Variation of the South Pacific Tropical Water in an ocean data assimilation system (MRI- MOVE) and its relation to ENSO

Multivariate Ocean Variational Estimation (MOVE) System is an ocean data assimilation system developed in Japan Meteorological Agency/ Meteorological Research Institute (MRI). MOVE system can reproduce a faithful salinity field because coupled temperature-salinity EOF modes are employed in its 3DVAR analysis scheme. In MOVE system, Argo float and Satellite altimetry data, as well as in-situ observation data, are assimilated into MRI Community Ocean Model (MRI.COM). An assimilation experiment using MOVE System is performed between 1993-2004. We examined the variation of the South Pacific Tropical Water (SPTW) in the NINO4 region (5°S- 5°N, 120°E-240°W) using the assimilation product and found that SPTW plays an important role in ENSO. The amount of SPTW increases in the cold (La Nina) phase because of intensified vertical circulation induced by the equatorial upwelling. The increase of SPTW is accompanied by the increase of subsurface heat content because SPTW is relatively warm. SPTW, together with the fresh water on it, also forms a stable barrier layer and helps the rise of near-surface temperature. Then, the warm (El Nino) phase starts. In the warm phase, the opposite chain takes place and, finally, it shifts back to the cold phase. The result of heat and salinity budget analysis is consistent with this scenario. It should be emphasized that the salinity variation plays an essential role in the mechanism suggested above.