Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics

Ocean circulation

Arnaud Czaja

1. Ocean and Climate2. Key observations

3. Key physics

Part I

Ocean and Climate

(heat transport and storage)

Ha Ho

+

Poleward energy transport

=

Net energy loss at top-of-the atmosphere

Imbalance between and = energy (heat) storage

Poleward heat transport and storage are small…

oaoP HHPWRS ,120)1( 2

Energy exchanged at top-of-atmosphere :

Planetary albedo Solar constant

SeasonalHeat storage

)(10 A

o

SPW

dzdycTdxt

S

Q5

Bjerknes’ (1964) monograph. Data from Sverdrup (1957) & Houghton (1954)

Ha+Ho

Ha

Ho

PWdaycalunit 5.0/101 19

No

rth

wa

rd h

eat

tra

nsp

ort

EquatorPole

Heat transport: a long history of measurements…

Vonder Haar & Oort, JPO 1973.

PWyrcalunit 3.1/101 22

30N 50N 70N10N

No

rth

wa

rd h

eat

tra

nsp

ort

Ho

Ha

Ha+Ho

GERBEapproved!

Poleward heat transport at 24ºN

Pacific 0.76 +/- 0.3 PW

Atlantic 1.2 +/- 0.3 PW

Atlantic+Pacific 2 +/- 0.4 PW

“Across the same latitude, Ha is 1.7PW. The ocean therefore can be considered to be more important than the atmosphere at this latitude in maintaining the Earth’s budget”.

Hall & Bryden, 1982; Bryden et al., 1991.

NB: 1PW = 10^15 W

Trenberth & Caron, 2001

GERBEapproved!

(ask more to Chris D.!)

Ha+Ho

Ha

Ho

Wunsch, JCl. 2005.

GERBEapproved!

Ganachaud & Wunsch, 2003

Sometimes effects of heat storage and transport are hard to

disentangle

• Is the Gulf Stream responsible for “mild” European winters?

“Every West wind that blows crosses the Gulf Stream on its way to Europe,and carries with it a portion of this heat to temper there the Northern windsof winter. It is the influence of this stream upon climate that makes Erin the“Emerald Isle of the Sea”, and that clothes the shores of Albion in evergreenrobes; while in the same latitude, on this side, the coasts of Labrador are fastbound in fetters of ice.”

Maury, 1855.

Eddy surface airtemperature from NCAR reanalysis(January, CI=3K)

WARM!

COLD!

Lieutenant Maury “The Pathfinder of the Seas”

Model set-up (Seager et al., 2002)

• Full Atmospheric model

• Ocean only represented as a motionless “slab” of 50m thickness, with a specified “q-flux” to represent the transport of energy by ocean currents

FseaairS

OOO QQt

ThC

Atmosphere

seaairQ

FQ

Seager et al. (2002)

Q3

Heat storage and Climate changeThe surface warming due to +4Wm-2 (anthropogenicforcing) is not limited to the mixed layer…

How thick is the layer is a key question to answer to predict accurately the timescale of the warming.

Ho = 50m

Ho = 150m

Ho = 500m

NB: You are welcome todownload and run the model :

http://sp.ph.ic.ac.uk/~arnaud

Ensemble mean model resultsfrom the IPCC-AR4 report

Q1

Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100)

Q4

Part II

Some key oceanic observations

World Ocean Atlas surface temperature

ºC

Thermocline

World Ocean Atlas Salinity (0-500m)

psu

The “great oceanic conveyor belt”

• Temperature Not changed by absorption/emission of photons.

• Salinity. No phase change in the range of observed concentration.

The ocean is conservative below the surface (≈100m) layer

Conservative nature of the ocean

50km 10km 2km

Spatial variations oftemperature and salinityare similar on scales fromseveral hundreds of kms to a few kms.

Salinity on 1027.6 kg/m3 surface

Ferrari & Polzin (2005)

Matsumoto, JGR 2007

“Circulation” scheme

“Circulation” scheme

Two “sources” of deep water:

NADW: North Atlantic Deep Water

AABW: Antarctic Bottom Water

Williams & Follows (2009)

In – situ velocity measurements

Location of “long”(~2yr) currentmeters

Dep

th

Amplitude oftime variability

From Wunsch (1997, 1999)NB: Energy at period < 1 day

was removed

1 yr

NB: Same velocity vectors but rotated

Moorings in the North Atlantic interior (28N, 70W = MODE)

Schmitz (1989)

(ask more to Ute and Chris. O.!)

