Transcript Ekman Transport

(Pinet)
Major ocean current systems
Surface patterns extend as deep as 1000 m
Ekman Transport
Westerlies
The red box...
Southward
transport
Trades
Ekman transport is
proportional to wind stress
 greater transport for
greater wind stress
Northward
transport
At the convergence,
water piles up and sinks,
thus depressing the
thermocline and
deepening the nutricline!
Convergence (sea surface pile-up)
Sea surface
Pressure
gradient force
Coriolis force
Velocity into page
Geostrophic Balance
Pressure gradient force balanced by Coriolis force requires that the
velocity be into the page, along the pressure lines, not across them
(opposite to our daily experience of a “ball rolling downhill”).
Geostrophy – a frictionless balance between the pressure gradient
and the Coriolis acceleration – generates currents that move
around the ‘hill’
Snapshot of ocean temperature – shows lots of eddies…
Warm-core
ring
Cold-core
ring
PERMANENT THERMOCLINE
km
0
1
2
3
4
5
6
Vertical Temperature Section along 36o North
Gulf Stream
STRONG NORTHWARD FLOW
Open Gyre
SLOW SOUTHWARD FLOW
km
0
1
2
Med Outflow
Part of Thermohaline Circulation
3
4
5
6
Wind-Driven Circulation Section along 36o North
(Fuglister, 1960)
km
0
1
2
3
4
5
6
Vertical Salinity Section along 36o North
(Fuglister, 1960)
Permanent Thermocline
NORTH
SOUTH
NORTH-SOUTH VERTICAL SALINITY SECTION
Atlantic Ocean
How does equatorial upwelling occur?
ITCZ
Generalized picture of equatorial currents
NEC
SEC
ECC
EUC
ECC and EUC: geostrophic currents;
ECC: runs along doldrums w/ little wind opposition
Surface water transport from E to W
wind
W
Mixed surface layer
thermocline
(EUC: flows below wind driven layer)
E
Current velocity (cm/s) of EC
ECC
SEC
S
EUC
N
NEC
Pacific – ‘normal’ equatorial flow
Trade
winds
El Niño sets in
• Decrease in trade winds in central, W Pacific
• Accompanying decrease in horizontal pressure
gradient
• thermocline sinks from 20 to 100 m, upwelling
continues, but upwelled water is warm (29C, not
16-18 C)
• Change propagates W to E in 2-3 months
• SST increases, region of ascending moist air over
W Pacific expands E
End of El Niño
• Begins in W Pacific: N, S of Equator, thermocline
rises, cooler water to surface--> cooler SST
• Lower T--> lower atm convection, trade winds
switch to normal pattern
Review of global wind and current
systems (idealized)