3. (Pinet)

4. Major ocean current systems4

5. Surface patterns extend as deep as 1000 m5

6. Convergence Ekman Transport
The red box...
Ekman Transport
Westerlies
Southward transport
Northward transport
Convergence
Trades
At the convergence, water piles up and sinks, thus depressing the thermocline and deepening the nutricline!
Ekman transport is proportional to wind stress  greater transport for greater wind stress

7. Convergence (sea surface pile-up)

8. Pressure gradient force
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”).

9. Geostrophy – a frictionless balance between the pressure gradient
and the Coriolis acceleration – generates currents that move
around the ‘hill’

10. Snapshot of ocean temperature – shows lots of eddies…
Warm-core
ring
Cold-core
ring 10

11. Vertical Temperature Section along 36o North
PERMANENT THERMOCLINE km 1 2 3 4 5 6
Vertical Temperature Section along 36o North

12. Wind-Driven Circulation Section along 36o North
Gulf Stream
STRONG NORTHWARD FLOW
Open Gyre
SLOW SOUTHWARD FLOW km 1 2 3
Med Outflow
Part of Thermohaline Circulation 4 5 6
Wind-Driven Circulation Section along 36o North
(Fuglister, 1960)

13. Vertical Salinity Section along 36o Northkm 1 2 3 4 5 6
Vertical Salinity Section along 36o North
(Fuglister, 1960)

14. Atlantic Ocean Permanent Thermocline
NORTH
SOUTH
NORTH-SOUTH VERTICAL SALINITY SECTION
Atlantic Ocean

16. How does equatorial upwelling occur?
ITCZ

17. Generalized picture of equatorial currents

18. ECC EUC NEC SEC ECC and EUC: geostrophic currents;
ECC: runs along doldrums w/ little wind opposition

19. Surface water transport from E to W
wind W E
Mixed surface layer
(EUC: flows below wind driven layer)
thermocline

20. Current velocity (cm/s) of EC

22. ECC
NEC
SEC S
EUC N

23. Pacific – ‘normal’ equatorial flow

25. Trade winds

26. 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

27. 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

28. Review of global wind and current systems (idealized)