W. Brechner Owens

Mid-depth recirculation observed in the interior Labrador and Irminger seas by direct velocity measurements

The Labrador Sea is one of the sites where convection exports surface water to the deep ocean in winter as part of the thermohaline circulation. Labrador Sea water is characteristically cold and fresh, and it can be traced at intermediate depths (500–2,000 m) across the North Atlantic Ocean, to the south and to the east of the Labrador Sea. Widespread observations of the ocean currents that lead to this distribution of Labrador Sea water have, however, been difficult and therefore scarce. We have used more than 200 subsurface floats to measure directly basin-wide horizontal velocities at various depths in the Labrador and Irminger seas. We observe unanticipated recirculations of the mid-depth (∼700 m) cyclonic boundary currents in both basins, leading to an anticyclonic flow in the interior of the Labrador basin. About 40% of the floats from the region of deep convection left the basin within one year and were rapidly transported in the anticyclonic flow to the Irminger basin, and also eastwards into the subpolar gyre. Surprisingly, the float tracks did not clearly depict the deep western boundary current, which is the expected main pathway of Labrador Sea water in the thermohaline circulation. Rather, the flow along the boundary near Flemish Cap is dominated by eddies that transport water offshore. Our detailed observations of the velocity structure with a high data coverage suggest that we may have to revise our picture of the formation and spreading of Labrador Sea water, and future studies with similar instrumentation will allow new insights on the intermediate depth ocean circulation.

Observations of Open-Ocean Deep Convection in the Labrador Sea from Subsurface Floats*

Abstract The occurrence and extent of deep convection in the Labrador Sea in winters 1996/97 and 1997/98 is investigated from measurements of over 200 neutrally buoyant subsurface Profiling Autonomous Lagrangian Circulation Explorer (PALACE) and Sounding Oceanographic Lagrangian Observer (SOLO) floats. In addition to providing drift velocity data and vertical profiles of temperature and salinity, 55 floats are equipped with vertical current meters (VCMs). Time series of vertical velocity (derived from measured pressure and vertical flow past the float) and temperature are obtained from the VCM floats. Mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997, but no deeper than 1000 m in 1998. Deep mixed layers were concentrated in the western basin, although a number of deep mixed layers were observed southwest of Cape Farewell and also north of 60°N. The highest variance in vertical velocity and the lowest mean temperatures were found in t...

Oceanic observations of stratified Taylor columns near a bump

Abstract Analyses of two hydrographic sections and records from moored instruments show that the flow over a 400-m high seamount, centred at 36°N, 55°W in the recirculation region of the Gulf Stream system, resembles a stratified Taylor column. Potential density sections indicate a bottom intensified uplifting over the seamount with a vertical scale of approx. 3 km. Time series of relative vorticity deduced from the current meter records show a significant correlation with the bathymetry and a similar vertical scale as seen in the density structure. Estimates of the vorticity balance nominally 1000 m above the bottom demonstrate that the balance is between the advection of relative vorticity and vertical vortex stretching which is consistent with Taylor column behavior. Objective stream function maps at the same depth show closed contours near the seamount for most of the nine-month duration of the moored array. The results suggest that the bathymetry distorts the larger scale mean circulation so as to produce large amplitude, small scale variability which should be taken into account before one extrapolates isolated measurements from moored instruments.

Anomalous Anomalies in Averaged Hydrographic Data

Abstract A comparison of a recently assembled hydrographic database for the North Atlantic with the Lovitus atlas shows striking differences in the vicinity of the Gulf Stream and the North Atlantic Current. On isopycnal surfaces in the main thermocline, isolated pools of warm, saline water are found in the Levitus database but are absent in the new database. Using synoptic data as a proxy for temporally averaged climatological data, it is shown that the anomalous features can be accounted for by the differences in the averaging process. To produce a gridded database from irregularly spaced station data, Levitus averaged the data on pressure surfaces while the new database was prepared with averaging an potential density surfaces. It is shown that averaging on a pressure surface in an area of sharply sloping isopycnals produces a water mass with a θ–S signature uncharacteristic of the local water mass(es). The anomalous potential temperatures and salinities that result are compared to the large-scale wate...

Climatic warming of Atlantic intermediate waters

Abstract Interdecadal temperature variability of the Atlantic Ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990s. A comprehensive reanalysis of North Atlantic sections and the inclusion of South Atlantic sections show that warming seen previously in the North Atlantic extends to the South Atlantic. The largest statistically significant changes occur on pressure surfaces between 1000 and 2000 decibars (db). Over this pressure range and for latitudes between 32°S and 36°N, temperatures have warmed by ∼0.5°C century−1. At 48°N a cooling of ∼3°C century−1 occurred between the 1950s and 1980s. These isobaric temperature trends are decomposed into ones along surfaces of constant neutral density, and ones due to the vertical movement of neutral surfaces. The two components are associated with different processes. In the southern North Atlantic (8°–36°N) the subthermocline warming between the 1950s and 1980s appears to be due primarily to downward displacements of n...

Deep Currents in the Central Subarctic Pacific Ocean

Abstract Sections of closely spaced CTD stations along Longs. 165°W, 175°W and 175°E, in combination with 14-month current records from the central longitude, define two deep, nearly zonal currants, with speed increasing upward, in the subarctic Pacific. One flows eastward above the Aleutian Rise and Aleutian Trench, and appears to be a concentration of geostrophic flow forced by the bottom topography. The other flows westward along the Aleutian Island Arc, and is the northern-boundary current predicted by deep-circulation theory. Both currents reach to the sea surface, the boundary current being simply the deep part of the Alaskan Stream. The current records were too few to permit better than rough estimates of volume transports but to the extent that they could be combined with thermal-wind calculations they suggest, at 175°W, (1) a transport of 28 × 106 m3 s−1 for the Alaskan Stream, of whch 5 × 106 m3 s−1 was found below 1500 m, and (2) a transport of around 20 × 1O6 m3 s−1 for the eastward jet, of wh...

