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Harmful algal blooms (HABs) are increasing globally, causing economic, human health, and ecosystem harm. In spite of the frequent occurrence of HABs, the mechanisms responsible for their exceptionally high biomass remain imperfectly understood. A 50-y- old hypothesis posits that some dense blooms derive from dinoflagellate motility: organisms swim upward during the day to photosynthesize and downward at night to access deep nutrients. This allows dinoflagellates to outgrow their nonmotile competitors. We tested this hypothesis with in situ data from an autonomous, ocean-wave- powered vertical profiling system. We showed that the dinoflagellate Lingulodinium polyedra’s vertical migration led to depletion of deep nitrate during a 2020 red tide HAB event. Downward migration began at dusk, with the maximum migration depth determined by local nitrate concentrations. Losses of nitrate at depth were balanced by proportional increases in phytoplankton chlorophyll concentrations and suspended particle load, conclusively linking vertical migration to the access and assimilation of deep nitrate in the ocean environment. Vertical migration during the red tide created anomalous biogeochemical conditions compared to 70 y of climatological data, demonstrating the capacity of these events to temporarily reshape the coastal ocean’s ecosystem and biogeochemistry. Advances in the understanding of the physiological, behavioral, and metabolic dynamics of HAB-forming organisms from cutting-edge observational techniques will improve our ability to forecast HABs and mitigate their consequences in the future. [ABSTRACT FROM AUTHOR]

The future Surface Water and Ocean Topography (SWOT) mission aims to map sea surface height (SSH) in wide swaths with an unprecedented spatial resolution and subcentimeter accuracy. The instrument performance needs to be verified using independent measurements in a process known as calibration and validation (Cal/Val). The SWOT Cal/Val needs in situ measurements that can make synoptic observations of SSH field over an O(100) km distance with an accuracy matching the SWOT requirements specified in terms of the along-track wavenumber spectrum of SSH error. No existing in situ observing system has been demonstrated to meet this challenge. A field campaign was conducted during September 2019–January 2020 to assess the potential of various instruments and platforms to meet the SWOT Cal/Val requirement. These instruments include two GPS buoys, two bottom pressure recorders (BPR), three moorings with fixed conductivity–temperature–depth (CTD) and CTD profilers, and a glider. The observations demonstrated that 1) the SSH (hydrostatic) equation can be closed with 1–3 cm RMS residual using BPR, CTD mooring and GPS SSH, and 2) using the upper-ocean steric height derived from CTD moorings enable subcentimeter accuracy in the California Current region during the 2019/20 winter. Given that the three moorings are separated at 10–20–30 km distance, the observations provide valuable information about the small-scale SSH variability associated with the ocean circulation at frequencies ranging from hourly to monthly in the region. The combined analysis sheds light on the design of the SWOT mission postlaunch Cal/Val field campaign. [ABSTRACT FROM AUTHOR]

