Neutral Density Surfaces Research Articles

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre‐WOCE hydrography are used to study the water‐mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.

The Contribution of Salt Fingers to Vertical Mixing in the North Atlantic Tracer Release Experiment*

The North Atlantic Tracer Release Experiment (NATRE) was performed in an area moderately favorable to salt fingers. However, the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent. This is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems. Despite the lack of a staircase, optical shadowgraph profiles revealed that small-scale tilted laminae, previously observed in a salt-finger staircase, were abundant at the NATRE site. Using microstructure observations, the strength of salt-finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy (Γ, “the dissipation ratio”). By examining the dissipation ratio in a parameter space of density ratio (Rρ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence describes most dissipation occurring when Ri < 1, dissipation at larger Ri is better described by the salt-finger model. Based on the results of the parameter space analysis, a method is proposed for estimating the salt-finger enhancement of the diapycnal haline diffusivity (ks) over the thermal diffusivity (kθ). During April 1992 at the NATRE site, it was found that kθ = (0.08 ± 0.01) cm2 s−1 and ks = (0.13 ± 0.01) cm2 s−1 for the neutral density surface local to the tracer release isopycnal (σθ ∼ 26.75 kg m−3, z ∼ 300 m). The flux divergence of buoyancy was also computed, giving the diapycnal advection w∗ = −(1.7 ± 1.2) m yr−1. Moreover, divergence of vertical buoyancy flux was dominated by the haline component. For comparison, the tracer release method gave a diffusivity of ks = (0.12 ± 0.02) cm2 s−1 (May–November 1992) and a diapycnal velocity of w∗ = −(3 ± 1) m yr−1 (May 1992–November 1994) at this site. The above numbers are contrasted to diffusivity estimates derived from turbulence theory alone. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of ks is taken into account.

Long-term hydrographic changes at 52 and 66°W in the North Atlantic Subtropical Gyre & Caribbean

In July–September 1997 two hydrographic lines were done in the western N. Atlantic along longitudes of 52 and 66°W as part of the WOCE one-time hydrographic survey of the oceans. Each of these two lines approximately repeated earlier ones done during the International Geophysical Year(s) (IGY) and the mid-1980s. Because of this repeated sampling, long-term hydrographic changes in the water masses can be examined. In this report, we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35°N and compare our results to those presented by Bryden et al. (1996), who examined changes along a zonal line at 24°N, most recently occupied in 1992. Since this most recent 24°N section in 1992, substantial changes have occurred in the western part of the subtropical gyre at the depths of the Labrador Sea Water (LSW). In particular, we see clear evidence for colder, fresher Labrador Sea Water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24°N. At shallower depths inhabited by waters that are an admixture of Mediterranean (MW) and Antarctic Intermediate Waters (AAIW), our recent survey shows an increase in salinity, which can only be attributed to changes in water masses on potential temperature or neutral density surfaces. Furthermore, waters above the MW/AAIW layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40-year period spanned by our sections. These latter changes have been dominantly due to a vertical sinking of density surfaces as T/S changes in density surfaces are small, but depths of individual T/S horizons have increased with time. The net change since the IGY shows a mean temperature increase between 800 and 2500 m depth at a rate of 0.57°C/century with a corresponding steric sea level rise of 1 mm/yr, and a net downward heave with small values near the top and bottom, and a maximum rate of −2.7 m/yr at 1800 m depth. Changes in the deep Caribbean indicate a warming since the IGY due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth, but no significant change in the deep salinity.

