Sea ice in the Weddell Sea and its relationship with the South Atlantic Subtropical High and precipitation in South America

Abstract

This study aims to evaluate the position and intensity of the South Atlantic Subtropical High (SASH), related to sea ice extent (SIE) retraction and expansion in the Weddell Sea, assessing precipitation in South America. We assess the difference between atmospheric fields related to SIE (four most intense retraction events minus four most intense expansion events) in February. To this end, we used NSIDC SIE, ERA-5 reanalysis, CHIRPS precipitation, ICOADS SST, ONI/SAM indexes (CPC/NOAA). In the following month, under neutral ENSO and SAM, we observed tropospheric warming in the Weddell Sea and cooling in the mid-latitudes South Atlantic. There is a weakening of both the Weddell Sea circumpolar low and the high pressures between tropical and subtropical latitudes, in addition to the equatorward shift of the Ferrel cell. Therefore, SASH weakens and contracts, resulting in a reduction of the tropical Atlantic moisture supply to South America and negative precipitation anomalies in the tropical region - similar to the suppression pattern of the South Atlantic Convergence Zone. Our results suggest that SIE retraction (expansion) in the Weddell Sea may contribute to the weakening (strengthening) of the SASH and an early-ending (longer-ending) or drier-ending (wetter-ending) rainy season in tropical South America.

Key words
sea ice; south atlantic aubtropical high; weddell sea; south america; south american monsoon system

INTRODUCTION

The South Atlantic Subtropical High (SASH) is a semi-permanent high-pressure system, typically located between 15°-45°S and 45°W-15°E (Mächel et al. 1998). This system is characterized by counterclockwise circulation (with calm winds at the center and more intense winds at the edge), subsidence (corresponding to the descending branch of the Hadley cell) and divergence in the low troposphere (He et al. 2017). In this way, evaporation exceeds precipitation in the SASH region, influencing the South American Monsoon System (Vera et al. 2006, Raia & Cavalcanti 2008, Arraut et al. 2012, He et al. 2017).

A Monsoon System is regarded as the reversion in the low-level wind direction between summer and winter, when the annual average is removed from the seasonal composition, associated with changes in thermal contrasts between continents and oceans, due to the difference in specific heat (Zhou & Lau 1998, Mechoso et al. 2005, Silva & Kousky 2012, Kitoh et al. 2020; and references therein). This difference in atmospheric circulation results in dry winters and rainy summers, typical of the tropical South America (Rao et al. 1996, Kousky & Ropelewski 1997, Grimm 2003, Gan et al. 2004). The exception occurs between the mouth of the Amazon River and the northern Northeast Brazil, where the maximum precipitation occurs in the austral autumn (March-April-May) (Silva & Kousky 2012).

The life cycle of the South American Monsoon System is influenced by spatial and temporal variations in the moisture flow governed by the SASH position and intensity. In summer, there is a significant change in the wind over South America in relation to the period before the beginning of South American Monsoon System. The main variation occurs in the northerly winds over the southwestern tip of the Amazon, which become northwesterly; and in the easterly winds over eastern Brazil, which become northeasterly, due to the eastward displacement of SASH (Raia & Cavalcanti 2008). These changes in low-level circulation contribute to the moisture flow from the Amazon and tropical South Atlantic to the Monsoon region (Lenters & Cook 1995, Rao et al. 1996, Nogués-Paegle et al. 2002, Raia & Cavalcanti 2008, Penna et al. 2021). Both of which are important mechanisms for the precipitation formation associated with the South Atlantic Convergence Zone (SACZ; Kodama 1993, Lenters & Cook 1995, Mechoso et al. 2005, Arraut et al. 2012, Penna et al. 2021). After the beginning of South American Monsoon System, the northwesterly flow in the southwestern tip of the Amazon extends towards central South America, where there is a confluence with flow from SASH (Vera et al. 2006, Raia & Cavalcanti 2008). This is because the Andes mountain range acts as a barrier (Silva & Kousky 2012), where there is a regional intensification of this flow due to the low-level jet, which transports considerable moisture from the Amazon to the La Plata basin (Berbery & Barros 2002).

