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Open Access Publications from the University of California

Scholarly Works (20 results)

  • Article
  • Peer Reviewed
Faculty Publications (2003)
    Cover page: Tracking water movement to and from the ocean surface.Creative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2006)

    A conceptually new approach to diagnosing tracer-independent ventilation rates is developed. Tracer Green functions are exploited to partition ventilation rates according to the ventilated fluid’s residence time in the ocean interior and according to where this fluid enters and exits the interior. In the presence of mixing by mesoscale eddies, which are reasonably represented by diffusion, ventilation rates for overlapping entry and exit regions cannot meaningfully be characterized by a single rate. It is a physical consequence of diffusive transport that fluid elements that spend an infinitesimally short time in the interior cause singularly large ventilation rates for overlapping entry and exit regions. Therefore, ventilation must generally be characterized by a ventilation-rate distribution, , partitioned according to the time that the ventilated fluid spends in the interior between successive surface contacts. An offline forward and adjoint time-averaged OGCM is used to illustrate the rich detail that and the closely related probability density function of residence times provide on the way the ocean communicates with the surface. These diagnostics quantify the relative importance of various surface regions for ventilating the interior ocean by either exposing old water masses to the atmosphere or by forming newly ventilated ones. The model results suggest that the Southern Ocean plays a dominant role in ventilating the ocean, both as a region where new waters are ventilated into the interior and where old waters are first reexposed to the atmosphere.

      Cover page: The Ocean's Memory of the Atmosphere: Residence-Time and Ventilation-Rate Distributions of Water MassesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      Faculty Publications (2010)
      We use 39Ar in conjunction with CFCs, natural radiocarbon, and the cyclostationary tracers PO4*, temperature, and salinity to estimate the ocean's transit time distributions (TTDs). A maximum entropy method is employed to deconvolve the tracer data for the TTDs. The constraint provided by the 39Ar data allows us to estimate TTDs even in the deep Pacific where CFCs have not yet penetrated. From the TTDs, we calculate the ideal mean age, Γ, the TTD width, Δ, and the mass fraction of water with transit times less than a century, f 1. We also quantify the entropic uncertainties due to the nonuniqueness of the deconvolutions. In the Atlantic, the patterns of Γ and f 1 reflect the distribution of the major water masses. At the deepest locations in the North Atlantic Γ ≃ 300−100 +300 a, while at the deepest locations in the South Atlantic Γ ≃ 500−100 +200 a. The Pacific is nearly homogeneous below 2000 m with Γ ≃ 1300−50 +200 a in the North Pacific and Γ ≃ 900−100 +200 a in the deep South Pacific. The Southern Ocean locations have little vertical structure, with Γ ranging from 300 to 450 a with an uncertainty of about −40 +150 a. The importance of diffusion compared to advection as quantified by Δ/Γ has most probable values ranging from 0.2 to 3 but with large entropic uncertainty bounds ranging from 0.2 to 9. For the majority of locations analyzed, the effect of39Ar is to reduce f 1 and to correspondingly increase Γ by about a century. The additional constraint provided by 39Ar reduces the entropic uncertainties of f 1 by roughly 50% on average.
        Cover page: Improved constraints on transit time distributions from argon 39: A maximum entropy approachCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        Faculty Publications (2008)
         A novel diagnostic for advective-diffusive surface-to-surface paths is developed and applied to a global ocean model. The diagnostic provides, for the first time, a rigorous quantitative assessment of the great ocean conveyor's deep branch. A new picture emerges of a diffusive conveyor in which the deep North Pacific is a holding pen of long-residence-time water. Our diagnostic is the joint density, η, per unit volume and interior residence time, τ, of paths connecting two specified surface patches. The spatially integrated η determines the residence-time partitioned flux and volume of water in transit from entry to exit patch. We focus on interbasin paths from high-latitude water mass formation regions to key regions of re-exposure to the atmosphere. For non-overlapping patches, a characteristic timescale is provided by the residence time, τ ϕ, for which the associated flux distribution, ϕ, has its maximum. Paths that are fast compared to τ ϕ are organized by the major current systems, while paths that are slow compared to τ ϕ are dominated by eddy diffusion. Because ϕ has substantial weight in its tail for τ > τ ϕ, the fast paths account for only a minority of the formation-to-re-exposure flux. This conclusion is expected to apply to the real ocean based on recent tracer data analyses, which point to long eddy-diffusive tails in the ocean's transit-time distributions. The long-τ asymptotic path density is governed by two time-invariant patterns. One pattern, which we call the Deep North Pacific pattern, ultimately dominates a secondary redistribution pattern.
          Cover page: The path-density distribution of oceanic surface-to-surface transportCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          Faculty Publications (2007)

