Observational evidences on the modulation of the South American Low Level Jet east of the Andes according the ENSO variability

Abstract

Abstract. The differences on the phase and wavelength of the quasi-stationary waves over the South America generated by El Niño (EN) and La Niña (LN) events seem to affect the daily evolution of the South American Low Level Jet east of the Andes (SALLJ). For the austral summer period of 1977-2004 the SALLJ episodes detected according to Bonner criterion 1 show normal to above-normal frequency in EN years, and in LN years the episodes show normal to below-normal frequency.During EN and LN years the SALLJ episodes were associated with positive rainfall anomalies over the La Plata Basin, but more intense during LN years. During EN years the increase in the SALLJ cases were associated to intensification of the Subtropical Jet (SJ) around 30 • S and positive Sea Level Pressure (SLP) anomalies over the western equatorial Atlantic and tropical South America, particularly over central Brazil. This favored the intensification of the northeasterly trade winds over the northern continent and it channeled by the Andes mountain to the La Plata Basin region where negative SLP are found. The SALLJ cases identified during the LN events were weaker and less frequent when compared to those for EN years. In this case the SJ was weaker than in EN years and the negative SLP anomalies over the tropical continent contributed to the inversion of the northeasterly trade winds. Also a southerly flow anomaly was generated by the geostrophic balance due to the anomalous blocking over southeast Pacific and the intense cyclonic transient over the southern tip of South America. As result the warm tropical air brought by the SALLJ encounters the cold extratropical air from the southerly winds over the La Correspondence to: G. A. M. Silva (gyrlene@model.iag.usp.br) Plata basin. This configuration can increase the conditional instability over the La Plata basin and may explain the more intense positive rainfall anomalies in SALLJ cases during LN years than in EN years.

Impacts of different ENSO flavors and tropical Pacific convection variability (ITCZ, SPCZ) on austral summer rainfall in South America, with a focus on Peru

“…This is opposite to the westerly pattern and divergence over the western Amazon sometimes observed after El Niño has peaked, which leads to drought in the region, although the warming of the northern tropical Atlantic plays a more dominant role in this case (Espinoza et al, 2011;Marengo and Espinoza, 2016). A northerly transport anomaly prevails east of the Andes into SESA, associated with the enhancement of the SALLJ (Silva et al, 2009) …”

“…For example, Giannini et al (2001) numerically showed that the warm ENSO phase reduces rainfall in most parts of the tropical Atlantic basin because El Niño warms the free troposphere and thus forces the vertical stabilization of the tropical atmosphere. Furthermore, the warm phase of the ENSO enhances humidity flux from low latitudes to SESA through an enhanced South American low level jet (SALLJ) (Marengo et al, 2004;Silva and Ambrizzi, 2006;Silva et al, 2009), while the South Atlantic Convergence Zone (SACZ) at the same time moves southward of its climatological position (Robertson and Mechoso, 2000;Carvalho et al, 2004).…”

Eventos de Precipitação Extrema na Região do Centro de Lançamento de Alcântara

“…Os aspectos dinâmicos e termodinâmicos associados a esse descrição poderão ser abordados em trabalhos futuros utilizando dados de Reanálise e TSM, de forma análoga ao realizado em vários estudos (p. ex., Silva et al, 2009). Os compostos poderiam ser obtidos não somente para os dias de PE, mas também para o dia anterior e posterior ao de PE.…”

La Niña events before and after 1979 and their impact in southeastern South America during austral summer: role of the Indian Ocean

“…Several studies have documented the effects of El Niño Southern Oscillation (ENSO) on the climate of southeastern South America (SESA) (Ropelewski & Halpert 1987, 1989, Aceituno 1988, 1989, Pisciottano et al 1994, Grimm et al 1998, Grimm 2003, Cazes-Boezio et al 2003, Kayano et al 2009, Silva et al 2009, Zamboni et al 2010. Some of these studies found statistically significant effects in the austral spring of ENSO years and somewhat weaker effects during the austral autumn of the subsequent years, with a tendency for positive precipitation anomalies over SESA during El Niño and negative anomalies during La Niña (Pisciottano et al 1994, Grimm et al 1998.…”