Gulf Stream Temperature, Salinity, and Transport During the Last Millennium David Lund - University of Michigan

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

100 $RY4ORTUGAS

200 'REAT"AHAMA"ANK

300

400

Water Depth (m) Water 500

600

700

SYMBOLS CORETOPDATA 800 LINES *ANCTDLINES *ANCTDCASTS

23 24 25 26 27 28 density (S ) t 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 f * transport can be calculated only ∂z ρo ∂y if velocity at a given depth is known Dry Tortugas δ18O Bahamas δ18O Florida Current density cross-sections

200 yr BP 1100 yr BP Lower Gulf Stream transport during Little Ice Age

modern annual average

what do estimates L I A of salinity tell us about LIA conditions?

Lund et al., 2006 Dry Tortugas reﬂects tropical Atlantic salinity

Dry Tortugas salinity= 36.0 79ggc and 62mc

Levitus, 1994 North Atlantic surface salinity controlled by evaporation-precipitation rate

I T C Z I T C Z

evaporation-precipitation rate

daSilva et al., 1994 Calculating surface salinity using planktonic foraminifera

18 18 Globeriginoides ruber δ Ocalcite = δ Owater - 0.20(SST) + 2.98 Kim and OʼNeil, 1997 Lynch-Stieglitz et al., 1999

18 18 δ Owater = δ Ocalcite - 2.98 + 0.20(SST)

200 µm 5.5

5.0

1.4 4.5 1.2 4.0 1.0 (‰) 3.5

water 0.8 Mg/Ca (mmol/mol) O 18

δ 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) 18 Dry Tortugas δ Ocalcite increased during LIA

95% conﬁdence interval O (‰)

18 100yr running mean δ

G. ruber

Lund and Curry, 2006 Dry Tortugas sea surface temperature increased during LIA

Lund and Curry, 2006 18 Dry Tortugas δ Ow increased during LIA salinity

Lund and Curry, 2006 18 Dry Tortugas δ Ow record is replicable anomaly (‰) salinity water O 18 δ 1 $T M " x ! y # $y

! 18 The magnitude of δ Ow variability is due to either: A) inﬂuence of thermocline water or B) incorrect Mg/Ca calibration PA2009 LUND AND CURRY: FLORIDA CURRENT TEMPERATURE AND SALINITY PA2009 Calendar Age (yr BP) 0 200 400 600 800 1000

-1.6

(‰, PDB) -1.4 c O 18 ! -1.2 28.5 SST (ºC) 28.0

27.5

27.0 1.05 *

1.00

(‰, SMOW) 0.95 w * O

Schmidt, 1999 18 0.90 ! Craig & Gordon, 1965 36.4 Salinity

36.2 * 36.0

based on multivariate equations of deMenocal et al., 2007

18 Figure 11. (a) Modern d Ow-S regressions for the North Atlantic, including the tropics, subtropical gyre, and thermocline (data from G. A. Schmidt et al., Global seawater oxygen 18 database, 1999, available at http://www.giss.nasa.gov/data/o18data/). Since the thermocline is ventilated at midlatitudes to high latitudes, this line also represents the high-latitude surface North Atlantic. On the basis of modern annual 18 average salinity of 36.1 the Dry Tortugas sites plot along the tropical relationship (d Ow = 0.95 (shaded circle)) where E – P (evaporation minus precipitation) is negative. The Great Bahama Bank sites, with 18 average salinity of 36.2, plot along the subtropical gyre line (d Ow = 1.02 (shaded square)), where E – P is 18 positive. Today’s average d Ow-S for North Atlantic Deep Water (NADW) is noted with the solid square 18 [Craig and Gordon, 1965]. (b) Little Ice Age d Ow values at Dry Tortugas (solid circle) and Great Bahama Bank (solid square). Dry Tortugas values were 0.1% heavier than today, while those on the Great Bahama Bank were 0.2% lighter. Using the modern low-latitude slopes, this scenario implies the Dry Tortugas sites were saltier than the Bahamas, an unrealistic possibility. (c) Southward Hadley cell migration case (see text). Dry Tortugas values increase along the low-latitude line as before, but now the Bahamas are influenced by a greater proportion of high-latitude 18O-depleted water and therefore move along the 0.5% slope. The LIA salinity values remain reversed but to a lesser extent than the previous scenario. (d) An 18 increase in the mean thermocline d Ow of 0.1% moving all the lines upward along the 0.5% slope, easily 18 accounting for the LIA Dry Tortugas d Ow (solid circle) but also causing the Bahamas to increase by 0.1% (solid square), opposite the observed change. (e) A combination of the models in Figures 11c and 11d), 18 where the mean thermocline d Ow-S increases and the Hadley circulation migrates southward. This is the most realistic model for the data as it eliminates the pitfall of a reversed salinity gradient.