          The steady state bifurcation structure of the double-gyre wind-driven ocean circulation is examined in a shallow water model where the upper layer is allowed to outcrop at the sea surface. In addition to the classical jet-up and jet-down multiple equilibria, we find a new regime in which one of the equilibrium solutions has a large outcropping region in the subpolar gyre. Time dependent simulations show that the outcropping solution equilibrates to a stable periodic orbit with a period of 8 months. Co-existing with the periodic solution is a stable steady state solution without outcropping.

          A numerical scheme that has the unique advantage of being differentiable while still allowing layers to outcrop at the sea surface is used for the analysis. In contrast, standard schemes for solving layered models with outcropping are non-differentiable and have an ill-defined Jacobian making them unsuitable for solution using Newton’s method. As such, our new scheme expands the applicability of numerical bifurcation techniques to an important class of ocean models whose bifurcation structure had hitherto remained unexplored.

            Cover page: Bifurcation structure of a wind-driven shallow water model with layer-outcroppingCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            Faculty Publications (2006)
             We use a novel path-density transport diagnostic to trace out the deep branch of the ocean conveyor in a global circulation model. Our results suggest that the majority of the world's deep water is not transported back to the surface along the current systems of the standard great ocean conveyor (GOC). Standard GOC routes are evident only for waters with interior residence times, τ, less than about a thousand years, accounting for less than a quarter of the ventilation-to-re-exposure flux. Waters with longer τ are spread across the deep oceans by the “diffusive conveyor” and, by τ ∼ 3000 years, organized into a characteristic deep-North-Pacific pattern that is dominated by eddy diffusion. The observed depletion of oxygen and 14C in the deep N Pacific is consistent with a diffusive conveyor and should not be interpreted as evidence of an advective terminus of the GOC deep branch.
              Cover page: The diffusive ocean conveyorCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              Faculty Publications (2013)
              Nutrient transport and productivity teleconnections with the Southern Ocean are diagnosed in a data-assimilated circulation model coupled to a jointly optimized simple phosphorus cycling model. The North Atlantic has the strongest extratropical teleconnections with the Southern Ocean: phosphate (PO4) last utilized in the Southern Ocean sustains 29 ± 6% of the subpolar and 14 ± 6% of the subtropical, new production in the North Atlantic. A PO4 path-density diagnostic shows that these teleconnections are mediated by thermocline paths and reveals that most paths to anywhere north of 40°S lie in the deep Pacific. Forcing nearly complete Southern-Ocean nutrient utilization increases the overall number of paths to anywhere north of 40°S, but reduces the number of paths from the Pacific to the North Atlantic by trapping nutrients in return paths to the Southern Ocean. At the same time, the mean export-to-uptake transit times to anywhere north of 40°S increase, while the mean transit times to the North Atlantic decrease. Correspondingly, the amount of North-Atlantic production sustained by Southern-Ocean export increases in spite of decreased total production in response to Southern-Ocean nutrient trapping. The distributions of export-to-uptake, export-to-surface, and surface-to-surface transit times are computed and summarized in terms of their mean transit times and their mean interior residence times. The combined particle and advective–diffusive transport of nutrients is characterized by broad, skewed transit-time distributions, which result in mean residence times much longer than the mean transit times, in turn much longer than the most probable transit times.
                Cover page: Global teleconnections in the oceanic phosphorus cycle: Patterns, paths, and timescalesCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                Faculty Publications (2008)
                We present a computationally-efficient method for obtaining the fully spun-up state of a seasonally-varying global ocean biogeochemistry model. The solver uses a Newton–Krylov method to find the fixed points of the map that assigns to an initial state the value of the model state at the end of a one-year run. Apart from the preconditioner, which we describe in the paper, the method relies on a black-box public-domain Newton–Krylov solver that does not require the explicit construction of the model’s Jacobian matrix. Applied to the PO4 plus dissolved organic phosphorus (DOP) cycle of an Ocean Carbon Model Intercomparison Project II (OCMIP-2) type model, the solver is more than two orders of magnitude faster than the traditional time-stepping method for spinning up the model. The efficiency of the solver is illustrated by using the seasonally varying globally-gridded PO4 climatology to objectively optimize the parameters that control the mean lifetime of semi-labile DOP and the fraction of new production allocated to DOP. The optimization study demonstrates that the information in the seasonal variations of PO4 do not provide a significantly stronger constraint than the annually averaged data used in previous optimization studies.
                  Cover page: A fast Newton-Krylov solver for seasonally varying global ocean biogeochemistry modelsCreative Commons 'BY' version 4.0 license
                  • Thesis
                  • Peer Reviewed
                  UC Irvine Electronic Theses and Dissertations (2018)

