How much did Glacial North Atlantic Water shoal?

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 29 (2014): 190-209, doi:10.1002/2013PA002557.Observations of δ13C and Cd/Ca from benthic foraminifera have been interpreted to reflect a shoaling of northern source waters by about 1000 m during the Last Glacial Maximum, with the degree of shoaling being significant enough for the water mass to be renamed Glacial North Atlantic Intermediate Water. These nutrient tracers, however, may not solely reflect changes in water mass distributions. To quantify the distribution of Glacial North Atlantic Water, we perform a glacial water mass decomposition where the sparsity of data, geometrical constraints, and nonconservative tracer effects are taken into account, and the extrapolation for the unknown water mass end-members is guided by the modern-day circulation. Under the assumption that the glacial sources of remineralized material are similar to that of the modern day, we find a steady solution consistent with 241 δ13C, 87 Cd/Ca, and 174 δ18O observations and their respective uncertainties. The water mass decomposition indicates that the core of Glacial North Atlantic Water shoals and southern source water extends in greater quantities into the abyssal North Atlantic, as previously inferred. The depth of the deep northern-southern water mass interface and the volume of North Atlantic Water, however, are not grossly different from that of the modern day. Under this scenario, the vertical structure of glacial δ13C and Cd/Ca is primarily due to the greater accumulation of nutrients in lower North Atlantic Water, which may be a signal of the hoarding of excess carbon from the atmosphere by the glacial Atlantic.G.G. is supported by NSF grants OIA-1124880 and OCE-1301907, and the WHOI Ocean and Climate Change Institute.2014-09-1

Atlantic warming since the little ice age

Author Posting. © Oceanography Society, 2019. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Gebbie, G. Atlantic warming since the little ice age. Oceanography, 32(1), (2019):220-230, doi:10.5670/oceanog.2019.151.Radiocarbon observations suggest that the deep Atlantic Ocean takes up to several centuries to fully respond to changes at the sea surface. Thus, the ocean’s memory is longer than the modern instrumental period of oceanography, and the determination of modern warming of the subsurface Atlantic requires information from paleoceanographic data sets. In particular, paleoceanographic proxy data compiled by the Ocean2k project indicate that there was a global cooling from the Medieval Warm Period to the Little Ice Age over the years 900−1800, followed by modern warming that began around 1850. An ocean simulation that is forced by a combined instrumental-​proxy reconstruction of surface temperatures over the last 2,000 years shows that the deep Atlantic continues to cool even after the surface starts warming. As a consequence of the multicentury surface climate history, the ocean simulation suggests that the deep Atlantic doesn’t take up as much heat during the modern warming era as the case where the ocean was in equilibrium at 1750. Both historical hydrographic observations and proxy records of the subsurface Atlantic are needed to determine whether the effects of the Little Ice Age did indeed persist well after the surface climate had already shifted to warmer conditions.The author thanks Peter Huybers for collaborating on the Common Era temperature evolution, Lars Henrik Smedsrud for the encouragement to write this manuscript and compute heat fluxes, and to Ellen Kappel, Paul Durack, and Alex Sen Gupta for their handling of the manuscript. GG is supported by the James E. and Barbara V. Moltz Fellowship and NSF OCE-1357121. Correspondence and requests for materials should be addressed to the author

Cancelation of deglacial thermosteric sea level rise by a barosteric effect

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(12),(2020): 3623-3639, https://doi.org/10.1175/JPO-D-20-0173.1Sea level rise over the last deglaciation is dominated by the mass of freshwater added to the oceans by the melting of the great ice sheets. While the steric effect of changing seawater density is secondary over the last 20 000 years, processes connected to deglacial warming, the redistribution of salt, and the pressure load of meltwater all influence sea level rise by more than a meter. Here we develop a diagnostic for steric effects that is valid when oceanic mass is changing. This diagnostic accounts for seawater compression due to the added overlying pressure of glacial meltwater, which is here defined to be a barosteric effect. Analysis of three-dimensional global seawater reconstructions of the last deglaciation indicates that thermosteric height change (1.0–1.5 m) is counteracted by barosteric (−1.9 m) and halosteric (from −0.4 to 0.0 m) effects. The total deglacial steric effect from −0.7 to −1.1 m has the opposite sign of analyses that assume that thermosteric expansion is dominant. Despite the vertical oceanic structure not being well constrained during the Last Glacial Maximum, net seawater contraction appears robust as it occurs in four reconstructions that were produced using different paleoceanographic datasets. Calculations that do not account for changes in ocean pressure give the misleading impression that steric effects enhanced deglacial sea level rise.GG is supported by NSF OCE-1536380 and OCE-1760878.2021-06-0

Meridional circulation during the Last Glacial Maximum explored through a combination of South Atlantic δ18O observations and a geostrophic inverse model