During winter, as SASH is shifted westward, towards the Southeastern Brazil, regionally presenting low total precipitations, while in the Northeastern Brazil there is a southeastern moisture transport (Rao et al. 1996, Doyle & Barros 2002, Reboita et al. 2010). Precipitation is favored in the Northeast of Brazil when SASH is displaced to the south and west of its climatological position, since the winds from the northern branch of the SASH can intensify the trade winds that arrive on the coast of the Northeastern Brazil and, therefore, contribute to greater moisture transport from the ocean to the continent (Moscati 1991).

Sea surface temperature (SST) anomalies affect the atmosphere through changes in the latent and sensible heat fluxes of the oceans, resulting in anomalous warming /cooling patterns (Holton 2004). The location of SST, in relation to large-scale atmospheric circulation, is one of the conditions for these anomalies to produce atmospheric circulation changes (Shukla 1986). The sea ice edge is in a very sensitive region - south of the Southern Hemisphere baroclinic zone. Thus, Antarctic sea ice can affect atmospheric circulation, from the surface to mid-troposphere levels (Raphael et al. 2011, Kidston et al. 2011, Parise et al. 2015).

Sea ice is the most variable component of the climate system. In the Southern Hemisphere, 5 million km² of the Southern Ocean surface is frozen in February, reaching 17 million km² in September (Parkinson 2019). Sea ice directly affects the global energy balance and the thermal balance of the planet. Therefore, variations in Antarctic sea ice cover affect the thermal gradient between the Equator and the South Pole, altering meridional energy transport and, thus, weather (Watkins & Simmonds 1995, Godfred-Spenning & Simmonds 1996, Menéndez et al. 1999, Carpenedo et al. 2022) and climate (Raphael et al. 2011, Carpenedo et al. 2013, Parise et al. 2015, 2022, Carpenedo & Ambrizzi 2022, Queiroz et al. 2022) of the Southern Hemisphere.

Although there is evidence of the connection between Subtropical Anticyclones and sea ice (e.g., Raphael et al. 2011, Carpenedo & Ambrizzi 2016, Atiqah Azhar et al. 2020, Queiroz et al. 2022), this relationship is still poorly understood. Studies carried out through numerical modeling show that a reduction in Antarctic sea ice cover results in a decrease in the meridional temperature and pressure gradients, weakening the low-pressure circumpolar belt and displacing the polar jet, northwards (Cunningham & Bonatti 2011, Raphael et al. 2011). In this way, the Polar cell expands and weakens, so that the Ferrel cell shifts north (Raphael et al. 2011), which can affect the position and intensity of Southern Hemisphere Subtropical Highs, associated with the subsidence movements in the subtropics, around 30° latitude (Carpenedo & Ambrizzi 2016). Thus, the descending branch of the Ferrel cell, in the subtropics, is related to the existence of subtropical anticyclones.

Variations in SASH position can contribute to precipitation anomalies and even hydrological extremes over South America, thus amplifying environmental risk to society by directly influencing water supply for human consumption, hydropower production and agriculture. For example, the severe droughts of 2013-2014 and 2014-2015 in the Southeast Brazil were associated with the westward displacement of SASH, preventing the normal passage of extratropical cyclones and frontal systems and contributing to weaker SACZ events (Seth et al. 2015, Coelho et al. 2016). Therefore, a better understanding of the atmospheric and oceanic mechanisms related to SASH is of great importance to improve the weather and climate forecast of South America.

The objective of this study is to evaluate the SASH position and intensity in sea ice extent (SIE) retraction and expansion events in the Weddell Sea, assessing the associated atmospheric circulation and South America precipitation. We focus our analyses in the Weddell Sea SIE, a region in the South Atlantic sector of the Southern Ocean, with the largest SIE in the Southern Ocean (Zwally et al. 2002).

