OCCAM and AGORA


The OCCAM contribution to AGORA is in collaboration with the University of Edinburgh. Starting from the 1/4 degree OCCAM model state at the end of year 8, the simulation was extended for two years using wind stress and atmospheric pressure forcing derived from the 6-hourly ECMWF re-analysis fields for 1992 and 1993. As in the main OCCAM run, relaxation to Levitus was used to provide the surface heat and freshwater forcing. This was partly because of possible errors in the ECMWF re-analysis fields (particularly in the Southern Ocean) but also to estimate the effect of high frequency wind forcing alone. The 1993 period was re-run using the Cooper & Haines (1996) Lagrangian water displacement method to assimilate altimeter data for 1993.

Comparisons between sea level variance from the main run of the OCCAM model with climatological wind forcing and Topex/Poseidon observations (illustrated below for the Gulf Stream region) have shown that the model agrees well in areas of high variability such as the Gulf Stream and Agulhas retroflection regions, with similar pattern and magnitude (50cm RMS sea surface height anomaly). However the model appears not to propogate Rossby wave energy correctly away from these high energy source regions to the quieter parts of the world ocean. For example, in the Agulhas retroflection region, only a narrow band of high energy propagates into the South Atlantic and in the Gulf Stream, the model over-estimates the variance close to the main current core but under-estimates it some distance away.

In the run with 6-hourly wind and pressure forcing (below left for the Gulf Stream region), there is still a concentration of energy in the high energy regions, but there is a significant increase in energy elsewhere in the ocean . Preliminary analysis of the variance from the assimilation run (below right for the Gulf Stream region) shows much better general agreement with observations, with higher variability in the quiet regions and increased variability spreading away from the energetic regions.

Larger images may be viewed by clicking on the above.