Showing papers in "Journal of Physical Oceanography in 1997"

An Elastic–Viscous–Plastic Model for Sea Ice Dynamics

Abstract: The standard model for sea ice dynamics treats the ice pack as a visco‐plastic material that flows plastically under typical stress conditions but behaves as a linear viscous fluid where strain rates are small and the ice becomes nearly rigid. Because of large viscosities in these regions, implicit numerical methods are necessary for time steps larger than a few seconds. Current solution methods for these equations use iterative relaxation methods, which are time consuming, scale poorly with mesh resolution, and are not well adapted to parallel computation. To remedy this, the authors developed and tested two separate methods. First, by demonstrating that the viscous‐plastic rheology can be represented by a symmetric, negative definite matrix operator, the much faster and better behaved preconditioned conjugate gradient method was implemented. Second, realizing that only the response of the ice on timescales associated with wind forcing need be accurately resolved, the model was modified so that it reduces to the viscous‐plastic model at these timescales, whereas at shorter timescales the adjustment process takes place by a numerically more efficient elastic wave mechanism. This modification leads to a fully explicit numerical scheme that further improves the model’s computational efficiency and is a great advantage for implementations on parallel machines. Furthermore, it is observed that the standard viscous‐plastic model has poor dynamic response to forcing on a daily timescale, given the standard time step (1 day) used by the ice modeling community. In contrast, the explicit discretization of the elastic wave mechanism allows the elastic‐viscous‐plastic model to capture the ice response to variations in the imposed stress more accurately. Thus, the elastic‐viscous‐plastic model provides more accurate results for shorter timescales associated with physical forcing, reproduces viscous‐plastic model behavior on longer timescales, and is computationally more efficient overall.

Global Characteristics of Ocean Variability Estimated from Regional TOPEX/POSEIDON Altimeter Measurements

Abstract: Three years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to study characteristics of eddy variability over the World Ocean. The nature of the variability and its spatial structure are characterized in terms of the geographical distribution of eddy energy, as simple approximations of observed regional frequency and wavenumber spectra, and in terms of associated eddy time and space scales of sea surface height (SSH) variability and geostrophic velocity. Emphasis is put on summarizing characteristics typical for dynamically distinct regions of the World Ocean. This effort results in an attempt to describe the observed ocean variability in terms of universal spectral relations that depend only on few mean flow parameters such as the first-mode Rossby radius of deformation. Regional peculiarities follow naturally as deviations from the fundamental frequency and wavenumber spectra presented here. Frequency spectra of both variables can be summarized by three basic types represen...

A Neutral Density Variable for the World’s Oceans

Abstract: The use of density surfaces in the analysis of oceanographic data and in models of the ocean circulation is widespread. The present best method of fitting these isopycnal surfaces to hydrographic data is based on a linked sequence of potential density surfaces referred to a discrete set of reference pressures. This method is both time consuming and cumbersome in its implementation. In this paper the authors introduce a new density variable, neutral density γn, which is a continuous analog of these discretely referenced potential density surfaces. The level surfaces of γn form neutral surfaces, which are the most appropriate surfaces within which an ocean model’s calculations should be performed or analyzed. The authors have developed a computational algorithm for evaluating γn from a given hydrographic observation so that the formation of neutral density surfaces requires a simple call to a computational function. Neutral density is of necessity not only a function of the three state variables: s...

The Adriatic Sea General Circulation. Part I: Air–Sea Interactions and Water Mass Structure

Abstract: A comprehensive historical hydrographic dataset for the overall Adriatic Sea basin is analyzed in order to define the open ocean seasonal climatology of the basin. The authors also define the regional climatological seasons computing the average monthly values of heat fluxes and heat storage from a variety of atmospheric datasets. The long term mean surface heat balance corresponds to a heat loss of 19–22 W m−2. Thus, in steady state, the Adriatic should import about the same amount of heat from the northern Ionian Sea through the Otranto Channel. The freshwater balance of the Adriatic Sea is defined by computing the average monthly values of evaporation, precipitation, and river runoff, obtaining an annual average gain of 1.14 m. The distribution of heat marks the difference between eastern and western Adriatic areas, showing the winter heat losses in different parts of the basin. Climatological water masses are defined for three regions of the Adriatic: (i) the northern Adriatic where seasonal ...

