The potential role of low-level circulation on the transport of atmospheric moisture towards Central America and northern South America is studied with the Dynamical Recycling Model (DRM) for the period 1981-2015. The net 2D field for advection of moisture in the DRM is estimated based on two different approaches: from the Vertically Integrated Moisture Flux (VIMF) and from a linear combination of low-level winds (Ueff). The relative importance of low-level wind structures is inferred from the comparison of both DRM approaches. The Ueff approach yields larger long-term estimates of transport from the Pacific compared to the VIMF approach, largely associated with the effect of the low-level circulation over the eastern Pacific, known as the Choco Jet (CJ). Large differences in the transport estimated by both approaches also appear to be associated with different phases of El Niño-Southern Oscillation (ENSO), when differences in the low-level circulation take place over the eastern Pacific and the tropical Atlantic. Similar differences are found for anomalous activity of the CJ and the Caribbean Low-level Jet (CLLJ). The differences in the magnitude of long-term estimates and anomalies from both 2D approaches show that details in vertical wind shear are important for the estimation of water vapour transport. However, the sign of the anomalies and correlation patterns under different circulation regimes were the same for both methods. Furthermore, with both approaches, it is found that variability in the low-level circulation over the eastern Pacific and the tropical Atlantic is substantially related to changes in the vapour transport towards Central America and northern South America. In addition, the robustness of this signal suggests that a simple 2D tool like the DRM may be effectively used to study transport changes under different regimes, like different ENSO phases or climate change scenarios.

Abstract:The Dynamic Recycling Model (DRM) is used to analyze the seasonal patterns of water vapor transport to Central America (CAM) and their interannual variability, with special emphasis on the role of the Caribbean low-level jet (CLLJ) and the Choco jet (CJ) during the period 1980-2012, using information from the ERA-Interim reanalysis. Our results suggest that approximately 35% of mean annual atmospheric moisture transported to CAM comes from the Caribbean Sea. This transport is closely related to the CLLJ, showing that a strong (weak) jet induces a greater (smaller) transport from the Atlantic to CAM. On the other hand, transport from the Pacific exhibits a marked seasonality, responding to the intensity of the CJ, which during high intensity episodes stimulates an increase in transport of water vapor. Finally, a comparison of our results with the work by Durán-Quesada et al. (2017) using the 3D FLEXPART model, suggests that the DRM (which is 2D) has a bias in estimating the mean annual cycle of water vapor transport associated with the CLLJ wind shear. However, the DRM is able to capture the interannual variability of the moisture transport and its response to the El Niño-Southern Oscillation (ENSO) and low-level jets anomalies.

A study of the simulation of the Orinoco Low‐Level Jet (OLLJ) and the cross‐equatorial moisture transport over tropical South America using the Weather Research and Forecasting (WRF) model is presented. The focus is on the diurnal cycle and monthly means during one December‐January‐February (DJF) season (2003–2004). The sensitivity of the OLLJ and cross‐equatorial moisture transport to the representation of surface fluxes and turbulence is explored by using two different Land Surface Models (LSM) and three Planetary Boundary Layer (PBL) schemes. Different LSMs produce large differences in the sensible heat flux over the Orinoco basin, but no substantial differences in the flow at the entrance of the OLLJ. However, these changes in surface fluxes were associated with a change in the low‐level pressure gradient between the Orinoco and Amazon basins, and with up to 10% change in the simulated cross‐equatorial moisture transport. The largest effects from different PBL schemes were found in the low‐level flow near the Andes cordillera, and on the low‐level southward cross‐equatorial flow during the daytime. One of the PBL schemes exhibited a reduced moisture transport on the eastern flank of the Andes but enhanced southward transport around the core of the cross‐equatorial moisture flux. Another PBL scheme produced a stronger net cross‐equatorial moisture transport, with enhanced precipitation over the Andes‐Amazon region by up to 30% during the afternoon and up to 13% in the daily mean. Improved observations over the region are needed to evaluate which combination best represents the flow.