Gulf Stream Flow Research Articles

On Energy‐Aware Hybrid Models

AbstractThis study proposes deterministic and stochastic energy‐aware hybrid models that should enable simulations of idealized and primitive‐equations Geophysical Fluid Dynamics (GFD) models at low resolutions without compromising on quality compared with high‐resolution runs. Such hybrid models bridge the data‐driven and physics‐driven modeling paradigms by combining regional stability and classical GFD models at low resolution that cannot reproduce high‐resolution reference flow features (large‐scale flows and small‐scale vortices) which are, however, resolved. Hybrid models use an energy‐aware correction of advection velocity and extra forcing compensating for the drift of the low‐resolution model away from the reference phase space. The main advantages of hybrid models are that they allow for physics‐driven flow recombination within the reference energy band, reproduce resolved reference flow features, and produce more accurate ensemble forecasts than their classical GFD counterparts. Hybrid models offer appealing benefits and flexibility to the modeling and forecasting communities, as they are computationally cheap and can use both numerically‐computed flows and observations from different sources. All these suggest that the hybrid approach has the potential to exploit low‐resolution models for long‐term weather forecasts and climate projections thus offering a new cost effective way of GFD modeling. The proposed hybrid approach has been tested on a three‐layer quasi‐geostrophic model for a beta‐plane Gulf Stream flow configuration. The results show that the low‐resolution hybrid model reproduces the reference flow features that are resolved on the coarse grid and also gives a more accurate ensemble forecast than the physics‐driven model.

Gulf Stream Effects on Sea Level Oscillations: Enhancing Performance of a Coastal and Estuarine Model Nested into Global Model through Modified Boundary Conditions

This study investigates the effects of the gulf stream (GS) on sea-level oscillations across various time scales and assesses the performance of a coastal and estuarine model nested within a global model in simulating these variations. It aims to improve boundary conditions to simulate sea-level oscillations more accurately by considering the influence of GS flow. An inverse correlation is observed between observed sea-level oscillation and GS flow, which becomes more pronounced over longer time scales. Using Delft3D, a high-resolution coastal and estuarine model is developed to simulate circulation dynamics in the central Indian River Lagoon (IRL), FL, and adjacent coastal areas on the Florida east coast. The model is nested into the HYCOM (Hybrid Coordinate Ocean Model), and meteorological forcings are derived from the NARR (North American Regional Reanalysis) model. The model demonstrates satisfactory performance across key parameters, including tide, salinity, water temperature, and currents. However, there remains a noticeable difference between the modeled and observed data. To address this, the model is executed with modified flow boundary conditions at eastern boundary nodes, integrating HYCOM tide, and observing low-frequency sea-level variations. The implementation of the new boundary conditions results in an improved simulation of sea-level oscillations. This study presents the conceptual framework and detailed methodologies employed in the creation of a high-resolution model tailored for estuarine and coastal areas nested into global models capable of satisfactorily simulating sea-level oscillations even when the global model does not represent GS effects.

Interdependence in Coastal Tourist Territories between Marine Litter and Immediate Tourist Zoning Density: Methodological Approach for Urban Sustainable Development

The coastal strip, characterized by the urbanization of coastal tourist territories (CTTs), has expanded over decades through civil engineering, altering the shoreline dynamics and creating artificial beaches crucial for tourism. To examine the relationship between extensive land use in CTTs for tourism and residences and the presence of marine litter, a specific parametric study was conducted along the coast of Tenerife, the largest island in the Canary Islands. Due to Tenerife’s geographical location and exposure to the descending Gulf Stream flow, the coastal waters in the selected zone experience waste impact at both local and global scales. However, the presence of marine litter deposited by ocean currents is at a micro level and falls outside the scope of this report. This study parameterised urban reality in study areas, and the presence of macro waste has been parameterised using standardised units of measurement. This enables the establishment of source measurements that will contribute to preventative measures against this type of coastal pollution. The interdependence between tourist zoning, civil seafront engineering works along the seafront, and marine litter presence in inaccessible and visible areas for tourists requires a methodology to better understand waste origin and loading areas. This knowledge is crucial for an effective local monitoring system. A quantitative overlay reading methodology has been designed in the urban setting through calculations of urban densities, while examining the waste in these areas’ immediate infralittoral flooring through the use of visual underwater extraction. Anticipating the type and quantity of waste in each area will allow for the implementation of effective awareness, promoting action for preventative and corrective measures at the urban level. The results show a direct dependence between urban density and the presence of waste, as well as an equation that makes it possible to anticipate the amount of waste according to urban density and its relational vector. There is no discontinuity between them, as each area is affected by others to the extent that they establish the parametric continuity conditions determining each field. Therefore, it is possible to relate them beyond a one-on-one relationship. This approach fosters sustainable tourism development, reducing pressure on the sea and enhancing the utilisation of tourism revenues in measures to address waste-related challenges and promotes sustainable tourism development in Europe’s coastal regions.

