Calculations involving the transport of sensible heat, kinetic energy and momentum were used in this work to evaluate the influence of ST on the atmosphere global circulation. The synoptic systems in mid-latitudes are initiated by processes linked to the baroclinic instability, i.e. to the increase in the amplitude of atmospheric disturbances because of the vertical shear of the average wind. This is the conversion of available potential energy from the basic state to the disturbances. Baroclinic instabilities predominate in medium latitudes, acting in the so-called baroclinic zones, where the maximum horizontal temperature gradients are located and, consequently, where the jets in the high troposphere are located [3] [22]. STs were extracted from daily data at average atmospheric levels (500 hPa) of the ECHAM5/MPI-OM model and were temporarily classified using a high-pass filter to include only systems with growth and decay, at intervals smaller than six days, according to methodology previously described by [23].

The sensible heat transport to the north (E1), that is measured around the circle of latitude on the period is calculated using the definitions of the zonal and temporal average, and can be written as follows:

[ v T ] ︸ ( 1 ) = [ v ¯ ] [ T ¯ ′ ] ︸ ( 2 ) + [ v ′ T ′ ¯ ] ︸ ( 3 ) + [ v ′ T ′ ¯ ] ︸ ( 4 ) (E1)

where each term of equation represents:

(1) Latitudinal and temporal transport of average sensible heat toward the north.

(2) Contributions of the average southern circulation.

(3) Stationary vortex contributions.

(4) Transient vortex contributions.

It is important to emphasize that the largest contribution to (E1) is given by (4), the transient vortices (v'T').

In order to obtain the ST intensity by using transient wave statistics (E2), the kinetic Energy per unit mass (EKE) from extratropical cyclones trajectories was used seasonally. Thus, an estimation of the ST intensity between the subpolar and subtropical regions is obtained:

[ EKE = 1 2 u ′ + v ′ 2 ] (E2)

The first term of the equation is the kinetic energy of the zonal flux, and the second term is the kinetic energy of the southern flow. The bar denotes the mean time.

The difference between Meridional Transport of sensible heat flux for GW and PD scenario is shown in Figure 1. The potential energy exchanges of the basic state and the potential energy available for the disturbances, indicated by (v'T'), were analyzed seasonally for summer (December, January and February, DJF) and winter (June, July and August, JJA). The differences were compared between ECHAM-5/MPI-OM simulations (GW, from 2080 to 2100), and ERA-40 (PD, from 1980 to 2000).

According to Sato [24], the sensible heat flux is associated with the mechanical transfer of energy by conduction between two regions with different temperatures, and it is greater for warm than for cold regions. Thus, through the results obtained by [7], it is clear that in the BMC region the southern transport of sensitive heat during DJF is distributed in a more limited range latitudinally, suggesting a greater zonal symmetry, evidencing the presence of well-defined cyclogenetic regions on SwAO.

For winter (JJA), the systems activity is distributed over a wide range of latitudes. It is also during JJA that the southern transport of sensible heat reaches the highest values, which may be associated with the higher baroclinidade of the region at high levels during this season, evidenced by the presence of jet streams [3].

Comparing the differences between GW and PD, we draw attention to an intensification of the transports mainly to the polar side of the ST position. The Figure 1(a) shows smooth signal changes on the equatorial side of the ST medium position, emphasizing a transport toward the North, mainly on the BMC region. For JJA (Figure 1(b)) the region presents maximum transport to the South (higher values). Therefore, changes of the meridional transport of sensible heat associated to the increase of the greenhouse gas emissions in GW promoted an acceleration of the basic flow for both seasons (JJA and DJF).

The ST observed displacement for south is approximately 5˚ latitudes as previously observed by Freitas et al. (2018), meridionally distributed on a regular form. Hence, inducing a smaller (larger) cyclonic activity close from 50˚S (60˚S). Thus, we suggest that in the GW conditions, the ST activity remain persistent during all year and the heat transport toward south can reach the maximum values during JJA. For DJF the BMC region may have the sensible heat transport changed toward north.

According to [25], the ST displacement to the south and decrease in cyclogenetic activity observed during the DJF may have a relation with the reduction and displacement of the maximum baroclinic region toward the pole. The reason for this behavior is due to changes that can occur in meridional sensible heat gradient throughout the warm troposphere.

The BMC region is important due to presence of ocean fronts, responsible for intermediate water mass formation in the Atlantic Ocean as a whole [26] [27]. Assessing the redistribution of extratropical cyclones over the mid-latitudes region for the GW scenario, we found that transport becomes more intense toward north during DJF in the BMC. For JJA months, the maximum values occur toward south.

The intensification of the sensible heat flux in the BMC region is directly associated to the ST position changes. For the future scenario, the numerical experiment in BMC showed increases in the basic flow for both DJF and JJA. Following our results we suggest that, in a future scenario with larger than present GHG concentrations, larger cyclone activity in latitudes around 60˚S are to be expected, while in mid-latitudes (approximately 50˚S) less frequent cyclogenetic activity is to be observed, however more intense, than what is observed for the present scenario.

