A Brief History of the Joint Effect of Baroclinicity and Relief (JEBAR ...

A Brief History of the Joint Effect of
Baroclinicity and Relief (JEBAR) and its
Application to Gulf Stream Separation
MPO 611
Geophysical Fluid
Dynamics II
Spring 2008
Benjamin Shaw

A Brief History
●
Sverdrup ocean circulation
– Considers only wind stress, ignores bottom
topographic effects
●
Sarkisyan & Ivanov (1971), the fathers of
JEBAR
– Pointed out that you can't ignore bottom baroclinic
or bottom topographic effects
– These have tremendous impact on transport in
oceanic gyres

What is JEBAR?
●
The Joint Effect of Baroclinicity And Relief
– It is now well established that the combination of
baroclinicity and sloping bottom topography can
give rise to a driving force for the depth-averaged
flow (Mertz et. al., 1992)
●
Generally derived through some combination of depth
averaging and cross differentiating simplified
equations of motion

What is JEBAR?
●
There are several interpretations of JEBAR. It
has been described by each of the following:
– Geostrophic component of the correction to the
topographic stretching term to account for the fact
that the bottom velocity, not the depth-averaged
velocity, yields topographic vortex-tube stretching
(Mertz, et. al. 1992)
– Relates to bottom torque (curl of horizontal force by
the bottom on the fluid) (Mertz, et. al. 1992)
– Difference between bottom pressure torque and the
corresponding torque associated with the depth
averaged pressure (Myers, et. al. 1996)

Derivation of the JEBAR term
●
JEBAR effect obtained when a
vorticity equation is formed from
the depth-averaged momentum
equations (Huthnance 1984)
●
The result is Jacobian of
potential energy anomaly and
depth

●
There are several interpretations of the JEBAR
term,depending on the choice of derivation. This version
relates (in order):
1. Rate of change of vorticity of the depth averaged flow
2. Transport across contours of constant planetary vorticity
3. Topographic vortex stretching
4. Surface forcing/bottom dampening
5. JEBAR effect
(Mertz, et. al. 1992)

JEBAR Skeptics
●
●
Some disagreement exists regarding
the utility of the JEBAR effect
Cane et. al. 1998 argues that:
1) JEBAR term cannot be calculated consistently,
leading to imperfect cancellation and spurious
transport values
2) Calculated velocity field is very noisy with large
relative errors
3) JEBAR overestimates the true influence of
topography on oceanic transports, especially since
majority of transport is confined to upper layers

Application of JEBAR
●
Reasons for separation of Gulf Stream from North
American coast are unclear
– Occurs at zero wind stress curl?
– Bottom topography?
– DWBC effects?
– Cyclonic recirculation?
●
●
GCMs predict separation
~3000 km farther north
Myers, et. al. (1996) able
to accurately predict
separation point using FE
model including JEBAR

Myers et. al. (1996)

Separation of the Gulf Stream
●
Dependent on JEBAR effect in 3 key areas
– Offshore of separation
– North Atlantic Bight
– Central Irminger Sea
●
●
Underestimating JEBAR
results in overshooting
the actual separation
point
Which comes first?
Separation or JEBAR?
– Observed density field already has signature of
the separated Gulf Stream in it Myers et. al. (1996)

Conclusions
●
JEBAR effect:
...shows the importance of baroclinic
effects near the ocean bottom
...is an important factor in oceanic
gyre transport
...provides solution to issue of Gulf
Stream separation from North
American coast
...may be difficult to accurately
measure

References
Cane, M.A., V.M. Kamenkovich, A. Krupitsky, 1998: On the Utility and Disutility
of JEBAR. Journal of Physical Oceanography – Notes & Correspondence,
28, 519-526.
Greatbatch, R.J., A.F. Fanning, A.D. Goulding, S. Levitus, 1991: A Diagnosis of
Interpentadal Circulation Changes in the North Atlantic. Journal of
Geophysical Research, 96, 22,009-22,023.
Huthnance, J.M., 1984: Slope Currents and “JEBAR”. Journal of Physical
Oceanography, 14, 795-810.
Mellor, G., C. Mechoso, E. Keto, 1982: A diagnostic calculation of the Atlantic
Ocean. Deep Sea Research, 29, 1171-1192.
Mellor, G., 1999: Comments on “On the Utility and Disutility of JEBAR”. Journal
of Physical Oceanography – Notes & Correspondence, 29, 2117-2118.
Mertz, G., D.G. Wright, 1992: Interpretations of the JEBAR Term. Journal of
Physical Oceanography – Notes & Correspondence, 22, 301-305.
Myers, P.G., A.F. Fanning, A.J. Weaver, 1996: JEBAR, Bottom Pressure
Torque, and Gulf Stream Separation. Journal of Physical Oceanography, 26,
671-683.