Decadal and centennial variability of the southern semiannual oscillation simulated in the GFDL coupled GCM

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.