The Orinoco Low-Level Jet and Its Association with the Hydroclimatology of Northern South America

Abstract

We investigated the relationship between the frequency of occurrence of the Orinoco Low-Level Jet (OLLJ) and hydroclimatic variables over northern South America. We use data from the ERA5 atmospheric reanalysis to characterize the spatial and temporal variability of the OLLJ in light of the LLJ-classification criteria available in the literature. An index for the frequency of occurrence of an LLJ was used, based on the hourly maxima of wind speed. The linkages among the OLLJ, water vapor flux, and precipitation were analyzed using a composite analysis. Our results show that during December–January–February (DJF), the OLLJ exhibits its maximum wind speed, with values around 8–10 m/s. During DJF, the analysis shows how the OLLJ transports atmospheric moisture from the Tropical North Atlantic Ocean. During this season, the predominant pathway of the OLLJ is associated with an area of moisture flux divergence located over northeastern South America. During JJA, an area of moisture flux convergence associated with the northernmost location of the ITCZ inhibits the entrance of moisture from northerlies. We also show that the occurrence of the OLLJ is associated with the so-called cross-equatorial flow. During DJF, the period of strongest activity of the OLLJ is associated with the northerly cross-equatorial flow and dry season, whereas during JJA the southerly cross-equatorial flow from the Amazon river basin predominates and contributes to the rainy season over the Orinoco region.

The Orinoco low‐level jet during El Niño–Southern Oscillation

“…Moreover, we calculated standardized anomalies of zonal wind speed at 925 hPa averaged over the OLLJ index area (6°–8°N and 67°–69°W), which was selected according to the recent work by Builes‐Jaramillo et al . (2022). Finally, we calculated Pearson correlations between the centre of the NASH minus NET SLP gradient anomaly and the OLLJ wind speeds anomalies for each of the DJF months.…”

“…Recently, Builes‐Jaramillo et al . (2022) and Algarra et al . (2019) showed that the OLLJ is one of the mechanisms involved in the moisture transport from the tropical North Atlantic (TNA) towards the Amazon River basin and southern South America during December–February (DJF).…”

“…In contrast, during July–August (JJA) coinciding with the weakest winds along the OLLJ corridor, the cross‐equatorial flow (CEF) transports moisture from the Amazon to northern South America (Wang and Fu, 2007). The moisture flux through the jet corridor is associated with the annual cycle of the OLLJ and also with the annual cycle of precipitation over the Orinoco region: strong OLLJ winds coincide with the dry season (DJF) whereas weak jet winds occur during the wet season (JJA) (Builes‐Jaramillo et al ., 2022).…”

Rainwater Isotopic Composition in the Ecuadorian Andes and Amazon Reflects Cross-Equatorial Flow Seasonality

“…Equatorial South America, although not monsoonal per se [13,15,53,54], is linked to the SAM through LLJs (e.g., cross-equatorial flow, Orinoco) and its seasonal convection variability [24,25,36,55]. In the first case, the V-Index region (5 • S-5 • N, 65 • -75 • W; black boxes in Figure 1) was identified according to Wang et al [24] and captured the interhemispheric migration of rainfall crossing the equator.…”

“…Wang et al [24] showed that precipitation variability over South America involves significant meridional migration and reversal of low-level winds between the south and north of the equator. These wind corridors in the lower atmosphere are Low-level jets (LLJs) and transport water vapor that modulates weather and climate [20,25,26]. Hence, LLJs are vital structures in the water vapor transport in the atmosphere, feeding the precipitation systems [26].…”