3.1. Mean conditions in DJ
Figure 3a shows the long-term mean streamlines at 850 hPa. The large anticyclonic gyres in each hemisphere correspond to the subtropical highs of the northern and southern Atlantic Ocean. As is typical of winter and summer circulations, the winter North Atlantic anticyclone is more elongated in the zonal direction, while the summer South Atlantic anticyclone has a relatively larger meridional extent. Poleward of these cells the westerly flow dominates the midlatitudes. At low latitudes, the winter hemispheric trade winds extend from the west coast of Africa across the tropical Atlantic, with a strong meridional gradient of streamlines that is maximized near the northeast coast of South America. A portion of this flow is diverted into the Amazon basin and then southward into the subtropics, forming a counterclockwise gyre over tropical South America. This continental-scale anticyclonic gyre is related to the topographic forcing, the Coriolis force and the deep continental low over the CHA region that forces northeasterly winds from low latitudes to turn southward. The southward flow is channeled between the eastern slope of the Andes and the Brazilian plateau and shows a regional intensification due to the South American low-level jet (SALLJ: Saulo et al. 2000; Jones 2019).
Over Africa, the northeast trade flow circulation is strengthened by a high-pressure center located over the northwest of the continent in winter. This high-pressure center is part of the subtropical ridge that includes the Azores anticyclone over the north Atlantic and the Saharan High (Fig. 2a). Northwest Africa in winter is affected by the subsidence of the descending branch of the Hadley circulation, introducing unfavorable conditions for the occurrence of precipitation. The strong radiative cooling during the night contributes to the strengthening of the high pressure in this region. The topography of northwest Africa induces changes in the regional circulation and even influences the location of the intertropical convergence zone (ITCZ: Hagos and Cook 2005; Siongco et al. 2015). The cool dry winds that blow from the northeast or east in the Western Sahara desert towards the Gulf of Guinea are commonly referred to as the Harmattan (Burton et al. 2013; Fiedler et al 2015).
In the upper troposphere the circulation is reversed, with a flow from South America to northwest Africa across the tropical Atlantic Ocean (Fig. 3b). The anticyclonic circulation over South America at 15°S is the Bolivian High (BH) (Orrison et al. 2022), which forms as a dynamic response to the diabatic heating released by deep convection over the Amazon Basin and central Brazil (Silva Dias et al. 1983; Figueroa et al. 1995; Lenters and Cook 1997). Downstream, the trough over the tropical South Atlantic near the coast of Brazil is called the Nordeste Low (NEL: Virji 1981; Chen et al. 1999). Leaving northern South America, the streamlines tighten over the Amazon River estuary and to the north of Venezuela. Jet streams occur in the mid-latitudes of the Southern Hemisphere, along the east coast of North America, and in northern Africa. An upper-level midocean trough in the North Atlantic, extending into the WEM region, lies above the surface anticyclone.
Figure 3c shows the mean precipitation. In the Atlantic Ocean, the ITCZ is located north of the equator, with heavier precipitation on the South American side. The position of the ITCZ, associated with the low-level convergence of the moisture-laden trade winds, lies in the minimum pressure belt, and is centered at the latitude where the meridional component of the wind cancels out (Nobre and Shukla 1996). In contrast to the copious precipitation in the ITCZ, rainfall is nearly absent over the high-pressure cells of the subtropical oceans (Cherchi et al. 2018). Beyond the subtropical gyres, broad bands of precipitation extend across the extratropical oceans, especially in the North Atlantic, due to the boreal winter storm track.
During the austral summer in South America, a broad area of abundant precipitation extends from the Amazon basin to northern Argentina. The South Atlantic Convergence Zone (SACZ; Carvalho et al. 2004), a diagonal band of precipitation maxima, extends from southeastern Brazil to the southwestern Atlantic Ocean. On the Pacific Ocean coast, two regions with high precipitation are observed: in the northernmost part, associated with the presence of the ITCZ over the eastern Pacific north of 5°N, and in the Patagonian Andes, associated with the orographic effect over the westerlies. The dry regions include the Atacama Desert, on the coast of southern Peru and northern Chile, northeastern Brazil, at the eastern end of the continent, and Patagonia east of the Andes.
