Description: The spatial distribution of the subtropical salinity maximum is identified using historical and recent data from the eastern North Atlantic. In the regions with high frequency of occurrence of the salinity maximum, the relative contributions of advection, eddy diffusion and double diffusion to the salt balance below the maximum salinity layer are determined. McDougall's (1984, Journal of Physical Oceanography, 14, 1577–1589) salt balance equation for neutral surfaces is used in this analysis. The data base consists of two meridional CTD sections along 33° and 27°W between 10° and 35°N, mean temperature-salinity profiles in 5° × 5° squares presented by Emery and Dewar (1982), and mean velocity profiles in 3° × 3° squares evaluated by Stramma (1984, Journal of Marine Research, 42, 537–558). The tropical salinity maximum tongue is found to be quite persistent in its salinity value and its geographic distribution, but less clearly in its vertical or isopycnal position. Double diffusion due to salt-fingering appears to be an important process for the salt balance below the salinity maximum layer. An approximate estimate of the double-diffusive salt flux is obtained. Near the subtropical source region, the double-diffusive salt flux is balanced primarily by isopycnal advection; further to the south it is also balanced by isopycnal eddy diffusion. Maximum double-diffusive fluxes correspond in magnitude to the mean salt flux caused by the excess in evaporation at the surface in the central subtropics. The resulting isopycnal and diapycnal eddy-mixing coefficients derived by a linear inversion technique have the reasonable values of Ki = (11 ± 5) × 102 m2 s−1 and Kd = (4 ± 2) × 10−5 m2 s−1. Considering the intermittency of the double-diffusive process, limiting values for the mean eddy-mixing coefficients are determined by neglecting the contribution of the double-diffusive salt fluxes. This leads to Ki = (5 ± 2) × 102 m2 s−1 and Kd = (5 ± 1) × 10−5 m2 s−1 for the isopycnal and diapycnal mixing coefficients, respectively.

Description: Horizontal velocity and temperature measurements observed from a two-dimensional array of moored instruments, mooring Fl, are analysed to describe the near-surface internal wave field in the GATE (GARP Atlantic Tropical Experiment) C-scale area. Spectral properties indicate strong deviations from the Garrett and Munk (1972, 1975) deep ocean internal wave models. The frequency spectrum in the upper pycnocline is dominated by three energetic bands centered at 0.0127 (inertial frequency), 0.08 (M2-tidal frequency) and 3 cph. The latter frequency band does not correspond to the local Brunt Väisälä frequency (〈 10 cph) and contains about one half of the total internal wave energy of fluctuations with periods less than 10 hours. Cross-spectral analysis of the high frequency internal waves yields corresponding wavelengths of order 1 km consistent with westward propagating first mode wave groups, if the effect of Doppler shift due to a strong mean current is taken into account

Description: To study the EasternBoundaryCurrentsystem off Northwest Africa in detail several CTD/ADCP-sections and long-term mooring work were carried out in the channel between Lanzarote and Africa. The observations are compared with a fine-resolution model, which was developed in the framework of the CANIGO project. The water masses, which are observed in this area, are characterised and classified in density ranges. The current field shows a high spatial and temporal variability with maximum velocities of about 35 cm/s. Seasonal means as well as currents averaged across the channel are only a few cm/s. In the surface water a steady southward flow in the middle of the channel indicates the CanaryCurrent in this area. During fall a strong northward current is observed close to the African shelf. Though the CanaryCurrent strengthens during summer and fall due to an increase of the trade winds, the transport in the channel decreases or turns northward during that time due to the enhanced poleward current at the eastern side. A northward undercurrent with a mean velocity of +2.3 cm/s is observed at the African slope in 950 m depth. The poleward transport of AAIW increases during fall and a strong influence of relatively fresh AAIW is observed during that time. Most of the observations fit well to the results of the CANIGO model, but the occurrence of MW at the bottom of the channel and the corresponding southward flow cannot be resolved by the model.

Description: In the western equatorial Pacific the low-salinity core of Antarctic Intermediate Water (AAIW) is found at about 800 m depth between potential density levels Sigma-theta = 27.2 and 27.3. The pathways of AAIW and the degradation of its core are studied, from the Bismarck Sea to the Caroline Basins and into the zonal equatorial current system. Both historical and new observational data, and results from numerical circulation model runs are used. The observations include hydrographic stations from German and Japanese research vessels, and Eulerian and Lagrangian current measurements. The model is the JAMSTEC high-resolution numerical model based on the Modular Ocean Model (MOM 2). The general agreement between results from the observations and from the model enables us to diagnose properties and to provide new information on the AAIW. The analysis confirms the paramount influence of topography on the spreading of the AAIW tongue north of New Guinea. Two cores of AAIW are found in the eastern Bismarck Sea. One core originates from Vitiaz Strait and one from St. George's Channel, probably arriving on a cyclonic pathway. They merge in the western Bismarck Sea without much change in their total salt content, and the uniform core then increases considerably in salt content when subjected to mixing in the Caroline Basins. Hydrographic and moored current observations as well as model results show a distinct annual signal in salinity and velocity in the AAIW core off New Guinea. It appears to be related to the monsoonal change that is typically found in the near-surface waters in the region. Lagrangian data are used to investigate the structure of the deep New Guinea Coastal Undercurrent, the related cross-equatorial flow and eddy-structure, and the embedment in the zonal equatorial current system. Results from 17 neutrally buoyant RAFOS floats, ballasted to drift in the AAIW core layer, are compared with a numerical tracking experiment. In the model 73 particles are released at five-day intervals from Station J (2.5°N, 142°E), simulating currents at a moored time series station north of New Guinea. Observed and model track patterns are fairly consistent in space and season. Floats cross the equator preferably north of Cenderawasih Bay, with a maximum range in eddy-motion in this region north of New Guinea. The northward route at 135°E is also reflected in a low-salinity tongue reaching up to 3°N. At that longitude the floats seem to ignore the zonally aligned equatorial undercurrents. Farther to the east (139?145°E), however, the float observations are consistent with low-latitude bands of intermediate currents.