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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

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