An LETKF-based ocean reanalysis for the Asia-Oceania region using Himawari-8 SSTs and SMOS/SMAP SSS

With the global spread of the Argo floats and development of high-resolution ocean models, high-resolution regional ocean reanalysis datasets have been released around the North Pacific region. However, their coverage areas are limited to north of 10°N.

Since July 2015, a geostationary satellite “Himawari-8” with an infrared sensor has been measuring SSTs with a high spatiotemporal resolution of 2 km and 10 min. in the western Pacific region. Because of the high temporal resolution, daily composite map shows that coverage area of Himawari-8 satellite exceeds that of an orbital satellite “GCOM-W” with a microwave sensor. Furthermore, the infrared sensor enables us to capture fine nearshore SST structure.

For the above reason, this study establishes a one-way nest high-resolution ocean data assimilation system based on LETKF (Hunt et al. 2007; Miyoshi and Yamane 2007) with 10 ensemble members at 1-day interval around the Asia-Oceania region: the southeast Asia coastal region [98°–115°E, 0°–22°N] at a spatial resolution of 1/36° and 50 layers for fishery and marine environmental monitoring, and the large-scale western Pacific region [95°E–165°W, 50°S–50°N] at a spatial resolution of 1/12° and 50 layers for tropical cyclone studies. Himawari-8 SSTs, SMOS/SMAP satellite SSS, satellite SSH, and temperature/salinity derived from GTSPP and AQC Argo dataset have been assimilated. Freshwater fluxes are incorporated into the system for improvement of the salinity field, an IAU filter (Bloom et al. 1996) is implemented to avoid generation of spurious gravity waves, RTPS (Kotsuki et al. 2017) are used to inflate the analysis perturbations, and an AOEI technique (Minamide and Zhang 2017) is implemented to suppress erroneous analysis increments. The southeast Asia system demonstrates low salinity water transport from the Mekong river, and the western Pacific system shows the SST cooling with the passage of tropical cyclones.