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Article

Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic

by
Dingzhu Hu
* and
Zhaoyong Guan
Key Laboratory of Meteorological Disasters of China Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environment Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China
*
Author to whom correspondence should be addressed.
Remote Sens. 2022, 14(14), 3447; https://doi.org/10.3390/rs14143447
Submission received: 5 June 2022 / Revised: 4 July 2022 / Accepted: 14 July 2022 / Published: 18 July 2022
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Abstract

:
Using a stratosphere-resolving general circulation model, the relative effects of stratospheric ozone and greenhouse gases (GHGs) increase on the temperature and circulation in the Arctic stratosphere are examined. Results show that stratospheric ozone or GHGs increase alone could result in a cooling and strengthening extratropical stratosphere during February, March and April. However, the contribution of stratospheric ozone increases alone on the cooling and strengthening Arctic stratosphere is approximately 2 fold that of the GHGs increase alone. Model simulations suggested that the larger responses of the Arctic stratosphere to the ozone increase alone are closely related to the wave fluxes in the stratosphere, rather than the wave activity in the stratosphere. In response to the ozone increase, the vertical propagation of planetary waves from the troposphere into the mid-latitude stratosphere weakens, mainly contributed by its wavenumber-1 component. The impeded planetary waves tend to result from the larger zonal wind shear and vertical gradient of the buoyancy frequency. The magnitudes of anomalies in the zonal wind shear and buoyancy frequency in response to GHGs increase alone are smaller than in response to the ozone increase, which is in accordance with the larger contribution of stratospheric ozone to the temperature and circulation in the Arctic stratosphere.

1. Introduction

Stratospheric temperature and circulation play a critical role in stratosphere–troposphere coupling [1,2]. In recent decades, a growing number of studies proved that the weather and climate in the troposphere, even at the surface, can be significantly influenced by anomalies in the stratosphere [3,4,5,6,7,8]. Previous studies suggested that circulation anomalies in the stratosphere can influence Arctic Oscillations [3,9], storm tracks [5], the North Atlantic [10,11,12,13], tropopause [14], tropospheric jet [15,16], etc. Due to the importance of stratospheric temperature and circulation, a better understanding of the variability in temperature and circulation in the stratosphere could provide more information to improve the skills of extended-range forecasting and climate prediction in the troposphere [3,17].
The warming in the troposphere and cooling in the stratosphere during the past 50 years have been widely established [18]. Climate model simulations showed that the cooling in the global average lower stratosphere is largely attributed to anthropogenic factors including stratospheric ozone depletion and well-mixed greenhouse gases (GHGs) increases [18,19]. Increased GHGs could lead to the cooling in the stratosphere via increasing the long-wave cooling rate [20,21]. In addition to the directly radiative effects, the temperature and circulation in the stratosphere could also be influenced by GHGs increases via indirect radiation–chemical–dynamic feedback [1,22,23]. Some previous studies revealed that there are warming anomalies in the polar stratosphere accompanied with an accelerated Brewer–Dobson circulation caused by increased GHGs and the corresponding sea surface temperature (SST) anomalies via enhancing the planetary wave propagation [1,2,22,23,24]. Ozone loss was suggested to not only result in a cooler stratosphere by decreasing the in situ short-wave heating rate [25], but also play a dominant role in cooling the northern stratosphere via a positive dynamic feedback mechanism [26,27,28]. Therefore, the feedback and mechanisms of the GHGs increases, stratospheric ozone depletion, and temperature and circulation in the stratosphere are complex, and it is of great interest and importance to characterize the influence of well-mixed GHGs increases along with changes in the stratospheric ozone on the temperature and circulation in the stratosphere, especially in the Arctic stratosphere.
Ozone depletion was widely accepted to be the dominant cause behind the reported stratospheric temperature trends in the Southern Hemisphere (SH), as reviewed by Solomon [29]. Since Son et al. [15] found the substantial alteration of the circulation in the SH resulted from the appearance of the ozone hole in year 2008, the influence of stratospheric ozone depletion on the climate anomalies in the SH is under active discussion. For example, Antarctic ozone depletion was found to result in a substantial poleward shift of the midlatitude jet in the SH summer [30], a broadening of the Hadley cell [31] and a poleward extension of the subtropical dry zones [32], the increase in the summer precipitation in the subtropics of SH [33], a warmed surface and reduced sea ice extent [34]. However, these studies focused on the importance of Antarctic ozone depletion in modulating the tropospheric circulation in the SH. It is noted that because of the signature and implementation of the Montreal Protocol and its Amendments, the globally averaged total ozone column in recent decades exhibited an increasing trend after the decreasing trend from the late 1970s to late 1990s [18,29,35].
Some studies also discussed the effects of stratospheric ozone on the temperature and circulation in the Northern Hemisphere (NH). The modeling groups from the Coupled Model Intercomparison Project (CMIP) were used to understand the observed cooling in the tropical lower stratosphere [36]. However, a mix of other forcings, including black carbon, volcanic aerosols, and dust., in addition to ozone changes in the CMIP models made the results difficult to understand, especially unable to draw clear cause-and-effect relationships. Some studies tried to isolate the effects of stratospheric ozone [37]; however, they used the “fixed dynamic heating” radiative-convective model. Subsequently, Polvani and Solomon [38] explored the effect of ozone depletion on temperature trends in the tropical lower stratosphere using an atmospheric general circulation model and suggested that the response of tropical lower stratospheric temperature to different forcings including ozone, GHGs, and SSTs is highly additive. At high latitudes, if considering the radiative effects of stratospheric ozone recovery alone, there might be warming in the stratosphere in the NH caused by the recovery of ozone, which is opposite to the effects of well-mixed GHGs increase. Part of stratospheric cooling trend at high latitude in the NH during late winter and early spring can be attributed to the radiative response of the atmosphere to increasing abundance of well-mixed GHGs and decreasing abundance of stratospheric ozone [26,27,28]. However, using numerical simulations, Hu et al. [39] suggested that the stratospheric ozone recovery results in a cooler Arctic vortex in the stratosphere dynamically induced by the planetary wave activity modulation, which is different from that in the SH. Therefore, under the condition of continued increase in the GHGs and the ozone recovery, the influences of stratospheric ozone recovery alone or along with the corresponding well-mixed GHGs increase on the temperature and circulation in the high-latitude stratosphere in the NH remains unclear, and the relative importance of stratospheric ozone increase versus well-mixed GHGs increase effects on the temperature and circulation over the Arctic in the stratosphere is still uncertain.
To address the above questions, on the basis of Hu et al. [39], we further evaluated the relative contribution of stratospheric ozone and well-mixed GHGs increases on the Arctic stratosphere by using a high-top general circulation model. The underlying dynamic mechanism is also elucidated through time-slice experiments. Our results showed that the ozone increases result in a cooling Arctic stratosphere by modifying the wave fluxes in the stratosphere. This is consistent with the study by Hu et al. [39] that showed that the effects of stratospheric ozone recovery on the temperature in the Arctic stratosphere are mainly controlled by the dynamic cooling. However, there are some novelties between our paper and that of Hu et al. [39]. We found that cooling and strengthening in the stratosphere over the Arctic caused by the stratospheric ozone increase are larger than that caused by the GHGs increase, and the magnitudes of anomalies in the temperature and zonal winds in the lower stratosphere over the Arctic contributed by the stratospheric ozone recovery are approximately 2-fold that by GHGs increase. This paper is organized as follows: the model and setup of the experiments are described in Section 2. Temperature and circulation responses in the northern stratosphere to the well-mixed GHGs increases and stratospheric ozone recovery and the possible dynamic mechanisms are presented in Section 3. Discussion and conclusions are given in Section 4 and Section 5, respectively.

