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1.
Analysis of a quality-controlled database of Gulf Stream warm-core rings (WCRs) between 75° and 50°W during 1978–1999 demonstrates a significant correlation between WCR occurrences and variations in large-scale atmospheric forcing related to the state of the North Atlantic Oscillation (NAO). The mechanisms for linking the NAO with the rate of WCR occurrences are two-fold: (1) the influence of the NAO on Gulf Stream (GS) position, which could affect the interaction of the Gulf Stream with the New England Seamounts chain and thus allow for a higher/lower number of WCR occurrences; (2) the NAO-induced eddy kinetic energy (EKE) variability in the Gulf Stream region (GSR), which is indicative of the baroclinic instability processes necessary for WCR formation. Variability in GS movement is studied by analyzing annual mean positions of the Gulf Stream North Wall obtained from satellite-derived sea surface temperature (SST) frontal charts. Response of GSR EKE to fluctuations in the state of the NAO is examined with a numerical simulation of the North Atlantic basin from 1980–1999. The North Atlantic basin is simulated using a 1/6°-resolution eddy-resolving Regional Ocean Modeling System (ROMS) model that spins up with Southampton Oceanography Center (SOC) ocean-atmosphere atlas-derived atmospheric forcing fields. Model-derived EKE estimates are observed to be in good agreement with TOPEX/Poseidon altimeter-based EKE estimates as well as with results from other modeling studies for the North Atlantic basin. We suggest that lateral movement of the GS may not be the primary mechanism causing variation in the rate of WCR occurrences, because GS position is observed to respond at a lag of one year, whereas annual rates of WCR occurrences respond at 0-year lag to the NAO. Based on results from numerical simulations of the North Atlantic basin, adjustment to NAO-induced wind forcing is seen to impact the GSR EKE intensity and possibly the related baroclinic instability structure of the GS at 0-year lag. These results suggest that NAO-induced interannual variability in GSR EKE is the most likely mechanism affecting WCR occurrences. Numerical simulations show that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, possibly resulting in higher (lower) occurrences of WCRs.  相似文献   

2.
Philip Sura 《Ocean Dynamics》2010,60(1):155-170
This paper examines the physics of observed non-Gaussian sea surface temperature (SST) anomaly variability in the Gulf Stream system in a recently developed stochastic framework. It is first shown from a new high-resolution observational data set that the Gulf Stream system is very clearly visible as a band of negative skewness all the way from Florida, over Cape Hatteras, to the central North Atlantic. To get an idea about the detailed non-Gaussian variability along the Gulf Stream, probability density functions are calculated at several locations. One important observational result of this study is that the non-Gaussian tails of SST variability in the Gulf Stream system follow a power-law distribution. The study then shows that the observed non-Gaussianity is consistent with stochastic advection of SST anomalies in an idealized zonal current. In addition, stochastic advection is compatible with the observed northward eddy heat flux in the Gulf Stream, providing a new dynamical view at the heat balance in strong currents.  相似文献   

3.
Summary The annual mean distribution of the surface stress curl over the Northern Hemisphere has been estimated from the horizontal vorticity advection in the atmosphere by using the upper-wind statistics as published byCrutcher [2]3). The results are used to compute the wind-driven mass transport (Sverdrup transport) in North Atlantic and North Pacific. The calculated intensity of the Gulf Stream is largest at the latitude 35°N, where a mass transport of 45×106 tons sec–1 is obtained; for the maximum intensity of the Kuroshio current a value of 60×106 tons sec–1 is obtained.Research supported in part by the Section of Atmospheric Sciences, National Science Foundation, Grant GP-2561.The research for this study was started by the author at the Department of Meteorology and Oceanography, The University of Michigan, Ann Arbor, Mich.  相似文献   

4.
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5.
Abstract

This is a study of the influence of bottom topography of an ocean basin on the wind‐driven, barotropic ocean circulation. A detailed investigation is made of the role of vorticity transfer to the ocean bottom in the presence of varying topography. It is shown that the wind‐driven gyre over the topography of the North Atlantic has a transport in the western part of the basin only half of that obtained in an ocean of constant depth.  相似文献   

