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Publication - Dr William Roberts

    ENSO in the Mid-Holocene according to CSM and HadCM3

    Citation

    Roberts, WHG, Battisti, DS & Tudhope, AW, 2014, ‘ENSO in the Mid-Holocene according to CSM and HadCM3’. Journal of Climate, vol 27.

    Abstract

    The offline linearized ocean–atmosphere model (LOAM), which was developed to quantify the impact of
    the climatological mean state on the variability of the El Ni~ o–Southern Oscillation (ENSO), is used to
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    illuminate why ENSO changed between the modern day and early/mid-Holocene simulations in two climate
    modeling studies using the NCAR Climate System Model (CSM) and Hadley Centre Coupled Model
    (HadCM3). LOAM reproduces the spatiotemporal variability simulated by the climate models and shows
    both the reduction in the variance of ENSO and the changes in the spatial structure of the variance during the
    early/mid-Holocene. The mean state changes that are important in each model are different and, in both
    cases, are also different from those hypothesized to be important in the original papers describing these
    simulations. In the CSM simulations, the ENSO mode is stabilized by the mean cooling of the SST. This
    reduces atmospheric heating anomalies that in turn give smaller wind stress anomalies, thus weakening the
    Bjerknes feedback. Within the ocean, a change in the thermocline structure alters the spatial pattern of the
    variance, shifting the peak variance farther east, but does not reduce the overall amount of ENSO variance. In
    HadCM3, the ENSO mode is stabilized by a combination of a weaker thermocline and weakened horizontal
    surface currents. Both of these reduce the Bjerknes feedback by reducing the ocean’s SST response to wind
    stress forcing. This study demonstrates the importance of considering the combined effect of a mean state
    change on the coupled ocean–atmosphere system: conflicting and erroneous results are obtained for both
    models if only one model component is considered in isolation.

    Full details in the University publications repository