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 σθ = 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.

Description: The structure of the subtropical South Indian Ocean Countercurrent (SICC) is revealed by altimeter-derived absolute geostrophic surface velocities. It is a narrow, eastward-flowing current between 22° and 26°S confined to planetary wave trains which propagate westward through the Indian Ocean. Multi-year averaging identifies it as a well-defined current between Madagascar and 80°E, continuing with lower intensity between 90° and 100°E. It virtually coincides with the northern limit of Subtropical Underwater subduction. Geostrophic currents from hydrographic sections closely correspond to these surface patterns. Volume transports of the countercurrent down to 800 dbar are of order (107 m3 s−1). Evidence is provided for a narrow branch of the South Equatorial Current (SEC) approaching Madagascar near 18°S and feeding the southern East Madagascar Current (EMC) which appears to continue westward around the southern tip of Madagascar. It then partially retroflects and nourishes the SICC.

Description: Twenty years of time series observations from the deep-sea mooring KIEL276 are used to obtain information on the frequency and propagation of meddies (Mediterranean Water eddies), on long-term changes in flow properties, and on a possible relation to the North Atlantic Oscillation. The mooring was set at the nominal position 33°N, 22°W at a water depth of more than 5200 m in the northern Canary Basin. It is located near the southern boundary of the Azores Current (AC), which is part of the North Atlantic subtropical gyre, and also in the large-scale Mediterranean Water (MW) tongue originating from the Strait of Gibraltar. The amplitudes of time-varying vertical quasi-geostrophic modes and the correlation of current and temperature changes at levels in the MW and the North Atlantic Central Water above are used to identify meddies. A total of 10 meddies passed the mooring during the period 1980–2000. Half of the events can be related to earlier observations. Directional changes in meddy-related velocities are used to estimate speeds and directions of meddy propagation. Directions of propagation are very homogeneous, with all the 10 meddies observed propagating with a southward velocity component within a sector of 90°, and typical speeds are 2–3 cm/s. Meddy occurrence was uneven in time, with six meddies found during the first four years and only four meddies during the remaining 16 years. Decadal changes show the annual-mean and the fluctuating kinetic energy levels at the site changing from lower values in the 1980s to high values in the 1990s. This change appears to be correlated with variations in the North Atlantic Oscillation (NAO) index, with a delay in oceanic response of about 3 years. A conceptual model of AC meanders is used to identify meander signals in the upper-layer time series. The AC axis appears to be closer to the site during the 1990s than during the preceding decade and indicates a southward or southwestward displacement of the AC with increasingly positive values of the NAO index. Meddy frequency is lower when the AC gets closer from the north. A reduction in meddy occurrence in the region just south of the AC is possibly caused by the shear-induced blocking of some meddies crossing the front from the north.

Description: Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21 degrees S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55 degrees E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.