Overturning circulations, high-latitude ocean climate
fromabove and below: satellite altimetry and Seagliders
P.B. Rhines, withCharlie Eriksen, Sirpa Häkkinen

1

Kyoto FDEPS lectures 4-7 xi 2007
Dynamics of oceans and atmospheres
P.B. Rhines
University of Washington

DAY 4• 1. rotating, stratified fluids: oceans and atmospheres– vorticity: a vector-tracer in classical homogeneous fluids• geostrophic adjustment, thermal wind• 2. wave dynamics: fundamentals, group velocity, energetics, raytheory• potential vorticity (PV)– vortex stretching, Prandtl’ s ratio, geography of PV• 3. Rossby waves• 4. instability => geostrophic turbulence; subtropical gyres:dynamics, jets and gyres, topography effects• 5. Case study of topographic effect on atmospheric circulation:
Greenland and Atlantic storm track.
• 6. Teaching young undergraduates about the global environment?
• 7. Seminar: subpolar climate dynamics observed from above andbelow: meridional overturning circulations (MOCs) altimetry andSeagliders

Some ‘burning’ questions for which we thought weknew the answers:(i) What drives the global meridional overturning circulation (MOC) of the oceans -- buoyancy or mechanical mixing induced by winds and tides?
(ii) Is high-latitude sinking and the deep, cold branch of the MOC a dominant
member of the meridional heat and fresh-water transport?
(iii) Does the ocean circulation substantially warm western Europe?
More generally, does heat transport by oceanic general circulation affectatmospheric climate?
(iv) What are the paths of upwelling of deep waters in the global oceanic MOC?
(v) Where are the crucial sites for convection and water-mass transformation ?
(vi) How does wind-driven circulation interact with buoyancy-driven MOC
overturning?
(vi) What is the quantitative rate of water-mass production for the several
components of the North Atlantic DeepWater (for example, Labrador Sea
Water), and how are they altered before being ‘delivered’ to the global MOC?
(vii) How do convection and mixing drive diffusive overturning at many scales,
reaching to the distant circulation.

After Schmitz (1995)

Reid, J Marine Res 2005

Reid,JMR 2005

Oceanic overturning circulations: coexisting with ‘horizontal gyres of windforced circulation• MOCs have an easier time in the oceans than in theatmosphere:a ring of air moved 1000 km north gains westerly velocity
of 100 m sec-1 There is not enough energy available to utilize thismode: the Hadley cell is limited in north-south extent. Forces (eddymomentum flux from PV stirring) and non-symmetric circulation
are required to support extensive meridional excursion.

Stationary and transient waves or bottom topography
all allow poleward heat flux with small or zero Eulerianv-velocity, as in simple annulus experiment. Notesignificance of Rossby radius (NH/f where H =vertical scale of motion) the scale at which baroclinic
APE ~ KE

(31%)

consider the differences between tropics and Arctic…(a)at 60N latitude
the sunshine incident per unit area is 50% of the full intensity withthe sun overhead; (b) the albedo (whiteness) is greater
source: IPCC-01

Is the ocean MOC important to atmospheric climate?

Global meridional heat transport divides roughly equally into 3 modes:(Bryden & Imawaki 2002)
1. atmosphere (dry static energy) cpT + Ф
2. ocean (sensible heat)
3. joint atmosphere/ocean mode: water vapor/latent heat transport Lq
The three modes of poleward transport are comparable in amplitude, and distinct incharacter (sensible heat flux divergence focused in tropics, latent heat flux divergence focus in thesubtropics) (based on Keith (Tellus 1995) climatology, similar to more modern: Trenberth et al. J.Clim 2003)(residual
method, TOA
radiation
1985-89 and
ECMWF/NMC
atmos obs:
redundantobs(air-sea flux)
also available

90S
90N
the northern subtropics show extremely active upward air/Error est.: ± 9% at mid-latitude; Bryden est 2.0 ±0.42 pW at 24Nvery similar numbers from Trenberth & Stepaniak,QJRMS 04