Inter-annual sea level change and transgression along a barrier Island coast

On the decadal time scale of coastal planning and management, relative sea-level rise is usually considered a trend or constant rate rather than a dynamic, time varying influence on coastal change. However, the coastal ocean sea level along the east coast of Florida and coastal regions further north is, to a first order, controlled at the interannual and decadal time scale by the dynamics of North Atlantic circulation and Gulf Stream (GS) flow, Every coastal region has its own distinctive annual sea-level cycle, and longer-term intra- and inter-decadal sea-level oscillations. This paper addresses the influence of the strong Florida coastal ocean sea-level signal on topographic evolution of the shoreface, coastal sediment budgets, and potential for coastal transgression at the decadal time scale.

A hyper-parameterization method for comprehensive ocean models: Advection of the image point

Idealized and comprehensive ocean models at low resolutions cannot reproduce nominally-resolved flow structures similar to those presented in the high-resolution solution. Although there are various underlying physical reasons for this, from the dynamical system point of view all these reasons manifest themselves as a low-resolution trajectory avoiding the phase space occupied by the reference solution (the high-resolution solution projected onto the coarse grid). In order to solve this problem, a set of hyper-parameterization methods has recently been proposed and successfully tested on idealized ocean models. In this work, for the first time we apply one of hyper-parameterization methods (Advection of the image point) to a comprehensive, rather than idealized, general circulation model of the North Atlantic.The results show that the hyper-parameterization method significantly outperforms the coarse-grid ocean model by reproducing both the large- and small-scale features of the Gulf Stream flow. The proposed method is much faster than even a single run of the coarse-grid ocean model, requires no modification of the model, and is easy to implement. Moreover, the method can take not only the reference solution as input data but also real measurements from different sources (drifters, weather stations, etc.), or combination of both. All this offers a great flexibility to ocean modellers working with mathematical models and/or measurements.

Atlantic Ocean Circulation as the Driving Force for the Holocene Thermal Maximum and Millennial Scale Climatic Variability

The aim of this meta-study is to provide an understanding of the Atlantic Ocean Circulation as the driving force for the Holocene Thermal Maximum (HTM) and Millennial Scale Climatic Variability. In addition to continental ice sheets, during the Great Ice Age (GIA) there was also a 900-m thick floating ice shelf (FIS) in the Arctic Ocean. Below the FIS, it was likely freshwater. It is plausible that around 11,700 BP the Gulf Stream established the present flow route. Until around 10,800 BP the Grand Banks of Newfoundland (GBN) diverted part of the Gulf Stream flow west towards the north-eastern American coast, where the HTM existed between ca. 11,600 and 10,800 BP. Soon after the Gulf Stream was able to flow over the GBN, the HTM started in Ireland ca. 10,700 BP. By ca. 10,000 BP, the Gulf Stream arrived on the coast of Norway, which diverted the outflow south through the English Channel for approximately 2,000 years. In Northern and Central Europe, the HTM existed between ca. 9,500 and 7,000 BP. Due to the melting of ice, the outflow turned brackish, and was mainly surface currents, having a temperature of around 0℃. The formation of descending salty water (DSW), which creates the suction of the Gulf Stream, was low. Between ca. 6,000 and 4,500 BP, there was Mid-Holocene cooling. Then the Arctic Ocean water was salinated by double diffusive convection (DDC). When DSW formation increased to its present-day volume, the Neoglacial period started ca. 4,500 BP. Since then, outflow has taken place mainly through cold (approximately –2℃) undercurrents. At this time, the intensive millennial scale cycling of warmer and colder periods also started. When increasingly larger areas in Arctic Ocean are covered by insulating multi-year ice (MYI), DSW formation and the suction of the Gulf Stream decrease, and vice versa. The amplitude of the cycle is approximately 750 to 800 years.

