Showing papers in "Journal of Physical Oceanography in 2004"
TL;DR: In this paper, temperature and sea level variability within the Indonesian seas and southeast Indian Ocean are described based on expendable bathythermograph deployments along volunteer merchant shipping lines under way since 1983.
Abstract: Temperature and sea level variability within the Indonesian seas and southeast Indian Ocean are described based on expendable bathythermograph deployments along volunteer merchant shipping lines under way since 1983. These data resolve variability at time scales ranging from the intraseasonal to the interannual. A lagged partial regression technique reveals that anomalies from a mean seasonal cycle of temperature and sea level for seasonal to interannual time scales can be largely understood in terms of free Kelvin and Rossby waves generated by remote zonal winds along the equator of the Indian and Pacific Oceans, with local wind forcing appearing to play a minor role. About 60%–90% of sea level variability and 70% of thermocline temperature variability can be accounted for in this way. Variations in zonal Pacific equatorial winds force a response along the Arafura/ Australia shelf break through Pacific equatorial Rossby waves exciting coastally trapped waves off the western tip of New Guinea, wh...
300 citations
TL;DR: In this article, it is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide.
Abstract: Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May–June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ϵ, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slope's corrugations or internal tide generation at the shelf break was found at this site.
246 citations
TL;DR: In this paper, the effect of breaking waves on ocean surface temperatures and surface boundary layer deepening is investigated, and the modification of the Mellor-Yamada turbulence closure model by Craig and Banner and others to include surface wave breaking energetics reduces summertime surface temperatures when the surface layer is relatively shallow.
Abstract: The effect of breaking waves on ocean surface temperatures and surface boundary layer deepening is investigated. The modification of the Mellor‐Yamada turbulence closure model by Craig and Banner and others to include surface wave breaking energetics reduces summertime surface temperatures when the surface layer is relatively shallow. The effect of the Charnock constant in the relevant drag coefficient relation is also studied.
229 citations
TL;DR: A climatology of the winter mixed layer in the North Pacific Ocean was constructed using hydrographic data from historical archives and recent observational programs, including the World Ocean Circulation Experiment as mentioned in this paper.
Abstract: A climatology of the winter mixed layer in the North Pacific Ocean was constructed using hydrographic data from historical archives and recent observational programs, including the World Ocean Circulation Experiment. The main aim was to provide better knowledge about source areas of upper water masses. The authors have endeavored to preserve water properties near the frontal regions by keeping the smoothing scale as small as possible. The resulting climatology shows considerable differences in the mixed layer depth and its water properties from those derived from the World Ocean Atlas (WOA). Maps of the potential vorticity field of the North Pacific pycnocline are presented using the isopycnally averaged climatology, HydroBase. Three distinct lateral minima of potential vorticity are identified as Subtropical Mode Water (STMW), Central Mode Water (CMW), and Eastern Subtropical Mode Water (ESTMW), in the western, central, and eastern parts of the subtropical gyre, respectively. The HydroBase isopy...
215 citations
TL;DR: The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions was investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral flow to the momentum budget within the estuary.
Abstract: The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions is investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary. For all model runs, lateral circulation is about 4 times as strong during flood tides as during ebbs. This flood–ebb asymmetry is due to a feedback between the lateral circulation and the along-channel tidal currents, as well as to time-varying stratification over a tidal cycle. As the stratification is increased, the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself. When rotation is included, a time-dependent, cross-channel Ekman circulation is driven, and the tidally averaged, bottom lateral circulation is enhanced toward the right bank (when looking toward the ocean in the Northern Hemisphere). This asymmetry...
210 citations
TL;DR: In this article, satellite-tracked drifters were used to detect seasonal currents entering the South China Sea from the Philippine Sea through the Luzon Strait, with ensemble mean speeds of 0.7 ± 0.4 m s−1 and daily mean westward speeds that can exceed 1.65 m s −1.
