Subtropical Anticyclones (SA)s are semi-permanent high-pressure systems that are crucial components of the large-scale atmospheric circulation in both hemispheres. Climatologically, SAs in the Southern Hemisphere (SH) influence the hydrological cycle and precipitation in the subtropics and midlatitudes (Sturman and Tapper 1996; Tyson and Preston-Whyte 2000; Reboita et al. 2010). The precipitation variability and intensity of subtropical regions are reported to be changed in recent years due to changes in the area and strength of SH SAs that led to regional drought extremes (Burls et al. 2019). To understand how the weather and climate of SH subtropical and midlatitude regions will change in response to global warming, understanding how the SH SAs will respond is crucial. In this article, we use the word “change” to describe the difference between a model’s future projection (a warmer climate due to external forcings) and the respective control run (e.g. PiControl, Historical) of the same climate model.
In simulations conducted by the models participating in the Coupled Model Intercomparison Project (CMIP), it is well documented that the subtropical to midlatitude atmospheric circulation is significantly affected in both hemispheres under future warming (Li et al. 2012, 2013; Shaw and Voigt 2015; He et al. 2017; Song et al. 2018a; Fahad et al. 2020) (Fig. 1a & b). The mechanisms that drive SA changes in the Northern Hemisphere (NH) have been explored in a number of studies (Li et al. 2012; Shaw and Voigt 2015, 2016; He et al. 2017; Song et al. 2018a). The SH SAs have however received less attention, with open questions surrounding our understanding of what drives the simulated changes in the SH SAs during both the austral summer and winter season.
The SH SAs are projected to extend in the area and intensify along the poleward flank of the subtropics during both the austral summer (December-January-February or DJF) and winter (June-July-August or JJA) season (Li et al. 2013; Fahad et al. 2020). Li et al. (2013) found that all three SAs in the SH, namely the South Pacific SA (SPSA), South Atlantic SA (SASA), and South Indian SA (SISA) extend in area and strength due to increased land-sea thermal contrast due to intensified cooling over the ocean. This local diabatic heating mechanism is consistent with Wu and Liu (2003), which show a longwave, latent, and sensible heating pattern in the subtropical region drives changes in the climatological area and strength of the summertime SH SAs’.
Later, He et al. (2017) argued that changes in upper troposphere static stability have a significant influence on changes in the summertime SH SAs under global warming conditions. Using CMIP5 output He et al. (2017) analyzed changes in subsidence, low-level divergence, and rotational wind from 10oS-40oS focusing on the difference between future RCP8.5 projections (years 2050–2099) and historical (years 1950–1999). He et al. (2017) found that while the SPSA increases in strength; the SASA and SISA weaken in future climate during local summer even though decreased heating over the subtropical ocean supposedly acts to intensify the SH SAs. He et al (2017) argue that increased tropical upper troposphere static stability in a future climate leads to mean advection of stratification changes (MASC) that act to decrease the strength of the SASA and SISA during local summer (Ma et al. 2012; He et al. 2017). In the case of the SPSA, the contribution from the MASC mechanism is relatively small and the local diabatic heating decreases primarily acts to increase the SPSA’s strength in future climate. Using a linear baroclinic model, He et al. (2017) show that the changes in SH SAs are consistent with prescribed changes in the heating and tropospheric static stability. The inconsistency of the austral summer SASA and SISA change in future climate between Li et al. (2013) and He et al. (2017) is due to the choice of domains to define the SH SAs. Li et al. (2013) defined the centers of the SAs as where the 925hPa stream function is at a maximum over the subtropical Ocean, whereas He et al. (2017) regionally averaged between 10oS-40oS, this latter definition focuses largely over the equatorward flank of the SAs where the MASC mechanism has a larger effect.
