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Table 1 lists the four CMIP6 models with daily outputs available for AMIP experiments. The four models are BCC-CSM2-MR, FGOALS-f3-L, FGOALS-g3, and NESM3, with their full name extensions and affiliations listed in the second column of Table 1. BCC-CSM2-MR is developed by the National Climate Center, China Meteorological Administration, and has a moderate horizontal resolution (T106, i.e., 320 × 160 grids, longitude × latitude). FGOALS-f3-L is developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, and has a moderate (although “L” denotes “low resolution”) horizontal resolution (i.e., C96, i.e., 382 × 194 grids, longitude × latitude). FGOALS-g3 is also developed by LASG, but the atmospheric component is different and has a low horizontal resolution (i.e., 180 × 80 grids, longitude × latitude). NESM3 is developed by the Earth System Modeling Center, Nanjing University of Information Science and Technology, and has a low horizontal resolution (i.e., T63, 190 × 95 grids, longitude × latitude). For full details, readers are directed to the model descriptions (Cao et al., 2018; He et al., 2019; Wu et al., 2019; Li et al., 2020). All models have a model top around 1–2 hPa, incorporating the mid-to-lower stratosphere, where SSWs happen. In contrast, BCC-CSM2-MR and NESM3 (46 and 47 levels in total; 18 and 19 levels around 100–10 hPa) have a nicer vertical resolution than FGOALS-f3-L and FGOALS-g3 (32 and 26 levels in total; 8 and 7 levels around 100–10 hPa).
Model Full name (and affiliation) Ensemble members Resolution (and model top/levels around 100–10 hPa) Total SSWs (and D/S) Reference BCC-CSM2-MR Beijing Climate Center, Climate System Model version two, Medium Resolution (National Climate Center, China Meteorological Administration) 3 T106L46
(1.459 hPa / 18)9 + 13 + 8 (17/13 or 1.31) Wu et al., 2019 FGOALS-f3-L Flexible Global Ocean–Atmosphere–Land System model, Finite-volume version 3, Low Resolution (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences) 3 C96L32
(2.16 hPa / 8)12 + 12 + 7 (11/20 or 0.55) He et al., 2019 FGOALS-g3 Flexible Global Ocean–Atmosphere–Land System model, Grid-point version 3 (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences) 5 180×80L26
(2.194 hPa / 7)7 + 12 + 6 + 12 + 10 (19/28 or 0.68) Li et al., 2020 NESM3 Nanjing University of Information Science and Technology Earth System Model version 3 (Earth System Modeling Center, Nanjing University of Information Science and Technology) 5 T63L47
(1 hPa / 19)43 + 31 + 40 + 39 + 39 (116/76 or 1.53) Cao et al., 2018 Table 1. The four Chinese CMIP6 models used in this study. One of the DECK experiments, AMIP, is commonly available for the four models. The size of the AMIP runs for each model is listed in the third column, and all ensemble members are analyzed in the composite. D/S in the fifth column represents the ratio of the vortex displacement and split SSWs. The CMIP6 AMIP experiments start from 1979 and end in 2014.
Because daily data from AMIP experiments were available for all of the four models at the beginning of this study (October 2019), we use the AMIP outputs. BCC-CSM2-MR and FGOALS-f3-L have three ensemble members, while FGOALS-g3 and NESM3 have five ensemble members (see the third column of Table 1). All the AMIP experiments are forced by the same external forcings, but the initial fields are different. All the ensemble members from the four Chinese CMIP6 models are used in our paper. Considering that the CMIP6 AMIP runs are from 1979–2014, the extracted Japanese 55-year Reanalysis (JRA-55) from 1979–2014 (JRA-55; Kobayashi et al., 2015) is used as a baseline for model evaluations. The SSW events from different reanalyses show little difference, especially during the satellite era since 1979 (Rao et al., 2015; Butler et al., 2017), so only the JRA-55 reanalysis is shown. Variables used in our paper include zonal and meridional winds, heights, and temperatures at pressure levels. Because NESM3 does not provide daily heights, we also calculate Ertel’s potential vorticity (PV) as a substitute for height in the stratosphere.
