some ways ocean circulation affects climate: biological
TRANSCRIPT
Some ways ocean circulation affects climate:
• biological productivity gas exchange• direct transport of gases (e.g., the carbon cycle)• ice albedo• heat exchange with atmosphere
“Heat Budget”
of ocean tells us• how ocean stores and moves heat • how ocean exchanges heat with atmosphere
Several possibilities: including1.
Q = 0 (thermally passive ocean)2.
D –
H small, Q = ∫Et
dV
(ocean is thermal flywheel)3.
D –
∫Et
dV
small, Q = H (ocean moves heat around)
1 good approx global and annual average2 good approx some places for timescales ≤ seasonal3 good approx annual average
What determines Q(x,y)?Need to consider both atmosphere dynamics andglobal oceanic velocity
and temperature field.
Similar expressions exist for transport of salt and other tracers.
100120
140160
180200
220240
260280
20
30
40
50
60
−6000
−4000
−2000
0
When calculating H, we often choose domain so that most of the lateralboundaries are “walls”, except for a 1 or 2 zonal sections (as shown). u●n
= 0 on bottom, and if we can neglect u●n
through the top, H is simply meridional heat transport.
Ocean and Atmospheric Meridional Heat Transport
Ocean clearly important in tropicsLatest observations smaller ocean role at high latitudes(still significant uncertainties)
Trenbirth
and Caron, 2001: J Clim, 3433-3443
oceanatm
osph
ere
tota
l1 PW = 1015
W
Trenberth
& Caron (2001)
Rules for Density1)
Density hardly changesρ
≈
1000 kg/m3
= 1 gm/cm3
everywhere in ocean2) Density increases with S:
At T=0 and p=0, ρ(S=0) = 999.8, ρ(S=35)=1028.1
3)
Density increases with p:At S = 35 psu, T = 0 C, p = 4000 dbar, ρ
= 1046.4
4)
Density decreases with T5)
Rate of change w.r.t. T depends on T,p
|dρ/dT| smaller if water is colder
or deeper6)
In ocean, variations due to p are greatest, butvariations due to T & S more important:
u
responds to horizontal
ρ
variations (indep
of p)
Equation of State
Accurate formula for ρ(T,S,p) can be found in Gill (1982)Rough approximation:
ρ
= 999.83 + C(p) + β(p)S
–
α(T,p)T
–
γ(T,p)(35-S)TWhere
C ≈
5.05(p/1000 dbar) kg/m3
β
≈
.8 kg m-3
psu-1
α
≈
.071[1+.351(p/1000 dbar) + (.068 C-1)T] kg/m3
γ
≈
.003 kg m-3
psu-1
C-1
0 5 10 15 20 25 30−8
−6
−4
−2
0
2
in situ temperature (C)
dens
ity (
kg/m
3 )
ρ(p,T,S) − ρ(p,0,S), Standard (solid) and Approx (dashed)
p = 0 dbar, S = 0 psup = 4000 dbar, S = 35 psu
p = 0 dbar, S = 35 psu
0 10 20 30 40−30
−25
−20
−15
−10
−5
0
5
salinity (psu)
dens
ity (
kg/m
3 )
ρ(p,S,T)−ρ(p,35,T)
p=0,T=0
p=0,T=30
p=4000,T=0
0 10 20−5
−4
−3
−2
−1
0
density (kg/m3)
pres
sure
(10
3 dba
r)
ρ(p,S,T)−ρ(0,S,T)
T=0,S=0T=0,S=35
T=30,S=35
30 32 34 36 38 40
0
5
10
15
20
25
30
salinity (psu)
pote
ntia
l tem
pera
ture
(C
)
σθ(θ,S)
2022
24
26
28
30
30 32 34 36 38 40
0
5
10
15
20
25
30
salinity (psu)
pote
ntia
l tem
pera
ture
(C
)
σ4(θ,S)
36
38
40
4244
46
48
Potential Density and Some Nomenclature
If water has given θ, S, and p, its potential density
= ρ(T=θ,S,p=0).
This is a way to filter out uninteresting dependence on p.
Because density is always about 1000, useful to define“sigma-t”: σt
= ρ(T,S,0) –
1000“sigma-theta”: σθ
= ρ(θ,S,0) –
1000Can define with reference to other depths, such as
σ1 = ρ(θ,S,1000 dbar) –
1000
Net Annual-Average Heat Flux into Ocean
General pattern:absorbs heat near equator, loses heat at high latitudes
(as expected)Sea’s heat transport comparable to atmosphere:
Significant role in moderating meridional T gradientIndicates importance of ocean circulation.
