The Orinoco Low‐Level Jet: An Investigation of Its Characteristics and Evolution Using the WRF Model

Abstract

The structure and evolution of the low‐level jet over the Orinoco River basin is characterized using finer horizontal, vertical, and temporal resolution than possible in previous studies via dynamical downscaling. The investigation relies on a 5‐month‐long simulation (November 2013 to March 2014) performed with the Weather Research and Forecasting model, with initial and boundary conditions provided by the Global Forecast System analysis. Dynamical downscaling is demonstrated to be an effective method to better resolve the horizontal and vertical characteristics of the Orinoco low‐level jet (OLLJ), improving not only the representation of small‐scale jet streaks within the broader region of low‐level wind enhancement but also its diurnal and austral‐summer evolution. The OLLJ is found to be a single stream tube over Colombia and Venezuela with wind speeds greater than 8 m/s and four distinctive cores varying in height under the influence of sloping terrain. The OLLJ has its maximum monthly mean wind speed (13 m/s) and largest spatial extent (2,100 km × 400 km) in January. The maximum mean wind speeds (13–17 m/s) in the diurnal cycle occur in the early morning, whereas wind speeds are a minimum (8–9 m/s) in the late afternoon when a deep, convective boundary layer is present. The intraseasonal variability of the wind speed and potential temperature only presents significant periodicity in the diurnal and semidiurnal scales, with no other meaningful cycles evident during the austral summer.

Ground validation ofTRMM 3B43 V7precipitation estimates over Colombia. Part I: Monthly and seasonal timescales

“…It is also remarkable that the Andes, Caribbean and Pacific regions exhibit a bimodal annual cycle, with wet (dry) seasons in MAM and SON (JJA and DJF), while the Orinoco and Amazon regions exhibit unimodal annual cycles with a single dry season during DJF (Figure 2e–i). These spatiotemporal patterns can be explained by the meridional migration of the ITCZ which has been identified as the main modulating mechanism of intra‐annual variability over Colombia (Poveda, 2004; Poveda et al ., 2006), but also by the dynamics of diverse large‐scale phenomena that influence the moisture transport and precipitation regime of each region, mainly the three low‐level jets present in the country: (a) the Caribbean Low‐Level Jet (CLLJ) (Amador, 1998, 2008; Amador and Magana, 1999; Wang, 2007), which is active from northern South America to the Greater Antilles; (b) the Chocó Low‐Level Jet (Choco Jet) acting over the far eastern Pacific (Poveda and Mesa, 1999, 2000; Rueda and Poveda, 2006; Sakamoto et al ., 2011; Poveda et al ., 2014; Bedoya‐Soto et al ., 2019; Yepes et al ., 2019) and (c) the Corriente de los Andes Orientales (CAO) Low‐Level Jet or Eastern Andes Low‐Level Jet (Montoya et al ., 2001; Torrealba and Amador, 2010; Bedoya‐Soto et al ., 2019), also recently identified as Orinoco Low‐Level Jet (Jiménez‐Sánchez et al ., 2019), which constitutes the northernmost leg of the South American Low‐Level Jet (Berbery and Collini, 2000; Marengo et al ., 2004; Vera et al ., 2006). These mechanisms, together with complex soil–atmosphere interactions over the Andes and the Amazon and Orinoco River basins, contribute to the regional particularities that are evidenced in Figure 2 (Mejía et al ., 1999; Poveda, 2004).…”

Hydroclimate of the Andes Part I: Main Climatic Features

“…One of the LLJs to the east to the Andes circulates over northern South America from the Tropical North Atlantic through Guyana and eastern Venezuela. It becomes a LLJ blowing over the Venezuela-Colombian Llanos (plains) of the Orinoco River, socalled Corriente de los Andes Orientales (CAO), the Llanos jet, the Orinoco jet, or the Eastern Andes Jet (Montoya et al, 2001;Torrealba and Amador, 2010;Jiménez-Sánchez et al, 2019; see Figure 6). The easterly flow of the CAO reaches the eastern piedmont of the Andes as the northernmost leg of the SALLJ (Figure 6).…”

“…Over the Colombian and Venezuelan Llanos region, the CAO exhibits a clear annual cycle with peak velocities during DJF (8-12 ms −1 ) and lower velocities during JJA (2-3 ms −1 ). It is worth noticing that the annual cycle of the CAO wind strength is negatively correlated with rainfall over the Colombian Llanos (Rueda, 2014;Jiménez-Sánchez et al, 2019) and the western Amazon basin and positively correlated with rainfall over the south tropical Andes and southern Amazonia, where annual rainfall reaches 6000 mm (e.g., Figueroa and Nobre, 1990;Garreaud et al, 2003;Espinoza et al, 2009b;Giovannettone and Barros, 2009;Segura et al, 2019). At the eastern flank of the south tropical Andes, the moisture fluxes provided by the CAO produce high rainfall rates (5000-7000 mm yr −1 ) over the Andes-Amazon transition zone of Peru and Bolivia (Poveda et al, 2014;Espinoza et al, 2015;Chavez and Takahashi, 2017).…”

Uncertainty of runoff sensitivity to climate change in the Amazon River basin

“…We show that all precipitation datasets exhibit similar regional patterns of P with the highest values of annual precipitation occurring in the Colombian Amazon (northwestern region), while the lowest precipita-tion rates are depicted in Peru (western region), Bolivia (southwestern region), and some parts of Brazil (southeastern region). These observations coincide with macroclimatic factors, such as the migration of the Intertropical Convergence Zone (ITZC) and the South Atlantic Convergence Zone (SACZ), the activity of aerial rivers, and large-scale circulation patterns across South America, [70][71][72][73][74] including the Eastern Andean jet, also known as the Orinoco jet (northernmost leg of the SALJET), 75,76 and the two phases of the ENSO, 20,23,24 land surface-atmosphere interactions, 21,[77][78][79] vegetation activity, and precipitation recycling. [80][81][82] Regarding E p , we show that both E p products produce quite different spatial results.…”