1 soee3410: lecture 13 ocean mixed layer. 2 what is the mixed layer? why is it important? how do the...

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SOEE3410: Lecture 13

Ocean Mixed Layer

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Ocean Mixed Layer

• What is the mixed layer?

• Why is it important?

• How do the atmosphere and ocean-surface interact?

• Example of atmosphere – ocean-surface coupling

• Evidence of climate change?

SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

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Idealised vertical density profiles

Mixed Layer


PycnoclineHaloclineThermoocline

Temperature Salinity Density

Dep

th (

km

)

0

1

2

3

4

4

Cool skin

Warm layer

residualmixed layer

Thermocline

Surface temperature

Solar radiationEvaporation

~1

-10

cm

~ m

etr

es

< 1 mm

ocea

n

~

~

• When wind-driven mixing is very low, solar radiation warms a thin layer of water near the surface. This increases local stability and further suppresses mixing.

• Evaporation of water at the surface causes evaporative cooling, producing a cooler ‘skin’ layer at the top of the warm layer. This is convectively unstable, promoting mixing within the warm layer, and limiting the extent of the cooling in the skin layer.

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Mixed layer depth:January-March

Mixed layer depth:June-August


LEVITUS94: World Ocean Atlas 1994, an atlas of objectively analyzed fields of major ocean parameters at the annual, seasonal, and monthly time scales. http://ingrid.ldeo.columbia.edu/SOURCES/.LEVITUS94/

Mixed Layer Depths

6Isotherms slope equatorward in eastern in subtropics

Isotherms slope poleward in eastern subpolar basins

Confined high salinity regions in subtropics. SAtl > SPac

High salinity ‘escapes’ northward in N Atlantic

Levitus et al. World Ocean Atlas 2005

Observed surface temperature and salinity

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Ocean Mixed Layer







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Heat in the oceans

• Increase in the heat content of the oceans is an order of magnitude larger than the increase in the atmospheric and cryospheric heat content (Levitus et al., Science, 2001)

• Global climate change in response to long-term natural and/or anthropogenic forcing depends on the effectiveness of the ocean as a heat reservoir…

• But, the effectiveness of the ocean as a reservoir is curtailed by increasing thermal stratification which limits the extent to which surface signals can be transmitted to depth via mixing


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Incoming Short wave radiation

Qsw

NetLong-wave radiation

Qlw

Net latent heatQlat

Net sensible heatQsen

Qnet = Qsw+Qlw+Qlat+Qsen+Qad

Heat fluxes into a region of ocean:

Qad

AdvectionOcean

Atmosphere

Air-seainterface

+ve

+ve+ve+ve

-ve

-ve -ve

-ve

Heat budget equation


Turbulent Fluxes

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Climatic importance of the mixed layer

• Ocean - Atmosphere Interface: ~70 % of Earth’s surface

• The largest source of water vapour to the atmosphere

• Huge heat capacity of the oceans slows down climate change (thermal inertia)

• Deep ocean's slow response temperature may continue to rise for centuries after stabilization of greenhouse gas levels


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Ocean Mixed Layer







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Ocean–Atmosphere interactions:

• Oceans and atmosphere are interacting continuously

• Transferring: momentum,

heat,

fresh water


Wind stress Heat

Qnet

EvaporationPrecipitation & run-off

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Momentum transfer:

Wind Stress: w = CD a u 2

where:

w = Wind stress (Nm-2)

a = Density of air ~ 1 (kgm-3)

CD = Drag coefficient ~ 0.00145 (Dimensionless)

u = Wind speed (ms-1)

Air-sea Boundary => Wind and Wave Driven Mixing


~100m

Wind

Current velocity~2% wind speed

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Heat transfer

Solar heating: Short wave radiation penetrates into the water column to heat the water

Latent, sensible, longwave radiation (tend to cool the ocean at the surface):

• Latent heat flux: associated with evaporation of water at the surface• Sensible heat flux: associated with temperature difference between

the atmosphere and the ocean and the turbulent transfer of heat between the two fluids (can heat or cool surface)

• Long wave radiation: thermal (infra red) radiation associated with the ocean’s surface temperature

Heat transfer within water column results from turbulent heat fluxes (i.e. dependent on wind forced mixing or convection)

In addition, heat exchanges between the mixed layer & deeper watersMostly restricted to specific geographical regions: formation of cold, saline deep waters in polar regions; wind-driven upwelling of cold waters along coastal regions.