The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides information on changes to the Earth's gravity field, including ocean mass. Long‐term trends in GRACE data are often considered unreliable due to uncertainties in the corrections made to calculate ocean mass from the raw measurements. Here, we use an independent estimate of ocean mass from satellite altimetry and in situ density data from five mooring sites and repeat hydrography to validate trends in GRACE over the North Atlantic, finding substantial agreement between the methods. The root‐mean‐square difference between ocean mass changes calculated with this method from the mooring data and those measured by GRACE is 3.5 mm per decade, much lower than the mean signal of 15.6 ± 1.8 mm per decade for GRACE and 17.8 ± 5.2 mm per decade for the altimetry‐mooring estimate. The GRACE ocean mass data are then used to study the change in the deep circulation of the North Atlantic between the 1 April 2002 to 31 March 2009 and 1 April 2010 to 31 March 2017 periods, revealing a large‐scale anticyclonic circulation anomaly off the North American coast. The change is associated with an increase of 13.9 ± 3.3 Sv (1 Sv = 106 m3 s−1) of southward North Atlantic Deep Water flow in the interior between 30°N and 40°N, largely balanced by a northward anomaly of 10.7 ± 3.3 Sv for the boundary circulation. This implies an increased importance of interior pathways compared to the Deep Western Boundary Current for the spreading of North Atlantic Deep Water, which constitutes the lower limb of the Atlantic Meridional Overturning Circulation. Plain Language Summary: The Gravity Recovery and Climate Experiment (GRACE) satellites measure the Earth's gravity field, which can be used to study ocean currents. In this study, we validate long‐term trends in the data using independent ocean measurements to confirm that they reflect real changes in the ocean. In the Atlantic Ocean, North Atlantic Deep Water flows southward from its formation region near Greenland, spreading throughout the ocean basin. The flow of North Atlantic Deep Water is a crucial part of Earth's climate system, making it an important process to understand. Most of the water spreads southward in the so‐called Deep Western Boundary Current along the continental shelves of North America, but there is also a significant flow further offshore in the interior of the basin. We use the GRACE data to measure changes to these two pathways between the 2002–2009 and 2010–2017 periods. The results show that southward flow in the interior has strengthened during this time, with opposite changes near the continent. This marks a significant change to how this water spreads from the formation region. Key Points: GRACE ocean mass trends are validated using mooring and altimetry dataChanges reveal decadal variability in the deep circulation of the North Atlantic oceanFlow of North Atlantic Deep Water in the interior strengthened compared to the boundary circulation [ABSTRACT FROM AUTHOR]

We analyzed impacts of the 2014–2015 Pacific Warm Anomaly and 2015–2016 El Niño on physical and biogeochemical variables at two southern California Current System moorings (CCE2, nearshore upwelling off Point Conception; CCE1, offshore California Current). Nitrate and Chl‐a fluorescence were <1 μM and <1 Standardized Fluorescence Unit, respectively, at CCE2 for the entire durations of the Warm Anomaly and El Niño, the two longest periods of such low values in our time series. Negative nitrate and Chl‐a anomalies at CCE2 were interrupted briefly by upwelling conditions in spring 2015. Near‐surface temperature anomalies appeared simultaneously at both moorings in spring 2014, indicating region‐wide onset of Warm Anomaly temperatures, although sustained negative nitrate and Chl‐a anomalies only occurred offshore at CCE1 during El Niño (summer 2015 to spring 2016). Warm Anomaly temperature changes were expressed more strongly in near‐surface (<40 m) than subsurface (75 m) waters at both moorings, while El Niño produced comparable temperature anomalies at near‐surface and subsurface depths. Nearshore Ωaragonite at 76 m showed notably fewer undersaturation events during both warm periods, suggesting an environment more conducive to calcifying organisms. Planktonic calcifying molluscs (pteropods and heteropods) increased markedly in springs 2014 and 2016 and remained modestly elevated in spring 2015. Moorings provide high‐frequency measurements essential for resolving the onset timing of anomalous conditions and frequency and duration of short‐term (days‐to‐weeks) perturbations (reduced nitrate and aragonite undersaturation events) that can affect marine organisms. Plain Language Summary: The 2014–2016 Pacific Warm Anomaly‐El Niño sequence significantly changed ocean conditions in the California Current System, but impacts of these anomalies on nutrients, chlorophyll‐a, and pH, and comparisons to past El Niño events, have not been well described. We examined 9 years (2010–2018) of data from two ocean moorings that provide hourly measurements at fixed locations in the southern California Current System, allowing us to determine both subseasonal and multiyear ecosystem changes. We found that nitrate (important for primary production) and chlorophyll‐a were anomalously low in nearshore waters throughout the Warm Anomaly and El Niño, suggesting nutrient delivery to the surface ocean was reduced. The Warm Anomaly and El Niño both produced anomalously warm shallow temperatures, but only El Niño produced significantly warmer waters below 50‐m depth. Aragonite, a carbonate mineral important for shell production in calcifying marine organisms, showed elevated saturation state during both the Warm Anomaly and El Niño, suggesting the two anomalous periods produced favorable calcification conditions. Planktonic shelled molluscs, a key component of the oceanic food web, increased during both the Warm Anomaly and El Niño. Our findings may help predict impacts of future anomalies on nutrient availability and biological responses in the California Current System. Key Points: Nearshore nitrate and Chl‐a fluorescence were near zero for most of the Warm Anomaly‐El Niño but increased briefly in spring 2015Temperature increases were surface‐intensified across the region during the Warm Anomaly, while El Niño also affected subsurface watersPlanktonic mollusc biomass was elevated in springs 2014 and 2016 in conjunction with Warm Anomaly and El Niño aragonite supersaturation [ABSTRACT FROM AUTHOR]

