Climatologically Modeled Wave Field Analyses in the Western South Atlantic

Abstract

Wave height, period and direction are basic parameters for designing off-shore structures. Besides this direct application, knowledge of the regional characteristics of a wave field can also help in the selection of optimal regions for wave power energy plant design and installation. A wave climatology based on data generated by a WAVEWATCH III model simulation (NOAA WW3) for the Brazilian coast was analyzed and validated against statistical values derived from opportunity vessel measurements. The hindcast covered the period from January 1997 to December 2005 in a region between 5°N – 40°S and 10°W – 65°W. The grid used was uniform with a 0.25° spacial resolution. The boundary conditions were obtained from NOAA WW3 operational model and the atmospheric forcing from NOAA GFS model. The model results were calibrated with field data and detailed information about the simulation can be obtained in Alves et al. (2008) and Alves et al. (in press). Monthly averages of significant height, period and wavelength were calculated using 3 hour time resolution fields. Since a simple mean direction has small physical representativeness, the predominant direction (moda) and associated persistency were obtained from the data. The results were then compared with values from the U.S. Navy Marine Climate Atlas of the World. This Atlas has four points located within the selected model grid region. These points showed good agreement with wave period, height and direction persistency based on the WW3 simulation results. The wave climatology showed that the predominant wave direction from April to July was from S and SE in southern Brazil, associated with swells related to cold fronts. The S and SE swells were also responsible for the largest mean wave height (2.1 m) observed in the climatology. Another result that was validated with the literature was the E and NE predominant wave direction during the austral summer. This phenomenon is associated with winds originated from the South Atlantic High Pressure Center, which is a semi permanent high pressure center near Trindade Island. The wave climate in northern Brazil showed a predominant direction from the N during January to March, associated with the northern hemisphere winter storms. During the remaining months of the year, the predominant wave direction is E and NE associated with trade winds. The model results are still in a processing phase to produce extreme values, which will be more useful for coastal and off-shore structure design.