The Kinetic Energy (KE) balance allows a better understanding of the energetic processes related to the mechanisms of growth and decay of atmospheric disturbances at different scales. The differences of KE between GW and PD are shown in Figure 2(a) and Figure 2(b). Negative values are found in the north of the 45˚S (40˚S) for DJF (JJA). Still, positive values concentrate between 45˚S and 62˚S (47˚S and 60˚S) during DJF (JJA). The highest values of KE are found mainly in the Malvinas Islands region and are related to an increase in the activity of extratropical cyclones as well as to an intensification in the direction of the westward flux, also being conditioned to the seasonal variation of the Antarctic sea ice in the months of DJF and JJA. The results indicate that under GW conditions, more intense systems will likely act in these regions during JJA (Figure 2(b)). These regions of mid and high latitudes can become sources of more intense cyclogenesis, but will present a displacement Southwards when compared to PD.

Higher values of KE found during DJF under GW conditions (not shown) demonstrate that during this period of the year the BMC region may present larger baroclinic activity, consequently inducing changes in climate features of the south coast of South America during DJF. On the other hand, during JJA, there is a possibility of decreasing in KE magnitude, demonstrating that less systems will act in the BMC region.

The differences between GW and PD of the horizontal momentum transports (u'v') are presented in Figure 3(a) and Figure 3(b). This transport represents the changes between KE of the disturbances and KE of the basic state.

For the PD and GW climatology, there is intense zonal symmetry of the horizontal transport of momentum during DJF, where the vortices are confined to a narrow range of latitudes, indicating that the horizontal transport of momentum presents less spatial variability during DJF (because systems evolve and are kept within the jet). For the JJA period, the activity of these meteorological systems extends through a greater range of latitudes, also presenting an interruption of the flux over South America, possibly caused by the orography of the region [28] [29].

The differences for the horizontal transport of momentum, presented in Figure 3, indicate that the mainly differences during DJF (Figure 3(a)) are found on the East of South America, particularly around 55˚S. For JJA (Figure 3(b)) the highest values are centered on OAS, varying between 45˚S and 65˚S latitudes. The largest values of the anomalies presented above are found in a region of connection between the Antarctic and Southern Ocean, which is associated with the largest southern thermal gradient.

Comparing PD and GW scenario, the horizontal transport of momentum is intensified and increases the area of maximum activity of the polar side of the ST in the middle latitudes. During JJA (Figure 3(b)) the region of maximum activity is located around 50˚S, thus the horizontal transport of momentum remains more intense, contributing to the acceleration of the basic flow, while its greater zonal symmetry continues to occur during DJF (Figure 3(a)).

According to [1], SST in mid-latitudes is fundamental for the asymmetric distribution and ST intensification of the lower troposphere. In addition, the region area is characterized by a strong southern gradient, but, under GW conditions, the gradient is even more pronounced. These differences in SST are even more evident when comparing SST between PD and GW (Figure 4(a) and Figure 4(b)), it can be seen that SST shows a tendency to increase in all SAO to GW, becoming even more evident during JJA (Figure 4(b)). The greatest differences are observed following the Malvinas and Brazil Currents, with the highest values on the Malvinas Current.

We suggest that the SST strong increase in middle latitudes is determinant for asymmetry and intensification of ST, associated by the heat flux in GW conditions. Therefore, it can be assumed that when the atmosphere is exposed to high CO₂ concentrations, the associated SST gradient will be higher.

Thus, the changes in ST are likely to be related to changes in SST patterns, and might lead to faster development of eddies and differences in the asymmetric distribution of ST. Besides, the projections of climate change made in GW scenario should lead to a series of changes that may modify the general circulation of the atmosphere [30].

Sea ice is a key component of the climate system due to its role in radiative processes, mass exchange and momentum. According to [31], sea ice modulates ocean-atmosphere interaction in Antarctic region. The sea ice variability in Austral Ocean occurs at different time scales, having a significant impact in

processes involving deep water and and changes of the water bodies, more precisely in the Weddell Sea [32]. Thus, it is very important to evaluate the changes in sea ice thickness around Antarctic region and its association with ST.

For PD conditions the sea ice thickness annual average reaches a maximum of 3.5 meters in the Weddell Sea and Antarctic Peninsula [33]. Figure 5 shows a shrinkage of approximately 5˚ in the entire extent and thickness of sea ice, mainly in the Antarctic Peninsula and Weddell Sea. The differences shown clearly demonstrate a decrease in sea ice thickness up to 2.5 meters to north of the Antarctic Peninsula during DJF (Figure 5(a)) and up to 2.0 meters in JJA (Figure 5(b)). For the Weddell Sea region, the reduction is close to 1.0 (0.7) meter in DJF (JJA).

The differences shown in Figure 5 demonstrate an important point for the ST displacement to the South direction, which is the shrinkage in GW scenario from the heating influenced by the increase in CO₂ concentration. Also, it is noted that this decrease in sea ice may be of great relevance in the zonal baroclinicity, influenced by the thermal contrast between sea ice and ocean, agreeing with the results obtained by [34], who evaluated the interannual variations of GM around Antarctica for the period from 1979 to 2000, and its possible impacts on ST. The results obtained by [34] showed that sea ice has a very well established seasonal cycle and a significant annual variation, with anomalies in the sea ice extent showing a good agreement with the anomalous positioning of ST.

Thus, the displacement around 5˚ toward the South of the ST (Figure 1 and Figure 2), under GW conditions also can be attributed to anomalies in the sea ice extension. In this way, it can be assumed that a significant zonal baroclinic influence occurs due to the thermal contrast between sea ice and ocean.