1. Introduction
Sea level changes predicted by coupled atmosphere–ocean general circulation models (AOGCMs) participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) show a large spread. Several regions stick out, including the North Atlantic and the Arctic. Large differences exist specifically also in the Southern Ocean.
The Southern Ocean substantially affects both the global ocean circulation and climate. It is also taking up a major fraction of anthropogenic heat, which is stored in a belt around 45°S latitude, causing sea level to rise. The Southern Ocean circulation connects the ocean basins through the zonally unobstructed Antarctic Circumpolar Current (ACC). A key driving force of the ACC is the southern westerlies, which have intensified and shifted poleward as a result of human-caused carbon dioxide emissions (e.g., Saenko et al. 2005; Toggweiler 2009; Swart and Fyfe 2012; Thomas et al. 2015; Deng et al. 2022). This ultimately causes the ACC to shift poleward resulting in sea level change (Bouttes et al. 2012; Yang et al. 2020). Despite the Southern Ocean being a crucial component of the climate system and sea level change, processes leading to changing sea levels are not well understood and represented in current climate models (Fox-Kemper et al. 2021; Meredith et al. 2019).
The most prominent Southern Ocean sea level change is a meridional dipole pattern with an increase in dynamic sea level north of 50°S and a decrease south of 50°S (Bouttes et al. 2012; Couldrey et al. 2021; Yin et al. 2010; Zhang et al. 2022). By analyzing three versions of MPI-ESM sea level projections, Wickramage et al. (2023) demonstrated that all resolutions could produce this meridional dipole pattern. However, the finer details of these sea level changes are different from simulation to simulation, such as the magnitude of sea level change or latitudinal positions of the dipole boundaries (e.g., Figs. 1a,c). They found substantial spread due to different horizontal resolutions for the strong emission scenario Shared Socioeconomic Pathway (SSP)5-8.5 (the SSP5 with the level of radiative forcing 8.5 W m−2 in the year 2100; fossil-fueled development).
(a),(c) DSL change under SSP5-8.5 scenario and (b),(d) DSL change in FAF-sum (Δζsum = Δζheat + Δζstress + Δζwater) for (a),(b) HR and (c),(d) ER. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
To better understand and isolate the impact of changing surface fluxes on sea level in individual models, the Flux-Anomaly Forced Model Intercomparison Project (FAFMIP) experiment (Gregory et al. 2016) was designed as part of CMIP6 (Eyring et al. 2016). In FAFMIP, individual atmospheric forcing components are being perturbed in a prescribed way and consistent with changes inferred from CMIP-type forced experiments. These perturbations for FAFMIP are derived from a consistent set of 13 CMIP5 AOGCMs to allow easy comparison with model mean sea level change (Gregory et al. 2016). For each model, the 1pctCO2 climatological monthly mean of years 61–80 was considered to calculate the changes relative to twenty years of the preindustrial control run (piControl). The piControl is forced by fixed 1850 forcing, which includes greenhouse gases, ozone, and aerosol loadings from the year 1850. Gregory et al. (2016), Todd et al. (2020), Suzuki and Tatebe (2020), Couldrey et al. (2021), and Zhang et al. (2022) used the results to identify the impact of heat, freshwater, and wind stress on sea level change. Couldrey et al. (2021) and Zhang et al. (2022) focused on sea level changes and investigated the mechanisms acting in the ocean as a function of individual forcing agents. In addition, Zhang et al. (2022) identified substantial feedback between individual forcing agents, particularly from heat flux perturbations on the other forcing fields.
Previous studies have proposed mechanisms affecting Southern Ocean sea level change (Bouttes et al. 2012; Frankcombe et al. 2013; Chen et al. 2019). Among those, some have shown how different aspects of the SSP5-8.5 sea level change can be attributed to the changes in surface fluxes of heat, momentum, and freshwater (Bouttes et al. 2012, 2014). Given that numerical details matter, studying the sensitivity of sea level responses to forcing changes using the same model in different configurations can help in identifying causes. An example is shown in Figs. 1b and 1d, showing sea level changes under the same forcing but with different spatial resolutions. See also section 2 for more details. As the Southern Ocean is rich in mesoscale eddy activity with a first-order influence on regional dynamics, the impact of ocean model horizontal resolution is expected to be most visible in this region (e.g., Wickramage et al. 2023).
What might account for these changes in the various resolutions is an intriguing question. Therefore, we analyzed the FAFMIP experiment results for MPI-ESM, high resolution (MPI-ESM-HR), and eddy-rich (ER). The goal is to investigate how sensitive the sea level changes in the Southern Ocean are to changes in heat, wind stress (momentum), and freshwater fluxes at various model resolutions in MPI-ESM. Moreover, the sensitivity of these surface perturbations to the model resolutions may account for the diagnosed model differences in anomalies in the CMIP6 MPI-ESM SSP5-8.5 projections (henceforth, SSP5-8.5) or at least help identifying dynamical mechanisms behind the changes we see in Fig. 1. The manuscript is structured as follows: Section 2 describes the model experiments and methods; in section 3, we analyze sea level change in the Southern Ocean, followed by a summary and concluding remarks in section 4.
2. Materials and methods
a. MPI-ESM model simulations
The basis of our study is to evaluate the impact of eddy-rich ocean resolution relative to lower resolution. To this end, we employ two configurations of the Max Planck Institute for Meteorology Earth System Model (MPI-ESM): one with eddy-parameterized (HR) resolution and another with ER resolution. Both configurations feature an identical atmospheric component, allowing us to isolate the effects attributable to variations in oceanic resolution. The atmospheric component of the model in each case is ECHAM (Mauritsen et al. 2019), with the spectral resolution referring to a T127 truncation, corresponding to a horizontal resolution of roughly 100 km, and the vertical representation manifested by 95 hybrid levels (top level 0.01 hPa).