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Ocean mixed layer: polar regions

• Summer: shallow warm surface mixed layers isolate the newly formed deep water from the atmosphere; mean currents and mesoscale eddies steadily transfer the newly formed deep water into the abyssal ocean

• Winter: deep convection affects local SST directly, and larger scales indirectly through its effect on water mass properties and circulation

• The maximum depth of convection occurs at the end of the cooling season (Think: plot of heat loss in Central Labrador Sea, THC lecture)

• Deep convection depends on the balance between cumulative air-sea fluxes i.e. ice-melt, precipitation, and the oceanic advection of buoyancy


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Ocean Mixed Layer







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What is the El Nino-Southern Oscillation?

• oscillation in ocean-atmosphere system that occurs in tropical Pacific;

• occurs every 2-7 years;

• associated with unusually warm waters in eastern Pacific;

• impacts on weather around the globe – e.g. increased rainfall in southern US, bush-fires in Australia

important to understand the driving mechanisms and to be able to predict the onset.

El Nino – Southern Oscillation (ENSO)

Source: NOAA website

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Normal conditions:

•strong easterly trade winds across Pacific – from South Pacific High to Indonesian Low

•warm waters pile up in West Pacific

El Nino conditions:

•Indonesian Low and S Pacific High both weaken

•trade winds relax

•sea-surface “collapses”

Understanding ENSO

Source: NOAA website

thermocline

sea-surface0.5m

E

W E

W

sea-surface

thermocline

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Monitoring ENSO

Source: TOA website

Tropical Atmosphere-Ocean Project

• 1982/83 el Nino event was particularly strong & not expected

• since 1984, an array of moorings has developed

• now 70 moorings returning real-time oceanographic / meteorological data to shore via satellite

Lots of information / data at:http://www.pmel.noaa.gov/tao/index.shtml

http://www.pmel.noaa.gov/tao/index.shtml

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Ocean Mixed Layer







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Climate implications: atmosphere-ocean boundary

• Both coupled climate models and observations suggest:

the heat content of the ocean is increasing

• Rising SST and atmospheric water vapour is increasing :

potentially enhancing tropical convection, including thunderstorms, and the development of tropical storms


22Barnett et al., SCIENCE, VOL 309, 8 JULY, 2005

Ocean Warming (Internal/Solar/Volcanic Variability)


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Barnett et al., SCIENCE, VOL 309, 8 JULY, 2005

Ocean Warming (Anthropogenic Forced)


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(Webster et al., 2005, Science, Vol 309)


Ocean feedback on atmosphere: hurricane frequency & intensity I

25(Webster et al., 2005, Science, Vol 309, 1844-1846)


Ocean feedback on atmosphere: hurricane frequency & intensity II

26SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

Ocean feedback on atmosphere: hurricane frequency & intensity III

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• Ocean mixed layer is the connection between ocean and atmosphere

• Climatically, the ocean acts as a heat reservoir

• Air-sea fluxes transfer momentum, heat, freshwater => affect properties of mixed layer

• El Nino – example of large-scale effects of air-sea interaction

• Evidence of climate impact on ocean, and ocean on atmosphere

Summary – ocean mixed layer

1 soee3410: lecture 13 ocean mixed layer. 2 what is the mixed layer? why is it important? how do the...

Documents

region of ocean

layer of water

world ocean atlas

salinityocean mixed

marchmixed layer depth

cooler skin layer

global climate change

surface signals