A statistical description of the mean circulation and eddy variability in the northwestern Atlantic using SOFAR floats

Abstract Velocity data obtained from trajectories of SOFAR floats launched as part of several experiments carried out in the Northwest Atlantic during the last two decades are used to give a statistical description of the mean Gulf Stream and its recirculation. The distribution of eddy variability in the region is also presented. A mean Gulf Stream which bifurcates between 55° and 45°W to feed the North Atlantic Current and a southern recirculation can be clearly identified. A weak interior Sverdrup flow is also suggested. The spatial distribution of eddy variability is consistent with previous descriptions, showing a maximum near the Gulf Stream.

A New Algorithm for CTD Oxygen Calibration

Abstract An algorithm for converting the Beckman dissolved-oxygen probe variables with other data from a WHOI/Brown CTD/O2 system to oxygen concentration is presented. Improvements over earlier oxygen algorithms are inclusion of an oxygen current bias and time-lag correction using a nonlinear, least-squares calibration to the titrated oxygen bottle samples. The calibration technique uses a quasi-Newtonian minimization scheme available in scientific subroutine libraries such as the “IMSL” library. Oxygen probe parameters are found to be stable over several (as many as 25) stations and give typical rms errors in the deep water of 0.005 ml l−1 which is roughly the expected error for the bottle samples. Oxygen current biases can be equivalent to concentrations as large as 0.8 ml l−1. Typical oxygen lag correction values, associated with diffusion times through the probe membrane are in the range of 4–10 seconds. Examples from the North Pacific where the shallow oxygen minimum severely tests the algorithm and ...

Deep circulation in the northwest corner of the Pacific Ocean

Abstract We deployed for two years a line of nine current-meter moorings bearing instruments at depths of 2, 3, and 4 km running southeast of Hokkaido, to measure currents above the continental slope, Kuril Trench, and Hokkaido Rise. The mean flow was directed southwestward above the continental slope, northeastward above the trench and upper rise (except at one mooring), and westward onto the lower rise. The mean currents were highly barotropic, except above the continental slope, and unexpectedly swift (8 cm s −1 in the trench). The velocity pattern above the Hokkaido Rise is like that observed earlier above the Aleutian Rise at Long. 175°W, and may be due, as suggested for the latter, to varying topographic beta associated with the curvature of the bottom profile of the rise. Thermal-wind fields from three CTD sections along the mooring line, while consistent among themselves, were unlike the observed mean shear, and therefore useless for estimating mean transports. Estimates based on the direct current measurements alone, for depths greater than 2000 m, are 4×10 6 m 3 s −1 southwestward above the continental slope and 20×10 6 m 3 s −1 northeastward in the trench; but the former might be too small, the latter too large, by as much as 10×10 6 m 3 s −1 because of the relatively broad mooring spacing. These measurements, in combination with many others reported earlier, unequivocally describe swift deep southward flow along the inshore sides of the Izu-Ogasawara, Japan, and Kuril Trenches, and opposed flow along their offshore sides, as well as above their axes (except in the Izu-Ogasawara Trench). The southward flow may be, at least in part, the recirculation western-boundary current predicted for the northern North Pacific, although the oceanic geometry is different from, and more complicated than, that of the classic analytical predictive models. Reasons for the strong opposed flow are obscure. Water properties reveal that deep water spreads into the Izu-Ogasawara Trench, and probably into the Japan Trench as well, flows northward through the Kuril Trench, and, at least at some levels, around the northern end of the Emperor Seamount Chain to fill the long Aleutian Trench. This plus eastward flows through the Main Gap in the Emperor Seamount Chain and the passage between the Hess Rise and the Hawaiian Ridge seem to be the principal deep outflows from the Northwest Pacific Basin into the Northeast Pacific Basin. The Meiji Sediment Drift, lying along the eastern flank of the far northern Emperor Seamount Chain, is composed of material from the Bering Sea. To account for its deposition, we conjecture that the deep Kamchatka Current, presently carrying this material southward, splits at the latitude of the northernmost Emperors, one branch flowing eastward as a zonal jet, and continuing southward along the eastern flank of the Seamount Chain as a deep western-boundary current. Descriptive ambiguities and dynamical puzzles are considered.

Some preliminary results concerning deep Northern-Boundary currents in the North Pacific

Abstract Fourteen-month velocity records from five moorings along Long. 175°W, in combination with CTD and nutrient data, distinguish two pronounced, quite steady, deep zonal currents near the northern margin of the North Pacific. One flows westward at speeds of 1–3 cm s −1 along the slope of the Aleutian Island Arc, and appears to be the northern-boundary current required by deep circulation theory. The other flows eastward at about 1–2 cm s −1 just to the south above the Aleutian Trench and upper Aleutian Rise, and carries water that is slightly lower in temperature and dissolved silica, and slightly higher in salinity and dissolved oxygen, than that to the north or south. We believe that this eastward jet represents an extreme distortion of a level-bottom, interior flow pattern, as forced by the meridional variation in bottom slope on the southern flank of the Aleutian Rise, which imposes a varying effective “beta” on the system. A simple model of this effect is discussed.