South African Cold Fronts

Meteorological Physical Background

Mean circulation and vertical structure

The upper-tropospheric circulation over South America during the summer is characterized by a quasi-stationary wave made by the BH, a trough to the west, over the Pacific Ocean, and one to the east, over northeastern Brazil, known as the Northeast Low, NEL.

The BH has its climatological center located at approximately 17°S - 70°W and it is an important component of the South American Monsoon System (SAMS) in its mature stage.

Mean circulation at 200 hPa and accumulated precipitation in summer in the southern hemisphere. (Figure adapted from Garreaud, Aceituno, 2007)

In terms of its vertical structure, the BH presents a warm core below 150 hPa, with a maximum at 300 hPa. Lenters (1997) suggests that the presence of this core is due to subsidence in the western coast of South America. The BH presents a thin layer of cold air above 150 hPa, to the west of the area with large scale deep convection with overshooting tops of Cb's over the Amazon Basin. Adiabatic cooling takes place within the easterly flow. Adiabatic cooling takes place within the easterly flow, which leads to the movement of the cold layer west of the overshooting tops.

Vertical Cross Section of geopotential height anomaly (contours) and temperature anomaly (shaded). The white line indicates the 0°C isotherm and the darker shadings indicate higher temperatures. The region depicted in the figure lies between between 10°S and 25°S. (Figure adapted from Lenters y Cook, 1996).

Overview of the regional-scale features related to the Bolivian High

Two semi-permanent low pressure systems are related to the upper level BH. It has two different centers: the Chaco Low (CHL) northwest of Argentina, which is a warm core low located between Paraguay and Bolivia, and the Northwestern Argentina Low (NAL). While the structure of the CHL depends on thermal processes, the NAL encompasses both thermal and orographic processes (Saulo et al., 2010).

By the end of November CHL intensifies generating an increase in the pressure gradient between the center of South America and the northwest of Sahara, Africa. As a result of this, the intensity of the trade winds increases, producing an anomalous positive humidity flow towards the South American continent. The winds entering from the northeast are forced towards the south by the Andes, thus forming the South American Low Level Jet (SALLJ). These low level winds advect humid air. As a result, precipitation over Amazonia, central and southeastern Brazil and the South American Convergence Zone (SACZ) is increased.

At high levels, the BH and NEL make up a quasi-stationary short wave system which is intensified by the upward movement of the convective activity over Amazonia, the SACZ and the Central Andes. Moisture convergence at low levels and the latent heat release at mid-levels help sustain the upward movement and maintain a high and relatively warm troposphere in the region. These features are typical of the South American Monsoon System (Vera et al., 2006).

Contrary to what might be thought, studies such as Gandu and Silva Dias (1998) and Lenters and Cook (1997) show that the elevation of the Andes does not directly and significantly affect upper level circulation. Simulations indicate that the BH is formed even in the absence of the Andes. In spite of this, the mountain range indirectly intensifies the anticyclone by inducing precipitation in the Central Andes and by modifying precipitation in other regions.

Surface circulation associated with the Bolivian High LLJ: Low level Jet ITCZ: Intertropical Convergence Zone SACZ: South Atlantic Convergence Zone CHL: Chaco Low NAL: Northwestern Argentina Low H: Subtropical Anticyclones	Upper level circulation associated with the Bolivian High BH: Bolivian High NEL: Northeast Low

Seasonal variability of the Bolivian High

The seasonal variations of the BH are closely linked to the different stages of the South American Monsoon System.

Winter: the area of heavy precipitation located in the north of South America is related to the position of the ITCZ. The upper level anticyclonic center is not present over the continent.
Spring: at surface level, the easterly flow moves towards the south, advecting humidity and displacing the area of heavy precipitation towards the center of Amazonas. A weak anticyclonic center related to this convective activity starts forming over South America, slightly to the north of the climatological position of the BH. These shifts in precipitation and upper level circulation trigger the South American monsoon.
Summer: during the mature stage of the monsoon, precipitation reaches its maximum value in the center of South America, Central Andes and over the SACZ. The BH is intensified in response to this convection and the release of latent heat, and settles over the Bolivian plateau. The cyclonic center which forms over northeast Brazil downstream of BH is the NEL.
Autumn: polar systems entering the continent lead to a decrease in moisture and rapid decrease in available latent heat. This, together with the sun's shift northwards of the equator, causes the weakening of the subtropical ridge/BH. NEL is no longer present and the monsoon weakens.

Mean seasonal precipitation (shaded) and stream function at 200 hPa for southern hemisphere (a) Winter (June-July-August), (b) Spring (September-October-November), (c) Summer (December-January-February), and (d) Autumn (March-April-May). H: high pressure system , BH: Bolvian High and NEL: Northeastern Low. Figure adapted from Garreaud and Aceituno, 2007.

