Gulf Stream Temperature, Salinity, and Transport during the Last Millennium
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Gulf Stream Temperature, Salinity, and Transport
during the Last Millennium
David Lund - University of Michigan
• Part I - Geostrophic estimation of Gulf Stream flow
- Gulf Stream strength varied by ~10%
- Weaker vertical shear and transport during Little Ice Age
- Consistent with timing of North Atlantic cooling
• Part II - Sea-surface temperature and salinity
- Gulf Stream salinity increased during LIA
- Likely due to southward migration of the Inter-Tropical
Convergence Zone
• Part III - Linking the oceanic circulation and ITCZ
North Atlantic surface circulation
wind-driven gyre flow
NADW compensation
Schmitz, 1996
Gulf Stream volume and heat transports
31 Sv (1Sv = 1x106 m3s-1)
1.3 PW (1PW = 1015 W)
25-40% of heat transport due
to shallow gyre overturning
Talley, 1999
Schmitz, 1996
Larsen, 1992
Location of core sites in Florida Straits
Core retrieval and sampling
Benthic foraminiferal δ18O and density
Cibicidoides floridanus
increasing S and
decreasing T causes
sea water density
AND foraminiferal
δ18O to increase
200 µm
after Lynch-Stieglitz et al., 1999
Foraminiferal density estimates match modern observations
0
100 $RY 4ORTUGAS
200
'REAT "AHAMA "ANK
300
Water Depth (m)
400
500
600
700
SYMBOLS CORETOP DATA
800 LINES *AN CTD CASTS
23 24 25 26 27 28
density (St)
Lund et al., 2006
Estimating Gulf Stream transport
Modern techniques
- current meters ~ 29-31 Sv Pillsbury, 1890; Schott et al., 1988; Leaman et al., 1995
- submarine cable estimates of 29 Sv (Jan) to 33 Sv (July)
Wertheim, 1954; Baringer and Larsen, 2001
- geostrophic estimates ~ 28-30 Sv Montgomery, 1941; Schmitz and Richardson, 1968
Geostrophic estimation (using thermal wind equations)
---> vertical current shear is
∂v = - g ∂ρ proportional to horizontal
∂z ρof ∂x density gradient
∂u = g ∂ρ * transport can be calculated only
∂z ρof ∂y if velocity at a given depth is known
Dry Tortugas δ18O Bahamas δ18O
Florida Current density cross-sections
200 yr BP 1100 yr BPLower Gulf Stream transport during Little Ice Age
modern
annual
average
what do estimates
LIA of salinity tell us
about LIA
conditions?
Lund et al., 2006Dry Tortugas reflects tropical Atlantic salinity
Dry Tortugas
salinity= 36.0
79ggc and 62mc
Levitus, 1994North Atlantic surface salinity controlled by
evaporation-precipitation rate
ITCZ
ITCZ
evaporation-precipitation rate
daSilva et al., 1994Calculating surface salinity using planktonic foraminifera
Globeriginoides ruber δ18Ocalcite = δ18Owater - 0.20(SST) + 2.98
Kim and OʼNeil, 1997
Lynch-Stieglitz et al., 1999
δ18Owater = δ18Ocalcite - 2.98 + 0.20(SST)
200 µm 5.5
5.0
Mg/Ca (mmol/mol)
1.4
4.5
1.2
4.0
δ18Owater (‰)
1.0
3.5
0.8
0.6 3.0
Anand et al., 2003
0.4 2.5
34.5 35.0 35.5 36.0 36.5
20 22 24 26 28 30
Salinity (psu) Temperature (°C)Dry Tortugas δ18Ocalcite increased during LIA
95% confidence interval
G. ruber δ18O (‰)
100yr running mean
Lund and Curry, 2006Dry Tortugas sea surface temperature increased during LIA
Lund and Curry, 2006Dry Tortugas δ18Ow increased during LIA
salinity
Lund and Curry, 2006Dry Tortugas δ18Ow record is replicable
δ18Owater anomaly (‰)
salinity!