Figure 3e shows the vertically averaged moisture transport and the corresponding vector field. The source of moisture for South America is water evaporated from the ocean as air flows across the tropical Atlantic, either from the equatorial and low latitudes of the Northern Hemisphere, which upon penetrating the northern part of the continent produces a cross-equatorial transport of moisture, or from the equatorial side of the South Atlantic anticyclone. Moisture inflow from the tropical Atlantic to the continent is highest around the equator and also at the northern tip of South America, feeding the northern branch of the SALLJ (Jones 2019). In addition, evapotranspiration from Amazonian vegetation is also an important source of moisture for precipitation over South America (Dominguez et al. 2022). Further south, moisture transport is maximised along the central branch of the SALLJ, which is funneled between the Andes and the Brazilian Plateau, towards northern Argentina.
The southward flow of moisture through the SALLJ is facilitated by the Chaco Low, which is visible as a cyclonic curvature in the streamlines near 20°S east of the Andes in the CHA region (Fig. 3a and 3e). The Chaco Low interacts with the Amazon basin and the South Atlantic anticyclone, as noted by Seluchi and Marengo (2000). The pressure gradient between the Chaco Low and the high pressures in the western South Atlantic also promotes the flow of moisture east of the CHA region (Fig. 3e). The hot and humid tropical air flow to the east of the CHA region provides the necessary instability conditions for the development of precipitation in south eastern South America (SESA: Giles et al. 2021).
The convergence of vertically integrated moisture flux in tropical South America and east of the Andes is enhanced by the drag produced by the continent and the presence of the mountain range (Fig. 3f). Moisture convergence is also observed in the ITCZ, SACZ, and mid-latitude storm tracks, as well as to the west of the CHA region. Conversely, moisture divergence is mainly on the equatorial side of subtropical anticyclones over the Atlantic.
The fact that the lower tropospheric circulation associated with the SAMS in DJ extends from the WEM region to the CHA region, transporting moisture from the tropical Atlantic to Amazonia and from there to SESA, and that the simultaneous and opposite surface pressure variations in the WEM and CHA regions cause an increase in the pressure gradient (Fig. 2), suggests a possible link between the two continents. If such a link exists, we would expect the monsoon flow to intensify during the summer months as this gradient increases.
3.2 Spatial fingerprints of WEM, CHA and CHAWEM indices
Here, we analyse the spatial patterns of variability in the WEM and CHA regions both individually and in combination (Fig. 4). Figure 4a shows the correlation between the WEM index and pressure anomalies. The Atlantic Ocean, between approximately 50°N and 20°S, along with Africa, south western Europe and eastern North America, display positive pressure anomalies associated with increases in WEM pressure (positive correlations). Figure 4d displays the correlation between the WEM index and surface temperature. Increased WEM pressure is linked to lower temperatures in northern Africa, the Iberian Peninsula, and the tropical Atlantic north of the equator, and higher temperatures in eastern North America and northern Europe. This pattern is similar to that observed during the positive phase of the NAO in the boreal winter. In South America, higher temperatures are observed to the east of Amazonia. Figure 4g shows the correlation between the WEM index and wind intensity at 850 hPa. Positive (negative) correlations indicate wind strengthening (weakening) associated with higher WEM pressure. Boreal winter months with positive WEM index are associated with stronger winds in the mid-latitudes of the Northern Hemisphere and around 20°N, and weaker winds at 30°N. Over Africa, the Harmattan winds in the Sahara desert and the trade winds in the Gulf of Guinea become stronger. In the South Atlantic, the flow in the polar branch of the subtropical anticyclone intensifies.
The middle column in Fig. 4 shows the correlation between the CHA index and anomalies of pressure (Fig. 4b), temperature (Fig. 4e) and wind intensity at 850 hPa (Fig. 4h). During the austral summer, the intensified Chaco Low typically results in negative pressure anomalies across South America and a large portion of the Atlantic Ocean (Fig. 4b). Additionally, cold anomalies can be observed in Brazil and Patagonia, while warm anomalies occur to the west of Amazonia and in SESA (Fig. 4e). Furthermore, an enhanced SALLJ can be observed (see Fig. 4h). A deeper Chaco Low is also accompanied by an intensification of westerlies in Patagonia at 40°S (consistent with higher baroclinicity in that area), a weakening of westerlies south of the continent around the Drake Passage (which would suggest an association with the negative phase of the SAM), a weakening of the anticyclonic gyre in the South Atlantic (consistent with negative surface pressure anomalies), and an intensification of easterlies in the equatorial Atlantic and parts of West and Northwest Africa.
