Tziperman Eli

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Tziperman
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Eli
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  • Article
    Quantifying the dependence of westerly wind bursts on the large-scale tropical Pacific SST
    (American Meteorological Society, 2007-06-15) Tziperman, Eli ; Yu, Lisan
    The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Niño event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inherent part of the large-scale deterministic ENSO dynamics. Implications for ENSO predictability and prediction are discussed.
  • Article
    Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing
    (American Geophysical Union, 2006-11-10) Tziperman, Eli ; Raymo, Maureen E. ; Huybers, Peter ; Wunsch, Carl
    The consequences of the hypothesis that Milankovitch forcing affects the phase (e.g., termination times) of the 100 kyr glacial cycles via a mechanism known as “nonlinear phase locking” are examined. Phase locking provides a mechanism by which Milankovitch forcing can act as the “pacemaker” of the glacial cycles. Nonlinear phase locking can determine the timing of the major deglaciations, nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear. A consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct. Phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing. Nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record. Nonlinear phase locking can also result in the observed “quantization” of the glacial period into multiples of the obliquity or precession periods.
  • Article
    Westerly wind bursts : ENSO’s tail rather than the dog?
    ( 2005-12-15) Eisenman, Ian ; Yu, Lisan ; Tziperman, Eli
    Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Niño events and are believed to be a major factor in ENSO's dynamics. Because of their short time scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and characteristics of WWBs may depend to some extent on the state of ENSO components, implying that WWBs, which force ENSO, are modulated by ENSO itself. Satellite and in situ observations are used here to show that WWBs are significantly more likely to occur when the warm pool is extended eastward. Based on these observations, WWBs are added to an intermediate complexity coupled ocean-atmosphere ENSO model. The representation of WWBs is idealized such that their occurrence is modulated by the warm pool extent. The resulting model run is compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO results in an enhancement of the slow frequency component of the WWBs. This causes the amplitude of ENSO events forced by modulated WWBs to be twice as large as the amplitude of ENSO events forced by stochastic WWBs with the same amplitude and average frequency. Based on this result, it is suggested that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO's dynamics, and that WWBs should not be regarded as purely stochastic forcing. In the paradigm proposed here, WWBs are still an important aspect of ENSO's dynamics, but they are treated as being partially stochastic and partially affected by the large-scale ENSO dynamics, rather than being completely external to ENSO. It is further shown that WWB modulation by the large-scale equatorial SST field is roughly equivalent to an increase in the ocean-atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.
  • Technical Report
    Conceptual models of the climate : 2001 program of studies in geophysical fluid dynamics
    (Woods Hole Oceanographic Institution, 2003) Balmforth, Neil J. ; Tziperman, Eli ; Cessi, Paola ; Pierrehumbert, Raymond T.
    In 2001, the Geophysical Fluid Dynamics Summer Study Program grappled with Conceptual Models of the Climate. Eli Tziperman (Weizman Institute), Paola Cessi (Scripps Institution of Oceanography) and Ray Pierre- Humbert (University of Chicago) provided the principal lectures. This introduction gave us all a glimpse into the complex problem of the climate, both in the present, past and future, and even on other planets. As always, the next weeks of the program were filled with many seminars from the visitors, and culminated in the fellow's reports.
  • Article
    Process-based analysis of climate model ENSO simulations : intermodel consistency and compensating errors
    (John Wiley & Sons, 2014-06-30) Linz, Marianna ; Tziperman, Eli ; MacMartin, Douglas G.
    Systematic and compensating errors can lead to degraded predictive skill in climate models. Such errors may be identified by comparing different models in an analysis of individual physical processes. We examine model simulations of El Niño–Southern Oscillation (ENSO) in five Coupled Model Intercomparison Project (CMIP) models, using transfer functions to analyze nine processes critical to ENSO's dynamics. The input and output of these processes are identified and analyzed, some of which are motivated by the recharge oscillator theory. Several errors and compensating errors are identified. The east-west slope of the equatorial thermocline is found to respond to the central equatorial Pacific zonal wind stress as a damped driven harmonic oscillator in all models. This result is shown to be inconsistent with two different formulations of the recharge oscillator. East Pacific sea surface temperature (SST) responds consistently to changes in the thermocline depth in the eastern Pacific in the five CMIP models examined here. However, at time scales greater than 2 years, this consistent model response disagrees with observations, showing that the SST leads thermocline depth at long time scales. Compensating errors are present in the response of meridional transport of water away from the equator to SST: two different models show different response of the transport to off-equatorial wind curl and wind curl response to East Pacific SST. However, these two models show the same response of meridional transport to East Pacific SST. Identification of errors in specific physical processes can hopefully lead to model improvement by focusing model development efforts on these processes.
  • Thesis
    Mixing and general circulation dynamics : theory and observations
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1987-02) Tziperman, Eli
    This thesis studies the role of cross-isopycnal mixing in general circulation dynamics, from both the theoretical and observational points of view. The first two chapters discuss some theoretical aspects of cross-isopycnal mixing in the oceans. In chapter one, an integral constraint relating the interior stratification and air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. Two simple models are then used to examine the way the numerically small mixing, together with air-sea fluxes, determines the average vertical density stratification of the oceans, and the deep buoyancy driven circulation. In chapter two, a more complete model of a deep flow driven by cross isopycnal diffusion is presented, motivated by the Mediterranean outflow into the North Atlantic. Mixing in this model is responsible for the determination of the detailed structure of the flow and density field, while in the models of the first chapter it was allowed to determine only the average vertical density stratification. In chapter three, a hydrographic data set from the Mediterranean sea is analyzed by inverse methods. The purpose is to examine the importance of mixing when trying to explain tracer distributions in the ocean. The time-mean circulation and the appropriate mixing coefficients are calculated from the hydrographic data. We conclude that the numerically small cross isopycnal mixing processes are crucial to the dynamics, yet difficult to parameterize and measure using available hydrographic data.