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There are many SSW definitions in the literature (Butler et al., 2015), which is not the focus of this study. Because the WMO definition is one of the most popular algorithms in the literature and considering that our results can be easily compared with earlier studies (Charlton et al., 2007; Butchart et al., 2011; Charlton-Perez et al., 2013; Hu et al., 2014; Manzini et al., 2014; Rao et al., 2018, 2020a), we still use this WMO SSW identification algorithm. According to the WMO SSW definition, the major SSW onset time is the first day when zonal-mean zonal winds at 10 hPa and 60°N reverse from westerlies to easterlies (Charlton and Polvani, 2007). The eddy heat flux (
$ \overline{v{'}T{'}} $ ) is proportional to the vertical component of the Eliassen–Palm (EP) flux (Fz), as a representation of the upward propagation of planetary waves (Limpasuvan et al., 2004; Polvani et al., 2017; Rao et al., 2018). The daily climatology is the long-term mean of each calendar day, and the daily climatology for each variable is smoothed with a 31-day running mean before being subtracted from the full field to get the anomaly field.A vortex-centric diagnostic procedure developed by Seviour et al. (2013) is used to classify the SSW type. This method is established based on the geometry of the stratospheric polar vortex using the geopotential height or PV at 10 hPa. Two-dimensional vortex moments day by day are calculated in the procedure. Two parameters are required, including the centroid and aspect ratio of the stratospheric polar vortex represented by an equivalent ellipse (Mitchell et al., 2011; Seviour et al., 2016). Time series of the centroid and aspect ratio of the stratospheric polar vortex are calculated using a two-dimensional moment equation. The absolute and relative vortex moments (denoted by Mab and Jab) of the modified PV (or height) field are extracted in the Cartesian coordinate. The latitude of the vortex centroid and the aspect ratio of polar vortex during each SSW event are saved using two-dimensional moment diagnostics and geopotential heights (or PVs) on isobaric levels (Matthewman et al., 2009). Note that the results from geopotential height and PV are highly correlated (Seviour et al., 2013, 2016).
Following Seviour et al. (2013, 2016), an SSW is classified into the vortex split group if the aspect ratio of the vortex is above 2.4 for at least seven days. An SSW is classified into the vortex displacement group if the centroid of the vortex is situated equatorward of 66°N for at least seven days. This threshold-based method has been confirmed to present a similar classification of split and displaced vortices as conventional methods (e.g., Charlton and Polvani, 2007; Mitchell et al., 2011). To show the feasibility of the threshold-based method, examples of vortex displacement and split SSWs are provided in Fig. 1 from JRA-55 and four CMIP6 models. Obviously, for displacement SSWs, the vortex is far biased from the North Pole, resembling a comma-like shape (Figs. 1a–e). In contrast, for split SSWs, the vortex breaks into two comparable pieces in models, as observed in the selected sample from the reanalysis (Figs. 1f–1j). Although the PV (value range: 30–50 PVU, −PV is drawn for an easy comparison with other models) is diagnosed for the vortex parameters in NESM3, the displacement and split are also clearly present as in other models.
Figure 1. Examples of the two types of SSWs for (a, f) JRA-55 on 16 February 1981 and 14 March 1988, (b, g) BCC-CSM2-MR on 11 March 2013 and 11 February 1982, (c, h) FGOALS-f3-L on 24 March 2013 and 10 March 2014, (d, i) FGOALS-g3 on 9 March 2014 and 28 February 1982, and (e, j) NESM3 on 13 February 1981 and 18 February 1980. The left-hand column shows the height or PV at 10 hPa for vortex displacement SSWs, and the right-hand column shows the height or PV at 10 hPa for the vortex split SSWs. All examples in the four Chinese CMIP6 models are selected from the first AMIP run. Note that daily heights are unavailable for NESM3 and Ertel’s PVs is exclusively shown for this model (the PV sign is reversed for an easy comparison with other models; −PV value ranges: [−50, −30] PVU).
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In the JRA-55 reanalysis, 23 SSWs appear during 1979–2014 (~0.64 events per year; Table 2). However, the models (excluding NESM3) tend to underestimate the SSW frequency: 30 events in 108 years for BCC-CSM2-MR (i.e., 36 years from 1979–2014 in three AMIP runs; similar for other models), 31 events in 108 years for FGAOLS-f3-L, 47 events in 180 years for FGAOLS-g3, and 192 events in 180 years for NESM3 (see the fifth column of Table 1). The SSW frequency is 0.28, 0.29, 0.26, and 1.1 events per year for the four models, respectively. Namely, three models underestimate the SSW frequency by half, and NESM3 nearly doubles the observed SSW frequency. To get an overview of SSWs in the four CMIP6 models, the month-by-month distributions of SSWs are shown in Fig. 2. SSWs mainly occur in midwinter (January and February; unfilled bars in Fig. 1) in observations. Obviously, most models simulate a climate drift for SSWs, and more SSWs appear in late winter (February and March), and SSWs in NESM3 are nearly uniformly distributed in most wintertime months except February.
SSW date SSW type 22 Feb 1979 D 29 Feb 1980 D 6 Feb 1981 D 4 Dec 1981 D 1 Jan 1985 S 23 Jan 1987 D 8 Dec 1987 S 14 Mar 1988 S 21 Feb 1989 S 15 Dec 1998 D 26 Feb 1999 S 20 Mar 2000 D 11 Feb 2001 S 31 Dec 2001 D 18 Jan 2003 S 5 Jan 2004 D 21 Jan 2006 D 24 Feb 2007 D 22 Feb 2008 D 24 Jan 2009 S 9 Feb 2010 S 24 Mar 2010 D 7 Jan 2013 S Table 2. Onset dates of SSW events and the corresponding type of the stratospheric polar vortex (D indicates a vortex displacement and S indicates a vortex split) in the JRA-55 reanalysis (1979–2014). The ratio of the vortex displacement and split SSWs is 1.3 (13/10) in JRA-55 during 1979–2014.