Specification of eddy transfer coefficients in coarse resolution ocean circulation models

Abstract: Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging from horizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams (GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of a transformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-dimensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective ‘‘chimney’’ where the baroclinic zone is created by differential surface cooling; 2) spindown of a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a form for the horizontal transfer coefficient given by 2 fM 2 2 k 5 a l 5 a l, N ˇRi where Ri 5 f2N2/M4 is the large-scale Richardson number and f is the Coriolis parameter; M2 and N2 are measures of the horizontal and vertical stratification of the large-scale flow, l measures the width of the baroclinic zone, and a is a constant of proportionality. In the very different scenarios studied here the authors find a to be a ‘‘universal’’ constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the atmosphere. The magnitude of the implied k, however, varies from 300 m2 s21 in the chimney to 2000 m2 s21 in the wind-driven channel.

Sensitivity to Surface Forcing and Boundary Layer Mixing in a Global Ocean Model: Annual-Mean Climatology

Abstract: The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more accurate bottom topography are investigated in a coarse-resolution, global oceanic general circulation model. In contrast to forcing with prescribed fluxes, the bulk forcing utilizes the evolving model sea surface temperatures and monthly atmospheric fields based on reanalyses by the National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is governed by a nonlocal K-profile parameterization (KPP) and is matched to parameterizations of mixing in the interior. The KPP scheme is designed to represent well both convective and wind-driven entrainment. The near- equilibrium solutions are compared to a baseline experiment in which the surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, except where there is either conve...

The Adriatic Sea General Circulation. Part II: Baroclinic Circulation Structure

Abstract: In the second part of the paper dedicated to the Adriatic Sea general circulation, the horizontal structure of the hydrographic parameters and dissolved oxygen fields is described on a seasonal timescale. Maps of temperature and salinity climatological fields reveal the enhanced seasonal variability of the Adriatic Sea, which at the surface is associated with the major dilution effects of river runoff. The density and derived dynamic height fields show for the first time the baroclinic geostrophic structure of the general circulation. Winter is dominated by compensation effects between temperature and salinity fronts along the western coastline. The resulting baroclinic circulation is weak and suggests the presence of barotropic current components not accessible by the dataset. Spring and summer seasons have the smallest spatial scales in the temperature and salinity fields and stronger subbasin-scale gyres and current systems, which have been classified in a schematic representation of the circu...

A Simple Theory for the Fate of Buoyant Coastal Discharges

Abstract: A simple theory that predicts the vertical structure and offshore spreading of a localized buoyant inflow onto a continental shelf is formulated. The theory is based on two competing mechanisms that move the buoyant fluid offshore: 1) the radial spread of the lighter water over the ambient water, being deflected by the Coriolis force and producing an anticyclonic cyclostrophic plume, and 2) offshore transport of buoyant water in the frictional bottom boundary layer that moves the entire plume offshore while maintaining contact with the bottom. The surface expression of the cyclostrophic plume moves offshore a distance ys = 2(3g′h0 + υ 2i)/(2g′h0 + υ 2i)1/2f,where g′ is reduced gravity based on the inflow density anomaly, h0 is the inflow depth, υi is the inflow velocity, and f is the Coriolis parameter. The plume remains attached to the bottom to a depth given by hb = (2Lυih0f/g′)1/2,where L is the inflow width. Both scales are based solely on parameters of the buoyant inflow at its source. There...

Kinematics of the Pacific Equatorial Undercurrent: An Eulerian and Lagrangian Approach from GCM Results

Abstract: Three-dimensional monthly velocity fields from an ocean general circulation model are used to study the annual mean mass balance of the Pacific Equatorial Undercurrent (EUC). Eulerian diagnostics are used to evaluate the various meridional, vertical, and zonal mass fluxes related to the EUC. There are several distinct regimes along the equator, showing clear asymmetries between the western and eastern parts of the basin, and between the northern and southern edges of the EUC. Meridional fluxes are decomposed into pure Ekman divergence and geostrophic convergence, and it is shown that the asymmetries are mainly related to the spatial structure of the Ekman divergence, and thus to that of the trade winds. Lagrangian calculations are used to evaluate accurately the mass transfers between various sections of the EUC and between the EUC domain and the Tropics. The authors show that geostrophic convergence only ventilates the upper layers of the EUC and that the EUC really is a tongue of water flowing ...

