General Comment on Lab Reports:
v. good + corresponds to a lab report that:
• has structure (Intro., Method, Results, Discussion, an Abstract
would be a bonus)
• is well written (take your time to edit)
• shows all figures with figure captions (Fig.1: Time series of
atmospheric pressure at Buoy 44025 for the year 1997)
• the content of the figures is discussed and referenced in the
main text
• there is a minimum amount analysis of the observations – i.e.
analysis of what you see in the figures in the context of what
you know or you are now learning in the course (From the
curves shown in Fig X. we can see that there is a lag between
the annual temperature cycle for the atmosphere and the ocean.)
Chapter 4: The N.Atlantic Gyre
•Exploration of the east coast of America,
colonization, trading and whaling led to
the Western N.A. being charted earlier
than most other ocean regions
•The Gulf stream was the most notable
current and has been depicted on some of
the earliest maps.
•The Gulf Stream consists of water from
equatorial regions and re-circulated water
from the subtropical gyre
Maury’s Chart of the Gulf Stream
Subtropical gyres:
intense western currents
diffuse eastern currents
Maury: GS not wind driven
PGF resulting from hydraulic head
in the western side of the basin
(forced by NE trade winds) gives the
jet like quality to the GS (2 m/s in
some areas)
GS follows continental slope up to
Cape Hatteras then moves into
deeper water, meanders and forms
eddies (Paul Gayes Story)
The Subtropical Gyres:
setting the stage for generating a more realistic gyre
Ekman used an ideal, infinite ocean, no slopes in sea level, or
variations in salinity
Western Boundary Currents
Eastern Boundary Currents
fast, deep, narrow, warm
slow, shallow, broad, cool
Gulf Stream, Kuroshio
Canary, California
O(100 km) width, u = 2 m/s
O(1000 km) width, u = 0.25 m/s
Dynamic Systems: conservation of momentum (linear and angular)
Conservation of a tendency to rotate
Conservation of vorticity
Current shear – change in the
velocity in the direction
perpendicular to the flow direction.
Current shear will induce a
tendency to rotate in a water parcel.
Positive Vorticity: anticlockwise
Negative Vorticity: clockwise
All rotating objects on the Earth possess:
relative vorticity: rotation in relation the surface
of the Earth – ζ
Planetary vorticity: vorticity associated with the
rotation of the Earth – f
VORTICITY is a measurement of the rotation of a small
air/water parcel. It has vorticity when the parcel spins as it
moves along its path. Although the axis of the rotation can
extend in any direction, meteorologists/oceanographers are
primarily concerned with the rotational motion about an
axis that is perpendicular to the earth's surface. If it does
not spin, it is said to have zero vorticity. In the Northern
Hemisphere, the vorticity is positive when the parcel has a
counterclockwise, or cyclonic, rotation. It is negative when
the parcel has clockwise, or anticyclonic, rotation.
Planetary Vorticity and the Coriolis force:
Poleward decrease in linear eastward velocity
The angular velocity of the surface of the Earth increases with latitude
- anticlockwise rotation around a local vertical axis N.H.
- clockwise rotation around a local vertical axis S.H.
W sin(f) = angular velocity
2W sin(f) = Planetary Vorticity (f)
Q4.3
Conservation of Vorticity: relative to fixed space
Absolute Vorticity = relative + planetary vorticity
(f + z)
Absolute vorticity of a parcel of fluid (its vorticity as a result
of being on the Earth plus any vorticity relative to the Earth)
must remain constant in the absence of any of external forces
such as wind stress and friction
Q4.4
X
Angular Momentum:
m x r2 w
m = mass
r = distance to axis of rotation
w = angular velocity
D: could be from the sea surface to the top of the permanent thermocline,
or in the case of deep and bottom waters from the permanent thermocline
to the sea floor. The depth of the column of water will change as angular
velocity changes.
Conservation of Vorticity  Conservation of Potential Vorticity
(f + z)/D
D
For much of the ocean (away from coastal boundaries and large
current shear) f >> z. This results in ‘topographic steering’.
Q4.5
Question 4.6
Why Is There A Gulf Stream
Sverdrup model – flow due to
horizontal pressure gradients
and wind stress on the surface.
