Effect of the Subpolar Gyre on the eastern North Atlantic
description
Transcript of Effect of the Subpolar Gyre on the eastern North Atlantic
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Effect of the Subpolar Gyre on the eastern North Atlantic
Clare Johnson (SAMS)*, Mark Inall (SAMS), Toby Sherwin (SAMS)
NACLIM Annual Science Meeting 2013
www.sams.ac.uk
Introduction - Subpolar Gyre (SPG)
stronger
Subpolar G
yrew
eaker S
ubpolar Gyre
stronger gyre
weaker gyre
Satellite altimeter data from 1992 + (Häkinnen and Rhines, 2009)
Model data back to 1960 (Hätun et al., 2005)
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Methods
NAW: North Atlantic Water
ENAW: Eastern North Atlantic Water
WNAW: Western North Atlantic Water
mod-WNAW: modified Western North Atlantic Water
ENAW
WNAW NAW
mod-WNAW
UK
Iceland
Ellett Linetime-series
salinity
potemp.
nitrate
phosphate
• Time-series of upper water (200-700 dbar) properties from 1996 to 2011 from Ellett Line dataset.
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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)
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Results
NAW
NAW
NAW
NAW
ENAW
ENAW
ENAW
ENAW
WNAW
WNAW
WNAW WNAW
mod-WNAW
mod-WNAW
mod-WNAW mod-
WNAW
b.a.
c. d.
• 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.
NAW
NAW
NAW
NAW
ENAW
ENAW
ENAW
ENAW
WNAW
WNAW
WNAW WNAW
mod-WNAW
mod-WNAW
mod-WNAW mod-
WNAW
b.a.
c. d.
weakening Subpolar Gyre
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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.
Pro
port
ion
mod
-WN
AW
SP
GIndex (cm
)
Year
r -0.88
Mean value from S-θ and S-PO4
Value from S-θ only
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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)
c. d.
b.a.
e. f.
r -0.85r -0.86
r +0.87 r +0.81
r +0.88
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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 for θ and 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.
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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