Spatial and temporal structure of the Denmark Strait Overflow revealed by acoustic observations

In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill

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The Atlantic Meridional Overturning Circulation (AMOC) is part of a global redistribution system in the ocean that carries vast amounts of mass, heat, and freshwater. Within the AMOC, water mass transformations in the Nordic Seas (NS) and the overflows across the Greenland-Scotland Ridge (GSR) contribute significantly to the overturning mass transport. The deep NS are separated by the GSR from direct exchange with the subpolar North Atlantic. Two deeper passages, Denmark Strait (DS, sill depth 630 m) and Faroe Bank Channel (FBC, sill depth 840 m), constrain the deep outflow. The outflow transports are assumed to be governed by hydraulic control (Whitehead 1989, 1998). According to the circulation scheme by Dickson and Brown (1994), there is an overflow of 2.9 Sv (1 Sv = 1 Sverdrup = 106 m3 s–1) through DS, 1.7 Sv through FBC and another 1 Sv from flow across the Iceland%Faroe Ridge (IFR). To the south of the GSR, the overflows sink to depth and then spread along the topography, eventually merging to form a deep boundary current in the western Irminger Sea. During the descent, the dense bottom water flow doubles its volume by entrainment of ambient waters (e.g. Price and Baringer 1994) so that there is a deep water transport of 13.3 Sv once the boundary current reaches Cape Farvel (Dickson and Brown 1994). Thus the overflows and the overflow-related part of the AMOC account for more than 70% of the maximum total overturning, which is estimated from observations to be about 18 Sv (e.g. Macdonald 1998

Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution

Series of sensitivity tests were performed with a z-coordinate, global eddy-permitting (1/4º) ocean/sea-ice model (the ORCA-R025 model configuration developed for the DRAKKAR project) to carefully evaluate the impact of recent state-of-the-art numerical schemes on model solutions. The combination of an energy-enstrophy conserving (EEN) scheme for momentum advection with a partial step (PS) representation of the bottom topography yields significant improvements in the mean circulation. Well known biases in the representation of western boundary currents, such as in the Atlantic the detachment of the Gulf Stream, the path of the North Atlantic Current, the location of the Confluence, and the strength of the Zapiola Eddy in the south Atlantic, are partly corrected. Similar improvements are found in the Pacific, Indian, and Southern Oceans, and characteristics of the mean flow are generally much closer to observations. Comparisons with other state-of-the-art models show that the ORCA-R025 configuration generally performs better at similar resolution. In addition, the model solution is often comparable to solutions obtained at 1/6 or 1/10º resolution in some aspects concerning mean flow patterns and distribution of eddy kinetic energy. Although the reasons for these improvements are not analyzed in detail in this paper, evidence is shown that the combination of EEN with PS reduces numerical noise near the bottom, which is likely to affect current-topography interactions in a systematic way. We conclude that significant corrections of the mean biases presently seen in general circulation model solutions at eddy-permitting resolution can still be expected from the development of numerical methods, which represent an alternative to increasing resolution