<Li> Polar outbreaks: Polar outbreaks occur when cold dense polar air masses pass beneath warmer tropical air masses, significantly cooling subtropical South America . They occur as a result of anticyclogenesis in the Pacific subtropical high, which is constrained at a low level by the southern extent of the Andes mountain range . This leads to the formation of a slow - moving long wave that creates mountain - parallel, ageostrophic flow, causing incursions of high - latitude cold air . They generate an important drop in temperature and increase in pressure, resulting in regional precipitation for southern South America . These surges occur mainly during the winter but their impact on the precipitation is even greater during summer . </Li> <Li> Low Level Jet (LLJ): LLJs originate in a low pressure area over the northern Andes and provide moisture for subtropical latitudes . During summer, they operate as localised wind maximum within the lower 1--2 km of the atmosphere, channelled by the Andes, terminating in southeastern South America . They are controlled by Amazonian wind patterns, which are influenced and controlled by patterns of insolation . They transport large amounts of moisture from the Amazon basin to the monsoonal anitcyclone over Bolivia . A suppressed SACZ and increased convection in the sub-tropical plains is associated with a strengthening of the LLJ . These phases are linked to short - term extreme precipitation events in the plains of central Argentina . When the LLJ is weak, there is enhanced SACZ and suppressed convection to the south and extreme heat waves over the sub-tropical regions . It also generates turbulence through shear and participates actively as trigging mechanism for the formation of severe storm and Mesoscale Convective Systems over Paraguay, Northern Argentina and South of Brazil . </Li> <Li> Westerlies: South America experiences westerly winds in the middle latitudes, caused by the Coriolis force and associated geostrophic circulation patterns . They are more intense than their Northern Hemisphere counterparts due to the lack of continental landmass in the Southern Hemisphere . They reach their maximum speed in the troposphere, where they form jetstreams . In particular, over the southern tip of South America and the adjacent south Pacific, the westerlies are strongest during austral summer, peaking between 45 ° and 55 ° S. During the austral winter, the jetstream moves into subtropical latitudes (its axis is at about 30 ° S) and the low - level westerlies expand equatorward but weaken, particularly at ~ 50 ° S . The pressure gradients between the polar low - pressure belt and the Pacific high - pressure cell, combined with these westerlies, results in permanent anticyclogenesis . Northward penetration of atmospheric perturbations from the westerlies is possible when the southeast Pacific anticyclone is weakened or moves equatorward, allowing penetration of westerly storm tracks to latitudes as far north as 31 ◦ S. In the Andes, winter rains reach further north . During summer, the Pacific anticyclone shifts southward, impeding the northward migration of the westerlies . </Li> <Li> The Bolivian High: large anticyclonic circulation centred near 15 ° S, 65 ° W. It has been explained as the response of diabatic local heating in the Amazon region . The SACZ has a strong influence on the position and intensity of the Bolivian High </Li>

What runs the length of pacific south america