Atmospheric horizontal resolution affects tropical climate variability in coupled models

The effect of horizontal resolution on tropical variability is investigated within the
modified SINTEX model, SINTEX-F, developed jointly at INGV, IPSL and at the
Frontier Research System. The horizontal resolutions T30 and T106 are investigated
in terms of the coupling characteristics, frequency and variability of the
tropical ocean-atmosphere interactions. It appears that the T106 resolution is generally
beneficial even if it does not eliminate all the major systematic errors of the
coupled model. There is an excessive shift west of the cold tongue and ENSO variability,
and high resolution has also a somewhat negative impact to the variability
in the East Indian Ocean. A dominant two-year peak for the NINO3 variabilty
in the T30 model is moderated in the T106 as it shifts to longer time scale. At
high resolution new processes come into play, as the coupling of tropical instability
waves, the resolution of coastal flows at the Pacific Mexican coasts and improved
coastal forcing along the coast of South America. The delayed oscillator seems the
main mechanism that generates the interannual variability in both models, but the
models realize it in different ways. In the T30 model it is confined close to the
equator, involving relatively fast equatorial and near-equatorial modes, in the high
resolution, it involves a wider latitudinal region and slower waves. It is speculated
that the extent of the region that is involved in the interannual variability may be
linked to the time scale of the variability itself.

This research was partially supported
by the Italy–USA Cooperation Program of the
Italian Ministry of Environment and by the EU projects
ENSEMBLES and DYNAMITE.

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