                  The production of organic matter at the ocean's surface, its transport, and subsequent remineralization in the ocean interior maintains a vertical carbon and nutrient gradient that sequesters atmospheric CO2, controls the distribution of dissolved oxygen, and modulates the flux of nutrients to the euphotic zone that fuels marine ecosystems. Collectively we refer to the set of processes maintaining these gradients as the biological pump and they are the central focus of this thesis. To advance our understanding of the biological pump, this thesis uses models to analyze and quantify individual processes within the larger biological pump system. Chapter 1 introduces the key processes at question and outlines the specific goals of each research chapter. Chapter 2 uses empirical models and a global database of in-situ observations to examine the environmental drivers of carbon export efficiency, defined as the fraction of organic matter production that is transported from the surface to the interior. We find significant regional variability in the relationship between export efficiency and environmental parameters that are shown to have important consequences for the prediction of global export. Chapter 3 further develops empirical models of carbon export efficiency with a focus on regional export variability in the Southern Ocean. We find new evidence that silica controls Southern Ocean export efficiency via interactions with ocean temperature. Chapter 4 uses a global ocean circulation and biogeochemistry model to examine the sensitivity of the global oxygen distribution to changes in the depth at which sinking organic matter is remineralized. Model experiments demonstrate that raising the remineralization depth of organic matter leads to net oxygen uptake and simultaneous expansion of oxygen minimum zones. Chapter 5 examines the consequences of long term climate-driven changes in the biological pump for the productivity of marine ecosystems and fisheries. The thesis concludes in Chapter 6 with a discussion of the challenges and opportunities in understanding the biological pump in a changing climate, along with my recommendations for follow up research.

                    Cover page: Causes and consequences of the ocean's biological pumpCreative Commons 'BY-ND' version 4.0 license
                    • Article
                    • Peer Reviewed
                    Faculty Publications (2013)
                    We present a data-assimilated model of the ocean's phosphorus cycle that is constrained by climatological phosphate, temperature, salinity, sea-surface height, surface heat and freshwater fluxes, as well as chlorofluorocarbon-11(CFC-11) and natural Δ14C. Export production is estimated to be 5.8±2.0×1012 mol P/yr of which (26±6)% originates in the Southern Ocean (SO) south of 40°S. The biological pump efficiency, defined as the proportion of the ocean's phosphate inventory that is regenerated, is (39±7)%. Dividing the SO south of 40°S into a sub-Antarctic zone (SANTZ) and an Antarctic zone (ANTZ) separated by the latitude of maximum Ekman divergence, we estimate that the SANTZ and ANTZ account, respectively, for (23±5)% and (3±1)% of global export production, (17±4)% and (3±1)% of the regenerated nutrient inventory, and (31±1)% and (43±5)% of the preformed nutrient inventory. Idealized SO nutrient depletion experiments reveal a large-scale transfer of nutrients into circumpolar and deep waters and from the preformed to the regenerated pool. In accord with the concept of the biogeochemical divide, we find that nutrient drawdown in the ANTZ is more effective than in the SANTZ for increasing the efficiency of the biological pump, while having a smaller impact on production in regions north of 40°S. Complete SO nutrient drawdown would allow the biological pump to operate at 94% efficiency by short circuiting the transport of nutrients in northward Ekman currents, leading to a trapping of nutrients in circumpolar and deep waters that would decrease production outside the SO by approximately 44% while increasing it in the SO by more than 725%.
                      Cover page: Southern Ocean nutrient trapping and the efficiency of the biological pumpCreative Commons 'BY' version 4.0 license
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