The Influence of Wind and Waves on Spreading and Mixing in the Fraser River Plume

This study uses drifter-based observations to investigate the role of wind and waves on spreading and mixing in the Fraser River plume. Local winter wind patterns commonly result in two distinct forcing conditions, moderate winds from the southeast (SE) and strong winds from the northwest (NW). We examine how these patterns influence the spreading and mixing dynamics of the plume. Under SE winds, the plume thins, spreads, and turns to the right (north) upon exiting the river mouth. Mixing is initially intense in the region of maximum spreading, but it is short-lived. Under NW winds, which oppose the rightward tendency of the plume, the plume remains thicker, narrower, and flows directly across the Strait with a lateral front on its northern side. Mixing is initially lower than under SE forcing but persists further across the Strait. A Lagrangian stream-normal momentum balance shows that wind and interfacial stress under NW conditions compress the sea surface height anomaly formed by the river discharge and guide the flow across the Strait. This reconfiguration changes spreading and mixing dynamics of the plume; plume spreading, which drives intense mixing under SE winds, is shut down under NW winds, and mixing rates are consequently much lower. Despite the initially lower mixing rates, the region of active mixing extends further under NW winds, resulting in higher net mixing. These results highlight that the wind, which is often a primary cause of increased plume mixing, can also significantly influence mixing by changing the geometry of the plume. Rivers transport sediment, pollutants, and nutrients from inland regions to coastal seas. Where rivers meet the ocean, freshwater flows over the ambient salty seawater, forming a river plume. The quantities that the river transports into the ocean are mixed into the seawater along with the freshwater, and so it is vital to understand this mixing process. While ocean surface waves might be the most striking visual feature of the coastal ocean, at the Fraser River mouth, south of Vancouver, Canada, we find that wind is a much more important influence on river plume mixing than waves. Wind can influence mixing by changing the geometry of the plume to either favor or discourage intense mixing occurring in the system. This is a result of the wind encouraging or discouraging plume spreading, which is the primary cause of mixing close to the river mouth. Ocean surface waves, despite being a visually striking feature of this system, do not play a large role in mixing the Fraser River plume. Thus, in order to correctly predict river plume mixing, we must take into account wind conditions near the river mouth, while waves are less important.