2. Materials and Methods

2.1. Model

We used the Whole Atmosphere Community Climate Model, version 3 (WACCM3), adopting a horizontal resolution of 1.9° × 2.5° (latitude × longitude) and 66 vertical levels from the ground to 5.96 × 10−6 hPa (~145 km geometric altitude) in this study, to setup numerical experiments. WACCM3 is based on the software framework of the Community Atmospheric Model (CAM) [40] and it performs stratospheric processes well relative to observations and to other general circulation models [41,42]. More details of this numerical model can be found in Garcia et al. [43].

2.2. Numerical Simulations

Four 52-year-long time-slice integrations excluding heterogeneous chemistry are designed in this study. According to the configurations, the four experiments are denoted as REF, O3_Inc, GHGs_Inc, and GHGs + O3, respectively. The run O3_Inc denotes the sensitive experiment that changed ozone forcings compared to the reference run (denoted as REF), but the run GHGs_Inc denotes the sensitive experiment that only changed the well-mixed GHGs (including CO2, CH4, N2O, CFC-11, and CFC-12) values compared to the run REF. The detailed configurations of ozone, well-mixed GHGs (including CO2, CH4, N2O, CFC-11, and CFC-12), and SST forcings adopted in these four experiments are listed in Table 1. The specific values of well-mixed GHG used in the simulations, listed in Table 1, are adopted from the values in IPCC Fourth Assessment Report (AR4) B1 scenarios [18]. The SST and sea ice (hereafter denoted SSTs) used in all the runs are generated from an AR4 integration of the Community Climate System Model, version 3, which has been used in previous studies [44]. Ozone forcing used in this study is derived from the WACCM reference simulation performed for Chemistry-Climate Model Validation Activity 2 [45].
In REF, ozone forcing is adopted from the monthly mean climatologies in the period 1998–2002, the well-mixed GHGs are fixed in the 2000 values, and the SSTs adopted the monthly mean climatologies in the period 1995–2004 to avoid picking up El Niño-Southern Oscillation signals. Compared to run REF, run O3_Inc only changes ozone forcing, which is taken from the monthly mean climatologies in the period 2018–2022. Differences in the initial ozone fields between 2018–2022 (2020s, used in run O3_Inc) and 1998–2002 (2000s, used in run REF) are shown in Figure 1. The differences in the seasonal distribution and vertical extents of initial ozone field averaged over the polar cap 65°–90°N between 2018–2022 and 1998–2002 show that the NH ozone maximum in the Arctic stratosphere peaks in February, March and April (FMA) above 100 hPa (Figure 1a,b). Compared to run REF, the recovery signals of FMA-averaged ozone in the stratosphere over the Arctic exist in run O3_Inc (Figure 1c), with the largest recovery 15% in the lower- and mid-stratospheric Arctic (Figure 1d). Note that there are decreased ozone in the tropical lower stratosphere between runs O3_Inc and REF, which is possibly caused by the stronger tropical upwelling that brings more ozone poor air from the troposphere into the tropical stratosphere [23,41].
The run GHGs_Inc is identical to the reference integration REF in all the respects, except that it uses the year 2020 GHG values and monthly mean climatological SSTs in the period 2015–2024. The fourth run GHGs + O3 adopts the configurations of GHGs, SSTs and ozone forcing at year 2020 levels described in Table 1. By analyzing the differences between runs O3_Inc and GHGs_Inc with REF, respectively, we attempt to diagnose the effects of ozone increases in the stratosphere or GHGs increases alone and try to compare the relative effects of these two metrics on the temperature and circulation in the NH. While the fourth run GHGs + O3 was performed to evaluate the linearity of the response.

2.3. Methods

To visualize the propagation of planetary waves in the latitude-height plane, the Eliassen–Palm (EP) flux vector F → = ( F φ , F z ) and its divergence ∇∙F following Andrews et al. [46] is used in this paper.
We also used the quasi-geostrophic refractive index (RI) to understand the planetary wave propagation [47,48]. According to Chen and Robinson [48],
R I = q φ ¯ u ¯ − ( k a c o s φ ) 2 − ( f 2 N H ) 2
where the meridional gradient of the zonal-mean potential vorticity q φ ¯ is
q φ ¯ = 2 Ω c o s φ a − 1 a 2 [ ( u ¯ c o s φ ) φ c o s φ ] φ − f 2 ρ 0
Here, u z is the vertical shear of zonal wind, k is the zonal wavenumber, and N 2 is the buoyancy frequency. a ,   φ ,   f ,   Ω and ρ 0 are the radius of Earth, latitude, Coriolis parameter, Earth’s angular frequency, and background density of the atmosphere, respectively.

