1. 1 Review: Lectures 7-11 G. Cowles Introduction to Physical Oceanography MAR 555 School for Marine Sciences and Technology Umass-Dartmouth

2. 2 Outline What drives the general circulation Background: Coriolis, Ekman, Geostrophy Our subtropic gyre: the North Atlantic Our WBC: The Gulf Stream Gulf Stream Instability: Rings! The subpolar gyre: impact on GoM Equatorial Current structure and ENSO Pacific (better in every way) Ocean Circulation Indian Ocean Variability: the Monsoon Antarctic Circumpolar Circ Water Masses - Clues of Origin

3. 3 What is driving the ocean? Differential Heating: Seasonal and Meridional - Both direct (deep water formation) and indirect (winds) Coriolis (technically not driving) but quite influential! + Tides, Freshwater, etc.

4. 4 Hadley Cells: Differential Heating (Equator > Poles) + Coriolis + Effects due to convergence of longitude.

5. 5 Global Average Wind Field: Note correlation with Hadley Cells What is complicating the picture?

6. 6 Coriolis: Review We want to apply Newton's Law F=ma to see how the ocean will respond (a) to a given force (F) We also want our coordinate system to be attached to the spinning earth. That is the reference frame in which we make measurements. As we discussed in detail in the last review, Newton’s law doesn’t work in this reference frame because it is non- inertial (accelerating). Things that aren’t experiencing any observable force are accelerating, according to our measurements. We modify slightly Newton’s Law to give F + Fc = ma where Fc is the Coriolis force. In most engineering problems, Fc is negligible. For geophysical flows, it is extremely important.

7. 7 Coriolis: Review. Cont’d Solution: We modify slightly Newton’s Law to give F + Fc = ma where Fc is the Coriolis force. In most engineering problems, Fc is negligible. For geophysical flows, it is extremely important. f|V| |V| f|V| |V| f|V| |V| 45N??

8. 8 Geostrophic Currents: Review Key Force Balance: Coriolis vs. Pressure Gradient 1.0m 0.5m 0.0m FPG Pressure Gradients (not considering influence of density) FPG V Fc Fc = FPG

9. 9 Geostrophic Currents: Quiz 1.0m 0.5m 0.0m Southern Hemisphere? FPG 1.0 m 0.5m 0.0m 1010mb 1000mb 990mb

10. 10 How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing At Interface: No Slip Condition Tau_wind = f( u_wind, wave state, atmospheric stability, z_obs) Wind Tugs on the Surface Water Fwind Fcoriolis Assume the ocean is solid and glides effortless over the Earth Coriolis will act as soon as the wind imparts momentum into water column Will begin to steer column to the right (Northern Hem). Picture at Right has NOT yet reached a Steady State (Forces don’t Balance) V

11. 11 How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing, cont’d Fwind Now, Water column is made of many tiny layers each having their own velocity V(t) Wind will initiate motion in first layer, Coriolis will steer it. First layer imparts a stress on second layer, Coriolis will steer it further to the right Picture at Right is NOT yet reached a Steady State (Forces don’t Balance) Fc for a layer

12. 12 How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing, cont’d Bird’s Eye View Our only external force is Fwind (assume no bottom friction) When will we reach a steady state? Steady State: acceleration = 0, thus our Forces must sum to zero. Who can possibly oppose the winds stress? Coriolis? Once the Coriolis force acting on the water column as a whole opposes in direction and magnitude the Wind, we have reached a steady state. Is our column at the top left at a steady state? Fc for a layer Fwind V1 V2 V3 Fc2 Fc3 Fc for the column Fwind V_avg

13. 13 AVERAGE Current flows at 90 degrees to the Wind Direction: This is the Ekman Transport Surface Current is 45 degrees to the right (left) of the Wind in the Northern (southern) hemisphere Ekman transport confined to the wind-driven layer (depth depends on Turbulence, Coriolis) Key Results for Ekman forcing

14. 14 Complication 1: Non-Uniform Winds

15. 15 Complication 2: Non-Uniform Winds

16. 16 Complication 1: Non-Uniform Winds

17. 17 Complication 2: A Non-Infinite Ocean

18. 18 Global Average Wind Field: Note correlation with Hadley Cells What is complicating the picture?

19. 19 Global Ocean Circulation: Principal Features

20. 20 Our Subtropic Gyre: The North Atlantic Pacific Subtropic - Similar Features

21. 21 Gulf Stream: The NA WBC Width ~100km Depth ~1000m Velocities ~100cm/s (up to 200) Transport near Florida ~ 30 Sv (what is a Sv?) Departs coast near Hatteras Is unstable (small perturbations in front position will grow) ‘Gulf’ is from Gulf of Florida Is our Western Boundary Current Some Key Facts SST: (NOAA) Pacific has the Kuroshio - Also Unstable

22. 22 Instability of the Gulf Stream: Gulf Stream Rings Cold Core: South of Front See Level Depressed Isotherms Uplifted Warm Core: North of Front Sea Level Uplifted Isotherms Depressed Influences Gulf of Maine!

23. 23 The Subpolar Gyre: 4.8 0.2 4.8- 5.3 0. 7 max 4.1 0.6 0.5 0.1 0.35 0.140.26 0.38 GoM is Influenced!! Intense Cooling of NAC

24. 24 Equatorial Currents - Note the Counter Current Trade Winds cause westward Currents To “Pile Up” on the West sides of the basins. This results in a pressure gradient which forces Eastward currents along the doldrums where There is limited wind stress to oppose them.

25. 25 ENSO

26. 26 The Indian Ocean: Monsoons and Variability NH Winter,, cold in Asia: Dry Winds Blow Offshore of the Subcontinent - Similar to Pacific Atlantic NH Summer,, continent heats up, winds shift to SW, Dump moisture form Arabian Sea onto Subcontinent: Key feature - reversal of North Equatorial Current and Somali Current - Upwells in NH Summer

27. 27 Antarctic Circumpolar No landmass to obstruct Currents follow the very strong zonal winds in a loop around Antarctica Sea Surface is higher or lower as you move North from Antarctica? Is this an upwelling or downwelling system?

28. 28 Can You Explain The Variability in Primary Productivity ? 1 2 3 4 5 6

29. 29 excretion Deep NO 3 pool Vertical profile of NO 3 Sea Surface Mixed layer P grazing NH 4 NutroclinesTurbulent diffusion High nutrients uptake NO 3 Z N

30. 30 Divergence Opposing Currents Cyclonic: Upwelling Coastal Upwelling Anti-Cyclonic - Downwelling