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Effect of the Subpolar Gyre on the eastern North Atlantic

Effect of the Subpolar Gyre on the eastern North Atlantic. Clare Johnson (SAMS)*, Mark Inall (SAMS), Toby Sherwin (SAMS) * cljo@sams.ac.uk. NACLIM Annual Science Meeting 2013. Introduction - Subpolar Gyre (SPG). stronger gyre. weaker gyre.

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Effect of the Subpolar Gyre on the eastern North Atlantic

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  1. Effect of the Subpolar Gyre on the eastern North Atlantic Clare Johnson (SAMS)*, Mark Inall (SAMS), Toby Sherwin (SAMS) * cljo@sams.ac.uk NACLIM Annual Science Meeting 2013

  2. Introduction - Subpolar Gyre (SPG) stronger gyre weaker gyre Satellite altimeter data from 1992 + (Häkinnen and Rhines, 2009) Model data back to 1960 (Hätun et al., 2005)

  3. Methods • Time-series of upper water (200-700 dbar) properties from 1996 to 2011 from Ellett Line dataset. NAW: North Atlantic Water ENAW: Eastern North Atlantic Water WNAW: Western North Atlantic Water mod-WNAW: modified Western North Atlantic Water salinity potemp. nitrate phosphate

  4. Results • As Subpolar Gyre weakens: • salinity (r -0.86) • potemp. (r -0.85) • nitrate (r +0.87) • phosphate (r +0.81) • Threshold reached mid-2000s (SPG index -5 cm)

  5. Results • Mixing between: • (1) mod-WNAW from Subpolar Gyre. • (2) NAW and ENAW from boundary Subpolar and Subtropical Gyres. • Increased southern water masses when weak Subpolar Gyre. • WNAW not an important upper water mass. weakening Subpolar Gyre

  6. Results • Strong gyre: • ~ 50 % mod-WNAW from Subpolar Gyre. • ~ 50 % NAW and ENAW from southern source. • Weak gyre: • < 25 % mod-WNAW from Subpolar Gyre. • > 75 % NAW and ENAW from boundary Subpolar and Subtropical Gyres. r -0.88 Value from S-θonly Mean value from S-θand S-PO4

  7. Results • Since the mid-2000s, the Subpolar Gyre has been weaker than anything observed since 1992, or modelled since 1960-1970. • We suggest the maximum proportion of southern water masses (and minimum proportion subpolar water masses) reached. Time-series 5-300 m Bay of Biscay (ICES Report Ocean Climate 2009)

  8. Conclusions • Between 1996 and mid-2000s, upper water properties in the Rockall Trough primarily controlled by the extent of the Subpolar Gyre (1975 to mid-2000s forθand S). • As the gyre weakens, the upper waters become: warmer, saltier and reduced in nitrate and phosphate. • Linked to increased advection of southern-origin water masses (and decreased influence of subpolar-origin water masses) as the Subpolar Gyre contracts. • Since the mid-2000s the gyre has been particularly weak. Upper water properties no longer directly related to changing ratio of southern to subpolar water masses. • Instead suggest related to changes in properties of southern water masses. • Johnson, C. et al., 2013, Declining nutrient concentrations in the northeast Atlantic as a result of a weakening Subpolar Gyre, DSR I, 82, 95-107.

  9. The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299 NACLIM www.naclim.eu

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