The impact of greenhouse gas and ozone forcing on the Southern Hemisphere climate system

doi:10.21203/rs.3.rs-1585709/v1

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The impact of greenhouse gas and ozone forcing on the Southern Hemisphere climate system

https://doi.org/10.21203/rs.3.rs-1585709/v1

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Anthropogenic climate change in the Southern Hemisphere is driven by two forces, the greenhouse gas emissions (CO₂) and the stratospheric ozone levels. In the past, the combination of increasing greenhouse gas emissions and ozone depletion over Antarctica worked together leading to an increase in sea surface temperatures and a poleward shift of the storm tracks. With the ozone expected to recover by mid-century, however, the greenhouse gas and ozone forces will oppose each other, and the changes observed previously will begin to weaken or reverse. The role the greenhouse gases and the ozone recovery play in the Southern Hemisphere climate system is examined using Community Climate System Model, version 4 coupled ocean eddy-parameterized and eddy-resolving simulations. The greenhouse gas emissions and ozone levels are specified independently to represent two extremes, peak greenhouse gas emissions and a recovered ozone. In the eddy-parameterized simulations, the ozone recovery signal is found to be stronger on average. In the case of the eddy-resolving simulations, however, the increase in greenhouse gases is stronger especially in eddy-rich regions like western boundary current regions and the Antarctic Circumpolar Current, as the eddy-resolving simulations are able to resolve a more accurate eddy field in these highly active regions.

Greenhouse gases

stratospheric ozone

Southern Hemisphere climate system

CCSM4

ocean eddy-resolving model

As greenhouse gas (GHG) emissions continue to increase, how they interact with the ozone is of great importance in the Southern Hemisphere climate system. Previous studies have shown that the combination of increased GHG emissions and ozone depletion work together to accelerate the changes seen in the Southern Hemisphere. With the increased GHG emissions and ozone recovery, however, these two forces oppose each other and the stronger of the two forces determines the changes observed. This paper seeks to answer two questions. The first question is what forcing has a greater impact on the Southern Hemisphere climate system, increased greenhouse gas emissions or the ozone recovery? This question is especially important in areas where the most warming is absorbed, such as western boundary current regions. These regions are known to be poorly resolved in low-resolution climate models and thus motivates the second question. The second question this paper answers is how will these changes in the Southern Hemisphere differ once ocean mesoscale processes are included in the climate projections?

The depletion of the stratospheric ozone in the late 20th century due to anthropogenic climate change has induced changes in the Southern Hemisphere climate and atmospheric circulation (Fig. 1a). Ozone depletion causes the lower stratosphere to cool and the troposphere to warm, leading to a rise in the tropopause height (Lorenz and DeWeaver 2007). This warming and cooling leads to an intensification and a poleward shift in the westerly jet (Fyfe et al. 1999; Kushner et al. 2001; Cai et al. 2003; Marshall et al. 2004; Shindell and Schmidt, 2004, Arblaster and Meehl 2006; Kan et al. 2011; Rivière 2011), a poleward expansion in the edge of the Hadley circulation (Hu and Fu 2007; Lu et al. 2007; Johanson and Fu 2009: Hu et al. 2011: Stachnik and Schumacher 2011; Davis and Rosenlof 2012), and a poleward extension of the subtropical dry zones and shift of the storm tracks with more precipitation at higher latitudes (Gillett and Thompson 2003; Yin 2005; Previdi and Liepert 2007; Archer and Caldeira 2008; Kang et al. 2011; Lucas et al. 2014). The depletion of the stratospheric ozone has implications for the ocean as well with a change in the strength of the wind driven oceanic circulation (Russell et al. 2006) and an increase in sea surface temperatures (SSTs) and sea ice melting (Thompson and Solomon 2002; Turner et al. 2009a,b; Sigmond and Fyfe 2010; Bitz and Polvani 2012; Smith et al. 2012). The primary driver behind these observed changes in the Southern Hemisphere is the ozone depletion in combination with the increase in GHGs as these two forces work together to influence Southern Hemisphere climate change (Kushner et al. 2001; Cai et al. 2003; Gillett and Thompson 2003: Rauthe et al. 2004; Shindell and Schmidt 2004; Fyfe et al. 2007; Fogt et al., 2009; Son et al. 2010; Polvani et al. 2011a,b; Thompson et al. 2011).