Most importantly, the ocean resolution is increasing from a nominal horizontal resolution of 0.4° in HR to 0.1° in ER. All models share the same 40 unevenly spaced ocean vertical levels that range from 6 m at the surface to several hundred meters at depth (Jungclaus et al. 2013). The eddy-permitting ocean model used in the HR configuration features a nominal resolution of 44 km on a tripolar grid (TP04; Jungclaus et al. 2013), with a grid size of 20–10 km in the Southern Ocean. In contrast, the ER configuration employs an eddy-resolving 6-min (TP6M) ocean grid (von Storch et al. 2012, 2016), where the grid size scales with latitude, ranging from 5 to 6 km at 60°S to 2–3 km in the Weddell and Ross seas. While the TP6M grid resolves most mesoscale eddies, it does not fully capture those in the Arctic Ocean, Nordic seas, and marginal seas of the subpolar North Atlantic or on continental shelves.
Besides resolution, the use of the thickness diffusivity of the Gent–McWilliams (GM) parameterization (Gent and McWilliams 1990) also changed between runs, which accounts for tracer advection and diffusion caused by unresolved mesoscale eddies in the ocean. The coefficients are constant in time and selected to be proportional to the grid spacing. GM was turned on in HR and turned off in ER although it does not resolve eddies in all areas of the ocean. The mixing formulation based on Redi (1982) was used to parameterize isopycnal diffusivity in all configurations, with the same resolution-dependent coefficient scaled to 1000 m2 s−1 for a grid cell that is 400 km wide. In all three runs, the vertical viscosity and diffusivity are based on the Pacanowski and Philander (PP; Pacanowski and Philander 1981) vertical ocean mixing scheme (Marsland et al. 2003; Gutjahr et al. 2019). In the following, we determine future changes by subtracting the time mean of the historical period (1995–2014) from the SSP5-8.5 scenario (2080–99). Moreover, we used an ensemble of two members for HR and three members for ER (Putrasahan et al. 2024a,b,c).
b. FAFMIP experiments
Several earlier studies have provided detailed descriptions of the FAFMIP experimental setup (see Gregory et al. 2016; Couldrey et al. 2021; Zhang et al. 2022 for more details). Therefore, we only briefly outline the key experiments used in this study.
Surface flux perturbation experiments were carried out based on each MPI-ESM control run by imposing additional perturbations at the air–sea interface. Four experiments were used for the analysis: FAF-passiveheat (except ER), FAF-heat, FAF-stress (momentum), and FAF-water. FAF-passiveheat is the control FAFMIP experiment in which no external forcing is applied, but it includes an additional diagnostic tracer (T-added). The T-added, which is introduced to quantify the passiveheat uptake of the heat flux perturbation, was initialized to zero everywhere and is subject to the imposed surface forcing. Moreover, T-added is advected through the ocean by the unaltered circulation. The FAF-passiveheat experiment is not performed for the ER model, due to a lack of computational resources. The temporal mean of the last two decades (years 51–70) of the FAFMIP experiments was compared to the temporal mean of the last two decades (years 51–70) of FAF-passiveheat for HR, whereas the corresponding 20 years of the 1950-control (control) run were considered for ER. The 1950-control simulation is performed under conditions chosen to be representative of the period of the 1950s. For HR, the control experiments are forced by the fixed 1850 greenhouse gases (GHGs), aerosols, ozone, and solar irradiance (preindustrial conditions).
In FAF-water, the freshwater flux perturbation is derived from the sum of precipitation minus evaporation, river inflow, and water fluxes between floating ice (sea ice and icebergs) and seawater.
FAF-heat forcing is characterized by the robust warming over the Southern Ocean and in the North Atlantic and cooling north of the subtropical front in the Atlantic sector of the Southern Ocean, while the FAF-stress forcing features a strengthening and southward shift of the Southern Ocean westerlies (Fig. S1 in the online supplemental material). The FAF-water forcing reveals a distinctive pattern of “wet get wetter dry get dryer pattern” (Held and Soden 2006).
c. Analysis of experiment results
1) DSL change calculation
Since the contribution from the redistribution of mass between land and ocean is eliminated by the zero global mean condition, bottom pressure changes (ΔζBP = Δζ − ΔζST) in the MPI-ESM projections are entirely caused by mass redistribution within the global ocean.
2) Poleward shift of ACC calculation
3) Stratification calculation
4) Calculations of eddy heat and salt fluxes
The barotropic flux F0 = υ0S0, which carries the mass related to the input of volume by the surface freshwater flux, is removed from the total and mean advective salt fluxes, and the same is done for the heat fluxes by removing the surface heat flux F0 = υ0Tj. To calculate the barotropic transport of salt and heat F0, the mean salinity S0 and mean temperature Tj at each latitude band j were considered, respectively. The term υ0(y) is the velocity averaged over each latitudinal band y of the Southern Ocean.
5) Significance test
3. Sea level change in the Southern Ocean
a. Under the SSP5-8.5 scenario
In all SSP5-8.5 projections, the DSL increases north of the ACC and decreases south of it (Figs. 2a,b). Yet, there are significant discrepancies between the spatial patterns of expected DSL changes from different variants of MPI-ESM under the SSP5-8.5 scenario (Wickramage et al. 2023). The DSL increase north of 50°S in the South Atlantic and the south Indian Ocean sector is smaller in ER than in HR. The DSL decrease is smaller in the region south of Australia along 60°S in ER compared to HR [Figs. 1 and 11 in Wickramage et al. (2023)].