Circulation, mixing, and production of Antarctic Bottom Water

The Antarctic source of bottom water to the abyssal layer of the World Ocean is examined, as well as its large-scale flow pattern and ultimate entrainment rate into the deep water above. We make use of the available high-quality station data in the Southern Ocean to construct bottom maps of neutral density and mean property maps, including Chlorofluorocarbon (CFC), for the abyssal layer underneath a selected neutral density surface. The maximum density at the sill depth of Drake Passage is used to distinguish between the voluminous deep water mass that is a continuous component of the Antarctic Circumpolar Current from the relatively denser bottom water originated along the Antarctic continental margins. Based on water density, Antarctic Bottom Water (AABW) is defined here generically to include all volumes of non-circumpolar water of Antarctic origin. Over the shelf regime multiple localized sources of specific AABW types contribute to the abyssal layer of the adjacent Antarctic basins. Characteristics of these dense bottom waters reflect closely those observed on the parent Shelf Water mass. Spreading paths of newly-formed deep and bottom waters over the slope regime, and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the AABW layer. Interior mixing and interbasin exchanges of AABW are deduced from mean characteristic curves following the southern streamline of the Antarctic Circumpolar Current. Outflow and mixing of AABW from the Weddell Sea to the Argentine Basin is depicted using density and CFC distributions of two zonal hydrographic lines. Recirculation and mixing of deep and bottom waters within the Weddell Gyre are also detailed using a meridional section along the Greenwich Meridian. The strength of all localized sources of AABW combined is estimated by two independent approaches. An estimate of the total production rate of AABW is calculated based on the oceanic CFC budget for the AABW layer offshore of the 2500-m isobath. The sum of all downslope inputs of well-ventilated bottom water types underneath the top isopycnal must account for the measured CFC content in the bottom layer. The resulting total AABW production rate is about 8 Sv, which is a conservative figure that neglects the loss of CFC-bearing waters across the top isopycnal in recent years, whereas about 9.5 Sv is calculated assuming a well-mixed bottom layer. Making use of their transient nature, CFC distributions at the top of the AABW layer indicate that more direct and rapid entrainment of CFC-rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing. A second, more ad-hoc but independent oceanic mass budget of the bottom layer is also constructed. A typical basin-wide rate of deep upwelling of 3×10 −7 m s −1 requires 10 Sv (1 Sv=10 6 m 3 s −1) of newly-formed AABW to sink down the slope around Antarctica. We have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the AABW. It is expressed as a combination of wind, topographic, and turbulent components, which in turn are functions of the isopycnal depth, bottom depth, and bottom layer thickness. This non-uniform upwelling field yields about 12 Sv of AABW exported across the top isopycnal. Fortuitously, the overall average of upward speed at the top isopycnal (3.7×10 −7 m s −1) compares well with previous estimates of deep upwelling in the northern basins. A series of likely sites for strong vertical entrainment of AABW are clearly identified in the modeled distribution of deep upwelling, consonant with the observed CFC distributions on the top isopycnal. Altogether, regions with relatively high upwelling rates (>5×10 −7 m s −1) occupy only a quarter of the total areal extent of the top isopycnal, but they account for as much as 45% of the total vertical transport.

Intermediate water circulation and ventilation of the Indian Ocean derived from water-mass contributions

The intermediate water circulation and ventilation of the Indian Ocean is somewhat unique among the world oceans (in terms of the source waters). This has been studied with historical and recently obtained hydrographic data including potential temperature, salinity, dissolved oxygen, phosphate and silicate in a mixing model of applying optimum multiparameter analysis (OMP). The mixing model comprises three source water masses, Antarctic Intermediate Water (AAIW) (applied the transformed AAIW north of the Antarctic frontal zone and central South Indian Ocean), Indonesian Intermediate Water (IIW) and Red Sea Intermediate Water (RSIW) (including the influence of Persian Gulf Intermediate Water). A possible source from south of Australia has also been considered and accommodated into the water type definition of AAIW. This study was performed on six closely spaced neutral density surfaces which encompass the intermediate layer of the Indian Ocean from 500 m (in the northern Indian Ocean) to 1500 m (in the subtropical latitudes) with a distance of about 100-150 m between a pair of surfaces. Water-mass mixing contributions were plotted on the neutral surfaces and in three cross sections, the western Indian Ocean along 60E, the eastern Indian Ocean along 90E, and a zonal section along 10S. The intermediate water circulation and ventilation of the Indian Ocean can thus be inferred from the spreading paths and mixing patterns of these source water masses. A schematic intermediate water circulation of the Indian Ocean therefore emerges from the water-mass and dynamical information. The latter is derived from the acceleration potential (10 m 2 s -2 ) mapped on the neutral surfaces. The equatorward AAIW enters the Indian Ocean from the mid-ocean of the southern Indian Ocean, is advected with the subtropical gyre and transits to the north through the western boundary. In the western equatorial Indian Ocean, AAIW flows northeastward to eastward. At about 80E, AAIW bifurcates into northward and southward flows. The former continues into the Bay of Bengal through the western boundary (east of Sri Lanka) with up to 10% of the contribution. It returns southward in the eastern Bay of Bengal and along the Sumatra and Java Islands, zonally westward with IIW. The latter recirculates southward and then westward, forming a cyclonic gyre. The AAIW then turns southward into the Agulhas Current system through either side of Madagascar. AAIW contributes about 10-20% of its water into the equatorial Indian Ocean. Its northward flow in the western Indian Ocean is limited to 5N. IIW flows zonally westward and bifurcates into a northward and a southward flow in the western Indian Ocean. The direction of the latter is southward into the Agulhas Current system through either side of Madagascar. The former lows northward by the way of AAIW. Although AAIW does not flow into the Arabian Sea, IIW is found flowing into the Arabian Sea via the west coast of India. The main flow path of IIW into the Bay of Bengal is through the south of Sri Lanka. IIW largely contributes about 50-60% of its water into the Bay of Bengal. The northward flow of IIW is interrupted at the central equatorial region by the eastward AAIW. These circulations form two cyclonic gyres in the western and eastern equatorial Indian Ocean.

