Solar heating of the upper ocean is a primary energy input to the ocean-atmosphere system, and the vertical heating profile is modified by the concentration of phytoplankton in the water, with consequences for sea surface temperature and upper ocean dynamics. Despite the development of increasingly complex modeling approaches for radiative transfer in the atmosphere and upper ocean, the simple parameterizations of radiant heating used in most ocean models can be significantly improved in cases of near-surface stratification. There remains a need for a parameterization that is accurate in the upper meters and contains an explicitly spectral dependence on the concentration of biogenic material, while maintaining the computational simplicity of the parameterizations currently in use. Here, we assemble observationally-validated physical modeling tools for the key controls on ocean radiant heating, and simplify them into a parameterization that fulfills this need. We then use observations from 64 spectroradiometer depth casts across 6 cruises in diverse water bodies, 13 surface hyperspectral radiometer deployments, and broadband albedo from 2 UAV flights to probe the accuracy and uncertainty associated with the new parameterization. A novel case study using the parameterization demonstrates the impact of chlorophyll concentration on the structure of diurnal warm layers. The parameterization presented in this work will allow for better modeling of global patterns of sea surface temperature, diurnal warming, and freshwater lenses, without a prohibitive increase in complexity.

In this study, the subseasonal Antarctic sea ice edge prediction skill of the Copernicus Climate Change Service (C3S) and Subseasonal to Seasonal (S2S) projects was evaluated by a probabilistic metric, the spatial probability score (SPS). Both projects provide subseasonal to seasonal scale forecasts of multiple coupled dynamical systems. We found that predictions by individual dynamical systems remain skillful for up to 38days (i.e., the ECMWF system). Regionally, dynamical systems are better at predicting the sea ice edge in the West Antarctic than in the East Antarctic. However, the seasonal variations of the prediction skill are partly system-dependent as some systems have a freezing-season bias, some had a melting-season bias, and some had a season-independent bias. Further analysis reveals that the model initialization is the crucial prerequisite for skillful subseasonal sea ice prediction. For those systems with the most realistic initialization, the model physics dictates the propagation of initialization errors and, consequently, the temporal length of predictive skill. Additionally, we found that the SPS-characterized prediction skill could be improved by increasing the ensemble size to gain a more realistic ensemble spread. Based on the C3S systems, we constructed a multi-model forecast from the above principles. This forecast consistently demonstrated a superior prediction skill compared to individual dynamical systems or statistical observation-based benchmarks. In summary, our results elucidate the most important factors (i.e., the model initialization and the model physics) affecting the currently available subseasonal Antarctic sea ice prediction systems and highlighting the opportunities to improve them significantly.

Wind over the ocean generates near-inertial velocities. In the open ocean, horizontal variability in the inertial frequency and mesoscale vorticity generate internal waves that transport energy laterally and drive diapcynal mixing in remote locations. In the coastal ocean, horizontal variability is produced by the coastline. This study analyzes observations along a straight coastline in Lake Superior, which acts as a "natural laboratory" for the coastal ocean. Depth-profiles of velocity, temperature, and turbulent miscrostructure were collected during a 96hr repeat survey from 3 to 20km offshore in Aug 2018. Wind work was 2mW and generated 0.2m near-inertial velocities that were inhibited within two internal Rossby radii (6km) of the coast. The velocities are interpreted as a superposition of a "forced flow", which is horizontally uniform, and a "wave flow", associated with offshore propagating near-inertial waves. A 1D momentum equation skillfully predicts the horizontally averaged near-inertial velocities and the TKE shear production, which matches the 1mW observed TKE dissipation rate. The offshore propagating wave has an energy flux of 10W (m-coastline)-1 and a downward energy flux of 1mW . These results suggest that most near-inertial wind work is lost directly to TKE shear production, but some energy is transferred to offshore propagating waves that may help catalyze shear instability away from the coast.