Future anthropogenic climate change in the Southern Hemisphere is expected to be controlled by the ozone recovery and the continued increase in GHGs (Arblaster et al. 2011). The ozone is expected to recover within the next 50 years as a result of the implementation of the Montreal Protocol (Eyring et al. 2007; Son et al. 2009). Signed in 1987, the Montreal Protocol has phased out the production and consumption of ozone depleting substances, such as industrial chlorofluorocarbons, and has resulted in the recovery of the ozone (Solomon et al. 2016). The recovery of the ozone hole has significant implications for the Southern Hemisphere climate as the ozone and GHGs will no longer constructively interfere but rather, the two forces will begin to oppose each other. The ozone recovery will work to weaken or reverse the effects on the Southern Hemisphere circulation caused by ozone depletion and the increase in GHGs over the past few decades (Fig. 1b; Shindell and Schmidt 2004; Perlwitz et al. 2008; Son et al. 2008, 2009, 2010). Therefore, the poleward trends seen previously are expected to weaken and, pending future emissions, reverse and shift equatorward (Polvani et al. 2011b). How these two opposing forces, ozone recovery and increase in GHGs, will go on to affect the future of the Southern Hemisphere climate after the ozone recovers will rely heavily on future GHG emissions.

Previous studies have also tried to determine which of the two forces will dominate moving into the 21st century. Polvani et al. (2011a) asked a similar question using an atmosphere only model, however, and found that the recovery of the ozone cancels or reverses the atmospheric circulation changes associated with increased GHGs in the Southern Hemisphere summer. Similar results were found using simulations from chemistry-climate models and CMIP5 (Waugh et al. 2009; Wang et al. 2014) in that the ozone recovery will reverse or weaken the changes seen in the atmosphere over the first half of the 21st century. CMIP5 models, however, prescribe the changes to GHG and ozone levels simultaneously rather than independently, and therefore, identifying which forcing is more dominant is difficult.

Additionally, these previous studies were done using ocean eddy-parameterizing models rather than eddy-resolving models, thus missing important physical processes. Eddy-resolving models are important for the ocean as they are able to resolve the mesoscale field and how it interacts with the overall circulation. Previous studies have shown that mesoscale features, such as the Agulhas Current and retroflection, are better resolved in high resolution simulations (Cheng et al. 2016). These mesoscale features influence the high impact weather events and large-scale climate patterns seen in the atmosphere and the presence of eddies regulates the heat, salt, and volume transport observed in the ocean, especially in eddy-rich regions like western boundary currents. Resolving these regions is crucial as it has been shown previously that western boundary currents and their extensions are the hotspots of ocean warming, especially over the last few decades where they have been warming at a rate 3–4 times the global average (Wu et al. 2012; Oliver and Holbrook 2014; Goyal et al. 2021).

To improve on the results of previous studies, this study uses an eddy-resolving coupled-climate model, rather than atmospheric or chemistry-climate models, in order to better resolve the ocean dynamics. The ocean response to the ozone recovery in an eddy-resolving model has not been analyzed in previous studies and is the primary focus of this paper. Although atmosphere and chemistry-climate models extend higher in the atmosphere and resolve stratospheric processes better, they are forced by sea surface temperatures from CMIP models as opposed to being coupled to an ocean model (Arblaster et al. 2011). As mentioned previously, the CMIP5 models have coarse resolution in the ocean and atmosphere and do not extend as high in the atmosphere like the chemistry-climate models do, therefore affecting how the troposphere responds to external forcing (Son et al. 2008; Shaw et al. 2009). Resolving both stratospheric chemistry and ocean circulation has proven to be computationally expensive and has limited climate modeling in recent years.

The role the increased greenhouse gases and the ozone recovery play in the Southern Hemisphere climate system is examined using Community Climate System Model, version 4 (CCSM4) coupled ocean eddy-parameterizing and eddy-resolving simulations. The GHGs and ozone levels are specified independently to represent the two extremes, peak greenhouse gas emissions and a recovered ozone. This is the first time that changes in the Southern Hemisphere climate, with an emphasis on the ocean, as a response to greenhouse gas and ozone forcing are examined using an eddy-resolving model. This paper compares the individual responses to increased greenhouse gas emissions versus ozone recovery and the changes seen in eddy-parameterizing simulations versus eddy-resolving simulations.

