Showing papers in "Journal of Physical Oceanography in 1987"

Parameter Sensitivity of Primitive Equation Ocean General Circulation Models

Abstract: Experiments with a low resolution, primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation. Emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity. Scaling arguments suggest, and the numerical experiments confirm, that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity. For small vertical diffusivity, wind forcing dominates the solution. The vertical scale of the thermocline is set by the strength of the Ekman pumping, and there is a multiple gyre circulation in the upper layers. For large vertical diffusivity, diabatic surface forcing dominates the solution. Vertical diffusion controls the vertical scale of the thermocline, and there is a single large anticyclonic gyre in the upper layers. Both the meridionally and zonally inte...

Establishment of Deep Ocean Circulation Driven by Deep-Water Production

Abstract: A linear, two-layer baroclinic model of deep circulation driven by deep water production is formulated. In distinction to the Stommel-Arons model where a uniform middepth upwelling is prescribed, the present model determines upwelling internally by inclusion of a Newtonian damping term in the continuity equation. Analytical solutions for the steady state are derived using the equatorial beta-plane approximation, which show that the coefficient of damping in the upwelling term determines the nature of the flow. With a large damping coefficient, the deep western boundary current from the deep-water source region does not cross the equator, but rather separates along it. The current reaches the eastern boundary and then flows poleward in both hemispheres as an eastern boundary current. In the limit of very weak damping, the flow spins up to the Stommel-Arons state, with the western boundary current crossing the equator and with poleward flows in the interior. Numerical experiments on the spherical c...

A Diagnostic Ice–Ocean Model

Abstract: A coupled ice–ocean model suitable for simulating ice–ocean circulation over a seasonal cycle is developed by coupling the dynamic thermodynamic sea ice model of Hibler with a multilevel baroclinic ocean model (Bryan). This model is used to investigate the effect of ocean circulation on seasonal sea ice simulations by carrying out a simulation of the Arctic, Greenland and Norwegian seas. The ocean model contains a linear term that damps the ocean's temperature and salinity towards climatology. The damping term was chosen to have a three-year relaxation time, equivalent to the adjustment time of the pack ice. No damping, however, was applied to the uppermost layer of the ocean model, which is in direct contact with the moving pack ice. This damping procedure allows seasonal and shorter time-scale variability to be simulated in the ocean, but does not allow the model to drift away from ocean climatology on longer time scales. For the standard experiment, an initial integration of five years was per...

Estuary Plumes and Fronts in Shelf Waters: A Layer Model

Abstract: A layer model that treats fronts as discontinuities is developed to study the steady state behavior of shallow estuary plumes on the continental shelf. The complete range of earth rotation effect is evaluated from small-scale or nonrotating plumes (Kelvin number equal zero) to large-scale, rotating plumes (Kelvin number equal order one). Supercritical flow is assumed in the outlet channel and the method of characteristics is used to compute the flow downstream. Nonrotating plumes have strong boundary fronts and concentrate their greatest layer depth and mass transport offshore near the front, but form no coastal current. Rotating plumes have boundary fronts that weaken soon after discharge, form a turning region where Coriolis action deflects the flow toward shore, and subsequently set up a coastal current. Soon after its formation this coastal current is bounded offshore by a strong front called the coastal front, across which the momentum balance changes from nearly inertial in the turning regi...

Sampling Turbulence in the Stratified Ocean: Statistical Consequences of Strong Intermittency

Abstract: Turbulence and turbulent mixing in the ocean are strongly intermittent in amplitude, space and time. The degree of intermittency is measured by the “intermittency factor” σ2, defined as either σ2lnϵ, the variance of the logarithm of the viscous dissipation rate ϵ, or σ2lnχ, the variance of the logarithm of the temperature dissipation rate χ. Available data suggest that the cumulative distribution functions of ϵ and χ in stratified layers are approximately lognormal with large σ2 values in the range 3–7. Departures from lognormality are remarkably similar to those for Monte Carlo generated lognormal distributions contaminated with simulated noise and undersampling effects. Confidence limits for the maximum likelihood estimator of the mean of a lognormal random variable are determined by Monte Carlo techniques and by theoretical modeling. They show that such large σ2 values cause large uncertainty in estimates of the mean unless the number of data samples is extremely large. To obtain estimates of ...

