Surface Mixed Layer Research Articles

Influence of North Pacific subtropical mode water variability on the surface mixed layer through the heaving of the upper thermocline on decadal timescales

Decadal variability of the North Pacific subtropical mode water (STMW) and its influence on the upper thermocline and mixed layer are examined in a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA–WNP30). The STMW that forms south of the Kuroshio Extension becomes thick/cold and thin/warm on decadal timescales. These variations are subducted and advected to the south, where thick (thin) STMW causes the upper thermocline to heave up (down) above the STMW, producing cold (warm) temperature anomalies at subsurface depths, with especially large anomalies at the depths of the seasonal thermocline. Temperature anomalies also appear in the mixed layer from March to November, except in September. These anomalies have the same sign as the temperature anomalies of the STMW, although they are due not to the reemergence of the STMW at the surface but to the heaving of the upper thermocline. In the FORA–WNP30, because the formation of the mixed layer temperature anomalies owes much to the increment introduced by data assimilation, the mechanism remains unclear. A heat budget analysis of the mixed layer, however, suggests the importance of entrainment and/or vertical diffusion at the base of the mixed layer for conveying temperature anomalies from the upper thermocline to the mixed layer. The STMW also affects the mixed layer depth. A thick (thin) STMW shoals (deepens) the seasonal thermocline, enhancing (weakening) stratification at depths below the mixed layer and thus hindering (favoring) mixed layer development from July to September.

Observations of internal tidal dynamics Southwest of Malta in the central Mediterranean Sea

Internal tidal dynamics are investigated southeast of Malta in the central Mediterranean using temperature, salinity, and current data from moorings on the shelf, slope, and deep sea. The greatest variability below the mixed surface layer and the sharp thermocline (below 60 m) is at the diurnal K1 tidal frequency, with the signal being significantly enhanced from the deep sea to the shelf and being fortnightly modulated. Harmonic analysis shows that the diurnal vertical displacements at the shelf break can reach an amplitude of 15–25 m at 160 m depth. The corresponding vertical velocities are ∼0.1–0.2 cm s−1. Horizontal currents are maximum (10–15 cm s−1) at about the same depth. They rotate clockwise and counterclockwise above and below ∼160 m, respectively. The internal K1 tidal signal is interpreted as a northwestward propagating shelf wave with a phase velocity close to 1 m s−1 and a wavelength of the order of 100 km. Horizontal and vertical currents at semi-diurnal (M2) frequency are much smaller (less than 2–3 cm s−1and ∼0.02 cm s−1, respectively).

Dissolved trace metals (Fe, Mn, Pb, Cd, Cu, and Zn) in the eastern Indian Ocean

The distribution of trace metal micronutrients, which are crucial in many biotic enzymatic reactions, in the eastern Indian Ocean is not well understood. Here, we measured the concentrations of dissolved iron (dFe), manganese (dMn), lead (dPb), cadmium (dCd), copper (dCu), and zinc (dZn) at full depth in the water column of the eastern Indian Ocean (from the Bay of Bengal to the south of the equator). Throughout, dFe was replete in the surface mixed layer (SML). Moreover, the supply of dFe from below the SML and atmospheric deposition met the phytoplankton demand. In the thermocline, the subsurface maxima of dFe coincided with the dissolved oxygen (DO) minima. In addition, the distribution of Fe* in the Bay of Bengal indicated an additional input of dFe into the water column, apart from the biogenic organic particle regeneration. Unlike dFe, other trace metals were not enriched in the oxygen minimum zone, indicating that high Fe* value was caused by the release of dFe from the estuary or continental shelf under oxic conditions. In deep water, dFe, dMn, and dPb levels were low, whereas dCd, dCu, and dZn concentrations increased with depth. However, dFe and dMn levels were elevated in coastal bottom waters, where Fe* values were also high, indicating the supply of these metals from external sources. Exploring the relationship between trace metals and phosphate at the thermocline illustrated how oxygen-deficient conditions in the Bay of Bengal affect trace metal (dCd in particular) and phosphate ratios, compared with the global trend.

Effects of atmospheric low-level jets on the mixing process of a large tropical reservoir.