12 of 15 Higher LIA salinity driven by southward ITCZ migration

I T C Z I T C Z

evaporation-precipitation rate

daSilva et al., 1994 Coherent change in northern Venezuela precipitation anomaly (‰) saltier northern tropical Atlantic water

O LIA 18 δ drier northern Venezuela

Haug et al., 2001 Gulf Stream characterized by low transport and high surface salinity during the Little Ice Age 1 $T M " x ! y # $y

A function! of reduced windstress curl?

224 W.E. Johns et al. / Deep-Sea Research I 49 (2002) 211–243 O. Annual mean Sverdrup streamfunction 2 Sv of net inflow coming through the St. Vincent and St. Lucia passages just to its north. This is WESTERLIES consistent with the above expectations from Sverdrup theory, even though the total recircula- tion of water through these passages in the model is much smaller than the Sverdrup requirement. The reasons for this may have to do with the fact that the return current required by Sverdrup theory O. exiting the southern Caribbean cannot, in reality, be infinitesimally thin, in which case it will directly oppose the Sverdrup inflow and could lead to a significant reduction in the total water exchange. Partial blocking of the inflow by the narrow channels and the shallow Grenadines platform TRADES could also contribute to this limited exchange. In either case, an important result of both the model and the Sverdrup prediction is that the net wind- O. forced inflow from the Atlantic to the Caribbean south of 151N (near the latitude of Martinique) is approximately zero. Thus, in the model as well as 1 ∂Tx the Sverdrup prediction, the wind-driven inflow My ≈ feeding the Florida Current is derived entirely from β ∂y the subtropical gyre inflow which enters the Caribbean north of the climatological tropical/ Fig. 4. The annual mean Sverdrup streamfunction derived subtropical gyre boundary at B151N: from Hellerman and Rosenstein (1983) winds.Johns These areet al., the 2002 same winds used to drive the model simulations. The region 4.1.2. Model B: wind and MOC forced surrounding the Carribean Sea is magnified in the bottom panel The total inflow to the Caribbean and its for closer comparison with Fig. 3. distribution among the passages changes consider- ably with the addition of a 14 Sv MOC (Figs. 3b western Caribbean, off Panama and just south of and 5b). The transport of the Florida Current at Yucatan Channel. 271N rises by 11 to 32:3 Sv; nearly in agreement The mean transports for this model through with the established value of 31:5 Sv (Lee et al., each entry and exit passage bordering the Car- 1996). The distribution of the transport among the ibbean Sea are shown in Fig. 5a. The transport of various passages, and the differences between the the Florida Current in this model is only 21:3 Sv; two models, are illustrated more clearly in Fig. 6. roughly 10 Sv less than the actual Florida Current The largest changes occur in the southernmost transport. Combined with the northward trans- passages, where the 2:1 Sv outflow from Grenada port off Abaco Island in the northern Bahamas Passage in Model A is reversed to an inflow of ð3:3 SvÞ; the total northward flow along the 5:0 Sv (a net change of 7:1 Sv), and the inflow in western boundary is 24:6 Sv; just balancing the St. Vincent Passage increases from 1:1 to 5:2 Sv (a B25 Sv of Sverdrup flow incident on the western net change of 4:1 Sv). The total inflow to the boundary south of 271N: The largest inflow Caribbean through the Windward Islands pas- contribution to the Caribbean occurs through sages (Grenada, St. Vincent, and St. Lucia) is now Windward Passage ð7:4 SvÞ; with other passages 12:3 Sv; as compared to essentially zero in the contributing between 1 and 3 Sv of inflow. Trans- wind-driven model. port through the southernmost (Grenada) passage The remaining passages, with the exception of is out of the Caribbean at 2:1 Sv; balancing the Windward Passage, show smaller changes of L11703L11703 SAENGERSAENGER ET ET AL.: AL.: EXTRATROPICAL ATLANTIC ITCZ FORCING L11703