MATERIALS AND METHODS

Datasets

The National Snow and Ice Data Center has made SIE data available, since 11/01/1978. This data is an estimate from the brightness temperature of the SMMR (Scanning Multichannel Microwave Radiometer) and SSM/I (Special Sensor Microwave/ Imager) sensors. The brightness temperature was converted to sea ice concentration using the Bootstrap algorithm (Comiso 1995). The sea ice concentration represents an average estimate of sea ice cover, greater than 15% for each pixel (25 x 25 km resolution). SIE is calculated from this data and is defined as the total area covered by sea ice, with an average concentration greater than 15%. In this study, we used the SIE monthly time series (1981-2018) for the Weddell Sea sector (60°W-20°E).

We used the monthly atmospheric fields of the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis v5 (ERA5), from 1981 to 2018. ERA5 is the fifth generation of ECMWF atmospheric reanalysis, produced by the Copernicus Climate Change Service (C3S). In ERA5, the data assimilation produces atmospheric variable estimates for the entire globe, with horizontal resolution of 0.25° latitude and longitude (Hersbach et al. 2020). This study employed the following variables: vertical velocity (omega) and air temperature, between 1000 and 100 hPa; mean vertically integrated moisture divergence; 250-hPa zonal wind; 850-hPa zonal and meridional wind; and mean sea level pressure (MSLP).

The monthly precipitation time series was obtained from the Climate Hazard Group InfraRed Precipitation with Station data (CHIRPS), with an almost global coverage, from 50°S to 50°N and for all longitudes, in a grid of 0.05° x 0.05° and temporal resolution from 1981 to the present (Funk et al. 2015). CHIRPS incorporates satellite images with in-situ station data, generating precipitation time series at a grid point (Funk et al. 2015).

We used the Extended Reconstructed Sea Surface Temperature (ERSST) version 5 (ERSST.v5; Huang et al. 2017), derived from the International Comprehensive Ocean-Atmosphere Dataset (ICOADS). ERSST.v5 is a global monthly SST dataset, with horizontal resolution of 2° latitude and longitude and spatial completeness enhanced using statistical methods. This dataset uses SST from Argo floats above 5 m (Huang et al. 2017).

We used the monthly Southern Annular Mode (SAM) index from the Climate Prediction Center (CPC/NOAA). This index comes from the projection of daily 700-hPa geopotential height anomalies, poleward of 20°S, onto the loading pattern of the SAM mode.

The CPC/NOAA supplied the Oceanic Niño Index (ONI). El Niño (La Niña) events are defined by the persistence of quarterly SST anomalies (three-month moving average), considering a the ≥ +0.5°C (≤ -0.5°C) threshold of at least five consecutive quarters.

Methods

For the monthly SIE anomalies in the Weddell Sea, the climatological reference period used is 1981–2010, as recommended by WMO - World Meteorological Organization (2017). The SIE retraction events (negative anomalies) and expansion events (positive anomalies) were analyzed for February, which accounts for the month of minimum Antarctic SIE (Parkinson 2019). We analyzed the four most intense SIE retraction events (1984, 1988, 2005, and 2017) minus the four most intense SIE expansion events (1991, 2003, 2004, and 2014) in the Weddell Sea, similar to the methodology used by Pezza et al. (2008), for February and one month after (March), under neutral El Niño-Southern Oscillation (ENSO) and SAM. In order to ensure the same number of years for each composite and to maximize the physical patterns here discussed, we present difference maps between top and bottom events. To determine the neutral phase of the SAM, we considered monthly indexes between ±1.0 standard deviation (cf. Reboita et al. 2009). The neutral ENSO occurs when the SST anomaly intensity and persistence, during El Niño or La Niña events, are not satisfied (see Datasets).

To assess the SASH position and intensity, we used the area between 38°S-20°S/38°W-10°E as a reference region, according to Degola (2013). Within this area, the grid point with the highest monthly MSLP was defined as the centre position (latitude and longitude) and intensity (hPa) of the SASH (adapted from Sun et al. 2017). Furthermore, the SASH position and intensity were evaluated using the 1018 hPa (cf. Reboita et al. 2019) and 1020 hPa (cf. Fahad et al. 2020) isobar. We produced difference composites from atmospheric field and SST in the South Atlantic, during and one month after the four most intense SIE retraction events minus the four most intense SIE expansion events in the Weddell Sea. The statistical significance of the difference composites was obtained with the t-Student test, at 10% significance level (Wilks 2006).

