Gerold Siedler, JŘrgen Holfort, Walter Zenk, Thomas MŘller and Tiberius Csernok
Two major water masses dominate the deep layers in the Mariana and Caroline Basins: the Lower Circumpolar Water, arriving from the Southern Ocean along the slopes north of the Marshall Islands, and the North Pacific Deep Water reaching the region from the northeastern Pacific Ocean. Both water mass transports are components of the global thermohaline circulation. Hydrographic and moored observations and multi-beam echosounding were performed in the East Mariana and the East Caroline Basins to detail water mass distributions in the area. The Lower Circumpolar Water enters the East Mariana Basin from the east. At about 13░N, however, in the souhern part of the basin, a part of this water mass arrives from the north. Both hydrographic observations and moored current measurements lead to the conclusion that this water not only continues westward to the West Mariana Basin as suggested before, but also provides bottom water to the East Caroline Basin. The locations and depths of critical sills were identified by multi-beam echosounding at the Yap-Mariana Junction between the East and West Mariana Basins and at the Caroline Ridge between the East Mariana and East Caroline Basins. The throughflow is rather steady between the East and West Mariana Basins, while considerable variability is found at the Caroline Ridge. At both locations throughflow fluctuations are correlated with water mass property variations. The total transport to the two neighbouring basins is only about 1 Sverdrup (1Sv = 106 m3s-1), but has considerable impact on the water mass structure in these basins. Further above in the water column, the high-silica tongue of North Pacific Deep Water extends from the east to the far southwestern corner of the East Mariana Basin, with transports being mostly southward across the basin.