ITCZ response, while NA2 is designed to reflect the subtle cooling that may have characterized the LIA. Our second two simulations (XTA1, XTA2) are identical to NA1 and NA2, respectively, but apply ZD05 SSTs only north of 30°N. Our final two simulations (TNA1, TNA2) apply ZD05 SSTs only from the equator to 30°N. All simulations were run for 18 years, of which the final 14 years were analyzed. We assumed pre-industrial carbon dioxide con- L11703L11703 SAENGERSAENGER ET ET AL.: AL.: EXTRATROPICAL EXTRATROPICALcentrations, ATLANTIC ATLANTIC and ITCZ ITCZ assessed FORCING FORCING significant anomalies (95%)L11703L11703 using a two-tailed t-test. ITCZITCZ response, response, while while NA2 NA2 is is designed designed to to reflect reflect the the subtle subtle 3.coolingcooling Modeled that that may may Precipitation have have characterized characterized and Wind the the LIA. LIA. Stress Our Our second second two3.two simulationsModeled simulations Precipitation (XTA1, (XTA1, XTA2) XTA2) and are are Wind identical identical Stress to to NA1 NA1 and and NA2,NA2,[10] respectively, respectively, NA1 exhibits but but decreased apply apply annual ZD05 ZD05 average SSTs SSTs only only precipitation north north of of throughout3030°°N.N. Our Our the final final North two two Atlantic simulations simulations in response (TNA1, (TNA1, to TNA2) TNA2) cooler apply apply SSTs (FigureZD05ZD05 SSTs SSTs 2a). only only Negative from from theprecipitation the equator equator to to anomalies 30 30°°N.N. All All extend simulations simulations in a zonalwerewere run run band for for from 18 18 years,the years, eastern of of which which North the the Pacific final final to 14 14 West years years Africa were were FigureFigure 1. 1. ImposedImposed SST SST anomaly anomaly pattern pattern (shading) for 1 andanalyzed.analyzed. can exceed We We assumed assumed4 mm pre-industrial pre-industrial dayÀ . Consistent carbon carbon with dioxide dioxide previous con- con- CAM3 simulations (note separate scales). XTA simulations CAM3 simulations (note separate scales). XTA simulations studiescentrations,centrations, [Zhang and and and assessed assessedÀ Delworth significant significant, 2005; anomalies anomalies Stouffer (95%) (95%) et al. 2006; using using applied SSTs only north of 30 N (dotted line). TNA applied SSTs only north of 30°°N (dotted line). TNA Suttonaa two-tailed two-tailed and Hodson t-test. t-test. , 2007], increased precipitation immedi- simulationssimulations applied applied SSTs SSTs only only south south of of 30 30°N. Marine ately south of this zonal band can be interpreted as a (circles)(circles) and and terrestrial terrestrial (squares) (squares) proxy proxy records records indicating southerly displacement of the Atlantic ITCZ. Cooler NA1 wetterwetter (open/orange) (open/orange) and and drier drier (filled/green) (filled/green) LIA LIA conditions. SSTs3.3. Modeled Modeled also lead to Precipitation Precipitation positive sea-level and and pressure Wind Wind Stress Stress (SLP) anoma- NumbersNumbers correspond correspond to to Table Table 1. 1. Modern Modern seasonal seasonal ITCZ lies[[1010 throughout]] NA1 NA1 exhibits exhibits the North decreased decreased Atlantic annual annual (not shown) average average that precipitation precipitation strength- extremes are also shown. extremes are also shown. enthroughoutthroughout northeasterly the the North North wind Atlantic Atlantic stress at in in low-latitudes response response to to (Figurecooler cooler SSTs SSTs 2a). Enhanced(Figure(Figure 2a). 2a). northeasterly Negative Negative precipitation precipitation wind stress is anomalies anomalies most prominent extend extend along in in a a thezonalzonal northern band band from from coast the the of eastern eastern South America North North Pacific Pacific and in to to the West West southern Africa Africa FigurethatFigurethat LIA LIA 1. 