Observable large-scale impacts of tropical cyclones on subtropical gyre

The large-scale ocean circulation in an ocean basin was previously thought to be impacted cumulatively by all the overlying tropical cyclones (TCs). Based on idealized numerical experiments and altimetry observation, this study reveals that, unnecessarily by cumulative impacts, a single TC actually has the ability of plowing the large-scale sea surface height (SSH) field due to the TC-induced geostrophic response. This ability is dictated by the along-track length scale of the geostrophic response, i.e. the total track length. Some of observed along-track signals including SSH trough, jet and SSH rise can confirm the TC-induced large-scale impacts. Shortly after the TC passage, the observable large-scale signals are generally the SSH trough. However, the jet and the SSH rise easily emerge from the evolved SSH trough due to Rossby wave dispersion. By identifying and tracking the observable signals, this study demonstrates that the subtropical gyre primarily over [4-20 °N, 122-180 °E] are plowed by 9 typhoons (2015) into several large blocks of SSH trough and SSH rise. These long-lived SSH troughs and SSH rises dominate the upper-layer circulation from April to December in 2015. If the large-scale signals cannot be observed, the estimated TC-induced mean SSH decreases suggest that the large-scale impacts may still exist but merely cannot be seen intuitively. This study provides compelling observational evidence for the TC-induced large-scale impacts, further highlighting that TCs may play a non-negligible role in the upper ocean dynamics in the subtropical gyre.