(a),(b) The DSL change (m); (c),(d) the thermosteric sea level change (m); (e),(f) the halosteric sea level change (m); and (g),(d) the nonsteric sea level change (m) in the Southern Ocean between two decades of SSP5-8.5 mean (2080–99) relative to the historical mean (1995–2014) for (left) MPI-ESM-HR and (right) ER. The cyan contour is the zero DSL in the historical mean. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The thermosteric component of the sea level change contributes to the DSL increase north of 50°S and to the decrease in the Antarctic shelf regions (Figs. 2c,d). The increase is relatively smaller in ER than in HR, while the decrease south of 60°S extends northward from the shelf regions in ER more than it does in HR (Fig. 2d). Changes in both salinity and ocean mass (bottom pressure) are what cause the DSL decrease south of the ACC in HR (Figs. 2e,g). They also somewhat weaken the negative thermosteric effect on the Antarctic shelf regions in all the configurations (Figs. 2c,e,g). Similarly, the thermosteric component reduces the impact of the halosteric component in some regions to the south of ACC, especially in the Weddell, Ross, Bellingshausen, and Amundsen seas in HR (Figs. 2c,e). It is intriguing to note that the changes in the thermosteric and bottom pressure components are the primary drivers for the DSL decline south of 50°S in ER (Figs. 2d,h). The strong thermosteric and halosteric signals nearly cancel each other in HR east and west of New Zealand along 40°S and, to a smaller extent, also in ER. Moreover, the halosteric component of sea level change opposes the sea level increase by the thermosteric component in the South Atlantic and south Indian Ocean centered at 45°S. The most striking difference between the two models is that DSL decline south of 50°S is barely affected by the halosteric component in ER (Fig. 2f). Furthermore, a noteworthy distinction is that the eddy-rich configuration differs significantly from the parameterized simulation south of 50°S. In this region, it exhibits larger negative thermosteric and insignificant halosteric sea level changes.
Figure 1 also compares the result with FAF-sum (FAF-heat + momentum + water) to evaluate how well the sea level change responses can be represented by a linear superposition of responses to the individual perturbations. Even if the FAFMIP forcings are derived from 1pctCO2 runs, the FAF-sum or FAF-all is not identical to the 1pctCO2 run (Fig. S2). The FAF-sum and FAF-all show a larger magnitude of changes than that of the 1pctCO2 run. Nevertheless, the spatial patterns of sea level changes in FAF-sum are similar to the patterns resulted from 1pctCO2 forcing (Couldrey et al. 2021; Zhang et al. 2022). This also holds true when comparing the FAF-sum with the SSP5-8.5 scenario (Fig. 1). The discrepancies between SSP5-8.5 simulations and FAF-sum in North Atlantic possibly come from the “North Atlantic redistribution feedback” in FAF-heat, in which Atlantic meridional overturning circulation (AMOC) declines as a response to surface heat flux perturbation (Gregory et al. 2016; Couldrey et al. 2021). A weakened AMOC transports less heat northward, causing consequently cooler sea surface temperatures over the North Atlantic. This in turn induces additional ocean heat uptake, doubling the heat input to the region. The nonlinear response due to the combined changes in each component resulting from anthropogenic greenhouse gas emissions may also contribute to the difference. Despite these differences, we can still use FAFMIP experiments to evaluate underlying dynamics for sea level change in various resolutions.
Note: The DSL biases indicate that the ER has smaller sea level biases in the Southern Ocean (see Fig. S5).
b. FAFMIP MPI-ESM experiments
1) DSL change
To understand the drivers influencing the differences under the SSP5-8.5 scenario in the MPI-ESM configurations, we determine how various horizontal resolutions react to each surface flux by analyzing the sea level response to individual perturbations. The spatial pattern of sea level change caused by the heat flux forcing (Figs. 3a,b) nearly reflects the response of the SSP5-8.5 projections, with wind stress perturbation also having a small but considerable impact, particularly in ER (Fig. 3d). The freshwater perturbation in the eddy-parameterized model opposes the effects of heat and momentum perturbations south of 55°S (Fig. 3e). It is intriguing to note that ER responds differently from the eddy-parameterized model to freshwater perturbations, displaying no counteraction for the sea level decline (Fig. 3f). Instead, surface freshwater forcing in ER slightly enhances the effect of both heat and stress fluxes. The DSL increase north of 50°S is mainly the result of heat and wind stress perturbations in ER; surprisingly, HR exhibits a weaker response in FAF-stress. In the eddy-parameterized model, the freshwater perturbation counteracts the sea level decrease south of 60°S, whereas the eddy-rich model shows a negligible response, which in fact somewhat reinforces the DSL change.
The Southern Ocean DSL change (m) in the time mean of the final two decades (years 51–70) of FAF-experiments relative to the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (left) MPI-ESM-HR and (right) ER. (a),(b) Heat, (c),(d) momentum/stress, and (e),(f) freshwater experiments are illustrated. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
In the following, decomposition into the steric (thermosteric and halosteric) sea level changes will be taken into consideration independently for each perturbation. Nonsteric (bottom pressure) sea level change will not be further explored with the FAFMIP experiments since there was not a considerable resolution dependence under the SSP5-8.5 scenario.
2) FAF-heat
The thermosteric (Figs. 4a,b) and halosteric (Figs. 5a,b) sea level change in FAF-heat indicates that both components contribute to the sea level decline south of 50°S, while the sea level increase north of 50°S is mainly caused by the changes in temperature. There, the salinity changes counteract the thermosteric sea level change. The redistribution of water masses as a response to heat flux perturbation causes this salinity change as a result of advection changes. The added heat mostly accumulates north of 50°S (Figs. 6a,b and 7a,b) as a result of wind-driven convergence and subduction (Gregory et al. 2016), leading to a positive DSL change north of the ACC. The same phenomena result in a relatively small accumulation of heat south of 50°S and negative DSL. The DSL change pattern north of 50°S is produced by the sum of T-added (Figs. 6a,b) and T-redistributed (Figs. 6c,d) thermosteric components, while the DSL change south of 50°S is produced only by the T-added thermosteric component. Along with the wind-driven convergence of heat, the thermal expansion coefficient is also larger north of 50°S than near Antarctica where seawater is colder. The small expansion coefficient causes the thermosteric sea level increase south of 50°S to be smaller than the global mean, resulting in a negative thermosteric sea level anomaly despite the accumulation of heat (Figs. 4a,b). There, the T-added dominates, while T-redistribute somewhat opposes the effect. Likewise, north of 50°S, the redistribution of salt driven by the altered circulation reduces the effect of heat (Figs. 5a,b). As previously stated, the changes in the halosteric component strengthen the DSL decline south of 60°S in HR (Fig. 5a), whereas ER indicates an insignificant contribution (Fig. 5b). The halosteric sea level change mainly comes from the redistribution of salt. Pearson’s (pattern) correlation between T-redistributed and halosteric sea level change in FAF-heat gives a negative coefficient of 0.69 for HR and 0.53 for ER. Here, high correlations indicate a predominantly linear temperature–salinity relation in the regions of change.