The magnitude of δ18Ow variability is due to either:
A) influence of thermocline water or B) incorrect Mg/Ca calibration
LUND AND CURRY: FLORIDA CURRENT TEMPERATURE AND SALINITY P
Calendar Age (yr BP)
0 200 400 600 800 1000
!18O c (‰, PDB)
-1.6
-1.4
-1.2 28.5
SST (ºC)
28.0
27.5
* 27.0
!18O w (‰, SMOW)
1.05
1.00
0.95
*
Schmidt, 1999 0.90
Craig & Gordon, 1965
36.4
Salinity
36.2
* 36.0
based on multivariate equations of deMenocal et al., 2007Higher LIA salinity driven by southward ITCZ migration
ITCZ
ITCZ
evaporation-precipitation rate
daSilva et al., 1994saltier northern
tropical Atlantic
δ18Owater anomaly (‰)
LIA
Haug et al., 2001
drier northern
Coherent change in northern Venezuela precipitation
VenezuelaGulf Stream characterized by low transport and high surface salinity during the Little Ice Age
!
A function of reduced windstress curl?
224 W.E. Johns et al. / Deep-Sea Research I 49 (2002
O. Annual mean Sverdrup streamfunction
2 Sv of net in
and St. Luci
WESTERLIES consistent w
Sverdrup the
tion of water
much smaller
reasons for th
the return cu
O. exiting the so
be infinitesim
oppose the S
significant red
Partial block
channels and
TRADES could also co
either case, an
and the Sverd
O. forced inflow
south of 151N
approximatel
My ≈ 1 ∂Tx
the Sverdrup
feeding the Fl
β ∂y the subtropi
Caribbean no
Fig. 4. The annual mean Sverdrup streamfunction derived subtropical gy
from Hellerman and Rosenstein (1983) winds. Johns et al.,
These are the 2002
same winds used to drive the model simulations. The regionnomalies exhibit a zonal band of increased aridity jus
!
orthModel
of theresults
equator, although
suggest curl opposing
increases positive
when ITCZprecipita
precipita-
on migrates
anomalies to the south of this band are more subtle
southward
- forcedSignificant
igure 2. NCAR CAM3 using mean
SST annual precipitation
- SST anomalies (contours
(contours,
scaled to simulate
anomalies
!1 from hosing experiment LIA observations !1
mm day(Zhang) &and wind stress
Delworth, 2005) (vectors, dyn cm ) anomalies
or (a) NA1, (b) NA2, (c) XTA1, (d) XTA2, (e) TNA1, and
) TNA2. Shading highlights positive (dark/green) and
Saenger et al., 2009A role for the MOC?
warm HadCM3 surface
` temp. (ºC) anomalies
cold
HadCM3 precipitation
wet (cm/yr) anomalies
`
dry
Vellinga and Wu, 2004Conclusions
• Gulf Stream transport varied by ~10% during the last
millennium, but was 3±1 Sv lower during Little Ice Age
• Surface Gulf Stream salinity increased during the LIA,
most likely due to southward ITCZ migration
• Simultaneous transport and salinity variability implies tight
linkage between oceanic circulation and hydrologic cycle
on centennial time scales
• Southward migration of wind-field would likely enhance flow,
implying MOC was primary driver of LIA transport anomalyWater depth (m)
Holocene variability in Gulf Stream transport
a) Transport increased
Transport (Sv)
~ 4 Sv during Holocene
b)
Calendar Age (yr BP) Lynch-Steiglitz, et al., in press
ITCZwith
gure 4. a) Calculated geostrophic flow through the Florida Straits migrated southward
2-sigma error
Water depth (m)
imates. b) Geostrophic flow per unit depth (Sv m-1).! during Holocene
Haug et al., 2001
17 AUGUST 2001 VOL 293 SCIENCE www.sciencemag.orgDry Tortugas δ18Owater record mimics Δ14Catm
Lund and Curry, 2006North Atlantic region cooled by ~1°C during LIA
Greenland Temp. (oC)
Dahl-Jensen et al., 1998
Lamb, 1995
Marchitto and
deMenocal, 2003
Bond et al., 2001
Keigwin, 1996
deMenocal et al., 2000You can also read
