The South American Water Balance: The Influence of Low-Level Jets

Abstract

To study the climatology of the water balance over South America and analyze the influence of low-level jets (LLJs), a climate study of the water balance and its main components was performed, specifically in the Amazon and La Plata basin (LPB) region, from 1979 to 2008. The results showed that on average for the analysis period, the Amazon basin and LPB performed as a sink of moisture (ET < P) and as a moisture convergence for the regions, which accounted for approximately 62% and 43% of the precipitation, respectively. During the study period, 884 days with an occurrence of LLJs were observed, occurring most frequently during the winter and around 0000 and 0600 UTC. When considering the water balance for the days with LLJs, it was observed that the Amazon acts as a source of moisture, especially in the dry season, and that the LPB behaves as a sink during all months. The influence of the LLJ as a modulator for precipitation on the LPB is clear, as the precipitation is 32% higher during the LLJ events compared with days without LLJs. This main pattern shows that the moisture convergence trough of the LLJs is crucial for the water balance on the LPB, whereas evapotranspiration is a more important variable of the water balance on the Amazon basin with or without the LLJs.

The South American Low‐Level Jet: A New Climatology, Variability, and Changes

“…During summer, jet events are more influenced by the penetration of the northeasterly trade winds across the Amazon, whereas the South Atlantic subtropical high and the upper level subtropical jet stream are more likely to influence jet events during the fall and winter. These results support the description of large-scale circulation and moisture transport patterns presented in previous studies of the SALLJ (Marengo et al, 2004;Nascimento et al, 2016;Wang & Fu, 2004).…”

“…Previous studies (Marengo et al, 2002(Marengo et al, , 2004Nascimento et al, 2016) have identified SALLJ events by applying the Bonner criteria to reanalysis data. The Bonner criteria for identifying low-level jet events are as follows: (1) northerly winds (850 hPa) with speeds ≥12 m/s, (2) vertical wind shear between 850 and 700 hPa ≥ 6 m/s, (3) meridional component larger than the zonal component, and (4) meridional winds from the north.…”

“…The South American low-level jet (SALLJ) east of the Andes (Marengo et al, 2002;Nascimento et al, 2016;Oliveira et al, 2018;Vera et al, 2006) transports substantial amounts of moisture from the Amazon to the subtropics, influencing the development of deep convection and precipitation over the La Plata Basin (LPB). The densely populated LPB is the continent's second largest drainage basin, but arguably the most important in terms of its socio-economic significance.…”

“…The economy of the LPB is primarily based on agriculture and hydropower production, which are both dependent on and vulnerable to extreme precipitation events. Much of the available water vapor for precipitation in this region is supplied by the SALLJ (Berbery & Barros, 2002;Nascimento et al, 2016), which creates the necessary conditions for the formation and sustenance of large, long-lived mesoscale convective systems (MCSs; Salio et al, 2007;Zipser et al, 2004). MCSs often intensify over the LPB, causing disastrous flooding and debris flows that are responsible for numerous injuries, fatalities, and damages to crops and property.…”

A New Look at the Identification of Low-Level Jets in South America

“…LLJs are relevant components of the global atmospheric system because in several regions they are responsible for transporting heat and water vapor poleward into the extratropics (Stensrud 1996;Marengo et al 2004;Vera et al 2006). In subtropical South America, the north-northwesterly LLJ often observed east of the Andes mountain range, and referred to as the South American low-level jet (SALLJ) (Vera et al 2006), plays a crucial role in the hydrological cycle of the La Plata basin and surrounding areas (Berri and Inzunza 1993;Salio et al 2002, hereafter SNS02;Marengo et al 2004; M. G. Nascimento et al 2016). The SALLJ is a component of the environment that leads to deep convection in the La Plata basin since it sets up a moist, unstable environment and promotes low-level lifting at its exit region or as it impinges on the Andes foothills and Sierras de Córdoba (Salio et al 2007;Rasmussen and Houze 2011;Rasmussen et al 2014;Rasmussen and Houze 2016).…”