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Thomas W. N. Haine

Abstract

The global ocean overturning circulation carries warm, salty water to high latitudes, both in the Arctic and Antarctic. Interaction with the atmosphere transforms this inflow into three distinct products: sea ice, surface Polar Water, and deep Overflow Water. The Polar Water and Overflow Water form estuarine and thermal overturning cells, stratified by salinity and temperature, respectively. A conceptual model specifies the characteristics of these water masses and cells given the inflow and air–sea–land fluxes of heat and freshwater. The model includes budgets of mass, salt, and heat, and parameterizations of Polar Water and Overflow Water formation, which include exchange with continental shelves. Model solutions are mainly controlled by a linear combination of air–sea–ice heat and freshwater fluxes and inflow heat flux that approximates the meteoric freshwater flux plus the sea ice export flux. The model shows that for the Arctic, the thermal overturning is likely robust, but the estuarine cell appears vulnerable to collapse via a so-called heat crisis that violates the budget equations. The system is pushed toward this crisis by increasing Atlantic Water inflow heat flux, increasing meteoric freshwater flux, and/or decreasing heat loss to the atmosphere. The Antarctic appears close to a so-called Overflow Water emergency with weak constraints on the strengths of the estuarine and thermal cells, uncertain sensitivity to parameters, and possibility of collapse of the thermal cell.

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Stuart A. Cunningham and Thomas W. N. Haine

Abstract

A synoptic distribution of Labrador Sea Water (LSW) in the eastern North Atlantic is determined from a regularly sampled, but sparse (3° resolution), survey covering 39° to 54°N, 11° to 34°W during spring 1991.

The core of LSW can be defined by a minimum in potential vorticity (PV). Using property values at this minimum the authors infer the circulation of LSW. In addition, using a known source function for salinities at the core of LSW, estimates are able to be made of LSW vintages. The authors then compared the synoptic circulation to historical data.

Youngest, 1986 vintage, LSW crosses the Mid-Atlantic Ridge to the eastern basin between 48° and 51°N at 34°W. This water then flows north to the Iceland Basin and eastward to the Rockall Trough, where it was found to be of 1978 vintage. Tongues of low salinity, low temperature, and high oxygen extend southward on the eastern side of the Mid-Atlantic Ridge, indicating that LSW also flows southward in the eastern basin. At the southern edge of the survey the salinity and density of LSW increases.

Compared to historical data of Talley and McCartney for the years 1957–1964 the authors found 1) no coincident values of PV, with LSW now having much lower PV and 2) that the core of LSW is significantly fresher. These differences show that climate variability, which affect these properties at the source, has a dramatic impact on tracer distribution at middepth in the eastern North Atlantic.

Mediterranean Water is shown to overlap the LSW in a band 600 km wide spanning the eastern North Atlantic. Staircase structures on salinity profiles are not observed in the region, indicating that salt fingering if present, must be intermittent. This is contrasted with the work of Schmitz and McCartney who show that salt fingering is active south of 39°N.

In Part II of this paper, the authors examine the anomalies inherited from the boundary condition variability and examine the advective/diffusive balance for LSW.

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Thomas W. N. Haine and D. A. Cherian
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Martina M. Junge and Thomas W. N. Haine

Abstract

The authors address the question: What are the oceanic mechanisms that control North Atlantic sea surface temperature (SST) anomalies? The approach is to examine the sensitivity dynamics of a non-eddy-resolving North Atlantic Ocean general circulation model (GCM) using its adjoint. The adjoint GCM yields the sensitivity of end-of-winter SSTs to the prior ocean state and prior air–sea forcing over a seasonal cycle. Diagnosis of the sensitivity results identifies the oceanic mechanisms involved in controlling SST anomalies. The most effective way to alter SST is to change the local, contemporaneous air–sea heat flux. Wind stress and freshwater perturbations are ineffective over one year. Upstream, wintertime heat flux anomalies can cause SST fluctuations in the following winter but heat flux anomalies during summer weakly affect subsequent end-of-winter SSTs. The dominant mechanism is the end-of-winter detrainment of warmer or colder water and its subsequent entrainment downstream into the mixed layer the next winter. This process is more effective in the midlatitude and subpolar North Atlantic where deep winter mixed layers occur, than in the tropical–subtropical regions, which are characterized by a shallow mixed layer and a weak seasonal cycle. Mean-flow advection in the seasonal thermocline of the North Atlantic Current is moderately important in the subpolar gyre. Dynamical mechanisms, such as planetary waves and anomalous currents, are much less important over one year. The GCM results indicate that internal ocean anomalies forced by remote heat fluxes do affect SST variability. But, overall, contemporaneous winter heat flux anomalies are 3–30 times more effective at causing SST anomalies than heat flux anomalies from the previous winter. The loss of sensitivity to prior air–sea fluxes suggests that North Atlantic SST fluctuations are thus primarily a response to local, recent forcing.

