Atmosphere, Ocean, Climate
Dynamics
EESS 146B/246B

• The three-dimensional, helical structure of the flow in the subtropical gyres.
• Movement of water down isopycnals.
• Preferential selection of surface waters from the winter/spring in the pycnoline.
• The unventilated shadow zones.

Geostrophic velocity based on observations from floats
Figure from Kwon and Riser 2005

Gyre boundaries in N. Atlantic

Geostrophic velocity based on observations from floats
Figure from Kwon and Riser 2005
•The circulation in the gyres has both horizontal and vertical components.
•The motion of water in the gyres should be helical.

Helical motion
•The slope of the helix is determined by the ratio of the vertical to horizontal velocity.

Estimate of helix slope in the subtropical gyre suction
•The Ekman pumping is ~10 m/year.
•The meridional flow in the gyre is ~1 cm/s
Æ helix slope ~ 3x10 -5

Along isopycnal motion
•Isopycnal slope~(50 m)/(15 degrees latitude)~3x10 -5
•The combined downward and southward velocity in the gyre allows for the subduction of water from the surface along isopycnals into the ocean interior.
•The water leaves the surface where the isopycnals outcrop into the mixed layer.

Where do density surfaces outcrop?
Figure from Qiu and Huang 1995

Where does the water go once it is subducted?
Streamfunction on 26.5 kg m -3 isopycnal
Calculated using observations from PALACE floats
50W 60W 50W
Figure from Kwon and Riser 2004
40W

Mapping of the surface T-S distribution to the vertical by subduction
T,S T,S
T,S
T,S
Figure from Qiu and Huang 1995

Where does the water go once it is subducted?
Streamfunction on 26.5 kg m -3 isopycnal
Calculated using observations from PALACE floats
50W 60W 50W
Figure from Kwon and Riser 2004
40W

Structure of T,S, and density in the mixed layer in the early spring in the N. Atlantic
•T,S, and density in the mixed layer on a cruise in early April.

T-S relation in the early spring mixed layer

Vertical structure of T,S, and density downstream, in the pycnocline
Pot. Density
Pot. Temperature Salinity pycnocline
•The temperature and salinity decrease with increasing density.
•How does this T-S relation compare to what is observed in the mixed layers to the north?

T-S relation in the pycnocline

T-S relation in the early spring mixed layer

T-S relation in the pycnocline
•The vertical structure of T and S in the ventilated pycnocline is set by subduction of surface waters with properties characteristic of the late winter, early spring.

Stommel’s demon
•Waters leaving the mixed layer in the late spring, summer, and fall can be reentrained into the mixed layer in the following winter, and thus won’t stay in the permanent pycnocline.
•Waters subducted during the late winter/early spring are sheltered from reentrainment and thus will set T-S relation in the permanent pycnocline.

Shadow zones
•In the eastern regions of the subtropical gyres, the transport streamlines on isopycnals do not connect to the surface at an outcrop.
•The water in these so called shadow zones is therefore unventilated .

Unventilated shadow zone in the N. Pacific
•Depleted in oxygen.

Unventilated shadow zone in the N. Pacific
•Depleted in CFCs.

Unventilated shadow zone