The thermosteric sea level change (m) in the Southern Ocean for FAF-experiments relative to the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (left) MPI-ESM-HR and (right) ER. (a),(b) Heat, (c),(d) momentum/stress, and (e),(f) freshwater experiments are illustrated. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The halosteric sea level change (m) in the Southern Ocean for FAF-experiments relative to the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (left) MPI-ESM-HR and (right) ER. (a),(b) Heat, (c),(d) momentum/stress, and (e),(f) freshwater experiments are illustrated. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
(a),(b) The added thermosteric sea level (m) and (c),(d) the redistributed thermosteric sea level change (m) in the Southern Ocean for FAF-experiments relative to the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (left) MPI-ESM-HR and (right) ER. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
Zonally integrated heat content change (J m−2) in the Southern Ocean for the time mean of the final two decades (years 51–70) of FAF-experiments relative to the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment for (left) MPI-ESM-HR and (right) ER. (a),(b) Heat, (c),(d) momentum/stress, and (e),(f) freshwater experiments are illustrated. In all panels, stippling indicates statistically significant changes (95% confidence level). The contour lines represent the neutral density layers; the dashed lines represent forced experiments; and the solid lines represent the passiveheat/control experiment.
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The added thermosteric sea level response is similar in all the model configurations because the mean circulations among the simulations are similar (Figs. 6a,b). This causes the model discrepancies for thermosteric sea level change patterns to be nearly identical to the model differences in T-redistributed in the FAF-heat experiment. Furthermore, resolution-dependent differences in redistributed thermosteric sea level change, mainly in the Atlantic and Indian sections of the Southern Ocean, partially coincide with differences in DSL change in FAF-heat. Heat content increase centered around 40°S (Figs. 7a,b) becomes smaller with increasing spatial resolution. This results from the additional reduction in warm water advection from the Indian Ocean (von Storch et al. 2016; Gutjahr et al. 2019), because the Agulhas Current system is better represented in the ER model than in the eddy-parameterized model. Plus, the eddies induce the cooling of the intermediate ocean at around 40°S (von Storch et al. 2016; Gutjahr et al. 2019). With better resolution, the FAF-heat experiment shows a decrease in heat accumulation centered at 45°S. Additionally, an insignificant contribution from halosteric to DSL change is noted south of 55°S.
3) FAF-water
The freshwater flux perturbations counteract the dynamic sea level changes south of 55°S caused by heat and momentum flux forcing (Figs. 3c,f). This counteraction, which has been mentioned in other FAFMIP studies also (Gregory et al. 2016; Couldrey et al. 2021), occurs only in the eddy-parameterized configuration. This positive DSL change is mainly thermosteric (Fig. 4e), which is partially opposed by the halosteric component (Fig. 5e), except for the shelf region. There, the freshwater accumulation results in a sea level increase.
What causes the Southern Ocean thermosteric sea level to increase in the FAF-water experiment with parameterized eddies and what accounts for the different responses in the ER model? To answer this question, potential processes that could build up the heat in the water column and lead to thermal expansion will be considered in the following. The increased freshwater input leads to a decrease in ocean surface density and convection as well as an increase in oceanic stratification. Changes in ocean stratification are characterized by density gradient change with depth illustrated by the squared buoyancy frequency N2. Figure 8 illustrates the normalized squared buoyancy frequency changes. The eddy-parameterized model shows increasing stratification of the water column (Fig. 8a) that is largely absent in ER (Fig. 8b). This difference is partially related to the already stronger stratification of the ER base state, as changes are normalized by N2 of the control. The importance of the base state is discussed further below. Increased stratification results in less vertical oceanic heat transfer and more heat being stored at depth in the water shown as enhanced zonally integrated heat content (Fig. 7e). This contributes to the pronounced thermosteric sea level increase in the eddy-parameterized model (Fig. 4c), while less heat accumulation in deep layers (Fig. 7f) causes less thermal expansion in ER (Fig. 4f). The reduced vertical heat flux toward the surface layer due to stronger stratification ultimately tends to cool the ocean surface, promoting the increased sea ice concentration (Fig. 9a). The extended sea ice cover further enhances warming though reducing the net air–sea heat fluxes, leading to a positive feedback loop.
Normalized zonal-mean squared buoyancy frequency N2 changes in the Southern Ocean for the FAF-water experiment for the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (a) MPI-ESM-HR and (b) ER.
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
Changes in sea ice concentration (%) in the Southern Ocean for the FAF-water experiment for the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (a) MPI-ESM-HR and (b) ER. In all panels, stippling indicates statistically significant changes (95% confidence level).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The response of the sea level due to salinity change is not significant south of 55°S in ER (Figs. 2f, 3f, and 5f), whereas it shows a considerable signal in the eddy-parameterized model. The halosteric sea level decline in HR relates to an increase in salinity below the fresher surface layer south of 55°S (Figs. 10a,g). Freshwater perturbation is the key driver that causes model discrepancies in salinity changes under the SSP5-8.5 scenario in the deep layer between eddy-rich and eddy-parameterized simulations (Fig. 10; larger signal south of 55°S comes from FAF-water). The increased stratification of the water column keeps the salt in the Circumpolar Deep Water from mixing with the surface layer. The accumulation of excessive freshwater at the surface may lead to a northward export of freshwater in the Ekman layer and via sea ice advection. As a result, induced salinification in the eddy-parameterized model relative to ER causes a negative halosteric sea level south of 55°S (Figs. 5c,f). In the real world, the dense waters that originate on the continental shelf descend to the deep ocean along the continental slope while being mixed with the surrounding water. The increased stabilization of the water column reduces the mixing and downwelling of dense water in the eddy-parameterized simulations. On the shelf from where dense waters originate, the accumulation of freshwater results in a halosteric sea level increase (Fig. 5e). Other than that, there is no considerable FAF-water halosteric impact on the DSL change. The magnitude of the thermosteric sea level increase is largest in the Weddell Sea and Ross Sea sectors in HR.
