Ocean swell decay observed over relatively short distances with directional buoys

Abstract

Swells can propagate over large distances without significant energy loss. Despite several recently published papers about this subject, swell decay in the ocean is not yet completely understood. Most published works employed satellite data and imposed a threshold distance to compute the decay. Here, we analyse events of dispersive arrivals recorded by two directional buoys moored in deep water 915 km apart from each other in the southwest South Atlantic. The buoys are relatively closer to the storm’s position, in respect with the recently published results that impose a distance in general superior to three thousand kilometers. Spectral partitioning techniques are employed to isolate the swell components. Hence, the storm’s position is computed and also the storm features based on the characteristics of the vorticity field at 850 hPa level. The rate of energy variation is calculated using two distinct approaches presented in the literature for swell decay, one distanceonly and the other distance-frequency dependent. In addition, a method for swell decay based on the storm features is proposed. The inter-evaluation of the results shows that the distance-frequency dependent method best fits our data. The storm features based method produced equivalent results to those obtained using distancefrequency dependent relation for the event whose reference distance is about 2400 km. The average swell decay rate is 1.4 × 10−6m−1 , within reference distances inferior to 2500 km. That is one order of magnitude larger than the rate reported in the literature, for greater reference distances. Therefore, the decay rate is stronger for the waves closer to their generating storms. Furthermore, it is clear that the decay is not only dependent on distance, but wave frequency as well. The storm features may also play a role.