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We recently developed a dynamically coupled hydrological‐ocean modeling system that provides seamless coverage across the land‐ocean continuum during hurricane‐induced compound flooding. This study introduced a local inertial equation and a diagonal flow algorithm to the overland routing of the coupled system's hydrology model (WRF‐Hydro). Using Hurricane Florence (2018) as a test case, the performance of the coupled model was significantly improved, evidenced by its enhanced capability of capturing backwater and increased water level simulation accuracy and stability. With four model experiments, we present a framework to detangle, define, and quantify compound and nonlinear effects. The results revealed that the flood peaks in the lower Cape Fear River Basin and the coastal waters were contributed by inland flooding and storm surge, respectively. These two processes had comparable contributions to the flooding in the Cape Fear River Estuary. The compound effect was identified when the flood levels resulting from the combination of land and ocean processes surpassed those caused by an individual process alone. The compound effect during Hurricane Florence exhibited limited impact on flood peaks, primarily due to the time lag between the peaks of the storm surge and the inland flooding. In the period between the two peaks, the compound effect was salient and significantly impacted the magnitude and variation of the flood level. The nonlinear effect, defined as the difference between the compound flood level and the superposition of storm surge and inland flooding water levels, reduced flood levels in the river channels while increasing flood levels on the floodplain. Plain Language Summary: This study addresses the phenomenon of hurricane‐induced compound flooding, which arises when inland waters and storm surges coincide at the land‐ocean boundary. We've devised a hydrological‐ocean model that effectively covers such events. This model, enhanced with new algorithms, was tested using Hurricane Florence (2018) data, showing marked improvements in predicting water levels and tide effects. Our research delineates and quantifies the complex interplay between different flooding sources during such events. Key findings include the determination that both inland flooding and storm surges contributed equally to the flooding in the Cape Fear River Estuary. However, the overlapping impact of these processes, termed the "compound effect," was limited in its influence on peak flood levels, mainly due to the time gap between storm surge and inland flooding peaks. Another crucial discovery was the "nonlinear effect," which accounts for discrepancies in predicted flood levels. This effect tended to decrease flood levels in river channels but increased them on floodplains. Key Points: A local inertial equation and a diagonal flow algorithm were introduced to a newly developed dynamically coupled hydrological‐ocean modelStrong compound effect between hydrological and ocean processes occurred between their peaksThe nonlinear effect reduced the flood peaks in the river channels while amplifying them on the floodplains [ABSTRACT FROM AUTHOR]

A three-dimensional coupled hydrodynamic–biogeochemical model with multiple nutrient and plankton functional groups was developed and adapted to the Gulf of Mexico to investigate the role of nutrients and the complexity of plankton community in dissolved oxygen (DO) dynamics. A 15-year hindcast was achieved covering the period of 2006–2020. Extensive model validation against in situ data demonstrates that the model was capable of reproducing vertical distributions of DO and spatial distributions of bottom DO concentration, as well as their interannual variations. The study demonstrates that bottom DO dynamics and hypoxia evolution are significantly influenced by both physical processes and local biochemistry, with sedimentary oxygen consumption and vertical diffusion identified as key contributors. Summer hydrodynamics play a critical role in nutrient distribution and limitation: a notable expansion of Si limitation was simulated when coastal currents shifted eastward or northward. This effect, especially pronounced on the western part of the Louisiana–Texas shelf, underscores the importance of nutrient limitation in shaping DO dynamics. The model identifies a bi-peak primary production pattern in spring and early summer, aligned with satellite chlorophyll a variations, attributed to the complexity of the plankton community and interactions among different plankton groups. Our findings emphasize the necessity of integrating sophisticated plankton community dynamics into biogeochemical models to understand primary production variability and its impact on bottom hypoxia. [ABSTRACT FROM AUTHOR]

