Wind-driven circulation and upwelling may enhance particle and nutrient transport by internal waves

Internal waves, particularly those generated by tides, are important and widespread forces for cross-shelf exchange and mixing in the oceans. They can drive significant mass transport on continental shelves and entrain, accumulate, and deliver larvae and other planktonic particles to nearshore habitats. They also have the potential to drive local adaptation or acclimation to extreme events. A complex set of factors affect the characteristics, propagation, and particle transport potential of internal waves. We examined internal wave run-up events in Bahia Todos Santos near Ensenada, B.C., Mexico, identifying and characterizing them by their leading cold fronts, currents, and subsequent vertical excursions of the thermocline. We present evidence that wind stress modified internal wave events. Upwelling introduced dense bottom water into the bay system and wind-driven surface currents interacted with internal waves as they shoaled in the bay. Internal wave run-up that co-occurred with wind-driven onshore currents had greater: stratification in the bay, potential energy change across fronts, baroclinic surface and bottom currents, downward vertical currents surrounding fronts, and particle accumulation potential. Wind may have caused internal waves to break and form bores sooner. Internal waves were known to enhance nutrient and larval delivery in the northern bay prior to our study, but we have examined evidence suggesting the effect is augmented by the addition of wind-driven currents and the presence of upwelled water masses in and near the bay. We found that the particle accumulation potential increased more than two-fold on average for events that had experienced lagged wind stress and more than three-fold on average for those that had experienced wind stress right up to event onset at our moored stations. Increased baroclinic and barotropic currents in the direction of wave propagation and slightly slower onshore wave propagation speeds drove greater accumulation potential for the windier events. Subsequent internal wave activity, however, was higher for fronts under lighter wind conditions, which may help to make up for some of the reduced particle transport in events with weaker total surface currents. Understanding the phasing of coastal transport mechanisms that may act in concert is key to extending these type of insights to larger scale predictions related to the relationship between particle transport and coastal oceanographic conditions.