6.
Abstract

Applying a mixing-length calculation to potential vorticity rather than to momentum a new type of lateral friction appears in the oceanic mass transport equations. This friction is evaluated for the special case of horizontally homogeneous, quasi-geostrophic turbulence. The main effect is a westward force arising from the so-called β-term. This produces an additional southward interior transport and a strengthening of the western boundary current. A turbulent exchange coefficient KH = 108 em2s?1 is sufficient to give a Gulf Stream transport twice that obtained by the classical Sverdrup model.  相似文献   

7.
Abstract

An analysis is presented of the propagation of barotropic non-divergent oscillations along the western side of an ocean basin along which the persistent circulation in the basin is strongly intensified and laterally sheared. Because the Rossby number of a western boundary current is near unity, the properties of these waves are strongly affected by the steady circulation pattern. It is shown that for relatively long wavelengths, these waves can travel along the shelf in both directions; however, for a small range of short wavelengths they can only propagate northward and are unstable. Along the southeastern coast of North America, the unstable waves have wavelengths of order 150 km and periods of order 10 days. However, these waves can become stable oscillations in the deeper water northeast of Cape Hatteras. These oscillations are a possible explanation of the initiation of Gulf Stream meanders along the continental rise.  相似文献   

8.
A three-dimensional mesoscale numerical model is used to investigate mesoscale circulation over a Gulf Stream filament. Two numerical experiments are performed with different initial uniform ambient wind speeds (U=0.1 m s–1, 3.5 m s–1 and 7 m s–1) for a typical winter day. It is found that for both low and moderate winds, a closed mesoscale circulation forms over the Gulf Stream filament. When the Gulf Stream filament was removed, the model did not predict a mesoscale circulation. The modeled circulation over the filament is in agreement with the observations, suggesting that the atmospheric circulations over the filaments may be an important mechanism in the U.S. East Coast cyclogenesis.  相似文献   

9.

We present a binned annual product (BINS) of sea surface temperature (SST), sea surface salinity (SSS), and sea surface density (SSD) observations for 1896–2015 of the subpolar North Atlantic between 40° N and 70° N, mostly excluding the shelf areas. The product of bin averages over spatial scales on the order of 200 to 500 km, reproducing most of the interannual variability in different time series covering at least the last three decades or of the along-track ship monitoring. Comparisons with other SSS and SST gridded products available since 1950 suggest that BINS captures the large decadal to multidecadal variability. Comparison with the HadSST3 SST product since 1896 also indicates that the decadal and multidecadal variability is usually well-reproduced, with small differences in long-term trends or in areas with marginal data coverage in either of the two products. Outside of the Labrador Sea and Greenland margins, interannual variability is rather similar in different seasons. Variability at periods longer than 15 years is a large part of the total interannual variability, both for SST and SSS, except possibly in the south-western part of the domain. Variability in SST and SSS increases towards the west, with the contribution of salinity variability to density dominating that of temperature in the western Atlantic, except close to the Gulf Stream and North Atlantic Current in the southwest area. Weaker variability and larger relative temperature contributions to density changes are found in the eastern part of the gyre and south of Iceland.

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10.
Abstract

Analysis of a two-layer, flat-bottom, steady-wind driven, eddy-resolving general circulation model reveals a distinct separation in frequency of baroclinic and barotropic motion in the region distant from the model Gulf Stream. The far-field motions at periods less (greater) than about 100 days are predominantly barotropic (baroclinic), unlike the near-field, eddy-generating, free-jet region which contains barotropic and baroclinic energy throughout the modei frequency range. The far-field barotropic energy produces a peak in the model sea-level spectra between 25 and 50 days with a magnitude comparable to energy levels observed in spectra of sea level from oceanic island tide gauges. The far-field barotropic motion is clearly composed of large-scale, resonant, barotropic normal modes drive by mesoscale activity of the turbulent, free-jet region. Oceanic mesoscale turbulence may therefore provide for planetary normal modes an excitation mechanism distinct from atmospheric forcing. The open-ocean, barotropic, model response is very similar to that of a fluctuating-wind driven model, which suggests that atmospheric and intrinsic forcing of mid-ocean eddies may be of comparable importance.  相似文献   