Northward shifts of Canadian late deglacial drainage routes caused a stronger and warmer Canaries Current that enabled Holocene monsoons and a savanna on the western Sahara

This paper proposes an explanation for the well-watered savanna on the presently barren western Sahara Desert during the mid-Holocene and near the ends of other earlier Canadian deglaciations. Between 7,500 and 4,000 years before present, a humidity index indicates moist conditions and a savanna in the western Sahara. During this interval, a stronger and warmer Canaries Current return flow of the Gulf Stream nullified the effect of cold water that upwells off the northwest African coast due to the westward movement of surface water by trade winds. The return flow was stronger than today because less eastward Gulf Stream flow was lost to the northward flow of the Atlantic Meridional Overturning Current. The Canaries Current was warmer than today because cold southern Canadian meltwater no longer entered the Gulf Stream, and the high latitude climate was warmer then. Similar conditions probably prevailed at the ends of many other deglaciations, which were separated by 20,000 to 70,000 years due to orbital factors and variable glaciation. The savanna connections across the Sahara would have allowed each Hominin population to evolve in the isolated Moroccan-Algerian coastal zone to extend its range into the larger Africa. The intermittent savannas could therefore have played a significant role in the evolution of the many Hominin species found in the African fossil record over the last three million years.

Global sea level reconstruction for 1900–2015 reveals regional variability in ocean dynamics and an unprecedented long weakening in the Gulf Stream flow since the 1990s

Abstract. A new monthly global sea level reconstruction for 1900–2015 was analyzed and compared with various observations to examine regional variability and trends in the ocean dynamics of the western North Atlantic Ocean and the US East Coast. Proxies of the Gulf Stream (GS) strength in the Mid-Atlantic Bight (GS-MAB) and in the South Atlantic Bight (GS-SAB) were derived from sea level differences across the GS. While decadal oscillations dominate the 116-year record, the analysis showed an unprecedented long period of weakening in the GS flow since the late 1990s. The only other period of long weakening in the record was during the 1960s–1970s, and red noise experiments showed that is very unlikely that those just occurred by chance. Ensemble empirical mode decomposition (EEMD) was used to separate oscillations at different timescales, showing that the low-frequency variability of the GS is connected to the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The recent weakening of the reconstructed GS-MAB was mostly influenced by weakening of the upper mid-ocean transport component of AMOC as observed by the RAPID measurements for 2005–2015. Comparison between the reconstructed sea level near the coast and tide gauge data for 1927–2015 showed that the reconstruction underestimated observed coastal sea level variability for timescales less than ∼5 years, but lower-frequency variability of coastal sea level was captured very well in both amplitude and phase by the reconstruction. Comparison between the GS-SAB proxy and the observed Florida Current transport for 1982–2015 also showed significant correlations for oscillations with periods longer than ∼5 years. The study demonstrated that despite the coarse horizontal resolution of the global reconstruction (1∘ × 1∘), long-term variations in regional dynamics can be captured quite well, thus making the data useful for studies of long-term variability in other regions as well.

The long-term and far-reaching impact of hurricane Dorian (2019) on the Gulf Stream and the coast

