Inner‐Shelf Vertical and Alongshore temperature Variability in the Subtidal, Diurnal, and Semidiurnal Bands Along the central California coastline with headlands

The temporal, vertical, and alongshore variation in inner‐shelf temperature, T, across subtidal (ST), diurnal (DU), and semidiurnal (SD) bands on coastlines with headlands is not understood. Inner‐shelf T was observed with 20 moorings in 9‐ to 16‐m depth with high vertical density during Fall 2017 along 50 km of central California coastline with headlands. The ST‐first empirical orthogonal function is largely barotropic. ST warm water events are associated with northward‐propagating buoyant plumes and onshore advection of offshore water, with headland effects, particularly for stronger events. Previous plume arrival criteria are northward propagation biased. The DU vertical structure was mixed barotropic and linear baroclinic, without surface extrema. Inner‐shelf DU‐band temperature variability was always evident, largest north of and weakest south of two headlands. The DU‐T envelope was not modulated by ST stratification and was not linked to the modeled DU‐wind envelope. North of one headland, the alongshore first complex empirical orthogonal function of DU temperature has a previously unobserved southward 2‐m/s phase propagation, even though DU frequency is subcritical, that is, not wind forced. A frictional subcritical wave mechanism is proposed for the DU propagation. The SD‐T vertical structure varies alongshore, suggesting at different locations linear internal waves and nonlinear cold bores. SD‐T variability was incoherent with barotropic tides and decorrelates alongshore in 7.5 km, contrasting with a few kilometers offshore. The SD depth‐averaged energy varied strongly alongshore particularly north and south of the headlands, and stronger and weaker SD energy was linked to nonlinear and linear baroclinic vertical structures, respectively, which are headland influenced. Plain Language Summary How water temperature (T) varies both in the vertical, along the coast, and in time at both tidal and longer time scales has important implications in the distribution, retention, and settlement of marine biota and nutrients. Temperature variability in regions near coastal headlands is not understood. Twenty inner‐shelf moorings with a high vertical number of temperature sensors were deployed in 9‐ to 16‐m water depths along 50 km of the central California inner shelf to measure temporal, vertical, and alongshore T variability. At 36 hr or longer time scales, water temperature varies due to warm plumes that propagate northward or onshore that are affected by the headlands. Water temperature always varies on a daily cycle; however, this variation is not due to daily solar heating or to daily sea breezes. Instead, daily T variability propagates like a wave alongcoast, in a manner inconsistent with theory, and a new mechanism is proposed to explain the propagation. The twice‐daily T variability has vertical structure that varies strongly along the coast, suggesting different types of internal wave processes. The twice‐daily T variation was not directly related to the tides and decorrelates rapidly along the coast, counter to previous observations made a few kilometers farther offshore.