Intra-seasonal variability of the DWBC in the western subpolar North Atlantic

Abstract

The Deep Western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic is an important component of the deep limb of the Meridional Overturning near its northern origins. A network of moored arrays from Denmark Strait to the tail of the Grand Banks has been installed for almost two decades to observe the boundary currents and transports of North Atlantic Deep Water as part of an internationally coordinated Observatory for the Atlantic Meridional Overturning Circulation. The dominant variability in all of the moored velocity time series is in the week-to-month period range. While the temporal characteristics of this variability changes only gradually between Denmark Strait and Flemish Cap, a broad band of longer term variability is present farther along the path of the DWBC at the Grand Banks and in the interior basins (Labrador and Irminger Seas). The vigorous intraseasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high-frequency fluctuations. Here, the intra-seasonal variability at key positions along the DWBC path using both, observations and high resolution model data is quantified. The results are used to evaluate the model circulation, and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation. Topographic waves are found to be trapped by the steep topography all around the western basins, the Labrador and Irminger Seas. In the Labrador Sea, the high intra-seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior. There, the variability is also on intra-seasonal timescales, but at much longer periods around 50 days. We would like to thank reviewer #2 for his/her very careful evaluation of our manuscript, the constructive criticism and the recommendations for corrections and suggestions. Below, recommendations by the reviewer are in blue and our response in black.Review of "Intra-seasonal variability of the Deep Western Boundary Current in the western subpolar North Atlantic", Manuscript Number: PROOCE-D-13-00096 1 General comments: Using long current meter records in the western margin of the subpolar gyre, the authors investigate the intra-seasonal variability in the Deep Western Boundary Current. From the Denmark Strait to the Grand Banks they show -that the dominant variability is in the week-to-month period range -that 10 day periods dominate the variance, which they attribute to topographic Rossby waves -that at Flemish Cap and farther south, there is also variability at longer periods -that in the basin centers (Labrador and Irminger) the variance dominate at 50 day period and there is almost no variance at 10 days. Using a long time series in the DWBC at 53°N they also find seasonality in the intra-seasonal variability, with an offset of 6 months between the surface and the bottom. Then to validate a new high resolu...

High-resolution modeling of the mean flow and meso-scale eddy variability around the Grand Banks of Newfoundland

“…The alongtrack altimeter data provide estimates of SLA and surface geostrophic currents. Additional data from in situ observations, such as moored current-meter (Fischer et al 2014), and bottom pressure data (Hughes et al 2013;Elipot et al 2014), are needed to evaluate model solutions at depth. In turn, the model results can help to interpret the discrete in situ measurements.…”

Dynamics of North Atlantic western boundary currents

“…Since this time there have been many direct current measurements using moored instrumentation that confirm this equatorward flow pattern; in the subpolar gyre at 53 ∘ N Fischer et al, 2010Fischer et al, , 2015 at 47 ∘ N (Rhein et al, 2011;Mertens et al, 2014) and at the Grand Banks , close to the inter-gyre boundary at Line W 1 about 39 ∘ (see Figure 3-1) (Joyce et al, 2005;Toole et al, 2011;Peña-Molino et al, 2012), at Cape Hatteras (Pickart et al, 1990), at 26.5 ∘ N (Meinen et al, 2013a;Srokosz and Bryden, 2015), at 16 ∘ N (Kanzow et al, 2006(Kanzow et al, , 2008Köhler et al, 2014) and in the Southern…”

“…The additional measurement locations that we analyze include a time series at the exit of the Labrador Sea at 53 ∘ N 2 , where yearly shipboard hydrographic measurements have complemented a moored array across the DWBC since 1996 Fischer et al, 2010Fischer et al, , 2015. Our focus is on the shipboard data because they include salinity measurements.…”

“…Because the directly measured speed of the DWBC is on the order of 5 −1 (Toole et al, 2011;Schott et al, 2006;Fischer et al, 2015), and will only get slower with horizontal averaging, the slower advection and mixing solution is much more realistic. For these solutions, is most sensitive to the relative amplitudes of the Line W and Labrador Sea time series; smaller relative amplitudes indicate shorter , or a larger role for mixing.…”

Simulating the Agulhas system in global ocean models – nesting vs. multi-resolution unstructured meshes

“…They are not only utilized to study ocean dynamics and the role of the ocean in the climate system (e.g., Griffies and Treguier, 2013), but they also guide the establishment of ocean observing system and are used for analysis in conjunction with direct ocean observations (Fischer et al, 2014;Hirschi et al, 2003). In an interdisciplinary context, OGCMs play an important role by providing the physical circulation that determines the distribution of biogeochemical tracers or the spreading of marine organisms and particles (Duteil et al, 2014;Baltazar-Soares et al, 2014).…”