A dominant paradigm for mid-latitude air-sea interaction identifies the synoptic-scale atmospheric “noise” as the main driver for the observed ocean surface variability. While this conceptual model ...successfully holds over most of the mid-latitude ocean surface, its soundness over frontal zones (including western boundary currents; WBC) characterized by intense mesoscale activity, has been questioned in a number of studies suggesting a driving role for the small scale ocean dynamics (mesoscale oceanic eddies) in the modulation of air-sea interaction. In this context, climate models provide a powerful experimental device to inspect the emerging scale-dependent nature of mid-latitude air-sea interaction. This study assesses the impact of model resolution on the representation of air-sea interaction over the Gulf Stream region, in a multi-model ensemble of present-climate simulations performed using a common experimental design. Lead-lag correlation and covariance patterns between sea surface temperature (SST) and turbulent heat flux (THF) are diagnosed to identify the leading regimes of air-sea interaction in a region encompassing both the Gulf Stream system and the North Atlantic subtropical basin. Based on these statistical metrics it is found that coupled models based on “laminar” (eddy-parameterised) and eddy-permitting oceans are able to discriminate between an ocean-driven regime, dominating the region controlled by the Gulf Stream dynamics, and an atmosphere-driven regime, typical of the open ocean regions. However, the increase of model resolution leads to a better representation of SST and THF cross-covariance patterns and functional forms, and the major improvements can be largely ascribed to a refinement of the oceanic model component.
And on Top of All That Breitburg, Denise L.; Salisbury, Joseph; Bernhard, Joan M. ...
Oceanography (Washington, D.C.),
06/2015, Letnik:
28, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Oceanic and coastal waters are acidifying due to processes dominated in the open ocean by increasing atmospheric CO₂ and dominated in estuaries and some coastal waters by nutrient-fueled respiration. ...The patterns and severity of acidification, as well as its effects, are modified by the host of stressors related to human activities that also influence these habitats. Temperature, deoxygenation, and changes in food webs are particularly important co-stressors because they are pervasive, and both their causes and effects are often mechanistically linked to acidification. Development of a theoretical underpinning to multiple stressor research that considers physiological, ecological, and evolutionary perspectives is needed because testing all combinations of stressors and stressor intensities experimentally is impossible. Nevertheless, use of a wide variety of research approaches is a logical and promising strategy for improving understanding of acidification and its effects. Future research that focuses on spatial and temporal patterns of stressor interactions and on identifying mechanisms by which multiple stressors affect individuals, populations, and ecosystems is critical. It is also necessary to incorporate consideration of multiple stressors into management, mitigation, and adaptation to acidification and to increase public and policy recognition of the importance of addressing acidification in the context of the suite of other stressors with which it potentially interacts.
Abstract
Hundreds of full-depth temperature and salinity profiles collected by Deepglider autonomous underwater vehicles (AUVs) in the North Atlantic reveal robust signals in eddy isopycnal vertical ...displacement and horizontal current throughout the entire water column. In separate glider missions southeast of Bermuda, subsurface-intensified cold, fresh coherent vortices were observed with velocities exceeding 20 cm s
−1
at depths greater than 1000 m. With vertical resolution on the order of 20 m or less, these full-depth glider slant profiles newly permit estimation of scaled vertical wavenumber spectra from the barotropic through the 40th baroclinic mode. Geostrophic turbulence theory predictions of spectral slopes associated with the forward enstrophy cascade and proportional to inverse wavenumber cubed generally agree with glider-derived quasi-universal spectra of potential and kinetic energy found at a variety of locations distinguished by a wide range of mean surface eddy kinetic energy. Water-column average spectral estimates merge at high vertical mode number to established descriptions of internal wave spectra. Among glider mission sites, geographic and seasonal variability implicate bottom drag as a mechanism for dissipation, but also the need for more persistent sampling of the deep ocean.
Significance Statement
Relative to upper-ocean measurements of temperature, salinity, and velocity, deep ocean measurements (below 2000 m) are fewer in number and more difficult to collect. Deep measurements are needed, however, to explore the nature of deep ocean circulation contributing to the global redistribution of heat and to determine how upper-ocean behavior impacts or drives deep motions. Understanding of geographic and temporal variability in vertical structures of currents and eddies enables improved description of energy pathways in the ocean driven by turbulent interactions. In this study, we use newly developed autonomous underwater vehicles, capable of diving to the seafloor and back on a near daily basis, to collect high-resolution full ocean depth measurements at various locations in the North Atlantic. These measurements reveal connections between surface and deep motions, and importantly show their time evolution. Results of analyzing these vertical structures reveal the deep ocean to regularly “feel” events in the upper ocean and permit new comparisons to deep motions in climate models.
