Available energy of the world ocean

The available energy of the ocean is the excess of the sum of the ocean\u27s internal and gravitational energies with respect to its equilibrium state, which is in thermodynamic equilibrium and has the same total entropy as the ocean. The equilibrium state for the world ocean is rigorously determined to be an isothermal ocean with a temperature of 3.66˚C and a horizontally uniform absolute salinity that increases monotonically from 27.30 g kg–1 at the surface to 47.39 g kg–1 at a depth of 5.5 km. This salinity profile is consistent with a uniform relative chemical potential of 47.44 J g–1 salt. The ocean\u27s available energy is 220 × 1021 J or 630 MJ m–2. Most (72%) of the available energy is due to the internal energy difference between the ocean and its equilibrium state; the remaining 28% is due to the gravitational energy difference. The ocean\u27s available energy is shown to be concentrated vertically in the upper half kilometer and geographically in the tropics and subtropics. This distribution is accurately represented by the temperature variance from the equilibrium temperature. The contributions of sea ice and variable sea surface height to the available energy are estimated to be small

Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m−2 (4μE m−2 s−1) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that are shown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region

Understanding Coastal Carbon Cycling by Linking Top- Down and Bottom-Up Approaches

The coastal zone, despite occupying a small fraction of the Earth\u27s surface area, is an important component of the global carbon (C) cycle. Coastal wetlands, including mangrove forests, tidal marshes, and seagrass meadows, compose a domain of large reservoirs of biomass and soil C [Fourqurean et al., 2012; Donato et al., 2011; Pendleton et al., 2012; Regnier et al., 2013; Bauer et al.,2013]. These wetlands and their associated C reservoirs (2 to 25 petagrams C; best estimate of 7 petagrams C [Pendleton et al., 2012]) provide numerous ecosystem services and serve as key links between land and ocean

Photochemistry, mixing and diurnal cycles in the upper ocean

The interplay between ocean photochemistry and surface boundary-layer physics is explored in a range of analytical and numerical process models. For simple systems, key attributes of the photochemical distribution—diurnal cycle, surface concentration, and the bulk concentration difference across the “mixed layer”—can be expressed in terms of a small number of physical (vertical diffusivity) and photochemical (turnover timescale and production depth scale) scaling factors. A coupled, 1-D photochemical/physical model is used to examine the more general case with finite mixing rates, variable photochemical production and evolving boundary layer depth. Finite boundary layer mixing rates act to increase both the diurnal cycle and mean concentration at the surface. The diurnal cycle and mean surface concentration are further amplified by coupling between photochemistry and diurnal physics. The daily heating/cooling cycle of the upper ocean can lead to a significant reduction in mixing and boundary-layer depth during the day when photochemical production is at a maximum. Accounting for these effects results in additional surface trapping of photochemically produced species and significant enhancements of the surface diurnal cycle and daily mean. The implications of our model results for field data interpretation and global air-sea flux calculations are also discussed

Estuaries as Filters for Riverine Microplastics: Simulations in a Large, Coastal-Plain Estuary

Public awareness of microplastics and their widespread presence throughout most bodies of water are increasingly documented. The accumulation of microplastics in the ocean, however, appears to be far less than their riverine inputs, suggesting that there is a “missing sink” of plastics in the ocean. Estuaries have long been recognized as filters for riverine material in marine biogeochemical budgets. Here we use a model of estuarine microplastic transport to test the hypothesis that the Chesapeake Bay, a large coastal-plain estuary in eastern North America, is a potentially large filter, or “sink,” of riverine microplastics. The 1-year composite simulation, which tracks an equal number of buoyant and sinking 5-mm diameter particles, shows that 94% of riverine microplastics are beached, with only 5% exported from the Bay, and 1% remaining in the water column. We evaluate the robustness of this finding by conducting additional simulations in a tributary of the Bay for different years, particle densities, particle sizes, turbulent dissipation rates, and shoreline characteristics. The resulting microplastic transport and fate were sensitive to interannual variability over a decadal (2010–2019) analysis, with greater export out of the Bay during high streamflow years. Particle size was found to be unimportant while particle density – specifically if a particle was buoyant or not – was found to significantly influence overall fate and mean duration in the water column. Positively buoyant microplastics are more mobile due to being in the seaward branch of the residual estuarine circulation while negatively buoyant microplastics are transported a lesser distance due to being in the landward branch, and therefore tend to deposit on coastlines close to their river sources, which may help guide sampling campaigns. Half of all riverine microplastics that beach do so within 7–13 days, while those that leave the bay do so within 26 days. Despite microplastic distributions being sensitive to some modeling choices (e.g., particle density and shoreline hardening), in all scenarios most of riverine plastics do not make it to the ocean, suggesting that estuaries may serve as a filter for riverine microplastics

Riverine Carbon Cycling Over the Past Century in the Mid-Atlantic Region of the United States