RESULTS

Figure 1 shows the average February Weddell Sea SIE in the four most intense SIE retraction events, in the four most intense SIE expansion events, and climatology. It is possible to observe that the SIE average position in retraction events (red line) is to the south of the climatological position (green line), while there is no sea ice cover in the Weddell Sea, to the east of 30°W, in retraction events. The average SIE retraction events in the Weddell Sea is 1.08 × 10⁶ km², which corresponds to -21.1% deviation in relation to the climatology. On the other hand, the average position of SIE expansion events (blue line) is north of the climatological position (green line). The average Weddell Sea SIE expansion events is 1.91 × 10⁶ km², which corresponds to a +39.4% deviation in relation to climatology.

Figure 1
Mean SIE in the Weddell Sea in the four most intense SIE retraction events (red line; 1984, 1988, 2005, and 2017); in the four most intense SIE expansion events (blue line; 1991, 2003, 2004, and 2014); and climatology (green line; 1981-2010), in February. Source: ERA5 (ECMWF).

Figure 2 shows the statistical properties of the SASH intensity and position in the four most intense SIE retraction events and in the four most intense SIE expansion events. During the February SIE retraction events, the SASH is more intense than in the SIE expansion events (Figure 2a). In addition, a southward (Figure 2b) and eastward (Figure 2c) shifted of the SASH occurs during SIE retraction events. In the following month, the SASH intensity and variability is lower in the SIE retraction events than in the SIE expansion events (Figure 2d). In relation to the SASH latitudinal position, mean latitude is higher in SIE retraction events than SIE expansion events (Figure 2e). However, the 75th percentile and the maximum of the SASH latitudinal position is higher, indicating lower latitudes of SASH. An eastward (Figure 2f) shifted of the SASH persist during SIE retraction events.

Figure 2
Box plot of (a, d) intensity, (b, e) central latitude and (c, f) central longitude of SASH in February (top) and March (bottom) associated with the four most intense Weddell Sea SIE retraction events (1984, 1988, 2005, and 2017) and the four most intense Weddell Sea SIE expansion events (1991, 2003, 2004, and 2014), in February.

Figures 3a and 3b show the latitude-height cross section of the air temperature in the South Atlantic (20°W to 10°E) in SIE retraction events in relation to the Weddell Sea expansion events, in February. During the retraction events (Figure 3a) the air temperature is 0.3°C to 1.2°C higher than in expansion events, observed between 40° and 65°S, mainly from 1000 to 600 hPa. In the north, around 35°S, the air temperature differences reach -1.2°C, from 1000 hPa up to the mid-troposphere. In the subsequent month (Figure 3b), the SIE retraction event air temperatures reach 1.8°C warmer than in expansion events, observed south of ~55°S and from 1000 to 400 hPa. On the other hand, cooling down to -2.1°C (from 900 to 300 hPa) occurs between 40° and 50°S, extending towards 65°S in 200-150 hPa. Low-level warming occurs in tropical latitudes, between 20°S and close to the Equator, up to +1.5°C.

Figure 3
South Atlantic (20°W-10°E) zonally averaged vertical distribution of composite differences of (a, b) temperature (°C) and (c, d) vertical velocity (10^-2 Pa s^-1) in February (left) and March (right) associated with the four most intense Weddell Sea SIE retraction events (1984, 1988, 2005, and 2017) minus the four most intense Weddell Sea SIE expansion events (1991, 2003, 2004, and 2014), in February. Continuous (dashed) lines represent the mean vertical velocity of 0 Pa s^-1 in SIE retraction (expansion) events. Gridded areas are significant at the 10% level.

