Assessing the Moisture Transports Associated With Nocturnal Low-Level Jets in Continental South America

Abstract

Given the crucial role of low-level circulation in convective events, this study presents a climatological characterization of the moisture sources and sinks associated with the occurrence of nocturnal low-level jets (NLLJs) in South America. Six selected NLLJ cores are identified according to the jet index that considers a vertical wind speed shear of the lower troposphere at 00:00 local time (LT). The Lagrangian FLEXible PARTicle (FLEXPART) model was used to provide the outputs for tracking atmospheric air masses to determine the moisture sources and sinks for the NLLJ cores (Argentina, Venezuela, and the regions of Brazil: south—Brazil-S, southeast—Brazil-SE, north—Brazil-N, and northeast—Brazil-NE). The analysis is based on 37 years (1980–2016) of the ERA-Interim reanalysis. We found that the NLLJ index is stronger in the warm periods of a year (austral spring and summer) for the six selected regions. The NLLJ frequency is also higher in the warm months of the year, except in Brazil-NE where it is very frequent in all months. In Brazil-NE, the NLLJ also persists for 8 or more days, while the other NLLJs frequently persist for 1–2 days. The NLLJs occupy a broad low-level layer (from 1000 to 700 hPa) and exhibit a mean speed between 7 and 12 ms–1, which peaks mostly at 900 hPa. The moisture transport for each NLLJ shows that in addition to the intense local moisture sources, the NLLJs in Argentina and Brazil-S receive moisture from the tropical-subtropical South Atlantic Ocean and the Amazon basin, while the tropical-subtropical South Atlantic Ocean is the main moisture source for the NLLJ in Brazil-SE. Both moisture sources and sinks are stronger in the austral summer and fall. The NLLJ in Brazil receives moisture from the tropical South Atlantic (TSA) Ocean, which has weak seasonality. The moisture sources for the NLLJs in Brazil-N and Venezuela come from the tropical North Atlantic (TNA) Ocean in the austral summer and fall, while the TSA Ocean appears as an additional moisture source in the austral winter. This research contributes to improving our understanding of the NLLJs and their role in transporting moisture and controlling precipitation over the continent according to the seasons of a year, helping to improve seasonal climate forecasting.

New insights on Climate Change and Adaptation Research in Brazil

“…Finally, cluster 4 (blue in Fig. 2) is related to climatic and atmospheric studies which are more concerned with understanding the natural sciences basis of climate change (Braz et al, 2021). These studies are vital for building the climate science knowledge, as they form the basis upon which all the other more impacts-oriented studies are developed.…”

Concurrent extreme events of atmospheric moisture transport and continentalprecipitation: the role of landfalling atmospheric rivers

“…This general pattern is disturbed regionally by the action of meteorological structures associated with strong moisture transport, in which high humidity is combined with low-level wind. Examples of this are the high values of concurrent extremes in the NE Brazilian region in JJA affected by Low-Level Jet (LLJ) systems (Braz et al, 2021), or the moderate values in the SE of North America in JJA affected by tropical cyclones (Liu et al, 2021). To sum up, maxima of concurrence of extremes are found on extratropical continental coasts during winter, mostly affected by ARs with regional fingerprints of other major mechanisms of atmospheric moisture transport such as LLJs or tropical cyclones and the orographic forcing by large mountain barriers.…”

Observational study of the South American low‐level jet during the SALLJEX

“…The low‐level jet (LLJ) is a wind speed maximum that occurs in the lowest few kilometres of the atmosphere and is associated with vertical and horizontal shear (Stensrud, 1996). The LLJ is a key component of the Earth's climate as it can favour the development of deep moist convection by transporting warm and moist air and promoting ascent in the jet exit region (e.g., Means, 1954; Stensrud, 1996; Gimeno et al ., 2016; Algarra et al ., 2019; Braz et al ., 2021). LLJs are also important due to their effect on aerosols dispersion (Wei et al ., 2018; Ulke et al ., 2014), birds' migration (Wainwright et al ., 2016), aviation safety (Arkell, 2000; Madougou et al ., 2014), and wind energy (Gutierrez et al ., 2014; Gadde and Stevens, 2021).…”

“…(2016), and Braz et al . (2021) reviewed and synthesized more than six decades of LLJ studies worldwide. According to these studies, the nocturnal inertial oscillation resulting from the decoupling of the residual layer from the stable boundary layer (Blackadar, 1957) is an important mechanism in the LLJ formation over the U.S. Great Plains.…”