General Circulation of the Ocean Lecture 6 Lisa Goddard
Feb 04, 2016
General Circulation of the OceanLecture 6
Lisa Goddard
* 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).
Main points:
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
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.
The Ocean Transports:
Heat
Freshwater
Mean surface ocean currents
Schematic view of the Gulf Stream and the
North Atlantic Subtropical Gyre A bit of reality
Sea surface temperature (SST)
Surface salinity
Ocean Temperature
sAnnual means
(°C)1000 m
2000 m
3000 m500 m
200 m
0 m
200 m
Pacific stratification is very different from that of the Atlantic:There is no deep overturning in the Pacific
Pacific stratification is very different from that of the Atlantic:There is no deep overturning in the Pacific
PacificAtlantic
Vertical structure
thermocline halocline pycnocline
Mixed layer
Salinity-min, Antarctic Intermediate Water
Salinity-max, North Atlantic Deep Water
Cold Antarctic Bottom Water
Subtropical
Southern Ocean
Winter Sea IceFreezing surface water
Newly formed AABW
Oxy-max [NADW]
Oxy-min [thermocline]
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
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
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.
• 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.
Surface winds
JanuarySurface winds
Surface current
s
Simplified view of surface ocean
gyres Subpolar Gyre
Subtropical Gyre
Subtropical Gyre
Subpolar Gyre
Ekman flow (Ekman transport, Ekman spiral)A balance between Coriolis force and wind stress + friction in the water
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.
Ekman (1905)
Coastal Upwelling
upwelling of colder, nutrient-rich water
Geostrophic currents from Ekman transport
Dynamic Height at the Surface
Geostrophic flows balance the pressure gradients
Dynamic Heightrelative to 2000mFrom T, S data
at 1500m
at 0m
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.
Dynamic HeightFrom T, S datarelative to 2000m
Steric HeightCalculated from Wind stress data
The Sverdrup Solution
NADW
AABW
80% of the bottom water is too cold to be explained by the happenings of the North Atlantic “Esturary”.
NADW
AABW
X AABW sources
Already the Day After Tomorrow?Bogi Hansen, Svein Østerhus, Detlef Quadfasel,William Turrellwww.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004
The Oceans’ Role in Climate:
Heat
Freshwater
Transport
s:
Sea Surface Temperatures influence the heating pattern
driving the atmosphere