Mechanisms for variable North Atlantic–Nordic seas exchanges

Abstract

[1] The variable oceanic exchanges between the Nordic seas and the Atlantic proper have been investigated using an isopycnic coordinate ocean model for the period 1948-2007. Observed and simulated time series of volume transports in the Denmark Strait (DS), between Iceland and the Faroe Islands, and in the Faroe-Shetland Channel (FSC) are used to evaluate the model, and the model captures much of the variability. The inflow of Atlantic Water in the FSC and the outflow of light Polar Water in the DS and of dense Overflow Water in both FSC and DS are all found to covary with an atmospheric pattern resembling the North Atlantic Oscillation. An increase in the FSC inflow is associated with a decrease in the FSC overflow and an increase in the DS overflow. The exchanges' response to the atmospheric forcing is mainly of a barotropic nature, but they are also influenced by baroclinic processes. The modeled antiphase between FSC inflow and overflow is connected to a vertical displacement of the isopycnal separating the two water masses in the channel and along the path of the Norwegian Atlantic Slope Current, consistent with hydraulic control of the FSC exchanges.

The circulation of Icelandic waters – a modelling study

“…This independence of the coastal circulation on wind forcing is supported by the results of the numerical sensitivity experiments performed by Ólason (2006). Furthermore, our findings are in agreement with recent works on the forcing of the Faroe Current (FC) (Hansen et al, 2010;Richter et al, 2012;Sandø et al, 2012). Herein, the meridional gradient of sea surface height across the Arctic Front, caused by the density gradient or even by the removal of dense water by the overflow (Hansen et al, 2010), is identified as the basic forcing of the FC.…”

Using simulations of the last millennium to understand climate variability seen in palaeo-observations: similar variation of Iceland–Scotland overflow strength and Atlantic Multidecadal Oscillation

“…Also in BCM, strong Iceland-Scotland overflow is mainly caused by anomalously high SSH in the eastern part of the Nordic seas in accordance with Sandø et al (2012). The authors analyse an ocean-only simulation with a regional version of the ocean component of BCM forced with atmospheric reanalysis fields and suggest that variations of the overflow transport across the ISR are mainly of a barotropic nature.…”

“…According to the literature (e.g. Hansen and Østerhus, 2007;Jungclaus et al, 2008;Olsen et al, 2008;Sandø et al, 2012), the Iceland-Scotland overflow strength is affected by the pressure gradient across the ISR in the core depth of the overflow. Mechanism (ii) thus implies a similar variation of Iceland-Scotland overflow strength and AMO index due to the influence of the Nordic seas surface state and density structure on the pressure gradient across the ISR.…”

“…Hansen and Østerhus, 2007) and modelling (e.g. Olsen et al, 2008;Sandø et al, 2012) studies add the importance of the barotropic pressure gradient. Note that mechanism (ii) may also involve the large-scale ocean circulation through the transport of heat and salt from the subtropics into the Nordic seas.…”

Changes in the strength of the Nordic Seas Overflows over the past 3000 years

“…Modelling studies have frequently proposed an antiphasing of the volume transport between 480 these overflows over the GSR, caused by the differing effects of atmospheric forcing on the 481 deep water formation at the different convection sites in the Nordic Seas which supply the 482 overflows [Biastoch et al, 2003;Kohl, 2010;Serra et al, 2010;Sandø et al, 2012]. 483 Nonetheless, this relationship has not yet been clearly observed in the instrumental record.…”