Objectives -understand differences between ocean and atmosphere that cause the ocean to be the...

Objectives

-understand differences between ocean and atmosphere that cause the ocean to be the “memory” of the climate system.

-Understand how atmosphere and ocean are forced and how they interact.

-Investigate some large scale climate patterns.

Outline:

1. Differences between ocean and atmosphere.

1. Density-timescales

2. Forcing

2. Equations of motion.

1. F=ma

2. Purpose of each term

3. How do ocean and atmosphere interact?

1. Heat

2. Momentum

4. Weather patterns

1. Short term

2. Decadal

6300 km

6.3 km

12 km

Relative thicknesses of earth, ocean and troposphere (lower atmosphere).

Consequence: Ocean and atmosphere motions are mostly 2-dimensional.

Energy = mass x heat capacity x temperature (broadly)Energy = mass x heat capacity x temperature (broadly)

Heat capacity of air ~ 1000 J/kg deg KHeat capacity of air ~ 1000 J/kg deg K

Density of air ~ 1.2 kg/mDensity of air ~ 1.2 kg/m33 at sea level at sea level

Heat capacity of water ~ 4000 J/kg deg KHeat capacity of water ~ 4000 J/kg deg K

Density of water is 1000 kg/mDensity of water is 1000 kg/m33

So, 1 meter of water has roughly as much stored energy as the So, 1 meter of water has roughly as much stored energy as the whole lower atmosphere (troposphere)whole lower atmosphere (troposphere)

How do ocean and atmosphere differ? How do ocean and atmosphere differ?

- density and energy content- density and energy content

ConsequencesConsequences: :

-ocean slower, more massive, has longer timescales (more -ocean slower, more massive, has longer timescales (more “memory”). “memory”).

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

What is the primary source of energy for the earth?



Blue dashed line is outgoing radiation, red line is incoming radiation. So why aren’t tropics getting steadily hotter and poles steadily colder?

Solar radiation is greatest at the equator, and smaller at the poles.

Consequence: Ocean and atmosphere motions are driven by thermal gradient (difference between Tequator and Tpole).

Key Point: little incoming radiation absorbed by atmosphere.

Consequence: Ocean is heated from above, Atmosphere is heated from below. Ocean is more stable than the atmosphere.

Sources of energy to ocean and atmosphere.

1. Heat from: sun (shortwave), evaporation and precipitation (latent), conduction (sensible), emission (longwave)

2. Freshwater: precipitation and evaporation

3. Momentum: winds

QuickTime™ and aGIF decompressor


Consequences:

The only external input here is solar radiation. Otherwise, the interaction between ocean and atmosphere occurs as a system. What is lost by one component is gained by the other (or by the land surface or cryosphere).

Outline:



2. Forcing


1. F=ma



1. Heat

2. Momentum

4. Weather patterns

1. Short term

2. Decadal

3. Long term

How do we predict motions in the ocean and atmosphere?

From F=ma

Or a=F/m

Equations of Motion

€

∂u

∂t+ u

∂u

∂x+ v

∂u

∂y+ w

∂u

∂z= −

1

ρ

∂p

∂x+ f v + τ ˆ i + ν∇ 2u

∂v

∂t+ u

∂v

∂x+ v

∂v

∂y+ w

∂v

∂z= −

1

ρ

∂p

∂y− f u + τˆ j + ν∇ 2v

∂w

∂t+ u

∂w

∂x+ v

∂w

∂y+ w

∂w

∂z= −

1

ρ

∂p

∂z− g + ν∇ 2w

∂ρ

∂t+

∂ ρu( )∂x

+∂ ρv( )

∂y+

∂ ρw( )∂z

= 0

€

∂u

∂t+ u

∂u

∂x+ v

∂u

∂y+ w

∂u

∂z= −

1

ρ

∂p

∂x+ f v + τ ˆ i + ν∇ 2u

∂v

∂t+ u

∂v

∂x+ v

∂v

∂y+ w

∂v

∂z= −

1

ρ

∂p

∂y− f u + τˆ j + ν∇ 2v

∂w

∂t+ u

∂w

∂x+ v

∂w

∂y+ w

∂w

∂z= −

1

ρ

∂p

∂z− g + ν∇ 2w

∂ρ

∂t+

∂ ρu( )∂x

+∂ ρv( )

∂y+

∂ ρw( )∂z

= 0

aF/m

Pressure from earth’s rotation

and unequal heating

F/mWind F/m

Friction

Conservation of mass

Outline:



2. Forcing


1. F=ma



1. Momentum

2. Heat

4. Weather patterns

1. Short term

2. Decadal

3. Long term

Easterlies

Westerlies

Surface winds

Trades

Wind - Driven circulation

Consequences:

In a perfect stable world, oceans would only carry heat within the gyres, not between them.

Consequences: What we observe rarely looks like the “mean” picture described. Ocean and atmosphere are turbulent and hard to predict.