Abstract: Velocity observations near the surface made with Argos satellite-tracked drifters between 1989 and 2002 provide evidence of seasonal currents entering the South China Sea from the Philippine Sea through the Luzon Strait. The drifters cross the strait and reach the interior of the South China Sea only between October and January, with ensemble mean speeds of 0.7 ± 0.4 m s−1 and daily mean westward speeds that can exceed 1.65 m s−1. The majority of the drifters that continued to reside in the South China Sea made the entry within a westward current system located at ∼20°N that crossed the prevailing northward Kuroshio path. In other seasons, the drifters looped across the strait within the Kuroshio and exited along the south coast of Taiwan. During one intrusion event, satellite altimeters indicated that, directly west of the strait, anticyclonic and cyclonic eddies resided, respectively, north and south of the entering drifter track. The surface currents measured by the crossing drifters were much...
200 citations
TL;DR: In this paper, a three-axis pulse-to-pulse coherent acoustic Doppler profiler and acoustic resonators were used to reveal the turbulence and bubble field beneath breaking waves in the open ocean at wind speeds up to 14 m s−1.
Abstract: Observations with a three-axis pulse-to-pulse coherent acoustic Doppler profiler and acoustic resonators reveal the turbulence and bubble field beneath breaking waves in the open ocean at wind speeds up to 14 m s−1. About 55%–80% of velocity wavenumber spectra, calculated with Hilbert spectral analysis based on empirical mode decomposition, are consistent with an inertial subrange. Time series of turbulent kinetic energy dissipation at approximately 1 m beneath the free surface and 1-Hz sampling rate are obtained. High turbulence levels with dissipation rates more than four orders larger than the background dissipation are linked to wave breaking. Initial dissipation levels beneath breaking waves yield the Hinze scale of the maximum bubble size aH ≅ 2 × 10−3 m. Turbulence induced by discrete breaking events was observed to decay as e ∝ tn, where n = −4.3 is close to the theoretical value for isotropic turbulence (−17/4). In the crest region above the mean waterline, dissipation increases as e(z) ...
181 citations
TL;DR: In this article, the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed, and the authors combine heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system.
Abstract: The properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed. The net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin. Heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current. The objectives of this study are to understand better what controls the density of waters formed within the marginal sea, the temperature of the outflowing waters, the amount of downwelling, and the mechanisms of heat transport within the marginal sea. The approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system. The theory compares well to a series of eddy-resolving primitive equation model calculations. The downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy-driven region on the offshore edge of the boundary current. For most high-latitude regions, the horizontal gyre is expected to transport more heat than does the overturning gyre.
174 citations
TL;DR: In this paper, the variation of the solitary wave parameters can be described analytically through an asymptotic description as a slowly varying solitary wave, which possesses a soliton-like form with varying amplitude and phase.
Abstract: Internal solitary waves transform as they propagate shoreward over the continental shelf into the coastal zone, from a combination of the horizontal variability of the oceanic hydrology (density and current stratification) and the variable depth. If this background environment varies sufficiently slowly in comparison with an individual solitary wave, then that wave possesses a soliton-like form with varying amplitude and phase. This stage is studied in detail in the framework of the variable-coefficient extended Korteweg–de Vries equation where the variation of the solitary wave parameters can be described analytically through an asymptotic description as a slowly varying solitary wave. Direct numerical simulation of the variable-coefficient extended Korteweg–de Vries equation is performed for several oceanic shelves (North West shelf of Australia, Malin shelf edge, and Arctic shelf) to demonstrate the applicability of the asymptotic theory. It is shown that the solitary wave may maintain its sol...
165 citations
TL;DR: In this article, the authors estimate that spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate to the local wind speed and thereby extract momentum from the wind.
Abstract: In winds approaching hurricane strength, spray droplets proliferate. Once created, these droplets accelerate to the local wind speed in 1 s or less and thereby extract momentum from the wind. Because these droplets have substantial mass, they eventually plunge back into the ocean, delivering their horizontal momentum to the surface in the form of a spray stress. Inadequate information on the production rate and size distribution of spray droplets, however, hampered previous attempts to estimate the magnitude of this spray-mediated momentum exchange. This paper therefore uses recent estimates of the spray generation function to reconsider spray's ability to alter air–sea momentum exchange. Conservation of momentum requires that spray cannot enhance the air– sea stress beyond what the large-scale flow dictates. However, spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate. For a wind of 30 m s−1, spray supports about 10% of the surfa...
160 citations
TL;DR: In this article, the mean seasonal cycle of the western boundary currents in the tropical North Pacific Ocean is studied diagnostically by combining atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a simplified numerical model incorporating geostrophy, hydrostatics, continuity, and tracer conservation.