The anomalous vertical structure of SASH in SIE retraction events (relative to expansion events) is evaluated through latitude-height cross sections of the vertical velocity (omega; Figures 3c, 3d), which are a representation of the meridional circulation cells (Hadley, Ferrel and Polar). During the SIE retraction events (Figure 3c), relative to expansion events, there is a strengthening of the Ferrel cell ascending movement between 55° and 60°S (from 1000 to 200 hPa) and a weakening in the north of up to 350 hPa, at the limit between the ascending and subsident branch of the Ferrel cell. This strengthens the downward movements ~50°S. Thus, there is an expansion to the south of the Ferrel cell subsidence movement of up to ~50°S (Figure 3c, continuous line), that is, ~4° latitude south of the position observed in expansion events (Figure 3c, dashed line). In the following month (Figure 3d), there is a weakening of ascending branch of the Ferrel cell, between 50° and 65°S, in practically the entire troposphere. At the same time, there is a strengthening of ascending branch of the Ferrel cell in the south, between 65° and 70°S, and in the north, between 45° and 50°S, at the limit between the ascending and descending branch of the Ferrel cell. Thus, the subsidence movements in the southern limit are weakened and contracted to the north up to ~45°S (Figure 3d, continuous line), that is, ~5° north latitude in relation to the position observed in expansion events (Figure 3d, dashed line). To the north, up to ~30°S, there is a weakening of subsidence movements in practically the entire troposphere.

The MSLP difference between the Weddell Sea SIE retraction and expansion events, in February and March, are shown in Figures 4a and 4b, respectively. During SIE retraction events (Figure 4a) there is an increase in MSLP in the Southeast Atlantic, so that the 1018 hPa isobar is displaced to south-southeast (Figure 3a, continuous line) in relation to the expansion events (Figure 4a, dashed line), which is in agreement with southward displacement of Ferrel cell. In addition, in expansion events, the 1020 hPa isobar does not exist, unlike retraction events. This indicates that SASH is strengthened and displaces southeastward in retraction events (relative to expansion events). In much of the tropical South Atlantic, there is a reduction in MSLP, ratifying the southward displacement of SASH. In the subsequent month, in March (Figure 4b), there is an increase in MSLP in the east of the Weddell Sea, while in the South Atlantic, between 25° and 50°S, there is a reduction. The 1018 hPa isobar is contracted to the east in retraction events (Figure 4b, continuous line), relative to expansion events (Figure 4b, dashed line), in addition to the lack of a 1020 hPa isobar of retraction events, in agreement to weakening of the Ferrel cell. Thus, SASH weakens in retraction events when compared to SIE expansion events.

Figure 4
Composite differences of (a, b) MSLP (hPa), (c, d) 850-hPa wind vector and speed (shaded; ms^-1) and (e, f) 250-hPa zonal wind (ms^-1) in February (left) and March (right) associated with the four most intense Weddell Sea SIE retraction events (1984, 1988, 2005, and 2017) minus the four most intense Weddell Sea SIE expansion events (1991, 2003, 2004, and 2014), in February. Continuous (dashed) lines represent the mean 1018 and 1020 hPa isobar in SIE retraction (expansion) events, respectively. Gridded areas and yellow lines are significant at the 10% level.

During the SIE retraction events, relative to expansion events, there is a weakening of the 850-hPa winds in the subtropical South Atlantic, with easterly, northeasterly, and southeasterly wind anomalies (Figure 4c). The anomalous anticyclonic circulation in the South Atlantic, between 30º-60ºS, and anomalous cyclonic circulation northward indicate southward displacement of SASH, in agreement with MSLP results. In the northern South America, there is a weakening or inversion of the trade winds. It is possible to observe an anomalous inversion (i.e., weakening) of the South America low-level jet, although without statistical significance. In the month following the SIE events (Figure 4d) there is an extensive cyclonic anomaly in the South Atlantic, indicating a weakening of SASH and, consequently, of the southeast trade winds. In addition, the weakening of the northeast trade winds over northern South America during the retraction events (Figure 4c) persists into March.

