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Atlantic Hydrography and Circulation: A GEOTRACER’s View

Atlantic Ocean Circulation Why consider the circulation? –Sets up large scale property distributions Establishes major redox zones (e.g. OMZ) –Controls major biological regimes Nutrient supply to euph. zone and ventilation –Can be significant terms in conservation, distribution, or budget equations “Knowing” these may give us “J ” –May vary with time  –Informs sampling strategies, interpretation

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic? 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind-Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

A GEOTRACER’s View of Atlantic Hydrography and Circulation The Atlantic is not like any other ocean: “The Pacific is an ocean… …the Atlantic is a bloody bowling alley” - John Edmond Photo courtesy Ed Boyle

What’s Different about the Atlantic? It is the saltiest of the oceans –Due to the atmospheric salt bridge and Agulhas leakage –Robust deep convection areas –Makes the densest deep waters Origination of the global conveyor –Entry point of anthropogenic CO 2 and climate anomalies Strong driven meridional flow –Sensitive to global change

The North Atlantic is Salty “Classic” subtropical surface salt cells –High E-P –Ekman downwelling –Recirculation trapping North Atlantic subtropics dominated by: –Mediterranean outflow –Strong latent heat extraction in WBC –Recirculation trapping salt cell Curry et al, 2003

What’s Different about the Atlantic? It is the saltiest of the oceans It is the only ocean with monotonic northward (not poleward ) heat flux *Due to southward flow of colder water being replaced by inflowing warmer shallow water Talley, 2003

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic? 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind-Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

The Global Conveyor & the M.O.C. * *Meridional Overturning Cell Kuhlbrodt et al., 2007

The MOC Meridional transport of NADW complex from the north spills downward and southward, collides with and over-rides northward flowing AABW

Transient Tracers show leading edge of MOC GEOSECS Atlantic Section 1972 NS Tritium Radiocarbon

CFCs & Model (Schlitzer, 2007) 14 C & Model (Schlitzer, 2007) CFC Ages (Fine et al, 2002) NB: Propagation timescales of order decades Injection of deep water masses from both poles

Southward flow along western boundary and in western basin Eastern basins much more weakly ventilated Kortzinger et al, 1999 CFCs & Model (Schlitzer, 2007)

Time Evolving Transient Tracers in the Northern Subtropics: TTO, WOCE, & CLIVAR

DWBC Rapid velocity flow, > 10 cm/s Oxygenated core High tritium, CFCs, low “age”

Transit Along the DWBC T-He age varies linearly with distance downstream Core velocities (via geostrophy and current meters) ~ cm/s

Transit Along the DWBC Tracer velocity is ~ 1/10 th of “directly” measured speeds

Transit Along the DWBC Tracer speed ~ 1/10 th direct Dilution ~ 10x

Transit Along the DWBC Downstream propagation of tracer & climatic anomalies is “impeded” 10x by internal gyres, entrainment, and mixing Important implications for GEOTRACES sampling?

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

Atlantic Intermediate Waters: UNADW & AAIW Note difference with North Pacific: where I.W. salinity is Basically symmetric.

Upper NADW Largely Labrador Sea Water, but “salinified” by Mediterranean Water Influence Schlitzer, 2007 CFC-11 pmol/kg Fine et al, 2002

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic? 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind-Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

Export of surface water to polar sea

The Ventilated Upper Ocean Main entry point is by subduction –Buoyancy induction (b) – & Ekman Pumping (c) Subducted water flow largely geostrophic & conserves vorticity –“streamlines” constrained by geostrophy/vorticity conservation –Some areas (“shadow zones”) excluded by topology of outcrops or subduction zones and geostrophic dynamics Qiu and Huang, 1994

Shallow Atlantic Circulation and Subduction Qiu and Huang, 1994 Converse process is Obduction High nutrient tongues related both to upwelling regions and shadow zones

The Nutrient Spiral in the Subtropical Oligotrophic Gyres 1.Remineralized thermocline nutrients entrained into WBC 2.Diapycnal processes (mixing, STMW formation, obduction?) move nutrients onto lighter isopycnals 3.Detrainment, streamline splaying spreads into seasonal accessible layers 4.Winter convection, eddy heaving mines nutrients for new production

Shallow water transition in mid-section from obduction to subduction mode: Obduction brings material up Subduction brings material down Qiu and Huang, JPO , 1995

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic? 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind-Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

Climatic Changes in the Atlantic Occurring –at all levels –On many time-scales e.g., MOC freshening over last half of 20 th century Dickson et al, 2002

