On the structure and the Variability of the Filchner
Overflow Plume
E. Darelius, L.H. Smedsrud, S. Østerhus, A. Foldvik, and T.
Gammelsrød
(Submitted to Tellus November 2007, in revision)
The Weddell Sea is one of the major formation sites for Antarctic
Bottom Water. The cold and dense Ice Shelf Water that is formed
under the Filchner-Ronne Ice Shelf is a precursor to the Antarctic
Bottom Water, and it exits the ice-shelf cavity through the Filchner
Depression forming a dense plume on the continental slope. Ice Shelf
Water spills over the sill at a rate of 1.6 Sv (Foldvik, 2004).
Properties of the dense plume are described using data from all
available current meter records and CTD-stations in the region,
focusing mainly on meso-scale variability and three distinct
oscillations that have periods of about 35 hours, 3 days and 6 days.
The oscillations are close to barotropic and are seen in both
temperature and velocity records. The observations are compared with
theories on eddies generated in overflow plumes and on continental
shelf waves,
but none of them seems to describe well the phenomena observed.
Twenty-five years of CTD data (more than 300 profiles)
were synthesized to give a ``mean-picture" of bottom temperature and
plume thickness. Low temperatures and relatively thin layers of
dense plume water are observed at great depth in the vicinity of a
ridge, while profiles west of the ridge show higher temperatures and
thicker layers. The three pathways suggested by Foldvik (2004) are
coupled to theories on topographic steering by canyons and ridges
and discussed in relation to the presented data.
Topographic steering of dense overflows: laboratory experiments with V-shaped ridges and
canyons
E. Darelius
(Deep-Sea Research (2008), doi: 10.1016/j.dsr.2008.04.008)
Topographic corrugations such as canyons and ridges cross-cutting
the path of a dense plume may effectively steer all or part of the
plume downslope. A dynamical regime, in which the along-slope
transport is balanced by a return flow in the Ekman layer to
maintain a geostrophically balanced downslope flow along the
corrugation, has been proposed. An analytical model
(Darelius and Wåhlin, 2007; Wåhlin, 2002) incorporating these physics is compared
with laboratory experiments of dense gravity currents flowing down
sloping, V-shaped canyons and ridges in a rotating frame of
reference. The response of the flow to variations in four governing
parameters (slope, rotation, volume flux and reduced gravity) is
generally described well by the model. Vertical velocity profiles
resolving the Ekman spiral were obtained using a Laser Doppler
Velocimeter and they showed the secondary, transverse circulation
superimposed on the primary, downslope flow. A particle flowing down
the canyon/along the ridge can be expected to follow a helix-like
path, and dye released within the dense layer showed this. The
experiments support the dynamical regime proposed for
topographically steered flows, and verifies the analytical model.
The gravity current was observed to divide in two when the transport
capacity of the corrugation was exceeded; one part continued along
the slope and the other flowed downslope along the corrugation.
Laboratory observations of enhanced entrainment in the presence of submarine canyons and ridges
A. Wåhlin, E. Darelius, C. Cenedese and G. Lane-Serff
(Deep-Sea Research I (2008), doi: 10.1016/j.dsr.2008.02.007)
The continental slopes are often covered by small-scale topographic
features such as submarine canyons and ridges. Dense plumes flowing
geostrophically along the slope may encounter such features and all,
or part of, the dense water can be steered downslope. A set of
laboratory experiments was conducted at the rotating Coriolis
platform to investigate the effect of small-scale topography on
plume mixing. A dense water source was placed on top of a slope, and
experiments were repeated with three topographies: a smooth slope, a
slope with a ridge, and a slope with a canyon, and for three flow
regimes: laminar, wave-, and eddy-generating plumes. When a ridge or
a canyon was present on the slope the dense plume was steered
downslope and waves developed along the ridge and canyon wall,
regardless of the flow characteristics on the smooth slope. Froude
numbers were estimated, and were found to be higher for the
topographically steered flow than for flow on smooth topography. The
stratification in the collecting basin was monitored and the mixing
inferred. The total mixing increased when a ridge or a canyon was
present, and the increased entrainment rate was hence more important
than the shortened path. The difference in mixing levels between the
regimes was smaller when topography was present, indicating that it
was the small-scale topography and not the large-scale
characteristics of the flow that determined the properties of the
end product.