Changes in the physical and biogeochemical properties of water columns are frequently associated with cold fronts and mesoscale convective systems due to increased cloud cover. The effects of low-level jet (LLJ) events on thermal stratification and water quality, however, remain undescribed, particularly for tropical reservoirs. Here, water temperature time series are combined with meteorological data, LIDAR observations, ERA5 reanalysis data, and hydrodynamical modeling to investigate the impact of an event of LLJ over the Furnas hydropower reservoir in Brazil. The LLJ event was characterized by dry, intense, and persistent winds (~10 m s-1) blowing for more than 12 hours over the main fetch of the reservoir. In the downwind side of the lake, the surface mixed layer depth increased by 50% during the LLJ event. The changes to the water column were produced by a combination of wind-induced upwelling, shear-driven mixing, and nocturnal convective overturning, different from the heat balance expected during passing cold fronts and mesoscale convective systems. The results suggest that both momentum and heat fluxes during LLJ events need to be accounted for in lake modelings to reproduce the vertical mixing process.

Acoustic Sensing of Ocean Mixed Layer Depth and Temperature from Uplooking ADCPs

Abstract Properties of the surface mixed layer (ML) are critical for understanding and predicting atmosphere–sea ice–ocean interactions in the changing Arctic Ocean. Mooring measurements are typically unable to resolve the ML in the Arctic due to the need for instruments to remain below the surface to avoid contact with sea ice and icebergs. Here, we use measurements from a series of three moorings installed for one year in the Beaufort Sea to demonstrate that upward-looking acoustic Doppler current profilers (ADCPs) installed on subsurface floats can be used to estimate ML properties. A method is developed for combining measured peaks in acoustic backscatter and inertial shear from the ADCPs to estimate the ML depth. Additionally, we use an inverse sound speed model to infer the summer ML temperature based on offsets in ADCP altimeter distance during open-water periods. The ADCP estimates of ML depth and ML temperature compare favorably with measurements made from mooring temperature sensors, satellite SST, and from an autonomous Seaglider. These methods could be applied to other extant mooring records to recover additional information about ML property changes and variability.

Behavior of Auramine O in the Aqueous Solution of Two Kolliphors and Their Mixture

The studies on the behavior of Auramine O (AuO) at the water-air interface and in the bulk phase of the aqueous solution of Kolliphor® ELP (ELP) and Kolliphor® RH 40 (RH40) and their mixture were based on the results obtained from the measurements of the contact angle of water, formamide and diiodomethane on the polytetrafluoroethylene covered by the AuO layer, the surface tension of the aqueous solution of AuO, AuO + ELP, AuO + RH40, AuO + ELP + RH40, density and fluorescence intensity. Based on the obtained results, it was possible to determine components and parameters of the AuO surface tension, concentration and composition of the mixed monolayer, including AuO, ELP and RH40, as well as that of the mixed micelles, and to determine the Gibbs standard free energy of adsorption, micellization and AuO solubilization. The obtained results also showed that surface tension isotherms of the studied solutions can be described by the Szyszkowski equation and the exponential function of the second order and predicted by the Fainerman and Miller equation. In addition, the mixed surface layer composition can be predicted based on the contribution of the components of this layer to the water surface tension reduction.

Millennial-scale changes in abundance of brachiopods in bathyal environments detected by postmortem age distributions in death assemblage (Bari Canyon, Adriatic Sea)

Abstract Inferring the composition of pre-Anthropocene baseline communities on the basis of death assemblages (DAs) preserved in a surface mixed layer requires discriminating among recently-dead shells sourced by living populations and older shells from extirpated populations. Here, we assess the distribution of postmortem ages in the DA formed by the brachiopod Gryphus vitreus at 580 m depth in the Bari Canyon (Adriatic Sea), with no individuals collected alive. The Gryphus DA exhibits millennial time averaging (inter-quartile range = 1250 years) and two modes in abundance at 500 and 1750 years BP. As high abundance of species in time-averaged DAs can reflect passive accumulation of shells sourced by populations with low standing population density, we reconstruct changes in annual density on the basis of the abundance maxima detected in the distribution of postmortem ages and on the basis of estimates of per-specimen disintegration rate. We find that adults (>20 mm) achieved densities of at least 10–20 individuals/m 2 (assuming lifespan is 10 years), and the pulses in abundance were thus associated with a high population density in the past, followed by the decline over the last few centuries. We infer that bathyal populations were volatile during the Late Holocene, with brachiopods sensitive to siltation that was induced by temporal changes in sediment dispersal into the Bari Canyon due to deforestation and climatic changes.