Simulation of the Seasonal Cycle of the Tropical Pacific Ocean

Abstract: In a general circulation model of the tropical Pacific Ocean forced with climatological seasonally varying winds, equatorial upwelling and downwelling in adjacent latitudes play central roles in closing the oceanic circulation. The transport of the eastward North Equatorial Countercurrent decreases in a downstream direction because fluid is lost to downwelling into the thermocline where there is equatorward motion. Although this fluid converges onto the Equatorial Undercurrent, the latter's transport decreases because of equatorial upwelling. The upwelling, on the other hand, enhances the transport of the westward South Equatorial Current. Seasonally, the Countercurrent and South Equatorial Current are intense during the Northern Hemisphere summer and fall, at which time the thermocline has a pronounced trough near 3°N and a ridge near 10°N, and are weak in the spring when latitudinal thermal gradients are small and when the southeast trades are relatively weak. These variations are out of phase ...

3H and 3He in the Beta Triangle: Observations of Gyre Ventilation and Oxygen Utilization Rates

Abstract: Isopycnal maps of 3H–3He age (τ) with about 100 km resolution have been obtained for a 1000-km scale area in the eastern subtropical North Atlantic. The midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m−2 s−1. Analysis of the distributions within the context of advection-diffusion equations reveals that the larger scale 3H–3He age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of Ekman downwelling. Thus, isopycnal velocities can be estimated from 1/∇τ, but record only the velocity component normal to the outcrop. The velocities thus obtained agree well with geostrophic estimates, although the comparison is flawed by the fundamental mismatch in timescales between the two techniques. Backward extrapolation of the maps to zero-age outcrop positions indicate that the contribution of unventilated, older, recirculated water to the gyre flow above 600 m depth is small in this area (no more than 20%...

Dynamic Heights and Zonal Geostrophic Transports in the Central Tropical Pacific during 1979–84

Abstract: Dynamic height is calculated from XBT and surface salinity data in the central Pacific using a mean temperature–salinity (T–S) relation in the usual way below the thermocline but assuming isohaline water in the upper layer where the temperatures are isothermal. This scheme produces a better estimate of dynamic height than the use of a mean T–S relationship alone and produces significant improvements near the equator where small pressure gradients imply large geostrophic currents. During the El Nino of 1982–83, water of very low surface salinity was observed spanning the equator; this event is attributed both to extreme local rainfall and anomalous advection from the western Pacific. Geostrophic transports of the major surface currants are estimated for the period January 1979 through December 1984. The North and South Equatorial countercurrents are found to have the largest annual fluctuations, and the vertical displacements of the thermocline associated with these fluctuations are qualitatively ...

Shear instability in a highly stratified estuary

Abstract: Shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary. A field program involving high-resolution velocity and density measurements, as well as high-frequency acoustic imagery, allowed direct comparison of instantaneous Richardson number distributions to the occurrence of shear instability. The theoretical stability threshold of 0.25 is consistent with the measurements, based on estimates of gradients that contain the mean as well as fluctuations due to internal waves. An effective stability threshold based on mean gradients is found to be approximately one-third, reflecting a significant contribution of internal wave shear. The integral effect of the mixing process is to homogenize the gradients of velocity and density, producing linear profiles of these quantities across the pycnocline. A turbulent Prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin...

Structure and Variability of the Florida Current at 27°N: April 1982–July 1984

Abstract: Results of a two-year field experiment as part of the SubTropical Atlantic Climate Studies (STACS) program in the Straits of Florida are presented. Temperature and absolute ocean current observations were obtained by PEGASUS acoustic current profilers over 16 cruises during which repeated cross sections of the Florida Current were made at 27°N. Results are shown for the mean velocity and temperature fields, the perturbation horizontal kinetic energy and potential energy fields and for those energy conversion terms that could be computed directly from the data. The barotropic and baroclinic energy conversion terms, although small, indicate that the flow is stable for both types of perturbations. A 1arge part of the variability is contributed by short time scales (one week or less). The average and standard deviation of northward volume transport by the Florida Current during these cruises was (31.7 ± 3.0) × 106 m3 s−1. Barotropic and baroclinic contributions to the total heat flux across the North...