Extreme precipitation during Hurricane Florence, which made landfall in North Carolina in September 2018, led to breaches of hog waste lagoons, coal ash pits, and wastewater facilities. In the weeks following the storm, freshwater discharge carried pollutants, sediment, organic matter, and debris to the coastal ocean, contributing to beach closures, algae blooms, hypoxia, and other ecosystem impacts. Here, the ocean pathways of land‐sourced contaminants following Hurricane Florence are investigated using the Regional Ocean Modeling System (ROMS) with a river point source with fixed water properties from a hydrologic model (WRF‐Hydro) of the Cape Fear River Basin, North Carolina's largest watershed. Patterns of contaminant transport in the coastal ocean are quantified with a finite duration tracer release based on observed flooding of agricultural and industrial facilities. A suite of synthetic events also was simulated to investigate the sensitivity of the river plume transport pathways to river discharge and wind direction. The simulated Hurricane Florence discharge event led to westward (downcoast) transport of contaminants in a coastal current, along with intermittent storage and release of material in an offshore (bulge) or eastward (upcoast) region near the river mouth, modulated by alternating upwelling and downwelling winds. The river plume patterns led to a delayed onset and long duration of contaminants affecting beaches 100 km to the west, days to weeks after the storm. Maps of the onset and duration of hypothetical water quality hazards for a range of weather conditions may provide guidance to managers on the timing of swimming/shellfishing advisories and water quality sampling. Plain Language Summary: Hurricane Florence hit the southeastern United States in September 2018, causing historic rainfall and flooding. Floodwaters carrying contaminants traveled down rivers to the ocean, causing harmful algal blooms, beach closures, and other impacts. In the Cape Fear River Basin, North Carolina's largest watershed, open‐air lagoons used to store waste from industrial hog farming were breached, causing sewage to flow into the ocean in a coherent feature called a 'river plume' that moved in different patterns. Flooding impacts are increasing, but the movement of contaminants in river plumes following extreme events is not well understood. We used a numerical ocean model to simulate the Cape Fear River plume carrying a dye representing contaminants following Hurricane Florence. The plume initially traveled west hugging the coast due to Earth's rotation, alternately spreading east or offshore due to changes in the direction of winds blowing on the ocean. The river plume patterns led to a delayed onset and long duration of contaminants affecting beaches far down the coast, days to weeks after the storm. We also simulated several scenarios with different river discharge and winds. This information could help guide managers on when to perform water quality sampling and issue swimming or shellfishing advisories. Key Points: Pathways of land‐sourced contaminants in the aftermath of Hurricane Florence are investigated with an ocean model with hydrology sourceThe Cape Fear River plume had a westward coastal current and intermittent offshore storage modulated by wind direction and river dischargeBeaches 100 km west saw river‐sourced water quality hazards with delayed onset and long duration, from days to weeks after the storm [ABSTRACT FROM AUTHOR]

In this study, a novel ensemble regression model was developed for hypoxic area (HA) forecast in the Louisiana– Texas (LaTex) Shelf. The ensemble model combines a zero-inflated Poisson generalized linear model (GLM) and a quasi- Poisson generalized additive model (GAM) and considers predictors with hydrodynamic and biochemical features. Both models were trained and calibrated using the daily hindcast (2007–2020) by a three-dimensional coupled hydrodynamic– biogeochemical model embedded in the Reginal Ocean Modeling System (ROMS). A promising HA forecast is provided by the ensemble model with a low RMSE (3,204 km2), a high R2 (0.8005), and a precise performance in capturing hypoxic area peaks in the summers. To test its robustness, the model was further applied to a global forecast model and produces HA prediction from 2019 to 2020 with the adjusted predictors from the HYbrid Coordinate Ocean Model (HYCOM). Predicted HA shows a high agreement with the ROMS hindcast time series (RMSE=4,571 km2, R2=0.8178). Our model can also predict the magnitude and onsets of summer HA peaks in both 2019 and 2020 with high accuracy. To the best of our knowledge, this ensemble model is by far the first one providing fast and accurate daily HA predictions for the LaTex Shelf while considering both hydrodynamic and biochemical effects. This study demonstrates that it is feasible to perform regional ocean HA prediction using global ocean forecast. [ABSTRACT FROM AUTHOR]

Sea level rise and intense hurricane events make the East and Gulf Coasts of the United States increasingly vulnerable to flooding, which necessitates the development of computational models for accurate water level simulation in these areas to safeguard the coastal wellbeing. With this regard, a model framework for water level simulation over coastal transition zone during hurricane events is built in this study. The model takes advantage of the National Water Model's strength in simulating rainfall–runoff process, and D‐Flow Flexible Mesh's ability to support unstructured grid in hydrodynamic processes simulation with storm surges/tides information from the Advanced CIRCulation model. We apply the model on the Delaware Estuary to simulate extreme water level and to investigate the contribution of different physical components to it during Hurricane Isabel (2003). The model shows satisfactory performance with an average Willmott skill of 0.965. Model results suggest that storm surge is the most dominating component of extreme water level with an average contribution of 78.16%, second by the astronomical tide with 19.52%. While the contribution of rivers is mainly restricted to the upper part of the estuary upstream of Schuylkill River, local wind‐induced water level is more pronounced with values larger than 0.2 m over most part of the estuary. [ABSTRACT FROM AUTHOR]

