Gulf Stream Research Articles

A comparative analysis of the mesoscale thermohaline features across subarctic frontal zones in the Northern Hemisphere

Mesoscale eddies and their thermohaline features across the two subarctic frontal (SAF) zones, the Oyashio Extension (OE) and the Gulf Stream (GS), in the Northern Hemisphere are investigated and compared using both satellite-based observations and Argo float data. Our results show that the thermohaline properties (temperature, salinity and associated density) induced by mesoscale eddies have significant differences between the south and north sides of each SAF zone. The relationships between the sea surface height anomaly (SSHA) and the sea surface temperature anomaly (SSTA), called the H-T amplitude relationship, are examined and used to depict the mesoscale sea surface characteristics. In both the OE and GS regions, SSTAs on the north sides of the SAFs are more sensitive to the SSHA than that on their southern sides, and this sensitivity is stronger in the GS than in the OE. It is found that in the OE, the difference in the H-T amplitude relationships between the north and south sides is mainly due to the thermal expansion coefficient, which is contingent on the ambient sea surface temperature. In the GS, the difference between the north and south sides in the H-T amplitude relationships is mainly due to the change of the vertical profiles of mesoscale eddies, which are related to the ambient meridional density front.

The Atlantic Meridional Overturning Circulation and its Hypothetical Collapse

Collapse of the Atlantic Meridional Overturning Circulation (AMOC) is often invoked as an explanation of major past climate changes and as a major risk for future climate. Many of these arguments appear, from an observer’s point of view, as far-more definitive than is warranted. In the hypothetical event of a future collapse, the implications may be much less severe than those from many other elements of global change already underway. The Gulf Stream system, and its required return flow of mass, implies that changed circulations will nonetheless continue to carry significant amounts of heat, carbon etc., poleward even without any AMOC.

Variability and Trends of the Florida Current and Implications for the Future of the Gulf Stream

Pietrafesa, L.J.; Bao, S.; Gayes, P.T.; Carpenter, D.D., and Kowal, J.C., 2022. Variability and trends of the Florida current and implications for the future of the Gulf Stream. Journal of Coastal Research, 38(6), 1096–1103. Coconut Creek (Florida), ISSN 0749-0208. The Gulf Stream, the North Atlantic Ocean Basin Western Boundary Current, plays a key role in moderating the climate of the North Atlantic Ocean Basin and surrounding landmasses. However, the Gulf Stream has been widely reported in the public media to be slowing down in its volumetric transport to the north, related to the reported weakening of the Atlantic Meridional Overturning Circulation. The recent exposé claims that the Gulf Stream may be in the throes of an irreversible collapse. This would have serious implications for the future climate of not only the United States, Canada, Greenland, Iceland, Scandinavia, the British Isles, and Europe, but for the entire planet, per se. In this study, investigations of the volumetric flux of the Florida Current took place that constitutes the formative beginning of the Gulf Stream at the Florida Straits to determine if there is evidence of a slowing of this major Western Boundary Current at the latitude of its beginning source. Findings include multiple, well-defined frequency and amplitude modulated internal modes of current variability in the 38-year record of Florida Current volumetric transport, ranging from weekly to monthly to seasonally to annually to interannually and decadal. The time series reveals an overall slowing of approximately 0.98×106 meters3/second (equivalent to approximately one Sverdrup), or a 2.795% drop over the nearly four past decades of daily observations.

Mitigating Climate Biases in the Midlatitude North Atlantic by Increasing Model Resolution: SST Gradients and Their Relation to Blocking and the Jet

Abstract Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multimodel ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multimodel ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the storm track. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

Quantification of the Dominant Drivers of Acidification in the Coastal Mid‐Atlantic Bight