A Model for the Vertial Circulation of the Baltic Deep Water

Abstract: The time-dependent vertical circulation of the Baltic Proper is modeled using a horizontally integrated model of high vertical resolution. A seasonal pycnocline model computes the properties of the mixed layer. Below this an advection-vertical diffusion model computes the evolution of the salinity and temperature fields. A simple model for an entraining dense bottom current—which carries the intruding seawater and drives the vertical advection in the basin—is developed and used. In the derivation of entrainment velocity we it is shown that E(=we/u, where u is the speed of the bottom current) may be expressed in the well-known empirical constants m0 and Cd. The hypsographic features of the Baltic are accounted for in the model. The model is forced using realistic meteorological and hydrological time series. The inflow of dense seawater to the Baltic, with large fluctuations in flow rate and salinity, is realistically described and constitutes the upstream boundary condition for the bottom current....

Ocean Response to a Hurricane. Part I: Observations

Abstract: The response of the ocean to hurricanes was investigated using aircraft-deployable expendable current profilers (AXCP). The goals were to observe and separate the surface wave and surface mixed layer (SML) velocities under the storms and to map the across-track and along-track velocity and temperature response in the mixed layer and thermocline. Custom instrumentation was prepared, including slower failing AXCPs, and the AXCP equipment was installed on NOAA WP-3D aircraft. Research flights were made into two 1984 hurricanes: Norbert, in the eastern Pacific off Baja California (19°N, 109°W), and Josephine, off the east coast of the United States (29°N, 72°W). Thirty-one probes were deployed in each hurricane, and about half the AXCPs provided temperature and velocity profiles. Most velocity profiles exhibited strong surface wave contributions, slablike velocities in the SML, strong shears beneath the SML, and only weak flows in the upper thermocline. Separation of the surface gravity wave velociti...

A Numerical Study of Sea Ice and Ocean Circulation in the Arctic

Abstract: A sea-ice model based bulk-viscous plastic dynamics and 3-layer thermodynamics is coupled to a multilevel primitive equation model of the Arctic Ocean and Greenland Sea. The combined model is forced by inflow through the Faeroe-Shetland Channel and Bering Strait and by observed monthly atmospheric forcing and river runoff. A long-term integration produces a realistic cycle of ice cover, whose extent is strongly influenced by ocean heat transport. The wintertime maximum is controlled by northward heat transport of 0.4 petawatts in the Greenland Sea and by southward transport of ice and water through the Fram Strait. The summertime minimum extent of sea ice is influenced by subsurface flow through the Fram Strait of warm Atlantic water, which rises in winter and thins the ice lying over the Eurasian continental shelf and along the Alaskan and Siberian coasts. The oceanic circulation in the Canadian Basin is anticyclonic at all depths, but changes to cyclonic in the Eurasian Basin below 200 meters. ...

Near-Inertial Ocean Current Response to Hurricane Frederic

Abstract: Hurricane Frederic passed with 80 to 130 km of the U.S. Naval Oceanographic Office current meter arrays in water depths ranging from 100 to 470 m near the DeSoto Canyon region, and within 150 km of an Ocean Thermal Energy Conversion (OTEC) mooring in 1050 m of water. Excitation of near-inertial waves by the moving hurricane was observed throughout the water column along the canyon walls and at the OTEC site. The frequencies of the waves were blue-shifted between 1% to 6% above the local inertial frequency. The horizontal wavelength of 250 km is consistent with an energetic first baroclinic-mode response, but is considerably below the linear theory prediction of 550 km. The inferred vertical wavelengths of the immediate response exceeded 1000 m along the northern and eastern sides of the canyon since the currents throughout the water column increased within hours of the hurricane passage. Later, the vertical wavelengths were about equal to the water depth. The vertical group velocities associated ...

Infragravity Edge Wave Observations on Two California Beaches

Abstract: Wavenumber-frequency spectra of the infragravity (periods 20-200 sec) wave velocity field in the surf zone of two California beaches are estimated. Because the longshore arrays of biaxial electromagnetic current meters are relatively short (comparable to the wavelengths of interest), high resolution spectrum estimators are required. Model testing provides insight into the limits, capabilities and reliability of the estimators used in this paper. On all 15 days analyzed, between 42% and 88% of the longshore current variance at the array is contributed by low mode (n≤2) edge waves. (Percentage estimates are not made at a few frequencies because the array is positioned near nodes.) The low mode signal in the cross-shore velocity at the arrays is usually masked by unresolvable high mode and/or leaky waves. The percentage of cross-shore current variance at the array estimated unresolvable high mode is less than 35%, with one exception for which approximately 50% of the variance is mode 0 across a subs...