Model

This study analyzes model output from NCAR’s CCSM4 coupled-climate model (Gent et al. 2011). The ocean and atmosphere models used in CCSM4 are Parallel Ocean Program, version 2 (POP2) and Community Atmosphere Model, version 4 (CAM4), respectively. POP2 has 60 vertical layers with 10 m layer thickness in the first 100 m and slowly increasing to 250 m at 6000 m depth. CAM4 has 26 vertical layers in the atmosphere. The stratospheric and tropospheric ozone is calculated semi-offline using the interactive chemistry in CAM-Chem, the chemistry version of CAM (Lamarque et al. 2010, 2011, 2012; Eyring et al. 2013).

The six model simulations listed in Table 1 are the three different climate scenarios considered with eddy-parameterizing, low-resolution (1° in the atmosphere and ocean) and eddy-resolving, high-resolution (1/2° in the atmosphere and 1/10° in the ocean) output available for each scenario. The first scenario (L/HRC07) is the 20th century climate change simulation with corresponding CO₂ levels applied (Kirtman et al. 2012; Fig. 2a, blue line). The ozone in this experiment is kept constant at year 2000 levels, a time representative of a depleted ozone (Fig. 2b, red line). There are 100 years of data available in the eddy-parameterizing simulation and 70 years available in the eddy-resolving simulation. The next experiment (L/HRC08), the control experiment, also keeps year 2000 ozone levels (Fig. 2b, red line) but differs in that the CO₂ forcing is kept constant at year 2000 levels (379 ppm, Fig. 2a, red line). Like L/HRC07, 100 years of data is available in the eddy-parameterizing simulation and 70 years is available in the eddy-resolving simulation. The last experiment (L/HRC20) uses constant year 2000 CO₂ forcing (Fig. 2a, red line) like L/HRC08 but will set the ozone to levels from year 1955, representative of the recovered ozone expected by the mid-century (Fig. 2b, blue line). The eddy-parameterizing simulation also has 100 years of data; however, the eddy-resolving simulation only has 18 years of data available as it is new and still ongoing. Monthly mean outputs are considered in this study.

Table 1

List of the six CCSM4 experiments used in this study. Low-resolution is equal to 1° in both the atmosphere and ocean, high-resolution is 1/2° in the atmosphere and 1/10° in the ocean
Name	Resolution	Emission scenario	Ozone	Years
LRC07	Low	Historical CO₂	Year 2000 O₃	100 years
LRC08	Low	Year 2000 CO₂	Year 2000 O₃	100 years
LRC20	Low	Year 2000 CO₂	Year 1955 O₃	100 years
HRC07	High	Historical CO₂	Year 2000 O₃	70 years
HRC08	High	Year 2000 CO₂	Year 2000 O₃	70 years
HRC20	High	Year 2000 CO₂	Year 1955 O₃	18 years

The difference between the scenarios is used to extract the greenhouse gas and ozone responses. To get the results from increasing the greenhouse gases (L/HRC_GHG), the low emissions cooling scenario (L/HRC07) is subtracted from the high emissions warming scenario (L/HRC08). Since the ozone levels are the same between these two scenarios and the CO₂ levels are the only parameter changed, any difference observed is attributed to the increased CO₂ emissions. The same can be done for the ozone by subtracting the ozone depletion scenario (L/HRC08) from the ozone recovery scenario (L/HRC20) to get the ozone recovery signal (L/HRC_O3). The O₃ levels are the only parameter changed between these two scenarios while the CO₂ levels remain the same. Lastly, to find the net change (L/HRC_Total) and which forcing is more dominant, the sum of the two differences is taken.

Spatial maps

Using the results from L/HRC_GHG, L/HRC_O3, and L/HRC_Total, spatial maps are constructed for sea surface temperature, sea surface height, eddy kinetic energy, convective precipitation, surface temperature, zonal mean temperature, and zonal mean zonal wind. Because the change in GHG and ozone forcing is not equal, the results are normalized so that they are comparable between scenarios. L/HRC_GHG and L/HRC_O3 are then divided by the difference in the global mean surface temperature for their respective scenarios.

