UC Santa Barbara

The South American low-level jet (SALLJ) east of the Andes plays an important role in transporting moisture from the Amazon Basin towards the subtropics, influencing the development of convection and precipitation over the La Plata Basin. This research presents a comprehensive analysis of the spatial and temporal variability of the SALLJ across multiple time scales. A new algorithm is introduced for detecting low-level jet events based on seasonal-percentile thresholds of wind speed and wind shear. The algorithm was applied to ERA-Interim reanalysis data to develop a 38-year dataset of SALLJ days (1979-2016). Although the SALLJ occurs year-round, seasonal composites show that moisture transport associated with the jet is largest during austral summer. Trends in the SALLJ were analyzed over the climatological period and revealed an intensification of the northwesterly moisture flux over the SALLJ region in recent decades. The second part of this research examines the large-scale atmospheric influences that modulate the strength and direction of the SALLJ and consequently, influence local precipitation regimes. By applying principal component analysis and k-means clustering to atmospheric fields associated with SALLJ days, we identified four distinct subtypes of SALLJ events. Composites of SALLJ types revealed noticeable differences in the exit region of the jet and the location and intensity of precipitation. In part, these differences may be influenced by the behavior of barotropic wave trains as they cross over the Andes from the extratropical Pacific. To explore the potential predictability of the SALLJ, we investigated the influence of global climate variability on the strength and frequency of SALLJ events. On interannual time scales, the warm phase of the El Nino Southern Oscillation was found to enhance the strength and frequency of the SALLJ. On subseasonal scales, significant relationships were found between the frequency of SALLJ subtypes and certain phases of the Madden Julian Oscillation. This information provides valuable insight that can be used to improve weather forecasts and subseasonal-to-seasonal climate predictions over South America.