Description: Highlights: • We present results from a current meter array and hydrographic observations on the eastern flank of Reykjanes Ridge, Iceland Basin. • The June 2000–August 2002 average volume transport for the Iceland Scotland Overflow Water (ISOW) plume is 3.8±0.6 Sv. • Our flux estimate compares favorably with historical observations and recent model results. • Downstream drainage of ISOW through Charlie–Gibbs Fracture Zone only accounts for 60% of our ISOW transport estimate • Periods of stronger flow coincide with a higher fraction of ambient water in the ISOW plume. Abstract: Here we present results from a combined moored current meter/hydrography array deployed within the Iceland Scotland Overflow Water (ISOW) plume on the eastern flank of Reykjanes Ridge approximately 1000 km downstream of Faroe Bank Channel (FBC) between June 2000 and August 2002. Based on the array measurements during this period the ISOW plume exhibited a time mean volume transport of 3.8±0.6 Sv (standard error, 1 Sv=106 m3/s). The transport estimate favorably compares with other recent estimates obtained by different methods, confirming that the fate of the ISOW plume downstream of the array is far from being fully understood. Historical observations show that drainage of ISOW through Charlie–Gibbs Fracture Zone (CGFZ) only amounts to 60% of our upstream transport estimate. To date, no reliable transport estimates of the fractions of ISOW recirculating within the Iceland Basin or being drained through fracture zones other than CGFZ do exist. Our observed 2-years-long transport time series show pronounced subseasonal variability with a standard deviation of 1.3 Sv. Simultaneous hydrographic observations reveal, that temporal changes in the strength of the flow go along with changes in the water mass properties. Periods of stronger flow within the ISOW plume coincide with a reduction in salinity.

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: Dense Antarctic Bottom Water formed around the continent of Antarctica spreads northward in the Atlantic underneath North Atlantic Deep Water, gradually mixing and upwelling into it. This Antarctic Water forms a significant element of the meridional circulation in both directions: northward as bottom water and southward as deep water. It is important to determine the strength of each component to assess its role in ocean circulation. Such measurements are useful when made in constricted pathways because any flow is more clearly defined. A new set of fine-resolution hydrograhic measurements in the Hunter Channel of the South Atlantic Ocean has been obtained, which allow the geostrophic bottom flow there to be estimated for the first time. The northward flow through the Hunter Channel of water cooler than 2-degrees-C is thus estimated to be 0.7 X 10(6) m3 s-1.

Description: The Vema Channel represents a prominent location for the nortward flow of bottom water in the subtropical western South Atlantic. A recent multibeam echo-sounding survey of the Vema Sill on board F.S. Meteor revealed a narrow and shallow portion of the Vema Channel at 31°12′S, 39°24′W, the Vema Sill. The survey also showed the remarkably asymmetric shape of the sill region, suggesting an interaction between the bottom flow and the shape of the channel.

Description: Hydrographic data from two cruises in the Canary Basin (Meteor 57, July 1981; Poseidon 86, April 1982) are analysed with respect to current distribution and lateral heat flux in the Azores-Madeira region. The first part of the data base consists of long transects of XBT and G.E.K. measurements between Cape Finisterre (North West Spain) and the northern Canary Basin, where several year-long current meter records exist. Further information is obtained by thermosalinograph surface data and by expendable current profilers (XCP). Geostrophic currents are derived from XBT profiles, using the tight temperature-salinity relationship in the depth range of the Warmwassersphäre. The results compare well with the G.E.K. and XCP current observations. The second part consists of CTD data from an eddy resolving, box-shaped CTD survey (500 × 500 km2) centered at the mooring location (33°N, 22°W), The observations are supplemented by satellite-buoy trajectories. Horizontal parameter distribution is shown in terms of objectively contoured maps. Bands of spatially enhanced energetic structures, seen in the long transects are further resolved by the box survey as a deep jet-like current system cross the Canary Basin in a west-east direction. Associated with this Azores Current is a frontal zone with near-surface temperature and salinity steps of order 2 K and 0.3 practical salinity units. The dynamic topography field can be decomposed into a linear background field, a Rossby wave and a mesoscale eddy field. We find that major contributions to the meridional eddy heat flux are confined to the vicinity of the Azores current frontal zone. It is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence.

Description: As a component of the meridional overturning variability experiment in the tropical North Atlantic, a four-year-long time series of meridional transport of North Atlantic deep water has been obtained from moored end point measurements of density and bottom pressure. This study presents a quality assessment of the measurement elements. Rigorous pre- and post- deployment in situ calibration of the density sensors and subsequent data processing establish an accuracy of O(1.5 Sv) in internal transport in the 1200–5000 dbar range at subinertial time scales. A similar accuracy is reached in the bottom pressure-derived external transport fluctuations. However, for pressure, variability with periods longer than a deployment's duration (presently about one year) is not measurable. This effect is demonstrated using numerical simulations and a possible solution for detecting long-term external transport changes is presented.

Description: The Vema Channel represents the only major conduit through which the deepest and coldest (〈0.2 °C potential temperature) Antarctic Bottom Water (AABW) flows from the Argentine into the Brazil Basin. From 2003 to 2007 two current meter moorings were present on each side of the Vema Sill, close to the narrowest spot of the Vema Channel. The data from the moorings are compared with earlier current and temperature observations. On average the maximum current core lies ∼100 m above the bottom of the sill with a mean northward speed of 0.3 m s−1. Farther up in the water column where Lower Circumpolar Deep Water and North Atlantic Deep Water prevail, one finds a level of sluggish currents with a southward tendency in the sub-centimeter-per-second range. The lower boundary of a layer of ‘no’ motion was observed at ∼3700 m depth where the mean potential temperature amounts to 1.5 °C. The evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the Argentine and the Brazil Basin differs from the more stable attitude of deep horizontal currents. Starting with CTD observations in 1972 we find a steady increase of temperatures of the coldest AABW in the Vema Channel. This general trend of rising abyssal potential temperatures of almost 2 mKelvin per year is based on mostly annual CTD observations. The overall warming trend is fully compatible with our three-year moored temperature series in agreement with earlier records with high temporal resolution. Distinct frequently fluctuating horizontal current shear between the western and eastern sides of the Vema Sill may be explained by two different catchment areas for AABW at the mouth of the Vema Channel. One pathway originates at the American continental rise and advects bottom water in form of the deep western boundary current. A second pathway is supplied by an eastern boundary current along the Mid Atlantic Ridge in the Argentine Basin. Both source waters merge at the channel entrance, mix, and their respective strengths can alternate within the sill area.