Abstract: The mean seasonal cycle of the western boundary currents in the tropical North Pacific Ocean is studied diagnostically by combining atmospheric climatologies with drifter, satellite altimetry, and hydrographic data in the framework of a simplified numerical model incorporating geostrophy, hydrostatics, continuity, and tracer conservation. The approach enables the authors to diagnose the absolute 3D velocity field and to assess the seasonal cycle of sea surface height (SSH)/total transports. Errors are estimated by considering multiple datasets and averaging over the results of the corresponding diagnostic computations. Analysis shows that bifurcation of the North Equatorial Current (NEC) occurs at 14.3° ± 0.7°N near the Philippine coast. Meridional migration of the NEC bifurcation latitude is accompanied by quantitative changes in the partitioning of the NEC transport between the Kuroshio and Mindanao Current. In February–July, when the NEC transport is 58 ± 3 Sv (Sv ≡ 106 m3 s−1), the Kuroshio t...
TL;DR: In this article, a decade-long satellite altimeter data from the Ocean Topography Experiment (TOPEX)/Poseidon and European Remote Sensing Satellite-1 and-2 (ERS-1/2) missions are analyzed to investigate the eddy signals in the South Pacific Ocean.
Abstract: Decade-long satellite altimeter data from the Ocean Topography Experiment (TOPEX)/Poseidon and European Remote Sensing Satellite-1 and-2 (ERS-1/2) missions are analyzed to investigate the eddy signals in the South Pacific Ocean. High–eddy kinetic energy (EKE) bands with well-defined annual cycles are detected along the eastward-flowing surface currents of the South Tropical Countercurrent (STCC) between 21°–29°S and the South Equatorial Countercurrent (SECC) centered near 9°S. Overriding the westward-flowing South Equatorial Current (SEC), the STCC layer has the sign of its mean potential vorticity gradient opposite to that in the SEC layer, subjecting the vertically sheared STCC–SEC system to baroclinic instability. In austral winter, the STCC–SEC system is baroclinically more unstable than in other seasons because of the large vertical shear and weak stratification. This seasonal variation in the intensity of baroclinic instability is responsible for the seasonal modulation of the STCC's EKE fi...
TL;DR: In this article, a density-driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system, and the behavior of the dense fluid descending the slope has been investigated by systematically varying four parameters: the rotation rate, the bottom slope, the flow rate of the densities of dense fluid and the density of the fluid.
Abstract: A density-driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. The behavior of the dense fluid descending the slope has been investigated by systematically varying four parameters: the rotation rate, the bottom slope, the flow rate of the dense fluid, and the density of the dense fluid. Over a wide range of parameter values, the following three flow types were found: a laminar regime in which the dense current had a constant thickness behind the head, a wave regime in which wavelike disturbances appeared on the interface between the dense and fresh fluids, and an eddy regime in which periodic formation of cyclonic eddies in the fresh overlying ambient fluid was observed. All of the experiments revealed that increasing the slope angle and the density of the bottom fluid allowed the flow to evolve from the laminar to the wave regime. Furthermore, increasing rotation rate induced the formation of eddies. A theoretical solution for the downslope velocity field has been found using a steady-state model. Comparison between the theoretical and experimental downslope velocities gave good agreement. The wave regime was observed to occur for values of the Froude number greater than 1. The laminar regime was found for values of the Froude number less than 1. The amount of mixing between the dense and the ambient fluids was measured. Mixing increased significantly when passing from the laminar to the wave regime, that is, with increasing Froude number. Good agreement between the amount of mixing observed in the ocean and in the laboratory experiments is encouraging and makes the waves observed in the present experiments a possible candidate for the mixing observed during oceanic dense current overflows.
TL;DR: In this paper, an effective horizontal diffusivity parameterizing all up-estuary salt flux is calculated as a function of riverflow: results show that Willapa's volume-integrated salt balance is almost always far from equilibrium.