A New Look at Physico-Chemical Causes of Changing Climate: Is the Seasonal Variation in Seawater Temperature a Significant Factor in Establishing the Partial Pressure of Carbon Dioxide in the Earth’s Atmosphere?

Seasonal oscillations in the partial pressure of carbon dioxide (pCO2) in the Earth’s atmosphere, stronger in northern latitudes, are assumed to show that terrestrial photosynthesis exceeds respiration in summer, reducing the pCO2 in air but increasing its value in winter when respiration exceeds photosynthesis. We disagree, proposing that variation in the temperature of the surface mixing zone of seawater also reversibly regulates the pCO2 in air as a non-equilibrium process between air and seawater. We predict by thermal modelling that carbonate (CO32−) concentration in the surface mixed layer seawater declines in winter by conversion to bicarbonate with CaCO3 (calcite or aragonite) becoming more soluble and, proportional to the fall of temperature, calcite decalcifying more strongly, allowing more CO2 emission to air. Paradoxically, the increasing CO2 concentration in seawater favoring photosynthesis peaking in mid-summer declines simultaneously in autumn and early winter, forced by boundary layer fugacity into phase transfer to the atmosphere, supporting peak atmospheric pCO2 by late winter. These physico-chemical processes reverse in late winter and spring as seawater warms favoring calcification, fugacity forcing CO2 from the atmosphere as bicarbonate declines and carbonate increases, augmenting suspended calcite particles by several percent. Our numerical computation predicts that the larger range of thermal fluctuations in the northern hemisphere could reversibly favor absorption from air of more than one mole of CO2 per square meter in summer with calcite formation potentially augmenting shallow limestone reefs, despite falling pH, if there is a trend for increasing seawater temperature. Another assumption we challenge is that upwelling and advection from deeper water is the sole cause of increases in dissolved inorganic carbon (DIC) and alkalinity in surface waters, even in the southern hemisphere. Instead, some calcite dissolution is favored as water temperature falls near the surface. Standard enthalpy analysis of key DIC reactions indicates why this oscillation is more obvious in the northern hemisphere with seasonal variations in water temperature (ca. 7.1 °C) being almost twice those in the southern hemisphere (ca. 4.7 °C) with a greater depth of the surface mixing zone of seawater in the southern oceans. Questions remain regarding the relative rates of biotic and abiotic inorganic precipitation and dissolution of CaCO3 in the mixing zone. In summary, rapid biogenic calcification is favored by summer photosynthesis, but slower abiotic calcification is also more likely in warmer water. We conclude that the relative significance of terrestrial biotic and seawater abiotic processes in seawater on the seasonal oscillation in the atmosphere can only be assessed by direct seasonal measurements in seawater.

Controlling factors of the climatological annual cycle of the surface mixed layer oxygen content: A global view

The annual cycle of global dissolved oxygen content (O2C) and mean oxygen concentration in the surface mixed layer are estimated using monthly climatological oxygen fields from the World Ocean Atlas 2018 (WOA18). The largest seasonal variability in the mixed layer O2C occurs in the extra-tropics between 30° and 70° latitude of each hemisphere. A global view of the role of entrainment, air-sea flux, and biological activity in controlling the oxygen content/concentration annual cycle in the mixed layer is determined using an oxygen mass balance model. Based on the relative percentage from the mass balance model, entrainment is only a significant driver (contributing to 20-40% of the total changes) from mid-fall to early spring when the mixed layer deepens and transfers oxygen to deeper waters. Both the air-sea oxygen flux and biological activity show strong annual cycles and play critical roles in the annual cycle of O2C in the mixed layer. Air-sea oxygen flux is ingassing from late fall to early spring and outgassing between late spring and early fall. It is a substantial factor throughout the year and controls 40-60% of the oxygen changes in most months. Biological activity is a net source (production) in the spring and summer and a net sink (consumption) in the late fall and winter for the mixed layer oxygen content. Biological activity plays a more important role during spring/summer (40-60%) than that during fall/winter (10-30%) in controlling the overall oxygen change in each month. The model estimates a mean value (±SD) of 3.06±1.61 mol C m-2 yr-1 and a total of 863.7±73.8 Tmol C yr-1 for the global annual net ocean community production (ANCP) between 60°S and 60°N latitude, which are in fairly good agreement with previous studies.

