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