Tidal variability of the motion in the Strait of Otranto

摘要

Various current data, collected in the Strait of Otranto during the period1994–2007, have been analysed with the aim of describing the characteristicsof the tidal motions and their contribution to the total flow variance. Theprincipal tidal constituents in the area were the semi-diurnal (M2) and thediurnal (K1), with the latter one predominant. The total flow was, ingeneral, more energetic along the flanks than in the middle of the strait.Specifically, it was most energetic over the western shelf and in the upperlayer along the eastern flank. In spite of the generally low velocities (afew cm s?1) of the principal tidal constituents, the tidal variance hasa pattern similar to that of the total flow variance, that is, it was largeover the western shelf and low in the middle. The proportion of non-tidal(comprising the inertial and sub-inertial low-frequency bands) to tidal flowvariances was quite variable in both time and space. The low-frequencymotions dominated over the tidal and inertial ones in the eastern portion ofthe strait during the major part of the year, particularly in the upper andintermediate layers. In the deep, near-bottom layer the variance was evenlydistributed between the low frequency, diurnal and semi-diurnal bands. Anexception was observed near the western shelf break during the summer seasonwhen the contribution of the tidal signal to the total variance reached77%. This high contribution was mainly due to the intensification of thediurnal signal at that location at both upper and bottom current records(velocities of about 10 cm s?1). Local wind and sea level data wereanalysed and compared with the flow to find the possible origin of thisdiurnal intensification. Having excluded the sea-breeze impact on theintensification of the diurnal tidal signal, the most likely cause remainsthe generation of the topographically trapped internal waves and the diurnalresonance in the tidal response. These waves were sometimes generated by thebarotropic tidal signal in the presence of summer stratification and thestrong bottom slope. This phenomenon may stimulate diapycnal mixing duringthe stratified season and enhance ventilation of the near-bottom layers.