The Speed of Observed and Theoretical Long Extratropical Planetary Waves

Abstract: Planetary or Rossby waves are the predominant way in which the ocean adjusts on long (year to decade) timescales. The motion of long planetary waves is westward, at speeds $ 1c m s 2 1. Until recently, very few experimental investigations of such waves were possible because of scarce data. The advent of satellite altimetry has changed the situation considerably. Curiously, the speeds of planetary waves observed by TOPEX/Poseidon are mainly faster than those given by standard linear theory. This paper examines why this should be. It is argued that the major changes to the unperturbed wave speed will be caused by the presence of baroclinic east‐ west mean flows, which modify the potential vorticity gradient. Long linear perturbations to such flow satisfy a simple eigenvalue problem (related directly to standard quasigeostrophic theory). Solutions are mostly real, though a few are complex. In simple situations approximate solutions can be obtained analytically. Using archive data, the global problem is treated. Phase speeds similar to those observed are found in most areas, although in the Southern Hemisphere an underestimate of speed by the theory remains. Thus, the presence of baroclinic mean flow is sufficient to account for the majority of the observed speeds. It is shown that phase speed changes are produced mainly by (vertical) mode-2 east‐west velocities, with mode-1 having little or no effect. Inclusion of the mean barotropic flow from a global eddy-admitting model makes only a small modification to the fit with observations; whether the fit is improved is equivocal.

The Vertical Partition of Oceanic Horizontal Kinetic Energy

Abstract: To produce an interpretation of the surface kinetic energy as measured by altimeters, a survey is made of the vertical structure of kinetic energy profiles in a large number of globally distributed long current meter records. Although the data are geographically confined primarily to a latitude band in the North Pacific, to the North Atlantic, and to a few moorings in the South Atlantic, the results show, generally speaking, that most regions are dominated by the barotropic and first baroclinic modes. Because of the near-surface intensification of baroclinic modes altimeters primarily reflect the first baroclinic mode, and thus the motion of the main thermocline. There is good quantitative agreement, with a few exceptions, with estimates of the surface kinetic energy obtained from the TOPEX/POSEIDON altimeter and from vertical extrapolations to the surface of the mooring profiles. These results are consistent with previous suggestions that barotropic models have little skill in depicting variability as seen in the altimeter data. An EOF analysis is shown to produce fictitious mode coupling unless the dynamical modes have very different energy levels.

Mixing and Spreading of the Mediterranean Outflow

Abstract: Hydrographic and current profiler data taken during the 1988 Gulf of Cadiz Expedition have been analyzed to diagnose the mixing, spreading, and descent of the Mediterranean outflow. The θ–S properties and the thickness and width of the outflow were similar to that seen in earlier surveys. The transport of pure Mediterranean Water (i.e., water with S ≥ 38.4 psu) was estimated to be about 0.4 × 106 m3 s−1, which is lower than historical estimates—most of which were indirect—but comparable to other recent estimates made from direct velocity observations. The outflow transport estimated at the west end of the Strait of Gibraltar was about 0.7 × 106 m3 s−1 of mixed water, and the transport increased to about 1.9 × 106 m3 s−1 within the eastern Gulf of Cadiz. This increase in transport occurred by entrainment of fresher North Atlantic Central Water, and the salinity anomaly of the outflow was consequently reduced. The velocity-weighted salinity decreased to 36.7 psu within 60 km of the strait and decre...

A method for improved representation of dense water spreading over topography in geopotential-coordinate models

Abstract: A new technique for incorporating processes of near-bottom tracer transport over sloping topography in geopotential-coordinate ocean circulation models is presented. It is based on a “hybrid” approach, coupling a simple terrain-following bottom boundary layer model to the standard ocean general circulation model. Tests in simplified configurations show enhanced downslope spreading of passive and active (density) tracers. The method is helpful for both coarse-resolution global climate studies and biogeochemical coastal modeling studies.

