A quantitative evaluation of the role of the Argentinean Col and the Low Pressure Tongue East of the Andes for frontogenesis in the South American subtropics

摘要

Previous studies have found the South American subtropics to exhibithigh climatological frontogenesis in equivalent potential temperatureduring the austral summer. An important contribution to this patternis given by frontogenesis over the Argentinean Col (AC), whichseparates the Northwestern Argentinean Low (NAL) from transienttroughs to the south of it. The NAL and the Low Pressure Tongue eastof the Andes (LPT) promote efficient transport of Amazonian humidityto the subtropics during the incursion of transient disturbances overthe continent. The convergence of this strong warm and humid flow withmid-latitude air brought into the subtropics by the disturbance occurspreferentially in the neighborhood of the AC. The main difficulty inquantifying the contribution of the NAL, AC and LPT structure tofrontogenesis in the South American subtropics is the automaticdetection of the AC and LPT. In this paper an algorithm developed tothis end is briefly presented and applied to obtain statistics on therole of these structures in frontogenesis. Six-hourly data from ECMWFERA-40 Reanalysis over 21 austral summer periods (December–March) isused. Occurrences of the AC are highly concentrated between 34–39° S and66–69° W, being present in this region in 42% of the time instantsanalyzed. The spatial average of the positive values of thefrontogenesis over this region was calculated for each time step as ameasure of intensity and histograms were built for the cases when theAC was and was not found inside this region. Mean, median and mode arelarger for the distribution of cases with the presence of the AC. Inaddition, we present the frequency of occurrence of the AC as afunction of the frontogenesis, showing that it grows with theintensity of the frontogenesis, rising above the 0.955 quantile. Wehave not found any correlation between the AC frequency and thefrontolysis intensity.