“…Spherical harmonic analysis was adopted (Chapman & Bartels, 1940; Liu et al., 2021; Stening & Winch, 2013; Takeda, 2002; Xu, 2003; Xu & Li, 1994; Yamazaki, Yumoto, Uozumi, et al., 2011b; Zhao et al., 2014) to model the Sq internal and external seasonal current systems in each chain. The details of SHA method could be found in previous literature (e.g., Liu et al., 2021; Matsushita & Maeda, 1965; Yamazaki & Maute, 2017).…”