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q11N07, doi:10.1029/2006GC001383.The vertical profile of meridional transport in the South Atlantic is examined by combining paleoceanographic observations with a geostrophic circulation model using an inverse method. δ18Ocalcite observations along the margins of the South Atlantic show that upper-ocean cross-basin differences were weaker during the Last Glacial Maximum (LGM) than the Holocene. The δ18Ocalcite observations can be explained by a shift of water-mass properties without any change in the overturning circulation. Alternatively, they may indicate a reduced LGM cross-basin density difference and, via the thermal wind relation, a reduced vertical shear. Model inversions of δ18Ocalcite are found to require meridional transports different from the modern only after three assumptions are made: temperature and salinity distributions are spatially smooth, the relationship between salinity and δ18Owater is linear and spatially invariant, and LGM temperatures are known to within 1°C along the margins. The last assumption is necessary because an independent constraint on temperature or salinity is required to determine density from δ18Ocalcite observations. δ18Ocalcite observations are clearly useful, but before any firm constraints can be placed on LGM meridional transport, it appears necessary to better determine the relationship between δ18Ocalcite and density.P.H. was funded by the NOAA postdoctoral program in climate and global change, and G.G. was partially funded by NSF paleoclimate program ATM-0502482

Controllability, not chaos, key criterion for ocean state estimation

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nonlinear Processes in Geophysics 24 (2017): 351-366, doi:10.5194/npg-24-351-2017.The Lagrange multiplier method for combining observations and models (i.e., the adjoint method or "4D-VAR") has been avoided or approximated when the numerical model is highly nonlinear or chaotic. This approach has been adopted primarily due to difficulties in the initialization of low-dimensional chaotic models, where the search for optimal initial conditions by gradient-descent algorithms is hampered by multiple local minima. Although initialization is an important task for numerical weather prediction, ocean state estimation usually demands an additional task – a solution of the time-dependent surface boundary conditions that result from atmosphere–ocean interaction. Here, we apply the Lagrange multiplier method to an analogous boundary control problem, tracking the trajectory of the forced chaotic pendulum. Contrary to previous assertions, it is demonstrated that the Lagrange multiplier method can track multiple chaotic transitions through time, so long as the boundary conditions render the system controllable. Thus, the nonlinear timescale poses no limit to the time interval for successful Lagrange multiplier-based estimation. That the key criterion is controllability, not a pure measure of dynamical stability or chaos, illustrates the similarities between the Lagrange multiplier method and other state estimation methods. The results with the chaotic pendulum suggest that nonlinearity should not be a fundamental obstacle to ocean state estimation with eddy-resolving models, especially when using an improved first-guess trajectory.Geoffrey Gebbie was funded through the Ocean and Climate Change Institute of the Woods Hole Oceanographic Institution. Tsung-Lin Hsieh was funded by the Arthur Vining Davis Foundations Fund for Summer Student Fellows through the Woods Hole Oceanographic Institution

Subduction in an eddy-resolving state estimate of the northeast Atlantic Ocean

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2004Relatively little is known about the role of eddies in controlling subduction in the eastern half of the subtropical gyre. Here, a new tool to study the eastern North Atlantic Ocean is created by combining a regional, eddy-resolving numerical model with observations to produce a state estimate of the ocean circulation. The estimate is a synthesis of a variety of in-situ observations from the Subduction Experiment, TOPEX/POSEIDON altimetry, and the MIT General Circulation Model. A novel aspect of this work is the search for an initial eddy field and eddy-scale open boundary conditions by the use of an adjoint model. The adjoint model for this region of the ocean is stable and yields useful information despite concerns about the chaotic nature of eddy-resolving models. The method is successful because the dynamics are only weakly nonlinear in the eastern region of the subtropical gyre. Therefore, no fundamental obstacle exists to constraining the model to both the large scale circulation and the eddy scale in this region of the ocean. Individual eddy trajectories can also be determined. The state estimate is consistent with observations, self-consistent with the equations of motion, and it explicitly resolves eddy-scale motions with a 1/6º grid. Therefore, subduction rates, volume budgets, and buoyancy budgets are readily diagnosed in a physically interpretable context. Estimates of eddy subduction for the eastern subtropical gyre of the North Atlantic are larger than previously calculated from parameterizations in coarse-resolution models. Eddies contribute up to 40 m/yr of subduction locally. Furthermore, eddy subduction rates have typical magnitudes of 15% of the total subduction rate. To evaluate the net effect of eddies on an individual density class, volume budgets are diagnosed. Eddies contribute as much as 1 Sv to diapycnal flux, and hence subduction, in the density range 25.5 < σ < 26.5. Eddies have a integrated impact which is sizable relative to the 2.5 Sv of diapycnal flux by the mean circulation. A combination of Eulerian and isopycnal maps suggest that the North Equatorial Current and the Azores Current are the geographical centers of eddy subduction. The findings of this thesis imply that the inability to resolve or accurately parameterize eddy subduction in climate models would lead to an accumulation of error in the structure of the main thermocline, even in the eastern subtropical gyre, which is a region of comparatively weak eddy motions.This thesis has received support through grants, fellowships, and computer allocations. A NASA Earth System Science Fellowship has been my primary source of funding. In addition, computer time at the University of Texas has been provided by a NPACI PRAC grant, "State Estimates of the Time-Evolving Three-dimensional Ocean Circulation with Eddy Resolution." Grant #6857100 through CalTech and the Jet Propulsion Laboratory, as well as Grant #6892952 through NASA-Goddard Flight Center for the synthesis of the World Ocean Circulation Experiment

How well would modern-day oceanic property distributions be known with paleoceanographic-like observations?