But it is not wind stress that
matters, rather its torque
Ahh here we go again! Velocity (linear & angular), Force, Momentum
(linear & angular), Work, Energy, Torque and much more!!
Torque is a force that tends to rotate or turn things. (You generate a
torque any time you apply a force using a wrench, for example) –
torque equals force multiplied by distance. Remember ‘work’? Work is
simply the application of a force over a distance. The catch: the
distance only counts if it is in the direction of the applied force –
lifting an object, for example. And doing work is using energy!
The difference between torque and work is the concept of rotation.
Torque is work (distance times force) which causes rotation.
(think ‘vorticity!)
Sverdrup’s Results: the net amount of water transported by a given
pattern of wind stress depends not on the absolute value of the wind
stress but on its torque (its tendency to cause rotation, its ability to
supply relative vorticity to the ocean)
Did not account for a western boundary current.
Thinking About Meridional Transport:
M = b x torque of t
M: meridional transport kgs-1m-1
b: latitude dependent constant
M = b x curl(t)
M = 1/b x curl(t)
b: the rate of change of f with latitude
In the end: the total meridional flow is determined by the rate of
change of f at the latitude concerned and the torque, or curl, of the
wind stress.
1948: Henry Stommel onto the problem of the Gulf Stream
Considered the effects of a symmetrical wind field on a rectangular
basin for three different scenarios:
‘lines’ of wind
1.non rotating Earth
2.rotating with constant
Coriolis parameter f
3.rotating and Coriolis
parameter varies with latitude
Stommel added friction –
Sverdrup did not have it
equilibrium achieved so forces must balance (steady state)
Stommel’s Results
1. non rotating Earth
2. rotating with
constant Coriolis
parameter f
3. rotating and
Coriolis parameter
varies with latitude
Munk: extends Stommel’s domain up to 60 N, and improves the
representation of both wind and friction.
Considered friction with boundaries and friction associated with
both lateral and vertical current shear (eddy viscosity in both
horizontal and vertical dimensions; Ah and Az)
Result: reproduced
more realistic ocean
circulation features
Equations of Motion
Gulf Stream:
Extends from Straits of
Florida to Grand Banks
off Newfoundland
Flow structure on either
side of Cape Hatteras is
different
Shallow (800 m), follows
continental slope, well defined
T,S (mixture of water from
Antilles Current and water
recirculated in the Sargasso Sea
meanders on the order of 55km
North of Cape Hatteras the Gulf
Stream pulls away from the cont.
slope and moves into deeper
water (4-5 km)
meanders can exceed 350 km
Flow around Cape Hatteras 70100 svd
Flow reaches a maximum around
65W, 150 svd (106m3/s)
Flow begins to be more diffuse
as water enters the Azores
Current and the recirculatory
system
Continuity:
The formation of the NADW
(part of the recirculatory system),
which is most noticeable in
winter, strengthens the flow of
the Gulf Stream
Think about the link between
thermohaline and wind driven
circulation
Distribution of Temperature and Salinity Through the Straits of Florida
Geostrophic flow
calculation performs
reasonably well
Question 4.10
Geostrophic Currents: E-W transect extending from Cape Hatteras
Flow consists of narrow,
deep vertical filaments
separated by counter
currents
Up to the 1960, the idea of
counter currents were
rejected by the scientific
community, data was
sparse leaving room for
interpretation
velocities (m/s)
T,S Sections Chesapeake Bay – Bermuda
Measurements in 1932, could not capture
the deep filaments of the Gulf Stream or
the deeper counter currents.
Where is the Gulf Stream?
Note: the Sargasso Sea water is warmer
and more saline than the water on the
landward side. Salinity alone would make
the system unstable, the temperature
distribution has a greater effect on the
density distribution and slope of the
isopycnals
Insights From MODE (Mid-Ocean Dynamics Experiment
•Deployed neutrally buoyant floats and Sofar Drifters
•Resolution of mesoscale eddies, which were found to dominate
flow with periods greater than the tidal and inertial periods
•Fastest currents occurred near the surface, however the eddies
persisted down to at least 1500m. The axes of rotation were not
always vertical
Gulf Stream Eddies
cold core - warm core rings
Current shear generated
meanders
once the eddy has sheared off
the energy source becomes the
sloping isopycnals ‘dynamic
topography’
Back to Chapter 2:
Icelandic Low
&
Azores High