Figure 4c, f, i (right column in Fig. 4) show the correlations between the combined CHAWEM index and the pressure, temperature and wind intensity anomalies, respectively. The correlations of the WEM (left column) and CHA (central column) indices show opposite behaviors over much of the Atlantic. Pressure or wind intensity anomalies associated with a deepening of the Chaco Low tend to be opposite to anomalies associated with an increase in pressure in the WEM region (e.g., a decrease in pressure in the CHA region is associated with a decrease in pressure in the tropical Atlantic, but an increase in pressure in the WEM region is associated with an increase in pressure in that region). Due to the increasing pressure gradient between CHA and WEM, these opposite effects are mutually canceling, and the correlations between the CHAWEM index and the tropical Atlantic pressure anomalies are not significant (Fig. 4c). In general, the positive CHAWEM index is associated with a decrease (increase) in pressure in the continental and oceanic zones adjacent to the CHA (WEM) region. Similarly, in the case of correlations with wind intensity, CHA and WEM regions are associated with opposite correlations over large oceanic areas, especially in the extratropics of both hemispheres. However, there are exceptions, such as the intensification of the trade winds in the central Atlantic and the Gulf of Guinea, which is simultaneously favored from South America by the deepening of the Chaco Low and from northwest Africa by the intensification of anticyclonic conditions. Surface temperature decreases in the tropical Atlantic with the intensification of the pressure gradient (Fig. 4f). In South America, a higher pressure gradient between CHA and WEM regions is associated with higher temperatures in Amazonia, east of the Andes, and in SESA. Over Patagonia, however, the effect is the opposite.
3.3 Changes associated with strengthening of the CHA-WEM gradient
To assess the impact of an enhanced pressure contrast between the CHA and WEM regions on the SAMS, Fig. 5 shows the differences between the composite of months with a strong pressure gradient between CHA and WEM (i.e. strong CHAWEM index) and the long term climatology.
As the CHAWEM index strengthens, the Southern Hemisphere exhibits negative sea level pressure anomalies at mid-latitudes, with the largest anomaly found on the polar branch of the South Atlantic anticyclone. The opposite occurs in the Northern Hemisphere, with positive anomalies maximized over southern western Europe (Fig. 5a). The 850 hPa streamline anomalies show an intensification of winds in the eastern equatorial Atlantic, around 20°N and over northern Africa (Fig. 5b). In the Southern Hemisphere, a cyclonic anomaly centered at 45°S in the South Atlantic is consistent with a weakening of the westerlies south of 50°S. Over South America, the continental anticyclonic gyre, the SALLJ, the northerly flow in the CHA region, and the westerlies over Patagonia become more intense. An anomalous ridge form near the coast of Uruguay (30°S, 40°W) which reinforces the zonal gradient of the circulation between the CHA region and the western Atlantic when the Chaco Low deepens, causing the meridional flow to intensify southward over SESA.
Figure 5d, f shows the changes in vertically integrated moisture transport and surface temperature. The largest changes in moisture flux occur in the winter hemisphere, with more moisture being transported westward between the coast of North Africa and the Caribbean. The Gulf of Guinea presents moisture flux divergence in the north and convergence further south over the west coast of southern Africa. The westward moisture flux increases slightly north of the equator, at the latitude of the ITCZ. In northern South America, the moisture flux is enhanced by the trade winds that cross Venezuela. On the continent, the moisture flux increases towards the south, parallel to the Andes, with a maximum anomaly in the CHA region, which contributes to an increase in moisture convergence at the exit of the SALLJ in SESA. In the southern Atlantic, the moisture flux increases eastward between 30°S and 40°S.
Temperature increases are observed in SESA, especially in those areas where precipitation does not increase (e.g. Uruguay), and temperature decreases in Patagonia (Fig. 5f). This pattern, with cooling in Patagonia and warming in the subtropics, is reminiscent of the negative phase of the SAM (Garreaud et al. 2008). The maximum increase in specific humidity in the lower troposphere (not shown) is found in SESA, due not only to the enhanced convergence, but also to the surface warming. The enhanced convergence of water vapor leads to a significant increase in precipitation just north-east of this region. Temperature changes in the Northern Hemisphere (Fig. 5f) are similar to those associated with the positive phase of the winter NAO (cold anomalies in northern Africa, southern Europe, and eastern Canada, and warm anomalies in northern Europe and the United States).