Variability of the flow across the Solomon Sea's southern entrance was examined using end point subsurface moorings and seafloor pressure sensors, reconstructed velocity profiles based on satellite‐derived surface velocity and bottom pressure‐derived subsurface velocity, and 1993–2017 proxy volume transport based on satellite altimetry. The reconstructed velocity correctly represents the fluctuating surface flow and subsurface core providing a high‐frequency continuous observing system for this sea. The mean equatorward volume transport over 0‐ to 500‐m depth layer is 15.2 Sv (1 Sv ≡ 106 m3/s) during July 2012 to May 2017. The measurements resolve the full spectrum of the volume transport including energetic subseasonal variability that fluctuates by as much as 25 Sv over one week. At low‐frequency timescales, the study finds that linear Rossby waves forced by Ekman pumping in the interior of the Pacific influence not only seasonal fluctuations as found by previous studies but also interannual variability. As found previously, the El Niño–Southern Oscillation highly influences interannual volume transport. During the 2015/2016 El Niño, observations show the seasonal cycle to be suppressed from the second half of 2014, prior to the mature phase of the El Niño, to September 2016 along with an increase in across‐transect transport. At subseasonal timescales, local Ekman pumping and remote wind stress curl are responsible for a third of the subseasonal variance. The study highlights the importance of high‐frequency observations at the southern entrance of the Solomon Sea and the ability of a linear Rossby model to represent the low‐frequency variability of the transport. Plain Language Summary: The Solomon Sea, located in the tropical southwest Pacific Ocean, is characterized by energetic currents that transport water from the subtropical South Pacific to the equatorial Pacific. In this study, we quantify the amount of water being transported through the Solomon Sea to understand how and why it changes over time. Over the observing period (July 2012 to May 2017), a mean water volume of 15.2 Sv (1 Sv ≡ 106 m3/s) flows into the Solomon Sea at its southern entrance. The flow can change greatly over short time period (±25 Sv over 1 week) as being modified by winds over the Solomon Sea and further upstream over the Coral Sea. The observed inflow also has distinct interannual variability and a seasonal cycle with a minimum in March/April and a maximum in July/August. On these timescales, the flow fluctuates up to ±10 Sv as being influenced by the winds over the equatorial and tropical Pacific Ocean. During the 2015/2016 El Niño, our observations show that the equatorward flow via the Solomon Sea strengthens but the seasonal cycle is suppressed. We also showed that the flow can be efficiently monitored using only a pair bottom‐pressure sensors and satellite altimetry. Key Points: The observations reveal energetic subseasonal flow (37% of total variance) into the Solomon Sea associated with local and remote windsRossby waves propagating from the central Pacific Ocean are well correlated with the inflow variability at seasonal and longer timescalesThe inflow strengthens and the seasonal cycle is suppressed during the 2015/2016 El Niño [ABSTRACT FROM AUTHOR]

Abstract: Direct observations of the deep water circulation in the western Mediterranean Sea are presented, based on the analysis of autonomous profiling floats drifting at 1,200 and 1,900 m depth during the 1997–2002 period. The amount of water circulating in the basin is quantified, revealing several distinct gyres and boundary currents. It was also possible to follow the spreading of the newly formed Western Mediterranean Deep Water (nWMDW) and Tyrrhenian Deep Water (TDW), two main components of the deep water in the western Mediterranean, from their origin, based on their temperature and salinity signature. Both boundary currents and isolated eddies carrying the water into the interior are important for this. [ABSTRACT FROM AUTHOR]

