From historical hydrocasts in the Okinawa Trough region of the East China Sea (ECS), acoustic echo time from 700 dbar to the surface shows a tight relationship with temperature (T) except near the 100--200-dbar layer. This is caused by 2nd- or higher-mode baroclinic variations. Significant out-of-phase correlation between upper and lower layer T from the hydrocasts confirms that 2nd-mode variations are strong. Furthermore, the 2nd-mode variations are dominant during the ebb tide period, which suggests they are caused by 2nd-mode internal tides (ITs) generated at the continental shelf break. For comparison, we investigate historical hydrocasts southeast of the Ryukyu Islands, where no significant 2nd-mode variations are found. A model simulating the M2 IT agrees qualitatively with these observations: it predicts relatively strong 2nd-mode IT energy in the ECS, but weak energy in the region southeast of the Ryukyu Islands.