Positive Regional Sea Level Anomalies in Southern Brazil Due to Changes in Austral Atmospheric Circulation

Pressure gradients and winds play an important role in Southern Hemisphere (SH) sea levels, which are currently associated with the positive trend of the Southern Annular Mode (SAM). This study investigated regional sea level anomalies (SLAs) in the southern coast Brazil using altimeter data (1993–2019), post-processed by the X-TRACK (CTOH/LEGOS). We observed a negative SLA from 1993 to 2009 and a positive SLA from 2010 to 2019, with upward trends throughout the evaluation period. We analyzed wind stress curl, pressure, and wind fields at sea level (FNMOC and ERA 5, respectively) in addition to sea surface temperature and height anomalies (SSTA/SSHA-OISST) in the South Atlantic Ocean (SAO) for 1993–2009 and 2010–2019. In relation to the first period, the second shows the enhancement in Hadley and Walker cells and trade winds, in addition to greater SSTA and SSHA in SAO. The SAO subtropical gyre and zonal winds at 45°S contribute to the intensification of the western boundary current. A greater pressure gradient between the SAO surface and the southeast of South America is noteworthy. Regionally, the positive SAM brings an increase in sea level to the study area, caused by greater wind stress and variability in heat flows.


Introduction
Coastal and oceanic sea levels vary in different ways both temporally and spatially. In coastal regions, where most of the global population lives, variability may occur across months or even days. This is why one of the greatest challenges for decision makers is to prepare for sea level rise (SLR) in coastal regions, as indicated by the Intergovernmental Panel on Climate Change 1 . On a regional scale, sea level studies consider areas of dozens to hundreds of kilometers, resulting in variations among different factors 2 . One of these factors is ocean-atmosphere coupling, which generates an interaction between wind and sea surface, near or far from the coast in this study 3 . In the South Atlantic Ocean (SAO), regional variability of sea level, at interannual and multidecadal scales, is produced by surface wind and heat flow anomalies 4 . Wind stress over oceanic surfaces causes undulation anomalies and variations in Ekman transport. These changes led to the coast by the rise in wave height and period, in addition to piles up water to the west. The hot water volume pumped from the tropics to higher latitudes will accelerate due to Ekman transport. This pumping causes a deepening in the tropical thermocline, raising local sea level and leading the cold water to superficial layers, which brings the thermocline to shallower levels and a decrease in sea level to the east.
The friction generated on the oceanic surface by the wind produces oceanic Rossby waves to the west, followed by SLR throughout the propagation track [5][6] .
The Southern Annular Mode (SAM) is described as a pressure gradient between the middle and high latitudes in the Southern Hemisphere (SH). Zonally symmetrical SAM variations also affect the variability in SH oceans at large scales [7][8] . The positive SAM phase is associated with intensification of the west winds over the circumpolar ocean (approximately 60°S) and weakening in the north. This process induces Ekman transport to the north in all longitudes of the circumpolar ocean and to the south around 30°S. Due to mass conservation, Ekman transport generates anomalous resurgence along the Antarctic continent, and subsidence at approximately 45°S. Positive SAM tendency in the last decades, associated with a high interannual variability of wind force, effectively adds to oceanic circulation variability of subtropical and subpolar oceans 9 . Many studies have identified changes in SH oceanic circulation in response to a positive SAM tendency 8,[10][11][12][13][14] .
In all SH oceans, positive SAM intensifies large gyres and increases the sea surface height, which, in turn, elevates sea surface temperature by isopycnal subsidence at 40°S 9,15 . Oceanic gyres represent large oceanic current systems caused by winds on the surface, especially by the vorticity provoked by wind stress 16 . Satellite images show consistent changes in the main subtropical gyres toward the pole in the last four decades. In addition, a propensity for warming in the west currents has been observed in the last century 17 . The western boundary currents (WBCs) of subtropical gyres transport warm water from the tropics to the pole, thus contributing to increasingly humid and warmer climates on adjacent continents.
Migration to the poles of extra tropical circulation agents, such as jet streams and west winds, show that all extra tropical circulation is moving to the south 18 . Two other tendencies have been associated in the extratropical region: positive SAM and Hadley cell expansion [19][20][21] . Climate models have shown that these tendencies are related to global atmospheric warming [22][23][24][25][26][27][28][29][30][31] .
The troposphere temperature rise and tropopause height drive the Hadley circulation toward the pole on the SH [32][33] . Oceanic gyres displacement, coupled with the change in direction of atmospheric extratropical circulation, widely affects ocean heat transport, regional SLR, and coastal ocean circulation 18 .
In the tropical Pacific, the SLR likelihood is three times higher than the global average, induced by trade winds intensification in the last decades 35 . In the tropical Indian Ocean, there was a sea level rise in some regions, boosted by the strengthening of Walker and Hadley cells 35 . Considering this global trend, this study discusses the possible causes of local sea variability in southern Brazil. For this purpose, we used three periods of altimetric monitoring of the sea level obtained from satellite sensors. We evaluated sea level variation in a WBC under the light of the strengthened atmospheric circulation (warmer), wind fields, wind stress curl (WSC), positive SAM tendency, and the tropic-pole teleconnections perspective over SAO.

