Autumn Warming of the Cold Intermediate Layer in the Labrador Shelf

Abstract

AbstractThe Labrador Shelf is integral to the North Atlantic Ocean's climate system, exerting a significant influence on both regional and global scales. This study examines continuous, high‐temporal resolution hydrographic profiles collected by two Argo floats located on the Labrador Shelf. Our focus centers on the subsurface changes. Among observed seasonal variations, the most pronounced and consistent change is a marked temperature increase during autumn in the Cold Intermediate Layer, which is also confirmed in an eddy resolving global reanalysis data set. Contrary to previous studies that attributed this warming to the autumnal deepening of the mixed layer, our analysis indicates that the warming extends to underneath the deepest mixed layer. Thus, mixed layer development cannot account for the observed warming in the deeper layer. On the other hand, analysis of velocity fields from reanalysis data set reveals active onshore intrusions at several locations, with a section at 58°N emerging as the northernmost hotspot. Budget analysis further indicates that the dominant factor driving autumn warming at Section 58°N is cross‐isobath advection that is associated with the intrusion of slope waters onto the shelf. Subsequently, the positive temperature anomaly at Section 58°N and other enhanced intrusion locations are transported downstream through along‐isobath currents, resulting in lagged yet intensified warming at lower latitudes. Our findings underscore the essential role of cross‐isobath intrusion, in combination with along‐isobath movements in governing seasonal temperature variability in the deep layer of the Labrador Shelf. Incorporating this mechanism is crucial for accurately hindcasting and forecasting bottom environmental conditions in the region.