A Lagrangian study of the surface circulation of a fjordic system: the effects of preceding weather conditions and local geomorphology

Variation in the surface and near surface circulation patterns of Effingham Inlet, a fjord on the west coast of Vancouver Island, Canada, were measured using Lagrangian drifters over two days during the early spring of 2010. The track data was compared to weather data collected from a local weather station. Drifter deployment sites were north and south of a known sill to assess the influence this bathymetric feature exerts on surface circulation. The drifters were based on designs similar the Davis Drifters and the Surface Velocity Program Holey Sock drogue. These drifters were equipped with tracking GPS recorders, enabling the drifters’ velocities to be calculated. Results are in agreement with previous studies that have found surface motions to be forced by the wind when fresh water terrestrial inputs were low. The deeper circulation patterns showed a slower tidal forcing. The local geomorphology also influenced the circulation. The tracks north of the sill in a shallow channel showed reduced complexity as the water was confined by the local bathymetry. The confined channel also had speeds recorded by the surface drifter of 0.6 m/s, more than twice that of the speed in a wide basin. This demonstrated the funneling effect of the local geomorphology on the water movements. Lagrangian study of fjordic surface circulation Page 5 of 22 Introduction The surface and near surface water circulation patterns of fjords are the result of a host of processes that link the terrestrial stream flow, surface winds, ocean tides and antecedent geological formation of the local bathymetry. The classic depiction of circulation within fjords is a two layer system consisting of a fresher, less dense, surface layer moving seaward over a deeper more saline layer (Hodgins, 1978, Stiegebrandt, 1980). This circulation varies from traditional estuaries as fjords are deeper due the glacial processes that carved the channels (Syvitski and Shaw, 1995). It has been shown by previous studies that the surface currents can be controlled by prevailing winds acting over the water for periods of 10 hours, if the fresh water input is low (Gade, 1963, Johannessen, 1968 and Svendsen and Thompson, 1978). Studies focusing on the deeper currents find that the circulation is generally the result of the tide (Farmer and Osborn, 1976, Hodgins, 1978). Fjords also have bathymetric features that lead to further variation in circulation. Sills are formed by the deposition of glacial sediment (Bennett 2001). Sills have been shown to influence deep circulation by limiting the flushing of deep water on their landward side (Cannon 1975, Leonov and Kawase 2009). This study further investigates the contribution to variation in the circulation patterns by the preceding weather conditions and local geomorphology. Lagrangian measurements from drifters were used to observe differences between the motions of the surface and near surface water. The preceding weather conditions were recorded at a weather station at Bamfield Marine Science Centre, 25 km to the southeast of study Inlet. Terrestrial fresh water inputs are from small rivers entering from the side of the channel, so precipitation acts as a good indicator of fresh water input. Deployment sites were north and south of a known sill in Effingham Inlet on the west coast of Vancouver Island, Canada (Figure 1).