The existence of a seasonally varying undercurrent along 8° N off the east coast of Sri Lanka is inferred from shipboard hydrography, Argo floats, glider measurements, and two ocean general circulation model simulations. Together, they reveal an undercurrent below 100-200 m flowing in the opposite direction to the surface current, which is most pronounced during boreal spring and summer and switches direction between these two seasons. The volume transport of the undercurrent (200-1000 m layer) can be more than 10 Sv in either direction, exceeding the transport of 1-6 Sv carried by the surface current (0-200 m layer). The undercurrent transports relatively fresher water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. Although the undercurrent is potentially a pathway of salt exchange between the Arabian Sea and the Bay of Bengal, the observations and the ocean general circulation models suggest that the salinity contrast between seasons and between the boundary current and interior is less than 0.09 in the subsurface layer, suggesting a small salt transport by the undercurrent of less than 4 % of the salinity deficit in the Bay of Bengal. [ABSTRACT FROM AUTHOR]

Measurements of near-surface oxygen (O2) concentrations and mixed layer depth from the K1 mooring in the central Labrador Sea are used to calculate the change in column-integrated (0-1700 m) O2 content over the deep convection winter 2014/2015. During the mixed layer deepening period, November 2014 to April 2015, the oxygen content increased by 24.3 ± 3.4 mol m−2, 40% higher than previous results from winters with weaker convection. By estimating the contribution of respiration and lateral transport on the oxygen budget, the cumulative air-sea gas exchange is derived. The O2 uptake of 29.1 ± 3.8 mol m−2, driven by persistent undersaturation (≥5%) and strong atmospheric forcing, is substantially higher than predicted by standard (nonbubble) gas exchange parameterizations, whereas most bubble-resolving parameterizations predict higher uptake, comparable to our results. Generally large but varying mixed layer depths and strong heat and momentum fluxes make the Labrador Sea an ideal test bed for process studies aimed at improving gas exchange parameterizations. [ABSTRACT FROM AUTHOR]

The existence of a seasonally varying undercurrent along 8° N off the east coast of Sri Lanka is inferred from Conductivity-Temperature-Depth profiles, Argo floats, glider measurements, and Ocean General Circulation Model outputs. Together, they reveal an undercurrent below 200 m that is approximately 140 km wide and can reach a maximum speed of 45 cm s-1 that hitherto has not been observed. The undercurrent, flowing in the opposite direction to the surface current, is most pronounced during boreal spring and summer and switches direction between these two seasons. The undercurrent transports relatively fresh water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. This suggests a pathway, independent of the surface circulation, whereby freshwater is removed and saline water is injected into the Bay of Bengal. [ABSTRACT FROM AUTHOR]

One of the major challenges to assessing the impact of ocean acidification on marine life is detecting and interpreting long-term change in the context of natural variability. This study addresses this need through a global synthesis of monthly pH and aragonite saturation state (Ωarag) climatologies for 12 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater partial pressure of CO2 and pH collected together since as early as 2010. Mooring observations suggest open ocean subtropical and subarctic sites experience present-day surface pH and Ωarag conditions outside the bounds of preindustrial variability throughout most, if not all, of the year. In general, coastal mooring sites experience more natural variability and thus, more overlap with preindustrial conditions; however, present-day Ωarag conditions surpass biologically relevant thresholds associated with ocean acidification impacts on Mytilus californianus (Ωarag < 1.8) and Crassostrea gigas (Ωarag < 2.0) larvae in the California Current Ecosystem (CCE) and Mya arenaria larvae in the Gulf of Maine (Ωarag < 1.6). At the most variable mooring locations in coastal systems of the CCE, subseasonal conditions approached Ωarag D1. Global and regional models and data syntheses of ship-based observations tended to underestimate seasonal variability compared to mooring observations. Efforts such as this to characterize all patterns of pH and Ωarag variability and change at key locations are fundamental to assessing present-day biological impacts of ocean acidification, further improving experimental design to interrogate organism response under real-world conditions, and improving predictive models and vulnerability assessments seeking to quantify the broader impacts of ocean acidification. [ABSTRACT FROM AUTHOR]