AbstractIn shallow coastal shelves like the Mid‐Atlantic Bight (MAB), ocean acidification due to increased atmospheric carbon dioxide (CO2) is compounded by highly variable coastal processes including riverine freshwater inputs, nutrient loading, biogeochemical influence, coastal currents and water mass mixing, and seasonal transitions in physical parameters. Past deconstructions of carbonate system drivers in the MAB have focused on nearshore zones or single season data, and thus lack the spatial and temporal resolution required to assess impacts to important species occupying the shelf. Deconstructing highly resolved data collected during four seasonal Slocum glider deployments in the MAB, this study uses a Taylor Series decomposition to quantify the influence of temperature, salinity, biogeochemical activity, and water mass mixing on pH and aragonite saturation state from sea surface to bottom. Results show that water mass mixing and biogeochemical activity were the most significant drivers of the carbonate system in the MAB. Nearshore water was more acidic year‐round due to riverine freshwater input, but photosynthesis reduced acidity at certain depths and times. Water mass mixing increased acidity in bottom water on the shelf, particularly in summer. Gulf Stream intrusions at the shelf break during fall acted to mitigate acidification on the shelf in habitats occupied by carbonate‐bearing organisms. The relationships quantified here can be used to improve biogeochemical forecast models and determine habitat suitability for commercially important fin and shellfish species residing in the MAB.

Interaction of Rossby waves with the Gulf Stream and Kuroshio using altimetry in a framework of a vortex layer model

In this paper, we show the potential of satellite altimetry to study the interaction of Rossby waves with the shear flow. The Miles-Ribner approach, which was developed in gas dynamics in the 1960 s, is used to describe Rossby waves interacting with the Gulf Stream and Kuroshio areas. The region of interaction is approximated by a nonzonal vortex layer. We apply the main formulations of the problem of a nonzonal vortex layer on the β-plane in the formulation of Miles-Ribner to observations in the real ocean. Earlier, we showed that the interaction of waves with a nonzonal flow gives rise to a new class of solutions, which is absent in the case of a zonal flow. This new class of solutions can be interpreted as the pure emission of Rossby waves by the nonzonal flow. We apply this theoretical approach to the areas of the Gulf Stream and Kuroshio as well. We use for analysis altimetry data available at Copernicus Marine Environment Monitoring Service. The analysis of Hovmöller diagrams in the areas under consideration confirms the previously obtained theoretical conclusions of the problem of the interaction of planetary waves with a nonzonal flow on the β-plane in the formulation of Miles-Ribner. The incident waves, as well as refracted and reflected waves are distinguished. The speed of refracted and reflected waves exceeds the speed of incident waves, which confirms the conclusions about the existence of mechanisms for the amplification of planetary waves when they interact with a nonzonal flow.

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.

Dispersal of juvenile leatherback turtles from different Caribbean nesting beaches: A model study

The Northwest Atlantic (NWA) leatherback turtle (Dermochelys coriacea) subpopulation was recently classified as endangered. It nests in the Wider Caribbean Region and includes five genetic stocks, all declining, albeit at different rates. The causes of decline are multiple and difficult to identify based on annual nest counts which integrate the effects of multiple stressors over the entire life history. Demographic models, however, show that survival during the juvenile pelagic stage is the main factor modulating population trends, but this life stage remains largely unobserved. This paper presents a suite of numerical simulations where juveniles from the five NWA stocks disperse under the combined effects of ocean currents and habitat-driven swimming movements. Simulations reveal when and where NWA juveniles likely disperse and, thus, the environmental conditions and anthropogenic threats they may encounter. Simulated individuals initially disperse following either the “Caribbean route,” inside the Caribbean Sea and the Gulf of Mexico (GoM), or the “Atlantic route” east of the Antilles Islands Arc. The percentage of individuals following one or the other route varies markedly with the stock of origin. Late dispersal in the eastern Atlantic Ocean and the Mediterranean Sea is similar in all stocks. Juveniles following the Caribbean route are rapidly entrained northwards by the Gulf Stream and incur a high risk of cold-induced mortality. This mostly affects the Florida stock and the Western Caribbean (WCA) stock nesting in Costa Rica, Panama, and Colombia. The Atlantic route is less lethal as individuals progress more slowly toward higher latitudes. Simulations also show that the percentage of WCA juveniles visiting the GoM is larger than for any other stock. The learned migration goal (LMG) hypothesis, which posits that adult sea turtles tend to exploit foraging areas previously identified at the juvenile stage, may thus explain why WCA adults are overrepresented in the GoM. Finally, our results suggest that the recently observed increase in the percentage of WCA adults migrating into the GoM could be linked to bycatch reduction measures implemented in 2003–2004, combined with an increase in the frequency of Loop Current intrusion and eddy-shedding events that started around the year 2000.

