Ocean and Atmosphere Coupling

El-nino -- Southern OscillationMartin Visbeck

DEES, Lamont-Doherty Earth Observatoryvisbeck@ldeo.columbia.edu

Outline

Review

Meridional heat and freshwater transferby the atmosphere and ocean

Fluxes across the sea surface interface:

Outline

Changes in equatorial Pacific ocean and atmosphere circulation associated with El Niño and La Niña events.

Two dynamic feedback processes which act to intensify El Niño and La Niña events.

Why this system oscillates and the time-scale of this oscillation.

Effects of El Niño/La Niña on regional and global climate.

Atmospheric Processes

Horizontal Forces: Sea Breeze

Sea Breeze /Monsoon

Seasonal cycle of heating produces the monsoon winds akin to the daily occurring sea-breeze.

Dynamic Meteorology

Pressure is routinely plotted on maps and isobars (contours of constant pressure) are drawn.

Dynamic MeteorologyThe Coriolis Force / geostrophic flow

Geostrophic Balance: f u = - ( p / y) /

Pressure Gradient Force (PGF) = Coriolis Force (CF)

Dynamic MeteorologyGeostrophic flow and Friction

The Ekman balance is important for forming weather patterns in the atmosphere:

General CirculationThe surface energy balance

I ~ T4Atmosphere/Ocean Heat Transport

General CirculationLarge scale energy transport

A non rotating planet might have a circulation like this.

What will the Coriolis force do?

General CirculationThe surface energy balance

What will the Coriolis force do?

A three cell structure emerges:

Hadley Cell

Mid-L. Cell

Polar Cell

General CirculationThe surface wind and pressure

Polar Easterlies

Westerlies

Trade Winds

ITCZ (Low)

Subtropical High

Subpolar Low

General CirculationThe surface pressure

NorthernWinter (January)

High over Land, Low over Ocean

General CirculationThe surface pressure

Northern Summer (July)

High over Ocean, Low over Land

General CirculationClimate Zones Evaporation - Precipitation

Evaporation: where air is subsiding

Precipitation: where air is rising

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

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

Ocean

Ocean Atmosphere Coupling

In low latitudes the ocean moves more heat poleward than does the atmosphere, but at higher latitudes the atmosphere becomes the big carrier.

Note the figure is old atmosphere to low...

The Oceans Role in Climate

The ocean role in climate would be zero if there were a impervious lid over the ocean, but there is not, across the sea surface pass heat, water, momentum, gases and other materials.

The Oceans Role in Climate

Much of the direct and diffuse solar short wave radiation that reaches the sea surface penetrates the ocean (the ocean has a low albedo), heating the sea water down to about 100 meters, depending on the water clarity. It is in this sunlit surface layer of the ocean that the process of photosynthesis can occur.

Solar heating of the ocean on a global average is 168 watts per square meter.

The Oceans Role in Climate

The ocean transmits electromagnetic radiation into the atmosphere in proportion to the fourth power of the sea surface temperature (°K). This radiation is at much longer wave lengths (greater than 10 microns, in the infrared range) than that of the sun, because the ocean surface is far cooler that the sun's surface.

The net long wave radiation from the ocean surface is surprisingly uniform over the global. Why? The infrared radiation emitted from the ocean is quickly absorbed and re-emitted by water vapor and carbon dioxide, and other greenhouse gases residing in the lower atmosphere.

The Oceans Role in Climate

Much of the radiation from the atmospheric gases, also in the infra red range, is transmitted back to the ocean, reducing the net long wave radiation heat loss of the ocean.

The warmer the ocean the warmer and more humid is the air, increasing its greenhouse abilities. Thus it is very difficult for the ocean to transmit heat by long wave radiation into the atmosphere, it just gets kicked back by the greenhouse gases, notably water vapor whose maximum concentration is proportional to the air temperature.

Net back radiation cools the ocean, on a global average by 66 watts per square meter.

The Oceans Role in Climate

When air is contact with the ocean is at a different temperature than that the sea surface, heat transfer by conduction takes place.

The ocean is on global average about 1 or 2 degrees warmer than the atmosphere so on average ocean heat is transferred from ocean to atmosphere by conduction.

The heated air is more buoyant than the air above it, so it convects the ocean heat into the lower atmosphere. If the ocean were colder than the atmosphere (which of course happens, just not quite as common as a warmer ocean) the air in contact with the ocean cools, becoming denser and hence more stable, more stratified.

The Oceans Role in Climate

As such it does a poor job of carrying the ocean 'cool' into the lower atmosphere. This occurs over the subtropical upwelling regions of the ocean (Cape Verde climate)

The transfer of heat between ocean and atmosphere by conduction is more efficient when the ocean is warmer than the air it is in contact with.

On global average the oceanic heat loss by conduction is only 24 watts per square meter.

The Oceans Role in Climate

The largest heat loss for the ocean is evaporation, (which links heat exchange with hydrological cycle). On global average the heat loss by evaporation is 78 watts per square meter.

Why so large? Its because of the large heat of vaporization (or latent heat) of water, a product of the polar bonding of the H2O molecule.

The water vapor leaving the ocean is transferred by the atmosphere eventually condensing into water droplets forming clouds, releasing its latent heat of vaporization in the atmosphere.

Evaporation and Latent Heat Flux

Scientist use what is called a "bulk formula" to calculate the evaporative heat flux. This is an empirically-derived formula (meaning it was derived after countless hours in the lab using real data, rather than derived from theoretical principals).

Qe = Lt W Ae air (qs - qa)

W is the wind speed in m/s

Ae = 1.3 x 10-3 (w/o dimension) is an exchange coefficient

qs is the saturated mixing ratio for water vapor in kg/kg

qa is the mixing ratio for water vapor in kg/kg

Lt = 2494 - 2.2T kJ/kg where T is the temperature of the water in ºC

air = 1.2 kg/m3 is the density of air

Evaporation and Latent Heat Flux

Qe = Lt W Ae air (qs - qa)

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

The Oceans Role in Climate

While the ocean gains heat in low latitudes and losses heat in high latitudes, the largest heat loss is drawn from the warm Gulf Stream waters off the east coast of the US during the winter. An equivalent pattern is found near Japan, where the Kuroshio current is influenced by the winter winds off Asia. It is in these regions that the atmosphere takes over as the major meridional heat transfer agent.

Ocean Atmosphere Coupling

To maintain an approximate steady state climate the ocean and atmosphere must move excess heat from the tropics to the heat deficit polar regions.

Additionally the ocean and atmosphere must move freshwater to balance regions with excess dryness with those of excess rainfall.

The movement of freshwater in its vapor, liquid and solid state is referred to as the hydrological cycle.

The Oceans Role in Climate

The annual freshwater flux between ocean and atmosphere reflects the water vapor content (relative humidity) of the atmosphere, resulting from the general circulation of the atmosphere. The dry regions of the subtropics where the air subsides along the poleward edges of the Hadley Cell; the rainy Intra Tropical Convergence Zone (ITCZ) where the trades winds of northern and southern hemisphere meet, forcing updrafts of air.

Climate Variability

Changes in equatorial Pacific ocean and atmosphere circulation associated with El Niño and La Niña events.