The Evolution of a Northward‐Propagating Buoyant Coastal Plume After a Wind Relaxation Event

After a relaxation of the regional southward, upwelling‐favorable winds along the central California coast, warm water from the Santa Barbara Channel propagates northward as a buoyant plume. As the plume transits up the coast, it causes abrupt temperature changes and modifies shelf stratification. We use temperature and velocity data from 35 moorings north of Pt. Arguello to track the evolution of a buoyant plume after a wind relaxation event in October 2017. The moorings were deployed September–October 2017 and span a ∼30 km stretch of coastline, including nine cross‐shelf transects that range from 17 to 100 m water depth. The high spatial resolution of the data set enables us to track the spatiotemporal evolution of the plume, including across‐front temperature difference, cross‐shore structure, and propagation velocity. We observe an alongshore current velocity signal that takes ∼10 hr to propagate ∼25 km alongshore (∼0.7 m/s) and a temperature signal that takes ∼34 hr to propagate the same distance (∼0.2 m/s). The plume cools as it transits northward, leading to a decrease in the cross‐front temperature difference and the reduced gravity (g’). The plume’s propagation velocity is nonuniform in space and time, with accelerations and decelerations unexplained by the alongshore reduction in g’ or advection by tidal currents. As the plume reaches the northernmost part of the mooring array, its temperature variability is obscured by internal waves, a prominent feature in the region. We focus on one relaxation event but observe five other similar events over the 2 months record. Plain Language Summary Coastal ocean conditions along the US west coast are affected by strong, regional‐scale winds that predominantly blow southward along California. Under “normal” conditions, these winds and other ocean processes cause a large difference between water temperature in the Santa Barbara Channel (warm) and outside the channel to the northwest (cold). When the regional winds weaken, warm water exits the channel as a buoyant plume and moves northward up the coast. Similar plumes occur elsewhere around the world, and these plumes have important ecological impacts because they transport larvae of coastal marine species. Using instruments in the water at 35 sites north of Pt. Arguello, we study how warm plumes travel along a ∼30 km stretch of coastline. Six of these northward‐propagating warm plumes were observed September–October 2017, and we primarily describe one event