Subsurface currents

Abstract: . Since December 2010, the MyOcean global analysis and forecasting system has consisted of the Mercator Ocean NEMO global 1/4° configuration with a 1/12° nested model over the Atlantic and the Mediterranean. The open boundary data for the nested configuration come from the global 1/4° configuration at 20° S and 80° N. The data are assimilated by means of a reduced-order Kalman filter with a 3-D multivariate modal decomposition of the forecast error. It includes an adaptive-error estimate and a localization algorithm. A 3-D-Var scheme provides a correction for the slowly evolving large-scale biases in temperature and salinity. Altimeter data, satellite sea surface temperature and in situ temperature and salinity vertical profiles are jointly assimilated to estimate the initial conditions for numerical ocean forecasting. In addition to the quality control performed by data producers, the system carries out a proper quality control on temperature and salinity vertical profiles in order to minimise the risk of erroneous observed profiles being assimilated in the model. This paper describes the recent systems used by Mercator Ocean and the validation procedure applied to current MyOcean systems as well as systems under development. The paper shows how refinements or adjustments to the system during the validation procedure affect its quality. Additionally, we show that quality checks (in situ, drifters) and data sources (satellite sea surface temperature) have as great an impact as the system design (model physics and assimilation parameters). The results of the scientific assessment are illustrated with diagnostics over the year 2010 mainly, assorted with time series over the 2007–2011 period. The validation procedure demonstrates the accuracy of MyOcean global products, whose quality is stable over time. All monitoring systems are close to altimetric observations with a forecast RMS difference of 7 cm. The update of the mean dynamic topography corrects local biases in the Indonesian Throughflow and in the western tropical Pacific. This improves also the subsurface currents at the Equator. The global systems give an accurate description of water masses almost everywhere. Between 0 and 500 m, departures from in situ observations rarely exceed 1 °C and 0.2 psu. The assimilation of an improved sea surface temperature product aims to better represent the sea ice concentration and the sea ice edge. The systems under development are still suffering from a drift which can only be detected by means of a 5-yr hindcast, preventing us from upgrading them in real time. This emphasizes the need to pursue research while building future systems for MyOcean2 forecasting.

Abstract: Recent observations within the Indonesian exit passages and internal seas highly resolve the arrival and passage of a semiannual Kelvin wave. In mid-May 1997, surface and subsurface currents were to the southeast at a mooring located south of Java in the South Java Current, while local wind forcing was northwestward. Subsequent northward fluctuations in the geostrophic current through Lombok Strait and in observed currents from two moorings located in Makassar Strait are commensurate with the speed and passage of a Kelvin wave through the region. The Kelvin wave was due to westerly wind forcing in the remote equatorial Indian Ocean during the semiannual April/May monsoon transition period. This was confirmed through a simple remote wind-forced analytical Kelvin wave model of velocity at the South Java Current mooring location and sea level in Lombok Strait and also in the numerical general circulation model of Murtugudde et al. [1998]. Warm temperature anomalies measured at the south Java mooring and within Makassar Strait are associated with the passage of the Kelvin wave. Salinity anomalies measured at the south Java mooring are consistent with an Indian Ocean source. The observed passage of the Kelvin wave during May 1997 unambiguously demonstrates for the first time that equatorial Indian Ocean remote wind forcing may on occasions influence the internal Indonesian seas

Abstract: Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coast in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.

Abstract: [1] Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.