A Simple Model of the Decadal Response of the Ocean to Stochastic Wind Forcing

Abstract: A simple linear model is used to estimate the decadal response of the extratropical ocean to wind stress forcing, assuming a flat bottom, a mean state at rest, and no dissipation. The barotropic fields are governed by a time-dependent Sverdrup balance, the baroclinic ones by the long Rossby wave equation. The ocean is bounded by a coast in the east and a radiation condition is used in the west. At each frequency, the baroclinic response consists of a forced response plus a Rossby wave generated at the eastern boundary. For zonally independent forcing, the response propagates westward at twice the Rossby phase speed. The wind stress is assumed to be stochastic with a white frequency spectrum, so the model represents the continuous excitation of the ocean interior by the weather fluctuations. The model predicts the shape and level of the frequency spectra of the oceanic pressure field and their variation with longitude and latitude. The baroclinic response is spread over a continuum of frequencies,...

The Air–Sea Momentum Flux in Conditions of Wind Sea and Swell

Abstract: During the Surface Wave Dynamics Experiment, direct measurements of momentum, heat, and water vapor fluxes were obtained from a mast on the foredeck of a SWATH (small water-plane area, twin hull) ship in deep water off the state of Virginia. Directional wave spectra were obtained simultaneously from a 6- or 3-wire wave-staff array mounted at the bow of the ship. One hundred and twenty-six 17-minute runs of flux and wave data obtained with the ship steaming slowly into the wind are examined for the effects of the relative direction of the wind sea and background swell on the momentum transfer. The adequacy of the inertial dissipation method, which depends on the high-frequency turbulent fluctuations for evaluating the wind stress, is also examined for any effects of swell. The results show that the presence of counter- and cross-swells can result in drag coefficients that are much larger than the value for a pure wind sea. The eddy correlation and inertial dissipation methods for measuring wind st...

Parameterization of Quasigeostrophic Eddies in Primitive Equation Ocean Models.

Abstract: A parameterization of mesoscale eddy fluxes in the ocean should be consistent with the fact that the ocean interior is nearly adiabatic. Gent and McWilliams have described a framework in which this can be approximated in z-coordinate primitive equation models by incorporating the effects of eddies on the buoyancy field through an eddy-induced velocity. It is also natural to base a parameterization on the simple picture of the mixing of potential vorticity in the interior and the mixing of buoyancy at the surface. The authors discuss the various constraints imposed by these two requirements and attempt to clarify the appropriate boundary conditions on the eddy-induced velocities at the surface. Quasigeostrophic theory is used as a guide to the simplest way of satisfying these constraints.

A Nonlinear Model of Internal Tide Transformation on the Australian North West Shelf

Abstract: A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.

Sediment Resuspension and Mixing by Resonantly Generated Internal Solitary Waves

Abstract: The observation of internal solitary waves (ISWs) propagating upstream along a strongly stratified bottom layer on the California shelf is reported. An increased concentration of particulates in the water column accompanies the passage of these ISW packets. The estimated local Richardson number in the bottom vicinity is around 1/4, and a vertical coefficient of eddy diffusivity of order 1022 m2 s21 is associated with the upstream propagating leading ISW. The leading ISW gave rise to reversed flow in an 8-m layer above the bottom. It is argued that the upstream propagating ISWs were generated by resonant flow over bottom topography. Internal waves generated in this way seem to be frequent in the record of a month-long experiment. Model results suggest that the ISWs can carry up to 73% of such generated long wave energy. The ocean conditions at the site are similar to those of other coastal sites, which suggests that the phenomenon described here may be common.

Tidally Driven Vorticity, Diurnal Shear, and Turbulence atop Fieberling Seamount

Abstract: Fine- and microstructure profiles collected over Fieberling Seamount at 32°26′N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides. The forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain: (i) an anticyclonic vortex cap of core relative vorticity − 0.5f, (ii) diurnal fluctuations of ±15 cm s−1 amplitude and 200-m vertical wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity κe ≅ 10 × 10−4 m2 s−1. The vortex cannot be explained by Taylor–Proudman dynamics because of its − 0.3fN2 negative potential vorticity anomaly. The ±0.3f fortnightly cycle in the vortex’s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit plain. They also exhibit a fortnightly cycle in their amplitude, pointing to...