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Timothy M. Hall and Thomas W. N. Haine

Abstract

The idealized age tracer is commonly used to diagnose transport in ocean models and to help interpret ocean measurements. In most studies only the steady-state distribution, the result of many centuries of model integration, has been presented and analyzed. However, in principle the transient solution provides more information about the transport. Here it is shown that this information can be readily interpreted in terms of the ventilation histories of water masses. A simple relationship is derived, valid for stationary transport, between the transient evolution, τ id(r, t), of the idealized age tracer and the “age spectrum,” G(r, t), the distribution of times t since a water mass was last ventilated. Namely, G(r, t) = −∂tt τ id(r, t). Implications of the relationship are discussed, and the relationship is illustrated with an idealized model.

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Thomas W. N. Haine and Timothy M. Hall

Abstract

A general theory to describe and understand advective and diffusive ocean transport is reported. It allows any passive tracer field with an atmospheric source to be constructed by superposing sea surface contributions with a generalized Green's function called the boundary propagator of the passive tracer equation. The boundary propagator has the interpretation of the joint water-mass and transit-time distribution from the sea surface. The theory thus includes the classical oceanographic idea of water-mass analysis and extends it to allow for a distribution of transit times from the sea surface. The joint water-mass and transit-time distribution contains complete information about the transport processes in the flow. It captures this information in a more accessible way than using velocity and diffusivity fields, however, at least for the case of sequestration and transport of dissolved material by the ocean circulation. The boundary propagator is thus the natural quantity to consider when discussing both steady-state and transient ocean tracers, including the inverse problem of interpreting tracer data in terms of ocean circulation. Two constraints on the shape of the transit-time distributions are derived. First, the asymptotic behavior for a steady, or time-averaged, circulation is exponential decay. Second, integrated over the whole ocean, the transit-time distribution from the sea surface cannot increase. The theory is illustrated using a one-dimensional advection–diffusion model, a box model, and a North Atlantic general circulation model.

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Kial D. Stewart and Thomas W. N. Haine

Abstract

The role of the ocean in Earth’s climate is fundamentally influenced by the locally dominant stratifying property (heat or salt), which in turn can be used to categorize the ocean into three classes: alpha, beta, and transition zone oceans. Alpha and beta oceans are regions where the stratification is permanently set by heat and salt, respectively. Transition zone oceans exist between alpha and beta oceans and are regions where the stratification is seasonally or intermittently set by heat or salt. Despite their large ranges of temperature and salinity, transition zone oceans are the most weakly stratified regions of the upper oceans, making them ideal locations for thermobaric effects arising from the nonlinear equation of state of seawater. Here a novel definition and quantification of alpha, beta, and transition zone oceans is presented and used to analyze 4 years (2010–13) of hydrographic data developed from the Argo profiling float array. Two types of thermobaric instabilities are defined and identified in the hydrographic data. The first type arises from the vertical relocation of individual water parcels. The second type is novel and relates to the effect of pressure on the stratification through the pressure dependence of the thermal expansion coefficient; water that is stably stratified for one pressure is not necessarily stable for other pressures. The upper 1500 m of the global ocean is composed of 67% alpha, 15% beta, and 17% transition zone oceans, with 5.7% identified as thermobarically unstable. Over 63% of these thermobarically unstable waters exist in transition zone oceans, suggesting that these are important locations for efficient vertical transport of water-mass properties.

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Thomas W. N. Haine and John Marshall

Abstract

A hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed. If a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface, then gravitational instability ensues in which buoyancy is drawn from depth by upright convection. But if spatial inhomogeneities in the ambient stratification or the forcing are present (as always exist in nature), then horizontal density gradients will be induced and, within a rotation period, horizontal currents in thermal-wind balance with those gradients will be set up within the mixed layer. There are two important consequences on the convective process:

  1. Upright convection will become modified by the presence of the thermal wind shear; fluid parcels are exchanged not along vertical paths but, rather, along slanting paths in symmetric instability. Theoretical considerations suggest that this slantwise convection sets the potential vorticity of the mixed layer fluid to zero but, in general, will leave it stably stratified in the vertical.
  2. The convective process ultimately gives way to a baroclinic instability of the horizontal mixed layer density gradients. The resulting baroclinic waves are important agents of buoyancy transport through the mixed layer and can be so efficient that the convective process all but ceases.