Depth-resolved photochemical production of hydrogen peroxide in the global ocean using remotely sensed ocean color

Hydrogen peroxide (H2O2) is an important reactive oxygen species (ROS) in natural waters, affecting water quality via participation in metal redox reactions and causing oxidative stress for marine ecosystems. While attempts have been made to better understand H2O2 dynamics in the global ocean, the relative importance of various H2O2 sources and losses remains uncertain. Our model improves previous estimates of photochemical H2O2 production rates by using remotely sensed ocean color to characterize the ultraviolet (UV) radiation field in surface water along with quantitative chemical data for the photochemical efficiency of H2O2 formation. Wavelength- and temperature-dependent efficiency (i.e., apparent quantum yield, AQY) spectra previously reported for a variety of seawater sources, including coastal and oligotrophic stations in Antarctica, the Pacific Ocean at Station ALOHA, the Gulf of Mexico, and several sites along the eastern coast of the United States were compiled to obtain a “marine-average” AQY spectrum. To evaluate our predictions of H2O2 photoproduction in surface waters using this single AQY spectrum, we compared modeled rates to new measured rates from Gulf Stream, coastal, and nearshore river-outflow stations in the South Atlantic Bight, GA, United States; obtaining comparative differences of 33% or less. In our global model, the “marine-average” AQY spectrum was used with modeled solar irradiance, together with satellite-derived surface seawater temperature and UV optical properties, including diffuse attenuation coefficients and dissolved organic matter absorption coefficients estimated with remote sensing-based algorithms. The final product of the model, a monthly climatology of depth-resolved H2O2 photoproduction rates in the surface mixed layer, is reported for the first time and provides an integrated global estimate of ∼21.1 Tmol yr−1 for photochemical H2O2 production. This work has important implications for photo-redox reactions in seawater and improves our understanding of the role of solar irradiation on ROS cycling and the overall oxidation state in the oceans.

Periodic boundary layer separation and lateral intrusions observed above a sloping lakebed

AbstractLateral intrusions flowing from the boundaries of lakes and oceans might influence basin‐scale distributions of heat, chemical solutes, sediments, and organisms. Here, observations of numerous intrusions from multiple locations in a small lake were used to examine the relation of intrusions to a lakewide internal seiche, and to estimate mean exchange between sloping bottom boundary layers (BBLs) and the lake interior. BBL waters were observed periodically separating from the bed and flowing laterally offshore. Boundary layer separation was initiated where strong downslope flows encountered upslope‐propagating temperature fronts. These separation events propagated upslope, coherent with the vertically propagating internal seiche, resembling previously reported cases of upslope‐propagating internal bores. Following separation, jets flowed offshore immediately above temperature fronts, transporting boundary layer water at least 61 m into the interior. Jet propagation could be traced upwards, from the bed at 9.4 m depth to the base of the surface mixed layer. Mean offshore transport was most intense within the metalimnion (4–7 m depth). Intruding intermediate‐temperature thermocline water was likely supplied both by river inputs and by boundary mixing of cold and warm waters. These findings suggest that periodic jets, resulting from seiche‐induced boundary layer separation, transported significant quantities of water from the boundary layer toward the stratified lake interior, with possible implications for distributions of sediments, solutes, and organisms.

Habitat compression indices for monitoring ocean conditions and ecosystem impacts within coastal upwelling systems

Upwelling ecosystems are characterized by intense seasonal productivity that supports highly dynamic species populations, high diversity of mid and upper trophic levels, and a myriad of important fisheries. Climate variability and long-term change will impact upwelling intensity, timing and persistence, thereby potentially threatening resilience of coastal food webs and stability of ecosystem services. The spatial footprint of cool upwelled waters in the surface mixed-layer supports trophic transfer and ultimately the productivity of fisheries. The spatial area of upwelled water in eastern boundary upwelling systems can vary dramatically in response to both local and remote atmospheric forcing. These variations contribute to dynamic habitats that impact the structure, function, and spatial characteristics of marine ecosystems. We quantified the variability in “cool-water thermal habitat” area as a new ecological indicator, the Habitat Compression Index (HCI), for the nearshore (within 150 km of the coast) waters of the California Current Large Marine Ecosystem (CCLME). The HCI can be easily updated from ocean model products and satellite observations of sea surface temperature. We describe standardization of the HCI, regional variability, and evaluate HCI relationships with other indicators commonly used to inform ecosystem context within the CCLME. Importantly, our approach to calculating the HCI is easily extendable to other upwelling ecosystems. Further, we discuss the management context of the HCI such as assessing risk of whale entanglement in a highly profitable fishery, and implications for monitoring ecosystem shifts in coastal upwelling systems and the fisheries they support.

