Ocean Circulation

The Layered Ocean The oceans have a well-mixed surface layer of approximately 100 meters(300 feet) Layers of increasing density from 100 meters to 1000 meters Below 1000 meters, the waters of the deep ocean are relatively homogeneous

Pycnocline 100 meters to 1000 meters~ the density changes rapidly with depth

If the density of water increases with depth, the water column is stable If more dense water is on top of the less dense water, the column is unstable An unstable column cannot persist. Vertical overturn of water takes place. Isopycnal – if a water column has the same density with depth, there is neutral stability It is easily vertically mixed with wind, waves and currents

Thermocline 100 meters to 1000 meters ~ the temperature decreases rapidly with depth

Below the thermocline, the temperature is relatively uniform. Isothermal – water temperature is unchanging with depth

Halocline At Middle Latitudes: 100 meters to 1000 meters – the salinity increases rapidly with depth

Below 1000 meters, the salinity remains unchanging with depth. Isohaline – if salinity is constant with depth

Density Driven Circulation Processes that increase water’s density at the surface cause convective mater movement ~ Vertical Circulation – top to bottom exchange of water. Because density is controlled by surfaces changes in temperature & salinity, we call this: Thermohaline Circulation

Upwelling & Downwelling Continuity of flow – Because water is a fixed quantity in the oceans, it cannot be accumulated or removed at given locations without movement of water between these locations. Downwelling Zones – areas of thermohaline circulation where water sinks Upwelling Zones – areas of thermohaline circulation where water rises

Upwelling & Downwelling

Downwelling - transports oxygen rich surface water to depth Upwelling – returns water with dissolved, decay-produced nutrients that have accumulated at depth to the surface. (acts as fertilizers)

Upwelling & Downwelling Upwelling & Downwelling can also be caused by wind-driven surface currents. Surface Convergence – surface waters are driven together by wind or against a coast Water sinks or downwells Surface Divergence – wind blows surface waters away from each other or away from a coast Water from below is upwelled

Upwelling & Downwelling Speed of upwelling or downwelling at the surface:0.1 to 1.5 meters per day Thermohaline flow at the mid-ocean depth: 0.01 cm per day *** water caught at the mid-ocean depth may spend 1000 years at these depths before reaching the surface***

The Layered Ocean The structure of an ocean is determined by the properties of the layers of water present under the sea surface. Each layer received its characteristic salinity, temperature & density at the surface Density controls the depth to which the water sinks The thickness & horizontal extent of each layer are related to the rate of its formation and the size of the surface region from which it came

The Layered Ocean The layers of water are associated with depth zones: Surface (to 200 meters) Intermediate (300 meters to 2,000 meters) Deep ( 2,000 meters to 4,000 meters) Bottom (below 4,000 meters)

The Layered Ocean North Atlantic Deep Water (NADW) Begins at 50° -60°N ~ boundary of the Gulf Stream, Greenland & Labrador currents in the Norwegian Sea

The Layered Ocean Antarctic Intermediate Water (AIW) 40° S ~ warmer and less dense than NADW so it remains above it

The Layered Ocean Antarctic Bottom Water (ABW) At the edge of Antarctica, very dense water formed by sea ice formation – Densest water in the world’s oceans

The Layered Ocean South Atlantic Surface Water (SASW) 60° S divergence moves north Rising North Atlantic Deep Water (NADW)