Rivers are an important component of the terrestrial-aquatic ocean continuum as they serve as a conduit for transporting carbon from the land to the coastal ocean. It is essential to track the fate of this carbon, including how much carbon is buried in the riverbed, outgassed to the atmosphere, and exported to the ocean. However, it is often difficult to quantify these carbon transport processes on the watershed scale because observational data obtained by field surveys can only be used to estimate the magnitude of these processes at distinct points. In this study, we used a coupled terrestrial-aquatic ecosystem model to assess the century-long full carbon budget of the riverine ecosystem across the watersheds of Chesapeake Bay and Delaware Bay. In addition, we examined the individual and combined impacts of climate change and anthropogenic activities on these terrestrial ecosystems and the resultant CO2emissions of their associated rivers. We found that climate variability and land conversion (from cropland to impervious surfaces and forest) are the most important factors governing the long-term change in riverine carbon dynamics. We also highlighted the importance of riverine CO2 emissions in the overall regional carbon budget

Potential climate-change impacts on the Chesapeake Bay

We review current understanding of the potential impact of climate change on the Chesapeake Bay. Scenarios for CO2 emissions indicate that by the end of the 21st century the Bay region will experience significant changes in climate forcings with respect to historical conditions, including increases in CO2 concentrations, sea level, and water temperature of 50–160%, 0.7–1.6m, and 2–6C, respectively. Also likely are increases in precipitation amount (very likely in the winter and spring), precipitation intensity, intensity of tropical and extratropical cyclones (though their frequency may decrease), and sea-level variability. The greatest uncertainty is associatedwith changes in annual streamflow, though it is likely that winter and spring flows will increase. Climate change alone will cause the Bay to function very differently in the future. Likely changes include: (1) an increase in coastal flooding and submergence of estuarine wetlands; (2) an increase in salinity variability on many time scales; (3) an increase in harmful algae; (4) an increase in hypoxia; (5) a reduction of eelgrass, the dominant submergedaquatic vegetation in the Bay; and (6) altered interactions among trophic levels, with subtropical fish and shellfish species ultimately being favored in the Bay. The magnitude of these changes is sensitive to the CO2 emission trajectory, so that actions taken now to reduce CO2 emissions will reduce climate impacts on the Bay. Research needs include improved precipitation and streamflow projections for the Bay watershed and whole-system monitoring, modeling, and process studies that can capture the likely non-linear responses of the Chesapeake Bay system to climate variability, climate change, and their interaction with other anthropogenic stressor

Relative impacts of global changes and regional watershed changes on the inorganic carbon balance of the Chesapeake Bay

The Chesapeake Bay is a large coastal-plain estuary that has experienced considerable anthropogenic changeover the past century. At the regional scale, land-use change has doubled the nutrient input from rivers and led to an increase in riverine carbon and alkalinity. The bay has also experienced global changes, including the rise of atmospheric temperature and CO2. Here we seek to understand the relative impact of these changes on the inorganic carbon balance of the bay between the early 1900s and the early 2000s. We use a linked land–estuarine–ocean modeling system that includes both inorganic and organic carbon and nitrogen cycling. Sensitivity experiments are performed to isolate the effect of changes in (1) atmospheric CO2, (2) temperature,(3) riverine nitrogen loading and (4) riverine carbon and alkalinity loading. Specifically, we find that over the past century global changes have increased ingassing by roughly the same amount (∼30 Gg-C yr−1) as has the increased riverine loadings. While the former is due primarily to increases in atmospheric CO2, the latter results from increased net ecosystem production that enhances ingassing. Interestingly, these increases in ingassing are partially mitigated by increase temperatures and increased riverine carbon and alkalinity in-puts, both of which enhance outgassing. Overall, the bay has evolved over the century to take up more atmospheric CO2 and produce more organic carbon. These results suggest that over the past century, changes in riverine nutrient loads have played an important role in altering coastal carbon budgets, but that ongoing global changes have also substantially affected coastal carbonate chemistry

Increased nitrogen export from eastern North America to the Atlantic Ocean due to climatic and anthropogenic changes during 1901-2008

We used a process-based land model, Dynamic Land Ecosystem Model 2.0, to examine how climatic and anthropogenic changes affected riverine fluxes of ammonium (NH4+), nitrate (NO3-), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) from eastern North America, especially the drainage areas of the Gulf of Maine (GOM), Mid-Atlantic Bight (MAB), and South Atlantic Bight (SAB) during 1901-2008. Model simulations indicated that annual fluxes of NH4+, NO3-, DON, and PON from the study area during 1980-2008 were 0.0190.003 (mean1 standard deviation) TgNyr(-1), 0.180.035TgNyr(-1), 0.100.016TgNyr(-1), and 0.043 +/- 0.008TgNyr(-1), respectively. NH4+, NO3-, and DON exports increased while PON export decreased from 1901 to 2008. Nitrogen export demonstrated substantial spatial variability across the study area. Increased NH4+ export mainly occurred around major cities in the MAB. NO3- export increased in most parts of the MAB but decreased in parts of the GOM. Enhanced DON export was mainly distributed in the GOM and the SAB. PON export increased in coastal areas of the SAB and northern parts of the GOM but decreased in the Piedmont areas and the eastern parts of the MAB. Climate was the primary reason for interannual variability in nitrogen export; fertilizer use and nitrogen deposition tended to enhance the export of all nitrogen species; livestock farming and sewage discharge were also responsible for the increases in NH4+ and NO3- fluxes; and land cover change (especially reforestation of former agricultural land) reduced the export of the four nitrogen species