Recent changes of the thermal structure in Three Gorges Reservoir, China and its impact on algal bloom in tributary bays

Despite the fact that the water temperature of the Yangtze River has changed substantially due to the construction of Three Gorges Reservoir (TGR), little is known about how the thermal structure of TGR responded to the implementation of two large upstream reservoirs in 2013. We investigated the seasonal variations of water temperature and discharge in the middle reaches of the Yangtze River by analyzing multi-year (2004–2018) hydrological and water temperature data. The effects of upstream dams on the thermal regime, density currents and phytoplankton blooms in TGR were examined. The results show that water temperature in TGR increased in spring (+1.1 °C) and winter (+0.8 °C) seasons after the filling of upstream reservoirs compared to the years before. Longer duration of stratification is associated with a smaller discharge and a longer water residence time. Moreover, the increase of inflow, which was at least partially due to upstream dam construction, lead to the weakening of stratification during the spring season. As a consequence of the higher water temperature in TGR in spring, the density-driven exchange flows between the main reservoir and tributary bays have changed. Overflows became more frequent, which promoted the deepening of the surface mixing layer, and resulted in a dramatic reduction in the frequency and biomass of phytoplankton blooms in tributary bays of TGR.

Evaluating the effects of a symmetric instability parameterization scheme in the Xisha-Zhongsha waters, South China Sea in winter

As one of the important submesoscale instabilities, symmetric instability (SI) widely exists in the ocean surface mixed layer (SML), which enhances the vertical material transport in the SML and also the exchanges between the SML and the ocean interior. Due to the small spatial scales of SI, O (10 m–1 km), which are not resolved by most current ocean models, the application of SI parameterization is an alternative choice in the coming decades to include the SI effects in ocean models and improve the model performance. In this study, we evaluate the impacts of SI in a realistic configuration with the SI parameterization scheme applied in the Xisha-Zhongsha waters, South China Sea in winter by using the Coastal and Regional Ocean Community Model (CROCO) version of the Regional Ocean Modeling System. Compared to the SI-lacking case, the SI energy source, the geostrophic shear production, is increased and elimination of anticyclonic potential vorticity is revealed in the SI-parameterized case. According to the energy analysis, multi-scale interactions are also influenced by the SI. The effective wind energy input is reduced, and the potential energy release in the SML is suppressed. Moreover, the SI scheme makes the SML depth shallower and closer to the reanalysis one. This work demonstrates a good performance of the SI scheme applied in regional models in representing SI effects.

Non‐Local Eddy‐Mean Kinetic Energy Transfers in Submesoscale‐Permitting Ensemble Simulations

AbstractUnderstanding processes associated with eddy‐mean flow interactions helps our interpretation of ocean energetics, and guides the development of parameterizations. Here, we focus on the non‐local nature of Kinetic Energy (KE) transfers between mean and turbulent reservoirs. Transfers are interpreted as non‐local when the energy extracted from the mean flow does not locally sustain an growth of energy in the turbulent flow, or vice versa. The novelty of our approach is to use ensemble statistics to define the mean and the turbulent flow. Based on KE budget considerations, we first rationalize the eddy‐mean separation in the ensemble framework, and discuss the interpretation of a mean flow driven by the prescribed (surface and boundary) forcing and a turbulent flow u′ driven by non‐linear dynamics sensitive to initial conditions. We then analyze 120‐day long, 20‐member ensemble simulations of the Western Mediterranean basin run at resolution. Our main contribution is to recognize the prominent contribution of the cross energy term to explain non‐local energy transfers, which provides a strong constraint on the horizontal organization of eddy‐mean flow KE transfers since the cross energy term vanishes identically for perturbations orthogonal to the mean flow . We also highlight the prominent contribution of vertical turbulent fluxes for energy transfers within the surface mixed layer. Analyzing the scale dependence of non‐local energy transfers supports the local approximation usually made in the development of meso‐scale, energy‐aware parameterizations for non‐eddying models, but points out to the necessity of accounting for non‐local dynamics in the meso‐to‐submeso scale range.

