Recent changes in the South America low-level jet

Charles Jones

doi:10.1038/s41612-019-0077-5

Abstract

The South America Low-level Jet (SALLJ) is a climatological feature with a critical role in the spatiotemporal distribution of precipitation in South America. While previous studies have focused on the mechanisms and variability of the SALLJ in the central Andes (i.e., southeast Peru, Bolivia, and Paraguay), the occurrence of the low-level jet in the eastern slopes of the northern Andes (i.e., northeast Peru, Colombia, and Venezuela) and its linkages to the central region have not been previously explored. This study shows that the SALLJ in the northern branch exceeds 10 m s−1 during September–February and the frequency can be as high as in the central region. Moreover, the SALLJ can occur simultaneously or separately in both regions. When the SALLJ is active only in the northern branch, wind speeds are very low from Bolivia towards southeast South America (SESA) and precipitation is significantly reduced (1–4 mm day−1) from climatology. In contrast, precipitation increases over eastern Bolivia by 2–4 mm day−1. Composites suggest that the SALLJ northern branch is forced by a large-scale circulation pattern with the enhancement of the North Atlantic Subtropical High driving northeasterly winds over the northern Andes. Further analysis shows that the frequency and intensity of the SALLJ in the northern Andes has substantially increased in the last 39 years.

Highlights

Mechanisms proposed to explain the South America low-level jet (SALLJ) include[5,8,24]: (1) deflection of the easterly winds by the Andes as they flow over the Amazon basin and veer to north-northwest around the Andes “elbow”, (2) local processes driven by sloping terrain and boundary layer diurnal cycle, and (3) pressure gradient perturbations associated with cross Andes flow
This study investigated the nocturnal low-level jet that forms on the eastern slopes of the northern Andes in Venezuela, Colombia, and northeast Peru
The results show that the frequency of the low-level jet in the northern Andes can be as high as in the central Andes, and the low-level jet may occur simultaneously or separately in both regions

Summary

Introduction

The South America low-level jet (SALLJ) was first recognized in the 1980s with the aid of sparse radiosonde station data and satellitederived cloud winds.[1,2,3,4,5,6,7] The South America low-level jet experiment (SALLJEX)[8] and the advent of reanalysis datasets have significantly contributed to the understanding of the SALLJ and its influence in the South American climate.[7,9,10,11,12,13,14] The SALLJ reaches very high wind speeds in the eastern central Andes around Bolivia and is present year round.[4,15,16,17] The SALLJ exhibits significant intraseasonal, interannual, and multidecadal variations.[4,7,8,13,15] the low-level jet transports large amounts of moisture from the Amazon Basin towards the subtropics of South America[12,18,19] and intense mesoscale convective systems (MCS), and heavy precipitation frequently develop near its exit region.[20,21,22,23]Perhaps because the SALLJ reaches maximum wind speeds in the central Andes and is clearly identified in vertical wind profiles in Santa Cruz de La Sierra, Bolivia, and Mariscal Estigarribia, Paraguay, previous studies of the SALLJ have focused primarily on the mechanisms and variability of the jet in the central Andes.[4,7,8,12,16,24] Much less attention has been given to SALLJ incidences in the northern Andes defined here as the region including Venezuela, Colombia, and northeast Peru. The South America low-level jet (SALLJ) was first recognized in the 1980s with the aid of sparse radiosonde station data and satellitederived cloud winds.[1,2,3,4,5,6,7] The South America low-level jet experiment (SALLJEX)[8] and the advent of reanalysis datasets have significantly contributed to the understanding of the SALLJ and its influence in the South American climate.[7,9,10,11,12,13,14] The SALLJ reaches very high wind speeds in the eastern central Andes around Bolivia and is present year round.[4,15,16,17] The SALLJ exhibits significant intraseasonal, interannual, and multidecadal variations.[4,7,8,13,15] the low-level jet transports large amounts of moisture from the Amazon Basin towards the subtropics of South America[12,18,19] and intense mesoscale convective systems (MCS), and heavy precipitation frequently develop near its exit region.[20,21,22,23]. We suggest that changes in the SALLJ in the northern Andes are linked to remotely forced suppressed convection and cooling in the tropical eastern Pacific in the last several decades

Methods

Results

Conclusion