Configuration and Validation of an Oceanic Physical and Biogeochemical Model to Investigate Coastal Eutrophication in the Southern California Bight

Abstract

The Southern California Bight (SCB), an eastern boundary upwelling system, is impacted by global warming, acidification, and oxygen loss and receives anthropogenic nutrients from a coastal population of 20 million people. We describe the configuration, forcing, and validation of a realistic, submesoscale‐resolving ocean model as a tool to investigate coastal eutrophication. This modeling system represents an important achievement because it strikes a balance of capturing the forcing by U.S. Pacific Coast‐wide phenomena, while representing the bathymetric features and submesoscale circulation that affect the transport of nutrients from natural and human sources. Moreover, the model allows simulations at time scales that approach the interannual frequencies of ocean variability. The model simulation is evaluated against a broad suite of observational data throughout the SCB, showing realistic depiction of the mean state and its variability with satellite and in situ measurements of state variables and biogeochemical rates. The simulation reproduces the main structure of the seasonal upwelling front, the mean current patterns, the dispersion of wastewater plumes, as well as their seasonal variability. Furthermore, it reproduces the mean distributions of key biogeochemical and ecosystem properties and their variability. Biogeochemical rates reproduced by the model, such as primary production and nitrification, are also consistent with measured rates. This validation exercise demonstrates the utility of using fine‐scale resolution modeling and local observations to identify, investigate, and communicate uncertainty to stakeholders to support management decisions on local anthropogenic nutrient discharges to coastal zones.

Seasonal nearshore ocean acidification and deoxygenation in the Southern California Bight

“…Numerical models may offer the best approach to address these questions, but most existing models are currently too coarse to resolve the spatiotemporal biogeochemical variability documented in this study. Existing models typically range in horizontal resolution from 1 to 4 km or larger (e.g., 22,76,[78][79][80][81][82] ) and therefore fail to reveal the full extent of ecologically relevant OAH dynamics in the proximal nearshore environment due to spatial and temporal averaging (e.g., 22 ). However, there is an increasing recognition that higher model resolution is required to accurately reproduce and forecast biogeochemical and biological impacts of climate change in this region as important differences have emerged in down-scaled model simulations 83 .…”

Enhancement of Oceanic Eddy Activity by Fine‐Scale Orographic Winds Drives High Productivity, Low Oxygen, and Low pH Conditions in the Santa Barbara Channel

“…CTRL is initialized and forced from the 1-km model using the same atmospheric forcing, starting on 01/1997 and ending on 12/2000. A detailed description of the model set up and configuration is presented in Kessouri et al (2021). We carried out an additional simulation (SMTH) over a 3-month period, initialized from CTRL on 1 June 2000.…”

Effects of urban eutrophication on pelagic habitat capacity in the Southern California Bight

“…The SCB model domain, which extends from Tijuana Mexico to Pismo Beach (U.S. Central California coast) and about 200-km offshore, is part of a nested configuration. Model nests scale from a 4-km horizontal resolution configuration spanning the entire CCS, to a 1-km resolution grid covering much of the California coast (latitude < 40.25 • N), to a 0.3-km grid in the SCB, where our investigations of local anthropogenic inputs were focused 10,67 . This grid, shown in Fig.…”

“…The model is run with a time step of 30 s, and outputs are saved as 1-day averages. More information on the model setup and forcing is provided in other works 63,64,67 .…”

“…ROMS-BEC has been validated for atmospheric forcing, physics, and biogeochemistry including O 2 , carbonate saturation state, primary productivity, and hydrographic parameters at a West Coast-wide scale 63 and, within the SCB, at scales at which anthropogenic nutrients inputs influence coastal eutrophication 67 .…”