An Application of the Peaks Over Threshold Method to Estimate Extreme Winds Over the South Atlantic Ocean

Abstract:

Given the growing investment in coastal activities, such as industrial and residential settings, proper understanding of oceanographic and meteorological phenomena over such areas became very important. The winds play a major role in this context, being the main source of energy for gravity waves generation in the ocean, and determining the characterization of severe weather conditions. In this study, a statistical analysis of extreme values was applied to wind data from National Centers for Environmental Prediction and National Center for Atmospheric Research reanalysis (NCEP-I) grid points with 2.5º of spacial resolution and results from a simulation with the BRAMS model in the Southwest Atlantic Ocean region with 0.25º of resolution. The Peaks Over Threshold (POT) technique was applied and the analysis focused on the behavior of extreme values according to the wind direction and the resolution of the original data. The period of analysis goes from 1982 to 2011 and the domain goes from 40ºS to 5ºN latitude and 70ºW to 10ºW longitude. The POT method demanded that peaks chosen for analysis were independent and identically distributed and a minimum interval of 48 hours was given to separate the subset sampled for analysis. The peak excesses above a determined threshold were adjusted to the Generalized Pareto Distribution and extrapolation to 50 years return periods was built in each grid point. General large-scale patterns of 50-yr return values were similar for both datasets used. However, more details were verified in the analysis of simulation results with BRAMS, given the dependence of the methodology to the resolution of the original set. Thus, the greater detailing suggests the inclusion of mesoscale features originating these extreme values. In the northern part of the domain, extreme winds were weaker and prevailed from north, northeast and east, given the influence of the trade winds and the positioning of the South Atlantic Subtropical High. On the other hand, in the southern area, the extreme values were more severe and all wind directions were significant. This result suggests the influence of a larger number of phenomena originating the extreme value, such as front passages and cyclones.