Results
To obtain the SLA for the study area, 966 cycles were used for each of the two tracks covering it. By averaging the points and cycles, it was possible to obtain the annual SLA to the southern coast of Brazil  On the surface, between the equatorial region and medium latitudes, the pressure gradient resulted from the dominance of the SAO subtropical high; it is worth noting the presence of high pressure on the west coast. The subtropical high was fortified with a medium center of 1025 hPa (Fig. 2a, b).
Between the first and second periods (1993-2009), when the SLA survey by satellite was already available, the meridional anomalous wind field continued to prevail towards the tropic, even though heat flow was better organized from the equator to the pole. Over the SAO, the main atmospheric flow originated from cold air in the Weddell Sea east towards the region of study. Thus, colder air anomalies were present in the region of the study. The subtropical high presented intensification and expansion in its acting area ( Fig. 3a, b).  Meanwhile, at sea level, between the equatorial region and the mid-latitudes, the pressure gradient intensified throughout the study period, strengthening the characteristics of the subtropical high domain and positive SAM. The subtropical high remains fortified with an average center of 1021 hPa, which is more centralized over the ocean and away from the continents. This repositioning adds to the development of a low-pressure area from north to south over South America, from Bolivia to Patagonia, which was not observed in the previous decades (Fig. 6a, b). It is our understanding that this low-pressure area results not only from the subtropical high of the Atlantic and Pacific but also from Brazil's current intensification and continental warming. At the same time, the Walker and Hadley cells weakened and expanded.
During this period, we observed the greatest shift and low-pressure belt change around the Antarctic.
It began to organize three low centers dislodged to the north with the Amundsen-Bellingshausen Sea low and Lazarev and Dumont d'Urville-Davis Sea lows. We also observed the intensification of the pressure gradient between the high subtropical region toward the Lazarev Sea.