Some Effects of the Wind on Rings

Abstract: Some simple air-sea momentum transfer models, which include sea surface velocity and temperature, are considered for their effects on Gulf Stream rings. Perturbing the stress calculation with sea surface velocity results in a “top drag”, which causes interior motions to decay. Numerical experiments with equivalent barotropic quasi-geostrophic dynamics and reasonable estimates for the top drag suggest that this mechanism can amount for a significant fraction of observed isotherm subsidence rates in rings. Perturbing the stress calculation with a temperature sensitive drag coefficient produces a dipolar Ekman pumping field over a ring. For an eastward directed wind, the result is a tendency for the ring to self-propagate to the south. Integral constraints can be used to estimate the meridional propagation rate precisely, and for reasonable stress and thermal anomaly values, the estimate compares well with observations.

Assimilation of altimeter Eddy fields in a limited-area quasi-geostrophic model

Abstract: Techniques for the synoptic analysis, vertical inference, dynamical adjustment, and forecast of altimetric and deeper in situ data are presented as a first step towards the design of continuous assimilation schemes in limited-area oceanic domains. A year-long time series of streamfunction maps, denoted Mark 2, drawn in the POLYMODE area of the western North Atlantic is used as a benchmark for various tests and simulations. An original projection/extension scheme using empirical modes of density and/or pressure anomalies is used to obtain a first guess of the three-dimensional structure of the currents, starting from surface topography only. In the Mark 2 domain, this technique works well, since the first empirical mode is surface intensified and largely dominant. An alternative approach is to incorporate deeper data, e.g., float trajectories below the main thermocline. The first-guess currents are specified as initial and boundary condition of the Harvard 6-level quasi-geostrophic open ocean mode...

Wave Attenuation and Wave Drift in the Marginal Ice Zone

Abstract: Surface gravity waves in a viscous rotating ocean are studied theoretically when they penetrate an area covered by highly concentrated brashlike ice. The motion is described by a Lagrangian formulation, and the brash is modeled by a viscous Newtonian fluid. Results for wave attenuation and wave drift are obtained in the asymptotic limit of a thin, very viscous upper layer. The derived damping rate compares favorably with field data from the marginal ice zone (MIZ). The drift velocity in the ocean exhibits a marked maximum in the viscous boundary layer near the ice-ocean interface. At the outer edge of the boundary layer it exceeds the inviscid Stokes drift by a factor of 7/4. Computed values for the mean viscous drag on the ice induced by the wave motion show that this effect may compete with the frictional effected of the wind in packing the ice. Finally it is demonstrated that the integrated horizontal mass transports in the open ocean and under the ice do not match, which leads to upwelling in...

Intraseasonal Oscillations in Sea Level along the West Coast of the Americas

Abstract: Hourly observations of coastal sea level at stations from Peru to British Columbia are analyzed for low-frequency content. A space-time contour plot of sea level, from four years of data during the 1971–75 period, shows the meridional structure of the seasonal cycle and interannual variability associated with the 1972–73 El Nino. Oscillations with intraseasonal periods of 36–73 days are also evident, coherent over alongshore distances of several thousand kilometers. Further investigation using spectral methods and empirical orthogonal function analysis in the frequency domain reveals, in particular, that intraseasonal sea level variability has a peak in spectral density along the coasts of South America, Central America and Mexico, with high coherence from near the equator (Tumaco, 2°N) north to central California (34°N) and south to at least Callao (12°S). Phase propagation north of the equator is poleward at 150–200 kilometers/day. Atmospheric pressure, alongshore wind stress and wind stress cu...

