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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2019)

This thesis explores the physical connections between surface processes and the global ocean overturning circulation, which is a critical component of the climate system.

Chapter 2 discusses the influence of Southern Ocean surface buoyancy forcing on the global deep ocean stratification. It shows that Southern Ocean surface buoyancy forcing exerts a strong control on the global deep ocean stratification below 2000 m depth.

Chapter 3 investigates the impact of Southern Ocean surface buoyancy forcing on the depth of the Atlantic Meridional Overturning Circulation (AMOC). It concludes that diapycnal mixing diminishes the influence of Southern Ocean surface buoyancy forcing on the AMOC depth and that the North Atlantic surface conditions can have a substantial influence on the AMOC depth.

Chapter 4 explores the influence of North Atlantic surface conditions on the Southern Ocean circulation. It highlights the importance of North Atlantic surface conditions in shaping the AMOC vertical structure.

Chapter 5 develops a framework that connects the AMOC depth to the surface density distributions in both the Southern Ocean and the North Atlantic.

Chapter 6 examines the role of the Southern Ocean in the AMOC variability. It highlights the importance of the Indo-Pacific ocean when the overturning circulation is not in steady state. Using a variety of models, it shows that changes in the Indo-Pacific component of the overturning circulation can compensate changes in the AMOC. This compensation decreases as the variability timescale becomes longer.

Chapter 7 is on a somewhat different subject. It investigates the influence of sea ice velocity biases on the simulated Antarctic sea ice extent trend during recent decades. It uses a state-of-the-art coupled climate model with the simulated ice velocity field replaced with a satellite-derived observational estimate to show that correcting the ice velocity bias could substantially improve the simulated Antarctic sea ice extent changes.

    Cover page: Surface constraints on the global ocean overturning circulation: Southern Ocean vs North Atlantic
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2021)
    The Antarctic sea ice area expanded significantly during 1979-2015. This is at odds with state-of-the-art climate models, which typically simulate a receding Antarctic sea ice cover in response to increasing greenhouse forcing. Here, we investigate the hypothesis that this discrepancy between models and observations occurs due to simulation biases in the sea ice drift velocity. As a control we use the Community Earth System Model (CESM) Large Ensemble, which has 40 realizations of past and future climate change that all undergo Antarctic sea ice retreat during recent decades. We modify CESM to replace the simulated sea ice velocity field with a satellite-derived estimate of the observed sea ice motion, and we simulate 3 realizations of recent climate change. We find that the Antarctic sea ice expands in all 3 of these realizations, with the simulated spatial structure of the expansion bearing resemblance to observations. The results suggest that the reason CESM has failed to capture the observed Antarctic sea ice expansion is due to simulation biases in the sea ice drift velocity, implying that an improved representation of sea ice motion is crucial for more accurate sea ice projections.
      Cover page: Observed Antarctic sea ice expansion reproduced in a climate model after correcting biases in sea ice drift velocity
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      Paleoclimate proxy data suggest that the Atlantic Meridional Overturning Circulation (AMOC) was shallower at the Last Glacial Maximum (LGM) than its preindustrial (PI) depth. Previous studies have suggested that this shoaling necessarily accompanies Antarctic sea ice expansion at the LGM. Here the influence of Southern Ocean surface forcing on the AMOC depth is investigated using ocean-only simulations from a state-of-the-art climate model with surface forcing specified from the output of previous coupled PI and LGM simulations. In contrast to previous expectations, we find that applying LGM surface forcing in the Southern Ocean and PI surface forcing elsewhere causes the AMOC to shoal only about half as much as when LGM surface forcing is applied globally. We show that this occurs because diapycnal mixing renders the Southern Ocean overturning circulation more diabatic than previously assumed, which diminishes the influence of Southern Ocean surface buoyancy forcing on the depth of the AMOC.
        Cover page: Does Southern Ocean Surface Forcing Shape the Global Ocean Overturning Circulation?
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2020)
        AbstractClimate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.
          Cover page: Transient Overturning Compensation between Atlantic and Indo-Pacific Basins
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2020)
          Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.
            Cover page: Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic
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