3. Results

3.1. Relative Contributions of Ozone and GHGs Increase

Differences in the temperature averaged over FMA among runs O3_Inc, GHGs_Inc, GHGs + O3, and REF are shown in Figure 2. In response to the ozone increase in the stratosphere, there is a statistically insignificant warming in the troposphere, significant cooling in the lower stratosphere but warming in the tropical mid-stratosphere, the largest cooling locates in the lower stratosphere over the Arctic (Figure 2a). As expected, temperature responses in the tropical stratosphere to ozone increase alone are in phase with the differences in the initial ozone fields between O3_Inc and REF (Figure 1c). It is interesting that ozone increase in the stratosphere leads to a statistically significant cooling in the lower stratosphere over the Arctic (Figure 2a), with the largest cooling there by about −1.6 K during FMA. Recall that ozone increase in the stratosphere has a direct radiative effect and indirect dynamic effects on the stratosphere. The directly radiative effects tend to result in a warmer Arctic stratosphere, but the dynamic feedback might cause a cooler Arctic stratosphere via reducing the planetary wave activity. Our result here is consistent with Hu et al. [39], which showed a cooling in the stratospheric Arctic vortex during boreal winter related to the ozone increase and suggested that the indirect dynamic contribution of ozone increase is larger than that of directly radiative effects.
Temperature in the NH responses to well-mixed GHGs increase are different from those to ozone increase. GHGs increase leads to a significant warming in the troposphere and a cooling in the stratosphere (Figure 2b), consistent with previous studies [1,2]. The maximum cooling over the Arctic in the lower stratosphere caused by the well-mixed GHGs increase is about −0.8 K (Figure 2b), which is nearly half of the cooling there caused by the ozone increase alone (Figure 2a). Differences in the temperature between different runs in different seasons suggested that the larger contribution of ozone on the stratospheric temperature over the Arctic depends on the seasons, only occur in boreal winter and spring (figure not shown). The combined effects of ozone and GHGs increases on the temperature in the NH (Figure 2c) are similar to that of GHGs increase alone, but with larger cooling in the lower stratosphere over the Arctic. To check the linearity of the response, Figure 2d shows the differences between Figure 2b,c. The spatial pattern of the temperature differences is similar to that in response to the ozone increase in the stratosphere alone (Figure 2a), indicating that the temperature responses of our model are approximately linear, except in the polar troposphere and mid-stratosphere.
Following the thermal wind relationship, changes in the temperature correspond to changes in the zonal winds. Figure 3 shows the differences in the zonal winds in the NH between runs O3_Inc and REF, GHGs_Inc and REF, and GHGs + O3 and REF. The zonal wind exhibits the statistically significant positive anomalies over the Arctic in the stratosphere between runs O3_Inc and REF, along with an insignificant weakened subtropical westerly jet (Figure 3a). In response to the GHGs increase, there are positive anomalies in the zonal winds in the stratosphere over the Arctic and in the upper troposphere at mid latitudes, indicating the strengthened zonal winds over the Arctic in the stratosphere and strengthened subtropical westerly jet in the troposphere (Figure 3b). This is consistent with previous studies [1,22]. The spatial distribution of zonal wind differences between GHGs + O3 and REF (Figure 3c) is similar to that between GHGs_Inc and REF. It is noted that the magnitudes in zonal wind differences related to the GHGs and ozone increase together are larger than those related to the GHGs increase only, suggesting that the effects of ozone increase in the stratosphere on the circulation cannot be neglected. This can also be obtained in Figure 3d, showing the zonal wind differences between Figure 3b,c. Overall, anomalous zonal winds over the Arctic in the stratosphere to ozone and GHGs increase alone are in accordance with those of temperature, with the larger cooling and strengthening over the Arctic caused by ozone increase alone but smaller cooling and strengthening caused by GHGs increase alone.
To better quantify the relative contributions of ozone and GHGs increase on the temperature and zonal winds over the Arctic, Figure 4 further gives the FMA mean differences in the temperature and zonal winds averaged over 65°–90°N among O3_Inc, GHGs_Inc, GHGs + O3 and REF, respectively. Apparently, in response to the ozone or well-mixed GHGs increase alone, both cooling and strengthening occur in the lower stratosphere over the Arctic. The magnitudes of the cooling and strengthening in response to the ozone increase are larger than those to GHGs increase, e.g., the temperature anomalies averaged over 50–200 hPa related to ozone or GHGs increase alone are −1.18 K and −0.58 K, respectively, whereas the corresponding zonal wind anomalies are 1.25 m s−1 and 0.51 m s−1, respectively. Therefore, the contribution of ozone increase in the stratosphere in our model to the temperature and zonal winds in the lower stratosphere over the Arctic is approximately 2 fold that of GHGs increase, consistent with the results in Figure 2 and Figure 3.

3.2. Possible Dynamic Mechanisms

3.2.1. Wave Activity Responses

Changes in the temperature and circulation in the Arctic stratosphere are closely related to the wave activity in the stratosphere [49,50,51], and the predominant waves propagated from the troposphere into the stratosphere are wavenumber-1 and -2 [52]. Figure 5 shows the differences in the zonal deviation of geopotential height averaged over 45°–75°N and its wavenumber-1 (WN1), and wavenumber-2 (WN2) components between O3_Inc and REF, and GHGs_Inc and REF in FMA, and the climatologies in REF are superimposed. Anomalies in the zonal deviation of geopotential height averaged over 45°–75°N caused by either ozone increase (Figure 5a) or GHGs increase (Figure 5d) are mostly in-phase with its climatologies. The WN1 components induced by the ozone increase (Figure 5b) or GHGs increase (Figure 5e) alone are also in-phase with their climatologies, but the WN2 components are nearly out-of-phase with their climatologies (Figure 5c,f). This implies that the ozone or GHGs increase alone could lead to strengthened wave activity, which accompanies with strengthened WN1 waves but weakened WN2 waves. The total wave activity in response to the ozone or GHGs increase exhibits a wavenumber-1 pattern (Figure 5a,d), similar to their WN1 components (Figure 5b,e). This further implies that the dominant contribution of ozone and GHGs increase to the total wave activity is from the WN1 components. It is worth pointing out that the wave activity responses in the stratosphere are not consistent with the temperature and zonal wind responses in the extratropical stratosphere shown in Figure 2, Figure 3 and Figure 4. There might be other processes in controlling anomalies in the temperature and circulation in the stratosphere.