Figures 4e and 4f show the 250-hPa zonal wind in SIE retraction events in relation to the expansion events in the Weddell Sea. In February (Figure 4e) we can see that in retraction events there is a strengthening of the polar jet in the South Atlantic and a weakening of the subtropical jet in the north (relative to expansion events). In the following month, in March (Figure 4f), the opposite pattern occurs to that observed in February, with a weakening of the polar jet and a strengthening of the subtropical jet. This alternating pattern of weak polar jet and intense subtropical jet suggests a shift towards the north of the atmospheric circulation (Raphael et al. 2011), corroborated by the shift of 5° latitude to the north of the Ferrel cell, in the month following the SIE retraction events (Figure 3c), relative to expansion events. Consequently, there is a weakening and contraction of SASH.

The mean vertically integrated moisture divergence during the SIE retraction events in relation to the expansion events (Figure 5a) presents anomalous divergence (decreased vertically integrated moisture) in the subtropical South Atlantic, between 50° and 30°W. On the other hand, in northwest South America and in central-east Argentina, there is an anomalous convergence (increased vertically integrated moisture). In the month following the SIE events (Figure 5b), the area of anomalous divergence expands to southeastern Brazil and northwestern South America, while an extensive area of anomalous convergence can be observed between northeastern Brazil, towards the extratropical South Atlantic, accompanied by a persistent increase in vertically integrated moisture in central-east Argentina.

Figure 5
Composite differences of (a, b) mean vertically integrated moisture divergence (10⁴ kg m^-2s^-1) and (c, d) SST (°C; over the ocean only) and precipitation (mm; over the continent only) in February (left) and March (right) associated with the four most intense Weddell Sea SIE retraction events (1984, 1988, 2005, and 2017) minus the four most intense Weddell Sea SIE expansion events (1991, 2003, 2004, and 2014), in February. Gridded areas are significant at the 10% level.

Analyzing the SST difference between the SIE retraction and expansion events (Figure 5c), it is possible to observe an anomalous warming south of 60°S over the Weddell Sea, in the tropical South Atlantic, along the coast of northeastern Brazil, and in the equatorial South Atlantic. Precipitation reduction occurs in central and northwestern Argentina, northeast and central-northern Brazil, Bolivia and southern Peru. Increased precipitation occurs in central-eastern Argentina and northwestern South America. In the following month, in March (Figure 5d), positive SST anomalies persist over the Weddell Sea. SIE retraction events demonstrate that there is an SST cooling in the extratropical South Atlantic (~25°-55°S) and an SST warming in the tropical South Atlantic (relative to expansion events). This pattern is associated with negative precipitation anomalies, between southeastern Colombia towards southeastern Brazil, as well as areas in eastern Bolivia and northern Paraguay. Positive precipitation anomalies occur in parts of north and northeastern Brazil, western Bolivia and central-east Argentina.

DISCUSSION AND CONCLUSIONS

SIE retraction and expansion events in the Weddell Sea during February can influence the intensity and position of SASH and precipitation in South America, in the subsequent month, via Ferrel cell.

Figure 6 presents the schematic representation of the differences between the atmospheric circulation in March, associated with the SIE retraction and expansion events in the Weddell Sea during February. The air temperature difference, between the retraction and expansion events, shows a warming in high latitudes over the Weddell Sea as a thermally direct response to the SIE retraction. Lower SIE results in an albedo reduction and greater absorption of solar radiation, which leads to ocean surface warming and heat flows from the ocean to the atmosphere (Menéndez et al. 1999, Cunningham & Bonatti 2011). Variations in the SIE affect the air temperature, not only on the surface. Warming, due to SIE retraction events, occurs from the surface to the mid-troposphere. This result was also observed in previous studies through numerical experiments (Raphael et al. 2011, Parise et al. 2015). In the mid-latitudes of the South Atlantic, the difference is negative, indicating an air temperature reduction in SIE retraction events in relation to the expansion events (up to -2.1°C), with greater differences in the subsequent month (March) and in the mid-troposphere (600-400 hPa). Thus, in relation to the expansion events, the warming in the high latitudes over the Weddell Sea and the cooling in the middle latitudes of the South Atlantic suggest a reduction in the meridional thermal gradients, in the month following February SIE retraction events.