• So, ventilation of the tropics by atmosphere + ocean MOC’s
provides ~ 5 pW (5 x 1015 W); distributed over the area of the
Earth between 0N and 30N, averages 5 x 1015 W/πR2 =39 W m-2, delivering the same amount per m2 to the Earth northof 30N.
Fully as much heat is carried in the atmosphere by
0.8 Sverdrups (megatonnes s-1) moisture flux ~ 2 pW as by dry
static energy flux. (using the heat of vaporization, 2.25 MJ kg-1)(It is useful to talk about both oceanic and atmosphericmass (water or air) transports in Sverdrups (Sv):
Gulf Stream 30-120 Sv
Antarctic Circumpolar Current~ 180 Sv
Atlantic MOC ~ 16-20 Sv
westerly winds/jet stream ~ 500 Sv
atmospheric MOC ~ 50 -100 Sv

Hydrostaticpressure difference= vapor pressure ofwater (as a functionof temperature

An evacuated glass vessel with water in it illustrates the Clausius-Clapyron relation
between vapor pressure of water and temperature. The water is pushed from the vessel in
my hand to the ‘cold ball’, and the vapor pressure difference between the two ends is close
to the hydrostatic pressure measured by the column’s vertical displacement. One can fillout the curve and see the greater sensitivity (to temperature) of water vapor production at
high, ‘tropical’ temperature. This all works because we shake the vessel so that a thin filmof water lies under my warm hand. It illustrates a key variable in the climate system.
When shaken this water ‘clinks’ like metal,vapor cavities opening up and slamming shut.

Moisture flux during high NAO: concentration in the highlatitude storm tracks of the ~ 2 petawatts of latent heat flux … which is~0.7 Sverdrup (0.7 megatonnes/sec) of freshwater flux
1993 JFM 1000mb moisture flux 1 Jan 1993 velocity column integrated
water vapor (red=high, blue=low)
and w.v. flux along
50N and 60N (yellow curves)16

cold-air outbreaks: a source of deep convection(surface air temperature, 2 Jan 1993)

2. Atmosphere/ocean forcing
Effect on subpolar gyre of the Atlantic, and Greenland Sea:enhance air/sea heat flux: much intensified at higher model
resolution
T95 (210 km grid)
T255
T799 (25km grid)

25xii2004

17i2005
18

Ocean heat transport by MOC
Trenberth & Caron
JClim01

merid. heat transport at 35N: 78% A, 22% O; 18N: 50% A, 50% Ovorticity colors
units of f

25-27 Dec 04

15-17 Jan 05

altitude colorshPaDownslope winds increase wavedrag (by Bernoulli) here in a layer of CO2

Effect on subpolar gyre of the Atlantic, and Greenland Sea:enhance air/sea heat flux: much intensified at higher model
resolution
T95
T255
T799

Principal eof of sea surface elevation, 1992-2006, which is mostly a simple
trend, showing deceleration of the subpolar Atlantic gyre over 15 years
Häkkinen & Rhines 2004 SciencePrincipal eof of sea surface elevation, 1992-2006, which is mostly a simple trend,
showing deceleration of the subpolar Atlantic gyre over 15 years
Häkkinen & Rhines 2004 Science(update using only TOPEX/Poseidon and Jason-1 data, time period covered: October 1992 to March 2005;

To get SSH /VELOCITY in any individual point one needs
to multiply the value of the spatial pattern (left)by the time series value on the right) SSH has units of cm
velocities are normalized so they are dimensiolessSeasatGeosatERS-1/2,
TOPEX/Poseidon Jason-1

24

satellite altimetric height (AH)sea surface trend, 1992-2006

global warming signal + gyre scale dynamics: expansion & acceleration of subtropical gyres, poleward migrationof polar fronts; equatorward migration of Gulf Stream, decel of Atl SP gyre, accel of NPac SP gyre25

Altimetric surface
height trend

Steric surface
height trend

figure from D. Roemmich (NASA OSTST, Hobart, 2007);
Willis, Roemmich & Cornuelle, 2002 JGR

26

Igor Yashayaev,
2006
altimetricheightand stericheight:a 2-mode
descriptionof thecirculation

1960s dynamicheight maximum
equal to today’s....GSA….
paradox: the highlatitude world isvery barotropic!
27

accompanying surface SP gyre deceleration is the
opening of the eastern Atlantic meridional pathway tostronger advection of subtropical waters by NAC
extension (Hatun et al.Science 2005).

