HFa for lag=-1 to 14 reflecting the aforementioned combination of the blocking events and trough. The decomposition reveals that the interference term mainly contributes to the HFa increase and that the fraction of the interference term (or nonlinear term) in HFa is limited between 25-75%. It is also found that the stratospheric zonal wind is weak for the SSW-triggering blocking events. In contrast, the non-SSW blocking events exhibit a temporal extension in HFa increase and a larger spread among the events.
Finally, we compared groups of blocking events that have similar HFa values for lag=-4 to 15 but different contributions from the interference term (or nonlinear term). The groups did not exhibit statistically significant differences in 500 hPa geopotential height wave fields or stratospheric zonal wind evolutions. This result suggests that the limitation occurs just by chance, although its possible dynamical implications require further investigation.
This study explored the connection between Atlantic blocking events and SSWs using the ERA5 reanalysis data. Studies like the present one are expected to improve our understanding of the generation mechanism of SSWs, and also predictions for SSWs. In summary, Atlantic blocking evens tend to lead to SSWs when they have large HFa for lag=-4 to 15 and the stratospheric jet is weak around lag=0. We further hope to reveal possible dynamical effects and roles of the interference and nonlinear terms.
This study investigated the connection between Atlantic blocking events and SSWs by using the ERA5 reanalysis data. Blocking events over the Atlantic region are known to weaken the stratospheric jet. The data period for the present study is 54 years long from 1970/1971 to 2023/2024 (NDJF). We followed Barriopedro et al. (2006) for identifying blocking events and Charlton and Polvani (2006) for defining SSW. Furthermore, we focused on heat flux anomalies (HFa) by waves (deviation from the zonal mean) in 45°-75°N, 200 hPa as an index for the propagation of planetary waves to the stratosphere. We examine HFa by decomposing it into the interference term between the climatological and anomaly fields, and the nonlinear anomaly term.
First, we examine time evolutions of HFa, and the zonal mean zonal wind at 60°N, 10hPa using the blocking onset as lag=0 [day]. The zonal wind weakens a few days after the onset for blocking events that accompany HFa increases around the onset.
Next, we perform a composite analysis of the blocking events based on the intensity of HFa for lag=0 to 11. It shows that when the blocking events exhibit large HFa, it is mainly contributed by a combination of the defined Atlantic blocking events with an intensified trough to their east, which tilts westward with height. The westward tilt is associated with a confinement of the blocking events (positive phase part of geopotential height waves) below 50 hPa.
Furthermore, we compared SSW-triggering blocking events to their non-SSW counterparts. The former exhibits significantly larger HFa for lag=-1 to 14 reflecting the aforementioned combination of the blocking events and trough. The decomposition reveals that the interference term mainly contributes to the HFa increase and that the fraction of the interference term (or nonlinear term) in HFa is limited between 25-75%. It is also found that the stratospheric zonal wind is weak for the SSW-triggering blocking events. In contrast, the non-SSW blocking events exhibit a temporal extension in HFa increase and a larger spread among the events.
Finally, we compared groups of blocking events that have similar HFa values for lag=-4 to 15 but different contributions from the interference term (or nonlinear term). The groups did not exhibit statistically significant differences in 500 hPa geopotential height wave fields or stratospheric zonal wind evolutions. This result suggests that the limitation occurs just by chance, although its possible dynamical implications require further investigation.
This study explored the connection between Atlantic blocking events and SSWs using the ERA5 reanalysis data. Studies like the present one are expected to improve our understanding of the generation mechanism of SSWs, and also predictions for SSWs. In summary, Atlantic blocking evens tend to lead to SSWs when they have large HFa for lag=-4 to 15 and the stratospheric jet is weak around lag=0. We further hope to reveal possible dynamical effects and roles of the interference and nonlinear terms.