Assimilation of sub-surface temperature profiles from Argo floats in the Indian Ocean in an Ocean General Circulation Model

Effects of assimilation of Argo temperature profiles in an Ocean General Circulation Model (OGCM) are studied in the tropical Indian Ocean. Two assimilation experiments have been performed during the first five months of 2004. In the first one, monthly averaged Argo profiles have been assimilated using nudging technique and in the second, Cressman technique has been used for assimilating daily Argo profiles. The differences in the isothermal layer depth (ILD), depth of the 20 degrees C isotherm (D20) and sea surface temperature (SST) have been examined. The impact of assimilation has also been assessed using independent observations from one of the Indian Ocean Triton buoys. Errors in SST at the Triton buoy location are reduced by 37 and 16% respectively, in the two experiments. ILD and D20 show significant improvement in terms of error reduction in both the experiments, implying improvement in the mixed layer and the thermocline region. The impact of assimilation using Cressman technique is more on ILD and D20 as is evident from the coefficient of determination. This is because in the first experiment monthly averaged data have been assimilated leading to a comparatively poor representation of the high frequency variability of the parameters. Temperatures at the surface, 50 and 100 m depths during the forecast period (June-August 2004) have also been compared with Triton buoy observations. Also, SST in the forecast phase has been compared with available National Institute of Ocean Technology buoy measurements.