This shift can also be seen with surface drifters….
28

early1991-95

middle1996-2000‘departure’
Lagrangianmean flow: drifters
released in a
subtropical Gulf
Stream box (red,blue)
red: speed> 30
cm/seccyan: drifters
arriving at box
Häkkinen & Rhines
2007

late

2001-200629

Sea-surface height anomaly vs. distance and time: subpolar Atlantic
time/latitude at 35W longitude

North Atlantic Current
time/longitude at 46.9N latitude

68N

48N
1992

2005
30

Where is air-sea heat flux most intense? January (W m-2)
(SOC/NOC1.1a climatology based on COADS)The air/sea heat flux seen by the atmosphere(latent+sensible+long-wave rad)and by the ocean (latent+sensible+long-wave + shortwave solar rad)the sun heats the ocean, but does not cool the atmosphere: so theright-hand figure shows much bigger warming of the atmospherethan the left-hand figure

Annual average ratio of convergence of heat flux by ocean
circulation divided by annual average heating of the atmosphereby ocean: (LH+SH+LW+SW)/(LH+SH+LW)

Held et al. (J.Clim.02) 100mb-1000mb diabatic heating for January

viscous overturning in a rotating cylider:
the radial/vertical plane transmits stress from the top plate (which is at restin the laboratory frame) and the bottom of the cylinder (which is rotating)

sugar
syrup

Overturning cells in an annulus of
fluid between concentric cylinders(the inner cyclinder is rotating, theouter cylinder is stationary(Taylor-Couette flow).
The cells transmit torque between
the solid cylinders more strongly
than would pure viscous diffusion.
(The same 2D equations govern
thermal convection, and theNusselt number expresses theanalogous increase in heat flux
above the diffusive rate).

Sink-driven flow in a rotating, stratified fluid: the cyclonic spin of the fluid
would be resisted by bottom Ekman friction (and all radial inflow
concentrated there in this tornado vortex); However, stable stratification
resists and forces continuing MOC within the fluid. The azimuthal velocity
and shear is so great that it tears up the stratification (originally 2 layers)

GFD lab, Univ of Washington

Dense plume flowing down a sloping valley in a rotating fluid(model of dense downslope flows in the Weddell Sea)
Elin Darelius, Univ of Washington GFD lab
particle paths are helical,
with Ekman driven meridional
overturning transmitting theboundary stress into thefluid. (Looking up the sloping
valley)Ψ(y,z) meridional streamfunction x-averaged
The zonally averaged
overturning streamfunction, Ψ(y,σ) in density space (σ=potential density)
North Atlantic/Arctic model of
Häkkinen driven by NCEP
winds and temperatures
This image of the ocean circulation is theusual output of climate models; many
essential processes are made invisible…theeast-west detail of the previous slides.
These ‘details’ are likely to be essential tounderstanding the global ocean transports.
The tendency for dominant sinking south ofGreenland in low-resolution climate models
is widespread: here in density- latitude
space the streamfunction reveals higher
latitude sinking and dense overflows.
The difference is expected from the eastwest tilt of potential density surfaces, sothat equal and opposite meridional velocities
at the same depth z may have very different
densities.

y-potential density space

35S

90N
Bailey, Hakkinen, Rhines
Climate Dynamics 2005

41

The sinking region of the deep circulation is not usuallycorrectly resolved in ocean models; here a bottom boundary
layer parameterization improves the sinking of dense water
Nakano & Suginahara 2002 JPO

Lumpkin & Speer’s JPO 03 discussion of the Atlantic MOC, here plotted against potential
density and latitude. Even though we know there is much east-west structure (boundary
currents, horizontal gyres as in Reid’s maps) the zonally averaged MOC ‘looks like’ thesimple 2-dimensional box models of the circulation

