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

Wind Driven Gyres

Symmetric gyre

Wind Driven Gyres

Wind Torque in Gyres

Need process to balance the constant addition of negative

wind torque

Curl of the wind stress…

• Model of steady subtropical gyre

• Includes rotation and horizontal friction

f = constant

f = 2 sin

Stommel’s Experiments

Stommel’s Experiments

• 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

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

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

Symmetric Gyre

Western Intensification

• Friction plays a role due to

wind stress curl (input of -)

sidewall friction (input of +)

+

+

WBC EBC

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!!

Symmetric Gyre

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!!

Potential Vorticity

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

Potential Vorticity

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

• My = 1/(f/y) curl w = (1/) curl w

-> Sverdrup dynamics

Munk’s Solution

Currents

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)