Observable Large-Scale Impacts of Tropical Cyclones on the Subtropical Gyre

Abstract

Abstract 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 to plow 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 the observed along-track signals, including the 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°E–180° is plowed by nine typhoons (2015) into several large blocks of SSH troughs and SSH rises. 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 nonnegligible role in the upper-ocean dynamics in the subtropical gyre. Significance Statement This study aims to demonstrate the ability of a typhoon to affect the large-scale ocean dynamics. The ability manifests as some along-track signals in altimetry observations, including sea surface height trough, jet, and sea surface height rise, which can be frequently observed after some typhoons in 2015. The sea surface height field in the western North Pacific is continuously plowed by these typhoons into several large blocks of sea surface height troughs and rises. These long-lived sea surface height troughs and rises dominate the upper-layer circulation from April to December in 2015. This study indicates that typhoons play a vital role in the upper-ocean dynamics in the western North Pacific.