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  • Ye, Z. and W. W. Hsieh, 2008: Changes in ENSO and associated overturning circulations from enhanced greenhouse gases by the end of the 20th century. Journal of Climate, 10.1175/2008JCLI1580.1. In press.

With data from 12 coupled models in the 4th Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC-AR4), climate under
year 2000 greenhouse gas (GHG) + aerosol forcing was compared with
climate under pre-industrial conditions. In the tropical Pacific, the warming
in the mean sea surface temperatures (SST) was found to have an El
Nino-like pattern, while both the equatorial zonal overturning
circulation and the meridional overturning circulation weakened under
increased GHG forcing.

For the El Nino-Southern Oscillation (ENSO), the asymmetry in
the SST anomalies between El Nino and La Nina was found to be
enhanced under increased GHG, for both the ensemble model data and the
observed data (1900-1999). Enhanced asymmetry between El Nino and La
Nina was also manifested in the anomalies of the zonal wind stress, the
equatorial undercurrent and the meridional overturning circulation in the
increased GHG simulations. The enhanced asymmetry in the model SST
anomalies was mainly caused by the greatly intensified vertical nonlinear
dynamic heating (NDH) anomaly (i.e. product of the vertical velocity anomaly
and the negative vertical temperature gradient anomaly) during El Nino (but
not during La Nina). Under increased GHG, the enhanced
positive NDH anomalies during El Nino when time-averaged over the whole
record would change the SST mean state by an El Nino-like pattern.

Last Updated: 2008-08-21

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