Discussion and conclusion
Variations in ocean heat transport and regional sea level rise on the SAO were detected 18 . The authors attributed these variations to the combination of extratropical atmospheric circulation and oceanic gyres toward the pole. From the coupled displacement of these two systems, it was possible to discuss regional sea level variations on the southern coast of Brazil.
This study identified an SLA inversion from negative to positive in 2010. Sea level rise in SAO has previously been identified through satellite data. The authors attributed this elevation to the steric height (halosteric and thermo steric) and changes in the oceanic mass 36 . This elevation is coupled with movements to the subtropical, subantarctic, and polar fronts to the south, associated with the strengthening of the west winds to the west. The southernmost continental shelf of Brazil is considered by scholars to be an area with long term high SLR propensity. Previously, southern Brazil and Uruguay coasts presented higher sea level values in summer and autumn, partially due to seasonal wind variability and solar radiation cycles 37 .
The oceanic front displacement is simultaneous to the extratropical atmospheric circulation migration, resulting in two main reasons: positive annular mode tendencies 19,20 and tropic expansion 21  From 2009, the heat flows intensified in both ways, although with an inversion in relation to the first period, suggesting anomalous growth between the tropic and the pole (vice versa). Over the SAO, there is also a south-to-north inversion of the eastern Atlantic fluxes and anomalous north-south from the west, originating from the study area to the Antarctic Peninsula and Weddell Sea, which is also responsible for the Hadley cell anomalies with anomalous heat and humidity transport to the south [61][62][63] .
In relation to the Coriolis force in this process, it induces oceanic streams and sea surface high differences enforced by geostrophic balance and the pressure gradient. Our SSHA and SSTA records in SAO WBC between 1993 and 2019 show variations compatible with those of previous studies. The Brazil Current front, observed through SSTA and SSHA, presents 6° of latitude variation (1993-2008) 64 . Regional sea surface height development is observed in mid latitudes in both hemispheres, while in high latitudes, the tendency is below the global average. Anticyclones and cyclone centers over subtropical and subpolar gyres are characterized by the regional sea surface height, which is used to detect oceanic gyre localization by ridges and valleys. These ridges and valleys are generated by downwelling and upwelling processes.
Observing them between mid and subpolar latitudes, we noticed an increase in the sea level, together with the migration of subtropical and subpolar fronts towards the pole 64 .
These fronts are the confluence and limit the subtropical, subpolar, and polar gyres. The subtropical fronts and gyres have been migrating decennially, 0.07° towards the poles 18 . According to ocean circulation theory, oceanic gyres are propelled by wind on the surface, more specifically the WSC 16  generates rotation of all ocean circulation at mid-latitudes and together, intensifies subtropical gyres towards the pole. This process causes the WSC vary implying changes in sea level (Fig. 8) 10 . Modeling has demonstrated that extratropical atmospheric circulation undergoes systematic changes over gases, forcing the greenhouse effect, implying, consequently, in oceanic gyres 11,13,14,18,19,65 . Shifting in gyres promotes a pronounced rise band at sea level over medium latitudes. This pattern overlaps with the global sea level average rise, causing an extra threat to the islands and coastal regions at medium latitudes 66 . As part of global circulation, the large-scale change in the oceanic gyre has the potential to reshape the circulation of the ocean over the tropics, close to coastal regions, and also to change meridional overturning circulation 18,67 .

Figure 8. Comparison between sea level anomalies (left axis) from 1993 to 2019 in the study area and Southern Annular Mode index from Nan and Li (right axis)
The consistent changes between 1993 and 2009 and between 2010 and 2019 in the SLA, SSTA, and SSHA were accompanied and influenced by significant variations in the pressure fields on the surface, winds, and temperature flows between high and medium latitudes. Regional sea level internal variability is one of the contributing factors for its increase, although recent studies have shown that anthropogenic forcing is an accelerating factor 68 . Climate models that use increased greenhouse gas effects as a variable suggest that the change observed in oceanic gyres is a consequence of global warming, as the displacement of atmospheric circulation leads to changes in gyres and subtropical fronts 18

Material and method
Satellite sea level altimetric measures in coastal areas provide lower quality data because of the interference of obstacles that these surveys find (radar interaction with the solid surface of the ground and geophysical corrections-suspended particles in the atmosphere, reversed barometer, wind and tide effects, sea level variations among others). The software X-TRACK, designed by the Laboratoire d'Etudes en Géophysique et Óceanographie Spatiale (LEGOS [69][70], uses a post-processing algorithm that reexamines altimetry data from the seashore. Sea level anomalies (SLAs) were projected onto reference tracks with an interval of approximately 6-7 km between each point (1 s). Each processing step was performed on a regional basis with a low-pass filter (with a 40 km cutoff frequency  Fig 1). Thus, we observed and compared the wind field before the era of spatial altimetry and within satellite coverage time from Topex-Poseidon and Jason.
Wind stress curl (WSC) data were taken from the U.S. Navy Fleet Numerical Meteorology and