A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modeling Benchmarks
Summary (2 min read)
1. Introduction
- Over the last decade their understanding of the relationship between the stratosphere and troposphere has been radically altered.
- Given the prominent role of SSW events, it is somewhat surprising that relatively few attempts have been made to establish a comprehensive climatology of SSWs; this is the aim of the current work which, encompasses two related papers.
- This study is also closely related to that of Limpasuvan et al. (2004, hereafter LIM04).
2. Sudden warming identification and classification algorithm
- In this section the authors describe the key tool that they have developed for the present study: an algorithm for automatically identifying and classifying SSWs.
- Thus, to avoid unnecessary complexity, the authors have not included the temperature gradient criterion1 in their algorithm.
- The authors algorithm, therefore, follows Nash et al. (1996) in that it identifies the vortex edges as the locations of maximum vorticity gradients, but it accomplishes this with no horizontal averaging.
- Before proceeding, a legitimate concern needs to be addressed: given the relatively short length of the datasets, less than 50 yr, one may wonder about the robustness of the numbers the authors have just presented, regarding the frequency and type ratio of SSWs.
4. Distribution of SSWs by month and year
- The second question the authors posed in the introduc5.
- As nC is increased, small-scale features in the p field became more prominent, and the number of differences between the algorithm and the subjective analysis increases by one or two sudden warmings.
- SSWs in the NCEP–NCAR dataset are shown by gray bars, and SSWs from the ERA-40 dataset are shown by black bars.
- From the distribution of all SSWs (Fig. 2a) one may conclude that, typically, most SSWs occur during midto late winter (January–February), with only a few SSWs occurring in November and December, and no midwinter warmings after March.
- Recent work by Manney et al. (2005) has shown that the period between 1998 and 2004 has the highest SSW activity of any period on record.
5. Dynamical differences between vortex displacements and splits.
- Having discussed the time distribution of SSWs, the authors now address the question of whether the two types of events, vortex displacement and vortex splitting, exhibit important dynamical differences.
- This result can be understood by considering the evolution of the polar cap temperature during SSWs.
- During the SSW (Fig. 7, middle column) large negative zonal mean zonal wind anomalies are found poleward of 50°N in the stratosphere, denoting the strong deceleration of the vortex that accompanies both types of SSW.
- Solid portions of each line indicate that the anomaly is significantly greater than zero at 0.10 confidence, calculated with a t test.
6. Tropospheric impact
- To address the last question posed in the introduction.the authors.
- In contrast, the geopotential height anomalies following the vortex splits have a much more global character and include positive maxima over central Eurasia and the Pacific Ocean (Fig. 10b), with the maximum in the Pacific directly over the center of action of the 1000-hPa NAM.
- Given that the anomaly patterns in the troposphere associated with vortex splits are complicated and might not all be directly related to stratospheric changes, diagnosing the relative impact of vortex displacements and vortex splits on the tropospheric flow as a whole is difficult.
- There is a marked seasonality in the NAM index in the stratosphere, with SSWs occurring later in the winter having a much smaller NAM index than those in midwinter.
7. Modeling benchmarks
- To date, GCM validation attempts have largely focused on comparing the statistics (e.g., the time mean) of model fields with those available from the reanalyses; see Pawson et al. (2000) for a recent summary.
- This table should be useful in validating numerical model simulations of SSWs.
- The numbers in the tables are constructed from the NCEP–NCAR reanalysis dataset.
- Following each SSW, the seasonal evolution of the troposphere should be disturbed, indicating the correct level of stratosphere–troposphere coupling.
8. Conclusions
- The authors have constructed a new climatology of major midwinter stratospheric sudden warming events, based on a new algorithm that they have developed to automatically extract SSWs from large datasets and distinguish between different types of SSWs.
- This new algorithm compares favorably with a subjective analysis of the data and is relatively easy to implement.
- The authors are able to provide the following answers, to the questions posed in the introduction: 1) SSWs occur with a mean frequency of 0.62 events per winter season.
- 2) The seasonal distribution of vortex splits and vortex displacements is very different.
- The NCEP––NCAR reanalyses were obtained from the IRI Ingrid data server at the Lamont-Doherty Earth Observatory of Columbia University.
Did you find this useful? Give us your feedback
Citations
1,022 citations
788 citations
Cites methods from "A New Look at Stratospheric Sudden ..."
...Detection of major SSWs follows the method described in Charlton and Polvani (2007) and Butler and Polvani (2011), wherein the ‘‘central date’’ of an SSW is the first day in which zonal-mean zonal winds at 10 hPa, 608N become westward....
[...]
575 citations
390 citations
Cites background from "A New Look at Stratospheric Sudden ..."
...Typically, GCMs have difficulties reproducing the observed SSW frequency (Charlton and Polvani 2007; Charlton et al. 2007)....
[...]
379 citations
References
7,052 citations
6,768 citations
4,270 citations
"A New Look at Stratospheric Sudden ..." refers background in this paper
...Short summaries can be found in Labitzke (1977) and Naujokat and Labitzke (1993)....
[...]
3,781 citations
"A New Look at Stratospheric Sudden ..." refers background in this paper
...In fact, the pattern of high geopotential height anomalies in the mid-Pacific is reminiscent of the positive phase of the Pacific–North American pattern (PNA; Wallace and Gutzler 1981)....
[...]
Related Papers (5)
Frequently Asked Questions (4)
Q2. Why is the vortex splits a counter to the expectation of the authors?
This is counter to their expectation that vortex splits would produce larger temperature anomalies in the middle stratosphere, as they are accompanied by a more substantial disturbance of the flow required to split the vortex.
Q3. What is the motivation for constructing the climatology the authors have just described?
Beyond contributing to a better understanding of SSWs, a further motivation for constructing the climatology the authors have just described is to be of practical help to the many modeling teams actively working to de-velop accurate GCMs of the stratosphere.
Q4. Why did the authors recalculate the diagnostics for vortex displacements?
Because of the difference in seasonality of the vortex displacement and vortex splitting events, the authors also recalculated all the diagnostics in this section for SSWs that occur in NDJF only.