Changes in salinity (psu) in the Southern Ocean for (a),(b) SSP5-8.5 scenario [(2080–99)–(1995–2014)], (c),(d) FAF-heat, (e),(f) FAF-stress, and (g),(h) FAF-water experiments for the time mean of the final two decades (years 51–70) of the FAF-passiveheat experiment (for ER, corresponding 20 years of control run was considered) for (left) MPI-ESM-HR and (right) ER. In all panels, stippling indicates statistically significant changes (95% confidence level). The contour lines represent the neutral density layers; the dashed lines represent forced experiments; and the solid lines represent the passiveheat/control experiment.
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The diversity of climate feedbacks is highly reliant on the control climate state in models (Bouttes and Gregory 2014; Hu et al. 2017). Therefore, we investigated the diversity of the unperturbed climatological fields in the control experiments of the two model versions. Here, we found stronger stratification (0.19 × 10−4 s−2) south of 60°S in the ER control mean state relative to HR, in which the mean state stratification is relatively weaker (for HR 0.14 × 10−4 s−2) associated with enhanced vertical mixing. Eddies influence the vertical structure of the ocean by enhancing vertical mixing and stratification and play a crucial role in the lateral and vertical transport of salt, smoothing out salinity gradients and preventing extreme salinity changes. This vertical structure affects how the ocean responds to surface forcings. When freshwater perturbation is applied to the sea surface, though a strengthening in stratification occurs in both ER and eddy-parameterized models from the initial state, stratification remains much stronger in ER than in HR, resulting in relatively less changes. This suggests that the ocean interior processes are affected by the horizontal resolution.
4) FAF-stress
The DSL change caused by wind stress forcing is predominantly thermosteric (Figs. 4c,d). The wind-driven convergence and subduction accumulate the deep-reaching heat centered at ∼45°S causing thermal expansion, predominantly in the Atlantic and Indian sections of the Southern Ocean (between 30° and 50°S) and slightly in the Pacific sector (Figs. 4c,d and 7a,b). The thermosteric sea level increase in the Pacific sector is slightly larger in HR than in ER; however, it is considerably smaller in the Atlantic and Indian sections in HR (Figs. 4c,d). Halosteric sea level change in FAF-stress is relatively smaller and opposes the thermosteric effect (Figs. 5c,d).
Poleward shifting and intensifying wind warm the subsurface in the northern part of the ACC (around 45°–55°S) and cool higher latitudes (Gupta and England 2006; Fyfe et al. 2007; Spence et al. 2010; Sallée 2018) as a result of the upwelling of subsurface cold water (Armour et al. 2016). The warming between 40° and 55°S is weaker in HR in comparison with ER (Fig. 7c). However, the high-latitude cooling weakens as model resolution increases with better-resolved mesoscale activities. This could be because explicitly resolved southward eddy heat transport is large, whereas the parameterized eddy heat transport is relatively insensitive to wind perturbation (Spence et al. 2010). Warmer water is transported from subtropical to subpolar regions primarily via eddy-induced meridional heat transport (e.g., Volkov et al. 2010). Furthermore, Yamazaki et al. (2021) argue that the southern boundary of the ACC, which controls the Antarctic thermal condition, shifts poleward as a response to the intensified westerlies. In addition, Hogg et al. (2008) found that the enhanced wind stress does not change the ACC transport much in the high-resolution model but enhances the eddy field in the Southern Ocean. They further stated that enhanced eddy field efficiently transports heat southward resulting in Southern Ocean warming. This can explain the larger warming centered around 45°S in ER relative to HR.
Here, we summarize the drivers for sea level change in the Southern Ocean. Changes in both surface heat and wind stress flux changes can explain the DSL change under the SSP5-8.5 scenario, with partial compensation resulting from changes in freshwater flux. Notably, in contrast to the projections in HR, the freshwater perturbation has a minor positive effect on the DSL change in ER. The discrepancies in resolution between eddy-parameterized and eddy-rich configurations for the thermosteric and halosteric sea level change south of 55°S are significant. The surface freshwater perturbation is the key driving factor. It is further evidenced by the vertical profiles of temperature and salinity changes (Figs. 7 and 10). Different from the well-stratified ER model, when freshwater forcing is applied to the sea surface, the HR configuration tends to exacerbate the stratification of the water column, which in turn keeps the heat and salt in the Circumpolar Deep Water from mixing with the surface layer. The salt and heat build up in deeper layers, causing the thermal expansion of the water column and a decrease in the halosteric sea level. The surface layer becomes cooler causing winter sea ice formation, which then results in a positive feedback loop. The salinity changes in ER are insignificant in particular south of 50°S. The DSL increase centered at 45°S is thermosteric and driven by both wind stress and heat flux perturbations. Wind stress perturbation induces heat convergence at around 45°S which is relatively smaller in parameterized simulation (Figs. 7c,d).
c. Changes in Drake Passage transport in different resolutions
The Drake Passage transport is considered in many studies to reflect changes in ACC strength in a warming climate (e.g., Cunningham et al. 2003; Behrens et al. 2016; Xu et al. 2020; Yang et al. 2023). Both observation (e.g., Böning et al. 2008) and model simulations (e.g., Hogg and Blundell 2006; Meredith and Hogg 2006; Hogg et al. 2008; Meredith et al. 2012; Munday et al. 2013; Stewart et al. 2023) have concluded that changes in Southern Ocean westerlies have little to no impact on the ACC transport but instead enhance the eddy field. This phenomenon is known as “eddy saturation” (Straub 1993; Hogg et al. 2008). Contrary to the anticipation that eddy saturation is able to limit transport increase related to wind stress enhancement, Wickramage et al. (2023) showed that the Drake Passage transport is increasing in the ER SSP5-8.5 simulation in comparison with the lower-resolution version of the same model (HR), where it is decreasing (Table 1 second column). Changes in FAF-sum exhibit a comparable pattern to those in the CMIP projections discussed by Wickramage et al. (2023), providing the opportunity to use FAFMIP experiments for a more in-depth explanation of model discrepancies.