For the GHG forcing, change per degree of warming:

$$L/HR{C}_{GHG}= \frac{L/HRC08-L/HRC07}{|\stackrel{-}{T{S}_{L/HRC08}}-\stackrel{-}{T{S}_{L/HRC07}}|}$$

For the O₃ forcing, changer per degree of cooling:

$$L/HR{C}_{O3}= \frac{L/HRC20-L/HRC08}{|\stackrel{-}{T{S}_{L/HRC20}}-\stackrel{-}{T{S}_{L/HRC08}}|}$$

The sum of these normalized maps is then used to get the net change:

$$L/HR{C}_{Total}=L/HR{C}_{GHG}+L/HR{C}_{O3}$$

The following figures (Figs. 3–10) will be used to show the differences between the eddy-parameterized and eddy-resolved simulations as well as the differences between the increased greenhouse gas emissions and ozone recovery. The first row shows the results from the eddy-parameterized experiments (LRC) and the second row shows the results from the eddy-resolved experiments (HRC). The first, second, and third column show results from L/HRC_GHG, L/HRC_O3, and L/HRC_Total, respectively.

Sea surface temperature

Beginning with the sea surface temperature for LRC_GHG (Fig. 3a), there is roughly one-degree warming everywhere except in the Southern Ocean where there is intense warming in the ACC of nearly three-degrees. With the increase in GHGs, there is warming everywhere in the Southern Hemisphere oceans except for one small region in the South Pacific. The same but opposite pattern is seen in the LRC_O3 scenario (Fig. 3b) with cooling seen throughout all the oceans and warming in that same small region in the South Pacific. The overall net change between the two scenarios is small (LRC_Total, Fig. 3c), indicating that the increased GHGs and ozone recovery largely cancel each other with respect to SST. In the Indian and Pacific Oceans, there is slight warming observed, demonstrating that the GHG forcing is stronger here, whereas there is cooling seen everywhere else showing that the ozone recovery signal is larger, especially over the ACC which is no longer as exposed to solar radiation with the recovery of the ozone layer.

In the eddy-resolving case, the results are similar to the eddy-parameterizing case with a few exceptions in each scenario. For HRC_GHG (Fig. 3d), there is warming nearly everywhere, like LRC_GHG, including over the ACC where the presence of ocean eddies leads to a strong increase in SSTs. This warming is seen in the eddy-rich regions like the Agulhas retroflection and the Brazil-Malvinas Confluence Zone. Unlike LRC_GHG, however, there is cooling in the lower ACC near Antarctica. The HRC_O3 (Fig. 3e) scenario shows equal but opposite results with cooling everywhere except near Antarctica as well. In contrast from the previous results, there is not a strong cooling seen over the ACC as in LRC_O3 nor a strong signal from the eddies as in HRC_GHG. This weaker signal is likely due to the increased heat trapped within the eddies in these regions weakening the intense cooling expected and seen in LRC_O3. Lastly, the overall net change seen in HRC_Total (Fig. 3f) is mostly dominated by the ozone recovery with cooling seen in most of the Southern Hemisphere. The eddy-rich regions identified previously undergo warming and cooling, highlighting the importance of both GHGs and ozone, however, overall, the increase in GHGs and the heat trapped within the eddies plays a larger role.

Sea surface height

Next, the results from the analysis of the sea surface height (SSH) are shown (Fig. 4). With an increase in GHGs, there is an increase in SSH seen everywhere except over the southern portion of the ACC and most of the South Atlantic in LRC_GHG (Fig. 4a). The areas where there is an increase (decrease) in SSH are areas where there will be an increase (decrease) in sea level as a result of anthropogenic climate change. The opposite pattern is then seen in the LRC_O3 scenario (Fig. 4b) with an increase in sea level expected over the southern ACC and South Atlantic as the ozone recovers. The net change is once again weak (Fig. 4c), but the pattern resembles that of the LRC_O3 case suggesting that the ozone recovery forcing is slightly more important in the low-resolution eddy-parameterizing model.

The patterns seen in both HRC_GHG (Fig. 4d) and HRC_O3 (Fig. 4e) closely resemble the patterns seen in their LRC equivalent with the exception of the eddies. In HRC_GHG, the magnitude of change is comparable to LRC_GHG and the eddies in the Agulhas retroflection and Brazil-Malvinas Confluence Zone show an increase in SSH. In HRC_O3, the magnitude is much smaller than what is seen in both HRC_GHG and LRC_O3 and the eddies are not associated with much change, similar to what was seen in the SST maps. Therefore, the SSH pattern seen in HRC_Total (Fig. 4f) closely resembles HRC_GHG unlike the LRC_Total which is weak overall and slightly resembles LRC_O3 poleward of 60°S demonstrating that the increased GHG emissions are more dominant in the high-resolution eddy-resolving model.

