Recent changes in the circulation and water mass structure of the Deep Western Boundary Current southeast of Cape Cod

Beatriz Peña-Molino, Terry M. Joyce and John M. Toole

AMOC Meeting,Miami, June 7, 2010

Dense water formation

In Stommel’s classical theory for the abyssal circulation dense water is exported southward along the Deep Western Boundary Current (DWBC).

(Talley and McCartney, 1982)

The distribution of Potential Vorticity suggests the LSW spreads southward mostly along the

western boundary

z

fPPV

Planetary Potential Vorticity

Without dense water formation there is no DWBC

(Gerdes and Koberle, 1995)

reduced forcing strong forcing

The Line W Program: monitoring the North Atlantic Deep Western Boundary Current

http://www.whoi.edu/science/PO/linew/about/index.htm

W3

(Toole et al, accepted in Deep-Sea Research)

Cross-stream(v>0)

Downstream (u>0)

In the rotated coordinate system, downstream velocities are normal to array (along-isobath

direction)

Integrated KE wavelet amplitude for the 15-60day at the bottom

The combined effect of rings and meanders reduces the southwestward flow

Leading joint-EOF (ring/meanders excluded)

The leading mode EOF of (normalized) PPV at W3

Upward migration of the PPV MINIMUM (anomaly).

(Yashayaev, 2007)

Changes at Line W follow changes in the Labrador Sea (6-8 years later)

Leading joint-EOF (ring/meanders excluded)

dLSW ISOW DSOW

DSOW dominated

dLSW dominated

The flow reorganizes around the upper and bottom velocity cores.

The Gulf Stream axis move north.

In a few words…

The PPV distribution along the North Atlantic DWBC suggests that the export of recently ventilated dLSW and DSOW have opposite phase.

Enhanced DWBC transports are associated with low (lower than the mean) PPV in the uLSW and DSOW depth ranges.

(Bower et al, 2009)

Floats vs. Eulerian view:

The export of LSW is an advective-diffusive process. The diffusive part pulls the water away from the boundary.

The LSW core deepens as we move away from the Labrador Sea

The LSW exported southward changes density (depth) from year to year.

GS-PC2

Line W-PC1

The abrupt change of the Gulf Stream is captured by the second mode EOF of the Gulf Stream position

Leading EOF of the surface geostrophic velocity correspond to domain-wide shifts in the latitude of the Gulf Stream

Filtering rings and meanders

A ring/meander index based on temperature and velocity anomaly extrema

0.1 = 1oC * 10cm/s change