The region surrounding Sri Lanka modulates monsoon-driven exchange between the Bay of Bengal and the Arabian Sea. Here, local circulation impacts the pathways followed by the boundary currents that drive exchange, thereby modulating mixing and water mass transformation. From 2013 to 2016, an international partnership conducted sustained measurements around the periphery of Sri Lanka, with the goal of understanding how circulation and mixing in this critical region modulate exchange between the Bay of Bengal and the Arabian Sea. Observations from satellite remote sensing, surface drifters, gliders, current meter moorings, and Pressure Inverted Echo Sounders capture seasonally reversing monsoon currents off the southern tip of Sri Lanka, trace the wintertime freshwater export pathway of the East India Coastal Current, and document the deflection of currents running along the east coast of Sri Lanka by cyclonic and anticyclonic eddies. Measurements also reveal energetic interleaving, indicative of mixing and stirring associated with these flows. Circulation inferred from satellite remote sensing and drifter tracks sometimes differs from that indicated by in situ sections, pointing to the need for observing systems that employ complementary approaches toward understanding this region. [ABSTRACT FROM AUTHOR]

One of the major challenges to assessing the impact of ocean acidification on marine life is the need to better understand the magnitude of long-term change in the context of natural variability. This study addresses this need through a global synthesis of monthly pH and aragonite saturation state (Ωarag) climatologies for 12 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater partial pressure of CO2 and pH collected together since as early as 2010. Mooring observations suggest open ocean subtropical and subarctic sites experience present-day surface pH and Ωarag conditions outside the bounds of pre-industrial variability throughout most, if not all, of the year. In general, coastal mooring sites experience more natural variability and thus, more overlap with pre-industrial conditions; however, present day Ωarag conditions surpass biologically relevant thresholds associated with ocean acidification impacts on Mytilus californianus (Ωarag < 1.8) and Crassostrea gigas (Ωarag < 2.0) larvae in the California Current Ecosystem (CCE) and Mya arenaria larvae in the Gulf of Maine (Ωarag < 1.6). At the most variable mooring locations in coastal systems of the CCE, subseasonal conditions approached Ωarag = 1. Global and regional models and data syntheses of ship-based observations tended to underestimate seasonal variability compared to mooring observations. Efforts such as this to characterize all modes of pH and Ωarag variability and change at key locations are fundamental to assessing present-day biological impacts of ocean acidification, further improving experimental design to interrogate organism response under real-world conditions, and improving predictive models and vulnerability assessments seeking to quantify the broader impacts of ocean acidification. [ABSTRACT FROM AUTHOR]

Event-scale phenomena, of limited temporal duration or restricted spatial extent, often play a disproportionately large role in ecological processes occurring in the ocean water column. Nutrient and gas fluxes, upwelling and downwelling, transport of biogeochemically important elements, predator-prey interactions, and other processes may be markedly influenced by such events, which are inadequately resolved from infrequent ship surveys. The advent of autonomous instrumentation, including underwater gliders, profiling floats, surface drifters, enhanced moorings, coastal high-frequency radars, and satellite remote sensing, now provides the capability to resolve such phenomena and assess their role in structuring pelagic ecosystems. These methods are especially valuable when integrated together, and with shipboard calibration measurements and experimental programs. [ABSTRACT FROM AUTHOR]

