[1] In this study, we evaluate results from an incremental strong constraint four-dimensional variational data assimilation (IS4D-Var) experiment applied to the circulation around the Hawaiian Islands using the Regional Ocean Modeling System (ROMS). Assimilated observations include (1) satellite-derived high-resolution swath radiometer sea surface temperatures (SST) and along-track altimetric sea surface heights (SSH) and (2) in situ temperature and salinity profiles from Argo floats, autonomous Seagliders, and shipboard conductivity-temperature-depth. Two assimilation configurations are compared: adjusting initial conditions versus adjusting both initial conditions along with atmospheric forcing. In the latter case, we compare two separate forcing products. For all experiments, we investigate how the assimilation alters the tidal, inertial, and mesoscale variability. Significant improvements in the observation-model fit are found for SST and salinity regardless of assimilation configuration or atmospheric forcing; however, significant change to the subsurface temperature is made when adjusting only initial conditions. Baroclinic tides are altered during the assimilation because of changes in the density field in regions of strong internal tide generation. Spurious inertial oscillations are found in assimilation circulations that are associated with the IS4D-Var increment when using either SST or SSH observations; however, this increase in the inertial energy had minimal effect on the mesoscale variability.
[1] This research examines the potential of ocean velocity observations generated from the maximum cross-correlation technique (MCC) applied to thermal infrared imagery to capture the total current at a high sampling rate, especially in the nearshore region, allowing for a novel view of the California Current. Comparison of 12 years of weekly ocean velocity fields derived from the MCC method with velocity observations from drifting buoys reveals strong correspondence between the data sets. The MCC method, however, is able to produce over $10 times the number of observations. Comparison of MCC velocities with geostrophic velocities from altimetry demonstrates differences that suggest ageostrophic currents in the MCC observations that are likely wind driven. The time-averaged mean velocity field from MCC observations reveals strong offshore jetlike features that extend off of coastline promontories. Eddy statistics from the MCC velocity observations indicate dramatically different dynamics in the nearshore and offshore regions. Differences in spatial statistics derived from the MCC observations compared to previous studies of the region are attributed to wind-driven currents in the observations that can dominate the variability of the current in the nearshore region.
[1] State estimation techniques have been well established in open ocean systems; however they are less often used in coastal applications due to nonlinearity. Using 4D-variational data assimilation in a triple one-way nested system, we investigate the processes that control coastal dynamics for a test case along the western coast of Oahu, Hawaii. All available observations are combined with the model dynamics for 13 months. Over this time, the residual error between the model and observations was improved by nearly 30% in the surface temperature and 34% in the alongshore ADCP currents. The barotropic and baroclinic tides are found dominate the local circulation; however, island and atmospheric interaction generates an island wake effect that is important to the subtidal dynamics of the region. The baroclinic tides exhibit well-defined energy paths, and the initial condition corrections, despite altering the density waveguide, have little influence on the propagation of the baroclinic energy, which is controlled by the propagation of baroclinic tides generated outside of the domain. We find the larger-scale, advected dynamics control the local surface circulation through boundary condition adjustment, accounting for 45% of the total corrections made via data assimilation system. The initial conditions controls little of the evolution of this local, coastal flow and has a short persistence. The wind stress control vector is important in the central region of the domain inducing flow toward the lee of the island. Our results reveal that coastal studies may not be initial value problems, rather they are forced problems that require a knowledge of the large-scale energy propagated into the region.
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