Figure 6
Conceptual diagram of the composite differences in March associated with the four most intense Weddell Sea SIE retraction events (1984, 1988, 2005, and 2017) minus the four most intense Weddell Sea SIE expansion events (1991, 2003, 2004, and 2014), in February.

The MSLP positive anomalies and the weakening of the Ferrel cell upward movements, between 50° and 65°S, through practically the entire troposphere, demonstrate that the circumpolar low over the Weddell Sea weakens in the month following SIE retraction events (relative to expansion events). Similarly, between the tropical and subtropical latitudes, where SASH is evident, there is a weakening of high pressures (negative MSLP anomalies and a weakening of downward movements, between 45° and 30°S, in practically the entire troposphere).

The meridional thermal gradient reduction, between the high latitudes over the Weddell Sea and the mid-latitudes of the South Atlantic in SIE retraction events (relative to expansion events) coincides with the apparent decrease in the meridional pressure gradients. Thus, we suggest that the pressure gradient between the Weddell Sea and the South Atlantic is influenced by SIE retraction (lower gradient) and expansion (higher gradient) events in the Weddell Sea. This occurs because there is an increase in heat flow from the ocean to the atmosphere during SIE retraction events, where the ocean surface temperature increases due to the greater absorption of solar radiation, resulting in adjacent atmospheric warming. The hot air is lighter, so that the pressure surfaces rises and the meridional pressure gradient decreases. The opposite occurs in SIE expansion events.

At higher levels, the polar jet weakens, between the northern Weddell Sea and the southern South Atlantic, while the subtropical jet is strengthened to the north, in the SIE retraction events (relative to expansion events). This alternating pattern of weak and intense winds suggests a northward shift of the atmospheric circulation (Raphael et al. 2011). In the mid-latitudes of the South Atlantic, between 40° and 50°S, the air temperature in the SIE retraction events is lower than in the expansion events between 900 and 300 hPa, suggesting that the pressure surfaces is lower and the meridional pressure gradient increases. Therefore, we observed a strengthening of Ferrel cell upward movements between 45° and 50°S, at the southern limit with the downward movements, which results in a northward shift of about 5° latitude in the SIE retraction events (at ~45°S), relative to expansion events. This difference observed further from the edge of the sea ice indicates that there may be some indirect dynamic adjustments.

The results presented here corroborate with Raphael et al. (2011), which analyzed the influence of the minimum, average and maximum Antarctic sea ice concentrations on the Southern Hemisphere atmospheric circulation, during the Southern Hemisphere summer. This was undertaken by employing numerical experiments with a coupled general circulation model. Whereas Raphael et al. (2011) analyzed the minimum and maximum sea ice concentration across the Southern Ocean, the present study analyzed SIE in retraction and expansion events only in the Weddell Sea. Nonetheless, both studies observe many similar characteristics. These results show that sea ice in the Weddell Sea has an important role in influencing the Southern Hemisphere atmospheric circulation, considering that the Weddell Sea has the largest SIE among the Southern Ocean sectors (Parkinson 2019).

Therefore, the weakening of Ferrel cell in the South Atlantic in the following month (in March) can be the effect of the Weddell Sea SIE retraction events in February. Complimentarily, there is a 5° latitude displacement of the Ferrel cell, towards the Equator (relative to expansion events). Hence, there is a weakening and contraction of SASH, especially in the western branch, which results in a weakening of the southeast trade winds. There is also a weakening of the northeast trade winds over northern South America.

The southeast and northeast trade winds are responsible for the moisture flow from the Amazon and the tropical North and South Atlantic to the South American Monsoon region (Carvalho & Cavalcanti 2016; and references therein). This represents an important mechanism for generating precipitation, associated to the South Atlantic Convergence Zone (SACZ; Kodama 1993, Lenters & Cook 1995, Marengo et al. 2004, Mechoso et al. 2005). This is because the Andes Mountain Range acts as a barrier (Silva & Kousky 2012), where there is a regional intensification of this circulation due to the South America low-level jet, transporting a considerable amount of humidity from the Amazon to the La Plata Basin (Berbery & Barros 2002). With the weakening of the trade winds observed in the month following the SIE retraction events (relative to expansion events), there is a reduction in the humidity supply from the tropical North and South Atlantic towards South America. This can be observed by the decreasing vertically integrated humidity, between southeastern Brazil and northwestern South America.