The upper ocean, including the biologically productive euphotic zone and the mixed layer, has great relevance for studies of physical, biogeochemical, and ecosystem processes and their interaction. Observing this layer with a continuous presence, sampling many of the relevant variables, and with sufficient vertical resolution, has remained a challenge. Here a system is presented that can be deployed on the top of deep-ocean moorings, with a drive mechanism at depths of 150-200 m, which mechanically winches a large sensor float and smaller communications float tethered above it to the surface and back down again, typically twice per day for periods up to 1 year. The sensor float can carry several sizeable sensors, and it has enough buoyancy to reach the near surface and for the communications float to pierce the surface even in the presence of strong currents. The system can survive mooring blowover to 1000-m depth. The battery-powered design is made possible by using a balanced energy-conserving principle. Reliability is enhanced with a drive assembly that employs a single rotating part that has no slip rings or rotating seals. The profiling bodies can break the surface to sample the near-surface layer and to establish satellite communication for data relay or reception of new commands. An inductive pass-through mode allows communication with other mooring components throughout the water column beneath the system. A number of successful demonstration deployments have been completed. [ABSTRACT FROM AUTHOR]

Many fixed oceanographic instruments and observing systems are deployed in the water column or on the seafloor for extended periods of time without any expression at the sea surface. To routinely communicate with such subsurface instruments in the deep ocean, here a system is presented that uses underwater gliders and commercially available acoustic modems for this task and its use is demonstrated with subsurface moorings and inverted echo sounders plus bottom pressure sensor (PIES). One recent glider mission spent 31 days in data retrieval dives, capturing 2 MB of error-free subsurface data. To acquire this volume, a total of 2.65 MB (including all retransmissions) were sent, with a success rate of 75%. A model for the energy usage of each phase of modem function was derived from laboratory measurements. While the model predicts that the glider would expend 0.21 J to acquire each data byte, the actual consumption of the glider in the field is 0.49 J byte−1. The inefficiency is due to overhead associated with establishment of the acoustic link and with the resending of data that is received with errors. Including all the time for negotiating the acoustic link and for the retransmission of erroneous data, the net data throughput are around 3 bytes s−1 in spite of the modem operating at 140 to 600 baud. Even with these limitations, the technique has shown to be useful and is being utilized routinely in a research project in the California Current to obtain data from horizontal distances up to 7 km from an instrument at depths up to 4000 m, transferring on average 6 kB of data in a day of acoustic communications. [ABSTRACT FROM AUTHOR]

Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s−1. They had core salinities and potential temperatures of 34.91-34.98 and 4.48°-5.34°C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28°C from noneddy values. All eddies had small (≪1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s−1. Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins. [ABSTRACT FROM AUTHOR]

This study presents observations of the cross-sectional structure of resonant response to sea/land breezes (SLBs) off Huntington Beach (HB) in the Southern California Bight (SCB). A resonant response to local diurnal wind stress fluctuations associated with SLB forcing occurs intermittently and produces strong diurnal oscillations of flow and temperature resulting from enhanced work of the diurnal local wind on the sea surface. At nighttime (daytime), a coherent cross-sectional circulation with offshore (onshore) currents in the surface layer (upper 15 m) and onshore (offshore) currents in the intermediate layer around 20 m are generated, with a three-layered vertical structure on the outer shelf. The authors find a net cross-shore eddy heat flux (net cooling of nearshore water) during the period of strong response to SLB, that is, a rectified mean heat flux and steeper isotherms resulting from the diurnal SLB fluctuations. The steepened mean isotherms are also found to be in thermal-wind balance with intensified mean equatorward flow, which thus can also be generated by the resonant SLB dynamics. Similar rectified onshore transport of other quantities is expected, relevant for biogeochemical processes. The distribution of maximum diurnal kinetic energy in time and across the shelf supports the concept that subinertial shears create the sufficient condition for resonant response to SLB forcing. [ABSTRACT FROM AUTHOR]

The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO2, reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO2. Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change. [ABSTRACT FROM AUTHOR]

The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring–summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales. [ABSTRACT FROM AUTHOR]

In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill. [ABSTRACT FROM AUTHOR]