Attribution of the ocean acidification (OA) signal in estuarine carbonate system observations is necessary for quantifying the impacts of global anthropogenic emissions on water quality, and informing managers of the efficacy of potential mitigation options. We present an analysis of observational data to characterize dynamics and drivers of seasonal carbonate system variability in two seagrass habitats of Puget Sound, WA, USA, and estimate how carbon accumulations due to anthropogenic emissions interact with these drivers of carbonate chemistry to determine seasonally resolved rates of acidification in these habitats. Three independent simulations of accumulation from 1765 to 2100 were run using two previously published methods and one novel method for estimation. Our results revealed persistent seasonal differences in the magnitude of carbonate system responses to anthropogenic emissions caused by seasonal metabolic changes to the buffering capacity of estuarine waters. The seasonal variability of and is increased (while that of is decreased) and acidification rates are accelerated when compared with open-ocean estimates, highlighting how feedbacks between local metabolism and can control the susceptibility of estuarine habitats to OA impacts. The changes in seasonal variability can shorten the timeline to exceedance of established physiological thresholds for endemic organisms and existing Washington State water quality criteria for pH. We highlight how estimation uncertainties manifest in shallow coastal waters and limit our ability to predict impacts to coastal organisms and ecosystems from anthropogenic emissions.

A nonlinear frequency-domain model and a probabilistic wave breaking model have been employed together to simulate the propagation of nearshore wave breaking and to provide estimates of related statistical quantities such as skewness and asymmetry. This combination of models requires a pre-specification of the frequency dependence of dissipation. Prior work has suggested that a frequency-squared weighting for the dissipation term is most appropriate via physical arguments. However, the original frequency distribution function significantly underpredicts the higher-order moments, particularly the accuracy of asymmetry predictions is in need of further improvement. An intensity of frequency dependence for the breaking-induced damping coefficient is introduced here to further adjust the dissipation function in order to increase the accuracy of asymmetry predictions. By correcting the frequency dependence function with a new form of frequency dependence in the breaking coefficient, the model results are in better agreement with the measurements of the spectrum and higher-order statistics, as well as with the free surface elevation measurements. It is also seen from testing the model with three different cases that the more evident the influence of the breaking mechanism is on the wave transformation process, the more pronounced the contribution of this modification is.

Despite their role in modulating the marine ecosystem, variability and drivers of low-oxygen events in the offshore northern Benguela Upwelling System (BenUS) have been rarely investigated due to the events' episodicity which is difficult to resolve using shipboard measurements. We address this issue using 4months of high-resolution glider data collected between February and June 2018, 100km offshore at 18°S. We find that oxygen (O2) concentrations in the offshore northern Benguela are determined by the subsurface alternation of low-oxygen Angola-derived water and oxygenated water from the south at 100-500m depth. We observe intermittent hypoxia (O2<60μmolkg-1) which occurs on average for ∼30% of the 4months deployment and is driven by the time-varying subsurface pulses of Angola-derived tropical water. Hypoxic events are rather persistent at depths of 300-450m, while they are more sporadic and have weekly duration at shallower depths (100-300m). We find extreme values of hypoxia, with O2 minima of 16μmolkg-1, associated with an anticyclonic eddy spinning from the undercurrent flowing on the BenUS shelf and showing no surface signature. Fine-scale patchiness and water mass mixing are associated with cross-frontal stirring by a large anticyclone recirculating tropical water into the northern BenUS. The dominance of physical drivers and their high variability on short time scales reveal a dynamic coupling between Angola and Benguela, calling for long-term and high-resolution measurements and studies focusing on future changes of both tropical O2 minima and lateral fluxes in this region.