Coupled physical-biogeochemical models can significantly reduce uncertainties in estimating the spatial and temporal patterns of the ocean carbon system. Challenges of applying a coupled physical-biogeochemical model in the regional ocean include the reasonable prescription of carbon model boundary conditions, lack of in situ observations, and the oversimplification of certain biogeochemical processes. In this study, we applied a coupled physical-biogeochemical model (Regional Ocean Modelling System, ROMS) to the Gulf of Mexico (GoM) and achieved an unprecedented 20-year highresolution (5 km, 1/22°) hindcast covering the period of 2000-2019. The model's biogeochemical cycle is driven by the Coupled Model Intercomparison Project 6-Community Earth System Model 2 products (CMIP6-CESM2) and incorporates the dynamics of dissolved organic carbon (DOC) pools as well as the formation and dissolution of carbonate minerals. Model outputs include generally interested carbon system variables, such as pCO2, pH, aragonite saturation state (OArag), calcite saturation state (OCalc), CO2 air-sea flux, carbon burial rate, etc. The model's robustness is evaluated via extensive model-data comparison against buoy, remote sensing-based Machine Learning (ML) predictions, and ship-based measurements. Model results reveal that the GoM water has been experiencing an ~ 0.0016 yr-1 decrease in surface pH over the past two decades, accompanied by a ~ 1.66 µatm yr-1 increase in sea surface pCO2. The air-sea CO2 exchange estimation confirms that the riverdominated northern GoM is a substantial carbon sink. The open water of GoM, affected mainly by the thermal effect, is a carbon source during summer and a carbon sink for the rest of the year. Sensitivity experiments are conducted to evaluate the impacts from river inputs and the global ocean via model boundaries. Our results show that the coastal ocean carbon cycle is dominated by enormous carbon inputs from the Mississippi River and nutrient-stimulated biological activities, and the carbon system condition of the open ocean is primarily driven by inputs from the Caribbean Sea via Yucatan Channel. [ABSTRACT FROM AUTHOR]

We introduce a sediment-induced light attenuation algorithm into the biogeochemical model of the Regional Ocean Modeling System (ROMS). A fully coupled ocean-atmospheric-sediment-biogeochemical simulation is carried out to assess the impact of sediment-induced light attenuation on primary production in the northern Gulf of Mexico during Hurricane Gustav in 2008. The new model shows a better agreement with satellite data on both the magnitude of nearshore chlorophyll concentration and the distribution of offshore bloom. When Gustav approaches, resuspended sediments shift the inner shelf ecosystem from a nutrient-limited one to light-limited. One week after Gustav's landfall, accumulated nutrient and favorable optical environment induces a post-hurricane algal bloom in the top 20 m of water column, while the productivity in the lower column is still light-limited due to unsettled sediment. Corresponding with the elevated offshore NO3 flux (38.71 mmol N/m/s) and decreased chlorophyll flux (43.10 mg/m/s), the post-hurricane bloom in the outer shelf is resulted from the cross-shelf nutrient supply instead of the lateral dispersed chlorophyll. Sensitivity tests indicate sediment light attenuation efficiency affects primary production when sediment concentration is moderately high. [ABSTRACT FROM AUTHOR]

We applied the newly developed WRF-Hydro model to investigate the hydroclimatic trend encompassing the three basins in Southwest Louisiana as well as their connection with large-scale atmospheric drivers. Using the North American Land Data Assimilation System Phase 2 (NLDAS-2), we performed a multi-decadal model hindcast covering the period of 1979–2014. After validating the model’s performance against available observations, trend and wavelet analysis were applied on the time series of hydroclimatic variables from NLDAS-2 (temperature and precipitation) and model results (evapotranspiration, soil moisture, water surplus, and streamflow). Trend analysis of model-simulated monthly and annual time series indicates that the regional climate is warming and drying over the past decades, specifically during spring and summer (growing season). Wavelet analysis reveals that, since the late 1990s, the anomaly of evapotranspiration, soil moisture, and streamflow exhibits high coherency with that of precipitation. Pettitt’s test detects a possible change-point around the year 2004, after which the monthly precipitation decreased from 140 to 120 mm, evapotranspiration slightly increased from 80 to 83 mm, and water surplus decreased from 60 to 38 mm. Changes in regional climate conditions are closely correlated with large-scale climate dynamics such as the Atlantic Multidecadal Oscillation (AMO) and El Niño Southern Oscillation (ENSO). [ABSTRACT FROM AUTHOR]