A numerical ocean circulation model of the Norwegian and Greenland Seas

The dynamics and thermodynamics of the Norwegian and Greenland Seas are investigated using a three-dimensional primitive equation ocean circulation model. The horizontal resolution of the model is 1° in the zonal direction and 0.5° in the meridional direction. The vertical structure is described by 15 levels. The model is driven by both annual mean and seasonally varying wind and thermohaline forcing. The connections of the Norwegian and Greenland Seas with the North Atlantic and Arctic Ocean are modelled with an open boundary condition. The simulated currents are in reasonable agreement with the observed circulation

On the parameterization of eddy transfer, Part II: Tests with a channel model

In Part I of this paper (Killworth, 1997), a new eddy parameterization scheme was presented. Here, the scheme is tested by comparing its predictions with those of multi-year averages from an eddy-resolving channel model. Its accuracy is similar to that of a tuned version of previous schemes. However, a tuned version of the new parameterization can reproduce both the long-term average of the eddy-resolving solution as well as the initial slumping of a narrow front. Both tuned schemes reproduced the bolus transport well. The new parameterization reproduces the observed feature that the diffusivity is maximal at mid-depth and minimal at surface and floor

On the parameterization of eddy transfer Part I. Theory

This is the first of three linked papers which develop an eddy parameterization scheme for mean flows which are wide compared with a deformation radius. The scheme is partly based on the behavior of potential vorticity and thickness fluxes in linear instability, where the former are downgradient (apart from a turning matrix, not present in channel models) and the latter are not precisely downgradient, except on an f-plane. The scheme leads to a diffusivity which varies quite strongly with depth and is smallest at surface and floor. Intrinsic delta-function fluxes also occur at surface and floor, and these are worked out in detail. It is shown that all such parameterization schemes (whether linked to linear instability or not) must satisfy a necessary consistency condition, in the form of a vertical integral. A uniform diffusivity does not satisfy this requirement unless it is defined to vanish at surface and floor. Two methods to compute approximate diffusivities efficiently are given, and their results compare well with exact results from instability theory

Buoyancy driven rotating boundary currents

The structure of boundary currents formed from intermediately dense water introduced into a rotating, stably stratified, two-layer environment is investigated in a series of laboratory experiments, performed for Froude numbers ranging from 0.01 to 1. The thickness and streamwise velocity profiles in quasi-steady currents are measured using a pH activated tracer (thymol blue) and found to compare favorably to simplified analytic solutions and numerical models. Currents flowing along sloping boundaries in a stratified background exhibit robust stability at all experimental Froude numbers. Such stability is in sharp contrast to the unequivocal instability of such currents flowing against vertical boundaries, or of currents flowing along slopes in a uniform background. The presence of a variety of wave mechanisms in the ambient medium might account for the slower and wider observed structures and the stability of the currents, by effecting the damping of disturbances through wave radiation.Comment: 9 pages with 2 figures to appear in Ann NYAS "Long range effects in physics and astrophysics

Interdecadal variability and oceanic thermohaline adjustment

Changes in the strength of the thermohaline overturning circulation are associated, by geostrophy, with changes in the east-west pressure difference across an ocean basin. The tropical-polar density contrast and the east-west pressure difference are connected by an adjustment process. In flat-bottomed ocean models the adjustment is associated with viscous, baroclinic Kelvin wave propagation. Weak-high latitude stratification leads to the adjustment having an interdecadal timescale. We reexamine model interdecadal oscillations in the context of the adjustment process, for both constant flux and mixed surface boundary conditions. Under constant surface flux, interdecadal oscillations are associated with the passage of a viscous Kelvin wave around the model domain. Our results suggest the oscillations can be self-sustained by perturbations to the western boundary current arising from the southward boundary wave propagation. Mixed boundary condition oscillations are characterized by the eastward, cross-basin movement of salinity-dominated density anomalies, and the westward return of these anomalies along the northern boundary. We suggest the latter is associated with viscous Kelvin wave propagation. Under both types of boundary conditions, the strength of the thermohaline overturning and the tropical-polar density contrast vary out of phase. We show how the phase relationship is related to the boundary wave propagation. The importance of boundary regions indicates an urgent need to examine the robustness of interdecadal variability in models as the resolution is increased, and as the representation of the coastal, shelf/slope wave guide is improved. (Abriged abstract)Comment: 20 pages, AGU LaTeX, 12 figures included using epsfig, to appear in JGR, complete manuscript also available at ftp://crosby.physics.mun.ca/pub/drew/papers/gp1.ps.g

Wave propagation and growth on a surface front in a two-layer geostrophic current

We study analytically and numerically small amplitude perturbations of a geostrophically balanced semi-infinite layer of light water having a surface front and lying above a heavier layer of finite vertical thickness which is at rest in the mean. In contrast with previous studies where the latter layer was infinitely deep we find that the equilibrium is always unstable regardless of the distribution of potential vorticity, and the maximum growth rates are generally much larger than in the one-layer case. The amplifying ageostrophic wave transfers kinetic energy from the basic shear flow as well as potential energy. Good quantitative agreement is found with the laboratory experiments of Griffiths and Linden (1982), and our model seems to be the simplest one for future investigations of cross frontal mixing processes by finite amplitude waves. The propagation speed of very low frequency and nondispersive frontal waves is also computed and is shown to decrease with increasing bottom layer depth

A numerical model study of the effects of interannual timescale wave propagation on the predictability of the Atlantic meridional overturning circulation

We investigate processes leading to uncertainty in forecasts of the Atlantic meridional overturning circulation (AMOC). A climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states. Baroclinic transports diverge on interannual timescales even though the ocean is not eddy-permitting. Interannual fluctuations of the model AMOC in the subtropical gyre are caused by westward propagating Rossby waves. Divergence of the predicted AMOC with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl. These windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw/dz. Consequently, errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary. Another source of divergence is transport anomalies propagating along the Gulf Stream (and other boundary currents). The propagation mechanism seems to be predominantly advection by mean currents, and we show that the arrival of westward propagating waves can trigger development of these anomalies. The mean state of the AMOC has a small effect on interannual predictability in the subtropical gyre, most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the AMOC in the subtropics. Eastern boundary windstress curl was more predictable at 45{degree sign}N when the AMOC was in a strongly decreasing state, but, unlike at 30{degree sign}N, no mechanism was found linking windstress curl fluctuations with deep transports