1. ITCZ ITCZImposedImposed displacements displacements SST SST anomaly anomaly were were likely likely pattern pattern accompanied accompanied (shading) (shading) for for by 11 Caribbeanandand can can exceed exceed where44 anomalies mm mm day dayÀÀ extend.. Consistent Consistent across with with the previous previous Central somesome degree degree of of low-latitude low-latitude cooling. cooling. CAM3CAM3 simulations simulations (note (note separate separate scales). scales). XTA XTA simulations simulations Americanstudiesstudies [ [ZhangZhang isthmus. and andÀÀ Delworth Delworth,, 2005; 2005; Stouffer Stouffer et et al. al. 2006; 2006; applied SSTs only north of 30 N (dotted line). TNA applied SSTs only north of 30°°N (dotted line). TNA SuttonSutton[11] Annualand and Hodson Hodson average,, 2007], 2007], results increased increased from precipitation precipitation NA2 are similar immedi- immedi- to simulations2.simulations2. Model Model applied appliedDescription Description SSTs SSTs and and only only Methods Methods south south of of 30 30°°N.N. Marine Marine thoseatelyately of south south NA1, of of suggesting this this zonal zonal an band band approximately can can be be interpreted interpreted linear response as as a a (circles)(circles) and and terrestrial terrestrial (squares) (squares) proxy proxy records records indicating indicating tosoutherlysoutherly the downscaled displacement displacement SST of of theforcing the Atlantic Atlantic (Figure ITCZ. ITCZ. 2b). Cooler Cooler A linear NA1 NA1 [[77]] We We performed performed six six simulations simulations using using the National wetterwetter (open/orange) (open/orange) and and drier drier (filled/green) (filled/green) LIA LIA conditions. conditions. regressionSSTsSSTs also also lead lead of NA1 to to positive positive and NA2 sea-level sea-level precipitation pressure pressure anomalies (SLP) (SLP) anoma- anoma- has a CenterCenter for for Atmospheric Atmospheric Research Research (NCAR) (NCAR) Community NumbersNumbers correspond correspond to to Table Table 1. 1. Modern Modern seasonal seasonal ITCZ ITCZ slopelieslies throughout throughout of 0.25 ± the the 0.1 North North that is Atlantic Atlantic close to (not (not the shown) shown) 0.22 expected that that strength- strength- for a AtmosphereAtmosphere Model Model version version 3 3 (CAM3) (CAM3) at at T42 T42 resolution extremesextremes are are also also shown. shown. perfectlyenen northeasterly northeasterly linear response. wind wind stress stress As at at in low-latitudes low-latitudes NA1, NA2 precipitation (Figure (Figure 2a). 2a). (2.8(2.8° byby 2.8 2.8° latitude-longitude,latitude-longitude, 26 26 vertical vertical levels) levels) [Collins ° ° anomaliesEnhancedEnhanced northeasterly northeasterly exhibit a zonal wind wind band stress stress of is is increased most most prominent prominent aridity along along just etet al. al.,, 2006]. 2006]. In In general, general, CAM3 CAM3 reproduces reproduces the the pattern and norththethe northern northern of the equator, coast coast of of although South South America America opposing and and positive in in the the precipita- southern southern amplitudeamplitude of of mean mean annual annual low-latitude low-latitude Atlantic Atlantic climate thatthat LIA LIA ITCZ ITCZ displacements displacements were were likely likely accompanied accompanied by by tionCaribbeanCaribbean anomalies where where to the anomalies anomalies south of extend extend this band across across are more the the Central Central subtle. variability,variability, as as well well as as interannual interannual migrations migrations of tropical somesome degree degree of of low-latitude low-latitude cooling. cooling. AmericanAmerican isthmus. isthmus. convergenceconvergence zones zones [ [HackHack et et al. al.,, 2006, 2006, HurrellHurrell et al., [[1111]] Annual Annual average average results results from from NA2 NA2 are are similar similar to to 2006]2006] suggesting suggesting that that it it is is appropriate appropriate for for investigating investigating the thosethose of of NA1, NA1, suggesting suggesting an an approximately approximately linear linear response response 2.mean2.mean Model Model state state of of Description Description the the ITCZ ITCZ during during and and Methods Methods the the LIA. LIA. However, However, as in toto the the downscaled downscaled SST SST forcing forcing (Figure (Figure 2b). 