A low-order wake interaction modeling framework for the performance of ocean current turbines under turbulent conditions

Understanding the effects of ambient turbulence (expressed often in terms of the turbulence intensity It) is critical to the development of predictive models for the performance of Ocean Current Turbine (OCTs). This paper describes a new, wake interaction modeling framework capable of capturing the detailed effects of turbulence on various performance parameters associated with OCTs that may be arranged in any arbitrary configuration. The model accounts for the effects of turbulence on the structure of the turbine wakes, specifically the extents of near- and far-wake regions, and the dependence of the transition point between the two regions on It. The analytical description for turbine wake is combined with an existing wake interaction model, the Unrestricted Wind Farm Layout Optimization (UWFLO) model to predict the global power output from an array of OCTs. The resulting modelling framework accurately captures the effect of inlet turbulence on the OCT farm power and efficiency, and can be applied to any array configuration. Results from the model are validated against Large Eddy Simulations (LES) in which the OCTs are modeled using the Blade Element Momentum (BEM) model, while the inlet flow is superposed with a synthetic turbulence field designed to approximate turbulence properties obtained from observational measurements of the Gulf Stream. The simulations show that OCT wakes recover faster at higher levels of inlet turbulence due to the enhanced entrainment and mixing between ambient flow and the wake, an effect that is captured by the modified UWFLO model.

Variations of acoustic ducting in the presence of gulf stream warm core rings at the New England shelfbreak

The physical oceanographic (PO) conditions at the New England shelfbreak have become more dynamic with increased warming and meandering of the gulf stream. Consequently, underwater sound propagation in the area has also become more variable. The most profound PO feature that breaks off from Gulf Stream instabilities is warm core rings, which have complex interactions with bathymetry and seasonal shelf water masses. Warm core rings can cause shelf water streamers to extend offshore, resulting in surface and/or subsurface acoustic ducting conditions. This study observed acoustic surface ducting and breakdown conditions induced by a shelf water streamer at the New England shelfbreak. Ducting conditions were variable when a subsequent warm core ring intruded the acoustic source and receiver network, mixing with the shelf water streamer and influencing the sound propagation during and after the acoustic duct breakdown period. The field observations were also compared with a broadband numerical model. With increasing gulf stream warm core ring formations, streamer induced surface ducts are becoming more relevant in shelf break acoustics. [Work supported by the Office of Naval Research.]

Shelfbreak processes affecting acoustic propagation in a changing ocean

Fundamental physical oceanographic processes affecting sound speed fields and acoustic propagation have been studied in shelfbreak regions for some time. Integrated observations extending back to the 1990s have related acoustic propagation variability to basic thermohaline structure as well as physical oceanographic processes causing significant variability in sound speed fields. Key shelfbreak processes such as frontal wave propagation, eddy-Shelfbreak Jet interactions, and wind-driven motions will be described. The influence of ocean warming and climate change impacts on these processes will be outlined with emphasis on the Middle Atlantic Bight in the northeastern United States. The impacts of changes in both the atmospheric Jet Stream as well as the Gulf Stream on the Shelfbreak Front and Jet and implications for acoustic propagation will be presented.

Sensitivity of ECMWF coupled forecasts to improved initialization of the ocean mesoscale

AbstractIt has been hypothesized that initial conditions derived from eddy‐resolving ocean models could benefit eddy‐permitting forecast systems at subseasonal lead times (2 weeks to 2 months) by providing improved ocean initial conditions and reduced sea‐surface temperature (SST) errors associated with a more realistic representation of mesoscale features and ocean fronts such as the Gulf Stream. Here, we test this hypothesis by running a series of coupled 32‐day reforecasts with ocean initial conditions derived from native resolution (i.e., eddy‐permitting) and higher resolution (i.e., eddy‐resolving) ocean reanalyses. Initial conditions derived from the eddy‐resolving reanalysis reproduce observed temperature and sea‐level variability associated with mesoscale ocean eddies more accurately. Furthermore, forecasts of ocean variables are significantly improved in the extratropics, particularly in regions of observed intense mesoscale variability. This result is relevant for the marine forecasting community and demonstrates the potential for improved probabilistic forecasts of ocean variables in affordable ensemble forecast systems. However, the impacts on the mean state and forecast skill in the atmosphere are more limited. Based on an evaluation of model biases as a function of lead time, we conclude that there is room for further improvements to the initialization of the Gulf Stream that could lead to more significant impacts on atmospheric skill. Furthermore, we demonstrate that our reforecasts underestimate the strength of mesoscale air–sea interaction in the vicinity of strong SST fronts by 20–40%. We speculate that the impacts of improved ocean eddy initialization on atmospheric forecast skill may be larger in coupled models with higher ocean and atmosphere resolutions that can simulate air–sea interaction in the midlatitudes more realistically.

