Introduction to Observational Physical Oceanography 12.808 Class 22, 3rd December, 2009

1. Introduction to Observational Physical Oceanography 12.808 Class 22, 3rd December, 2009 1:05 to 2:25 these slides are online at www.whoi.edu/class/12808

2. Where are we? • Ekman Layer (review) • Vorticity and Potential Vorticity • Sverdrup balance Last class • Review Sverdrup Balance • Western Boundary Currents • Other Currents • Sea-Level Rise Papers today’s class

3. Wind Driven Currents in the Ocean Wind inputs energy at the ocean’s surface and drives ocean currents Where the wind stress is t = raCdU U and the Drag coefficient Cd is roughly 1.5x10-3

4. Wind Driven Currents in the Ocean ‘Steady’ winds act to generate a net transport in the 50-100m wind-affected layer that is to the right (left) of the wind direction in the Northern (Southern) hemisphere  EKMAN TRANSPORT Ekman Transport occurs in the Ekman Layer (50-100m) Note units of M are kg m2/s, the solution is independent of the details of the momentum transfer…

5. Wind Driven Currents in the Ocean Divergence/Convergence of Ekman Transport gives rise to vertical motions which displace the thermocline wind ET wind ET upwelling Downwelling The displacement of the thermocline, in turn, drives motion in the much thicker deeper layers of the ocean We understand this by considering conservation of potential vorticity

6. Potential Vorticity PV = (relative + planetary vorticity)/thickness is the fluid equivalent of angular momentum for a solid body Suppose a water column with c=0 (i.e no local spin) is squashed to conserve PV it can do one two things • Start spinning anticyclonically (c<0) • Move equatorward (where f is smaller)

7. Wind Driven Currents in the Ocean Interior In the ocean interior, away from boundaries and bottom topography, the relative vorticity is very small therefore squashing (stretching) of water columns results in equatorward (poleward) flow Figure courtesy of L. Talley (UCSD)

8. Wind Driven Currents in the Ocean Interior Sverdrup Balance • About Sverdrup balance • It applies only to the interior • It applies only if there are eastern and western boundaries • The currents are geostrophic • Its direction depends on the Ekman convergence/divergence in the layer above • It does not include stratification, advection, friction in the interior

9. Western Boundary Currents So Ekman convergence/divergence drives equatorward/poleward flow in the interior of the ocean gyres – how is this flow compensated in a steady state ocean ? Through a poleward/equatorward flow along one of the boundaries which does not follow Sverdrup dynamics because it is affected by friction (which is important near the boundaries) But which boundary ? Let’s consider potential vorticity again  (Much of this theory is described in the works of Stommel and Munk. )

10. Western Boundary Currents E.g. the subtropical gyre The interior Sverdrup flow causes parcels to move equatorward to lower their vorticity (via planetary) The return flow (poleward) has to be associated with an input of positive vorticity since the parcels which enter the boundary current (south) will exit with higher vorticity (north) Thus the boundary current must achieve a positive vorticity input

11. Western Boundary Currents Western Boundary Current E.g. the subtropical gyre The interior Sverdrup flow causes parcels to move equatorward to lower their vorticity (via planetary) The return flow (poleward) has to be associated with an input of positive vorticity since the parcels which enter the boundary current (south) will exit with higher vorticity (north) Thus the boundary current must achieve a positive vorticity input Eastern Boundary Current

12. Western Boundary Currents Western Boundary Current The boundary current must therefore be on the western side since frictional effects (on a western boundary current only) will result in a positive input of vorticity (relative) to match the change in latitude of the parcels as they re-connect with the Sverdrup interior Another way to think about this is to think that the frictional boundary layer must balance the negative vorticity input by the wind. Eastern Boundary Current

13. A Western Boundary Current - The Gulf Stream

14. There are also Eastern Boundary Currents … • Wind driven by along-shore winds • shallow flow • Equatorward • Deeper Poleward Undercurrent • Associated with the alongshore pressure gradients which feeds the upwelled waters Northern Hemisphere Figure from Talley et al. California Current – North Pacific; Peru Current – South Pacific Canary Current – North Atlantic; Benguela Current – South Atlantic

15. And Equatorial Currents • Mainly driven by the easterly trade winds (in the Atlantic and Pacific) • But with a complex system of current and undercurrents Meridional sections of temperature (white) and zonal current (red is eastward cm/s) from www.pmel.noaa.gov/pubs/outstand/kess258

16. Currents in the Southern Ocean – the only ones with no zonal boundaries From http://www.eng.warwick.ac.uk Transport 120-150 Sv !!!

17. A system of fronts Note that the isopycnals slope all the way to the bottom Density Cross section across the Antarctic Circumpolar Current The current has three streams associated with the three fronts (dark shading): SF = Southern acc Front, PF = Polar Front, and SAF = Sub-antarctic Front. From Orsi (2000).

18. Surface Properties in the Southern Ocean

19. Westerlies around the Southern Ocean convergence-  downwelling divergence  upwelling

20. Upwelling/Downwelling in the Southern Ocean

21. Wind-driven gyres in the large scale ocean circulation

22. Influence of the ocean circulation on SST T0

23. Influence of the ocean circulation on Salinity Distribution S(z=0)

24. Influence of the ocean circulation on Primary Production 0 150 300 450 g C/m2/yr Annual Global Primary Production From http://marine.rutgers.edu/opp/Production/

25. Influence of the ocean circulation on Primary Production Productivity Circulation