2b). A A linear linear otherother AGCMs, AGCMs, CAM3 CAM3 tends tends to to overestimate overestimate Caribbean [[77]] We We performed performed six six simulations simulations using using the the National National regressionregression of of NA1 NA1 and and NA2 NA2 precipitation precipitation anomalies anomalies has has a a Centerprecipitation,Centerprecipitation, for for Atmospheric Atmospheric particularly particularly Research Research in in boreal boreal (NCAR) (NCAR) summer summer and Community Community autumn slopeslope of of 0.25 0.25 ± ±1 0.1$ 0.1T that that is is close close to to the the 0.22 0.22 expected expected for for a a M " x ! Atmosphere[Atmosphere[HackHack et et al. al.,, Model Model 2006; 2006; BiasuttiBiasutti version version et et 3 3 al. al. (CAM3) (CAM3),, 2006]. 2006]. at at T42 T42 resolution resolution y perfectlyperfectly linear linear# $ response. response.y As As in in NA1, NA1, NA2 NA2 precipitation precipitation [[88]] Given Given the the possibility possibility that that LIA LIA SST SST and and ITCZ vari- (2.8(2.8°° byby 2.8 2.8°° latitude-longitude,latitude-longitude, 26 26 vertical vertical levels) levels) [ [CollinsCollins anomaliesanomalies exhibit exhibit a a zonal zonal band band of of increased increased aridity aridity just just abilityability was was associated associated with with a a weaker weaker AMOC, AMOC, all all simulations ! etet al. al.,, 2006]. 2006]. In In general, general, CAM3 CAM3 reproduces reproduces the the pattern pattern and and northnorth of of the the equator, equator, although although opposing opposing positive positive precipita- precipita- werewere forced forced using using the the North North Atlantic Atlantic SST SST pattern pattern from the Model results suggest curl increases when ITCZ amplitudeamplitude of of mean mean annual annual low-latitude low-latitude Atlantic Atlantic climate climate tiontion anomalies anomalies to to the the south south of of this this band band are are more more subtle. subtle. variability,‘‘hosing’’variability,‘‘hosing’’ experiment experiment as as well well as as of of interannual interannualZhangZhang and and Delworth Delworth migrations migrations[2005] of of tropical tropical (here- migrates southward convergenceinafterconvergenceinafter referred referred zones zones to to as as ZD05). ZD05). [ [HackHack Briefly, Briefly, et et al. al.,, ZD052006, ZD05 2006, applied appliedHurrellHurrell a et0.6 et al. al. Sv,, 2006]freshwater2006]freshwater suggesting suggesting forcing forcing that that to to the the it it is is high-latitude high-latitude appropriate appropriate North North for for investigating investigating Atlantic Atlantic (55 the the°– mean75mean75°°N,N, state state 63 63°°W–4W–4 of of the the°°E)E) ITCZ ITCZ for for 60 60 during during years years the the in in LIA. LIA.an an ocean-atmosphere ocean-atmosphere However, However, as as in in otherglobalotherglobal AGCMs, AGCMs, general general circulation circulation CAM3 CAM3 tends tends model. model. to to In In overestimate overestimate response response to to this Caribbean Caribbean forcing, precipitation,theprecipitation,the entire entire North North particularly particularly Atlantic Atlantic cooled, cooled, in in boreal boreal with with summer summer some some SST SST and and anomalies autumn autumn [reaching[reachingHackHack et et al. al.99,,°° 2006;C. 2006;C. BiasuttiBiasutti et et al. al.,, 2006]. 