Ocean circulation and coupling with the atmosphere

Arnaud Czaja

1. Ocean heat storage & transport

2. Key observations

3. Ocean heat uptake and global warming

4. Mechanisms of ocean-atmosphere coupling

Part I

Ocean heat storage and transport

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

Q4

Trenberth & Caron, 2001

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

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”

Matsumoto, JGR 2007

“Circulation” scheme

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

Q5

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)

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”

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 £

Part III

Ocean heat uptake and anthropogenic forcing of climate

change

Heat storage and Climate change

The surface warming due to +4Wm-2 (anthropogenicforcing) is not limited to the mixed layer.

Heat exchanges between the mixed layer and deeper layers control the timescale of the surface warming.

Anthropogenic forcing

Net surface ocean heating

Upper ocean cooling via diabatic processes

Upper ocean cooling via massexchange with deep ocean

Weak vertical ocean heat transport

Anthropogenic forcing

Net surface ocean heating

Upper ocean cooling via diabatic processes

Upper ocean cooling via massexchange with deep ocean

Large vertical ocean heat transport

The Environmental Physics Climate Model

Tropics

ExtraTropicsOcean

OH2AT

1ST 2ST

1OT 2OT

AH

OH

AtmosphereTA1

Hea

t co

nten

t (J

)

http://www.sp.ph.ic.ac.uk/~aczaja/EP_ClimateModel.html

Upper (0-750m) ocean heat content vs TOA imbalance: observations

Wong et al (2006)

Mechanisms of heat exchange between upper and deep layers

• Wind driven circulation pumping down of warm subtropical waters; upwelling of cold, high

latitude waters.

• Buoyancy driven circulations sinking of dense water and upwelling of light water (= overturning circulations + eddy driven + convection).

• Mixing isopycnal diffusion and breaking internal gravity waves. Q1

Ocean heat uptake in wind driven gyres

• Global downward ocean heat transport driven by winds.

• Strength:

1233 430

10.410 KWmyr

mwc Ekpo

Levitus (1988)

Williams & Follows (2012)

Buoyancy driven circulations and ocean heat uptake :

• Total temperature change in the 10th decade after 2XCO2 (idealised ocean basin)

• Temperature change due to change in ocean currents

• Temperature change in absence of change in ocean currents.Xie and Vallis (2011)

Cooling

Interior mixing & ocean heat uptake

Osborne (1998)Upward heat flux

Downward heat flux

Verticalheat flux(Wm-2)

+100

-100

Equator North PoleSouth Pole

deep

er

Motions in the ocean are not isotropic: “neutral” surfaces

• In the simplest case of a waterworld at rest, a fluid parcel does work against the buoyancy force when displaced upward or downward. Motions along z=cst are energetically neutral.

SolidEarth

g

02

1 22 hNW ref z

gN ref

refref

2where

Z=0Z=h

Reference density

Motions in the ocean are not isotropic: “neutral” surfaces

• In the real ocean, neutral surfaces take the shape of a bowl due to the distortion of spheres by the seafloor topography, surface heating, cooling and winds.

Neutral surfaces in the Atlantic

WOCE A16

NB: These surfaces can be approximated as surfaces of constant density (“isopycnals”).

Neutrally energeticdisplacements

The movie…