Fronts and frontogenesis during summer: geometrical and dynamical aspects and the influence over rainfall on the South American subtropics / Frentes e frontogênese no verão: aspectos geométricos, dinâmicos e impactos sobre as chuvas nos subtrópicos da América do Sul

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Three different aspects related to summer precipitation over subtropical South America (SA) and the continental pattern of humidity transport in that season are studied. Firstly, a climatological point of view is adopted and it is shown that most of the humidity that enters SA through its northern coast, up to Par´a, comes from the tropical Northern Atlantic. It is also shown that most of the humidity that leaves that part of the ocean heads towards inland SA, crossing over this coast. In the subtropics and extra-tropics, the spatial distributions of the climatological daily standard deviation of the meridional humidity transport and of its magnitude are notoriously similar to each other, as well as to that of the climatological rains. This points towards an important contribution from transient disturbances to the rain and humidity transport patterns. In the following part of the work, some new aspects pertaining the interaction between the North Western Argentinean Low (NAL) and transient troughs south of it are brought to light. The combination of these two elements brings about the formation of a geometrical structure in the pressure field. The influence of this structure over the wind field is documented and explained. One of the important elements of this structure is the saddle that necessarily occurs between the NAL and the transient trough. This saddle is a preferred spot for convergence and deformation of the wind field, both warm and cold advections, frontogenesis in equivalent potential temperature and rain. In addition to this saddle, the shape and geographical location of its separatrices reveal the essential aspects of the referred structure. Both extend from the tropics to the midlatitudes, intersecting themselves on the saddle point, which is located in the subtropics. The flow over the orbits that make up these separatrices reflects the flow of the wind, which therefore aproaches the saddle both from the tropics and from the midlatitudes. Two of the orbits which locally make up the separatrices merge away from the saddle forming a self connection of the saddle which contains the NAL in its interior. This allows for the formation of an extension of the transient trough, which wraps over the self connection, and is here called the Low Pressure Tongue East of the Andes (LPTEA). The LPTEA geostrophically forces the development of a moisture corridor all the way from Amazonia to the midlatitudes, allowing the water vapour which has flowed over the forest to fuel the transient disturbance. It is ilustrated how, in the absence of a saddle self connection, the geometry of the wind field causes the transient disturbance to be fuelled by humidity coming directly from the South Atlantic. This is probably the typical winter situation. It is found that the geometrical structure described above is very common during summer, but does not always lead to frontogenesis and rain formation. Organized events, lasting a few days, in which frontogenesis and rain formation are strongly influenced by this structure are identified. Their occurence is associated with the incursion of transient disturbances coming from the Pacific. These events are separated in two categories, which can be distinguished by the displacement of the frontogenesis: towards the northeast (NE events) and towards the east (LE events). Both are studied through composits and have their basic structure described in conceptual models. The structural differences between the two categories are caused by the behaviour of the transient disturbance in the high levels. In the last part of the work, the link between the Low Pressure Tongue and its saddle and frontogenesis over the subtropics of SA is statistically evaluated. It is shown that the saddle favours frontogenesis. Furthermore, the presence of the saddle becomes more statistically likely the higher the frontogenesis, approaching 100% for very high values. A statistical link is also shown to exist between the occurrence of the saddle over northern central Argentina and rain slightly to the northeast. The rain attains maximum intensity one day after the detection of the saddle. A similar link is found between intense frontogenesis and rain over the same regions.

ASSUNTO(S)

américa do sul south america pressure frontogênese frentes fronts humidity transport separatrices summer transporte de umidade meteorology pressão saddle sela verão rainfall separatrizes frontogenesis meteorologia chuva

ACESSO AO ARTIGO

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