The role of lowlevel circulation on water vapour transport to central and northern South America: Insights from a 2D Lagrangian approach

Abstract

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.