Ueber die Eigenschaften der oberflaechennahen Zirkulation im zentralen Nordatlantik. Analyse eines Driftbojendatensatzes

Abstract

An analysis of a drifting buoy data set is presented. The objective is to arrive at a self contained description of the properties of the near-surface circulation (drogue depth 100m) in the central North Atlantic Ocean. A necessary pre-analysis step was the removal of all data from undrogued buoys. The physical parameters are deduced by averaging the data in 2 x 3 boxes. The minimum amount of data that is necessary to get nearly statistically stable results is determined by an empirical quality criterion. All important currents in the investigation area are reproduced by the near surface mean velocity field. Using a stream function approach a separation of the mean velocity field into a divergent and a non-divergent part shows that the flow is nearly non-divergent. The distribution of eddy kinetic energy (EKE) is very inhomogeneous. EKE is isotropic and concentrated along the mean currents which therefore determine its distribution. The greatest part of the total kinetic energy consists of EKE, but there are regional variations. A comparison of the deduced mean velocity field and the EKE distribution with the results of an eddy-resolving model reveals qualitative and quantitative discrepancies. The analysis of the Reynolds stress terms shows an energy transfer from the eddy field to the mean circulation in the vicinity of North Atlantic Current. Meridional sections along 30 W reveal significant seasonal variations of EKE in many regions of the North Atlantic Ocean. Regional differences in their seasonal cycle are also noticeable. The spectral energy distribution of Lagrangian and Eulerian spectra with equal total energy level is different. Due to the shorter Lagrangian time scale the cut-off frequency is at higher frequencies as in the Eulerian case. The position of the cut-off frequency varies depending on the area under consideration and characterises a special current regime. It turns out that it is not possible to analyse tidal currents with drifting buoy data. A basinwide distribution of the energy of inertia motions is deduced by complex demodulation. (orig.)SIGLEAvailable from TIB Hannover: RN 3292(244) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman