Abstract
Long period surface ocean waves generated by atmospheric disturbances represent a worldwide coastal hazard that cannot be readily and accurately predicted. They can cause severe damage to coastal settlements and loss of life in extreme events. With a view to improving knowledge of these coastal motions, this manuscript presents observational and theoretical results related to long waves, with periods in the range of a few minutes to several days, generated by atmospheric disturbances moving over shelf topography or by distant storms. This work aims to contribute to a better understanding of how atmospherically forced wave energy can be released as free long wave energy.
Using wavelet signal processing, the sea level response to several storms obliquely impacting the Atlantic coast of Canada is investigated, focusing on a particular large long wave event related to Hurricane Florence in September 2006. The results showed that Florence released a free Continental Shelf Wave (CSW) with a period similar to the passage time of the storm-forced wave over the shelf (approximately 28 h). This study presents direct measurements of a high alongshore group speed (11.4 ± 5.9 m s−1), in the manner of free barotropic CSW, by examination of sea level wavelet power spectra at different locations. Furthermore, using cross-wavelet analysis of pairs of stations, an exceptional phase speed of 16.0 ± 5.1 m s−1was found, larger than had been previously observed for a free CSW.
With a view to investigating the link between forced long waves under atmospheric disturbances moving over varying topography and free long waves in a rotating ocean, a two-dimensional analytical model is developed. Free transients generated by storm-forced waves crossing step changes in water depth at particular angles are shown to resonate by exciting a range of barotropic free waves. It is demonstrated that three kinds of resonances and four types of trapped free waves can be excited by rotationally influenced forced waves obliquely crossing bottom topography and/or coastline. The results are applicable to storms obliquely crossing the continental shelf. This model may contribute to understanding how a particular combination of the storm characteristics can result in destructive coastal events with time scales encompassing the typical meteotsunami period band (tens of minutes) and storm surges with periods of several hours or days.
Long period waves can also be generated by offshore moving storms which do not cross the shelf or the coast. Long waves can reduce underkeel clearances and cause problematic surging and resonant motions for vessels in and around ports and harbours during calm weather conditions. Their generation and propagation mechanism has been the subject of several postulations. Based on observations of Far Infra Gravity Waves (FIGWs) with periods in the range of several minutes to tens of minutes on the coasts of New Zealand and the Chatham Islands, a semi-empirical predictor for FIGWs is suggested. This prediction is based on a strong correlation between FIGWs and several spectral characteristics of gravity waves of shorter periods (typically 3-20s). The possible reasons for the underprediction of several long wave events are also discussed.