Over the Ross Sea shelf, annual primary production is limited by dissolved iron (DFe) supply. Here, a major source of DFe to surface waters is thought to be vertical resupply from the benthos, which is assumed most prevalent during winter months when katabatic winds drive sea ice formation and convective overturn in coastal polynyas, although the impact of these processes on water-column DFe distributions has not been previously documented. We collected hydrographic data and water-column samples for trace metals analysis in the Terra Nova Bay and Ross Ice Shelf polynyas during April-May 2017 (late austral fall). In the Terra Nova Bay polynya, we observed intense katabatic wind events, and surface mixed layer depths varied from ∼250 to ∼600m over lateral distances <10km; there vertical mixing was just starting to excavate the dense, iron-rich Shelf Waters, and there was also evidence of DFe inputs at shallower depths in the water column. In the Ross Ice Shelf polynya, wind speeds were lower, mixed layers were <300m deep, and DFe distributions were similar to previous, late-summer observations, with concentrations elevated near the seafloor. Corresponding measurements of dissolved manganese and zinc, and particulate iron, manganese, and aluminum, suggest that deep DFe maxima and some mid-depth DFe maxima primarily reflect sedimentary inputs, rather than remineralization. Our data and model simulations imply that vertical resupply of DFe in the Ross Sea occurs mainly during mid-late winter, and may be particularly sensitive to changes in the timing and extent of sea ice production.

Along the west coasts of North, Central, and South America, sea surface temperature (SST) fronts are important for circulation dynamics and promoting biological activity. Prevailing equatorward winds during summer results in offshore Ekman transport and upwelling along the coast, where fronts often form between cold, upwelled water and warmer offshore waters. The interannual variability in winds, coastal upwelling, sea level anomalies, and SST in these regions have been linked to the El Niño-Southern Oscillation (ENSO), however SST fronts have received less attention. Here, we investigate the interannual variability of SST fronts off North, Central, and South America using satellite SST data spanning 1982-2018. Anomalies of fronts within 0-300km offshore indicate interannual variability that coincides with ENSO events in most regions. Frontal activity generally decreases during El Niño events and increases during La Niña events. The decrease in fronts off Peru and Chile during El Niño coincides with the seasonal peak in frontal activity, while off the United States the decrease occurs when frontal activity is at a seasonal minimum. We also utilized satellite measurements of wind stress and sea level anomaly to investigate how ENSO oceanic and atmospheric forcing mechanisms affect frontal activity. Decreases in frontal activity during El Niño events are largely due to oceanic forcing (i.e., coastal Kelvin waves) off Central and South America and to both oceanic forcing and atmospheric teleconnections off the United States. This study furthers our understanding of the influence of ENSO on coastal upwelling regions in the eastern Pacific Ocean.

The Chukchi Sea is an increasing CO2 sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO2 exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO2 (pCO2) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO2 flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface pCO2 and thus the air-sea CO2 flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO2 uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO2 removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC-based NCP was higher than nitrate-based NCP by 66%-84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non-Redfield C:N uptake ratio can adequately reproduce observed pCO2 and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO2 uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO2 uptake of the Chukchi ecosystem to climate change.

We identified anomalously warm sea surface temperature (SST) events during 1980–2019 near the major upwelling center at Punta Lavapié in the central Chile‐Peru Current System, using the European Centre for Medium‐Range Weather Forecasts reanalysis and focusing on time scales of 10 days to 6 months. Extreme warm SST anomalies on these time scales mostly occurred in the austral summer, December through February, and had spatial scales of 1000s of km. By compositing over the 37 most extreme warm events, we estimated terms in a heat budget for the ocean surface mixed layer at the times of strongest warming preceding the events. The net surface heat flux anomaly is too small to explain the anomalous warming, even when allowing for uncertainty in mixed‐layer depth. The composite mean anomaly of wind stress, from satellite ocean vector wind swath data, during the 37 anomalous warming periods has a spatial pattern similar to the resulting warm SST anomalies, analogous to previous studies in the California Current System. The weakened surface wind stress suggests reduced entrainment of cold water from below the mixed layer. Within 100–200 km of the coast, the typical upwelling‐favorable wind stress curl decreases, suggesting reduced upwelling of cold water. In a 1000‐km area of anomalous warming offshore, the typical downwelling‐favorable wind stress curl also decreases, implying reduced downward Ekman pumping, which would allow mixed‐layer shoaling and amplify the effect of the positive climatological summertime net surface heat flux.