Eddy kinetic energy

The eddy kinetic energy (EKE = 0.5(u'²+v'²)) is calculated for each scenario and across the LRC scenarios, there is a weak EKE signal since it is not an eddy-resolving model (Fig. 5a-c). There is a small signal in the equatorial region with opposite values of high and low EKE in the LRC_GHG (Fig. 5a) and LRC_O3 (Fig. 5b) scenarios. The pattern from LRC_Total (Fig. 5c), although small, more closely resembles the pattern from LRC_O3, again suggesting that the ozone recovery forcing is slightly stronger than the increased GHG forcing in the eddy-parameterized model, agreeing with the previous SSH figure (Fig. 4).

The presence of eddies is very evident in the HRC EKE maps (Fig. 5d-f) as the changes in EKE are seen throughout the Southern Hemisphere in both HRC_GHG and HRC_O3 with the HRC_GHG map being more active. In HRC_GHG (Fig. 5d) there is an increase in the ACC, the open Pacific Ocean, along the coast of Africa, through the Mozambique Channel, and the Agulhas retroflection associated with an increase in GHG emissions. There is a decrease in EKE in the equatorial region, near the EAC and Brazil-Malvinas Confluence Zone, and parts of the Agulhas retroflection. The values of EKE are not as high in HRC_O3 (Fig. 5e). There is an increase in the equatorial Pacific and the subtropical Pacific and Indian Oceans, and a decrease seen in the majority of the ACC. It is clear that between the two forces, the increase in GHG is more dominant. The pattern seen in HRC_Total (Fig. 5f) is strikingly similar to HRC_GHG. With the increase in GHG emissions, there is an intensification of the winds and an increase in windstress creating a more energetic and excited eddy-field with as a result of eddy saturation (Meredith et al. 2012; Morrison and McC. Hogg 2013; Marshall et al. 2014).

Zonal mean ocean temperature

The zonal mean ocean temperature is calculated at each latitude throughout the Southern Hemisphere (Fig. 6). With the increase in GHGs (Fig. 6a), there is an increase in the ocean temperature observed throughout the entire vertical structure of the ocean with the exception of one small region of cooling found near 35°S at 1000 m. The greatest warming in this scenario is found poleward of 40°S with strong increases at the surface and 2000 m. Once again, a similar but opposite pattern is seen in LRC_O3 (Fig. 6b) with cooling observed nearly everywhere. The overall change in LRC_Total (Fig. 6c) shows that the ozone recovery is the stronger of the two forces. In the LRC scenarios (Fig. 6a-c), the impact of increased GHGs and ozone recovery are seen at depth suggesting a strong mixing component in the eddy-parameterizing simulations as the changes are nearly uniform throughout the water column, especially towards Antarctica.

The HRC maps show a very different response than the LRC maps however, with an overall weaker, less uniform response. In HRC_GHG (Fig. 6d), the strongest increase in temperature is found at the surface equatorward of 50°S. In addition to the surface, there are strong increases in temperature found down to the intermediate depths near Antarctica, 50°S, and near the equator, all locations of increased EKE (Fig. 5d) with the eddies from the ACC, western boundary currents, and equatorial currents likely influencing the increase in temperature observed. In HRC_O3 (Fig. 6e), there is cooling observed nearly everywhere with the strongest decrease in temperature found in the upper 1000 m and little change seen at deeper depths. The decrease in temperature found at 50°S is located at the same latitude where there is a strong zonal mean decrease in EKE (Fig. 5e). HRC_Total (Fig. 6f) shows the ozone recovery signal is only stronger in the upper 500 m equatorward of 50°S, and that the increase in GHGs play a larger role elsewhere, especially at the intermediate depths in eddy-rich regions. The HRC maps also show that the decrease (increase) in SST observed near Antarctica (Fig. 3a,b) with the increased GHGs (ozone recovery) occurs only at the surface in the upper 200 m and there is an increase (decrease) found at the depth directly below.