Abstract Hurricane Dorian (28-August to 6-September 2019) was one of the most powerful hurricanes ever recorded in the Atlantic Ocean; it had disastrous impact on the Bahamas, before moving along the southeastern coast of the U.S. The unusual track of Dorian followed the track of hurricane Matthew (2016)- both hurricanes moved along the Gulf Stream (GS) without making a significant landfall and both seemed to weaken the flow of the GS by almost 50%. In the case of Dorian, the transport of the Florida Current (FC) measured by the cable across the Florida Straits had dropped from 34.7 Sv (1 Sv = 106 m3/s) on 22-August before the storm, to 17.1 Sv on 4-September (the lowest recorded value since measurements started in 1982). Two questions that this study tried to answer are: 1. Did the disruption that Dorian caused to ocean currents off the Florida coast affect the large-scale Gulf Stream (GS) dynamics downstream? and 2. Was there a long-term impact on the GS flow and on coastal sea level? Satellite altimeter data showed that the signal of the hurricane's impact on reducing the GS flow near the Florida coast is seen as far as 4000 km downstream along the GS path 50 days later. This long period of a weakened GS flow can elevate coastal sea level and increase flooding in the days and weeks after offshore storms already disappeared. The observed FC transport was found to be significantly correlated with the downstream GS velocity as far as 50°W and was anti-correlated with sea level along the entire U.S. East Coast. The density and velocity anomaly created by the hurricane's cooling and mixing near the Florida coast seemed to propagate downstream with the GS flow at ~1 m/s, but slow-moving baroclinic waves with propagation speed of ~0.1 m/s were also observed along the GS path. The results of this study may have implications for the indirect impact of storms on large-scale ocean circulation, coastal processes and the response of coastal ecosystems to offshore changes.

Analysis of the changing patterns of seasonal flooding along the U.S. East Coast

Sea level rise (SLR) is causing acceleration in the frequency and duration of minor tidal flooding (often called “sunny-day” or “nuisance” flooding) along the U.S. East Coast. Those floods have a seasonal pattern that often follows the monthly mean sea level anomaly which peaks in September–October for stations between New York and south Florida. However, there are large differences between coasts: for example, over 75% of the minor floods occur during the fall in Florida, but during the spring and winter in Boston. Various data and forcing, such as tide gauge records, surface temperatures, winds, long-term tidal cycles, and the Gulf Stream flow, were analyzed to examine potential drivers and mechanisms that can contribute to the seasonal pattern of floods. The seasonal water temperature cycle, with maximum temperatures in August, could not by itself explain the seasonal sea level pattern, but two mechanisms that significantly correlate with the seasonal sea level and flooding patterns are the annual and semi-annual tidal cycles (correlation of ~ 0.97) and changes in the Gulf Stream (GS) flow (correlation of − 0.6; the GS shows a maximum decline in September–October during the period of peak flooding). The combination of the seasonal pattern of tropical storms and high coastal sea level can explain the high frequency of fall flooding along the Southeastern U.S. coasts, while winter storms have more influence on the northeastern coasts. In recent decades however, the seasonal pattern seemed to have shifted so that increased flooding is seen on the northeastern coasts during spring and summer, while in the Mid-Atlantic and southeastern coasts, a dramatic increase in flooding is seen almost exclusively during the fall. A long-term change in the mean zonal wind pattern along the coast can contribute to the recent shift in the seasonal flooding pattern. The study can help regional adaptation and resilience planning for flood-prone coastal cities and communities.

The Distinction Between the Gulf Stream and Its North Wall

AbstractDownstream of Cape Hatteras, the Gulf Stream (GS) is bounded to the north by a sharp temperature front known as the North Wall (NW). Previous studies have generally assumed that variations of the NW and GS are equivalent. Using satellite sea surface height to identify the GS and the 15 °C isotherm at 200‐m depth to represent the NW, this paper examines their similarities and differences during 1993–2016. The NW and GS are geographically close and vary similarly only to the west of 71°W. Downstream of that, they rapidly diverge—and the variances of their latitudes increase by more than a factor of 2—as the GS flows past the New England Seamounts. Evidence is presented to show that the difference in properties of the NW and the GS is related to the presence of mesoscale eddies in the region separating them.

Numerical modeling of the impact of hurricanes on ocean dynamics: sensitivity of the Gulf Stream response to storm’s track