The ocean bottom is the Earth's least explored region, and the bottom mixed layer (BML) is the pathway for communication between the ocean interior and the ocean floor. In this study, we used ...full‐depth conductivity‐temperature‐depth profiles archived by the World Ocean Circulation Experiment Program to obtain the first approximation of the global distribution of the oceanic BML thickness, HBML, by applying an integrated method (Huang, Cen, et al., 2018, https://doi.orag/10.1175/jtech-d-18-0016.1). We found that the median HBML values were 40, 42, and 64 m in the Atlantic, Indian, and Pacific Oceans, respectively, and 47 m globally. Statistically, the peak values for the median HBML were around 20°N or 20°S, and it had weak dependence on the buoyancy frequency, where a thin HBML corresponded to strong stratification. In addition, the median HBML became thicker with the ocean depth (D), according to HBML = 26.34 + 0.85e(D/1271.8).
Plain Language Summary
There is an increasing demand in observing the abyssal ocean, the oceanic bottom mixed layer (BML) is the water column communicating with the ocean interior and underlying, where interrelated physical, geochemical, and biological processes actively take place. However, the basic knowledge of the oceanic mixed layer thickness and its spatiotemporal variability is lacking. By using full‐depth conductivity‐temperature‐depth profiles archived by the World Ocean Circulation Experiment Program, we, to the first approximation, show a global distribution of the oceanic BML thickness, with the application of an integrated method. The findings of the oceanic BML thickness in different oceans and its dependencies on latitude and ocean depth would be attractive to various scientific fields.
Key Points
The global distribution of the oceanic bottom mixed layer thickness was assessed with the full‐depth WOCE data
Statistically, the oceanic bottom mixed layer thickness becomes thicker exponentially with depth in the abyssal ocean
Basement formation pressures and temperatures were recorded from 1997 to 2017 in four sealed‐hole observatories in North Pond, an isolated ∼8 × 15 km sediment pond surrounded by thinly sedimented ...basement highs in 7–8 Ma crust west of the Mid‐Atlantic Ridge at ∼23°N. Two observatories are located ∼1 km from the southeastern edge of North Pond where sediment thickness is ∼90 m; the other two are ∼1 km from the northeastern edge where sediment thickness is 40–50 m. Sediments are up to 200 m thicker in the more central part of the pond. The borehole observations, along with upper basement temperatures estimated from seafloor heat flux measurements, provide constraints on the nature of low‐temperature ridge‐flank hydrothermal circulation in a setting that may be typical of sparsely sedimented crust formed at slow spreading ridges. Relative to seafloor pressures, basement formation pressures are modestly positive and increase with depth, except for a slight negative differential pressure in the shallowest 30–40 m in one northeastern hole. Although the observatory pairs are ∼6 km apart, the lateral pressure gradient in basement between them is very small. Formation pressure responses to seafloor tidal loading are consistent with high basement permeability that allows for vigorous low‐temperature circulation with low lateral pressure gradients. In contrast, there is significant lateral variability in upper basement temperatures, with highest values of ∼12.5°C beneath the thickly sedimented southwest section, lower values near the edges, and lowest values near the southeast edge. The results are key to assessing past and recent models for the circulation system.
Plain Language Summary
Low temperature hydrothermal systems in young oceanic seafloor have important integrated thermal and chemical effects, but flow patterns are weakly constrained by direct observations and are poorly understood, especially in sparsely sedimented settings that are characteristic of much of the world's seafloor. We present 20 years of observations of seafloor and formation pressures and temperatures recorded by sealed borehole observatories beneath a North Atlantic ridge‐flank sedimented basin (“North Pond”) surrounded by sparsely sedimented basement highs—the first such long‐term data from this common type of setting. The observations indicate that there is a vigorous low‐temperature hydrothermal flow system beneath the sediments of North Pond as previously concluded, but lateral pressure gradients in uppermost basement are very low; these observations require the permeability of uppermost basement to be very high. Our determinations of uppermost basement temperatures show greater variations than previously inferred, with the highest temperatures beneath the most thickly sedimented section. This is inconsistent with prior conceptual models of unidirectional hydrothermal flow in uppermost basement beneath the sediment pond, but more supportive of recent models with upflow beneath the most thickly sedimented section of the pond, accompanied by lateral flow toward the pond perimeter where mixing with seawater occurs.
Key Points
Formation pressures in igneous basement beneath North Pond sediments are positive except in the upper few tens of meters near the pond edge
Lateral pressure gradients in upper igneous basement are very low, implying very high permeabilities (∼10−10 m2) and vigorous flow
Basement temperatures are highest beneath the thickest sediments, unsupportive of prior models for unidirectional flow in igneous basement
We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate ...of near sea surface temperature (NSST) at the Last GlacialMaximum (LGM; circa 23–19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled ocean–sea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled ocean–atmosphere–ice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.
Mapping and sampling four sections of the slow‐spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are ...characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m‐resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle‐dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial‐high and axial valley (∼59.5°N). (c) Across‐axis variations in the fault density and sediment thickness provide similar widths for the neo‐volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat‐topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V‐shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by‐pass the normal subaxial magmatic plumbing system, erupting over a wider‐than‐normal area.