Wind-Induced Destratification in Chesapeake Bay

Abstract: Multiyear continuous observations of velocity and salinity in the Chesapeake Bay indicate that wind-induced destratification occurs frequently from early fall through midspring over large areas of the estuary. Storm-driven breakdown of summer stratification was observed to occur near the autumnal equinox in two separate years. Surface cooling plays an important, though secondary, role in the fall destratification by reducing the vertical temperature gradient in the days prior to the mixing event. Large internal velocity shear precedes mixing events, suggesting a mechanism involving the generation of dynamic instability across the pycnocline. Destratification is shown to fundamentally alter the response of the velocity field to subsequent wind forcing; in stratified conditions, response is depth-dependent, while after mixing a depth-independent response is observed.

The Intraseasonal Oscillation in Eastern Pacific Sea Levels: How Is It Forced?

Abstract: Daily sea level and surface winds at eastern Pacific shore locations and equatorial islands, together with gridded five-day averages of 850 mb winds, have been analyzed for the 1979–84 period to determine how the 40–60 day intraseasonal oscillation of eastern Pacific sea levels is forced, as described by Spillane et al. for 1971–75. The oscillation was also present in 1980–84 from Callao, Peru, to San Francisco, with maximum energy near 52–57 days and band limits of 43 and 65 days. During 1980–84 there was no evidence for forcing of the large-scale oscillation in the eastern Pacific, although a local contribution of forcing was superimposed on the remote signal at the California stations. Interannual fluctuations in amplitude were evident in the sea level time series, consistent with those of the corresponding wind oscillation in the western equatorial Pacific. The oscillation was best developed in both variables in 1980–82 and became weak or nonexistent during the recovery phase of the 1982–83 E...

Wind and Wave Induced Currents in a Rotating Sea with Depth-varying Eddy Viscosity

Abstract: A theory is presented for time-dependent currents induced by a variable wind stress and wave field in deep water away from coastal boundaries. It is based on a second-order perturbation expansion of a version of the Navier-Stokes equations in Lagrangian coordinates. The Coriolis effect and the effect of a depth-dependent eddy viscosity are included. (The eddy viscosity is taken to depend on the Lagrangian vertical coordinate ĉ.) Partial differential equations are derived for the vertical and time variation of the mass transport velocity, together with boundary conditions at the sea surface. The vertical variation of the eddy viscosity causes an extra source term to appear in the equation for the evolution of the current profile. This additional source of momentum within the water column is exactly balanced by an extra term in the surface boundary condition, which in turn represents the contribution to wave dissipation caused by the eddy viscosity within the water column being different from its s...

The Madeira Mode Water

Abstract: Quasi-homogeneous layers in vertical profiles of temperature and salinity in the eastern North Atlantic near Madeira indicate the existence of a subtropical Mode Water in the Eastern Basin. Temperature sections show a maximum horizontal extent of at least 500 km. The frequency distribution analysis of homogeneous layers in a historical XBT dataset shows a Mode Water formation region near and to the north of Madeira. This Mode Water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection. It disappears almost completely, due to mixing, before the next winter. Volume estimates suggest that this Madeira Mode Water in the eastern Atlantic accounts for 15–20% of the total Central Water formation in the corresponding density range as obtained from tracer studies in the North Atlantic gyre.

Velocity Structure in the Mixed Layer during MILDEX

Abstract: During October–November 1983, a MIxed Layer Dynamics EXperiment (MILDEX) took place off the coast of California. As a part of MILDEX, a number of sensors were deployed from the Research Platform FLIP in an effort to monitor the flow structure in the near-surface mixed layer. Profiling current meters (VMCMs) measured velocities down to 150 m on an hourly basis. Other VMVMs were set at fixed depths. Depths of interest were examined using a package which measured three-component velocities and displayed them in FLIP's laboratory in real time. The current meters often observed sequences of downwind-directed jets, with maximum velocities exceeding 25 cm s−1. Downwelling velocities of equal magnitude were also observed. The strongest currents were found 10 to 35 m below the surface, at mid-depth in the mixed layer. Surface convergences associated with these jets were visualized by scattering tracers (computer cards) on the sea surface. These features suggest Langmuir circulation. Six Doppler sonars wer...

