*4.2. The Trapped Core and Breaking Stage*

The trapped core is defined as the particle, which was trapped in the wave and carried along at constant propagating speed [29] and occurs when the water velocity exceeds the propagation speed of the wave [30]. It is formed when ISWs shoal from deep to shallow water. As the ISW reaches its breaking limit, it forms a subsurface trapped core [31]. A trapped core was identified in the measured ISW zonal velocity (inside the white line representing the ISW phase speed in Figure 3a). The upper portion of the trapped core was beyond the shipboard ADCP range.

The breaking stage can be defined according to [32] by comparing the peak current velocity *umax* and phase speed *c*. When *umax* > 0.8 *c* in a continuously stratified fluid, the ISW would break. The ISW presented in this paper had values of *umax* = 2.93 m/s and *c* = 1.76 m/s, with peak current velocity much larger. Obviously, the peak current velocity was much larger than the estimated phase speed. The ISW was close to or already in the process of breaking.

The above results are consistent with those in [5], which present 41 ISWs captured in April 1999 and 2000 on the continental slope of the northern South China Sea during the Asian Seas International Acoustics Experiment (ASIAEX) [5]. One ISW among them was similar to our observations, with the largest westward current exceeding 2.4 m/s right near our research area (details shown in Table 1). This suggests that when one ISW approaches the breaking stage, its velocity could be significantly larger than before.


**Table 1.** Observations in this study and in [5].
