Transit time of deep and intermediate waters in the Gulf of St. Lawrence

Abstract

The transit time of the subsurface waters in the hypoxic and acidified Gulf of St. Lawrence (GSL) is poorly understood, despite its strong influence on physical and biogeochemical water properties. Three estimates of the transit time of the deep waters between Cabot Strait and the head of the Laurentian Channel, a deep channel cutting through the GSL, have been published up to now. Here, using lagrangian tracking experiments in a regional ocean model, we provide a new estimate of the transit time in the deep layer (> 225 m) of the GSL, as well as the first estimate of the transit time in the intermediate layer (50–175 m). Our estimate for the deep layer is 3.2 ± 0.7 years. The transit time in the intermediate layer (1.2 ± 0.5 years) is nearly three times faster than in the deep layer. The deep waters travel mainly up the Laurentian Channel, whereas most of the intermediate waters first transit through the Esquiman and /or Anticosti Channels. Our results also highlighted the impact of the seasonal changes in large-scale circulation on the transit times of the particles seeded at Cabot Strait. In summer and fall, the circulation is relaxed, and subsurface waters transit slowly but more directly upstream, leading to faster transit times. In winter and spring, the circulation is intensified but many particles get caught in large gyres prevalent during these seasons, leading to slower average transit times.