2006]. ÀÀ [[8989]] GivenOur GivenOur first first the the two two possibility possibility simulations simulations that that (NA1, (NA1, LIA LIA SST SST NA2) NA2) and and prescribe ITCZ ITCZ vari- vari- the abilityZD05abilityZD05 SST SSTwas was associated associated pattern pattern in in with withthe the Atlantic Atlantic a a weaker weaker north north AMOC, AMOC, of of the the all all equator simulations simulations and wereusewereuse climatological climatological forced forced using using SSTs SSTs the the North North elsewhere elsewhere Atlantic Atlantic (Figure (Figure SST SST 1). 1). pattern pattern NA1 from from uses the the Figure 2.- forcedSignificant NCAR CAM3 using mean SST annual- SST anomalies precipitation scaled to simulate (contours, anomalies1 from hosing experiment LIA observations 1 ‘‘hosing’’full‘‘hosing’’full magnitude magnitude experiment experiment of of cooling, cooling, of of ZhangZhang which which and and reaches reaches Delworth Delworth 9 9°°CC[2005][2005] in the (here- extra-(here- mm day(ZhangÀ ) & and Delworth, wind 2005) stress (vectors, dyn cmÀ ) anomalies inaftertropicalinaftertropical referred referred North North to to Atlantic. Atlantic. as as ZD05). ZD05). NA2 NA2 Briefly, Briefly, is is identical identical ZD05 ZD05 applied applied to NA1, a a 0.6 0.6 but Sv Sv for (a) NA1, (b) NA2, (c) XTA1, (d) XTA2, (e) TNA1, and freshwateruniformlyfreshwateruniformly reduces reduces forcing forcing to to SST SST the the anomalies anomalies high-latitude high-latitude by by 78%North 78% North such such Atlantic Atlantic that (55 (55 extra-°°–– (f) TNA2. Shading highlights positive (dark/green)Saenger et al., 2009 and 75tropical75tropical°°N,N, 63 63 Atlantic Atlantic°°W–4W–4°°E)E) cooling cooling for for 60 60 does does years years not not in in exceed exceed an an ocean-atmosphere ocean-atmosphere 2 2°C. NA1 is negative (light/orange) precipitation anomalies that are globalintendedglobalintended general general to to be be circulation circulationan an idealized idealized model. model. simulation simulation In In response response that that will will to to yield this this forcing, forcing, a clear significant at 95%. thethe entire entire North North Atlantic Atlantic cooled, cooled, with with some some SST SST anomalies anomalies reachingreaching 99°°C.C. 2 of 5 [[99]] Our OurÀÀ first first two two simulations simulations (NA1, (NA1, NA2) NA2) prescribe prescribe the the ZD05ZD05 SST SST pattern pattern in in the the Atlantic Atlantic north north of of the the equator equator and and useuse climatological climatological SSTs SSTs elsewhere elsewhere (Figure (Figure 1). 1). NA1 NA1 uses uses the the FigureFigure 2. 2. SignificantSignificant mean mean annual annual precipitation precipitation (contours, (contours, 11 11 fullfull magnitude magnitude of of cooling, cooling, which which reaches reaches 9 9°°CC in in the the extra- extra- mmmm day dayÀÀ )) and and wind wind stress stress (vectors, (vectors, dyn dyn cm cmÀÀ )) anomalies anomalies tropicaltropical North North Atlantic. Atlantic. NA2 NA2 is is identical identical to to NA1, NA1, but but forfor (a) (a) NA1, NA1, (b) (b) NA2, NA2, (c) (c) XTA1, XTA1, (d) (d) XTA2, XTA2, (e) (e) TNA1, TNA1, and and uniformlyuniformly reduces reduces SST SST anomalies anomalies by by 78% 78% such such that that extra- extra- (f)(f) TNA2. TNA2. Shading Shading highlights highlights positive positive (dark/green) (dark/green) and and tropicaltropical Atlantic Atlantic cooling cooling does does not not exceed exceed 2 2°°C.C. NA1 NA1 is is negativenegative (light/orange) (light/orange) precipitation precipitation anomalies anomalies that that are are intendedintended to to be be an an idealized idealized simulation simulation that that will will yield yield a a clear clear significantsignificant at at 95%. 95%.