Buoyancy/Stratification:

(Thermal and freshwater forcing)

Ideal gas law PV=nRT…

And density = mass/volume

if you keep all constant then, T -> V but

P -> T

Also, as S ->

So warm water rises, cold water sinks

And fresh water rises, salty water sinks,



Density (or temperature) from South to North - pycnocline or thermocline = maximum upper ocean density or temperature gradient.



Thermohaline circulation.



Consequences: Cold water made at poles encircles the globe, gradually upwelling to the surface. This also occurs at mid-latitudes. (Storing surface conditions for ~ 500 years)

Outline:



2. Forcing


1. F=ma



1. Momentum

2. Heat

4. Weather patterns

1. Short term

2. Decadal

3. Long term

http://www.thecoolroom.org/education/upwelling.htm

Coastal upwelling

Consequence: Relatively short changes in wind (several days), drive large ocean changes that affect ecosystems down to physics. Feedbacks!

http://storm.rsmas.miami.edu/~tfaber/nick/Sarantis/wind2.gif

El Nino - La Nina or ENSO

1860 - 2000

120E 160E 120W160W

29C (84F)Surface water

29C29C

>29C>29C

<29C<29C

Maximum Tuna Catch

19951995

19941994

19931993

19921992

19911991

19901990

19891989

19881988

A11.008 STAAC2

La NiñaOct., 1983

El NiñoOct., 1997

Sea Surface ChlorophylSea Surface Chlorophyl

Sea Surface Temperature (SST) distribution

October-March air temperature anomalies

DJF Precipitation Anomalies

North American climate anomalies during warm phase PDO

Above average winter and springTemperatures in Northwestern NorthAmerica, below average temperaturesIn the southeastern US.

Above average winter and springRainfall in the southern US and Northern Mexico, below average Precipitation in the interior PacificNorthwest and Great lake regions.

Table 1: summary of North American climate anomalies associated with extreme phases of the PDO.

climate anomalies Warm Phase PDO

Cool Phase PDO

Ocean surface temperatures in the northeastern and tropical Pacific

Above average Below average

October-March northwestern North American air temperatures

above average Below average

October-March Southeastern US air temperatures

below average Above average

October-March southern US/Northern Mexico precipitation

Above average Below average

October-March Northwestern North America and Great Lakes precipitation

Below average Above average

Northwestern North American spring time snow pack

below average Above average

Winter and spring time flood risk in the Pacific Northwest

Below average Above average

TOPEX/Poseidon satellite measures the sea surface height. The image shows a horsehoe of higher than (warm) water in the western Pacific (red and white)and lower than average (cool) blue and purple water in the eastern Pacific.

Beginning of a cold PDO phase in 1999

Impact of PDO on west-coast ecosystem productivity

Warm PDO phase: enhanced coastal ocean productivity in Alaska but inhibited productivity in Washington and Oregon.

Cold PDO phase: reversed fortune.

Quotes from News:

August/September 1972 (Pacific Fisherman)

“Bristol Bay (Alaska) salmon run a disaster” “Gillnetters in the lower Columbia received an unexpected bonus when the largest run of spring chinook since counting began in 1938 entered the river.”

1995 Yearbook (Pacific Fishing)

“Alaska set a new record for its salmon harvest in 1994, breaking the record set the year before” “ Columbia spring chinook fishery shut down; west coast troll coho fishing around.

Positive NAO Index * The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low.* The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track.* This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland* The eastern US experiences mild and wet winter conditions

• Negative NAO Index

• * The negative NAO index phase shows a weak subtropical high and a weak Icelandic low.

• * The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway.

• * They bring moist air into the Mediterranean and cold air to northern Europe

• * The US east coast experiences more cold air outbreaks and hence snowy weather conditions.

• * Greenland, however, will have milder winter temperatures

• source http://www.ldeo.columbia.edu/NAO by Martin Visbeck

The NAO index is defined as the anomalous difference between the polar low and the subtropical high during the winter season (December through March)

How MeasuredInstrumented records are limited in their ability to examine decadal to centennial scale climate variability. Paleo proxies, including tree rings, ice cores and corals, are now being used in an effort to determine the climate dynamics and forces involved.

Fig. 3. Mean spatial distribution of C. finmarchicus and C. helgolandicus abundances in the Northeast Atlantic and the North Sea, during years of low and high NAO. Colourscales are proportional to log-abundances of the species. Log-abundances of each species over that area have been estimated for each month from 1962 to 1992 by means of kriging interpolation. This procedure resulted in a series of regular maps from which values have been averaged over the periods of high and low NAO (respectively 60 and 72 months).

Objectives

-understand differences between ocean and atmosphere that cause the ocean to be the “memory” of the climate system.

-Understand how atmosphere and ocean are forced and how they interact.

-Investigate some large scale climate patterns.

-Density! Leads to differences in energy content, memory, types of forcing.

-Sun, but interactions between ocean and atmosphere occur through momentum (wind) forcing,and through buoyancy

(heating and freshwater).

-Local processes (short time scale), ENSO 2-3 year timescale,

NAO, PDO decadal timescales.

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