Analyzing the CMIP5 and CMIP6 models, Fahad et al. (2020) show that Sea Level Pressure (SLP) and 925hPa wind associated with SAs intensifies in the center and along the poleward flank in the SH comparing historical (years 1950–1999) to future projections (RCP8.5 for CMIP5 and SSP585 for CMIP6) (years 2050–2099). Fahad et al. (2020) defined the 1020hPa isobar area as the center of the SH SAs, and 25oS-45oS as the latitudinal extent of the domain for the austral summer (DJF), and 20oS-40oS as the latitude domain for the austral winter (JJA), Findings of SH SAs change in the future climate compared to the historical multi-model mean (MMM) are consistent between Li et al. (2013) and Fahad et al. (2020) for austral summer. However, Fahad et al. (2020) show that reduced baroclinic eddy growth in the future climate acts to intensify SAs along their poleward flank in both seasons. By using the metric for the Philipps criterion (Phillips 1954) to decompose the changes in baroclinic instability into the contribution from static stability changes and wind shear change, Fahad et al. (2020) find that an increase in tropospheric static stability is the dominant driver of the baroclinic eddy growth reduction leading to intensified SH SAs along their poleward flank
The climatological influence of the tropical diabatic heating, especially heating over the Inter Tropical Convergence Zone (ITCZ) and the South Pacific convergence zone (SCPZ) on SAs in both hemispheres are documented in several studies (Hoskins 1996; Chen et al. 2001; Nigam and Chan 2009; Fahad et al. 2021). Future changes in NH SAs are reported by Song et al. (2018) to be related to the change in the tropical precipitation (and latent heating) resulting from a delay in the boreal spring to summer seasonal cycle of tropical rainfall and heating under global warming conditions. Changes in the onset of the rainfall band over the tropical ocean affect the Hadley circulation in the NH leading to a change in the zonal mean component of NH SAs (Song et al. 2018b, a). With the exception of the MASC mechanism (Ma et al. 2012, He et al. 2017), the influence of tropical diabatic heating changes under global warming conditions on SH SAs has received much less attention compared to their NH counterparts.
Focusing on the midlatitude atmospheric circulation, several studies have investigated the relative importance of the fast atmospheric response due to the direct radiative forcing component of CO2 versus the slower atmospheric response due to the indirect forcing component related to the patterns of Sea Surface Temperature (SST) warming (Shaw 2014; Simpson et al. 2014; Grise and Polvani 2014; Shaw and Voigt 2015). For the summertime NH atmospheric circulation change in a warming climate, Shaw and Voigt (2015) show that the North Pacific SA responses to an increase in the direct radiative forcing from the elevated atmospheric CO2 concentrations oppose those from the indirect SST warming. Using an atmospheric general circulation model forced with prescribed SSTs, Shaw and Voigt (2015) found the North Pacific SA increases in strength during boreal summer due to increased direct radiative forcing, however, this weakens in strength due to the SST warming patterns. In coupled model simulations these opposing responses to direct radiative forcing and indirect SST warming lead to a small net change of the North Pacific SA’s strength under future warming. There is also a similar tug-of-war response between these two forcings found on upper troposphere atmospheric circulation in the North Pacific (Shaw and Voigt 2015).
As with the influence of tropical diabatic heating, the response of the SH SAs to direct CO2 radiative forcing (fast atmospheric forcing), and indirect SST warming (slow oceanic forcing) has received much less attention compared to their NH counterparts. This study aims to determine which of these two forcings primarily drives changes in the SH SAs. Using the Community Earth System Model version 2 (CESM2) coupled ocean-atmosphere experiments from CMIP6, and prescribed SST Community Atmospheric Model 6 (CAM6) numerical experiments, we explore how the direct CO2 radiative forcing versus indirect SST warming patterns drive the SH SAs during both austral summer and winter. We further investigate the relative contribution of these two forcing mechanisms to future changes in tropical diabatic heating and how changes in the tropical diabatic heating alone act as a remote forcing on the SH SAs. The sections of this study are structured as follows: Sect. 2 describes the experimental design, numerical model configurations, and data analyzed from different experiments. Results from the analysis are presented in Sect. 3. Finally, in Sect. 4, the discussion and conclusions from the results are documented.