Abstract: Results from 3 yr of hydrographic time series are shown for Willapa Bay, Washington, a macrotidal, partially mixed estuary whose river and ocean end members are both highly variable. Fluctuating ocean conditions— alternations between wind-driven upwelling and downwelling, and intrusions of the buoyant Columbia River plume—are shown to force order-of-magnitude changes in salinity gradients on the event (2‐10 day) scale. An effective horizontal diffusivity parameterizing all up-estuary salt flux is calculated as a function of riverflow: results show that Willapa’s volume-integrated salt balance is almost always far from equilibrium. At very high riverflows (the top 15% of observations) the estuary loses salt, on average, while at all other riverflow levels it gains salt. Under summer, low-riverflow conditions, in fact, the effective diffusivity K is large enough to drive a net increase in salinity that is 3‐6 times the seaward, river-driven salt flux. This diffusion process is amplified, not damped, by increased tidal forcing, contrary to the expectation for baroclinic exchange. Furthermore, K varies along the length of the estuary as ;5% of the rms tidal velocity times channel width, a scaling consistent with density-independent stirring by tidal residuals. To summarize Willapa’s event- and seasonal-scale variability, a simple diagnostic parameter space for unsteady estuarine salt balances is presented, a generalization from the Hansen and Rattray steady-state scheme.
TL;DR: In this paper, a large eddy simulation (LES) of the ocean mixed layer was performed in which both wave breaking and Langmuir circulation are realized, and the effects of wave breaking were found to be mainly limited to the near-surface zone of the upper few meters.
Abstract: Large eddy simulation (LES) of the ocean mixed layer was performed in which both wave breaking and Langmuir circulation are realized. Wave breaking was represented by random forcing consistent with the observed near-surface turbulence, and Langmuir circulation was realized by the Craig–Leibovich vortex force. High- resolution simulations were carried out using parallel computing with or without each contribution, wave breaking and Langmuir circulation, with an aim to clarify their respective roles in the ocean mixed layer. The effects of wave breaking were found to be mainly limited to the near-surface zone of the upper few meters. Langmuir circulations below it are not significantly modified, although they become somewhat weakened and less coherent. Under the influence of wave breaking, however, the turbulence production in the upper-ocean mixed layer becomes dominated by the turbulent kinetic energy flux, contrary to the case of the atmospheric boundary layer where it is dominated by shear prod...
TL;DR: In this article, the authors investigated the origin, pathway, and destination of water occupying the surface layer of the eastern equatorial Pacific Ocean (Nino-3 region; 5 8S-58N, 1508-908W).
Abstract: The nature of subtropical-tropical water mass exchange in the Pacific Ocean is investigated, focusing on the origin, pathway, and destination of water occupying the surface layer of the eastern equatorial Pacific Ocean (Nino-3 region; 5 8S-58N, 1508-908W). Simulated passive tracers and their adjoint are employed to explicitly follow the circulation of specific water masses accounting for advective and diffusive effects and their time variabilities. The evolution of the forward passive tracer and adjoint passive tracer can be identified as describing where the tracer-tagged water mass goes and from where it comes, respectively. Over 10 years on average, water mass of the Nino-3 region can be traced back to eastern subtropical thermocline waters of the Northern (27%) and Southern Hemispheres (39%). The Nino-3 water subsequently returns to these subtropical latitudes in the upper ocean. In contrast to the hypothesized ''subtropical cell,'' however, this circulation is an open circuit with water returning to the western regions of the two hemispheres (subtropical gyres) and to the Indian Ocean, instead of returning to its origins. The representative transit time scale from the subtropics to the Tropics is 10- 15 yr. Temporal variability causes the tropical circulation inferred from a time-mean state to differ significantly from the average circulation. In particular, stirring due to nonseasonal, intra-annual variability significantly enhances the transport magnitude of the so-called interior pathways relative to that of the circuitous low-latitude western boundary pathways. Such short-circuit in the subtropical-tropical exchange may help better to explain tracer distributions, such as the observed midbasin tritium maximum in the equatorial Pacific Ocean. Significant differences in circulation pathways are also identified that are associated with El Nino and La Nina events. The strength of the subtropical-tropical water mass exchange is estimated to have weakened during the 1990s.
TL;DR: In this article, an idealized model is used to study the restratification of the Labrador Sea after deep convection, with emphasis on the role of boundary current eddies shed near the west coast of Greenland.
Abstract: An idealized model is used to study the restratification of the Labrador Sea after deep convection, with emphasis on the role of boundary current eddies shed near the west coast of Greenland. The boundary current eddies carry warm, buoyant Irminger Current water into the Labrador Sea interior. For a realistic end-of-winter state, it is shown that these Irminger Current eddies are efficient in restratifying the convected water mass in the interior of the Labrador Sea. In addition, it is demonstrated that Irminger Current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the Labrador Sea.