3.2.2. Anomalies in the Wave Fluxes

To understand anomalies in the temperature and circulation in the extratropical stratosphere, we further examine the response of wave fluxes to ozone and GHGs increase. Figure 6 shows the differences in the EP flux divergence at 75°N between O3_Inc and REF, and GHGs_Inc and REF. EP flux divergence both in response to ozone and GHGs increase alone exhibits statistically significant positive anomalies above 100 hPa, with the larger anomalies caused from the former (the black line in Figure 6a). This is consistent with the temperature and zonal wind responses (Figure 1 and Figure 2). From this, we conclude that the larger contribution of ozone increase to the Arctic temperature and zonal winds in the lower stratosphere is closely related to the wave fluxes anomalies in the stratosphere.
Recall that the dominant contribution to the total wave activity in response to ozone or GHGs increase is from their WN1 components; therefore, the WN1 and WN2 components of EP flux divergence anomalies are further shown in Figure 6b,c, respectively. It is interesting that anomalies in the WN1 component of EP flux divergence related to ozone or GHGs increase are comparable, which are statistically significant in the lower stratosphere (Figure 6b). However, anomalies in the WN2 component of EP flux divergence caused by ozone increase alone are larger than those by GHGs increase alone, along with the statistically significant positive anomalies in the WN2 component of EP flux divergence in the stratosphere in response to ozone increase (Figure 6c). By looking at Figure 5 and Figure 6 together, it may conclude that the larger cooling and strengthening over the Arctic in the stratosphere related to ozone increase in our simulation is closely related to the stronger wave-mean flow interactions, rather than the wave activity in the stratosphere.
The EP flux responses (vectors) in different experiments are shown in Figure 7. Ozone increases in the stratosphere tend to weaken the upward propagation of planetary waves in the northern extratropical stratosphere (Figure 7a), with statistically significant downward anomalous EP flux at mid latitude in the stratosphere. This is consistent with the results in Hu et al. [39], they showed more details about the weakened upward propagation of planetary waves over the Arctic in the stratosphere caused by the stratospheric ozone recovery. There is a slightly increased upward propagation of planetary waves caused by the well-mixed GHGs increase in the polar stratosphere, accompanied with a statistically significant decrease in the wave fluxes at mid latitude in the stratosphere (Figure 7b). By a comparison between Figure 7a,b, it is easy to find that the magnitude of the decreased wave fluxes in the stratosphere related to the ozone increase is larger than that to the GHGs increase, which is consistent with the larger contribution of ozone increase on the temperature and circulation in the extratropical stratosphere. Ozone increases result in larger decreased upward EP flux at mid latitude in the stratosphere (Figure 7a), which coincides with the larger negative temperature anomalies (Figure 2) and larger positive zonal wind anomalies (Figure 3) in the extratropical lower stratosphere. This suggested that the ozone increase might suppress the EP flux convergences in the extratropical lower and middle stratosphere and accordingly strengthen the circulation in the stratospheric over the Arctic.
In order to better understand the wave propagation changes, Figure 8a shows the FMA mean differences in the total eddy heat flux averaged over 70°–90°N among O3_Inc, GHGs_Inc, GHGs + O3 and REF, because of the good representation of the eddy heat flux to wave energy propagating into the stratosphere [27]. Total eddy heat flux above 200 hPa exhibits negative anomalies in response to ozone increase alone (black line in Figure 8a) but positive anomalies in response to the GHGs increase alone (grey line in Figure 8a). This indicates that ozone increases tend to result in weakened upward propagation of planetary waves into the stratosphere but GHGs increase could strength the wave propagation. Similar to the differences in the total eddy heat flux, negative anomalies in the WN1 component of eddy heat flux related to ozone increase alone can be observed, but well-mixed GHGs increase results in the positive anomalies in the WN1 (Figure 8b). Whereas the WN2 component of eddy heat flux in the lower stratosphere exhibits negative and positive anomalies under the condition of ozone or GHGs increase alone, respectively (Figure 8c). By a careful comparison of Figure 8a–c, the differences in the total eddy heat flux caused by both the ozone and GHGs increase alone are mainly contributed by its WN1 component.
Overall, the cooling and strengthening in the Arctic stratosphere related to ozone increase in the stratosphere shown in Figure 2, Figure 3 and Figure 4 tend to be caused by the weakened vertically propagating WN1 waves from the troposphere into the mid-latitude stratosphere. Contributions from the ozone increase in the stratosphere alone is larger than that from the well-mixed GHGs increase alone.

