Incoherent signatures of internal tides in the Tokara Strait modulated by the Kuroshio

•Prominent incoherence of internal tides in the Tokara Strait is quantified.•Mean incoherence is ∼54 % for semidiurnal constituents and ∼75 % for that of diurnal.•Background flow affects variability of incoherence by refracting the propagation rays.•The northward (southward) shift of the Kuroshio path increases (decreases) the incoherence.