TL;DR: In this article, the plausibility of mesoscale eddy generation through local baroclinic instability of weak midocean gyre flows was examined using a statistically steady, two-layer quasigeostrophic turbulence model driven by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction.
Abstract: This paper examines the plausibility of mesoscale eddy generation through local baroclinic instability of weak midocean gyre flows. The main tool is a statistically steady, two-layer quasigeostrophic turbulence model driven by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction. A wide range of friction strengths is investigated. In the weakly damped limit, flow is nearly barotropic, and the horizontal length scale of barotropic energy increases with decreasing friction, consistent with previous studies. The strongly damped limit, explored here for the first time, is equivalent barotropic (lower-layer velocities are nearly zero) and features an increase in the horizontal scale of potential energy with increasing friction. Current-meter data suggest that midocean eddies lie between the barotropic and equivalent barotropic limits. In accord with this suggestion, the moderately damped regime of the model compares well to observations of eddy amp...
TL;DR: In this paper, a mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes.
Abstract: The current system east of the Grand Banks was intensely observed by World Ocean Circulation Experiment (WOCE) array ACM-6 during 1993–95 with eight moorings, reaching about 500 km out from the shelf edge and covering the water column from about 400-m depth to the bottom. More recently, a reduced array by the Institut fur Meerskunde (IfM) at Kiel, Germany, of four moorings was deployed during 1999–2001, focusing on the deep-water flow near the western continental slope. Both sets of moored time series, each about 22 months long, are combined here for a mean current boundary section, and both arrays are analyzed for the variability of currents and transports. A mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes. The offshore part of the combined section is dominated by the deep-reaching North Atlantic Current (NAC) with currents still at 10 cm s−1 near the bottom and a total northward transport of about 140 Sv (Sv ≡ 106 m3 s−1), with the details depending on the method of surface extrapolation used. The mean flow along the western boundary was southward with the section-mean North Atlantic Deep Water outflow determined to be 12 Sv below the σθ = 27.74 kg m−3 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below σθ = 27.74 kg m−3, resulting in a large net northward flow in the western part of the basin. From watermass signatures it is concluded that the deep NAC is not a direct recirculation of DWBC water masses. Transport time series for the DWBC variability are derived for both arrays. The variance is concentrated in the period range from 2 weeks to 2 months, but there are also variations at interannual and longer periods, with much of the DWBC variability being related to fluctuations and meandering of the NAC. A significant annual cycle is not recognizable in the combined current and transport time series of both arrays. The moored array results are compared with other evidence on deep outflow and recirculation, including recent models of different types and complexity.
TL;DR: In this article, the role of the Southern Ocean in global climate is examined using three simulations with a coupled model employing geometries different only at the location of the Drake Passage (DP), and the results of three main experiments are examined: 1) a simulation with DP closed, 2) an experiment with DP at a shallow (690 m) depth, and 3) a realistic DP experiment.
Abstract: The role of the Southern Ocean in global climate is examined using three simulations with a coupled model employing geometries different only at the location of Drake Passage (DP). The results of three main experiments are examined: 1) a simulation with DP closed, 2) an experiment with DP at a shallow (690 m) depth, and 3) a realistic DP experiment. The climate with DP closed is characterized by warmer Southern Hemisphere surface air temperature (SAT), little Antarctic ice, and no North Atlantic Deep Water (NADW) overturn. On opening the DP to a shallow depth of 690 m there is an increase in Antarctic sea ice and a cooling of the Southern Hemisphere but still no North Atlantic overturn. On fully opening the DP, the climate is mostly similar in the Southern Hemisphere to DP at 690 m, but the model now simulates NADW formation and a warming in the Northern Hemisphere. This suggests the North Atlantic thermohaline circulation depends not only on the existence of a DP throughflow, but also on the dep...
TL;DR: In this paper, a simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°45°N, 40°75°W) during the period from November 1992 to December 1999.
Abstract: A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40°–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in t...
TL;DR: In this paper, a simple theory is proposed for steady, two-dimensional, wind-driven coastal upwelling that relates the dynamics and the structure of the cross-shelf circulation to the stratification, bathymetry, and wind stress.