Direct ship observations

NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot

Surface currents measured from Space

y

Pfu

o

1

Time mean sea surface height Standard deviation of sea surface height

“Geostrophic balance”

Momentum balance

East to westacceleration

North

East

Rotationrate f/2

East to westdeceleration

f V up

NB: f = 2 Ω sinθ

Geostrophic balance!


North

East

Rotationrate f/2


f V up

HighPressure

LowPressure

10-yr average sea surface height deviation from geoid

Subtropical gyres

10-yr average sea surface height deviation from geoid

Antarctic Circumpolar Current

Subpolar gyres

ARGO floats (since yr 2000)

Coverage by depths

Coverage by lifetime

T/S/P profiles every 10 days

Sv2010max

All in-situ observations can be interpolated dynamically using numerical ocean models

136101 smSv

From Wunsch (2000)

Overturning Streamfunction(Atlantic only)

RAPID – WATCH array at 26N

Q2

RAPID – WATCH array at 26N

14 m

illion

s £

The movie…

Part IIIKey physics

Because T is conserved by fluid motion the temperature structure simply reflects transport by waves

and mean currents

Sea surface

Ocean bottom

Upward heattransport

Downward heattransport

No internalheat source/sink

=

Z

X, Y

Zo

Sea surface

Ocean bottom




=

Z

X, Y

Zo

This simply happenswhen warm water goesup or cold water goes down

Sea surface

Ocean bottom




=

Z

X, Y

Zo

This happens when warm water goesdown or cold water goes up…

Sea surface

Ocean bottom




=

Z

X, Y

Zo

Requires mechanical forcing (winds/tides)!

“Historical” view

Sea surface

Ocean bottom

Z

X, Y

Zo


Sea surface

Ocean bottom

“Conveyor-belt”upwelling/downwelling

Z

X, Y

Zo

Broecker, 2005NB: 1 Amazon River ≈ 0.2 Million m3/s

Q6


Sea surface

Ocean bottom


“Small scale”wave breaking=

Z

X, Y

Zo

x,yz

C W

Q7

Internal waves

• Waves inducing displacement of density surfaces whose restoring mechanism is gravity.

• Frequency of linear wave is between the Coriolis frequency f (T~10h in midlatitudes) and the buoyancy frequency N (T=10mn in upper ocean; 100mn in deep ocean)

“Small scale” wave

breaking strength (Naveira-Garabato, 2006)

Numerical model results

-2ºX2º horizontal resolution

-Single basin

-No wind

-Surface heating-cooling

-Small scale wave breakingparameterised by a constantdiffusivity coefficient K

(cm²/s)

(Sv)

From Vallis (2000)

2/3K slope

Conveyor belt strength


Sea surface

Ocean bottom


Small scalewave breaking=

Z

X, Y

Zo

x,yz


Sea surface

Ocean bottom

“Conveyor-belt”upwelling


Z

X, Y

Zo

x,yz

A very bold statement!

-Is the ocean circulationdriven by tides?

-Can hurricanes drive the“conveyor belt”?


Sea surface

Ocean bottom

“Conveyor-belt”upwelling


Z

X, Y

Zo

x,yz10,000km ≈km

In-situ observations are dominated by a “meso-scale” (≈100km)

Infrared basedsurfacetemperature

KE spectra(surface)

Alternative paradigm

Ocean bottom

Z

X, Y

Zo


Ocean bottom

“Meso-scale” wavesupwelling/downwelling

Z

X, Y

Zo


Ocean bottom


Wind forced “pumping”

=

Z

X, Y

Zo

Momentum balance


North

East

Rotationrate f/2


f V up

Ekman balance!


North

East

Rotationrate f/2


f V up

Windstress

Wind forced pumping

Trade winds(≈10º latitude)

Sea surfaceX

Westerly winds (≈ 45º latitude)

DownwellingUpwellingUpwelling

Ekman layer


Ocean bottom


Wind forced “pumping”

=

Z

X, Y

Zo

Lab experiments-Rotating tank-Pump warm fluiddown from a moreslowly rotating disk

Wind strength

Depth of warm lens

From Marshall (2003)

Results from realistic coupled models

• Upper ocean: 0-2500m,

wT by the resolved

flow is downward and

balanced by upward

heat flux due to eddy

advection.

• Abyssal ocean: below

2500m, very weak but

positive upward heat

transport by the

resolved flow, opposed

by downward diffusive

heat transport.Gnanadesikan et al. (2007)

NB: >0 means upward

Friday’s session

Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics

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Transcript of Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics