Synoptic climatology of warm fronts in Southeastern South America

Abstract

This study shows a synoptic climatology of warm fronts in Southeastern South America (SESA). Data from Climate Forecast System Reanalysis (CFSR) was used to identify warm fronts from 1979 to 2010. The identification method was based on the magnitude of meridional gradient of 850-hPa equivalent potential temperature ( e) and 850-hPa wind fields. Composites of the most important atmospheric variables were constructed from 1 day before until 1 day after the formation of the warm front. An average frequency of two warm fronts per month is observed, with higher frequencies in austral winter. Most warm fronts precede the formation of extratropical cyclones over Uruguay and form because of the southward movement of previous cold/stationary fronts. Warm fronts form on average around southern Paraguay, northeastern Argentina and western part of southern Brazil and Uruguay, coupled to the eastern edge of the Chaco Low (CL) and the Northwestern Argentinean Low (NAL) where north/northwesterly flow predominates. An upper-level wave of wavenumber eight supports warm frontogenesis. Location and intensity of synoptic systems associated with a warm front event differ from winter to summer. Elevated instability is commonly present near warm fronts, and the average warm-front slope is 1 : 110, agreeing with other studies. Instability indices increase after the warm-front passage, leading to greater rainfall 1 day after the warm front forms.

On the Wind Contribution to the Variability of Ocean Currents Over Wide Continental Shelves: A Case Study on the Northern Argentine Continental Shelf

“…There were between one and three reversal events each month that lasted between 3 and 14 days. This is in agreement with the frequency of passage of cold and warm atmospheric fronts over the region (Garreaud, ; Ribeiro et al, ; Solman & Orlanski, ). The composite of SLP for those days (Figure S7) shows that these reversal events coincide with the presence of a high‐pressure system located SE of the mooring sites that induces an alongshore wind stress component to the west over the location of the deployments, and to the SW northeast of the deployments.…”

“…The detailed analysis of the time series shows that sudden current reversals throughout the water column are observed in response to the passage of atmospheric fronts that reverse the direction of the dominant wind, similar to earlier observations obtained in the midshelf region further south (Rivas, ). Since the study area is characterized by the frequent passage (one to three events per month) of both low‐ and high‐pressure atmospheric systems coming from the west and southwest with a duration of 3 to 14 days (Garreaud, ; Ribeiro et al, ; Solman & Orlanski, ), current reversals are expected to be a recurrent phenomenon. The frequency of passing pressure systems is higher in winter, as the semipermanent high‐pressure system over the southwestern Atlantic Ocean, centered at ~30°S, displaces equatorward, and hence, the region becomes more affected by more intense cyclogenic activity (Martos & Piccolo, ).…”

A climatology of quasi‐linear convective systems and associated synoptic‐scale environments in southern Brazil

“…In the 850‐hPa composites (Figure ), the Chaco Low over northwestern Argentina appears as a trough, promoting intense quasi‐geostrophic flow from the Amazon Basin towards SB. Enhanced meridional equivalent potential temperature ( θ e ) gradient is collocated with 1,000–700‐hPa moisture flux convergence (MFC; Banacos and Schultz, ) over Uruguay and SB, where strong positive θ e advection indicate the baroclinic zone described in Figure is a warm front (Ribeiro et al ., ). The MFC is stronger in T1SVR composites and favours greater synoptic‐scale ascent.…”

“…In T2 and T3 cases, there is no clear cold advection over central Argentina, but the warm front is noted over SB and Uruguay. Warm fronts are a common feature in this region and the warm sector is generally associated with high thermodynamic instability (Ribeiro et al ., ).…”

Changes in South Atlantic Cyclones due Climate Change

“…However, the influence of moisture transport and convergence at low-level remain, which may explain the intense cyclones generated in LA PLATA and SE-BR regions during the winter. The development of the orographic low, called NAL, on the lee side of Andes reinforces the warm and moist advection in LA PLATA region, decreasing the low-level stability during cyclonegenesis (Seluchi and Saulo, 1998;Seluchi et al, 2003;Ribeiro et al, 2016). Moreover, stratospheric PV intrusions also affect genesis and cyclone intensification in this area, as seen in distribution maps and reported by Iwabe and da and Funatsu et al (2004).…”