Time-mean anomalies of transport through Drake Passage. The two standard deviations are shown in the bracket.
In all simulations, the ACC transport is amplified by the wind stress perturbation [9 Sv (1 Sv ≡ 106 m3 s−1) in HR and 6 Sv in ER]. As anticipated, the circumpolar transport is less sensitive to the induced westerlies in ER than in parameterized simulations. Why then do we notice a substantial increase in ACC transport in ER and an overall reduction in HR under the SSP5-8.5 scenario?
The surface freshwater water forcing results in a comparatively large decrease (11 Sv in HR and 1.3 Sv in ER), which significantly deviates HR and ER from each other. In addition, a reduction in ACC transport (2.58 Sv in ER and 2 Sv in HR) is caused by the surface heat flux perturbation in both models without substantially differing from one another. Assuming a linear response to all perturbations, both FAF-heat and FAF-water changes together cause a net reduction in the ACC transport. Even if the change in wind stress increases the ACC transport in HR more than it does in ER, wind stress–induced changes are opposed by the large impact from freshwater and heat flux changes, while the net changes in ER primarily result from only the wind stress perturbation.
d. Shift of Antarctic Circumpolar Current
The strong sea level change gradient over the ACC is a result of the enhanced and poleward-shifted westerlies (Bouttes et al. 2012; Frankcombe et al. 2013), which move the ACC poleward. The magnitude of the poleward movement of ACC varies with longitude and is hardly resolution specific (Fig. 11). The mean position of ACC, nevertheless, is model dependent, where ER is positioned somewhat further north compared to HR (Fig. 11a). Zonally averaged southward shift of ACC in the SSP5-8.5 projections is 0.47° (±0.31°) and 0.40° (±0.30°), respectively. However, the zonal-mean shift is not significant in FAFMIP experiments (the table in Fig. 11). Thus, it cannot be used to explain the zonal-mean shift under the SSP5-8.5 scenario. Although this is the case for the zonal mean, the shift of ACC also depends on the location and it shows significant variations between 60°W and 0°–120°E (when the shift is significant, it is illustrated by the light colors). In response to the wind stress perturbation, ER experiences a substantially larger rise in sea level north of 50°S between 60°W and 0°–120°E (Fig. 3d) in comparison with HR (Fig. 3c). Examining the ACC shift in FAF-stress (Fig. 11f) reveals that the zonal mean of the shift is 0.36° larger in ER than in HR between 60°W and 0°–120°E. It is evident that the differences in poleward shift of ACC reflect some of the resolution discrepancies in DSL change between 60°W and 0°–120°E, with ER showing a larger shift and greater DSL increase which is thermosteric (Fig. 4d). Although the FAFMIP results do not fully explain the ACC shift in the SSP5-8.5 simulations, it is clear that changes in wind stress are the primary cause of the southward shift, particularly between 60°W and 120°E. In this region, the responses to heat and freshwater flux perturbations counterbalance the effects of momentum flux changes (Figs. 11d,j).
(a),(c),(e),(i) The mean position of ACC (°) and (b),(d),(f),(j) the shift of ACC (°). The control state (solid line) was established by considering two decades of historical mean (1995–2014) for both (a) and (b), whereas it is the final two decades (years 51–70) of the FAF-passiveheat experiment or the corresponding 20 years of control run (for ER) in the FAFMIP experiments in (c)–(j). The control state is compared to (a),(b) SSP5-8.5 (2080–99); (c),(d) surface heat flux perturbation; (e),(f) surface momentum flux perturbation; and (c),(d) surface freshwater flux perturbation. The shift was calculated by zonal surface geostrophic velocity between two decades of forced mean (dashed line and light colors) relative to the time mean of two decades in the control state (solid line and dark colors) for MPI-ESM-HR (red) and ER (blue). The zonal average ACC shift is indicated in the table for each resolution and each run. Statistically significant shift (95% confidence level) is indicated by the light colors in (b), (d), (f), and (j).
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
e. Eddy-induced heat and salt flux in the ER simulation
Wunsch (1999) and Stammer (1998) underscored the significance of zonally integrated eddy transports, particularly in regions like western boundary currents, the tropics, and the ACC. Consequently, the heat and salt content in eddy-active regions can be considerably affected by the eddy-induced temperature and salinity anomalies, which in turn can have an impact on sea level change. Without resolving eddies, low-resolution models often overestimate the mean advective transport because they do not account for the dissipative and redistributive effects of eddies. This overestimation can lead to an exaggerated accumulation of heat in certain regions such as north of 50°S (Figs. 2c,d).
High-resolution numerical ocean models enable us to consistently quantify both time-mean and eddy contributions to the meridional and vertical heat and salt transport. These transport changes influence thermosteric and halosteric sea level changes. To analyze this further, we separate the “eddy” term, which arises from the correlation between velocity and tracer fluctuations, from the transport by the time-averaged circulation for ER. The eddy heat and salt fluxes play a crucial role in maintaining the overall salt and heat balance of the Southern Ocean over time. We investigated the meridional and vertical eddy heat transports in the Southern Ocean in ER (0.1°/TP6M), comparing the historical period with the SSP5-8.5 scenario. We chose to exclude the HR configurations from our analysis because the resolution of the TP04 (0.40°) simulations is insufficient to accurately represent the mesoscale eddies. Additionally, the contributions from the GM parameterization are nearly zero in HR in the Southern Ocean.
