1. Western Intensification Subtropical gyres are asymmetric & have intense WBC’s Western intensification is created by the conservation of angular momentum in gyre Friction driven boundary current is formed along the western sidewall Maintains the total vorticity of a circulating water parcel

2. Wind Driven Gyres

3. Symmetric gyre Wind Driven Gyres

4. Wind Torque in Gyres Need process to balance the constant addition of negative wind torque Curl of the wind stress…

5. Model of steady subtropical gyre Includes rotation and horizontal friction f = constant f = 2  sin  Stommel’s Experiments

6. Stommel showed combination of horizontal friction & changes in Coriolis parameter lead to a WBC Need to incorporate both ideas into an explanation of western intensification

7. Western Intensification Imagine a parcel circuiting a subtropical gyre As a parcel moves, it gains negative vorticity (wind stress curl) Gyre cannot keep gaining vorticity or it will spin faster and faster Need process to counteract the input of negative vorticity from wind stress curl

8. Western Intensification Conservation of potential vorticity (f +  )/D Assume depth D is constant (barotropic ocean) Friction (i.e., wind stress curl) can alter (f +  ) In the absence of friction Southward parcels gain  to compensate reduction in f Northward parcels lose  to compensate increase in f

9. Symmetric Gyre

10. Western Intensification Friction plays a role due to wind stress curl (input of -  ) sidewall friction (input of +  ) + + WBC EBC

11. Western Intensification In a symmetric gyre, Southward: wind stress input of -  is balanced +  inputs by  ’s in latitude & sidewall friction Northward:  ’s in latitude result in an input of -  along with the wind stress input of -  This is NOT balanced by +  by sidewall friction Need an asymmetric gyre to increase sidewall friction in the northward flow!!

12. Symmetric Gyre

13. Western Intensification In a symmetric gyre, Southward: wind stress input of -  is balanced +  inputs by  ’s in latitude & sidewall friction Northward:  ’s in latitude result in an input of -  along with the wind stress input of -  This is NOT balanced by +  by sidewall friction Need an asymmetric gyre to increase sidewall friction in the northward flow!!

14. Potential Vorticity

15. Western Intensification In a asymmetric gyre, Southward: wind stress input of -  is balanced +  inputs by  ’s in latitude & sidewall friction Northward:  ’s in latitude result in an input of -  along with the wind stress input of -  This IS balanced by LARGE +  from sidewall friction Total vorticity balance is satisfied & we have an asymetric gyre

16. Potential Vorticity

17. Role of Wind Stress Curl Spatial  ’s in wind stress control where Ekman transports converge Where changes in  w = 0, the convergence of Ekman transports = 0 This sets the boundaries of gyres M y = 1/(  f/  y) curl  w = (1/  ) curl  w -> Sverdrup dynamics

18. Munk’s Solution

19. Currents

20. Western Intensification Intense WBC’s create a source of positive vorticity that maintains total vorticity balance Creates asymmetric gyres & WBC’s Boundary currents are like boundary layers Wind stress curl &  ’s in Coriolis parameter with latitude are critical elements Can be extended to quantitatively predict water mass transport (Sverdrup theory)