34 research outputs found
Horizontal Stratification during Deep Convection in the Labrador Sea
No full textDeep convection—the process by which surface waters are mixed down to 1000 m or deeper—forms the primary downwelling of the meridional overturning circulation in the Northern Hemisphere. High-resolution hydrographic measurements from Seagliders indicate that during deep convection—though water is well mixed vertically—there is substantial horizontal variation in density over short distances (tens of kilometers). This horizontal density variability present in winter (January–February) contains sufficient buoyancy to restratify the convecting region to observed levels 2.5 months later, as estimated from Argo floating platforms. These results highlight the importance of small-scale heterogeneities in the ocean that are typically poorly represented in climate models, potentially contributing to the difficulty climate models have in representing deep convection
Determining vertical water velocities from Seaglider
No full textVertical velocities in the world's oceans are typically small, less than 1 cm/s, posing a significant challenge to observation techniques. Seaglider, an autonomous profiling instrument, can be used to estimate vertical water velocity in the ocean to about half a centimeter per second. Using a Seaglider flight model and pressure observations, vertical water velocities are estimated along glider trajectories in the Labrador Sea before, during and after deep convection. Results indicate that vertical velocities in the stratified ocean agree with theoretical WKB-scaling of w, and in the turbulent mixed layer, scale with buoyancy and wind forcing. We estimate that accuracy is within 0.6 cm/s. Due to uncertainties in the flight model, velocities are poor near the surface and deep apogees, and during extended roll maneuvers. Some of this may be improved by using a dynamic flight model permitting acceleration, and by better constraining flight parameters through pilot choices during the mission
Instability and energetics in a baroclinic ocean
No full textSubmitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute and the Woods Hole Oceanographic Institution August 1975This thesis is made of two separate, but interrelated parts.
In Part I the instability of a baroclinic Rossby wave
in a two-layer ocean of inviscid fluid without topography,
is investigated and its results are applied in the ocean.
The velocity field of the basic state (the wave) is characterized
by significant horizontal and vertical shears, non-zonal
currents, and unsteadiness due to its westward propagation.
This configuration is more relevant to the ocean
than are the steady, zonal 'meteorological' flows, which
dominate the literature of baroclinic instability. Truncated
Fourier series are used in perturbation analyses.
The wave is found to be unstable for a wide range of
the wavelength; growing perturbations draw their energy from
kinetic or potential energy of the wave depending upon
whether the wavelength, 2πL, is much smaller or larger than
2πLρ, respectively, where Lρ is the internal radius of deformation. When the shears are comparable dynamically,
L~Lρ , the balance between the two energy transfer processes is very sensitive to the ratios L/Lρ and U/C as well,
where U is a typical current speed, and C a typical phase
speed of the wave. For L = Lρ they are augmenting if
U C.
The beta-effect tends to stabilize the flow, but perturbations
dominated by a zonal velocity can grow irrespective
of the beta-effect.
It is necessary that growing perturbations are comprised
of both barotropic and baroclinic modes vertically.
The scale of the fastest growing perturbation is significantly
larger than L for barotropically controlled flows
(L < Lρ ), reduces to the wave scale L for a mixed kind
(L ~ Lρ ) and is fixed slightly larger than Lρ for baroclinically controlled flows (L > Lρ ).
Increasing supply of potential energy causes the normalized
growth rate, αL/U, to increase monotonically as
L → Lρ from below. As L increases beyond Lρ,
the growth rate αLρ /U shows a slight increase, but soon
approaches an asymptotic value.
In a geophysical eddy field like the ocean this model
shows possible pumping of energy into the radius of deformation
(~ 40 km rational scale, or 250 km wavelength) from
both smaller and larger scales through nonlinear interactions,
which occur without interference from the beta-effect.
The e-folding time scale is about 24 days if
U = 5 cm/sec and L = 90 km. Also it is strongly suggested
that, given the observed distribution of energy versus
length scale, eddy-eddy interactions are more vigorous than
eddy-mean interaction, away from intènse currents like the
Gulf Stream. The flux of energy toward the deformation
scale, and the interaction of barotropic and baroclinic
modes, occur also in fully turbulent 'computer' oceans, and
these calculations provide a theoretical basis for source of
these experimental cascades.
In Part II an available potential energy (APE) is defined
in terms appropriate to a limited area synoptic density
map (e.g., the 'MODE-I' data) and then in terms appropriate
to time-series of hydrographic station at a single geographic location (e. g., the 'Panulirus' data).
Instantaneously the APE shows highly variable spatial
structure, horizontally as well as vertically, but the vertical
profile of the average APE from 19 stations resembles
the profile of vertical gradient of the reference stratification.
The eddy APE takes values very similar to those of
the average kinetic energy density at 500 m, 1500 m and
3000 m depth in the MODE area.
In and above the thermocline the APE has roughly the
same level in the MODE area (centered at 28°N, 69° 40'W) as
at the Panulirus station (32° 10'N, 64° 30'W), yet in the
deep water there is significantly more APE at the Panulirus
station. This may in part indicate an island effect near
Bermuda.This research has been supported by the National Science
Foundation grant IDO 73-09737, formerly GX-36342
Circulation and transport in the western boundary currents at Cape Farewell, Greenland
No full textThe circulation and volume transports in the western boundary currents around Cape Farewell, Greenland, are derived from full-depth hydrographic and velocity measurements from August-September, 2005. The western boundary currents from surface to seafloor transport 40.5 ± 8.1 Sv southwards in the Irminger Sea, and 53.8 ± 10.8 Sv northwards in the Labrador Sea. The Deep Western Boundary Current (DWBC, defined as water with potential density greater than 27.80 kg m-3) transports 12.3 ± 2.5 Sv southwards in the Irminger Sea. The deep water transport is reduced south of Cape Farewell, where it changes flow direction from southwards to northwards (the south corner). At a section over the Eirik Ridge, a bathymetric feature extending southwest of Cape Farewell, the DWBC transports 8.7 ± 1.7 Sv westwards. The reduction in transport at the south corner is associated with decreased velocities within the deepest layers, and the volumetric loss of the most saline deep water types. The observations suggest that the part of the shallow and deep western boundary currents diverge at the south corner. Downstream in the eastern Labrador Sea the deep water transport is increased to 19.7 ± 3.9 Sv northwards, with the addition of recirculating denser deep waters. The representativeness of the results from the semi-synoptic survey are discussed with reference to companion current meter measurements of the DWBC
Convection above the Labrador continental slope
No full textThe Labrador Sea is one of the few regions of the World Ocean where deep convection takes place. Several moorings across the Labrador continental slope just north of Hamilton Bank show that convection does take place within the Labrador Current. Mixing above the lower Labrador slope is facilitated by the onshore along-isopycnal intrusions of low-potential-vorticity eddies that weaken the stratification, combined with baroclinic instability that sustains slanted mixing while restratifying the water column through horizontal fluxes. Above the shelf break, the Irminger seawater core is displaced onshore while the stratification weakens with the increase in isopycnal slope. The change in stratification is partially due to the onshore shift of the “classical” Labrador Current, baroclinic instability, and possibly slantwise convection