Simulation of Barotropic and Baroclinic Tides off Northern British Columbia

Abstract: The tidal response of northern British Columbia coastal waters is studied through simulations with a three-dimensional, prognostic, primitive equation model. The model is forced at the boundaries with the leading semidiurnal and diurnal constituents and experiments with stratified and homogeneous fluid are compared. The barotropic response shows good agreement with previously published studies of tides in the region. A comparison with tide gauge measurements indicates that average relative rms differences between observations and the model surface elevation field are less than 5% for the largest constituents. An internal tide is generated in cases where the model is initialized with a vertical stratification. Diagnostic calculations of the baroclinic energy flux are used to identify regions of generation and propagation of internal tidal energy. With a representative summer stratification, the integrated offshore flux is about 0.5 gigawatts, higher than previously estimated from theoretical model...

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment

Abstract: Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15‐20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3‐7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Boundary Mixing and the Dynamics of Three-Dimensional Thermohaline Circulations

Abstract: Boundary mixing is implemented in an ocean general circulation model such that the vertical mixing coefficient ky is nonzero only near side boundaries and in convection regions. The model is used in a highly idealized configuration with no wind forcing and very nearly fixed surface density to investigate the three-dimensional dynamics of the thermohaline circulation. For ky 5 20 3 1024 m2 s21 and lower, the meridional overturning strength to great accuracy is proportional to ; meridional heat transport is proportional to . The circulation 2/3 1/2 kk y y patterns resemble those from runs with uniform vertical mixing, but vertical motion is entirely confined to the boundary regions. Near the western boundary, there is upwelling everywhere. Near the eastern boundary, there is a consistent pattern of downwelling above upwelling, with downwelling reaching deeper at high latitudes; this pattern is explained by convection and vertical advective‐diffusive balance underneath. For ky 5 30 3 1024 m2 s21 and higher, no steady solutions have been found; the meridional overturning oscillates on a timescale of about 25 years. A time-averaged thermally direct overturning cell is not supported dynamically because convection extends longitudinally across the entire basin, and upwelling near the western boundary does not lead to densities higher than at the eastern boundary. Assuming uniform upwelling in the west, level isopycnals near the equator, and level isopycnals along the eastern boundary south of the outcropping latitude permits the analytic determination of convection depth at the eastern wall and hence the density difference between the eastern and western walls. This difference is at most one-quarter the surface density difference between high and low latitudes, and agrees in magnitude and latitudinal dependence with the numerical experiments. Scaling arguments estimate overturning strength as of the order of 10 3 106 m3 s21 and confirm the 2/3 power dependence onky. The derivation also gives a dependence of overturning strength with latitude that agrees qualitatively with the numerical results. The scaling for the dependence of meridional heat transport on latitude agrees well with the model results; scaling for heat transport amplitude agrees less well but correctly predicts a weaker dependence on ky than maximum overturning.

Thermostad Distribution in the North Pacific Subtropical Gyre: The Central Mode Water and the Subtropical Mode Water

Abstract: The upper thermal structure of the midlatitude North Pacific is studied with the use of all the bathythermograph data compiled in the global ocean temperature and salinity profiles released by the National Oceanographic Data Center. Climatological temperature data are prepared for each 2.5° × 5° (latitude × longitude) rectangle. The upper layer of the subtropical gyre is characterized by two types of thermostads or mode waters: one in the western basin known as North Pacific subtropical mode water (NPSTMW) and the other in the central basin to be named North Pacific central mode water (NPCMW). The NPCMW thermostad lies centered around 160°W between the Kuroshio Extension and the Kuroshio bifurcation front. Its local core temperature ranges from 10° to 13°C with a somewhat zonally elongated pattern, in contrast to the more uniform core temperature at 16°–17°C of the NPSTMW thermostad lying centered at 150°–160°E south of the Kuroshio Extension. The climatological map of the wintertime mixed layer ...

The Response of a Steep-Sided, Narrow Canyon to Time-Variable Wind Forcing

Abstract: The response of a relatively narrow (∼7 km wide) and deep (∼450 m deep) steep-sided (up to 45° bottom slope) submarine canyon to strong wind forcing is explored using data from an 18-element moored array as well as CTD surveys in the vicinity of Astoria submarine canyon. The data are used to describe spatial patterns and phase relationships between lateral velocity, vertical velocity, temperature, relative and stretching vorticity, alongshelf wind, and the flow incident on the canyon. Upwelling within the canyon is simultaneous and spatially uniform to zero order, and vertical velocity is highly correlated and in phase with alongshelf wind. Vertical velocity within the canyon is not related to flow incident on the canyon except during strong upwelling. Above the canyon, temperature, rather than vertical velocity (time rate of change of temperature), is in phase with wind. Estimated vertical velocities within the canyon were as great as 50 m d−1 (upward) during upwelling and 90 m d−1 (downward) du...

