The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO(2), 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 CO(2), 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 CO(2). 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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Near‐bottom diurnal thermocline shoaling and diurnal baroclinic currents were observed from time series data of water temperature, salinity, pressure, and velocity collected from August 2006 to ...January 2007 in the nearshore zone off the Huntington Beach where the local inertial frequency is higher than the diurnal frequency (poleward of the diurnal critical latitude). During the stratified season, the deep offshore cold (dense) water, when shoaled into shallow water along the bottom, was often trapped for a few hours and dissipated in the nearshore region (water depth of 10–20 m) during the ebb phase of the diurnal surface tide. It appears that perturbations propagating onshore along the shoaled isopycnals can form an upslope surging front and an internal bolus. Our data allow the application of theoretical/lab criteria for internal hydraulic jumps, internal bolus formation, and internal wave breaking and all criteria are fulfilled during a part of the tidal shoaling cycle. Nonlinear advection associated with the internal boluses causes higher (close to ω−3) spectral falloff rate of near‐bottom temperature with frequency ω than the canonical Garret‐Munk spectra (ω−2) in the range of 0.1–1.0 cph, implying strong scattering of tidal energy toward smaller scales. We are able to directly calculate the offshore eddy heat flux by cold water intrusion onto the shelf resulting from this process. Similar impacts and implications are expected for the biogeochemical quantities, as well as for the role of subinertial internal waves on turbulent mixing in the immediate proximity of the generating region, i.e., sloping bottom.
Key Points
Observational evidence of near‐bottom intrusion of deep water onto the shelf
Cross‐shelf transport variability in the coastal ocean
Role of subinertial (diurnal) internal tides in inducing the intrusion
Low‐oxygen and low‐pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal ...seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low‐oxygen and low‐pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen off Southern California, reveal large additional oxygen deficiencies relative to the seasonal cycle during the latest La Niña event. While some changes in oxygen related to the isopycnal depression/uplifting during El Niño/La Niña are not unexpected, the observed oxygen changes are 2–3 times larger than what can be explained by cross‐shore exchanges. In late summer 2010, oxygen levels at mid‐depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4–5 ml/L. The extra uplifting of isopycnals related to the La Niña event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Niña‐caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Niña, the seasonal oxygen minimum and the La Niña anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes.
Key Points
La Nina can amplify low oxygen events creating hypoxia even late in the season
Extra isopycnal uplifting, decreased productivity, strengthened undercurrent
A La Nina earlier in the season could cause even more extreme hypoxia
As harmful algae blooms are increasing in frequency and magnitude, one goal of a new generation of higher spectral resolution satellite missions is to improve the potential of satellite optical data ...to monitor these events. A satellite-based algorithm proposed over two decades ago was used for the first time to monitor the extent and temporal evolution of a massive bloom of the dinoflagellate Lingulodinium polyedra off Southern California during April and May 2020. The algorithm uses ultraviolet (UV) data that have only recently become available from the single ocean color sensor on the Japanese GCOM-C satellite. Dinoflagellates contain high concentrations of mycosporine-like amino acids and release colored dissolved organic matter, both of which absorb strongly in the UV part of the spectrum. Ratios <1 of remote sensing reflectance of the UV band at 380 nm to that of the blue band at 443 nm were used as an indicator of the dinoflagellate bloom. The satellite data indicated that an observed, long, and narrow nearshore band of elevated chlorophyll-a (Chl-a) concentrations, extending from northern Baja to Santa Monica Bay, was dominated by L. polyedra. In other high Chl-a regions, the ratios were >1, consistent with historical observations showing a sharp transition from dinoflagellate- to diatom-dominated waters in these areas. UV bands are thus potentially useful in the remote sensing of phytoplankton blooms but are currently available only from a single ocean color sensor. As several new satellites such as the NASA Plankton, Aerosol, Cloud, and marine Ecosystem mission will include UV bands, new algorithms using these bands are needed to enable better monitoring of blooms, especially potentially harmful algal blooms, across large spatiotemporal scales.
Abstract
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Loop Current is the main mesoscale feature of the Gulf of Mexico oceanic circulation. With peak velocities above 1.5 m s
–1
, the Loop Current and its mesoscale eddies are of interest to ...fisheries, hurricane prediction and of special concern for the security of oil rig operations in the Gulf of Mexico, and therefore understanding their predictability is not only of scientific interest but also a major environmental security issue. Combining altimetric data and an eddy detection algorithm with 8 years of deep flow measurements through the Yucatan Channel, we developed a predictive model for the Loop Current extension in the following month that explains 74% of its variability. We also show that 4 clusters of velocity anomalies in the Yucatan Channel represent the Loop Current dynamics. A dipole with positive and negative anomalies towards the western side of the Channel represents the growing and retracted phases respectively, and two tripole shape clusters represent the transition phases, the one with negative anomalies in the center associated with 50% of the eddy separation events. The transition between these clusters is not equally probable, therefore adding predictability. Finally, we show that eddy separation probability begins when the Loop Current extends over 1800 km (~27.2°N), and over 2200 km of extension, eddy detachment and reattachment is more frequent than separation. These results represent a step forward towards having the best possible operational Loop Current forecast in the near future, incorporating near real-time data transmission of deep flow measurements and high resolution altimetric data.
Abstract
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•North Atlantic Current (NAC) is observed with altimetry, floats, moorings, and XBTs.•Decadal NAC variability is traced back to sea level anomalies and density signals.•Uncertainty estimates quantify ...sensor and sampling errors in NAC observations.•Observed NAC variability is larger than the uncertainty estimates.
Large-scale geostrophic flow across an oceanographic section can be computed from in situ density profiles and satellite-borne sea level anomaly measurements at the section endpoints. Here, uncertainties are estimated and compared for observations that span the North Atlantic Current. Out of multi-year mooring records from two sites, Porcupine Abyssal Plain and Central Irminger Sea, 16 months from the 2002–2005 period are available that have sufficient data coverage to observe this current. Transport and uncertainty estimates from these moorings are compared to results from Argo floats and expendable bathythermographs. The latter have longer data sets and provide decadal time series with semiannual resolution in time. Typical uncertainties, which take sensor errors and sub-sampling the eddy field into account, are below 2 Sv of volume transport for the mooring- and float-based estimates and just below 3 Sv for the expendable bathythermographs. Peak-to-peak variability occurs on decadal time scales and is about 11 Sv, well above the observational uncertainty. We can therefore have confidence that the observed signal reflects true changes in the ocean. The time series is further extended to 25 years duration, using solely the altimetry data by proxy. The aim of this study is to add value to such time series by understanding and quantifying the uncertainties and consistency between methods. The methodology is applicable at other locations as well.
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