Slantwise Convection in the Irminger Sea

AbstractThe subpolar North Atlantic is a site of significant carbon dioxide, oxygen, and heat exchange with the atmosphere. This exchange, which regulates transient climate change and prevents large‐scale hypoxia throughout the North Atlantic, is thought to be mediated by vertical mixing in the ocean's surface mixed layer. Here we present observational evidence that waters deeper than the conventionally defined mixed layer are affected directly by atmospheric forcing in this region. When northerly winds blow along the Irminger Sea's western boundary current, the Ekman response pushes denser water over lighter water, potentially triggering slantwise convection. We estimate that this down‐front wind forcing is four times stronger than air–sea heat flux buoyancy forcing and can mix waters to several times the conventionally defined mixed layer depth. Slantwise convection is not included in most large‐scale ocean models, which likely limits their ability to accurately represent subpolar water mass transformations and deep ocean ventilation.

Recent Changes of Phytoplankton Bloom Phenology in the Northern High‐Latitude Oceans (2003–2020)

AbstractEighteen years (2003–2020) of data on remotely‐sensed ocean color in high northern latitudes show a significant increase (0.56% yr−1) in double blooms of phytoplankton. Starting in 2015 the rate of increase rose to 1.10% yr−1. Double blooms have become especially prevalent in regions north of 50°N in the North Atlantic, Gulf of Alaska and southern Chukchi Sea. Our correlation analysis suggests that double bloom occurrences are dependent on latitude and seasonal changes in both sea surface temperature (SST) and mixed layer depth (MLD), particularly in the subpolar North Pacific (SNP) and polar oceans (65°–75°N), although we cannot attribute the increased double blooms to specific causes. Single blooms indicated by Chlorophyll a (Chl a) dominate in subpolar‐polar waters and occur close to the timing of the Particulate Inorganic Carbon (PIC) peak (i.e., June–August). In the SNP single bloom, Chl a peaks occur earlier than PIC peaks. This interval was shorter during 2016–2020, which is associated with warming anomalies. In the southern part of the Chukchi Sea, however, we found that PIC peaks occur earlier than Chl a peaks, suggesting a potentially different succession pattern between calcifiers and other functional groups. We speculate that in the near future double blooms of phytoplankton will become a prevalent feature in high‐latitude oceans with concurrent changes in the timing of blooms and seasonal phytoplankton succession in the surface ocean due to climate change.

Seasonal Dynamics of Dissolved Iron on the Antarctic Continental Shelf: Late-Fall Observations From the Terra Nova Bay and Ross Ice Shelf Polynyas.

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.

Rapid vertical exchange at fronts in the Northern Gulf of Mexico.

Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region’s dead zone.

Coastal freshening drives acidification state in Greenland fjords

Greenland's fjords and coastal waters are highly productive and sustain important fisheries. However, retreating glaciers and increasing meltwater are changing fjord circulation and biogeochemistry, which may threaten future productivity. The freshening of Greenland fjords caused by unprecedented melting of the Greenland Ice Sheet may alter carbonate chemistry in coastal waters, influencing CO2 uptake and causing biological consequences from acidification. However, few studies to date explore the current acidification state in Greenland coastal waters. Here we present the first-ever large-scale measurements of carbonate system parameters in 16 Greenlandic fjords and seek to identify the drivers of acidification state in these freshening ecosystems. Aragonite saturation state (Ω), a proxy for ocean acidification, was calculated from dissolved inorganic carbon (DIC) and total alkalinity from fjords along the east and west coast of Greenland spanning 68–75°N. Aragonite saturation was primarily >1 in the surface mixed layer. However, undersaturated—or corrosive––conditions (Ω < 1) were observed on both coasts (west: Ω = 0.28–3.11, east: Ω = 0.70–3.07), albeit at different depths. West Greenland fjords were largely corrosive at depth while undersaturation in East Greenland fjords was only observed in surface waters. This reflects a difference in the coastal boundary conditions and mechanisms driving acidification state. We suggest that advection of Sub Polar Mode Water and accumulation of DIC from organic matter decomposition drive corrosive conditions in the West, while freshwater alkalinity dilution drives acidification in the East. The presence of marine terminating glaciers also impacted local acidification states by influencing fjord circulation: upwelling driven by subglacial discharge brought corrosive bottom waters to shallower depths. Meanwhile, discharge from land terminating glaciers strengthened stratification and diluted alkalinity. Regardless of the drivers in each system, increasing freshwater discharge will likely lower carbonate saturation states and impact biotic and abiotic carbon uptake in the future.