S

O2

sea
surfaceThe ACC is the only ocean current
with The Problem (how to flow meridionally, given
the absolute angular momentum constraint)..yet it has
ample topographic bottom slopes tolean on: these clearly balance thezonal wind stress that drives thisAAIWgreatest of all ocean currents
This may be a dominant site of
upwelling in the global MOC(with respect z and potential
NADW
density)
Salinity at 24W longitude

Oceanic
meridional
overturning
strength vs.
meridional
baroclinic
dynamic
heightgradient in
HADCM3(Thorpe et al 2001)

A change in the MOC transport may be associated with some measurable change in the meridional
density gradient. HadCM3 finds a very close correlation between Atlantic overturning rate and the S-N
gradient of steric height from 30S - 60N through the W Atlantic. But, there is a possible oversensitivity ofmodels to subpolar buoyancy/Labrador Sea.

Observations of the MOC in the Labrador Sea andIcelandScotland Ridge:
Temperature, salinity, oxygen, fluorescence, particle scattering,vertical velocity, depth-avereaged horizontal velocity all for 0.5
Watts powerfor publications visit
www.ocean.washington.edu/research/gfd/papers-rhines.html

the classic way to do subpolar hydrography (R/V Knorr, R.Pickart photo)47

Charlie Eriksen with a potential customer48

Wait for a nice day and zoom out to deep water…

49

50

potential temperature along glider tracks: ix2003-iv2005-Arctic waters from Baffin Bay;-warm Irminger Sea water from boundary
current along w. Greenland-thin, cold, low-salinity surface layer advected
over-top of Labrador Sea from Greenland coast

Seaglider ribbon section viewed from NE;

Labrador Sea looking from NW

51

The ribbon pulled taut: Seaglider 014: 1200 profiles of temperature, salinity, oxygen, chlorophyll… Oct 04-April 05

Davis Strait
cold Arctic waters

Warm Irminger
Sea water

winter deep
convection

cold,fresh intense
Greenland fine
52 sructur
shelf wateradvection of low salinity surface layer off the west Greenland coast
shapes both deep convection region, Labrador Sea Water production andprimary spring plankton bloom (Hatun, Eriksen & Rhines 2007 JPO)

black contours:
Lavender ARGO
streamfunction
grey shades: altimetric
EKE
colors: depth of winterconvection in 1968.
from Pickart et al. 2002)

53

Cold shelf water (purple) streams
off the Greenland shelf. Seagliders
014 and 015 are embedded in thisjetAqua & Terra satellite SST day 089 05
54

The Greenland waters reaching out over the Labrador Sea also carry strong
primary productivity with them…as seen in SeaWIFS ocean color (May 2004)

55

SeaWiFS ocean color: 2005 days 91-120;
two Seagliders pass through this bloom

56

oxygenfluorescence

Seaglider 016 spring 2005 1800 km Labrador Sea track57

channels and conduits for heat- and fresh-watertransport

5. Deep Ocean overturning circulations at the ‘headwaters’ of the global MOC
exchanges across the Iceland-Faroes-Shetland Ridge and Channel
Iceland
IcelandFaroesFaroes420-480840

latitude600

E Longitude59

Hansen & Osterhus PiO 2000
60

Water-mass transformation on Θ/S plane:
Faroe-Bank Channel Mauritzen et al. 2005 DSR: mixing downstream of the final sill dilutes the dense cold
water, with impact on the global MOC

61

Potential temperature to 7 November 2007 Seaglider
104 Iceland-Faroe Ridge
686 hydrographic profiles during this 3 month
missionTemperature and dissolved oxygen profiles
dive 343 in Faroe Shetland Channel

Temperature section following glider 101 (Nov 2006-March 2007)