11.
Eddy momentum fluxes, i.e. Reynold stresses, are computed for the latitude bands of the Gulf Stream and Kuroshio extensions using 13 years of data from the merged satellite altimeter product of Le Traon et al. The spatial pattern and amplitude of the fluxes is remarkably similar to that found by Ducet and Le Traon using the 5 years of data that were available to them. In addition to updating the work of Ducet and Le Traon, we provide new insight into the role played by the underlying variable bottom topography, both for determining the structure of the eddy momentum fluxes seen in the satellite data and for influencing the way these fluxes feedback on the mean flow. While there is no clear evidence that eddies locally flux momentum into the eastward jets of the Gulf Stream and Kuroshio extensions, a clearer picture emerges after zonally integrating across each of the North Atlantic and North Pacific basins. We argue that the eddy momentum fluxes do indeed drive significant transport, a conclusion supported by preliminary results from a 3-D model calculation. We also present evidence that in the North Pacific, the Reynolds stresses are important for driving the recirculation gyres associated with the Kuroshio extension, taking advantage of new data from both observations and high-resolution model simulations.  相似文献   

12.
Abstract

An attractive explanation for the observed spatial growth of the Gulf Stream meanders is that the meanders are spatially growing unstable waves. The results of a calculation based on a simple two-layer model of baroclinically unstable flow presented here support this idea. The model is a familiar one with the energy for the growth of the meander perturbations coming from the potential energy available in the geostrophic tilt of the interface between the two layers due to their velocity shear. In order to distinguish between spatial and temporal growth, it IS necessary to assume that the meanders are generated in a localized region, or equivalently, that the meanders are upstream disturbances which are amplified as they enter a region of unstable flow. This assumption is implemented mathematically through the use of a Green's function which governs the propagation of the meanders. Analysis of the spatial and temporal characteristics of the Green's function leads to a criterion which must he satisfied if the meanders arc to grow spatially. This criterion is that the mean flow velocity must be sufficiently greater than the velocity shear, Um > √2 Us, in order to have spatial growth. This simply means that the growing meanders must be washed downstream faster than they spread upstream, or equivalently the spatial growth is due to downstream advection of growing disturbances. The actual Gulf Stream flow is in fair agreement with this criterion.  相似文献   

13.

The time‐dependent meandering in a thin baroclinic jet over bottom topography is discussed in the quasi‐geostrophic approximation. The motion of the axis of the jet is taken to be vertically coherent and the axis itself is defined as inextensible. The motion is examined from a frame of reference moving with the axis but fixed at an arbitrary longitude in terms of an open ocean spatial initial value problem. The velocities of the axis and of the jet are quasi‐geostrophic, and vorticity conservation for the first non‐geostrophic components constrains the evolution of the axis and gives a path equation. The spatial linearized stability problem is studied and the jet is found to be baroclinically unstable to path disturbances of sufficiently high frequency which amplify downstream. An exact solution is obtained to the nonlinear path equation over a flat bottom with no ß‐effect. The evolution of the path of these unstable meanders is such that the path closes itself and forms rings (at which point the analysis breaks down). It is proposed that the baroclinic jet processes studied here are fundamental to the dynamics of Gulf Stream meandering and isolated eddy production.  相似文献   

14.
It is known that dissipative adjustment must occur in the cross-equatorial dynamics of a deep western boundary current (DWBC) that is in planetary-geostrophic balance away from the equator. Theoretical modelling and numerical simulations suggest that the dissipative zones correspond to “small” isolated zonally-elongated regions within the trough and crest of a nonlinear stationary equatorial planetary wave that is formed as the DWBC flows eastward along the equator. An internal frictional boundary layer theory is advanced to describe the leading order structure of the DWBC in the dissipative regions, which asymptotically matches with the large scale inviscid flow characteristics in the equatorial region.  相似文献   

15.
Abstract

Laboratory experiments and analysis of shallow water equations in a rotating fluid show that channel flow is governed by the ratio of the width of the channel to the Rossby radius of deformation R= √[g&Delta;ρH/ρf 2]. Flows through narrow ocean openings exhibit blocking and clear evidence of hydraulic control. These imply that formulae can be derived for width, volume flux, and velocity scales of the currents. A new version of the constant potential vorticity problem is solved, and it is shown to predict volume flux within 22% of the zero potential vorticity results. Next a systematic method of predicting volume flux through ocean passages is described. Some examples are given from the Denmark Straits overflow and the flow of Antarctic Bottom Water into the western Atlantic Ocean. Two-layer flows and counter-flows with rotation in a narrow passage, the so-called lock exchange flow problem, duplicate flows at a number of important straits and openings to bays. A potential vorticity formulation is reviewed. The flows in the mouths of various bays such as Funka Bay in Hokkaido, Japan, Spencer Gulf in South Australia, and Chesapeake Bay in the United States has R < width of the mouth, and the two currents are separated by a front. The width of the front and the density difference can be predicted with good results.  相似文献   

16.
Abstract

Unstable waves in a western boundary current are investigated in a full three-dimensional, numerical model. A numerical integration is carried out which traces the evolution of a growing wave on an initially uniform current with vertical shear. As indicated in earlier analytic studies based on simpler 2-layer models (Orlanski, 1969) the current is baroclinically unstable for the observed parameter range of the Gulf Stream.

