Reconstruction of Three Dimensional Ocean Structure From Sea Surface Data: An Application of isQG Method in the Southwest Indian Ocean

Using sea surface data, including sea surface temperature (SST), sea surface height (SSH), and sea surface salinity (SSS), from two reanalysis data sets as input, we explore the potential for retrieving subsurface three dimensional (3 D) structures in the Southwest Indian Ocean (SWIO) based on the "interior+surface quasi geostrophy" (isQG) method. We find that the isQG method is particularly successful south of 35°S corresponding to greater SSH anomaly (SSHA) and to higher ratio of the barotropic mode (BT) and the first baroclinic mode (BC1). Meanwhile, higher resolution surface data may provide more coherent retrieved results with original reanalysis data in the inactive eddy regions north of 35°S, where coarser dataset cannot resolve mesoscale signals. Based on a long time series reconstruction by the Simple Ocean Data Assimilation (SODA) surface data from 2011 to 2015 in the SWIO, our spectrum correlation analysis shows that the isQG solution achieves a better skill near the surface layer and under the thermocline depth with horizontal scale larger than 50 km, but it degrades from ~30 to 200 m depth south of 35°S. In contrast, the spectral correlation north of 35°S is relatively low, with correlation coefficient beyond (below) 0.5 for signals with a wavelength longer (shorter) than 200 km near surface layer and from 250 to 500 m depth. Mechanisms about the regional dependent isQG skill are also discussed. This study should have important implications for both operational oceanography and ocean dynamics research in the SWIO, including the greater Agulhas system.