Geoffrey (Jake) Gebbie
Research Associate, Harvard University
Visiting Scientist, Massachusetts Institute of Technology

Contact Information:
Department of Earth and Planetary Sciences
24 Oxford St., Cambridge, MA 02138 USA
Phone: 617-495-4865
Fax: 617-496-7411
E-mail: Email Address for Jake Gebbie


Primary collaborators:
Peter Huybers, Eli Tziperman, Carl Wunsch
Climate Dynamics Group at Harvard University

Research Interests

Paleoceanography: Estimating Past Ocean Circulation

The combined use of observations and models is not limited to the traditional problems of physical oceanography. One new avenue of research is to gain perspective on the potential changes to ocean circulation by looking to the past as an example. I recently applied an inverse method to paleoceanographic data in the South Atlantic Ocean to understand the changes in the meridional overturning circulation there. The project showed that although the paleo-observational record is limited, the methods of modern physical oceanography can be applied to other problems with success. In particular, this research has led to specific recommendations for future observations.

  • Gebbie, G., and P. Huybers: "How old is the ocean?: Radiocarbon-based estimates with multiple ocean pathways," Manuscript in preparation.

  • Gebbie, G., and P. Huybers: "Meridional circulation during the Last Glacial Maximum explored through a combination of delta-18-O observations and a geostrophic inverse model," Geochem. Geophys. Geosyst., 7, Q11N07, doi:10.1029/2006GC001383, 2006. (An edited version of this paper was published by AGU. Copyright 2006 American Geophysical Union. Further reproduction or electronic distribution is not permitted.)

  • Huybers, P., G. Gebbie, and O. Marchal: "Can paleoceanographic tracers constrain meridional circulation rates?," J. Phys. Oceanogr., 37 (2), 394-407, doi:10.1175/JPO3018.1, 2007. (Accepted for publication in the Journal of Physical Oceanography. Copyright 2007 American Geophysical Union. Further reproduction or electronic distribution is not permitted.)

    ENSO Dynamics and Predictability

    The El Nino-Southern Oscillation phenomenon directly influences droughts, floods, and seasonal climate shifts around the world. Presently, it is not agreed whether the coupled system is driven by external stochastic noise or whether the chaotic nature of the coupled system creates interannual variability in the tropical Pacific. This question is important because it affects how we model the tropical Pacific, and consequently it affects our prospects for extended seasonal-to-interannual prediction. To proceed, I have focused on bursts of strong westerly wind along the equator, known as Westerly Wind Bursts. To investigate the dynamical link between the wind bursts and the ocean, I analyzed observations and developed a statistical model of the process. Then I coupled my wind burst model to a ocean general circulation model and a linear atmospheric model that has been used by my postdoctoral host, Eli Tziperman, and collaborators at the Geophysical Fluid Dynamics Laboratory. This novel approach to the problem shows that the SST-wind burst link may be a critical dynamical process that creates interannual variability.

  • Gebbie, G., and E. Tziperman: "Incorporating a semi-stochastic model of ocean-modulated westerly wind bursts into an ENSO prediction model," submitted, 2008.

  • Gebbie, G., and E. Tziperman: "Predictability of SST-modulated westerly wind bursts," submitted, 2008.

  • Gebbie, G., I. Eisenman, A. Wittenberg, and E. Tziperman: "Could westerly wind bursts help predict El Nino? ," Bull. Amer. Meteorolo. Soc., 88 (9), 2007.

  • Gebbie, G., I. Eisenman, A. Wittenberg, and E. Tziperman: "Modulation of Westerly Wind Bursts by Sea Surface Temperature: A Semistochastic Feedback for ENSO," J. Atmos. Sci., 64, doi:10.1175/JAS4029.1, 2007. ( Copyright 2007 American Meteorological Society )

    The next step is to use this model to make predictions, and I have led a collaboration with the Jet Propulsion Laboratory and FastOpt, Inc., to develop an adjoint to the coupled model. To my knowledge, this is the first attempt to create a dynamically-consistent, statistically-rigorous initialization scheme for seasonal-to-interannual predictions with a fully coupled model.

  • Gebbie, G., and the ECCO/GODAE Group: "The MOM4 Tangent-Linear and Adjoint Project," Code Documentation , 2007.

  • The MOM4 Tangent-Linear and Adjoint Project Website



    Ocean State Estimation

    Despite many observational and theoretical studies of the North Atlantic Ocean, the processes which create and destroy water-masses are poorly quantified. Subduction, the movement of water from the surface layers to mid-depths, is the main process that sets the water-mass properties of the subtropical North Atlantic Ocean. Subduction is greatly important because it allows communication between the atmosphere and the immense reservoir of the interior ocean. Instead of using field observations alone, as typical of many studies of subduction, I used a combination of satellite altimetry and an ocean general circulation model, along with the Subduction Experiment observational survey, to reconstruct the North Atlantic ocean state in my Ph.D. thesis. Specifically, I used the adjoint method of data assimilation with the ECCO Consortium numerical codes based upon the MITgcm . To technically accomplish this research, I added an open boundary scheme to the model for use in regional configurations, and I performed multiple model runs on massively-parallel supercomputers, including 40,000 hours at the San Diego Supercomputer Center. My thesis concluded that the oceanic mesoscale eddy field subducts water at a substantial rate, and that the neglect of this process in climate models would lead to substantial biases over decadal timescales. This is ultimately important because coarse-resolution climate models can not be considered reliable until they either resolve the eddy field or they accurately parameterize it. With the machinery developed in my thesis, I plan on extending this research to the subpolar gyre of North Atlantic to quantify the differences between mass transport and water-mass transformation in the meridional overturning circulation.

    An overview of the general research topic may be of interest to a wider audience.

  • Gebbie, G.: "Does Eddy Subduction Matter in the Northeast Atlantic Ocean?," J. Geophys. Res., 112, C06007, doi:101029/2006JC003568, 2007.

  • Gebbie, G., P. Heimbach, and C. Wunsch: "Strategies for Nested and Eddy-Permitting State Estimation," J. Geophys. Res., 111, C10073, doi:10.1029/2005JC003094, 2006. (An edited version of this paper was published by AGU. Copyright 2006 American Geophysical Union.)

  • Ph. D. Thesis :   "Subduction in an Eddy-Resolving State Estimate of the Northeast Atlantic Ocean," MIT/WHOI, 2004.

  • Animation of Potential Temperature at 310 meters from a Nested, Eddy-Permitting State Estimate.