Understanding Ocean Circulation Dynamics: Wind-Driven & Thermohaline Forces

1. General Circulation of the Ocean Lecture 6 Lisa Goddard

2. Main points: • * The ocean is forced from the surface by fluxes of momentum and buoyancy (heat and freshwater). • * The wind driven circulation is by far the more energetic and for the most part resides in the ocean’s top kilometer. • * Theory for the wind-driven circulation: Ekman, geostrophy • * Most of the stratification is in the top km or so • * The sluggish thermohaline circulation forces ocean overturning reaching in some regions to the sea floor, resulting in the formation of the major water masses of the global ocean: • North Atlantic Deep Water (NADW) • Antarctic Bottom Water (ABW).

3. Outline • Describe: Surface Currents • Describe Ocean Structure: temperature and salinity; the surface mixed layer • Wind Driven Ocean Circulation: Ekman, Geostrophic Flow, Sverdrup relation, Stommel western boundary • The buoyancy-driven thermohaline circulation

4. Water on the Planet • The ocean holds 98% of the 1.4 billion cubic kilometers of water on the planet. • Exchange of this water between ocean, atmosphere and land forms the global hydrological cycle.

5. The Ocean Transports: Heat Freshwater

6. Mean surface ocean currents

7. A bit of reality Schematic view of the Gulf Stream and the North Atlantic Subtropical Gyre

8. Sea surface temperature (SST)

9. Surface salinity

11. Ocean Temperatures Annual means (°C) 0 m 1000 m 200 m 200 m 2000 m 500 m 3000 m

12. Atlantic Pacific Pacific stratification is very different from that of the Atlantic: There is no deep overturning in thePacific

14. Vertical structure

15. Winter Sea Ice Freezing surface water Mixedlayer Oxy-min [thermocline] thermocline halocline pycnocline Salinity-min, Antarctic Intermediate Water Salinity-max, North Atlantic Deep Water Oxy-max [NADW] Subtropical Southern Ocean Cold Antarctic Bottom Water Newly formed AABW

17. Ocean and Atmosphere • Both are shallow(thin layers of fluid) • Both are rotating rapidly • Both are stratified fluids (usually stably, with lighter fluid on top) • Rotation and Buoyancy are important: • Geophysical Fluids

18. Ocean vs Atmosphere • The ocean has sidewall boundaries. • The ocean has a definitive top while the atmosphere does not. • The ocean is almost incompressible. • The atmosphere is driven primarily by thermal forcing at its lower boundary; the oceans are driven primarily mechanically driven from the top. • The atmosphere has significant internal diabatic heating (latent heat release; radiation); the oceans do not. • The oceans are salty, the atmosphere is moist and cloudy • The ocean is dense (~1000 times air), with a large heat capacity and large inertia. 2.5m of water holds as much heat as the whole depth of the atmosphere

19. Wind-driven: by the wind stress* acting as a drag on the sea surface • Thermohaline: by buoyancy fluxes of heat and freshwater between the ocean and atmosphere creating a contrast between lighter and denser water masses. The Ocean Circulation is forced by the atmosphere * wind stress is a vector proportional in strength to the square of the wind speed and its direction is in the direction of the wind.

20. Surface winds

21. January Surface winds Surface currents

22. Simplified view of surface ocean gyres Subpolar Gyre Subtropical Gyre Subtropical Gyre Subpolar Gyre

23. Ekman flow (Ekman transport, Ekman spiral) A balance between Coriolis force and wind stress + friction in the water Ekman (1905) The vertically averaged Ekman flow - the Ekman transport -is 90°to the right (left) of the wind in the Northern (Southern) hemisphere. It is proportional to the square of the wind speed and its strength is 2-5% of the wind speed.

24. Coastal Upwelling upwelling of colder, nutrient-rich water

25. Geostrophic currents from Ekman transport

26. Dynamic Height at the Surface Geostrophic flows balance the pressure gradients

27. Dynamic Height relative to 2000m From T, S data at 1500m at 0m

28. Modeling of mean wind-driven circulation • Sverdrup, Stommel, and Munk laid the foundations of the modern theory of ocean wind-driven circulation in a series of papers by between 1947 and 1951. • Sverdrup showed that the curl of the wind stress drives a north-south mass transport, and that this can be used to calculate currents in the ocean away from western boundary currents. • Stommel showed that western boundary currents are required for flow to circulate around an ocean basin when the Coriolis parameter varies with latitude. Munk showed how to combine the Sverdup & Stommel solutions. • The observed circulation in the ocean is very turbulent. many years of observations may need to be averaged together to obtain a stable map of the mean flow.

29. Dynamic Height From T, S data relative to 2000m The Sverdrup Solution Steric Height Calculated from Wind stress data

31. NADW AABW

32. NADW AABW X AABW sources 80% of the bottom water is too cold to be explained by the happenings of the North Atlantic “Esturary”.

34. Already the Day After Tomorrow? Bogi Hansen, Svein Østerhus, Detlef Quadfasel,William Turrell www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

35. The Oceans’ Role in Climate: Transports: Heat Sea Surface Temperatures influence the heating pattern driving the atmosphere Freshwater