Abstract: A simple theory is proposed for steady, two-dimensional, wind-driven coastal upwelling that relates the dynamics and the structure of the cross-shelf circulation to the stratification, bathymetry, and wind stress. The new element is an estimate of the nonlinear cross-shelf momentum flux divergence due to the wind-driven cross-shelf circulation acting on the vertically sheared geostrophic alongshelf flow. The theory predicts that the magnitude of the cross-shelf momentum flux divergence relative to the wind stress depends on the Burger number S = αN/f, where α is the bottom slope, N is the buoyancy frequency, and f is the Coriolis parameter. For S ≪ 1 (weak stratification), the cross-shelf momentum flux divergence is small, the bottom stress balances the wind stress, and the onshore return flow is primarily in the bottom boundary layer. For S ≈ 1 or larger (strong stratification), the cross-shelf momentum flux divergence balances the wind stress, the bottom stress is small, and the onshore return ...
TL;DR: In this article, the deep circulation and related transports of the southern Labrador Sea are determined from direct current observations from ship surveys and a moored current-meter array, and measurements covered a time span from summer 1997 to 1999 and show a well-defined deep boundary current extending approximately out to the 3300-m depth contour and weak reverse currents farther offshore.
Abstract: The deep circulation and related transports of the southern Labrador Sea are determined from direct current observations from ship surveys and a moored current-meter array. The measurements covered a time span from summer 1997 to 1999 and show a well-defined deep boundary current extending approximately out to the 3300-m depth contour and weak reverse currents farther offshore. The flow has a strong barotropic component, and significant baroclinic flow is only found in the shallow Labrador Current at the shelf break and associated with a deep core of Denmark Strait Overflow Water. The total deep-water transport below σΘ = 27.74 kg m−3 was 26 ± 5 Sv (Sv ≡ 106 m3 s−1) comprising Labrador Sea Water (LSW), Gibbs Fracture Zone Water (GFZW), and Denmark Strait Overflow Water (DSOW). Intraseasonal variability of the flow and transport was high, ranging from 15 to 35 Sv, and the annual means differed by 17%. A seasonal cycle is confined to the shallow Labrador Current; in its deeper part, where the mean flow is still strong, no obvious seasonality could be detected. The transport of the interior anticyclonic recirculation was estimated from lowered acoustic Doppler current profiler stations and geostrophy, yielding about 9 Sv. Thus, the net deep-water outflow from the Labrador Sea was about 17 Sv. The baroclinic transport of GFZW and DSOW referenced to the depth of the isopycnal σΘ = 27.80 kg m−3 is only about one-third of the total transport in these layers. Longer-term variations of the total transports are not represented well by the baroclinic contribution.
TL;DR: In this article, a two-dimensional theory of entrainment of a buoyant coastal plume to wind-induced upwelling was developed, where the primary assumption is that competition between wind-driven vertical mixing and lateral buoyancy forcing in the region where the isopycnals slope upward to intersect the surface results in continual entraining at the offshore edge of the plume.
Abstract: To better understand the response of a buoyant coastal plume to wind-induced upwelling, a two-dimensional theory is developed that includes entrainment. The primary assumption is that competition between wind-driven vertical mixing and lateral buoyancy forcing in the region where the isopycnals slope upward to intersect the surface results in continual entrainment at the offshore edge of the plume. The theory provides estimates of the buoyant plume characteristics and offshore displacement as a function of time t, given the wind stress, the characteristics of the buoyant plume prior to the onset of the wind forcing, and a critical value for the bulk Richardson number (Ric). The theory predicts that, for t ≡ t/ts, the plume density anomaly decreases as (1 + t)−1, the thickness increases as (1 + t)1/3, the width increases as (1 + t)2/3, and the plume average entrainment rate decreases as (1 + t)−2/3. Here ts = 2Ao/(RicUE) is the time for entrainment to double the cross-sectional area of the pl...
TL;DR: In this paper, the authors investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999.
Abstract: Month-long simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception and the Santa Ba...
TL;DR: In this paper, the authors examined the northward intrusion of Antarctic Intermediate Water (AAIW) using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean.
Abstract: The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertica...
TL;DR: The time dependence of the local relation between sea surface temperature (SST) and thermocline depth in the central and eastern equatorial Pacific Ocean is analyzed for the period 1990-99, using subsurface temperature measurements from the Tropical Atmosphere-Ocean Array/Triangle Trans-Ocean Buoy Network (TAO/TRITON) buoy array as mentioned in this paper.
