Rainer J. Zantopp

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

Moored observations of western boundary current variability and thermohaline circulation at 26.5°N in the subtropical North Atlantic

Abstract A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation. Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer,...

Circulation and Deep-Water Export at the Western Exit of the Subpolar North Atlantic

The current system east of the Grand Banks was intensely observed by World Ocean Circulation Experiment (WOCE) array ACM-6 during 1993-95 with eight moorings, reaching about 500 km out from the shelf edge and covering the water column from about 400-m depth to the bottom. More recently, a reduced array by the Institut fur Meerskunde (IfM) at Kiel, Germany, of four moorings was deployed during 1999-2001, focusing on the deep-water flow near the western continental slope. Both sets of moored time series, each about 22 months long, are combined here for a mean current boundary section, and both arrays are analyzed for the variability of currents and transports. A mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes. The offshore part of the combined section is dominated by the deep-reaching North Atlantic Current (NAC) with currents still at 10 cm s 21 near the bottom and a total northward transport of about 140 Sv (Sv ( 106 m3 s 21), with the details depending on the method of surface extrapolation used. The mean flow along the western boundary was southward with the section-mean North Atlantic Deep Water outflow determined to be 12 Sv below the su 5 27.74 kg m23 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below su 5 27.74 kg m23, resulting in a large net northward flow in the western part of the basin. From watermass signatures it is concluded that the deep NAC is not a direct recirculation of DWBC water masses. Transport time series for the DWBC variability are derived for both arrays. The variance is concentrated in the period range from ;2 weeks to 2 months, but there are also variations at interannual and longer periods, with much of the DWBC variability being related to fluctuations and meandering of the NAC. A significant annual cycle is not recognizable in the combined current and transport time series of both arrays. The moored array results are compared with other evidence on deep outflow and recirculation, including recent models of different types and complexity.

Variability of Structure and Transport of the Florida Current in the Period Range of Days to Seasonal

Abstract Current measurements with five repeated moored arrays were carried out from April 1982 to June 1984 across the Florida Current between West Palm Beach and Grand Bahama Island; a reduced sixth array was continued until June 1985. Transport were calculated directly by vertical integration of the 40-hour low-passed northward current components and extrapolation to the surface using the mean vertical shears ova the extent of the mooring. These transports compared well with 96 transport sections measured by PEGASUS and with transports determined by cable voltage measurements. During 1982–84 the transport variations ranged between 20 and 40 Sv (Sv = 106 m3 s−1) with a mean of 30.5 Sv and standard deviations of the 40-hour low-passed data of ±3 Sv. However, monthly mean deviations from the mean annual cycle were only of order ± 1 Sv, indicating that the climate relevant long-period variations of this current are fairly small. The volume transport shows a continuous spectrum with no particularly energeti...

Break-up of the Atlantic deep western boundary current into eddies at 8° S

The existence in the ocean of deep western boundary currents, which connect the high-latitude regions where deep water is formed with upwelling regions as part of the global ocean circulation, was postulated more than 40 years ago. These ocean currents have been found adjacent to the continental slopes of all ocean basins, and have core depths between 1,500 and 4,000 m. In the Atlantic Ocean, the deep western boundary current is estimated to carry (10–40) × 106 m3 s-1 of water, transporting North Atlantic Deep Water—from the overflow regions between Greenland and Scotland and from the Labrador Sea—into the South Atlantic and the Antarctic circumpolar current. Here we present direct velocity and water mass observations obtained in the period 2000 to 2003, as well as results from a numerical ocean circulation model, showing that the Atlantic deep western boundary current breaks up at 8° S. Southward of this latitude, the transport of North Atlantic Deep Water into the South Atlantic Ocean is accomplished by migrating eddies, rather than by a continuous flow. Our model simulation indicates that the deep western boundary current breaks up into eddies at the present intensity of meridional overturning circulation. For weaker overturning, continuation as a stable, laminar boundary flow seems possible.