Precipitation anomalies (Fig. 5e) show less precipitation along the ITCZ in the Atlantic, especially on its equatorial side. Also, more precipitation is observed north of the ITCZ near South America and south of the Gulf of Guinea, suggesting a shift of the ITCZ north on the South American side and south on the African side. In the Northern Hemisphere, precipitation decreases over the North Atlantic anticyclone zone and especially over southern Europe and adjacent areas. In South America, precipitation decreases over most of Brazil and the SACZ, while precipitation increases downstream of the SALLJ, to the east of the CHA region, and to the south of the South Atlantic Anticyclone. These changes are consistent with the changes in the 850 hPa circulation and moisture convergence discussed above.
Figure 5c shows circulation changes at 200 hPa. The upper branch of the Hadley circulation in the tropical North Atlantic is weakened (southward streamline anomalies between 15°N and 5°S). These changes are in accordance with the weakening of the ITCZ. Over South America at about 15°S, the BH and, downstream over the western South Atlantic, the NEL, are weaker for high values of the CHAWEM index.
For clarity, Fig. 6 schematises the climatology and the main anomalies in the upper troposphere: the weakening of the BH and the NEL is indicated by solid curved arrows near 15°S (Fig. 6b). Consequently, the 200 hPa monsoon return flow (i.e. the flow from the BH, northward near the Brazilian coast and eastward in the tropical North Atlantic) weakens, as indicated by the westward anomalies over South America at 15°S and the southward anomalies in the tropical Atlantic near the Brazilian coast (outlined arrow). This would imply that the strengthening of the surface gradient between WEM and CHA is not sufficient to ensure an enhanced return flow into the winter hemisphere from the reversed upper tropospheric pressure gradient, which would contradict the initial hypothesis that the strengthening of the lower tropospheric pressure gradient should enhance the monsoon circulation (Zhou and Lau 1998). However, the BH weakening fosters the intensification of the cross-equatorial flow towards the Northern Hemisphere over northern South America (filled arrow in Fig. 6b). The intensification of the return flow to northwest Africa is completed across the northern Atlantic with an intensification of the westerlies at about 20°N. In the active CHAWEM months, therefore, the 200 hPa anomalies favor a northward return flow to North Africa over northern South America rather than over eastern Brazil. On the other hand, to the south of South America, changes in the upper tropospheric circulation appear to be influenced by extratropical dynamics. The streamlines anomalies at 200 hPa suggest a wave pattern with a cyclonic gyre off the southern coast of Chile, followed by an anticyclonic gyre around the SESA coast (anomalies linked by the dashed arrow in Fig. 6b). This latter anticyclonic anomaly, in turn, contributes to the weakening of the westerly winds at about 20°S between the BH and the NEL.
Figure 7a displays a vertical cross-section of the meridional wind across the equator for the climatological mean. The cross-equatorial flow towards the summer hemisphere can be seen in the lower half of the troposphere, and the opposite occurs above the 400 hPa level over northern South America, west of about 30°W. Over the equatorial Atlantic, the meridional wind is weaker, and its direction is reversed (northward near the surface and southward in the upper troposphere). Over Africa, northerly winds are observed in the lower troposphere and southerly winds in the upper troposphere. As the CHAWEM index strengthens (Fig. 7b), small negative anomalies are observed in the middle and lower troposphere to the east of the Andes (between 65°W and 80°W), indicating a slightly more intense southward flow. In the upper troposphere of this continental area, the northward return flow is stronger. These differences suggest that the cross-equatorial circulation tends to be stronger over South America, although only marginally so near the surface. As previously noted, there are prominent southward anomalies in the upper troposphere over the Atlantic between 0°W and 50°W.
The circulation of the BH is evident at 200 hPa in Fig. 7c (climatological mean vertical cross-section at 17°S), with northerly winds west of 60°W and southerly winds east of the Andes, initiating the return of the monsoon circulation to the winter hemisphere over the eastern half of South America. The southern Atlantic, to the east of the NEL in the upper troposphere, is dominated by northerly winds. In the lower troposphere, wind direction is reversed. Northerly winds reach their maximum intensity over the continent in proximity to the Andes and near the east coast. The anomalies that appear as the pressure gradient between the CHA and WEM intensifies (Fig. 7d) confirm the weakening of the BH (positive anomalies to the west of the Andes and negative anomalies to the east in the upper troposphere). Further to the east, the succession of positive and negative anomalies at higher levels over the continent and Atlantic Ocean is associated with the complex pattern of anomalies described in Fig. 5c. In particular, the weakening of the NEL corresponds to the negative anomaly at 30°W and the positive anomaly at 15°W. In the lower troposphere, northerly winds intensify over South America, especially between the Andes and the Brazilian Plateau.