3.2.3. Wave Propagation Responses

A question arises as to which processes lead to anomalies in the wave fluxes in the stratosphere. Previous studies suggested that the wave fluxes in the stratosphere are closely related to the condition of wave propagation around the tropopause [23,47]. Increases in CO2 and the corresponding SSTs can weaken the polar vortex via changing the propagation properties of waves [22,23]. Some studies suggested that the stratospheric ozone also has important effects on the stratospheric Arctic vortex via modulating the wave propagation around the tropopause [39]. Therefore, the wave propagation conditions in the NH around the tropopause in response to ozone or GHGs increases need further examine.
In theory, for a given wavenumber k , the second term of RI shown in Equation (1), ( k a c o s φ ) 2 , is fixed. Change of RI’s third term, ( f 2 N H ) 2 , is insignificant than the first term, q φ ¯ u ¯ . Previous studies revealed that changes of the RI at mid and high latitudes are mainly explained by the change in its meridional gradient of the zonal-mean potential vorticity q φ ¯ [47,53]. The vertical structure of the zonal wind and buoyancy frequency is the key components of the meridional gradient of potential vorticity q φ ¯ , thus they are the key factors to influence the wave propagation in the atmosphere [47,54].
Figure 9 shows the differences in the vertical shear of the zonal wind and the buoyancy frequency square (N2) between O3_Inc and REF, and GHGs_Inc and REF. Both ozone and GHGs increase alone result in the increased zonal wind shear at high latitude in the stratosphere, accompanied by the decreased N2 over the Arctic around the tropopause. Hence, the planetary waves are more likely to propagate into the stratosphere under these conditions. Note that anomalies in the zonal wind shear and the vertical gradient of N2 caused by the ozone increase alone are much larger than that by the GHGs increase alone. Previous studies suggested that the smaller vertical shear of zonal wind and vertical gradient of N2 tend to enhance the probability of wave propagation [23,55,56]. Therefore, the larger zonal wind shear and vertical gradient of N2 related to the ozone increase alone might impede the planetary wave propagation, which is consistent with the weakened propagation of waves into the stratosphere (Figure 7 and Figure 8). The smaller zonal wind shear anomalies caused by the GHGs increase alone are in accordance with the stronger propagation of planetary wave, facilitating smaller cooling and strengthening in the Arctic stratosphere.

4. Discussion

The Arctic stratosphere plays an important role in the coupling between the stratosphere and troposphere. To better understand the impacts of the Arctic stratosphere on the climate or weather in the troposphere, and even at the surface, firstly, variabilities of the temperature and circulation in the stratosphere over the Arctic should be investigated clearly. Previous studies revealed that the temperature in the stratosphere is largely controlled by anthropogenic factors including stratospheric ozone depletion and well-mixed greenhouse gases (GHGs) increases [18,19]. Both stratospheric ozone depletion and well-mixed GHGs increase lead to a cooling in the stratosphere via the direct radiation effects [20,21] and indirect radiation–chemical–dynamic feedback [1,22,23]. However, numerical studies indicated that the stratospheric ozone will recover in the 21st century with the decreased ODSs under the impact of the Montreal Protocol and its amendments [57]. There might be warming in the northern stratosphere related to the stratospheric ozone increases in the view of directly radiation effects. This is opposite to the cooling effects of well-mixed GHGs increase. Thus, under the condition of the ozone concentration in the stratosphere increases and GHGs increases in the 21st century, it is still unclear how the temperature and circulation in the Arctic stratosphere will change. To address this question clearly, the relative effects of the ozone and GHGs increase alone should be evaluated. Using a high-top general circulation model, the effects of both ozone and well-mixed GHGs increase on the temperature and circulation in the Arctic stratosphere and the possible mechanisms were investigated. Interestingly, both the ozone increases alone and GHGs increase alone result in cooling and strengthening in the Arctic stratosphere during boreal spring, with the larger contribution of the ozone increases.
Some studies revealed that the impacts of stratospheric ozone loss on the cooling in the lower stratosphere are dominantly controlled by a positive dynamic feedback mechanism [26,27,28]. However, in the middle and upper stratosphere, ozone loss tends to result in the weakened polar vortex along with an increase in stratospheric temperature and wave fluxes via possible negative dynamic feedback [44,58]. Furthermore, our study suggested that the ozone increases affecting the temperature in the Arctic stratosphere are also mainly by the dynamic processes, and the ozone increases result in a cooling Arctic stratosphere, which is not opposite to the effects of the ozone loss. Specifically, our results showed that the ozone increases result in a cooling Arctic stratosphere by modifying the wave fluxes in the stratosphere. This is consistent with the study by Hu et al. [39] that showed that the effects of stratospheric ozone recovery on the temperature in the Arctic stratosphere is mainly controlled by the dynamic cooling. On the basis of Hu et al. [39], we further evaluated the relative contribution of stratospheric ozone and GHGs increases. Results showed that the temperature and zonal winds in the lower stratosphere over the Arctic in response to the stratospheric ozone increases is approximately 2 fold that of with GHGs increase. It is worth to pointing out that these results are dependent on seasons.
One may wonder whether the temperature in the Arctic stratosphere will decrease under the future climate. Firstly, according to our results, the larger contribution of ozone increases on the temperature over the Arctic in the stratosphere depends on different seasons. In the future climate, whether the temperature in the stratosphere over the Arctic will decrease might also depend on seasons. Secondly, several recent studies suggested that there is a decreasing trend in the ozone in the NH after the 2000s [59,60,61,62,63]. However, numerical results suggested that the upper stratospheric ozone will recover due to de decreased ODSs from the 1990s [35,57,64,65]. Therefore, the trends in the ozone concentrations in different levels of stratosphere and in different regions are inconsistent, and the trends in the Arctic stratosphere after the 2000s are still under debate. Thirdly, in addition to the stratospheric ozone and GHGs, there are many other factors to affect the temperature and circulation in the northern stratosphere, such as QBO [66], solar cycle [67], SSTs [23,68,69,70], and polar vortex [71]. To examine the changes in the temperature over the Arctic in the stratosphere in future, the contributions from different factors should be considered. In the future, it is necessary to examine the combined effects and relative contributions of different factors on the temperature and circulation in the northern stratosphere. Therefore, we think whether the temperature over the Arctic in the stratosphere will decrease or not is still an open question.