Sound propagation measurements using an autonomous underwater vehicle acoustic array in the New England shelf break acoustics network

The New England shelf break is a highly dynamic region, which experiences complex spatial and temporal water-column variations due to interactions with warm core rings originating from the gulf stream. This widely varying ocean environment leads to changes in sound speed and acoustic propagation. Acoustic payload-equipped autonomous underwater vehicles (AUVs) are advantageous for sound propagation measurements in such environments due to their ability to detect changes with greater spatial resolution compared to fixed moorings. An AUV-towed acoustic array was tested and deployed in the New England Shelf Break Acoustics (NESBA) experiment in May 2021. The acoustic AUV system was comprised of a modified REMUS 600 vehicle, a hull-mounted 3.5 kHz transducer, and a towed multi-channel linear hydrophone array. The AUV sound source was tested at the Dodge Pond Naval Facility to characterize the effect of AUV body resonance, and the resulting calibration was incorporated into the data processing. Propagation paths between the AUV, acoustic moorings, and a ship-towed sound source were studied to investigate the acoustic effects of varying physical oceanographic conditions and biological scattering layers. These measurements also enabled investigation of the local seabed conditions and sub-bottom layering structure. [Work supported by the Office of Naval Research.]

Long-term evolution of ocean eddy activity in a warming world

Mesoscale ocean eddies, an important element of the climate system, impact ocean circulation, heat uptake, gas exchange, carbon sequestration and nutrient transport. Much of what is known about ongoing changes in ocean eddy activity is based on satellite altimetry; however, the length of the altimetry record is limited, making it difficult to distinguish anthropogenic change from natural variability. Using a climate model that exploits a variable-resolution unstructured mesh in the ocean component to enhance grid resolution in eddy-rich regions, we investigate the long-term response of ocean eddy activity to anthropogenic climate change. Eddy kinetic energy is projected to shift poleward in most eddy-rich regions, to intensify in the Kuroshio Current, Brazil and Malvinas currents and Antarctic Circumpolar Current and to decrease in the Gulf Stream. Modelled changes are linked to elements of the broader climate including Atlantic meridional overturning circulation decline, intensifying Agulhas leakage and shifting Southern Hemisphere westerlies.

Evidence of Spatial Stability in Core Fauna Community Structure of Holopelagic Sargassum

As Sargassum biomass continues to increase globally, it is critical to develop a better understanding of how it functions as habitat, therefore, community structure of Sargassum-associated organisms was examined from 11 sampling locations spanning the tropical Florida Straits to the more temperate Gulf Stream off the coast of Savannah, Georgia from May to September 2018 using a combination of modified shrimp trawls and dip nets. A total of 5413 organisms were collected from Sargassum habitat representing 14 species from 10 families. A core group of organisms (Platynereis dumerilii, Litiopa melanostoma, Portunus sayi, Portunus spinimanus, Leander tenuicornis, and Latreutes fucorum) were found throughout the entirety of the geographic range surveyed. This core community did not vary significantly with increasing distance to shore or latitude, nor did it correlate with environmental variables such as salinity and temperature. However, community structure did vary with clump size, with larger clumps harboring more speciose communities. The Sargassum community in the Florida Straits and Gulf Stream appear to provide habitat for a consistent group of epifaunal organisms. In turn, this stable group offers a consistent prey source for a variety of important, higher trophic level organisms.

