every part of the shock front will not oscillate in the same phase, resulting in a phase lag between the shock front on and around the equatorial plane and the top of the shock front ($r^T_s$). For simplicity, we record the variation of shock location with time at the disc equatorial plane which is shown in [*top-left*]{} panel of Fig. 3. Note that there exists quasi-periodicity in the variation of shock location with time. We identify eight shock locations within a given oscillation period that are marked in open circles. The respective velocity field and density contours (solid, online red) of the flow in the $r-z$ plane is shown in the rest of the panels of Figs. 3(a-h). Higher density and extra thermal gradient force in the CENBOL region causes a fraction of in falling matter to bounce-off as outflow. When shock front oscillates, post-shock volume also oscillates which induces a periodic variation of driving force responsible to vertically remove a part of the in falling matter. As the shock reaches its minimum (Fig. 3c), the thermal driving of outflow is weak, so the spewed up matter falls back. The post-shock outflow continues to fall as the shock expands to 