Fates and Travel Times of Denmark Strait Overflow Water in the Irminger Basin (original) (raw)
2013, Journal of Physical Oceanography
The Denmark Strait Overflow (DSO) supplies about one third of the North Atlantic Deep 6 Water and is critical to the global thermohaline circulation. Knowledge of the pathways of 7 DSO through the Irminger Basin and its transformation there is still incomplete however. We 8 deploy over 10,000 Lagrangian particles at Denmark Strait in a high resolution ocean model 9 to study these issues. The particle trajectories show that: First, the mean-position and 10 potential density of dense waters cascading over the Denmark Strait sill evolve consistently 11 with hydrographic observations. These sill particles transit the Irminger basin to the Spill Jet 12 section (65.25 o N) in 5-7 days and to the Angmagssalik section (63.5 o N) in two-three weeks. 13 Second, the dense water pathways on the continental shelf are consistent with observations 14 and particles released on the shelf in the Strait constitute a significant fraction of the dense 15 water particles recorded at the Angmagssalik section within 60 days (∼ 25%). Some particles 16 circulate on the shelf for several weeks before they spill off the shelf break and join the 17 overflow from the sill. Third, there are two places where the water density following particle 18 trajectories decreases rapidly due to intense mixing: southwest of the sill and southwest of 19 the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, 20 the overflow particles exhibit a wide range of densities. 21 1 23 waters formed in the Arctic Ocean and the Nordic Seas. The dense waters pass through the 24 Irminger Basin toward the North Atlantic where they supply about one third of the North 25 Atlantic Deep Water, a major component of the global thermohaline circulation (Dickson 26 and Brown 1994). The DSO transmits the climate signals from its source regions, modified 27 en route by mixing and entrainment, and affects the properties throughout the water column 28 in the North Atlantic (Dickson et al. 2008; Yashayaev and Dickson 2008). 29 The 620-m deep Denmark Strait (DS) sill is the main gateway for dense waters exiting 30 the Greenland Sea to the Irminger Basin and is a key location for observing the DSO at 31 the start of its transit to the North Atlantic (Dickson et al. 2008). Measurements show that 32 the dense overflow through the sill is fast (speeds frequently exceed 1 m/s) and occurs as 33 pulses of dense water (referred to as boluses) cascading to the deep water south of the sill 34 at intervals of 2-5 days. On longer timescales, DSO appears as a steadier and hydraulically 35 controlled flow with a mean transport of approximately 3 Sv (1 Sv = 10 6 m 3 s −1 ; Käse and 36 Oschlies 2000; Macrander et al. 2007; Jochumsen et al. 2012). DSO temperature and salinity 37 vary on a timescale of a few-days, owing to mesoscale activity and intense mixing processes 38 near the sill (Rudels et al. 1999; Tanhua et al. 2005). The seasonal signals in the DSO 39 transport and properties are weak (Dickson and Brown 1994; Jochumsen et al. 2012). The 40 overflow composition exhibits interannual-to-decadal variations, however, most likely linked 41 to changes in the upstream source waters or pathways (Rudels et al. 2002a). These changes 42 in turn are possibly linked to atmospheric forcing and in particular to variations in the North 43 Atlantic Oscillation (Yashayaev and Dickson 2008; Serra et al. 2010).
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