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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:

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slide1

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.
slide2

Outline:

  • Differences between ocean and atmosphere.
    • Density-timescales
    • Forcing
  • Equations of motion.
    • F=ma
    • Purpose of each term
  • How do ocean and atmosphere interact?
    • Heat
    • Momentum
  • Weather patterns
    • Short term
    • Decadal
slide3

12 km

6.3 km

6300 km

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

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

slide4

How do ocean and atmosphere differ?

- density and energy content

Energy = mass x heat capacity x temperature (broadly)

Heat capacity of air ~ 1000 J/kg deg K

Density of air ~ 1.2 kg/m3 at sea level

Heat capacity of water ~ 4000 J/kg deg K

Density of water is 1000 kg/m3

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

Consequences:

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

slide6

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

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

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

slide7

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.

slide8

Sources of energy to ocean and atmosphere.

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

Freshwater: precipitation and evaporation

Momentum: winds

slide15

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).

slide16

Outline:

  • Differences between ocean and atmosphere.
    • Density-timescales
    • Forcing
  • Equations of motion.
    • F=ma
    • Purpose of each term
  • How do ocean and atmosphere interact?
    • Heat
    • Momentum
  • Weather patterns
    • Short term
    • Decadal
    • Long term
slide19

a

F/m

Pressure from

earth’s rotation

and unequal

heating

F/m

Wind

F/m

Friction

Conservation of mass

slide20

Outline:

  • Differences between ocean and atmosphere.
    • Density-timescales
    • Forcing
  • Equations of motion.
    • F=ma
    • Purpose of each term
  • How do ocean and atmosphere interact?
    • Momentum
    • Heat
  • Weather patterns
    • Short term
    • Decadal
    • Long term
slide21

Easterlies

Surface winds

Westerlies

Trades

slide22

Wind - Driven circulation

Consequences:

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

slide23

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

slide24

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,

slide25

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

slide27

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)

slide28

Outline:

  • Differences between ocean and atmosphere.
    • Density-timescales
    • Forcing
  • Equations of motion.
    • F=ma
    • Purpose of each term
  • How do ocean and atmosphere interact?
    • Momentum
    • Heat
  • Weather patterns
    • Short term
    • Decadal
    • Long term
slide29

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!

slide37

Maximum Tuna Catch

1995

1994

1993

>29C

29C

1992

<29C

1991

1990

1989

1988

120E

160E

160W

120W

29C (84F)

Surface water

A11.008 STAAC2

slide38

Sea Surface Chlorophyl

La Niña

Oct., 1983

El Niño

Oct., 1997

slide40

North American climate anomalies during warm phase PDO

October-March air temperature anomalies

Above average winter and spring

Temperatures in Northwestern North

America, below average temperatures

In the southeastern US.

Above average winter and spring

Rainfall in the southern US and

Northern Mexico, below average

Precipitation in the interior Pacific

Northwest and Great lake regions.

DJF Precipitation Anomalies

slide42

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

slide43

Beginning of a cold PDO phase in 1999

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.

impact of pdo on west coast ecosystem productivity
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.

slide45

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

slide46
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

slide48

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 Measured

Instrumented 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.

slide52

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).

slide53

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.