Decadal Changes in the Mode Waters in the Midlatitude North Pacific

Abstract: Temporal changes in the properties of the North Pacific subtropical mode water (NPSTMW) and the North Pacific central mode water (NPCMW), which occurred around the mid-1970s, are investigated using temperature data composited for the two decades bounded by the mid-1970s: 1966–75 and 1976–85. Properties of these mode waters changed greatly after the mid-1970s. The colder NPCMW was formed and widely distributed during 1976–85. In the NPSTMW formation area, warmer water occupied the southwestern part, and colder water occupied the northeastern part during 1976–85. The cause of this change is discussed with regard to the heat flux and wind stress data. The cooling can be explained as a result of changes in surface heat flux and heat divergence in the Ekman layer, that is, a larger amount of heat released from the ocean surface and an increased southward Ekman transport of cold water due to intensification of the westerlies. In particular, the latter plays a dominant role in the observed cooling. On t...

Circulation and Low-Frequency Variability near the Chilean Coast: Remotely Forced Fluctuations during the 1991–92 El Niño

Abstract: Results are reported from the first long, recording current meter observations over the slope off Chile. These observations, at 308S during the 1991-92 El Nino event, are analyzed together with observations of currents at a local deep sea site; local wind and sea level; sea level from the Peru and Chile coasts; and wind, temperature, and currents from the equatorial Pacific. Mean poleward flow of 12 cm s21 was observed within the Peru-Chile Undercurrent over the slope. Mean flow in the depth range of Antarctic Intermediate Water was not distinguishable from zero in the presence of strong, low-frequency (LF) variability, which dominated slope currents at all depths. The strongest LF fluctuations had periods of about 50 days, but periods of 10 and about 5 days were also observed. Significant, local wind forcing of slope currents was only found in the period band 6-10 days and may be related to coastal-trapped waves in the atmosphere. Our analysis shows that free, coastal-trapped waves in the ocean, arriving from the north, dominated the LF variability over the shelf and slope off northern and central Chile during the 1991-92 El Nino event. Strong 50-day period fluctuations there started their journey about two months earlier—and 15 000 km farther up the coastal-equatorial waveguide—near the date line in the equatorial Pacific as equatorial Kelvin waves forced by westerly wind events of similar period. Upon reaching the South American coast, these waves forced coastal- trapped waves, which propagated along the Peru coast into the study region. Likewise, a scenario of equatorial- trapped waves forcing coastal-trapped waves may explain 10-day as well as 6-day and 4.5-day period coastal- trapped waves off Chile stemming from mixed Rossby-gravity and inertia-gravity waves trapped at the equator. Since the large, 50-day period, coastal-trapped waves may strongly modify coastal upwelling source water, such remotely forced waves may have a significant influence on the pelagic ecosystem off Chile, at least during El Nino events.

Modeling Sea Ice as a Granular Material, Including the Dilatancy Effect

Abstract: A dynamic sea ice model based on granular material rheology is presented. The sea ice model is coupled to both a mixed layer ocean model and a one-layer thermodynamic atmospheric model, which allows for an ice albedo feedback. Land is represented by a 6-m thick layer with a constant base temperature. A 10-year integration including both thermodynamic and dynamic effects and incorporating prescribed climatological wind stress and ocean current data was performed in order for the model to reach a stable periodic seasonal cycle. The commonly observed lead complexes, along which sliding and opening of adjacent ice floes occur in the Arctic sea ice cover, are well reproduced in this simulation. In particular, shear lines extending from the western Canadian Archipelago toward the central Arctic, often observed in winter satellite images, are present. The ice edge is well positioned both in winter and summer using this thermodynamically coupled ocean–ice–atmosphere model. The results also yield a sea ic...