5. Conclusions

Our results show that both ozone and GHGs increase alone tend to result in negative anomalies in the temperature and positive anomalies in the zonal wind in the extratropical stratosphere in boreal spring, with the larger cooling and strengthening in the stratosphere over the Arctic caused by the ozone increase but smaller cooling and strengthening caused by the GHGs increase. The contribution of ozone increase in our model to the temperature and zonal winds in the lower stratosphere over the Arctic is approximately 2 fold that of GHGs increase.
Further analysis suggested that the larger cooling and strengthening in the Arctic stratosphere in response to the ozone increase are closely related to the anomalous wave fluxes in the stratosphere, rather than the wave activity in the stratosphere. Ozone increase-related vertical propagation of planetary waves from the troposphere into the mid-latitude stratosphere exhibits weakened anomalies, much larger than that caused by the GHGs increase. The weakened anomalous upward propagation of planetary waves is mainly contributed by the WN1 component. Compared to the GHGs increase, the ozone increase tends to result in larger zonal wind shear and vertical gradient of the buoyancy frequency, which might impede the planetary wave propagation. The smaller anomalous zonal wind shear caused by the GHGs increase compared to that caused by the ozone increase are in accordance with the stronger propagation of planetary wave, facilitating smaller cooling and strengthening in the Arctic stratosphere.

