Person: Gebbie, Geoffrey A
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Gebbie
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Geoffrey A
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Gebbie, Geoffrey A
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Publication How is the ocean filled?(Wiley-Blackwell, 2011) Gebbie, Geoffrey A; Huybers, PeterThe ocean surface rapidly exchanges heat, freshwater, and gases with the atmosphere, but once water sinks into the ocean interior, the inherited properties of seawater are closely conserved. Previous water-mass decompositions have described the oceanic interior as being filled by just a few different property combinations, or water masses. Here we apply a new inversion technique to climatological tracer distributions to find the pathways by which the ocean is filled from over 10,000 surface regions, based on the discretization of the ocean surface at 2° by 2° resolution. The volume of water originating from each surface location is quantified in a global framework, and can be summarized by the estimate that 15% of the surface area fills 85% of the ocean interior volume. Ranked from largest to smallest, the volume contributions scaled by surface area follow a power-law distribution with an exponent of −1.09 ± 0.03 that appears indicative of the advective-diffusive filling characteristics of the ocean circulation, as demonstrated using a simple model. This work quantifies the connection between the surface and interior ocean, allowing insight into ocean composition, atmosphere-ocean interaction, and the transient response of the ocean to a changing climate.Publication Predictability of SST-Modulated Westerly Wind Bursts(American Meteorological Society, 2009) Gebbie, Geoffrey A; Tziperman, EliWesterly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations.Publication Incorporating a Semi-Stochastic Model of Ocean-Modulated Westerly Wind Bursts into an ENSO Prediction Model(Springer Wien, 2009) Gebbie, Geoffrey A; Tziperman, EliPrediction models of the El Niño-Southern Oscillation (ENSO) phenomenon often represent westerly wind bursts (WWBs), a significant player in ENSO dynamics, as stochastic forcing. A recent paper developed an observationally motivated semi-stochastic statistical model that quantifies the dependence of WWBs on large-scale sea-surface temperature. This WWB model is added here to a hybrid coupled model, thus activating a two-way SST-WWB feedback. The WWB model represents both the deterministic and stochastic elements of WWBs and thus is especially appropriate for ensemble ENSO prediction experiments. An ensemble of retrospective forecasts is performed for the years 1979–2002. Overall statistical measures of predictability are neither degraded nor improved relative to the hybrid, coupled general circulation model, perhaps because of the limitations of the hybrid coupled model and the initialization procedure used. While the present work is meant as a proof-of-concept, it is found that the addition of the WWB model does improve the prediction of the onset and the development of the large 1997 warm event, pointing to the potential for ENSO prediction skill improvement using this approach.Publication Can Paleoceanographic Tracers Constrain Meridional Circulation Rates?(American Meteorological Society, 2007) Huybers, Peter; Gebbie, Geoffrey A; Marchal, OlivierThe ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.Publication Meridional Circulation During the Last Glacial Maximum Explored Through a Combination of South Atlantic δ18O Observations and a Geostrophic Inverse Model(American Geophysical Union, 2006) Gebbie, Geoffrey A; Huybers, PeterThe vertical profile of meridional transport in the South Atlantic is examined by combining paleoceanographic observations with a geostrophic circulation model using an inverse method. δ 18Ocalcite 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 δ 18Ocalcite 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 δ 18Ocalcite 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 δ 18Owater 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 δ 18Ocalcite observations. δ 18Ocalcite 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 δ 18Ocalcite and density.Publication Modulation of Westerly Wind Bursts by Sea Surface Temperature: A Semistochastic Feedback for ENSO(American Meteorological Society, 2007) Gebbie, Geoffrey A; Eisenman, Ian; Wittenberg, Andrew; Tziperman, EliWesterly wind bursts (WWBs) in the equatorial Pacific are known to play a significant role in the development of El Niño events. They have typically been treated as a purely stochastic external forcing of ENSO. Recent observations, however, show that WWB characteristics depend upon the large-scale SST field. The consequences of such a WWB modulation by SST are examined using an ocean general circulation model coupled to a statistical atmosphere model (i.e., a hybrid coupled model). An explicit WWB component is added to the model with guidance from a 23-yr observational record. The WWB parameterization scheme is constructed such that the likelihood of WWB occurrence increases as the western Pacific warm pool extends: a “semistochastic” formulation, which has both deterministic and stochastic elements. The location of the WWBs is parameterized to migrate with the edge of the warm pool. It is found that modulation of WWBs by SST strongly affects the characteristics of ENSO. In particular, coupled feedbacks between SST and WWBs may be sufficient to transfer the system from a damped regime to one with self-sustained oscillations. Modulated WWBs also play a role in the irregular timing of warm episodes and the asymmetry in the size of warm and cold events in this ENSO model. Parameterizing the modulation of WWBs by an increase of the linear air–sea coupling coefficient seems to miss important dynamical processes, and a purely stochastic representation of WWBs elicits only a weak ocean response. Based upon this evidence, it is proposed that WWBs may need to be treated as an internal part of the coupled ENSO system, and that the detailed knowledge of wind burst dynamics may be necessary to explain the characteristics of ENSO.