Lecture 9: Recirculation gyres
Atmosphere, Ocean, Climate
Dynamics
EESS 146B/246B
Reasons for western intensification
• Energetics: The wind-work is positive for gyres
whose circulation is closed by western boundary
currents.
• Vorticity balance: The frictional torque
associated with bottom stress in a western
boundary current counteracts that associated
with the wind-stress curl.
• Westward propagation: during the spin-up of the
circulation, energy propagates to the west, piling
up at the western boundary.
Closing the circulation in the western boundary
Transport in western
boundary current
-Transport in the
interior
Depth averaged circulation in the Pacific
•The Sverdrup relation predicts a transport in the Kuroshio of ~50 Sv.
Observed transport in the Kuroshio
Observations
Numerical model
Figure from Jayne et al 2009
Sv
•Observations of the Kuroshio taken as part of the Kuroshio Extension System
Study (KESS) as well as numerical simulations show that the transport in the
current increases from 45 Sv where it leaves the coast to 115 Sv offshore.
•The enhanced transport is associated with a recirculation gyre to the south of
the current.
Observations of the transport in the Gulf Stream
87 Sv
Figure from Rossby et al 2005
•Ship-based observations of the transport in the Gulf Stream at 70 W reveal
that the transport is 87 Sv, nearly three times that predicted by the Sverdrup
relation.
Recirculation gyre south of the Gulf Stream
•The recirculation gyre is responsible for a 5 fold increase in the transport.
•Why does the Sverdrup relation under predict the transport?
Recirculation gyres
• Potential vorticity (PV)
• Interpreting the Sverdrup relation in terms
of the PV
• Geostrophic contours
• PV in a stratified ocean
• Homogenization of PV in recirculation
gyres.
The potential vorticity
•Conservation of mass + conservation of angular momentum
POTENTIAL VORTICITY
CONSERVED
Frictional change of potential vorticity
•Frictional torques change the angular momentum and the potential
vorticity as well:
•For steady flows advection of PV balances the frictional sources/sinks of PV:
•Which in the limit of a constant depth ocean yields:
Sverdrup relation
•The transport in the gyre is inversely proportional to the PV gradient.
Geostrophic contours
•In the absence of frictional torques the PV is conserved which for steady, low
Rossby number flows implies:
•The flow is parallel to contours of constant f/h, these contours are known
as geostrophic contours.
Geostrophic contours in the N. Atlantic
Figure
courtesy of
Peter Rhines,
UW
•Geostrophic contours are far from zonal Æ topography shapes the depth
averaged PV field
•Suggest that the Sverdrup relation should do a poor job of predicting the
circulation. But outside of the recirculation gyre it does a good job, why?
Density variations insulate the circulation
from the topography
•The circulation is surface intensified because of density variations, more
specifically because of its baroclinicity.
•Taylor columns in the upper layer of the ocean (i.e. above the thermocline)
feels the interface rather than the ocean bottom.
The potential vorticity in a layered ocean
•Conservation of mass + conservation of angular momentum
POTENTIAL VORTICITY
CONSERVED
Structure of the potential vorticity in the N. Atlantic
h
•In the recirculation gyre, h
increases to the north, which
weakens the gradient in the PV.
Consequence of the weak PV gradient
•For steady flows advection of PV balances the frictional sources/sinks of PV:
•The transport in the gyre is inversely proportional to the PV gradient.
•The weak PV gradient in recirculation gyres implies that the net transport in
the gyes should be stronger than that predicted by the Sverdrup relation, as is
observed.
•Why is the PV gradient weaker in the recirculation gyres?
•Due to mixing of the PV by eddies that are shed off of the western
boundary currents.
y (km)
Mixing of potential vorticity by eddies
Surface
density
x (km)
y (km)
PV
x (km)