The study is focused on the disruption that a storm can cause to ocean stratification and ocean currents in a region dominated by a western boundary current and meso-scale variability. Sensitivity experiments with a regional numerical ocean model of the US East Coast are used to simulate different hurricane tracks to study the impact on the Gulf Stream (GS) flow, surrounding waters and coastal sea level. Realistic simulations of Hurricane Matthew (October 2016) using surface wind and heat flux from NOAA’s operational coupled forecast system were compared with idealized artificial hurricanes with tracks located at different distances from the coast (~ 200–600 km). Despite the limitation of representing realistic wind patterns by an analytical formula, coastal storm surge near the hurricane was simulated quite well. The height of the coastal storm surge was found to be very sensitive to the location of the hurricane track relative to the coast, but the impact of the hurricane on the GS flow was found to be less sensitive to the exact hurricane track, though the maximum influence was when the hurricane track passed ~ 100 km east of the GS with winds over the GS opposing the current direction. Hurricanes that passed within hundreds of kilometers from the GS caused disruption in the GS dynamics and weakening in the downstream flow of the GS that can last for many days after the storm disappeared. This indirect impact of hurricanes on the GS can elevate sea level along long stretches of the coast. The impact of a hurricane on a region dominated by meso-scale variability is complex, creating unpredictable spatial changes in temperatures and currents. After the hurricane disappeared and without additional surface heating, it may take the stratification as much as 2 months to recover to pre-hurricane conditions by advection alone. This lasting impact of a storm on ocean dynamics is consistent with observations that show minor tidal coastal flooding that lasts for days after hurricanes passed offshore.

Cyclic Hominid Evolution in A Moroccan-Algerian Coastal Refuge: The Last Million Years

To explain the abundance of species of genus Homo in the fossil record of Africa south of the Sahara, the small Moroccan-Algerian coastal zone that was isolated by the barren Sahara is proposed to have been a refuge in which cyclic evolution occurred. A dry climate in combination with a small population enabled natural selection to generate new sub species or species during each climate cycle. As generalized from the last two major glaciations, in each cycle three coastal zone climates of differing aridity occurred, depending on the latitudinal zonality of high latitude Gulf Stream flow. When initially isolated with minimal zonality (strong northward North Atlantic Drift of Gulf Stream water), the coastal climate was like today’s, with warm summers and mild winter rains. Subsequently during intervals of ice sheet growth with intermediate zonality (weaker Drift), winters were colder, the climate was drier, and the environmental stresses increased. Finally, with the quite strong or complete zonality associated with Northern Hemisphere deglaciations (little or no Drift), extreme aridity often reduced the inhabitable area of the coastal zone. When each Eurasian deglaciation was completed, the isolation was probably briefly interrupted, as it was in the mid Holocene, by a well-watered savanna that developed across the Sahara. The savannas enabled each small and genetically modified population to increase and extend its range southward into the larger Africa. The pulses of evolution are directly related to glacial cycles by way of Earth’s orbital eccentricity and precession of the equinox. The intervals of coastal zone isolation usually lasted almost 22,000 years, which is the time needed for the precession of the equinox to move summer around on Earth’s orbit from one perihelion point to the next where monsoons are strong, and deglaciation and the savannas tend to occur. However, isolations as long as ~76,000 years also are found in the record because Eurasian ice sheet growth sometimes resumed before deglaciation was complete. In the last million years there may have been at least 18 pluvial savanna intervals when populations of new species or sub species of hominids would have extended their range by expanding on the savanna into the larger Africa or Eurasia. Periodic pulses of evolution of primitive hominids probably also occurred much earlier in the Pliocene with brief savannas but without large Northern glaciations. Generation of new species of hominids in the coastal zone and their injection into the larger Africa by savanna connections may therefore have been largely responsible for the abundance of genus Homo and predecessors in the fossil record and for our own Homo sapiens that we know today.

On the interaction between a hurricane, the Gulf Stream and coastal sea level

Tropical storms and hurricanes in the western North Atlantic Ocean can impact the US East Coast in several ways. Direct effects include storm surges, winds, waves, and precipitation and indirect effects include changes in ocean dynamics that consequently impact the coast. Hurricane Matthew [October, 2016] was chosen as a case study to demonstrate the interaction between an offshore storm, the Gulf Stream (GS) and coastal sea level. A regional numerical ocean model was used, to conduct sensitivity experiments with different surface forcing, using wind and heat flux data from an operational hurricane-ocean coupled forecast system. An additional experiment used the observed Florida Current (FC) transport during the hurricane as an inflow boundary condition. The experiments show that the hurricane caused a disruption in the GS flow that resulted in large spatial variations in temperatures with cooling of up to ~ 4 °C by surface heat loss, but the interaction of the winds with the GS flow also caused some local warming near fronts and eddies (relative to simulations without a hurricane). A considerable weakening of the FC transport (~ 30%) has been observed during the hurricane (a reduction of ~ 10 Sv in 3 days; 1Sv = 106 m3 s−1), so the impact of the FC was explored by the model. Unlike the abrupt and large wind-driven storm surge (up to 2 m water level change within 12 h in the South Atlantic Bight), the impact of the weakening GS on sea level is smaller but lasted for several days after the hurricane dissipated, as seen in both the model and altimeter data. These results can explain observations that show minor tidal flooding along long stretches of coasts for several days following passages of hurricanes. Further analysis showed the short-term impact of the hurricane winds on kinetic energy versus the long-term impact of the hurricane-induced mixing on potential energy, whereas several days are needed to reestablish the stratification and rebuild the strength of the GS to its pre-hurricane conditions. Understanding the interaction between storms, the Gulf Stream and coastal sea level can help to improve prediction of sea level rise and coastal flooding.