Large meanders of the jet in the western boundary current are noticeable within 10 days. Finite amplitude effects, which can be investigated by the numerical model, reduce the growth rate of the disturbance by nearly an order of magnitude compared to linear theory. Comparison with observations indicate that the meanders of the Florida Current between Miami and Hatteras are probably baroclinically unstable waves.  相似文献   

17.
Abstract

One of the central unsolved theoretical problems of the large scale ocean circulation is concerned with explaining the very large transports measured in western boundary currents such as the Gulf Stream and the Kuroshio. The only theory up to now that can explain the size of these transports is that of non-linear recirculation in which the advective terms in the momentum equations became important near the western boundary. In this paper an alternative explanation is suggested. When bottom topography and baroclinic effects are included in a wind-driven ocean model it is shown that the western boundary current can have a transport larger than that predicted from the wind stress distribution even when the nonlinear advective terms are ignored. The explanation lies in the presence of pressure torques associated with bottom topography which can contribute to the vorticity balance in the same sense as the wind stress curl.

Three numerical experiments have been carried out to explore the nature of this process using a three dimensional numerical model. The first calculation is done for a baroclinic ocean of constant depth, the second for a homogeneous ocean with an idealized continental slope topography, and the third for a baroclinic ocean with the same continental slope topography. The nature of the vorticity balance and of the circulation around closed paths is examined in each case, and it is shown that bottom pressure torques lead to enhanced transport in the western boundary current only for the baroclinic case with variable depth.  相似文献   

18.
Recent oceanographic observations and a retrospective analysis of nutrients and hydrography over the past five decades have revealed that the principal source of nutrients to the Gulf of Maine, the deep, nutrient-rich continental slope waters that enter at depth through the Northeast Channel, may have become less important to the Gulf's nutrient load. Since the 1970s, the deeper waters in the interior Gulf of Maine (>100 m) have become fresher and cooler, with lower nitrate (NO3) but higher silicate (Si(OH)4) concentrations. Prior to this decade, nitrate concentrations in the Gulf normally exceeded silicate by 4–5 μM, but now silicate and nitrate are nearly equal. These changes only partially correspond with that expected from deep slope water fluxes correlated with the North Atlantic Oscillation, and are opposite to patterns in freshwater discharges from the major rivers in the region. We suggest that accelerated melting in the Arctic and concomitant freshening of the Labrador Sea in recent decades have likely increased the equatorward baroclinic transport of the inner limb of the Labrador Current that flows over the broad continental shelf from the Grand Banks of Newfoundland to the Gulf of Maine. That current system now brings a greater fraction of colder and fresher deep shelf waters into the Gulf than warmer and saltier offshore slope waters which were previously thought to dominate the flux of nutrients. Those deep shelf waters reflect nitrate losses from sediment denitrification and silicate accumulations from rivers and in situ regeneration, which together are altering the nutrient regime and potentially the structure of the planktonic ecosystem.  相似文献   

19.
3/He4He measurements at two stations in the Atlantic show that the deep water (> 2 km) contains far less excess3He than our previous measurements have shown for the Pacific Ocean. The3/He4He ratio anomaly (relative to atmospheric3/He4He) is approximately 5% for the deep Atlantic compared to about 20% for the deep Pacific. The North Atlantic3He profile shows much more structure than the South Atlantic profile, with maxima observed at 500 m, 1900 m, and 3200 m. The maxima at 500 m and 1900 m are probably due to in situ tritium decay, whereas the 3200 m maximum cannot be due to tritium, and is probably due to leakage of3He into the Atlantic water from the mantle. It seems significant that maxima in the trace elements Cu, Zn and Fe have also been observed at 3200 m at this station by Brewer, Spencer and Robertson.  相似文献   

20.
Tal Ezer 《Ocean Dynamics》2018,68(10):1259-1272
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.  相似文献   

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