Typical values O(50 W/m2)Some interesting details:
heat gain largely in east, heat loss in westPacific heat gain especially bigAtlantic heat loss especially big
Annual Avg Net Heat Flux
−150 −100 −50 0 50 100 150
Evaporation/Precipitation Patterns
Evap. minus Precip
(and runoff) drives S variationsNo (significant) salt fluxes through surface,But freshwater fluxes can increase/decrease S dilution.General pattern:
Net precip
in tropics and high latitudesNet evap
in subtropics
Typical values Order(1 m/yr)
from COADS, cm/yr
Annual Avg Precipitation
0 50 100 150 200 250 300 350
from COADS, cm/yr
Annual Avg −Evaporation
−200 −150 −100 −50 0
from COADS, cm/yr
Annual Avg P−E
−150 −50 50 150 250
Surface Property Distributions
Temperature: general pattern as would be expectedwarm in tropics, cold at high latitudesup to about 6 C winter-summer differenceSST lags land temperature
Salinity: general pattern follows forcinghigh values in subtropical evaporation regionslow values in tropics and high latitudessome regional extreme values
Density: dominated by T variations2-D patterns: zonal and inter-basin differences
indications of effects of ocean currents?aT and S data from Levitus
et al. (1994), World Ocean Atlas
Mar SST
0 4 8 12 16 20 24 28 32
Sep SST
0 4 8 12 16 20 24 28 32
cold tongue
Mar SSS
28 32 33 34 35 36 37 38 42
Mar SS Density
20 21 22 23 24 25 26 27 28
Sep SS Density
20 21 22 23 24 25 26 27 28
High-Resolution SST SnapshotShows More Structure
Pacific Atlantic
Meridional-Depth Profiles
Warm tropical water is just a thin layer at top of oceanmost of ocean is < 5 Cwarm layer has “double bowl”
shape
structure very similar in Atlantic and PacificSalinity has more complicated structure
subsurface “tongues”
of fresh watercreate minima and maxima in S(z) in placesindications of deep flow?
T and S data from Levitus
et al. (1994), World Ocean Atlas (meridional sections)And World Ocean Atlas (2001) (global sea surface properties)
Surface Mixed Layer
Top of ocean relatively homogenous: “mixed layer”
Mixed layer grows thicker via:• wind stirring• convection due to cooling/salinification
Mixed layer grows thinner via:• solar heating• desalinification
(precipitation)
Hence ML depth varies greatly in space/time10 –
1000 m depth
Shallower at low latitudes, daytime, summerDeeper at high latitudes, night, winter
Near-global ocean model,
Hirst
et al. (1996, JPO)
NADW
AABW
CDW
Oceanic Overturning Circulation(s)
Sub-Tropical Cells
North Atlantic Deep Water
Antarctic Bottom Water
Circumpolar Deep Water
STC
Tomczak
and Godfrey, Regional Oceanography, ch
15, after Bainbridge (1980)
Pot.Temp.
Salinity
Oxygen
How do we know the DMOC exists?
Tracers hint at deepflows in Atlantic:
Relatively saltyNorth Atlantic Deep Water(NADW) moves southward at2 –
4 km depth
Relatively fresh Antarctic Bottom Water (AABW)moves northward near bottom
Relatively fresh Antarctic Intermediate Water(AAIW) moves northward atabout 1 km depth.
Smethie
et al. (2000, JGR)
More tracerEvidence(freons
fromAtmosphereSince 1940s)
Pickart
and Smethie
(1993, JPO)
Geostrophic velocity shows DWBC in N Atl(from density measurements + “Pogo”
floats)
Section 3
Schmitz (1996), cited in Ocean Circulation and Climate
Schematic of Global DeepMeridional Overturning Circulations
NADW FORMATION RATE13 Sv
(Rintoul
1991)15 Sv
(Ganachaud
Wunsch
2000)20 Sv
(Talley 2003)
Wyrtki
and Kilonsky, 1984 0 10 20-10latitude
dept
h (m
)
Equator has important vertical structure:
Surface flow: westwardSubsurface Flow: eastward
“Equatorial Undercurrent”Flow upward along thermocline
(see previous slide)Important part of Subtropical Cellcolder subtropical water flows to equator flows E’wardand upwells returns poleward as Ekman transport
El Nino makes Cold Tongue go away
Wind-Driven Gyres
Consider water
in basin forced by wind stress at surface:
Closed basin: watermust form GYRES.
Direction based onwind shear
Can get water flowing inopposite directionto wind
top view
Ocean Gyres (clockwise and counter-clockwise) follow the Wind Curl
Surface circulation and depth-averaged circulation shows gyres whichare generally aligned with the regions of positive and negative wind curl(see observations on following slides).
Reid (1997, Prog
Oceanogr)
Kuroshio
E. Australian Cur.
NoteStrongWesternBoundaryCurrents
Observed Pacific Surface Geostrophic Flow
subpolar
gyre
subtropical gyre
Peru
-Chi
le C
ur
subtropicalgyre
Antarctic Circumpolar Current
N. Eq. Counter Cur. S. Eq. Cur.
N. Eq. Cur.
Oyashio
−100 −80 −60 −40 −20 0 20
0
20
40
60
Smoothed Annual Mean Surface Velocity from Drifters
Arrow length proportional to (speed)1/2
based on Hansen and Poulain
(1996)
subpolar
gyre
subtropical gyre
Gulf Stream
N. Eq. Cur.
Eq. Counter Cur.
Braz
il Cu
r.
Falkland/Malvinas Cur.
BenguelaCur.
SEC
Reid (1989, Prog
Oceanogr)
subtropical gyre
Antarctic Circumpolar Current
Observed Surface South Atlantic Geostrophic Flow
Agul
has
Cur
subtropical gyre
Antarctic Circumpolar Current
Observed Surface Indian Ocean Geostrophic Flow
Eq. Counter Cur.
S. Eq. Cur.
How much heat transport is associated with Subtropical Cells?
latitude
dept
h (m
)
Pacific T at 210 E
−45 −30 −15 0 15 30 45−500
−400
−300
−200
−100
0
φ
−20
0
20