Annual cycle of subsurface thermal structure in the Tropical Atlantic ocean

Abstract: The subsurface thermal structure in the tropical Atlantic Ocean (30°N–20°S, East of 80°W) is studied on the basis of an extensive data bank of subsurface soundings. Calendar monthly maps are presented showing mixed layer depth, base of thermocline, thermocline thickness, and vertical temperature gradient across the thermocline. These maps are complemented by vertical cross sections depicting mixed layer depth, base of thermocline, and selected isotherms: a zonal profile along the equator (50°W–10°E), a meridional transect across the Eastern Atlantic (4°N–18°S), and a meridional section across the Central Atlantic (30°N–18°S). The basinwide subsurface thermal structure is dominated by the annual cycle of the surface wind field with extrema around April and August. The mixed layer is relatively shallow between 20°N and 10°S, with overall grater depth in the western as compared to the eastern portion of the basin. Two systems of annual cycle variation of mixed layer depth stand out. (i) Along the eq...

Flow through Discovery Gap

Abstract: A narrow gap (Discovery Gap) in the East Azores Fracture Zone at 37°N in the eastern Atlantic provides a channel for the exchange of bottom water between the Madeira and Iberian abyssal basins. A detailed survey defines its length (150 km), width (10–50 km), depth of sills (near 4800 m) and sediment thickness (0–500 m). Year-long measurements of deep flow are made from six moorings and ten current meters. Mean flows are about 5 cm s−1. These data are supplemented by 10 days of float tracking near 4700 m and numerous density profiles within and around the gap. A persistent southwest-northeast flow of dense water is found in Discovery Gap, and the flux of water colder than a potential temperature of 2.05°C is measured to be (0.21 ± .04) × 106 m3 s−1. This discharge spreads over an area of about 1011 m2 beyond the Gap exit, where it is warmed both by geothermal heating and by mixing with overlying water. An estimate of 1.5 to 4 cm2 s−1 is derived for the diapycnal diffusivity, similar to values dete...

Reconstructed Mediterranean Salt Lens Trajectories

Abstract: The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

The Fine-Structure of Nearshore Tidal and Residual Circulations Revealed by H.F. Radar Surface Current Measurements

Abstract: Using the Ocean Surface Current Radar (OSCR) developed by the Rutherford–Appleton Laboratory (UK), 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square. Tidal analyses of these data show that the currents associated with the predominant M2 constituent sweep smoothly and regularly through the area, unaffected by the finer topographic features. Moreover, contours of the amplitude of the M2 semi-major axis are mutually consistent to a precision of better than 0.5 cm s−1. Statistical analyses of these data indicate that the standard error of OSCR current measurements is less than 4 cm s−1. By contrast the major higher harmonic constituent, M4, shows pronounced, but ordered, spatial variability. Relating the observed distributions for M4 and M2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes. Standard relationships between residual surface currents and...

An Inertial Theory of the Equatorial Undercurrent

Abstract: An inertial nonlinear model of the equatorial undercurrent is presented. The model is a simple two-layer model whose lower layer represents the undercurrent. The flow in the lower layer preserves potential vorticity and Bernoulli function. The former includes the relative vorticity f the current and the latter includes the current's kinetic energy. The required relation between the potential vorticity and the Bernoulli function is determined by matching the solution far from the equator with the ventilated thermocline theory of Luyten et al. The model describes an eastward-accelerating undercurrent fed by a general wedge-shaped meridional circulation pattern. The general character of the meridional and zonal flow, as well as the magnitudes of the undercurrent velocity, the current width and thermocline depth agree reasonably well with observations.

The Penetration of Tritium into the Tropical Pacific

Abstract: The persistence of subsurface tritium maxima coincident with the Equatorial Currents is used to show that advection along isopycnals by the mean wind-driven circulation is the dominant process in the at most 14-year time scale for the penetration of high northern latitude water to the equator (above 26.2 sigma-theta). Ventilation of the equatorial Pacific thermocline from the north contrasts sharply with the equatorial Atlantic thermocline which is ventilated from the south. The most striking manifestation of the North Pacific circulation is evidenced by a tritium maximum and salinity minimum at the equator between 145° and 125°W located above 25.6 sigma-theta. It shows that regardless of time of sampling the eastern&sol/central equator has received the moat high latitude water, probably as a consequence of recirculation by the Equatorial Currents. Between the same meridians there is a tritium maximum on and north of the equator at the surface, which is interpreted as an expression of upwelling. ...