Abstract

Transport time scales are helpful to determine aquatic ecosystem health and sensitivity to pollution. Water age was studied in a Patagonian fjord in Chile with an unstructured three-dimensional hydrodynamic model (MIKE 3) coupled with a high-resolution atmospheric model (WRF). Water age is defined as the time a water parcel has remained since its entrance to a system. Water age is represented in the Puyuhuapi fjord and adjacent Jacaf channel with two tracers: i) freshwater from rivers and ii) ocean water. Hydrodynamic model results showed reasonable stratification structure in the fjord and response to wind forcing. The fjord had high water age values (300–400 days) during autumn-winter due to increased wind stress, which favored vertical mixing and entrainment of subpycnocline waters of higher age. Wind stress decreased during spring-summer, which favored stratification and decreased water age (~100 days). The change of water age at intermediate (50–100 m) and deep (>100 m) layers is asynchronous. Intermediate layers have their lowest annual water age (100 days) during autumn-winter, when deep waters, especially in the northern part of the Puyuhuapi fjord, have the highest annual water age (>600 days). Increased inflows of dense waters in spring-summer above a sill of the Puyuhuapi fjord (~100 m) renew the deep upstream basins, causing their lowest annual water age values (400 days) at a time when intermediate waters have their highest values.