Convective precipitation

The convective precipitation is analyzed rather than the total precipitation due to its stronger signal and its relationship with SST. In LRC_GHG (Fig. 7a), there is an increase in convective precipitation over the equatorial regions, especially the Pacific, over all of Australia, southern South America, eastern South Africa, and over the western boundary current regions. With the increase in greenhouse gas emissions, there is warming in the ocean and release of latent heat to the atmosphere and therefore, an increase in precipitation. There is a decrease in convective precipitation over northern and central South America and the rest of Africa. Once again, the same but opposite pattern is seen in LRC_O3 (Fig. 7b). The overall net change seen in LRC_Total (Fig. 7c) is very small, however, the pattern is similar to the one seen for the ozone recovery forcing, with the exception of Australia, that shows a total increase in precipitation as in the increased GHG scenario.

The results from the eddy-resolving scenarios vary from the eddy-parameterizing scenarios. HRC_GHG (Fig. 7d) shows an increase over eastern and southern Africa, tropical Indian Ocean, Pacific islands, the equatorial Pacific Ocean, and western South America. There is a decrease seen over parts of Africa, Australia, just south of the equatorial Pacific, and eastern South America. The HRC_O3 (Fig. 7c) convective precipitation pattern, surprisingly resembles the LRC_GHG scenario (Fig. 7a), especially with the same precipitation found over the Pacific. There is a decrease in convective precipitation seen across all Southern Hemisphere land as the ozone recovers in the high-resolution case. The GHG forcing dominates everywhere in HRC_Total (Fig. 7f) except over land like HRC_O3 suggesting that over the open ocean the GHG forcing is more important as the ocean absorbs more CO₂ and over land, the ozone forcing is more important. Additionally, HRC_Total differs from LRC_Total in that over land there is a decrease in convective precipitation found compared to an increase in the eddy-parameterizing simulation.

Surface temperature

The surface temperature (2 m temperature) results do not differ much from the SST results seen previously. In LRC_GHG (Fig. 8a), there is strong warming everywhere in the Southern Hemisphere, especially over the ACC. Like the SST, there is one small region of cooling found in the South Pacific and a region in South America. The same but opposite pattern is seen in LRC_O3 (Fig. 8b) with cooling everywhere other than the tiny patches seen in the South Pacific and South America that are warming. The net change in LRC_Total (Fig. 8c) is weak once again but more closely resembles the ozone forcing aside from the precipitation over South Africa and South America which have increasing surface temperatures and are responding to the increase in GHGs. Australia, Chile, and eastern South Africa have a cooling pattern as in the ozone case, likely because these are desert regions and undergo extreme cooling.

The HRC maps show similar results to the LRC maps. HRC_GHG (Fig. 8d) has warming nearly everywhere other than below 60°S where there is some strong cooling over Antarctica possibly related to changes in the sea ice. There is a strong increase in surface temperature over land and in the eddy-rich regions as in the LRC case. For HRC_O3 (Fig. 8e) there is cooling everywhere in the Southern Hemisphere excluding the area around Antarctica which shows warming. The intense cooling seen over the ACC does not exist in the eddy-resolving case as in the SST case. Finally, the HRC_Total (Fig. 8f) is dominated by the ozone recovery everywhere other than the eddy-rich regions and below 60°S that show the increase in GHG emissions is more important. Similar to the SSTs, the heat trapped within the eddies is overpowering the cooling seen from the ozone signal and showing a strong net warming change overall.

Zonal mean atmospheric temperature

The next two figures (Fig. 9 and Fig. 10), motivated by Polvani et al. (2011a), examine the response to the zonal mean atmospheric temperature and zonal mean zonal wind. The zonal mean temperature throughout the atmosphere is calculated at each latitude (Fig. 9). In LRC_GHG (Fig. 9a), there is warming seen throughout the Southern Hemisphere except in the lower stratosphere poleward of 50°S, where the ozone is depleted. The strongest changes in temperature are seen at 100 mb with an increase (decrease) of over two-degrees equatorward (poleward) of 50°S. The LRC_O3 (Fig. 9b) map shows a similar but opposite pattern to the LRC_GHG map, with a stronger signal seen in the lower stratosphere poleward of 50°S. Warming of much greater than two-degrees is seen in this ozone recovery scenario. The recovery of the ozone above Antarctica means most of the incoming solar radiation will be absorbed by the lower stratosphere in this region and there will be less heating throughout the troposphere. This intense warming is present in LRC_Total (Fig. 9c) suggesting that the ozone recovery is the dominant force in the lower stratosphere. The rest of the atmosphere has little to zero temperature change with the two forces cancelling each other out.

