Temporal variations of HL1 mode velocity pattern. Top: HMI $V_{\mathrm{zonal}}^{(-)}$ in the first half of 2021 applied with a band-pass filter around ${\nu }_{HL1}^{\mathrm{synodic}}$. The slope of the striped pattern in $V_{\mathrm{zonal}}^{(-)}$ is highlighted by the tilted red lines. The Δτ (time lag) and Δλ (latitudinal separation) that determine the slope of the striped pattern Δτ/Δλ are indicated in the plot (the two legs of the red right triangle). Bottom: Δτ/Δλ as a function of time, using the same color conventions as in Fig. 7.