ENSO in the Mid-Holocene according to CSM and HadCM3

William H G Roberts, David S. Battisti, A. W Tudhope

Research output: Contribution to journalArticle (Academic Journal)peer-review

17 Citations (Scopus)

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.
Original languageEnglish
JournalJournal of Climate
Volume27
Issue number3
DOIs
Publication statusPublished - 1 Feb 2014

Keywords

  • El Nino
  • ENSO
  • Paleoclimate

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