HRC_GHG (Fig. 9d), shows a similar response to LRC_GHG with cooling in the lower stratosphere but this cooling extends beyond the high-latitudes and up to the equator in the top 100 mb. There is also slight cooling seen at the surface in the high-latitudes corresponding to the cooling seen in the surface temperature map (Fig. 8d). HRC_O3 (Fig. 9e), also shows warming in the lower stratosphere at the high-latitudes but it is not as strong as in LRC_O3. There is, however, strong cooling of more than two-degrees seen equatorward of 55°S between 200 and 600 mb that is not present in LRC_O3. The overall net change seen in HRC_Total (Fig. 9f) is slightly different from LRC_Total as there is only a small region of the lower stratosphere warming at the high latitudes and a cooling associated everywhere else. The upper 100 mb looks to be controlled by the increase in greenhouse gases whereas the rest of the Southern Hemisphere shows a response similar to the ozone recovery scenario.

Zonal mean zonal wind

The same calculation for the temperature is done for the zonal wind (Fig. 10) and the patterns seen in the LRC scenarios (Fig. 10a-c) are all relatively weak. In LRC_GHG (Fig. 10a), there is an increase in the zonal wind found in the upper 200 mb at the mid-latitudes and a slight increase in the westerlies and towards the equator. There is a decrease seen at the trade wind latitudes. The opposite is shown for the LRC_O3 (Fig. 10b) scenario with a slight decrease in the westerlies, but between the two scenarios there seems to be no poleward or equatorward shift. The responses in both scenarios are nearly equal with LRC_Total (Fig. 10c) showing very little change with the exception of the upper 100 mb resembling the LRC_O3 scenario.

The HRC simulations (Fig. 10d-f) show much more change than their LRC equivalents. The pattern seen in HRC_GHG (Fig. 10d) is similar to LRC_GHG but much stronger. There is a strong increase in the westerlies and upper troposphere equatorward of 55°S. There is a strong weakening of the westerly jet and upper troposphere seen in HRC_O3 (Fig. 10e). In the HRC simulations, the position of the westerly jet shifts equatorward in the HRC_O3 scenario compared to the HRC_GHG scenario, signifying the recovery of the ozone and the reversal of the westerly jet latitude poleward shift observed in the HRC_GHG scenario. HRC_Total (Fig. 10f) shows that equatorward of 50°S the increase in GHG emissions dominates and poleward of 50°S the ozone recovery is more important, and the westerly jet is weakening and shifting equatorward.

Comparing the zonal mean temperature and zonal mean zonal wind results (Fig. 9 and Fig. 10) to those in Fig. 1 from Polvani et al. (2011a), who use an atmospheric model (CAM3, 2.8° ⋅ 2.8° horizontal resolution), the two are in agreement. With an increase in GHG emissions, Polvani et al. (2011a) find a decrease in the temperature at the high-latitudes in the lower stratosphere that extends to the equator in the upper 100 mb and an increase in temperature everywhere else in the Southern Hemisphere, a result consistent with HRC_GHG (Fig. 9d). In their ozone recovery simulation, they find intense warming in the lower stratosphere more closely resembling LRC_O3 (Fig. 9b) than HRC_O3. The net change results from Polvani et al. (2011a) continue to show an increase in this region at the high-latitudes with cooling at the very top of the atmosphere, features that are both seen LRC_Total and HRC_Total (Fig. 9c,f). The results from the zonal mean zonal wind calculation in Polvani et al. (2011a) agree with the HRC simulations shown previously (Fig. 10d-f). The result of increasing GHGs is an increase and poleward shift of the westerlies that can also be seen in HRC_GHG (Fig. 10d). The more intense weakening and equatorward shift seen with the ozone recovery is found in HRC_O3 (Fig. 10e). And the overall trend is a equatorward shift and weakening of the westerlies, especially in the upper 200 mb consistent with what is found in HRC_Total (Fig. 10f).

This paper, using CCSM4, addresses two questions, what external forcing plays a greater role in the Southern Hemisphere climate system, increased greenhouse gas emissions or ozone recovery. And how does the Southern Hemisphere climate change in a low-resolution eddy-parameterizing model versus a high-resolution eddy-resolving model? Using six simulations, L/HRC07 (20th century climate change forcing), L/HRC08 (constant 2000 CO₂ and ozone levels), and L/HRC20 (ozone recovery) the impact the GHGs and ozone have on the Southern Hemisphere in an eddy-parameterizing and eddy-resolving model is studied in both the atmosphere and ocean.

