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North Atlantic Subpolar Gyre Dynamics in the Absence of Major Freshwater Forcing

Quillmann, Ursula 1 ; Jennings, Anne E 2 ; Marchitto Jr., Thomas M 3 ; Andrews, John T 4 ; Haflidason, Haflidi 5 ; Hall, Ian R 6

5 Univeristy of Bergen
6 Cardiff University

Pronounced changes in the water mass properties of the high-latitude North Atlantic have taken place over the last decade. Substantial evidence has been provided that these changes are associated with subpolar gyre (SPG) dynamics. Presently the SPG oscillates between an extended mode and a contracted mode (Fig 1). Preliminary results of records are presented starting at ~8000 cal yr BP, after the freshwater forcing from the decaying Laurentide ice sheet had waned. The marine sediments are expected to record SPG dynamics (Fig 2). NEAP4K lies within the SPG during its extended mode but lies to the east of the SPG during its contracted mode. Core JM96-1216 in the Denmark Strait should be sensitive to varying amounts of Atlantic Water carried northward by the Irminger Current (IC), which present-day observations show is enhanced during the contracted SPG mode. For comparison, cores MD99-2259 and MD99-2256 lie on the SW Iceland shelf and record the upstream water mass properties. HM119-04GC, in the main stream of North Atlantic Current entering the Nordic Seas is hypothesized have a stronger Atlantic water signal during the extended SPG mode, when more Atlantic water reaches the Norwegian Sea. δ18Oseawater and temperature were reconstructed by paired measurements of δ18Ocalcite and Mg/Ca ratio of planktonic foraminifera. Faunal assemblages and mineralogy are available for some of the cores. These preliminary results are part of an overarching project to reconstruct the evolution of the SPG during the Holocene and map the northern and eastern boundaries of the gyre at different time slices.


Fig 1.

The two end members of the subpolar gyre modes as recognized over the past 50 years and their relationship with the North Atlantic Oscillation (Sarafanov et al., 2009, their fig 3). Darker (lighter) arrows indicate surface currents transporting colder, subpolar (warmer, more saline subtropical) water with the area “E” marking the location where the Atlantic waters are entrained into the Iceland Scotland Overflow Water. Dashed lines indicate pathways of Nordic Seas overflow waters. Other abbreviations: SPG - Subpolar Gyre, STG - Subtropical Gyre, c - convection (size of circles represent intensity of convection), IB - Iceland Basin, LS - Labrador Sea, RT- Rockall Trough, RR - Reykjanes Ridge. Shaded areas: <500 m water depths. RR). Figure: after Sarafanov, Falina, Mercier, Lherminier, and Soko. 2009. \"Recent changes in the Greenland–Scotland overflow-derived water transport inferred from hydrographic observations in the southern Irminger Sea\"


Fig 2.

Map of study sites with surface currents. The study sites were chosen to capture the SPG in its extreme extended and contracted modes. Solid lines with arrows indicate warm currents and dashed lines cold currents



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