Meridional Overturning and Dianeutral Transport in az-Coordinate Ocean Model Including Eddy-Induced Advection

The present study examines the marked changes in the patterns of meridional overturning and dianeutral motion that occur upon introduction of the Gent and McWilliams (GM) scheme for eddy-induced transport into a coarse-resolution global ocean model. Results from two versions of the ocean model are compared. The first version does not have the GM scheme and uses a standard background horizontal diffusivity. The second version includes the GM scheme and has zero horizontal diffusivity. Both versions include a weak vertical diffusivity and isoneutral tracer diffusion as implemented by Cox. First, representations of the meridional overturning circulation computed via integration along (i) level, (ii) potential density, and (iii) neutral density (γ) surfaces are compared. Differences between the level surface representations are similar to those noted in previous studies. Differences between the density surface representations (not previously studied) are major at high densities (γ or σθ > 27.0). The residual Deacon cell of the first version has completely vanished in the GM version. The separate direct Antarctic and deep circulation cells of the first version are fully merged in the GM version. In these respects, the solution of the GM version is broadly more consistent with that of the FRAM eddy-permitting simulation. However, introduction of the GM scheme does not aid long-standing model problems of excessive upwelling of deep water into the thermocline in the Pacific and excessive penetration of Antarctic Bottom Water into the North Atlantic. The marked differences between the versions noted above imply marked differences in the dianeutral transport of fluid. A direct calculation of dianeutral transport shows that this transport is much weaker in the Southern Ocean and in the western boundary currents in the GM run. A breakup of the dianeutral transport into components resulting from the individual model mixing processes shows that the sole factor responsible for these qualitative changes is the absence of dianeutral motion induced by horizontal diffusive fluxes in the GM version. The interior dianeutral transport in this version is characterized by widespread and gentle upward motion induced by vertical diffusive fluxes. The results are shown to be insensitive to details of the surface boundary restoration. Consequences for our understanding of (i) the role of the Deacon cell as a tracer transport mechanism and (ii) the nature of the eddy-induced transports as provided by the GM scheme are discussed. In particular, although the total effective transport in the ocean interior is nearly isoneutral in the GM case, the eddy-induced part of that transport is shown to have a large dianeutral component and so cannot be interpreted as an isoneutral “bolus” transport.

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 Meridional Overturning Cells of a World Ocean Model in Neutral Density Coordinates

Abstract A comparison is made of the meridional overturning circulation in a coarse-resolution World Ocean model when the integration is performed along (i) level, (ii) potential density, and (iii) neutral density surfaces. In the level-surface calculation, all the usual cells are evident, including the Atlantic “conveyor,” the Deacon cell, and the direct Antarctic cell. In the potential or neutral density calculations, all cells remain present; however, the Deacon cell is greatly reduced in strength (to just a few Sverdrups). An analysis of the thermodynamics underlying the dianeutral motion is conducted. Most dianeutral motion results from fluxes associated with the vertical diffusivity and the (unphysical) horizontal diffusivity. Caballing is not important, despite the inclusion of isopycnal diffusivity. The mechanism of the residual Deacon cell involves densification near 40°5 resulting from fluxes associated with the horizontal diffusivity. Horizontal diffusivity results in substantial dianeutral mot...