The authors illustrate and quantify these ideas by numerical experiment with a highly resolved nonhydrostatic Navier–Stokes model. Uniform spatial cooling at the surface of a resting, stratified fluid in a 2½-dimensional model on an f plane, in which zonal strips of fluid conserve their absolute momentum, causes energetic vertical overturning. A well-mixed boundary layer develops over a depth that is accurately predicted by a simple 1D law. In contrast, differential surface cooling induces a mixed layer front. Fluid parcels, made dense at the surface, sink along slanting trajectories in intense nonhydrostatic plumes. After cooling ceases the Ertel potential vorticity within the convective layer is indeed found to be vanishingly small, corresponding to convective neutrality measured in the absolute momentum surfaces that are tilted from the vertical by the horizontal vorticity of the thermal wind.

In analogous fully three-dimensional calculations, the absolute momentum constraint is broken, and the convection at first coexists with, but is ultimately dominated by, a baroclinic instability of the mixed layer. For typical mixed layer depths of 500 m stability analysis predicts, and our explicit calculations confirm, that baroclinic waves with length scales O(5 km) develop with timescales of a day or so. By diagnosis of fully developed mixed layer turbulence, the authors assess the importance of the baroclinic eddy field as an agency of lateral and vertical buoyancy flux through the layer. A novel scaling for the lateral buoyancy flux due to the baroclinic eddies is suggested. These ideas are based on analysis of several experiments in which the initial stratification, rotation rate, and buoyancy forcing are varied, and the results are compared to previous attempts to parameterize the effects of baroclinic instability. There is a marked difference between the scaling that accounts for the resolved experiments and the Fickian schemes used traditionally in large-scale ocean models.

Finally, consideration of the results in light of high-resolution mixed layer hydrographic surveys in the northeast Atlantic suggests mixed layer baroclinic instability may be very important at fronts. The authors speculate that the process exerts a large influence on the character of newly subducted thermocline water throughout the extratropical ocean.

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Thomas W. N. Haine and Deepak A. Cherian

The dynamics of the rotating shallow-water (RSW) system include geostrophic f low and inertial oscillation. These classes of motion are ubiquitous in the ocean and atmosphere. They are often surprising to people at first because intuition about rotating f luids is uncommon, especially the counterintuitive effects of the Coriolis force. The gyroscope, or toy top, is a simple device whose dynamics are also surprising. It seems to defy gravity by not falling over, as long as it spins fast enough. The links and similarities between rotating rigid bodies, like gyroscopes, and rotating fluids are rarely considered or emphasized. In fact, the dynamics of the RSW system and the gyroscope are related in specific ways and they exhibit analogous motions. As such, gyroscopes provide important pedagogical opportunities for instruction, comparison, contrast, and demonstration. Gyroscopic precession is analogous to geostrophic flow and nutation is analogous to inertial oscillation. The geostrophic adjustment process in rotating fluids can be illustrated using a gyroscope that undergoes transient adjustment to steady precession from rest. The controlling role of the Rossby number on RSW dynamics is reflected in a corresponding nondimensional number for the gyroscope. The gyroscope can thus be used to illustrate RSW dynamics by providing a tangible system that behaves like rotating fluids do, such as the large-scale ocean and atmosphere. These relationships are explored for their potential use in educational settings to highlight the instruction, comparison, contrast, and demonstration of important fluid dynamics principles.

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Stuart A. Cunningham and Thomas W. N. Haine

Abstract

Deep wintertime convection in the Labrador Basin produces Labrador Sea Water (LSW), which spreads throughout the North Atlantic. The LSW is not formed with constant characteristics but varies on decadal timescale. These changes in the source of LSW propagate throughout the North Atlantic, having a dramatic impact on middepth properties in regions far removed from the Labrador Basin. Here, by means of a property variance correlation analysis on isopycnal surfaces through the core of LSW, the authors show that the boundary anomalies of LSW survive the effects of mixing over the length and timescales taken for LSW to reach the eastern North Atlantic. Typically, the anomaly amplitude is reduced by a factor of 5 during transit, and from a simple estimate the integrated timescale for this reduction is seven years. Uncertainties in the oxygen anomaly analysis exclude specific interpretation, but constrain the boundary condition variability to about ±5%. The lateral effect of unresolved processes resembles isopycnic Fickian diffusion; and an estimate of the mean isopycnal eddy diffusion coefficient through the LSW core is 950±300 m2 s−1, with a mixing length scale of 166 ±30 km.

The authors examine the advective-diffusive balance for salinity and oxygen and calculate the horizontal advective flux divergence and diffusion. Contributions from the vertical advection and diffusion terms are small and can be neglected. The uncertainties in the oxygen balance prohibit any firm conclusions regarding the time rate of change of this property. However, for salinity an unsteady contribution up to ∼0.008±0.005 psu yr−1 is needed to close the budget. On σ1.5=34.64 kg m−3, comparing the unsteady contribution to the source function in the Labrador Basin the age range of LSW was found to be 10±3 years.

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