Climatic Changes in the Atlantic Occurring –At intermediate water levels, e.g., Labrador Sea

Climate changes in NASTMW Based on isopycnal analysis of salinity, oxygen and tritium- 3 He Jenkins, 1982

The Atlantic Hydrography and Circulation 1.What’s So Different About the Atlantic? 2.A Drama in 4 Acts: 1.The Lower Limb of the M.O.C 2.The Intermediate & Mode Waters 3.The Shallow Wind-Driven Gyres 4.The Times They are a-Changin’ 3. A Short Bibliography of Salient References

Some References Curry, R.G. and McCartney, M.S., Ocean gyre circulation changes associated with the North Atlantic Oscillation. Journal of Physical Oceanography, 31: Curry, R.A., Dickson, R.R. and Yashayev, I., A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 426(6968): Dickson, R.R. et al., Rapid freshening fo the deep North Atlantic Ocean over the past four decades. Nature, 416(6883): Doney, S.C. and Jenkins, W.J., Ventilation of the deep western boundary current and abyssal Western North Atlantic: Estimates from tritium and 3 He distributions. Journal of Physical Oceanography, 24(3): Fine, R.A., Rhein, M. and Andrie, C., Using CFC effective age to estimate propagation and storage of climate anomalies in the deep western North Atlantic Ocean. Geophysical Research Letters, 29(24): doi: /2002GL Jenkins, W.J., On the climate of a subtropical ocean gyre: Decade timescale variations in water mass renewal in the Sargasso Sea. Journal of Marine Research, 40S: Jenkins, W.J., Studying Thermocline Ventilation and Circulation Using Tritium and 3He. Journal of Geophysical Research, 103(C8): Jenkins, W.J. and Doney, S.C., The Subtropical Nutrient Spiral. Global Biogeochemical Cycles, 17(4): 1110 doi: /2003GB

More References… Joyce, T.M., and P. Robbins, The long-term hydrographic record at Bermuda, Journal of Climate, 9 (12), Kortzinger, A., Rhein, M. and Mintrop, L., Anthropogenic CO 2 and CFCs in the North Atlantic Ocean - a comparison of man-made tracers. Geophysical Research Letters, 26(14): Kuhlbrodt, T. et al., On the driving processes of the Atlantic meridional overturning circulation. Reviews of Geophysics, 45(RG2001): doi: /2004RG Marsh, R., Hazeleger, W., Yool, A. and Rohling, E.J., Stability of the thermohaline circulation under millennial CO2 forcing and two alternative controls on Atlantic salinity. Geophysical Research Letters, 34(L02605): doi: /2006GL Poole, R. and Tomczak, M., Optimum multiparameter analysis of the water mass structure in the Atlantic Ocean thermocline. Deep-Sea Research I, 46(11): Qiu, B. and Huang, R.X., Ventilation of the North Atlantic and North Pacific: subduction versus obduction. Journal of Physical Oceanography, 25(10): Reid, J.L., On the total geostrophic circulation of the North Atlanic Ocean: flow patterns, tracers, and transports. Progress in Oceanography, 33(1): 1-92.

Yet More References… Sarmiento, J.L., Gruber, N., Brzezinski, M. and Dunne, J., High- latitude control of thermocline nutrients and low latitude biological productivity. Nature, 427(6969): Schlitzer, R., Assimilation of radiocarbon and chlorofluorocarbon data to constrain deep and bottom water transports in the world ocean. Journal of Physical Oceanography, 37(2): Schmitz, W.J., Jr., On the interbasin-scale thermohaline circulation. Reviews of Geophysics, 33(2): Serreze, M.C. et al., The large-scale freshwater cycle of the Arctic. Journal of Geophysical Research-Oceans, 111(C11010): doi: /2005JC Sundby, S. and Drinkwater, K., On the mechanisms behind salinity anomaly signals of the northern North Atlantic. Progress in Oceanography, 73: Talley, L.D., Shallow, intermediate, and deep overturning components to the global heat budget. Journal of Physical Oceanography, 33(3): Yashayaev, I., Hydrographic changes in the Labrador Sea, Progress in Oceanography, 73:

Atlantic Hydrography and Circulation: A GEOTRACER’s View The basic challenges in the Atlantic: strongly advective system (relative to Pacific) How to distinguish modest source/sink terms against big V spatial aliasing high abyssal eddy energy/structure temporal representativeness Have strong unsteady terms in equations But on the Plus Side: It’s a smaller ocean Logistically more accessible Well studied long time series Temporal changes may be a useful “signal”