On the structure and the Variability of the Filchner
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
(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