Tropical and Extratropical Wave Disturbances Impact on the Amazon Basin Precipitation: Subseasonal to Intraseasonal Timescales

Abstract

Esta investigación presenta observaciones que asocian perturbaciones de ondas tropicales y extratropicales en la organización de la precipitación sobre la Cuenca Amazónica (BA). Los resultados sugieren que en la variabilidad de baja frecuencia intraestacional (banda de 30 a 70 días), la Oscilación de Madden-Julian es uno de los principales mecanismos atmosféricos que modulan la variabilidad de la precipitación sobre la región amazónica, jugando un papel fundamental, principalmente durante el invierno austral. A escalas de mayor frecuencia, las ondas de Kelvin acopladas convectivamente juegan un papel relevante en la organización de la convección sobre la Amazonía. Se encuentran dos tipos principales de ondas Kelvin sobre el Amazonas: ondas Kelvin con un precursor del Pacífico que se propaga hacia el Amazonas y ondas Kelvin con un precursor de América del Sur. Ondas de Kelvin con precusor en el Pacífico oriental representa aproximadamente el 35% del total de eventos. Los casos con precursor en América del Sur están asociados con la actividad de los trenes de olas extratropicales, responsables del 40% del total de eventos, o convección “in situ“ que excita localmente las ondas Kelvin, que representan el 25% restante.

Observational evidences linking tropical and extratropical wave disturbances in organizing precipitation over the Amazon basin (AB) are presented. The time scale of the wave disturbances considered in this thesis are from intraseasonal (low- and high-frequency intraseasonal) to subseasonal. The results are based on observed, satellite, and reanalysis datasets. The results suggest that on 30-70-day low-frequency, the Madden-Julian oscillation (MJO) is one of the main atmospheric mechanisms modulating intraseasonal rainfall variability over the Amazon region, playing an important role, particularly during the austral winter. Despite of the fact that the AB region extends over tropical region, the 30-70-day low-frequency intraseasonal are not strictly associated to the forcing produced by the equatorially propagating MJO. In addition, it is suggested that convective-based MJO indices are able to better account the influence of the MJO on intraseasonal precipitation. The 10-30-day high-frequency intraseasonal (HFIS) activity account for about 22% of the total Amazon low-frequency intraseasonal rainfall events and it is part of large-scale pattern observed over South America during the onset of austral rainfall season (October-November) and during the dry season (May-September). It seems that during the peak of the wet season (December-April), the Northwest-Southeast orientation of the precipitation seems to be influenced by Rossby wave trains not previously documented, with a southeastern South America (SESA)-South America convergence zone (SACZ) pattern. At higher time-scales, the Convectively Coupled Kelvin waves (CCKW) has a relevant role in organizing convection over the Amazon. They are more active during the transition seasons (austral spring and autumn), with peaks in November and March. The vertical structure observed on both radiosonde and reanalysis data have confirmed the existence of a second baroclinic mode as widely documented in previous works. CCKW propagate slightly faster over central and eastern Pacific (24.0 m s-1), slow down over the Amazon (13.5 m s-1) and then continues their propagation over the tropical Atlantic at about 20.8 m s-1. Two main types of Kelvin waves over the Amazon are found: Kelvin waves with a Pacific precursor that propagates into the Amazon, and Kelvin waves with a South America precursor. CCKW associated with a preexisting Kelvin in the eastern Pacific account for about 35% of the total events. The cases with South America precursor are associated with either pressure surges from extratropical wave trains activity, responsible for 40% of the total events, or “in situ“ convection that locally excite Kelvin waves, accounting for the remaining 25%.