22 of of 5 5 A role for the MOC?

warm HadCM3 surface ` temp. (ºC) anomalies cold

HadCM3 precipitation wet (cm/yr) anomalies ` dry

Vellinga and Wu, 2004 Conclusions

• 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 anomaly R EPORTS over much of the tropical Atlantic, as indi- which plays a major role in modulating ter- Although independent evidence supports cated by lake-level data from west Africa rigenous input to this basin on glacial-inter- our interpretation of the metal data in terms (10), pollen data from northern South Amer- glacial time scales (6), has probably not var- of precipitation and runoff, Fe and Ti con- ica (11), and general circulation model ied so much during the Holocene as to influ- tents can also vary as a result of dilution by (GCM) simulations of regional climate dur- ence Fe and Ti concentrations over the inter- biogenic sediment components. Calcium car- ing this period (12). The wettest conditions of val of this study (17). bonate, derived mostly from coccolithopho- the last 14 ky, as inferred from high Fe and Ti Large century-scale variations in precipita- rids and planktic foraminifera, typically values, occurred between 10.5 and 5.4 ka tion are inferred between ϳ3.8 and 2.8 ka, makes up 25 to 35% of the Holocene sedi- during the Holocene “thermal maximum,” a a)based on amplitudes of the Fe and Ti signals ments at Site 1002 (6). Diatoms are locally period associated with wet conditions in Af- that) are comparable to the difference between important as well (18). Tradewind-driven up-

rica (10, 13) and with the most depleted theSv Younger Dryas and the Holocene “thermal welling is the dominant cause for changes in oxygen isotope values in Cariaco Basin fora- maximum.” Higher but variable precipitation is Cariaco biological export production during minifera (14). The subsequent Holocene dry- also indicated from 1.05 to 0.7 ka, the interval the Holocene (1). Hence, changes in up- ing trend has been previously recognized in a of time sometimes referred to as the “Medieval welling might be expected to affect the sed- lacustrine record from Haiti (15) and in pol- Warm Period.” This was followed by the “Little imentary concentrations of Ti and Fe through len records from nearby Lake Valencia (16), Ice Age,” marked by drier conditions in the dilution, leading to ambiguity in the interpre- affirming the broad regional importance of Cariaco ( Transport region and punctuated by three notable tation of the metal data in the absence of the Cariaco Basin sediment record. Sea level, precipitation minima. additional information. However, bulk accu- b) Fig. 3. Bulk Ti content of Cari- aco Basin sediments from ODP Site 1002 versus age, spanning (a) the last 14 ky (3-point running mean); (b) the last 5 ky (3-point running mean); and (c) the last 1.2 ky (raw data and 3-point running mean).

Higher Ti content reﬂects ) greater terrigenous input from riverine runoff and is interpret- m ed to indicate greater precipitation and a more northerly Holocene variability in Gulf Stream transport mean latitude of the ITCZ.

a) Transport increased ) Water depth ( Water Sv ~ 4 Sv during Holocene Transport ( Transport

b) Calendar Age (yr BP) Lynch-Steiglitz, et al., in press

Figure 4. a) Calculated geostrophic ﬂow through the Florida StraitsITCZ with migrated 2-sigma southward error ) estimates. m b) Geostrophic ﬂow per unit depth (Sv m-1).! during Holocene Water depth ( Water

Haug et al., 2001

1306 17 AUGUST 2001 VOL 293 SCIENCE www.sciencemag.org

Calendar Age (yr BP)

Figure 4. a) Calculated geostrophic ﬂow through the Florida Straits with 2-sigma error estimates. b) Geostrophic ﬂow per unit depth (Sv m-1).! 18 14 Dry Tortugas δ Owater record mimics Δ Catm

Lund and Curry, 2006 North Atlantic region cooled by ~1°C during LIA C) o

Dahl-Jensen et al., 1998

Greenland Temp. ( Lamb, 1995

Marchitto and deMenocal, 2003

Bond et al., 2001

Keigwin, 1996 deMenocal et al., 2000