Observations and operational model simulations reveal the impact of Hurricane Matthew (2016) on the Gulf Stream and coastal sea level

In October 7–9, 2016, Hurricane Matthew moved along the southeastern coast of the U.S., causing major flooding and significant damage, even to locations farther north well away from the storm’s winds. Various observations, such as tide gauge data, cable measurements of the Florida Current (FC) transport, satellite altimeter data and high-frequency radar data, were analyzed to evaluate the impact of the storm. The data show a dramatic decline in the FC flow and increased coastal sea level along the U.S. coast. Weakening of the Gulf Stream (GS) downstream from the storm’s area contributed to high coastal sea levels farther north. Analyses of simulations of an operational hurricane-ocean coupled model reveal the disruption that the hurricane caused to the GS flow, including a decline in transport of ∼20Sv (1Sv=106m3s−1). In comparison, the observed FC reached a maximum transport of ∼40Sv before the storm on September 10 and a minimum of ∼20Sv after the storm on October 12. The hurricane impacts both the geostrophic part of the GS and the wind-driven currents, generating inertial oscillations with velocities of up to ±1ms−1. Analysis of the observed FC transport since 1982 indicated that the magnitude of the current weakening in October 2016 was quite rare (outside 3 standard deviations from the mean). Such a large FC weakening in the past occurred more often in October and November, but is extremely rare in June-August. Similar impacts on the FC from past tropical storms and hurricanes suggest that storms may contribute to seasonal and interannual variations in the FC. The results also demonstrated the extended range of coastal impacts that remote storms can cause through their influence on ocean currents.

High‐frequency internal waves and thick bottom mixed layers observed by gliders in the Gulf Stream

AbstractAutonomous underwater gliders are conducting high‐resolution surveys within the Gulf Stream along the U.S. East Coast. Glider surveys reveal two mechanisms by which energy is extracted from the Gulf Stream as it flows over the Blake Plateau, a portion of the outer continental shelf between Florida and North Carolina where bottom depths are less than 1000 m. Internal waves with vertical velocities exceeding 0.1 m s−1 and frequencies just below the local buoyancy frequency are routinely found over the Blake Plateau, particularly near the Charleston Bump, a prominent topographic feature. These waves are likely internal lee waves generated by the subinertial Gulf Stream flow over the irregular bathymetry of the outer continental shelf. Bottom mixed layers with O(100) m thickness are also frequently encountered; these thick bottom mixed layers likely form in the lee of topography due to enhanced turbulence generated by O(1) m s−1 near‐bottom flows.

On the Evaluation of Seasonal Variability of the Ocean Kinetic Energy

AbstractThe seasonal cycles of the mean kinetic energy (MKE) and eddy kinetic energy (EKE) are compared in an idealized flow as well as in a realistic simulation of the Gulf Stream (GS) region based on three commonly used definitions: orthogonal, nonorthogonal, and moving-average filtered decompositions of the kinetic energy (KE). It is shown that only the orthogonal KE decomposition can define the physically consistent MKE and EKE that precisely represents the KEs of the mean flow and eddies, respectively. The nonorthogonal KE decomposition gives rise to a residual term that contributes to the seasonal variability of the eddies, and therefore the obtained EKE is not precisely defined. The residual term is shown to exhibit more significant seasonal variability than EKE in both idealized and realistic GS flows. Neglecting its influence leads to an inaccurate evaluation of the seasonal variability of both the eddies and the total flow. The decomposition using a moving-average filter also results in a nonnegligible residual term in both idealized and realistic GS flows. This type of definition does not ensure conservation of the total KE, even if taking into account the residual term. Moreover, it is shown that the annual cycles of the three types of EKEs or MKEs have different phases and amplitudes. The local differences of the EKE cycles are very prominent in the GS off-coast domain; however, because of the spatial inhomogeneity, the area-mean differences may not be significant.