Two state-of-the-art profiling floats were equipped with novel optode-based oceanographic oxygen sensors. Both floats were simultaneously deployed in the central Labrador Sea gyre on 7 September 2003. They drift at a depth of 800 db and perform weekly profiles of temperature, salinity, and oxygen in the upper 2000 m of the water column. The initial results from the first 6 months of operation are presented. Data are compared with a small hydrographic oxygen survey of the deployment site. They are further examined for measurement quality, including precision, accuracy, and drift aspects. The first 28 profiles obtained are of high quality and show no detectable sensor drift. A method of long-term drift control is described and a few suggestions for the operation protocol are provided. [ABSTRACT FROM AUTHOR]

The so-called equatorial stacked jets are analyzed with ship-board observations and moored time series from the Atlantic Ocean. The features are identified and isolated by comparing vertical wavenumber spectra at the equator with those a few degrees from the equator. Mode-filtering gives clear views of the jets in meridional sections, the typical extent being ±1° in latitude. The vertical structure can be well described (explaining 82% of the variance) by N[sup -1]-stretched cosines, with a Gaussian amplitude tapering in the vertical. The stretched wavelengths are somewhat variable. Fitting jets of a fixed (stretched) wavelength to four moored sensors in the depth range 1300-1900 m, allows one to track the vertical phase of the jets with an rms error of 30°-45°. The resulting fit from a 20-month moored time series shows long periods of unchanging jet conditions and intermittent times of high variability. There is no significant vertical propagation on these timescales nor a seasonal reversal. Using a composite from many different experiments, interannual variability is visible, however. A possible mechanism for the stacked jets is inertial instability, resulting from background meridional shears at the equator. A condition is that the Ertel potential vorticity becomes zero somewhere, due to meridional asymmetries in the zonal flows. The ship-board observations show that this may be approximately fulfilled by the instantaneous zonal low-mode flows at various depths, resulting from an excess of zonal momentum south of the equator most of the time. Inertial instability should act to redistribute this zonal momentum, and our mooring data show indeed persistent northward momentum flux, but not at the depth levels expected. The momentum transport might suggest that the jets can also flux or mix other properties across the equator. [ABSTRACT FROM AUTHOR]

An international acoustic tomography experiment had been deployed in the western Mediterranean from January to October 1994. Seven transceivers spanned the region between France, Algeria, the Balearic Islands, and Sardinia/Corsica. The basin (maximum range 600 km) was ''illuminated'' by a single HLF-5 source, the remaining sources had a shorter range but could communicate with their neighbors. Extensive hydrographic measurements and analysis of historical data accompanied the observations, including a large-scale survey during deployment and recovery and a 2-weekly XBT section along the main transmission path. Some paths went along/through complicated topography, and in addition a shipboard acoustic survey was carried out to study the propagation into shallow water. All instruments remained operational for the entire period, and 6 of the 7 had 100% return of received data. The receptions show a high S/N ratio with well-resolved arrivals that agree well with acoustic propagation predictions. The seasonal variation of the travel times appears to be a good measure of the cycle of the large-scale heat content. The 3-D basin-averaged temperature is estimated to have an accuracy of 3%-4% of the seasonal amplitude. A system like this, installed with shore-cabled instruments, should be a valuable tool for monitoring interannual and climate-scale water mass variability. [ABSTRACT FROM AUTHOR]

The Strait of Gibraltar Acoustic Monitoring Experiment was conducted during April-May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, transport measurements in the Strait of Gibraltar, as well as to explore the acoustic scattering caused by the internal wave bores generated in the Strait. Three different approaches to monitoring the flow were explored: (i) high-frequency (2-kHz) reciprocal transmissions; (ii) high-frequency (2-kHz) horizontal arrival angle measurements of ray bending due to currents; and (iii) one-way transmissions from a low-frequency (250-Hz) source to a vertical receiving array on the opposite side of the Strait. Extensive independent measurements of the temperature, salinity, and velocity fields were also made. The deep-turning rays are stable and yield differential travel times that give reasonable values for the current components along the acoustic paths. Horizontal arrival angles (phase differences) are strongly correlated with rough estimates of the current perpendicular to the acoustic path made using current meter data, suggesting that this approach is feasible at ranges up to at least 15 km. Finally, the acoustic signals show the effects of the nonlinear internal waves present in the Strait. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]