The eddy-induced heat (Fig. 12a) and salt (Fig. 13a) transports play a significant role in the net meridional heat and salt transport. The meridional eddy heat and salt fluxes are essentially negative, implying that mesoscale eddies transport heat and salt southward. Both transports are characterized by a maximum at about 40°S, where approximately 0.8 Petawatt (PW) of heat is carried southward by the eddies. The eddy heat transport is virtually zero south of 60°S. The meridional heat transport due to time-mean circulations is primarily northward.
(a) Decomposition of the global meridional transport of heat (PW) and (b) eddy tendency forcing Eυ in the ER (TP6M) simulation. The historical period is over 1995–2014 (dotted lines) and the future period is over 2080–99 under the SSP5-8.5 scenario (solid lines). The pink curves are the total meridional transports
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
(a) Decomposition of the global meridional transport of salt (Gg s−1) and (b) eddy tendency forcing Eυ in ER (TP6M) simulation. The historical period is over 1995–2014 (dotted lines) and the future period is over 2080–99 under the SSP5-8.5 scenario (solid lines). The pink curves are the total meridional transports
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
In the present-day state, we demonstrate that most of the poleward meridional heat transport in the Southern Ocean is due to the eddy heat transport, which surpasses the contributions from mean advective transports between 38° and 53°S, leading to a net poleward transport. However, by the end of the twenty-first century, the changes in meridional heat transports indicate intensification and a poleward shift of both eddy and mean components (Fig. 12a). The profiles of the tendency forcing show that the mean advective warming maximum is between 38° and 40°S, while eddy-induced cooling weakens this warming (Fig. 12b). Morrison et al. (2016) used a global eddying climate model to investigate the response of meridional heat transport in a warming scenario. They found in the region around 40°S eddy transports carry additional heat northward due to a reduction in southward eddy heat transports. In contrast, increased southward eddy heat transports almost balance similarly increased northward transports of the mean circulation in ER (Fig. 12a).
Eddies can redistribute heat more evenly across latitudes, which can mitigate excessive localized warming. Between 40° and 45°S, the positive eddy tendency forcing (as shown in Fig. 12b) indicates that eddies induce the warming in this region, while the mean advective transport partially counterbalances this warming. However, the warming is larger in the eddy-parameterized model between 40° and 45°S relative to ER (Figs. 2c,d). This is because the eddy-parameterized model cannot accurately capture the full impact of eddy effects and may not fully replicate the heat redistribution capabilities of actual eddies, leading to an overestimation of localized warming. In contrast, local heating is balanced by the redistribution of heat by eddies in ER through mixing and heat flux. As a result, ER shows a moderate thermosteric sea level rise compared to the eddy-parameterized model (Figs. 2c,d).
Both eddy and mean salt transports are southward, causing a net southward salt transport. Particularly, the eddy contribution is large around 40°S, where it comes close to the transport by the mean circulation. The sum of eddy salt transports and salt transports due to the time-mean circulation (pink line in Fig. 13a) is negative (poleward) with a maximum of around 40°S. These southward salt transports in both components increase under the SSP5-8.5 scenario. Additionally, divergences of transports contribute to freshening north of 42°S with the maximum being at 21°S (pink line in Fig. 13b) and convergences of transports to salinification south of 42°S. Both freshening and salinification increased at the end of the twenty-first century. The tendency forcings indicate that the eddy-induced freshening (35 Kg m−1 s−1) north of 40°S dominates the mean advective salinification (27 Kg m−1 s−1) during the historical period. Eddy-induced freshening and mean advective salinification increase under the SSP5-8.5 scenario by about 3 and 1 Kg m−1 s−1, respectively. This pattern mirrors between 40° and 50°S, indicating eddy-induced salinification and mean advective freshening. South of 47°S, the net salinification increases under the SSP5-8.5 scenario, contributing to the decrease in halosteric sea level at the end of the twenty-first century.
Eddies play a crucial role in the lateral and vertical transport of salt, smoothing out salinity gradients and preventing extreme salinity changes. Our results suggest that the divergence of the eddy-induced salt transport opposes that of the mean advective salt transport at most of the latitude bands. Without this eddy compensation, salinification in the Southern Ocean would be larger, given an unchanged mean component. Since compensation generally holds for the changes in a warming climate, the missing contribution of the changing eddy transports may explain the larger halosteric sea level change in the parameterized version (Fig. 2e) in locations where the eddy transports are large. In this respect, it is worth noting that the eddy salt transport is considerably small south of 50°S.
Last, we used the FAFMIP experiments to decompose the effects of heat, momentum, and freshwater flux change on meridional heat and salt transports (Figs. 14 and 15). For meridional heat transport, we found that the changes related to the different surface forcing components contribute with similar tendencies to the changes seen under the SSP5-8.5 scenario (Fig. 14a). However, the magnitude and shift (shift of maximum at 40°S) are different between them. We found that the wind stress is the predominant driver responsible for the amplification of eddy-induced southward heat transport at 40°S, resulting in an increase of approximately 0.1 PW and the poleward shift of the maximum. Additionally, heat and freshwater flux perturbations lead to an approximate increase of 0.05 PW in the southward eddy-induced heat transport near 40°S. North of 40°S, only heat flux enhances the eddy-induced southward heat transport, while other flux perturbations reduce the transport. It is worth noting that only changes in wind stress result in an increase in the southward eddy-induced heat transport south of 40°S.
(a) Decomposition of the global meridional transport of heat (PW) and (b) eddy tendency forcing Eυ in ER (TP6M) simulation. The time mean of the final two decades (years 51–70) of the FAFMIP experiments, relative to the corresponding 20 years of control run. The dashed lines are the total meridional transports
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
(a) Decomposition of the global meridional transport of salt (Ggs−1) and (b) eddy tendency forcing Eυ in ER (TP6M) simulation. The time mean of the final two decades (years 51–70) of the FAFMIP experiments, relative to the corresponding 20 years of control run. The dashed lines are the total meridional transports
Citation: Journal of Climate 37, 24; 10.1175/JCLI-D-24-0063.1
The changes in surface heat and freshwater flux forcings do not increase the southward eddy-induced heat transport south of 40°S (Fig. 14a). However, we observed an increase in the eddy-induced heat transport till about 57°S in response to wind stress.