Propagation and Decay of Forced and Free Baroclinic Rossby Waves in Off-Equatorial Oceans*

Abstract: Baroclinic Rossby wave motions in the off-equatorial oceans are investigated with emphasis on how eddy dissipation can influence the propagation of the height anomalies when both the forced wave response to wind in the interior ocean and the free wave response originating along the ocean’s coastal and topographic boundaries are present. By explicitly estimating the decay scale for the long baroclinic Rossby wave, the authors show that the forced wave patterns at all off-equatorial regions appear to propagate westward at 2 cr, where cr is the phase speed of the long baroclinic wave. The presence of the boundary-generated, free Rossby waves in the low latitudes, however, reinforces the 1cr phase propagation in the combined height anomaly fields. Toward higher latitudes, this reinforcement weakens as the boundary-generated, free waves become highly dissipative; as a result, the forced wave motion becomes more dominant, which works to increase the apparent phase speed up to 2cr. In the subpolar regions where the annual baroclinic Rossby waves become evanescent, an apparent phase speed higher than 2cr is observed when an annual, standing wave response and a propagating wave response with an interannual frequency coexist. Stronger annual and interannual wind fluctuations over the Southern Hemisphere subpolar regions than over the Northern Hemisphere subpolar regions suggest that this coupling, and the phase speed higher than 2cr, are more likely to be detected in the Southern Hemisphere subpolar oceans.

Mixed Layer Deepening Due to Langmuir Circulation

Abstract: The interaction between wind-driven Langmuir circulation and preexisting stratification is examined in order to elucidate its role in the deepening of the ocean surface mixed layer. For linear stratification, a numerical model suggests that Langmuir cells initially engulf water and create a homogeneous surface layer. The depth h of this layer can be understood in terms of a Froude number Fr = wdn/(Nh), where wdn is the maximum downwelling velocity generated by Langmuir circulation in homogeneous water and N is the buoyancy frequency. Numerical results show that Fr is a constant ≈ 0.6. Using computed values of wdn, this implies that the rapid mixed layer deepening stops at h = cu*/N in which u* is the water friction velocity and the coefficient c is about 10 for fully developed seas. Alternatively, the deepening is arrested when the buoyancy jump Δb at the mixed layer base reaches about 50u2*/h. The above formula, compared with the Price, Weller, and Pinkel value of 0.65 for the bulk Richar...

Internal Waves in the Strait of Messina Studied by a Numerical Model and Synthetic Aperture Radar Images from the ERS 1/2 Satellites

Abstract: A new numerical two-layer model is presented, which describes the generation of internal tidal bores and their disintegration into internal solitary waves in the Strait of Messina. This model is used to explain observations made by the synthetic aperture radar (SAR) from the European Remote Sensing satellites ERS 1 and ERS 2. The analysis of available ERS 1/2 SAR data of the Strait of Messina and adjacent sea areas show that 1) northward as well as southward propagating internal waves are generated in the Strait of Messina, 2) southward propagating internal waves are observed more frequently than northward propagating internal waves, 3) sea surface manifestations of southward as well as northward propagating internal waves are stronger during periods where a strong seasonal thermocline is known to be present, 4) southward propagating internal bores are released from the sill between 1 and 5 hours after maximum northward tidal flow and northward propagating internal bores are released between 2 and 6 hours after maximum southward tidal flow, and 5) the spatial separation between the first two internal solitary waves of southward propagating wave trains is smaller in the period from July to September than in the period from October to June. The numerical two-layer model is a composite of two models consisting of 1) a hydrostatic “generation model,” which describes the dynamics of the water masses in the region close to the strait’s sill, where internal bores are generated, and 2) a weakly nonhydrostatic “propagation model,” which describes the dynamics of the water masses outside of the sill region where internal bores may disintegrate into internal solitary waves. Due to a technique for movable lateral boundaries, the generation model is capable of simulating the dynamics of a lower layer that may intersect the bottom topography. The proposed generation–propagation model depends on one space variable only, but it retains several features of a fully three-dimensional model by including a realistic channel depth and a realistic channel width. It is driven by semidiurnal tidal oscillations of the sea level at the two open boundaries of the model domain. Numerical simulations elucidate several observed characteristics of the internal wave field in the Strait of Messina, such as north–south asymmetry, times of release of the internal bores from the strait’s sill, propagation speeds, and spatial separations between the first two solitary waves of internal wave trains.