Hydrographic variability of Denmark Strait Overflow Water near Cape Farewell with multi-decadal to weekly time scales

Abstract

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. a b s t r a c tData in temperature and salinity from near-bottom layer of Denmark Strait Overflow Water in the Irminger Sea near 601N are presented. These are hydrographic section data from data archives, literature, and CTD observations along the former WOCE AR7E line, as well as data from continuous measurements at two locations. For both salinity and temperature, hydrographic variability was present at all time scales that could be analysed, from multi-decadal to weekly. A mechanistic explanation for the primal cause of the hydrographic variability is still lacking. The direct cause of the observed variability has to be sought in variations of the properties of the source water types involved in the overflow across the sill in Denmark Strait, variations in the mutual ratios of these source water types, and in inhomogeneities in the DSOW layer. The strongest changes occurred at the longest time scales, as is often found for ultimately randomly forced geophysical time series. However, a significant annual cycle of the DSOW, not reported before, was observed at the foot of the East Greenland continental slope, likely a deterministic contribution to the hydrographic variability signal.

Atlantic Deep Water Formation Occurs Primarily in the Iceland Basin and Irminger Sea by Local Buoyancy Forcing

“…The entrainment of ambient water by the overflow does not seem to play a major role in the transformation of upper waters to densities larger than σ MOC . Nevertheless, the entrainment is not negligible in the subsequent transformation of the deep waters within the lower limb of the AMOC (Dickson & Brown, 1994;van Aken & de Jong, 2012), and thus on the deep water properties exported southward.…”

“…The transformation of surface water by air-sea fluxes is computed using a well-developed method based on a linearized version of the equation of state (Speer & Tziperman, 1992;Tziperman, 1986;Walin, 1982). Assuming steady state, the amount of water transformed across an isopycnal outcrop toward higher or lower densities by net heat and freshwater fluxes is computed for each month and each isopycnal σ following the equations below.…”

“…The transformation of surface water by air-sea fluxes is computed using a well-developed method based on a linearized version of the equation of state (Speer & Tziperman, 1992;Tziperman, 1986;Walin, 1982).…”

Entrainment and Energy Transfer Variability Along the Descending Path of the Denmark Strait Overflow Plume

“…Recent work has helped to clarify the volume transport through the Denmark Strait sill, and has highlighted the high temporal variability of the overflow, dominated by scales of 2-5 days (e.g., Jochumsen et al, 2017;von Appen et al, 2017). From the 600 m deep sill, the plume descends to more than 2,500 m at Cape Farewell (van Aken & de Jong, 2012), increasing its volume transport from 3.2 Sv (Jochumsen et al, 2017) to 6 Sv (Dickson et al, 2008), although 5.2 Sv is already reached in the first 200 km (Dickson & Brown, 1994;Girton & Sanford, 2003). The increase in volume transport is mainly a result of entrainment, when the plume's water mixes with lighter and warmer ambient water.…”

Warming‐to‐Cooling Reversal of Overflow‐Derived Water Masses in the Irminger Sea During 2002–2021

“…This was partly attributed to an upstream freshening of the source waters in the Nordic Seas, and partly to mixing with upper and intermediate water masses southeast of Iceland and in the Irminger Seas that were themselves freshening at equal or greater rates. A reversal of this trend in the late 1990's was subsequently observed, with ISOW and DSOW entering a warming and salinification phase until the year 2006 at least (Holliday et al, 2008;Sarafanov et al, 2010Sarafanov et al, , 2007van Aken & de Jong, 2012). Here, we pursue those efforts and analyze the recent variability in ISOW and DSOW properties in the Irminger Sea using nine biennial occupations (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) of the Portugal-Greenland A25-Ovide section (Daniault et al, 2016;Mercier et al, 2015), the 2017 occupation of the Reykjanes Ridge Experiment (RREX) sections (Petit et al, 2018), 6 yr of OSNAP mooring data (2014-2020), and about 6 yr (2016-2021) of continuous Deep-Argo observations.…”