Downward flow of dense water leaning on a submarine ridge
E. Darelius and A. Wåhlin
Deep Sea Research I (2007), Volume 56(7)
Large-scale dense bottom currents are geostrophic to leading order,
with the main flow direction along the continental slope. Bottom
friction makes the water descend to greater depths, but only at a
small angle to the horizontal. Here the effect of a submarine ridge
that intersects the slope is considered. It is shown that the
presence of a submarine ridge greatly enhances the downward
transport. By leaning against the ridge it is possible for the dense
water to flow downhill, perpendicular to the depth contours, even
though the first order dynamics are geostrophic. The requirement for
downward flow next to the ridge is that the frictional transport
that it induces is sufficiently large to counteract geostrophic
advection along the isobaths and out of the ridge region. The
dynamics is similar to that of downward flow in submarine canyons,
but ridges appear to be more effective in channeling the dense water
downhill, in particular for narrow ridges/canyons with small seaward
slope of the ridge/canyon axis. The downward flow is analyzed using
a simplified analytical model and the results are compared to data
from the Filchner Overflow, which agrees qualitatively with the
model.
Ovädren blir kraftigare när världen blir varmare
E. Darelius, Svenska Dagbladet, September 2007
De tropiska orkanerna har blivit kraftigare de senaste 30 åren. Vinden är starkare, varar längre - och förstör mer. Forskarna pekar på klimatförändringar och stigande temperaturer som förklaring.
Antarktis ger liv åt världshaven
E. Darelius, Svenska Dagbladet, September 2007
Vita vidder, snö, is och evig kyla. Polområdena är vackra, vilda - och väldigt viktiga när forskarna försöker förstå hur havet, atmosfären och allt annat hänger samman. I Antarktis mäktiga isvärld finns många av svaren.
Det stora blå
E. Darelius, AKA nummer 25, February 2006, p.22-24.
Allt är vatten sa Thales en gång för länge sedan. I en trång föreläsningssalpå studentcentret var det svårt att tro honom - men här, i fören på forskningsskeppet Polarstern, med vatten, vatten och bara vatten framför mig får hans ord en annan mening.
Antarktis i sikte!
E. Darelius, Hallands Nyheter, 17 February, 2005
Oceanografen från Getinge berättar i HN om sin fantastiska resa.
On the Influence of Small-scale Topography on Dense Plumes, with a special focus on the Filchner Overflow Plume
E. Darelius, PhD-thesis (2007) Geophysical Institute, University of Bergen
Supervisors: T. Gammelsrød, A. Wåhlin and S. Østerhus
Dynamical processes of
importance to dense overflow plumes are studied, with a special
focus on the Filchner Overflow plume, Antarctica. The Filchner
Overflow Plume consists of cold and dense Ice Shelf Water formed
under the Filchner-Ronne Ice Shelf in the southwestern Weddell Sea,
which spills over the sill of the Filchner Depression at a rate of 1.6 Sv.
The influence of small-scale topography on plume paths and
mixing is explored theoretically and experimentally, and it is shown
that submarine ridges, like canyons, can steer dense plume water
downslope. An analytical model describing the topographically
steered flow is developed and applied to a number of idealized ridge
topographies, allowing for an estimation of their transport
capacity, i.e. the maximum amount of water that they can channel
downslope. Model results are compared with observations from the
Filchner area and with laboratory experiments, and they agree well.
The secondary circulation associated with these flows is visualized
and measured in the laboratory using a Laser Doppler Velocimeter.
The model is applied to two prominent ridges crosscutting the
continental slope in the Filchner area, and their transport capacity
is estimated to be 0.3 and 0.6 Sv respectively; a substantial part
of the total outflow. The effect of topography (ridge, canyon,
smooth slope) and flow regime (laminar, eddy, wave) on mixing was
studied experimentally. It was found that for the topographically
steered flows the increased entrainment rate was more important than
the increased descent rate and that the total mixing increased. The
small-scale topography proved to have a larger impact on mixing than
the large-scale characteristics of the flow. Dense overflow plumes
are an important link in the large-scale ocean circulation and we
show that small-scale topography (10 km) has a large impact on the
plume-path and entrainment and that it should be
considered when describing or modeling these flows.
In addition, data from moored instruments and CTD data from
the Filchner region are analyzed. The mooring data show pronounced
oscillations in the temperature and velocity records with periods of
about 35 hours, 3 days and 6 days that existing theories cannot
explain satisfactorily. The oscillations are generally barotropic,
episodic and strongest east of the two ridges. Twenty-five years of
CTD data are synthesized to give a mean picture of the plume, and
these data do not support the three plume pathways proposed in
previous work.
Circulation and Water Mass formtion in an Arctic Fjord
E. Darelius, Cand. Scient. Thesis (2003) Geophysical Institute, University of Bergen and University Centre on Svalbard
Supervisors: P. Haugan and F. Nilsen