Wyville-Thompson Basin

Iceland-Faroe Ridge
multiple crossings of Faroe-Bank Channel

64Seaglider 101 9 June-30 Aug 2008: temperature section
on south slope of Iceland-Faroe Ridge (plus north-south section at end…like
Poseidon section of Meincke, 1976). Thin, cold bottom layer
0357Z
encountered widely
31 Aug 07

Patrolling east-west along southern flank of Iceland-Faroe Ridge

Iceland-Faroe Ridge
south <-> north65SouthNorth
Iceland Faroe Ridge ‘Poseidon’
1977 Section (Meincke 1978)66

close to the Poseidon section on the previous
slide, a Seaglider section…blue-light particle
backscatter showing biological activity andpotential density, salinity, Iceland-Faroe
Ridge, SG101 Aug 200767

fluorescence and oxygen saturation

68

Seaglider 104 dive 218 15 x 2007 Iceland Faroe Ridge

69

Seeking the cold, thin overflows on the Iceland-Faroe Ridge.

70

Kyoto FDEPS lectures 4-7 xi 2007
Dynamics of oceans and atmospheres
P.B. Rhines
University of Washington

ALL DAYS• 1. rotating, stratified fluids: oceans and atmospheres– vorticity: a vector-tracer in classical homogeneous fluids• geostrophic adjustment, thermal wind• 2. wave dynamics: fundamentals, group velocity, energetics, raytheory• potential vorticity (PV)– vortex stretching, Prandtl’ s ratio, geography of PV• 3. Rossby waves• 4. instability => geostrophic turbulence; subtropical gyres:dynamics, jets and gyres, topography effects• 5. Case study of topographic effect on atmospheric circulation:
Greenland and Atlantic storm track.
• 6. Teaching young undergraduates about the global environment?
• 7. Seminar: subpolar climate dynamics observed from above andbelow: meridional overturning circulations (MOCs) altimetry andSeagliders

•
•
•
•
•
•

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

FDEPS Lectures, November 2007
P.B. Rhines, Oceanography and Atmospheric Sciences, University of Washington
Rhines@ocean.washington.edu
www.ocean.washington.edu/research/gfd
These lectures will address the dynamics of oceans and atmospheres, as seen through theory, laboratory
simulation and field observation. We will look particularly at high latitudes and climate dynamics of the ocean circulation
coupled to the atmospheric storm tracks. We will emphasize the dynamics that is difficult to represent in numericalcirculation models. We will discuss properties of oceans and atmospheres that are both fundamental, unsolved questionsof physics, and are also important, unsolved problems of global environmental change.
Lecture 1:
Is the ocean circulation important to global climate ? Does dense water drive the global conveyor circulation?
Fundamental questions about oceans and atmospheres that are currently under debate.
The field theory for buoyancy and potential vorticity.
Basic propagators: Rossby waves and geostrophic adjustment.
Potential vorticity: inversion and flux.
Lecture 2:
How do waves and eddies shape the general circulation, gyres and jet streams?
Almost invisible overturning circulations.
Lessons from Jupiter and Saturn.
The peculiar role of mountains, seamounts and continental-slope topography.
Lecture 3:
Dynamics of ocean gyres and their relation with the global conveyor circulation.
Water-mass transport, transformation and air-sea exchange of heat and fresh water.
Ocean overflows and their mixing.
Decadal trends in the global ocean circulation.
Lecture 4:
Heat, fresh-water, ice: convection in oceans and atmospheres and the texture of geophysical fluids.
Lecture 5:
Teaching young students about the global environment using the GFD laboratory: science meets energy andenvironment in the lives of Arctic natives
Seminar:
Exploring high-latitude ocean climate with Seagliders and satellites

These are unusual times we live in….

 Congratulations Nobel Peace
Laureates;
 Fundamentals of our basic scienceof circulation and climate areunder debate (what drives the global
oceanic overturning circulation?
is the ocean important to climate?...)

 We are about to hit the wall(the exponentials of global change); Our science is never far from the public interest…and the public
interest is disturbingly remote from the integrity of the biosphere;=>there has never been a more important time to engage in74
teaching broad environmental science to young people.

Kelvin waves, inertial waves in shallow rotating fluid