With SASH weakened, SST in the extratropical South Atlantic is relatively cooler. This is probably due to the weakening of the subsidence movements and the consequent weakening of the warming due to SASH-related adiabatic compression. In the tropical South Atlantic, SST is relatively warm. This pattern is associated with negative precipitation anomalies over the tropical South America, with a northwest-southeast orientation between southeast Colombia/ northwest Amazon and towards southeast Brazil, while in central east Argentina the anomalies are positively associated with a vertically integrated humidity increase.

The spatial organization of negative precipitation anomalies over South America is very similar to the suppression pattern of the SACZ (Nogués-Paegle & Mo 1997, Marengo et al. 2004, Vera et al. 2006, Muza et al. 2009). SACZ events occur 10% in March (Ambrizzi & Ferraz 2015), which is the beginning of the rainy season´s decay in South America, coinciding with the convection displacement towards the Equator (Vera et al. 2006, Liebman et al. 2007), Bombardi et al. (2014) showed that, in the positive phase of the South Atlantic Dipole, similar to the SST difference pattern observed in the present study, there is a reduction in cyclogenesis and low density of cyclone trajectories on the southeastern coast of Brazil. This reduces precipitation over eastern South America, which appears to be associated with the organization of the SACZ. Saurral et al. (2014) observed a positive correlation between sea ice concentration in the Weddell Sea and precipitation over the SACZ region. At the same time, the correlation was negative (without statistical significance) over Uruguay and northeastern Argentina, indicating that lower sea ice concentrations in the Weddell Sea are associated with a weakening of SACZ activity and vice versa. These findings corroborate with the patterns observed in the present study. Thus, the results presented here suggest that SIE retraction events in the Weddell Sea may be related to an early or drier end of the rainy season in South America, while SIE expansion events may be related to a delayed or more humid end of the rainy season.

In northeastern Brazil there is an increase in vertically integrated humidity and positive precipitation anomalies, while SST is warm in the adjacent South Atlantic. Previous studies show that rainy events in northeastern Brazil accompany positive SST anomalies, south of the Equator, and the Atlantic Intertropical Convergence Zone is strengthened towards the warm waters (Nobre & Shukla 1996, Uvo et al. 1998, Silva & Kousky 2012).

In the last decades (1979-2014) Antarctic SIE has shown an increasing trend, followed by an abrupt reduction, greater than the loss of the Arctic SIE in the last three decades (Parkinson 2019, Eayrs et al. 2021). This same trend has been observed in the Weddell Sea (Parkinson 2019, Turner et al. 2020). Thus, if the SIE reduction trends observed in recent years persist, the results presented here suggest that there may be a weakening of SASH, due to the lower SIE in the Weddell Sea. Consequently, promoting a precipitation reduction in tropical South America, hampering even more water availability (hydropower generation, food production and public utilities), in a region with a large population concentration.

ACKNOWLEDGMENTS

CBC acknowledges the Brazilian National Institute of Science and Technology of Cryosphere (Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq – Research Grant 465680/2014-3). TA acknowledges the National Institute of Science and Technology for Climate Change Phase 2, under CNPq Grant 465501/2014-1; Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP Grants 2014/50848-9 and 2017/09659-6; CNPq under grants 304298/2014-0 and 301397/2019-8. We also acknowledge ECMWF for making ERA5 data available, the NSIDC for providing access to sea ice extent data and the usage of the CHIRPS dataset from the Climate Hazards Group. We thank MSc. Ricardo Burgo Braga for the translation and revisions of this work.

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Publication Dates

• Publication in this collection
  28 Nov 2022
• Date of issue
  2022

History

• Received
  4 Jan 2022
• Accepted
  25 July 2022