Author Contributions

Conceptualization, D.H.; methodology, D.H.; software, D.H.; validation, D.H. and Z.G.; formal analysis, D.H.; investigation, D.H.; resources, D.H.; data curation, D.H.; writing—original draft preparation, D.H.; writing—review and editing, Z.G.; visualization, D.H.; project administration, D.H. and Z.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China, grant number 42175072 and 41975073.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are grateful to W. Tian for the constructive comments and suggestions. We would thank NCAR for providing the WACCM3 model and the SPARC of the World Climate Research Project for providing the CCMVal-2 datasets.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Differences in the seasonal distribution of (a) vertical extents of initial ozone field averaged over 65°–90°N, and (b) horizontal extents of initial ozone field averaged over 10–100 hPa between 2018–2022 (2020s) and 1998–2002 (2000s). (c) Differences and (d) percentage differences in the FMA-averaged initial ozone fields between 2020s and 2000s.
Figure 1. Differences in the seasonal distribution of (a) vertical extents of initial ozone field averaged over 65°–90°N, and (b) horizontal extents of initial ozone field averaged over 10–100 hPa between 2018–2022 (2020s) and 1998–2002 (2000s). (c) Differences and (d) percentage differences in the FMA-averaged initial ozone fields between 2020s and 2000s.
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Figure 2. Differences (shading) in the temperature in FMA between runs (a) O3_Inc and REF, (b) GHGs_Inc and REF, and (c) GHGs + O3 and REF. (d) The differences between (c) and (b). The line contours represent the FMA mean of temperature in REF and the contour interval is 5 K. Stippled regions indicate the differences at/above the 90% confidence levels.
Figure 2. Differences (shading) in the temperature in FMA between runs (a) O3_Inc and REF, (b) GHGs_Inc and REF, and (c) GHGs + O3 and REF. (d) The differences between (c) and (b). The line contours represent the FMA mean of temperature in REF and the contour interval is 5 K. Stippled regions indicate the differences at/above the 90% confidence levels.
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Figure 3. Same as Figure 2, but for the differences in the zonal winds (shading) in FMA between runs (a) O3_Inc and REF, (b) GHGs_Inc and REF, (c) GHGs + O3 and REF, and (d) the differences between (c) and (b). The line contours represent the FMA mean of zonal winds in REF and the contour interval is 5 m s–1. Stippled regions indicate the differences at/above the 90% confidence levels.
Figure 3. Same as Figure 2, but for the differences in the zonal winds (shading) in FMA between runs (a) O3_Inc and REF, (b) GHGs_Inc and REF, (c) GHGs + O3 and REF, and (d) the differences between (c) and (b). The line contours represent the FMA mean of zonal winds in REF and the contour interval is 5 m s–1. Stippled regions indicate the differences at/above the 90% confidence levels.
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Figure 4. FMA mean differences in the (a) temperature and (b) zonal winds averaged over 65°–90°N between O3_Inc and REF (red lines), GHGs_Inc and REF (blue lines), and GHGs + O3 and REF (black lines).
Figure 4. FMA mean differences in the (a) temperature and (b) zonal winds averaged over 65°–90°N between O3_Inc and REF (red lines), GHGs_Inc and REF (blue lines), and GHGs + O3 and REF (black lines).
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Figure 5. Differences (line contours) in the zonal deviation of (a,d) geopotential height averaged over 65°–90°N in FMA, its (b,e) wavenumber-1, and (c,f) wavenumber-2 components between (a–c) O3_Inc and REF, and (d–f) GHGs_Inc and REF. The shadings represent the climatologies in REF. Stippled regions indicate the differences at/above the 90% confidence levels.
Figure 5. Differences (line contours) in the zonal deviation of (a,d) geopotential height averaged over 65°–90°N in FMA, its (b,e) wavenumber-1, and (c,f) wavenumber-2 components between (a–c) O3_Inc and REF, and (d–f) GHGs_Inc and REF. The shadings represent the climatologies in REF. Stippled regions indicate the differences at/above the 90% confidence levels.