Abstract: The time dependence of the local relation between sea surface temperature (SST) and thermocline depth in the central and eastern equatorial Pacific Ocean is analyzed for the period 1990‐99, using subsurface temperature measurements from the Tropical Atmosphere‐Ocean Array/Triangle Trans-Ocean Buoy Network (TAO/TRITON) buoy array. Thermocline depth anomalies lead SST anomalies in time, with a longitude-dependent delay ranging from 2 weeks in the eastern Pacific to 1 year in the central Pacific. The lagged correlation between thermocline depth and SST is strong, ranging from r . 0.9 in the east to r 0.6 at 1708W. Time-lagged correlations between thermocline depth and subsurface temperature anomalies indicate vertical advection of temperature anomalies from the thermocline to the surface in the eastern Pacific. The measurements are compared with the results of forced OGCM and linear model experiments. Using model results, it is shown that the delay between thermocline depth and SST is caused mainly by upwelling and mixing between 1408 and 908W. Between 1708E and 1408W the delay has a different explanation: thermocline depth anomalies travel to the eastern Pacific, where upwelling creates SST anomalies that in turn cause anomalous wind in the central Pacific. SST is then influenced by these wind anomalies.
TL;DR: In this paper, the surface ageostrophic velocity was calculated using the classical formula of the Ekman spiral, with the depth determined from an empirical formula, and the total amount of energy input over the global oceans for subinertial frequency was estimated as 2.4 TW averaged over a period from 1997 to 2002, or 2.3 TW averaged from 1948 to 2002.
Abstract: Wind stress energy input through the surface ageostrophic currents is studied. The surface ageostrophic velocity is calculated using the classical formula of the Ekman spiral, with the Ekman depth determined from an empirical formula. The total amount of energy input over the global oceans for subinertial frequency is estimated as 2.4 TW averaged over a period from 1997 to 2002, or 2.3 TW averaged over a period from 1948 to 2002, based on daily wind stress data from NCEP‐NCAR. Thus, in addition to the energy input to the near inertial waves of 0.5‐0.7 TW reported by Alford and by Watanabe and Hibiya, the total energy input to the Ekman layer is estimated as 3 TW. This input is concentrated primarily over the Southern Ocean and the storm track in both the North Pacific and North Atlantic Oceans.
TL;DR: Very high turbulent dissipation rates (above 5 1024 Wk g 21) were observed in the nonlinear internal lee waves that form each tide over a sill in Knight Inlet, British Columbia as discussed by the authors.
Abstract: Very high turbulent dissipation rates (above «5 1024 Wk g 21) were observed in the nonlinear internal lee waves that form each tide over a sill in Knight Inlet, British Columbia. This turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions. Away from the sill, turbulent dissipation was significantly lower ( «5 1027 to «5 1028 Wk g 21). Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately twothirds of the barotropic energy loss radiated away as internal waves, while the remaining one-third was lost to processes near the sill. Observed dissipation in the water column does not account for the near-sill losses, but energy lost to vortex shedding and near-bottom turbulence, though not measured, could be large enough to close the energy budget.
TL;DR: In this article, a combination of laser and radar aboard an aircraft is used to directly measure long gravity wave surface tilting simultaneously with nadir-viewing microwave backscatter from the sea surface.
Abstract: Combination of laser and radar aboard an aircraft is used to directly measure long gravity wave surface tilting simultaneously with nadir-viewing microwave backscatter from the sea surface. The presented dataset is extensive, encompassing varied wind conditions over coastal and open-ocean wave regimes. Laser-derived slope statistics and Ka-band (36 GHz) radar backscatter are detailed separately to document their respective variations versus near-surface wind speed. The slope statistics, measured for λ > 1–2 m, show good agreement with Cox and Munk's oil-slickened sea measurements. A notable exception is elevated distribution peakedness and an observed wind dependence in this likely proxy for nonlinear wave–wave interactions. Aircraft Ka-band radar data nearly mimic Ku-band satellite altimeter observations in their mean wind dependence. The present calibrated radar data, along with relevant observational and theoretical studies, suggest a large (−5 dB) bias in previous Ka-band results. Next, wave-...