Figure 2. Seasonal distribution of the total frequency of SSWs (units: number per year) from November to March for JRA-55 (hatched bars) and CMIP6 models (bars in gray shades).
Seasonal distributions of vortex displacement and vortex split SSWs from November–March are shown in Fig. 3. As seen in Fig. 3a, vortex displacement SSWs are nearly uniformly distributed in December–February, followed by March. This peak in February is successfully simulated in NESM3, although SSWs occur much more frequently in this model than in JRA-55. Consistent with the distribution of SSWs in Fig. 2, all the other three models simulate much fewer displacement SSWs, and SSWs are drifted to late winter (February and/or March).
Figure 3. Seasonal distribution of the frequency (units: events per year) of (a) vortex displacement SSWs and (b) vortex split SSWs in each wintertime month in the JRA-55 reanalysis during 1979–2014 and AMIP runs during 1979–2014 from four Chinese CMIP6 models.
A stronger seasonality of split SSWs than displacement SSWs is observed for JRA-55, comparing the unfilled bars in Figs. 3a and b. More split SSWs appear in midwinter (January–February) in observations, and far fewer are observed in other wintertime months. Such a seasonality of SSWs observed in JRA-55 is drifted one month later to February–March for most models except BCC-CSM2-MR. Such a climate drift can be tracked to the seasonal evolution of the stratospheric polar vortex, which tends to get strongest in February (January) in models (reanalyses) [Fig. 5 in Rao et al. (2015)]. Compared with the three other models, BCC-CSM2-MR is the only one of the four Chinese CMIP6 models that simulates a stratospheric QBO (Rao et al., 2020b, c), which might also affect SSWs.
Figure 5. Composite pressure–time evolution of the zonal mean zonal wind anomalies area-averaged over 55°–75°N (shading; units: m s−1) from day −20 to day 60 relative to the onset date for (a–e) vortex displacement SSWs and (f–j) vortex split SSWs for (top row) the JRA-55 reanalysis during 1979–2014, and (second–last rows) four Chinese CMIP6 models during 1979–2014. The last column (k–o) shows the difference of vortex split minus displacement SSWs in each dataset. Black contours mark the composite zonal wind anomalies/differences at the 95% confidence level according to the Student’s t-test.
In addition to their contrasting seasonal distributions for both types of SSWs in JRA-55, the difference can also be identified for their intensities. We use the warming anomalies in the stratospheric polar cap to denote the intensity of SSWs. The composite strength of SSWs in each month for each type is shown in Fig. 4. To reverse the polar night jet that usually reaches climatological maxima in midwinter (Rao et al., 2015), the polar vortex anomalies are expected to be stronger for midwinter SSWs than events in November and March. This expectation is observed in JRA-55 (< 15 K in November and March versus > 20 K in midwinter) and simulated in almost all models for both displacement and split SSWs. Although the SSW frequency is not satisfactorily simulated by most models, the contrast in strength between displacement and split is simulated by models to different degrees of success. Specifically, on average, the strength of split SSWs is larger than displacement SSWs in JRA-55, which is simulated in some models (especially in BCC-CSM2-MR and NESM3).
Model | Full name (and affiliation) | Ensemble members | Resolution (and model top/levels around 100–10 hPa) | Total SSWs (and D/S) | Reference |
BCC-CSM2-MR | Beijing Climate Center, Climate System Model version two, Medium Resolution (National Climate Center, China Meteorological Administration) | 3 | T106L46 (1.459 hPa / 18) | 9 + 13 + 8 (17/13 or 1.31) | Wu et al., 2019 |
FGOALS-f3-L | Flexible Global Ocean–Atmosphere–Land System model, Finite-volume version 3, Low Resolution (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences) | 3 | C96L32 (2.16 hPa / 8) | 12 + 12 + 7 (11/20 or 0.55) | He et al., 2019 |
FGOALS-g3 | Flexible Global Ocean–Atmosphere–Land System model, Grid-point version 3 (State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences) | 5 | 180×80L26 (2.194 hPa / 7) | 7 + 12 + 6 + 12 + 10 (19/28 or 0.68) | Li et al., 2020 |
NESM3 | Nanjing University of Information Science and Technology Earth System Model version 3 (Earth System Modeling Center, Nanjing University of Information Science and Technology) | 5 | T63L47 (1 hPa / 19) | 43 + 31 + 40 + 39 + 39 (116/76 or 1.53) | Cao et al., 2018 |