Plain Language Summary
Volcanic eruptions and faults growth are two important geologic processes taking place along seafloor spreading centers. Their variations in space and time are displayed in the morphology of the spreading centers. Investigating these morphological variations is key to understanding the deeper processes of the oceanic crust formation. South of Iceland, the Reykjanes Ridge is the location of increased volcanism due to the interaction between the mid‐ocean ridge and the Iceland hotspot. Using high‐resolution seafloor topographic data, chemical analyses of volcanic rock, and videos of the seafloor from a remotely operated vehicle, we investigated how volcanism and faulting change along the ridge. The increase in fault dimensions (height, length) with distance from the plume center is probably the result of the crust and mantle becoming cooler and stiffer and thus able to support larger faults. Fault density and thickness of the sediment covering the lava flows near the ridge axis are used to delimit the region of young volcanism. Seamounts often emplaced beyond that region. A peak in seamount abundance near 60°N suggests that the thick crust here is generated from numerous small batches of magma possibly resulting from a migrating instability in the melt production process beneath the axis.
Key Points
The distance from the plume center is not the only factor controlling tectonic and volcanic activity along the Reykjanes Ridge
Fault dimensions are primarily controlled by the variation of crustal thermal structure with distance from the hotspot
Flat‐topped seamount abundances peak where a V‐shaped ridge intersects the axis, consistent with a buoyant upwelling melting instability
This research links global climate to regional weather by considering Caribbean trade wind strength in the context of the large-scale Walker circulation across the Pacific-Atlantic basins, and ...localized processes involving air-sea interactions between freshwater flux, the ocean mixed-layer depth, and topographic channeling of airflow north of Colombia. Trade wind driven coastal upwelling in the southern Caribbean is enhanced by the Andes Mountains, and creates a focal point for summer climate variability. This emerges in empirical orthogonal function (EOF) analysis of June-July surface zonal winds in the period 1979-2022. Highest EOF loading occurs at 12° N, 75° W northwest of Colombia. Point-to-field correlations with the EOF time score reflect a Pacific-Atlantic thermal dipole and Walker circulation linked with the El Niño-Southern Oscillation (ENSO). As southern Caribbean trade winds weaken, run-off increases, the upper ocean becomes buoyant, and westward currents slacken. Composite differences show that slow trade-wind conditions in June-July induce a counter-current that spreads warm fresh water northeastward from Colombia. This plume disperses toward the Antilles Islands with sufficient memory to triple the number of tropical cyclones in August-September. A slow trade-wind case study in June 2011 emphasizes key air-sea interactions. Channeling of the large-scale airflow north of the Andes Mountains creates a narrow atmospheric bridge for transmission of ENSO signals.
We present regional sea-level projections and associated uncertainty estimates for the end of the 21 ˢᵗ century. We show regional projections of sea-level change resulting from changing ocean ...circulation, increased heat uptake and atmospheric pressure in CMIP5 climate models. These are combined with model- and observation-based regional contributions of land ice, groundwater depletion and glacial isostatic adjustment, including gravitational effects due to mass redistribution. A moderate and a warmer climate change scenario are considered, yielding a global mean sea-level rise of 0.54 ±0.19 m and 0.71 ±0.28 m respectively (mean ±1σ). Regionally however, changes reach up to 30 % higher in coastal regions along the North Atlantic Ocean and along the Antarctic Circumpolar Current, and up to 20 % higher in the subtropical and equatorial regions, confirming patterns found in previous studies. Only 50 % of the global mean value is projected for the subpolar North Atlantic Ocean, the Arctic Ocean and off the western Antarctic coast. Uncertainty estimates for each component demonstrate that the land ice contribution dominates the total uncertainty.
The central role played by the ocean's Atlantic Meridional Overturning Circulation (AMOC) in the uptake and sequestration of transient tracers is studied in a series of experiments with the Goddard ...Institute for Space Studies and Massachusetts Institute of Technology ocean circulation models. Forced by observed atmospheric time series of CFC‐11, both models exhibit realistic distributions in the ocean, with similar surface biases but different response over time. To better understand what controls uptake, we ran idealized forcing experiments in which the AMOC strength varied over a wide range, bracketing the observations. We found that differences in the strength and vertical scale of the AMOC largely accounted for the different rates of CFC‐11 uptake and vertical distribution thereof. A two‐box model enables us to quantify and relate uptake efficiency of passive tracers to AMOC strength and how uptake efficiency decreases in time. We also discuss the relationship between passive tracer and heat uptake efficiency, of which the latter controls the transient climate response to anthropogenic forcing in the North Atlantic. We find that heat uptake efficiency is substantially less (by about a factor of 5) than that for a passive tracer.
Key Points
The AMOC controls the depth and the strength of the ocean uptake of transient, passive tracers
Quantitatively, the rate and vertical extent of tracer sequestration scale linearly with the AMOC
Uptake efficiency for temperature is substantially less than that for a passive tracer
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