Numerical analysis of residence time distribution of solids in a bubbling fluidized bed based on the modified structure-based drag model

The residence time distribution (RTD) of solids and the fluidized structure of a bubbling fluidized bed were investigated numerically using computational fluid dynamics simulations coupled with the modified structure-based drag model. A general comparison of the simulated results with theoretical values shows reasonable agreement. As the mean residence time is increased, the RTD initial peak intensity decreases and the RTD curve tail extends farther. Numerous small peaks on the RTD curve are induced by the back-mixing and aggregation of particles, which attests to the non-uniform flow structure of the bubbling fluidized bed. The low value of t50 results in poor contact between phases, and the complete exit age of the overflow particles is much longer for back-mixed solids and those caught in dead regions. The formation of a gulf-stream flow and back-mixing for solids induces an even wider spread of RTD.

Structure, transport, and vertical coherence of the Gulf Stream from the Straits of Florida to the Southeast Newfoundland Ridge

Data from three independent and extensive field programs in the Straits of Florida, the Mid-Atlantic Bight, and near the Southeast Newfoundland Ridge are reanalyzed and compared with results from other historical studies to highlight the downstream evolution of several characteristics of the Gulf Stream's mean flow and variability. The three locations represent distinct dynamical regimes: a tightly confined jet in a channel; a freely meandering jet; and a topographically controlled jet on a boundary. Despite these differing dynamical regimes, the Gulf Stream in these areas exhibits many similarities. There are also anticipated and important differences, such as the loss of the warm core of the current by 42°N and the decrease in the cross-frontal gradient of potential vorticity as the current flows northward. As the Gulf Stream evolves it undergoes major changes in transport, both in magnitude and structure. The rate of inflow up to 60°W and outflow thereafter are generally uniform, but do exhibit some remarkable short-scale variations. As the Gulf Stream flows northward the vertical coherence of the flow changes, with the Florida Current and North Atlantic Current segments of the Gulf Stream exhibiting distinct upper and deep flows that are incoherent, while in the Mid-Atlantic Bight the Gulf Stream exhibits flows in three layers each of which tends to be incoherent with the other layers at most periods. These coherence characteristics are exhibited in both Eulerian and stream coordinates. The observed lack of vertical coherence indicates that great caution must be exercised in interpreting proxies for Gulf Stream structure and flow from vertically-limited or remote observations.

Energetics of Eddy–Mean Flow Interactions in the Gulf Stream Region

AbstractA detailed energetics analysis of the Gulf Stream (GS) and associated eddies is performed using a high-resolution multidecadal regional ocean model simulation. The energy equations for the time-mean and time-varying flows are derived as a theoretical framework for the analysis. The eddy–mean flow energy components and their conversions show complex spatial distributions. In the along-coast region, the cross-stream and cross-bump variations are seen in the eddy–mean flow energy conversions, whereas in the off-coast region, a mixed positive–negative conversion pattern is observed. The local variations of the eddy–mean flow interaction are influenced by the varying bottom topography. When considering the domain-averaged energetics, the eddy–mean flow interaction shows significant along-stream variability. Upstream of Cape Hatteras, the energy is mainly transferred from the mean flow to the eddy field through barotropic and baroclinic instabilities. Upon separating from the coast, the GS becomes highly unstable and both energy conversions intensify. When the GS flows into the off-coast region, an inverse conversion from the eddy field to the mean flow dominates the power transfer. For the entire GS region, the mean current is intrinsically unstable and transfers 28.26 GW of kinetic energy and 26.80 GW of available potential energy to the eddy field. The mesoscale eddy kinetic energy is generated by mixed barotropic and baroclinic instabilities, contributing 28.26 and 9.15 GW, respectively. Beyond directly supplying the barotropic pathway, mean kinetic energy also provides 11.55 GW of power to mean available potential energy and subsequently facilitates the baroclinic instability pathway.