For the sea surface temperatures in the eddy-parameterized simulations, the increase in GHGs and ozone recovery are found to be nearly equal but opposite of each other. In the case of the eddy-resolving simulations, the ozone recovery (cooling) dominates throughout the Southern Hemisphere with the exception of eddy-rich regions like the ACC, where the increase in GHGs is stronger (warming). The presence of eddies in these regions allow for more heat to be trapped and therefore, the signal is much stronger. In the eddy-parameterizing simulation, the ozone recovery plays a slightly larger role for the sea surface height, eddy kinetic energy, zonal mean ocean temperature, and convective precipitation. For the eddy-resolving simulation, however, the increase in GHGs dominates with the HRC_Total spatial maps closely resembling the increased GHG scenario (HRC_GHG). The surface temperature response in the eddy-parameterizing simulations is that over Africa and South America, the increase in GHGs is stronger (warming), but over Australia and the Southern Ocean, the ozone recovery is more important (cooling). In the eddy-resolving simulation, the ozone is stronger everywhere (cooling), including over land, except in eddy-rich regions and the Southern Ocean near Antarctica. The response of the surface temperature over Australia is unique in that Australia is seen to be cooling as a result of the ozone recovery in both simulations, despite Africa and South America warming because of the increased GHGs emissions in the eddy-parameterizing simulations. For the zonal mean temperature, the ozone recovery forcing dominates the lower stratosphere at the high-latitudes in the eddy-parameterizing simulations with almost no net change seen elsewhere. In the eddy-resolving simulations, however, the ozone forcing is strong throughout the Southern Hemisphere whereas both the GHGs and ozone affect the high-latitudes lower stratosphere. The increased GHGs and ozone recovery are nearly equal and opposite for the zonal mean zonal wind with no shift of the westerlies observed in the low-resolution eddy-parameterizing simulation. In the high-resolution eddy-resolving experiment, the increased GHGs dominate equatorward of 40°S and the ozone recovery poleward of 40°S with the westerly jet shifting equatorward overall. Both the external forcing (increased greenhouse gas emissions versus ozone recovery) and model resolution (eddy-parameterized versus eddy-resolving) are important in the spatial maps discussed previously.

The greenhouse gas emissions and stratospheric ozone levels are two of the key drivers in the Southern Hemisphere climate. Understanding how these two forces influence the climate is important for predicting future climate and preparing for climate risks. If the two forces continue to work together towards the most drastic climate scenario, more severe weather events can be expected. The increase in sea surface temperatures, sea level rise, and changes in precipitation patterns will lead to more droughts and flooding, affecting regional and local communities globally. If the ozone recovers and the Paris Agreement of 1.5°C warming is upheld, then it can be expected the climate will stabilize beginning by the mid-century. With the recovery of the ozone layer over Antarctica and net zero carbon emissions, there will be cooler temperatures and less sea ice melting, providing a very different future. Overall, predicting the future climate in the Southern Hemisphere proves to be challenging as it is highly dependent on many parameters such as the external forcing and model resolution as this paper shows.

Acknowledgments

The authors acknowledge the support from NOAA (NA18OAR4310293, NA15OAR4320064, NA20OAR4320472), NSF (OCE1419569, OCE1559151), and DOE (DE-SC0019433).

Funding

This study has been funded by NOAA (NA18OAR4310293, NA15OAR4320064, NA20OAR4320472), NSF (OCE1419569, OCE1559151), and DOE (DE-SC0019433).

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Houraa Daher and Ben P. Kirtman contributed to the study conception and design. Model simulations were run by both authors. Material preparation, data collection and analysis were performed by Houraa Daher. The first draft of the manuscript was written by Houraa Daher and both authors commented on previous versions of the manuscript. Both authors read and approved the final manuscript.

Data Availability

Data available upon request.

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The impact of greenhouse gas and ozone forcing on the Southern Hemisphere climate system

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Abstract

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Introduction

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Results

Sea surface temperature

Sea surface height

Eddy kinetic energy

Zonal mean ocean temperature

Convective precipitation

Surface temperature

Zonal mean atmospheric temperature

Zonal mean zonal wind

Conclusions

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