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 472–490, doi:10.1002/2015PA002917.Compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins, mid-ocean ridges, and bathymetric highs, where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells. Here we design an idealized experiment to test our predictive ability to reconstruct modern-day seawater properties by considering paleoceanographic-like data. We attempt to reconstruct the known, modern-day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics. When a modern-like suite of observations (Θ, practical salinity, seawater δ18O, inline image, PO4, NO3, and O2) is used from the sparse paleolocations, the state estimate is consistent with the withheld data at all depths below 1500 m, suggesting that the observational sparsity can be overcome. Physical features, such as the interbasin gradients in deep inline image and the vertical structure of Atlantic inline image, are accurately reconstructed. The state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales (at least 10,000 km horizontally and 1500 m vertically) and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used. When the sparse observations are more realistically restricted to the paleoceanographic proxy observations of δ13C, δ18O, and Cd/Ca, however, the large-scale property distributions are no longer recovered coherently. At least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large-scale property distributions of the Last Glacial Maximum.NSF Grant Numbers: 1124880, 11254222016-10-0

A century of observed temperature change in the Indian Ocean

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Wenegrat, J. O., Bonanno, E., Rack, U., & Gebbie, G. A century of observed temperature change in the Indian Ocean. Geophysical Research Letters, 49(13), (2022): e2022GL098217, https://doi.org/10.1029/2022GL098217.The Indian Ocean is warming rapidly, with widespread effects on regional weather and global climate. Sea-surface temperature records indicate this warming trend extends back to the beginning of the 20th century, however the lack of a similarly long instrumental record of interior ocean temperatures leaves uncertainty around the subsurface trends. Here we utilize unique temperature observations from three historical German oceanographic expeditions of the late 19th and early 20th centuries: SMS Gazelle (1874–1876), Valdivia (1898–1899), and SMS Planet (1906–1907). These observations reveal a mean 20th century ocean warming that extends over the upper 750 m, and a spatial pattern of subsurface warming and cooling consistent with a 1°–2° southward shift of the southern subtropical gyre. These interior changes occurred largely over the last half of the 20th century, providing observational evidence for the acceleration of a multidecadal trend in subsurface Indian Ocean temperature.GG is supported by U.S. NSF-OCE 82280500

Global-mean marine δ13C and its uncertainty in a glacial state estimate

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Quaternary Science Reviews 125 (2015): 144-159, doi:10.1016/j.quascirev.2015.08.010.A paleo-data compilation with 492 δ13C and δ18O observations provides the opportunity to better sample the Last Glacial Maximum (LGM) and infer its global properties, such as the mean δ13C of dissolved inorganic carbon. Here, the paleocompilation is used to reconstruct a steady-state water-mass distribution for the LGM, that in turn is used to map the data onto a 3D global grid. A global-mean marine δ13C value and a self-consistent uncertainty estimate are derived using the framework of state estimation (i.e., combining a numerical model and observations). The LGM global-mean δ13C is estimated to be 0:14h±0:20h at the two standard error level, giving a glacial-to-modern change of 0:32h±0:20h. The magnitude of the error bar is attributed to the uncertain glacial ocean circulation and the lack of observational constraints in the Pacific, Indian, and Southern Oceans. Observations in the Indian and Pacific Oceans generally have 10 times the weight of an Atlantic point in the computation of the global mean. To halve the error bar, roughly four times more observations are needed, although strategic sampling may reduce this number. If dynamical constraints can be used to better characterize the LGM circulation, the error bar can also be reduced to 0:05 to 0:1h, emphasizing that knowledge of the circulation is vital to accurately map δ13CDIC in three dimensions.GG is supported by NSF grants OIA-1124880 and OCE-1357121, the WHOI Ocean and Climate Change Institute, and The Joint Initiative Awards Fund from the Andrew W. Mellon Foundation

Data constraints on glacial Atlantic Water mass geometry and properties

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 1013-1034, doi:10.1029/2018PA003408.The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.National Science Foundation (NSF) Grant Numbers: OCE‐0750880, OCE‐1335191, OCE‐1558341, OCE‐1536380; Woods Hole Oceanographic Institution (WHOI) Grant Numbers: 27007592, 2700080