Biogeographic variations in diel vertical migration determined from acoustic backscattering in the northwest Atlantic Ocean

Active acoustic scattering techniques, which capitalize on the fact that different kinds of organisms scatter sound differently as the frequency changes, are uniquely suited to synoptic studies of zooplankton and fish distributions. Acoustic systems are particularly useful for documenting diel vertical migrations (DVM). Measurements with a hull mounted echosounder operating at frequencies of 43, 120, 200, and 420 kHz were made near-continuously on a 26-day cruise in the Northwest Atlantic Ocean in August 2011. The distribution of backscattering in relation to changing environmental quantities along the cruise track enabled characterization of the rates and amplitudes of diel vertical migrations and acoustic indices of pelagic animal abundance in relation to hydrographic regimes. The most pervasive phenomenon observed acoustically was a regular DVM evident along a series of six study transects. In the mode waters (∼18 °C) of the Sargasso Sea, a non-migrating layer was observed around 200 m, which was not present in the other surveyed regions (Gulf Stream, GS/Labrador Sea loop, Flemish Cap, Slope Water). A deeper (>450 m) non-migrating layer existed throughout the survey region. Migration speeds varied between 361 m/hour in the Sargasso Sea and 72.5 m/hour in the GS/Labrador Sea Loop. Acoustic backscattering at 43 kHz in the upper 500 m was highest in the Sargasso Sea (day overall median = −79.5 dB; night median = −76.5 dB) and lower in the more northern stations (all the other regions day = −86.8 dB and night = −81.1 dB). Variations in the depth of the DSL at the sampled locations are not limited by subsurface oxygen levels in the Northwest Atlantic and Northern North Atlantic.

Coherence and potential drivers of stock assessment uncertainty in Northeast US groundfish stocks

Abstract Failure to account for the impacts of climate and ecosystem change on stock dynamics can introduce uncertainty to stock assessments that can make meeting the objective of sustainable fisheries management challenging. The increased prevalence and magnitude of uncertainty in New England groundfish stock assessments (i.e. retrospective patterns) in recent years suggest that there may be common drivers impacting these stocks that are currently unaccounted for in the stock assessment. We examined the coherence in retrospective patterns across groundfish stock assessments and evaluated candidate drivers of retrospective patterns, including large-scale climate and ecosystem change, as well as significant management and monitoring changes. We found high coherence in moving window Mohn's rho time series for groundfish within the Gulf of Maine and Georges Bank areas. Fluctuations in Gulf of Maine groundfish Mohn's rho values were most strongly related to lagged bottom temperature and spiny dogfish biomass time series, whereas fluctuations in Georges Bank groundfish Mohn's rho values were strongly related to lagged time series of warm core rings formation from the Gulf Stream. Our identification of coherence in retrospective patterns across groundfish stocks by region supports the idea of common regional drivers with climate and ecosystem changes emerging as the leading contributing factors.

Improved Gulf Stream separation through Brinkman penalization

The advantage of a smooth representation of bathymetry in terrain-following σ-coordinate ocean models is compromised by the need to avoid numerical errors on steep slopes associated with pressure gradient discretization or spurious diapycnal diffusion. Geopotential z-coordinate models avoid these errors, but greatly underrepresent the interaction of flow with a topographic slope, especially when the bathymetry is underresolved. Hybrid coordinate models are also deficient because it is difficult to find a satisfactory compromise between z and σ coordinates. More general vertical coordinates (not just combinations of z and σ) can also be used, in particular for ocean interior, but without solving the problems associated with the representation of bathymetry. With volume penalization, we do not seek a compromise, but rather a correction to the usual coordinate systems that realistically recovers continuous and steep bathymetry. The Brinkman volume penalization method studied here is a modified version of the one introduced in Debreu et al.(2020) that simplifies the numerical implementation of the penalization, increases robustness and improves its computational performance for realistic long-term simulations, while preserving accuracy. We apply this penalization method to the Gulf Stream separation problem that has puzzled modelers for decades. The method improves the representation of the flow-topography interaction and achieves realistic separation of the Gulf Stream at resolutions as coarse as 1/8°. In addition, it provides a tool to separate the effect of eddy activity and topographic slope when changing grid resolution. This has never before been possible because at coarse resolution none of the usual coordinate systems can properly represent a steep continental slope. Our results show that realistic bathymetry is more important than eddy activity in ensuring realistic Gulf Stream separation, even though many recent studies tend to focus on the eddy activity. A steep slope can exert a stabilizing influence that promotes a strong mean slope current with strong inertia that helps it separate from the coast at the topographic curvature of Cape Hatteras. We anticipate that a successful topographic slope correction will be very valuable for climate models, as their current resolution is far from sufficient to represent western boundary currents (WBCs) using traditional coordinate systems. Our results suggest that a climate model with a 1/4° resolution using volume penalization — and perhaps also some parameterization of the eddy-mean flow interaction to energize the WBCs — could represent ocean circulation much more realistically than a model at the same resolution, but without volume penalization.

