Wind forcing of salinity anomalies in the Denmark Strait overflow

Abstract

Abstract. The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. The salinity of the overflow plume has been measured by an array of current meters across the continental slope off the coast of Angmagssalik, southeast Greenland since September 1998. During 2004 the salinity of the overflow plume changed dramatically; the entire width of the array (70 km) freshened between January 2004 and July 2004, with a significant negative salinity anomaly of about 0.06 in May. The event in May represents a fresh anomaly of over 3 standard deviations from the mean since recording began in 1998. The OCCAM 1/12 • Ocean General Circulation Model not only reproduces the 2004 freshening event (r = 0.96, p < 0.01), but also correlates well with salinity observations over a previous 6 year period (r = 0.54, p < 0.01), despite the inevitable limitations of a z-coordinate model in representing the mixing processes at and downstream of the Denmark Strait sill. Consequently the physical processes causing the 2004 anomaly and prior variability in salinity are investigated using the model output. Our results reject the hypotheses that the anomaly is caused by processes occurring between the overflow sill and the moorings, or by an increase in upstream net freshwater input. Instead, we show that the 2004 salinity anomaly is caused by an increase in volume flux of low salinity water, with a potential density greater than 27.60 kg m −3 , flowing towards the Denmark Strait sill in the East Greenland Current. This is caused by an increase in southward wind stress upstream of the sill at around 75 • N 20 • W four and a half months earlier, and an associated strengthening of the East Greenland Current.

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

“…In 48 day, from 24 May to 9 July 2004, the salinity decreased with 0.050, coincident with a sudden temperature decrease of 0.42 1C. The resulting salinity minimum at LOCO2 can be identified with as similar minimum, encountered at 3 moorings of the Angmagssalik array, spanning a distance of about 40 km at $ 631N around 1 May 2004 Hall et al, 2011). The 40 day transfer of this minimum over 31 latitude suggests a definitely higher advection velocity for anomalies (11 per 23 day or about 6.4 cm/s) than the velocity of 11 per 56 day, proposed by Holfort and Albrecht (2007).…”

“…Rudels et al (2002)) or via the North Icelandic Jet, a current from the Iceland Sea over the slope off northwest Iceland (Jonsson and Valdimarsson, 2004). While Holfort and Albrecht (2007) speculate that the sudden freshening is due to a change in the source waters of DSOW, Hall et al (2011) conclude however, from a systematic model analysis, that the salinity decrease is related to a wind driven increase in the volume transport in the overflow, so that more fresh bottom water enters the Irminger Sea, which takes more time to mix with the overlying saline NEADW to increase to standard salinities in the DSOW, DSOW S . So, the ultimate explanation for the observed shifts from a DSOW S dominated overflow to overflow with DSOW F characteristics and vice versa, although mediated by the wind forcing near Denmark Strait, still remains an open question.…”

“…8 (big black dots) illustrates the range from freshening events (DSOW F ) to ''normal'' circumstances (DSOW S ). According to Holfort and Albrecht (2007) and Hall et al (2011) the freshening events correlate with strong southerly winds upstream of the sill in Denmark Strait. Different paths for the supply of source waters for the DSOW may be involved here.…”

“…Comparison of these continuous data with the salinity data and meridional wind data by Holfort and Albrecht (2007) has revealed that the extreme freshening events with a time scale of about 6 months are also correlated with the meridional wind events over Denmark Strait . Comparison of the data with an OCCAM model simulation suggests that the cause of the 2004 low-salinity anomaly is not an increase in upstream net freshwater input but a wind driven increase in volume flux of low salinity water, with a high potential density, at around 751N, 201W four and a half months earlier, and an associated spin-up of the Greenland Sea Gyre (Hall et al, 2011). Temperature variation of DSOW is less well described, but from temperature measurements by a current meter array in and out of the DSOW core, also at $ 631N, Dickson et al (1999) derived a considerable variability in temperature of the bottom layer over the slope.…”

“…1, white bar) from the summer of 1998 to the summer of 2005 (with a 1 year data gap in 1999-2000) have revealed intra-annual variability Hall et al, 2011). Extreme freshening events by about 0.1 were observed between January and July in both 1999 and 2004.…”