Seasonal variability and north–south asymmetry of internal tides in the deep basin west of the Luzon Strait

• Internal tides in deep basin exhibit seasonal and spatial variations.
• ITs have stable vertical structures in different seasons.
• Diurnal ITs are strong both in winter and summer.
• Diurnal ITs are modulated by Luzon Strait tides but not seasonal thermocline.
• Diurnal ITs are more coherent than semidiurnal IT.

Temporal and spatial variability of internal tides (ITs) in the deep basin west of the Luzon Strait were examined, based on two 9-month mooring current observations from autumn 2008 to summer 2009. The baroclinic current measurements exhibited north–south asymmetry and temporal variation. At the southern site (19.5°N), the kinetic energy of diurnal IT was dominant over that of semidiurnal IT by a factor of 3 to 4, whereas at the northern site (~ 21°N), the diurnal energy was comparable to or a little larger than semidiurnal energy. At both sites, the motions of semidiurnal IT were recognized as seasonally invariant, while diurnal IT showed notable seasonal variation, namely, stronger in summer and winter but weaker in spring and autumn. The seasonal variation was mainly modulated by the astronomical tides in the Luzon Strait rather than by the seasonal thermocline. Both diurnal and semidiurnal ITs contained stronger coherent signals than incoherent counterparts at two sites, but the IT at the southern site was more coherent than that at the northern site. Diurnal IT is more coherent than the semidiurnal IT at each site. The spatial–temporal variations were probably due to IT interferences from various sources within the Luzon Strait and modulation by varying background conditions.