Ocean Stratification and Circulation

transcript

Martin VisbeckDEES, Lamont-Doherty Earth Observatory

visbeck@ldeo.columbia.edu

Outline

Review of Climate Variability

Ocean Stratification Ocean Circulation Review of Oceans Role in Climate

General Atmosphere Ocean CirculationThe surface energy balance

Top of atmosphere

Air-sea interface

seafloor

Imbalance of energy flux at the top can be balanced by:

Atmospheric Heat Transport

Oceanic Heat Transport

Radiative Energy Balance

The imbalance of the top of the atmosphere radiation implies that there must be an internal heat transport by the combined action of ocean and atmosphere of ~6 1015 W 30°N/S.

Top of atmosphere

Atmosphere

The Oceans Role in Climate

The annual heat flux between ocean and atmosphere is formed by the sum of all of the heat transfer process:

Solar radiation +168 Terrestial radiation -66 Evaporation -78 Heat conduction -24

Effect of Atmospheric Forcing on Ocean

Ocean Basins

The ocean composes 70.8% of the earth's surface.

Sea water fills the basins separating the continents with an average depth of 3795 meters.

Ocean Basins

The transition from the continental to the deep ocean or continental margins extend from the sea shore to around 2500 meters depth, it covers 40.7% of the ocean (29% of Earth surface).

The deep basins composes of extensive flat plains of 4000 to 5000 meters depth, and the mid-ocean ridge, marking the axis of sea floor spreading where the crust tectonic plates of the earth form.

The deep ocean covers about 59.3% of the ocean's surface (42% of Earth¹s surface).

Ocean Basins

The deepest ocean is found in the trenches where the plates are subducted, the Mariana Trench is 11035 meters deep (compared to the 8848 meter height of Mount Everest). If the solid earth were made into a flat plain, the sea water would cover the entire earth to a depth of 2440 meters.

If all of the water vapor in the atmosphere were converted to liquid it would cover the smoothed earth surface by about an 1 inch.

Ocean Basins

The ocean basins are divided into three main Oceans:

The Pacific Ocean is the largest and deepest (52% of the ocean area, mean depth of 4028 meters);

The Indian Ocean (20% area, mean depth of 3897 m) and

The Atlantic Ocean, the shallowest because of the rather narrow deep basins (25% area, mean depth of 3332 m).

The Arctic is considered part of the Atlantic Ocean;

The southern parts of the three Oceans are referred to as the Southern Ocean.

The northern hemisphere has less ocean than the southern hemisphere, only about 61% ocean versus 81% for the southern hemisphere.

Water in the Ocean

The ocean holds 98% of the 1.4 billion cubic kilometers of water on the planet.

Exchange of this water with the water vapor within the atmosphere is a key part of the global hydrological cycle.

Water Properties

In addition, the high heat capacity and density of water relative to the atmosphere, and the great amount of energy required to change its phases (solid to liquid to gas) makes the ocean a powerful factor of the Earth's climate system.

General CirculationThe seasonal cycle revisited

Strong Temperature contrast in high latitudes

Strong over Land...why?

January

Properties of Water and Climate

The ocean attenuates winter/summer and day/night changes in air temperature, and moves significant amounts of heat from low to higher latitudes, it provides moisture to the atmosphere.

Sea Surface Temperature

The link between ocean and climate depends on exchange of energy (mainly heat and radiation) and materials (water, gases) across the sea surface. T atmosphere 'sees', influences and responds to the sea surface temperature (SST), by way of sea-air heat flux. SST generally cools with increasing latitude, but important deviations from a pure latitudinal dependence occurs. These are generally due to the movement of sea water in both the horizontal and vertical directions. Temperature and density of ocean water are related inversely: warm water means low density, cold water means denser sea water.

Ocean Salinity

Sea water is about a 3.49% salt solution, the rest is freshwater.

The major salt constituent is Chloride

The more saline, the denser the sea water

Ocean Salinity

As the range of salt concentration in the ocean varies from about 3.2 to 3.8%, oceanographers, who refer to salt content as 'salinity', express salt concentration as parts per thousand; 34.9 ppt is the average salinity.

Evaporation and Precipitationand Ocean Surface Salinity

As sea water evaporates the salt remains behind, only the freshwater is transferred from the ocean to the atmosphere. A region of excess evaporation, such as the subtropics tend to become salty, while the areas of excess rainfall become fresher. Salinity reflects the workings of the hydrological cycle: the movement of freshwater through the earth/ocean/atmosphere system.

Sea Ice and Salinity

Sea ice formation also removes freshwater from the ocean, leaving behind a more saline solution. The resulting ice contains only part of the sea water salt, about 0.5% (5 ppt), hence ice formation like evaporation, concentrates salt in the remaining body of sea water.

Along the shores of Antarctica this causes very dense water (cold and salty), which in some regions in the Southern Ocean leads to deep reaching convection, called Antarctic Bottom water. Southern ocean ice exhibits lots of seasonal variability, and is generally only 0.5 meters thick.

The very low salinity of the Arctic prohibits the development of deep reaching convection. In sharp contrast is the Arctic sea where the sea ice is usually about 2 to 3 meters thick with a lesser amount of seasonality, and a water column which is very stratified. There is some evidence that global warming is reducing the area of year round sea ice in Arctic, but not (yet?) within the

Southern Ocean.

Stratification

What sets the stratification in the ocean?

Where is the warm water? Where is the cold water?

Ocean simulation in tank

Equator Pole

Basin Stratification

The surface and deep Atlantic is relatively saline. This water is derived from the sinking of chilled saline surface water in the northern North Atlantic, and is called North Atlantic Deep Water (NADW).

In contrast the deep Pacific is relatively fresh, as it experiences no deep convection of cooled salty surface water, its surface layer is too fresh and buoyant to sink.

Towards the sea floor, temperatures are near 0°C marking the presence of Antarctic Bottom Water derived from the dense water along the shores of Antarctica.

Intermediate Water

At the base of the thermocline is the low salinity Antarctic Intermediate water derived from the Antarctic Circumpolar Current.

How does the Ocean move heat?

Top of atmosphere

Atmosphere

Ocean Stratification and Circulation

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