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Figure 6. FMA mean differences in the (a) total EP flux divergence, its (b) WN1, and (c) WN2 components at 75°N between O3_Inc and REF (black lines), and GHGs_Inc and REF (grey lines). Circles and triangles indicate the differences at/above the 90% confidence levels.
Figure 6. FMA mean differences in the (a) total EP flux divergence, its (b) WN1, and (c) WN2 components at 75°N between O3_Inc and REF (black lines), and GHGs_Inc and REF (grey lines). Circles and triangles indicate the differences at/above the 90% confidence levels.
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Figure 7. Differences in the EP flux vectors (arrows, units: 104 kg s−2 for vertical vectors and 106 kg s−2 for horizontal vectors; only the EP flux vectors larger than 0.15 are shown) and its divergence (shading) in FMA between runs (a) O3_Inc and REF, and (b) GHGs_Inc and REF. The values in the dotted areas are significant at/above the 90% confidence level of the t-test.
Figure 7. Differences in the EP flux vectors (arrows, units: 104 kg s−2 for vertical vectors and 106 kg s−2 for horizontal vectors; only the EP flux vectors larger than 0.15 are shown) and its divergence (shading) in FMA between runs (a) O3_Inc and REF, and (b) GHGs_Inc and REF. The values in the dotted areas are significant at/above the 90% confidence level of the t-test.
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Figure 8. FMA differences in the (a) total eddy heat flux (units: K m s−1), its (b) WN1, and (c) WN2 components averaged over 70°–90°N between O3_Inc and REF (black lines), and GHGs_Inc and REF (grey lines).
Figure 8. FMA differences in the (a) total eddy heat flux (units: K m s−1), its (b) WN1, and (c) WN2 components averaged over 70°–90°N between O3_Inc and REF (black lines), and GHGs_Inc and REF (grey lines).
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Figure 9. Differences (shading) in the (a,b) zonal wind shear (unit: 10−4 s−1) and (c,d) N2 (unit: 10−5 s−1) in FMA between O3_Inc and REF (left panels), and GHGs_Inc and REF (right panels). The solid and dashed line contours represent the positive and negative FMA-mean zonal wind shear (unit: 10−2 s−1) and N2 (unit: 10−5 s−1) in REF run, respectively. Stippled regions indicate the differences at/above the 90% confidence levels.
Figure 9. Differences (shading) in the (a,b) zonal wind shear (unit: 10−4 s−1) and (c,d) N2 (unit: 10−5 s−1) in FMA between O3_Inc and REF (left panels), and GHGs_Inc and REF (right panels). The solid and dashed line contours represent the positive and negative FMA-mean zonal wind shear (unit: 10−2 s−1) and N2 (unit: 10−5 s−1) in REF run, respectively. Stippled regions indicate the differences at/above the 90% confidence levels.
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Table
Table 1. Configurations of ozone, GHGs (including CO2, CH4, N2O, CFC-11, and CFC-12), and SST fields adopted in the four experiments.
Table 1. Configurations of ozone, GHGs (including CO2, CH4, N2O, CFC-11, and CFC-12), and SST fields adopted in the four experiments.
ExperimentsOzoneCO2 (ppmv)CH4 (ppbv)N2O (ppbv)CFC-11 (pptv)CFC-12 (pptv)SSTs
REF1998–200237117573162675351995–2004
O3_Inc2018–202237117573162675351995–2004
GHGs_Inc1998–200241633619302144862015–2024
GHGs + O32018–202241633619302144862015–2024
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Hu, D.; Guan, Z. Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic. Remote Sens. 2022, 14, 3447. https://doi.org/10.3390/rs14143447

AMA Style

Hu D, Guan Z. Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic. Remote Sensing. 2022; 14(14):3447. https://doi.org/10.3390/rs14143447

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Hu, Dingzhu, and Zhaoyong Guan. 2022. "Relative Effects of the Greenhouse Gases and Stratospheric Ozone Increases on Temperature and Circulation in the Stratosphere over the Arctic" Remote Sensing 14, no. 14: 3447. https://doi.org/10.3390/rs14143447

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