Four typical upper level circulation patterns of the BH

The position and shape of the BH are modified by precipitation and different components of the atmospheric circulation. Ferreira et al. (2004) identify four types of configurations:

a) Bolivian High - Northeast Low

Stream function at 250 hPa, for January 2, 1999. (Adapted from Ferreira et al., 2004)

In this pattern the BH is located in its climatological position.
A cyclonic vortex (NEL) is located downstream and over the Atlantic Ocean, around NE Brazil. The generation of these vortices is due to the amplification of the upper level ridge associated with cold fronts moving towards the equator over South America (Kousky and Gan, 1981).
The transition zone between the upper and lower level systems (BH and NEL) favors divergence in the upper levels. This leads to convection and precipitation.
Between the subtropical ridge and the upper trough to the east, convective instability is maximized by the advection of cooler air aloft. Also, weak wind maxima of 35-50 kt tend to favor areas of upper divergence as they circle the trough, leading to further enhancement of convection.

b) Bolivian High - Northeast low - Anticyclonic Center

Stream function at 250 hPa, for January 7, 1999. (Adapted from Ferreira et al., 2004)

When the NEL intensifies and moves towards the continent the BH acquires a noticeable NW-SE orientation over SE Brazil. Depending on the intensification of the ridge, the BH can split into two anticyclones over SE Brazil and the adjacent ocean.
Ferreira et al. (2004) suggests that the formation of the AT to the south of the NEL is associated with an amplification of an upper level ridge due to the movement of cold fronts from Argentina to the SACZ region.
The cold fronts remain stationary for a few days establishing the SACZ and associated cloudiness.
NW-SE oriented band of convective clouds and precipitation is produced west of the ridge, together with a mid-latitude trough at upper levels. This pattern is generally associated with a frontal system.

c) Bolivian High -Anticyclonic

Stream function at 250 hPa, for January 19 , 1999. (Adapted from Ferreira et al., 2004)

This pattern is generally seen once the NEL dissipates, the AT intensifies and the BH moves towards the west and settles over the Pacific Ocean.
A mid-level trough develops over Argentina between the two anticyclonic circulations favoring convection in southeastern Paraguay and northeastern Argentina.
On the southeast flank of the AT convection is reinforced by upper level circulation, which produces stratiform precipitation.

d) Bolivian High - Trough

Stream function at 250 hPa, for January 12, 1999. (Adapted from Ferreira et al., 2004)	When a trough at high levels dominates the circulation over southern Brazil, the BH extends zonally and is centered approximately over northern Chile. The NEL is not observed in this case, so the cold fronts move freely towards the northeast.

Interaction with SACZ

Numeric simulations have shown that precipitation in SACZ modulates the intensity and position of the BH. The pattern that is responsible for the mutual influence between convection in the SACZ and the BH is known as the South American SeeSaw (SASS) Dipole.

a) SASS negative phase

When frontal systems reach southeastern Brazil, strong convergence at low levels increases the humid westerly flow deflecting the SALLJ towards the SACZ region. Under these conditions, convection over the SACZ is intensified and well organized, and extends to the southeast. Strong compensation subsidence inhibits precipitation over Paraguay, Uruguay and the subtropical regions of Argentina and Brazil. All these changes in the precipitation fields over South America weaken the BH and shift it to the west from its climatological position.

b) SASS positive phase

When the SACZ is weakened and to the south of its climatological location, the low level jet acquires an intense southern component along the east of the Andes. The BH is intensified and centered over the Bolivian Plateau.

Interaction between the Bolivian high and deep convection over northern Argentina

The BH may help to develop deep convection, especially in its periphery, due to the presence of diffluent and divergent flows.

Green: Geopotential height at 200 hPa
Orange: Isotachs at 200 hPa
Light blue: Geopotential at 500 hPa
Shaded red area: enhanced convection

Therefore two mechanisms may contribute to convective development:

a) Interaction with a mid-level short wave:

A short-wave trough moving south of the BH may generate sufficient divergence to cause the development of explosive convection and mesoscale convective systems (when other atmospheric parameters are favorable) over northern Argentina and southern Brazil.

b) Formation of intense high level jet:

Other mesoscale factors, such as convergence lines, gravity waves, convective rolls at the planetary boundary layer and surface inhomogeneities also affect the triggering of convection. These mechanisms play an essential role particularly in the northwestern region of Argentina, where they lead to a development of local circulations associated with the topography and the radiative heating (Vidal, 2014). Also, in this region the lee side perturbations of the airflow nearly perpendicular to the northern Andes of Chile can provide the triggering mechanism for diurnal convection.