Neutral surfaces and potential vorticity in the world's oceans

Several neutral surfaces are mapped in this paper and their properties are contrasted with those of potential density surfaces. It is shown that the Pacific is relatively forgiving to the use of potential density, while more care must be taken in the Atlantic and Indian oceans because of the larger compensating lateral gradients of potential temperature and salinity along neutral surfaces in these oceans. The dynamically important tracer, neutral‐surface potential vorticity (NSPV), defined to be proportional to f/h (where f is the Coriolis frequency and h is the height between two neutral surfaces), is mapped on several neutral surfaces in each of the world's oceans. At a depth of 1000m in the Atlantic and Indian oceans, the epineutral gradient of NSPV is different to the isopycnal variations of fN2 by as much as a factor of two (here N is the buoyancy frequency). Maps of isopycnal potential vorticity (IPV) resemble those of fN2, but the values of IPV are less by the simple factor μ, defined by μ = c[Rρ−1]/[Rρ−c], where Rρ is the stability ratio of the water column and c is the ratio of the values of α/β at the in situ pressure to that at the reference pressure (α and β being the thermal expansion and saline contraction coefficients, respectively). Layered models of the ocean circulation often take the vertical shear between layers (the thermal wind) to be given by the product of the interface slope and the contrast of potential density across the interface. The true thermal wind equation involves the interfaeial difference of in situ density, which is larger than the corresponding difference of potential density by the factor μ that is mapped in this paper, taking values up to 1.25 at a depth of 1000 m. This implies that the thermal wind is currently underestimated by up to 25% in layered ocean models. The differences between the slopes of neutral surfaces and potential density surfaces can be quantified Using the factory μ. The magnitudes of these slopes are illustrated here with contour maps and with vertical profiles, One would think that by choosing the reference pressure of potential density to be at the central pressure of a data set, the conservation equation of potential vorticity could be expressed with respect to these potential density surfaces with sufficient accuracy. Here it is shown that even the best potential density variable is significantly in error at thermoclinic frontal regions. This is linked to the fact that diapycnal velocities are not simply due to vertical mixing processes, but are also partly caused by epineutral mixing.

Neutral Surfaces

Scalar properties in the ocean are stirred (and subsequently mixed) rather efficiently by mesoscale eddies and two-dimensional turbulence along “neutral surfaces”, defined such that when water parcels are moved small distances in the neutral surface, they experience no buoyant restoring forces. By contrast, work would have to be done on a moving fluid parcel in order to keep it on a potential density surface. The differences between neutral surfaces and potential density surfaces are due to the variation of α/β with pressure (where α is the thermal expansion coefficient and β is the saline contraction coefficient). By regarding the equation of state of seawater as a function of salinity, potential temperature, and pressure, rather than in terms of salinity, temperature, and pressure, it is possible to quantify the differences between neutral surfaces and potential density surfaces. In particular, the spatial gradients of scalar properties (e.g., S, θ, tritium or potential vorticity) on a neutral surface can be quite different to the corresponding gradients in a potential density surface. For example, at a potential temperature of 4°C and a pressure of 1000 db, the lateral gradient of potential temperature in a potential density surface (referenced to sea level) is too large by between 50% and 350% (depending on the stability ratio Rp of the water column) compared with the physically relevant gradient of potential temperature on the neutral surface. Three-examples of neutral surfaces are presented, based on the Levitus atlas of the North Atlantic.

Neutral surfaces in the ocean: Implications for modelling

Surfaces of neutral static stability (neutral surfaces) along which lateral ocean mixing occurs, can be quite different to potential density surfaces. For example, the lateral gradients of potential temperature may differ in the two surfaces by up to a factor of four in some extreme but important regions. Vertical mixing processes cause vertical upwelling velocities that can be modelled as vortex‐stretching terms in layered models of ocean circulation. However, since potential density surfaces are commonly inclined with respect to neutral surfaces, lateral motion along neutral surfaces also gives rise to unwanted vertical motion through isopycnals that is not related to vertical mixing processes.When considering the conservation of potential vorticity in the real ocean, the differences between the depths of neutral surfaces and potential density surfaces becomes quite important. Neutral surface potential vorticity (NSPV), is introduced, defined to be proportional to f/h, where f is the Coriolis parameter and h is the vertical distance between adjacent neutral surfaces. NSPV is often significantly different (by factors of 2 or 3) to the two other commonly used potential vorticity variables, isopycnal potential vorticity, IPV, and fN².