Decadal and centennial variability of the southern semiannual oscillation simulated in the GFDL coupled GCM |
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Authors: | I Simmonds D J Walland |
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Institution: | (1) School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3052, AU |
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Abstract: | We investigate the behavior of the semiannual oscillation (SAO) in surface pressure and 500 hPa baroclinicity at high southern
latitudes in a 1000-year GFDL coupled ocean-atmosphere GCM run. The model represents this feature but is shown to underestimate
its amplitude and percentage of variance explained in the midlatitudes. South of 60 °S the simulation of the pressure oscillation,
although somewhat too weak, is considerably better. Our analysis reveals significant interannual, decadal and centennial variability
in the modeled SAO. While there is only a short historical record of observational data in the middle and high southern latitudes,
existing studies suggest that the strength of the SAO does show significant variability on at least the first two of these
time scales. Strong relationships between the semiannual cycles of surface pressure and baroclinicity are apparent in the
model output, reinforcing the findings in earlier studies that the differing annual march of temperature between the midlatitudes
and the Antarctic coast leads to a semiannual component in the baroclinicity and thence the surface pressure. We draw attention
to extended periods when the model SAO is weak and strong, and have investigated the nature of the circulation during each
period. The GFDL model results suggest that a significant proportion of the SAO variation was associated more with variations
in the strength of the winter pressure maximum rather than the springtime minimum. The extent to which this and other aspects
of the modeled longterm variability are related to actual atmospheric structure must await the availability of longer data
records.
Received: 11 November 1996 / Accepted: 28 July 1997 |
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