Next, we investigate the changes in meridional salt transport as a response to the different surface forcings (Fig. 15a). We found that the shape of changes in the meridional salt transport curves due to the FAFMIP forcings closely resembles those under the SSP5-8.5 scenario. Unlike the changes in eddy-induced heat transport, which is mainly driven by wind stress, the largest increase in eddy-induced salt transport occurs in response to the surface heat flux perturbation. The magnitude of the increase is similar for both wind stress and freshwater flux forcings. However, the wind stress and freshwater flux perturbations lead to a southward shift of the maxima at 40°S, while the shift is equatorward in response to the heat flux forcing (Fig. 15a).
In summary, the main driver of the eddy-induced cooling and freshening between 38° and 40°S, as well as the warming and salinification between 40° and 44°S, is the wind stress perturbation. The southward migration of the eddy fluxes particularly visible in FAF-stress is consistent with the poleward migration of the frontal systems at that latitude (e.g., Yang et al. 2016). However, eddy fluxes are dominated by smaller regions such that migration eddy fluxes may not be representative of the frontal migration at the same latitude. Between 34° and 37°S, the main driver of the eddy-induced cooling and freshening is, however, related to heat flux perturbations. Between 30° and 40°S, changes in eddy heat and salt fluxes mainly occur in eddy hotspots such as Agulhas Retroflection, the Brazil–Malvinas confluence zone, and the Tasman Sea, whereas the changes between 45° and 60°S are related to the eddy transport changes in the ACC region (Figs. 4 a,c,e,i).
4. Summary and concluding remarks
An analysis of FAFMIP experiments is carried out to comprehend how surface flux perturbations impact Southern Ocean sea level changes in different MPI-ESM configurations with different horizontal resolutions. The main patterns of DSL change in the MPI-ESM experiments agree with what was described in previous studies (Gregory et al. 2016; Couldrey et al. 2021; Zhang et al. 2022), indicating that our findings are likely applicable to other model systems.
Key findings include the following:
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In the SSP5-8.5 projections, the sea level change in the Southern Ocean is mainly driven by the steric effect and can be characterized as an increase north of 50°S and a decrease south of it, relative to the global mean.
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Changes north of 50°S are mainly thermosteric. In ER, both wind stress and heat flux perturbations play a role. In the eddy-parameterized model version, heat flux perturbations are more important.
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South of 50°S: Both mass redistribution and the steric effect contribute to the DSL decline. Heat and wind stress perturbations drive these negative DSL changes, while freshwater perturbation opposes the effect. In ER, the halosteric contribution is insignificant, while the contribution of the thermosteric component is larger.
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DSL changes related to salinity changes and to freshwater perturbation in general are insignificant in ER. The HR simulations show relatively smaller wind-induced heat convergence at ∼45°S, leading to insignificant DSL change north of 50°S.
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The strong positive sea level signal south of 50°S in HR is due to increased near-surface buoyancy and stratification from freshwater perturbation, which suppresses convection. This promotes sea ice formation, reducing melting. The increased sea ice further decreases vertical heat fluxes, creating a positive feedback loop. This leads to increased thermosteric sea level due to enhanced heat storage in deeper layers. In contrast, the mean stratification is relatively strong in ER (Fig. S8), which in turn shows a weaker response to freshwater perturbation.
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Contrary to expectations, the HR model shows a deceleration of ACC transport under SSP5-8.5 forcing (Wickramage et al. 2023). However, FAFMIP experiments confirmed that ER exhibits the least sensitivity in barotropic transport through Drake Passage to wind stress perturbations. The negative changes in Drake Passage transport in HR are due to opposing effects of surface freshwater and heat flux changes against wind stress perturbations.
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The mean position of ACC is more equatorward in ER, which also shows a larger poleward shift as response to the wind stress forcing, causing a positive sea level change between 60°W to 0° and 120°E, where heat and freshwater forcings show an equatorward shift.
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Eddy-induced transport impacts net meridional heat and salt transports in the Southern Ocean. Mesoscale eddies transport heat and salt southward, the peak being along the 40°S latitude band, where we observe a significant positive DSL change. By the end of the twenty-first century, these transports increased and shifted poleward. Direct eddy effects in ER are limited to small areas like the Agulhas Retroflection, the Brazil–Malvinas confluence zone, the Tasman Sea, and, to some extent, the Antarctic Circumpolar Current (ACC). Wind stress changes intensify eddy-induced heat fluxes, while heat flux perturbations mainly drive mean advective circulation. Eddy heat and salt fluxes between 35° and 40°S are negative, cooling, and freshening water masses, while between 40° and 45°S, they cause warming and salinification, counterbalancing the mean flux.
Our study confirms that high-resolution modeling will be required to capture the complex nature of regional sea level change. Coarse-resolution models with parameterized mesoscale eddy processes have limitations in accurately representing these dynamics. Eddy-resolving models have the capability to provide more accurate and reliable sea level projections, which are essential for developing effective adaptation strategies against future sea level rise.
Acknowledgments.
This study was funded by the German Research Foundation (DFG) as part of the Priority Programme SPP 1889 “Sea level.” Dian Putrasahan provided the data from the MPI-ESM-ER SSP5-8.5 experiment with the support of the DFG-funded Excellence Cluster “CLICCS” of Universität Hamburg. Deutsches Klimarechenzentrum (DKRZ) provided computing resources and access to CMIP6 data. This study contributed to the Centrum für Erdsystemforschung und Nachhaltigkeit (CEN) of Universität Hamburg.
Data availability statement.
MPI-ESM ScenarioMIP and FAFMIP output HR are available at https://esgf-data.dkrz.de/projects/cmip6-dkrz/. MPI-ESM-ER data are published on WDCC at https://www.wdc-climate.de.
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