Sensitivity of the tropical Pacific to a change of orbital forcing in two versions of a coupled GCM |
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| Authors: | F Codron |
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| Institution: | (1) Laboratoire de Météorologie Dynamique du CNRS, Paris, France E-mail: Codron@atmos.washington.edu, FR |
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| Abstract: | The changes of the variability of the tropical Pacific ocean forced by a shift of six months in the date of the perihelion
are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two
versions of the atmospheric model, varied by two parametrization changes. The first one concerns the interpolation scheme
between the atmosphere and ocean models grids near the coasts, the second one the advection of water vapor in the presence
of downstream negative temperature gradients, as encountered in the vicinity of mountains. In the tropical Pacific region,
the parametrization differences only have a significant direct effect near the coasts; but coupled feedbacks lead to a 1 °C
warming of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale
convergence area. The sensitivity of the seasonal cycle of equatorial SST is very different between the two experiments. In
both cases, the response to the solar flux forcing is strongly modified by coupled interactions between the SST, wind stress
response and ocean dynamics. In the first version, the main feedback is due to anomalous upwelling and leads to westward propagation
of SST anomalies; whereas the version 2 model is dominated by an eastward-propagating thermocline mode. The main reason diagnosed
for these different behaviors is the atmospheric response to SST anomalies. In the warmer climate simulated by the second
version, the wind stress response in the western Pacific is enhanced, and the off-equatorial curl is reduced, both effects
favoring eastward propagation through thermocline depth anomalies. The modifications of the simulated seasonal cycle in version
2 lead to a change in ENSO behavior. In the control climate, the interannual variability in the eastern Pacific is dominated
by warm events, whereas cold events tend to be the more extreme ones with a shifted perihelion.
Received: 14 December 1999 / Accepted: 24 May 2000 |
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