Distribution and predicted climatic refugia for a reef-building cold-water coral on the southeast US margin.

Climate change is reorganizing the planet's biodiversity, necessitating proactive management of species and habitats based on spatiotemporal predictions of distributions across climate scenarios. In marine settings, climatic changes will predominantly manifest via warming, ocean acidification, deoxygenation, and changes in hydrodynamics. Lophelia pertusa, the main reef-forming coral present throughout the deep Atlantic Ocean (>200 m), is particularly sensitive to such stressors with stark reductions in suitable habitat predicted to accrue by 2100 in a business-as-usual scenario. However, with new occurrence data for this species along with higher-resolution bathymetry and climate data, it may be possible to locate further climatic refugia. Here, we synthesize new and published biogeographic, geomorphological, and climatic data to build ensemble, multi-scale habitat suitability models for L. pertusa on the continental margin of the southeast United States (SEUS). We then project these models in two timepoints (2050, 2100) and four climate change scenarios to characterize the occurrence probability of this critical cold-water coral (CWC) habitat now and in the future. Our models reveal the extent of reef habitat in the SEUS and corroborate it as the largest currently known essentially continuous CWC reef province on earth, and also predict abundance of L. pertusa to identify key areas, including those outside areas currently protected from bottom-contact fishing. Drastic reductions in L. pertusa climatic suitability index emerged primarily after 2050 and were concentrated at the shallower end (<~550 m) of the regional distribution under the Gulf Stream main axis. Our results thus suggest a depth-driven climate refuge effect where deeper, cooler reef sites experience lesser declines. The strength of this effect increases with climate scenario severity. Taken together, our study has implications for the regional and global management of this species, portending changes in the biodiversity reliant on CWC habitats and the critical ecosystem services they provide.

The Atmospheric Response to Meridional Shifts of the Gulf Stream SST Front and Its Dependence on Model Resolution

Abstract The Gulf Stream (GS) plays a key role in shaping the North Atlantic climate. Moreover, the associated sea surface temperature (SST) front undergoes interannual-to-decadal variability that is thought to force significant atmospheric circulation anomalies. However, general circulation models do not accurately reproduce the atmospheric response to SST front variability as estimated from observations. In this work we analyze the atmospheric response to the GS SST front (GSF) shifts in a multimodel ensemble of atmosphere-only simulations forced with observed SSTs (1950–2014). The atmospheric response is found to be resolution dependent. Only the high-resolution simulations produce a wintertime response similar to observed anomalies. More specifically, (i) analysis of the atmospheric thermodynamic balance close to the GSF showed that the anomalous diabatic heating associated to the GSF displacement is mainly balanced by vertical motion and by meridional transient eddy heat transport (not the case for low-resolution models), while (ii) the large-scale response includes a meridional shift of the North Atlantic eddy-driven jet and storm track homodirectional to the GSF displacement. This atmospheric response is accompanied by changes in low-level baroclinicity close to and north of the GSF, resulting from the oceanic forcing and the zonal atmospheric circulation anomalies respectively. The low-level baroclinicity anomalies lead to changes in baroclinic eddy activity and, ultimately, in the jet via eddy–mean flow interaction. Considering the two-way nature of air–sea interactions, using historical atmosphere-only simulations is a powerful way to isolate the impact of realistic oceanic variability on the atmosphere. Our results suggest that interannual-to-decadal predictability may be higher than what low-resolution models currently indicate.