Asymmetry of Tidal Plume Fronts in an Eastern Boundary Current Regime

The Columbia River tidal plume or near-field is formed twice daily by the ebb outflow of the Columbia River. It is a part of a larger, anticyclonic plume bulge, which in turn is embedded in far-field plume and coastal waters. Because of the mixing caused directly and indirectly by plume fronts, the interaction of the tidal plume and bulge with the California Current upwelling regime plays a vital role in coastal productivity on the Oregon and Washington shelves. The tidal plume is initially supercritical with respect to the internal Froude number on all stronger ebbs. It is separated from the plume bulge by a front, whose properties are very different under upwelling vs. downwelling conditions. Under summer upwelling conditions, this front is sharp and narrow (only 50-100 m wide on its upwind or northern side) and marks a transition from supercritical to subcritical flow for 6-12 hours after high water. This sharp front is a source of turbulent mixing, despite the strong stratification. Because the tidal plume may overlie newly upwelled waters, these fronts can mix nutrients into the plume, enhancing primary productivity. Symmetry would suggest that there should be a sharp front south of the estuary mouth under summer downwelling conditions. Instead, the downwelling tidal plume front is usually broad (up to several km) and diffuse on the upstream side. Less mixing occurs, and the water immediately below the plume consists of old plume and surface ocean waters, both low in nutrients. There is also a second up-welling-downwelling asymmetry. Supercritical upwelling plume fronts often generate solitons trains as they slow and transition to a subcritical state. These soliton trains contribute to vertical mixing in the plume bulge and have a non-zero Stokes drift so that they transport low-salinity water across the tidal plume into the plume bulge. Under downwelling conditions, soliton forma-tion is uncommon. Moreover, solition formation almost always begins on the south side of the plume so that the front unzips from south to north. This implies that a frontal transition from supercritical to subcritical conditions first occurs on the south side tidal plume, regardless of whether this is the upwind or downwind side of the plume. This contribution describes and analyzes these two asymmetries using vessel data, SAR images and a vorticity analysis. Internal Froude number and plume depth are key parameters in distinguishing the upwelling and downwelling situations, and the observed asymmetries can be explained in terms of potential vorticity conservation. The tidal outflow embeds relative vorticity in the emerging tidal plume water mass. This vorticity controls the transition of the tidal plume front to a subcritical state and the timing and location of internal wave generation by plume fronts