Mean transport and seasonal cycle of the Kuroshio east of Taiwan with comparison to the Florida Current

Moored observations of Kuroshio current structure and transport variability were made across the channel between northeast Taiwan and the Ryukyu Islands at 24 degreesN from September 19, 1994, to May 27, 1996. This was a cooperative, effort between the United States and Taiwan. The moored array was designated PCM-1, for the World Ocean Circulation Experiment (WOCE) transport resolving array. The dominant current and transport variability occurred on 100-day timescales and is shown by Zhang et al. [2001] to be caused by warm mesoscale eddys merging with the Kuroshio south of the array causing offshore meandering and flow splitting around the Ryukyu Islands. An annual transport cycle could not be resolved from our 20-month moored record because of abasing from the 100-day period events. Sea level difference data were used to extend the transport time series to 7 years giving a variation in the range of the annual transport cycle of 4-10 Sv, with a mean range closer to 4 Sv. The seasonal maximum of 24 Sv occurred in the summer and the seasonal minimum of 20 Sv occurred in the fall. A weaker secondary maximum also occurred in the winter. The cycle of Kuroshio transport appears to result from a combination of local along-channel wind forcing and Sverdrup forcing over the Philippine Sea. Our estimate of the mean transport of the Kuroshio at the entrance to the East China Sea from the moored array is 21.5 +/- 2.5 Sv. The mean transpacific balance of meridional flows forced by winds and thermohaline processes at this latitude requires an additional mean northward flow of 12 Sv with an annual cycle of +/-8 Sv along the eastern boundary of the Ryukyu Islands. The mean transport and annual cycle of the Kuroshio were found to be in reasonable agreement with basin-scale wind-forced models. Remarkable similarities are shown to exist between the mean western boundary currents and their seasonal cycles in the Atlantic (Florida Current and Antilles Current) and Pacific (Kuroshio and boundary current east of Ryukyu Island chain) at the same latitude. However, detailed comparison shows that the mean Kuroshio is weaker and more surface intensified than the mean Florida Current, while the Kuroshio-transport variability is significantly larger.

Western Boundary Current Structure and Variability East of Abaco, Bahamas at 26.5°N

Abstract During April 1987 to June 1988, a four-mooring, transport-resolving current meter array was deployed in a section extending 70 km eastward from Abaco, Bahamas, at 26.5°N. Mean currents in the upper layer (≤800 m) showed a clockwise rotation that appears to be associated with a small scale, quasi-permanent, anticyclonic gyre centered just northeast or the Bahamas. Deep mean currents were persistenly southward and indicated a strong, deep jet, the Deep Western Boundary Current (DWBC), reaching core speeds of 20 cm s−1at 2500 m depth about 25 km seaward of the western boundary. Vertically integrated meridional volume transport over the section showed a surprisingly large variability, ranging from approximately 20 Sv northward to 70 Sv southward (1 Sv = 106m3s−1). The mean meridional volume transport was 30 Sv to the south, with about 3 Sv flowing northward above 800 m, which could be produced by a weak Antilles Current and 33 Sv flowing, southward below 800 m. The deep southward transport of 33 Sv i...

Interannual to decadal variability of outflow from the Labrador Sea

A decade of weak convection in the Labrador Sea associated with decreasing water mass transformation, in combination with advective and eddy fluxes into the convection area, caused significant warming of the deep waters in both the central Labrador Sea and boundary current system along the Labrador shelf break. The connection to the export of Deep Water was studied based on moored current meter stations between 1997 and 2009 at the exit of the Labrador Sea, near the shelf break at 5˚3N. More than 100 year -long current meter records spanning the full water column have been analyzed with respect to high frequency variability, decaying from the surface to the bottom layer, and for the annual mean flow, showing intra- to interannual variability but no detectable decadal trend in the strength of the deep and near-bottom flow out of the Labrador Sea.

The Shallow and Deep Western Boundary Circulation of the South Atlantic at 5°–11°S

Repeated shipboard observation sections across the boundary flow off northeastern Brazil as well as acoustic Doppler current profiler (ADCP) and current-meter records from a moored boundary array deployed during 2000–04 near 11°S are analyzed here for both the northward warm water flow by the North Brazil Undercurrent (NBUC) above approximately 1100 m and the southward flow of North Atlantic Deep Water (NADW) underneath. At 5°S, the mean from nine sections yields an NBUC transport of 26.5 ± 3.7 Sv (Sv ≡ 106 m3 s−1) along the boundary; at 11°S the mean NBUC transport from five sections is 25.4 ± 7.4 Sv, confirming that the NBUC is already well developed at 11°S. At both latitudes a persistent offshore southward recirculation between 200- and 1100-m depth reduces the net northward warm water flow through the 5°S section (west of 31.5°W) to 22.1 ± 5.3 Sv and through the 11°S section to 21.7 ± 4.1 Sv (west of 32.0°W). The 4-yr-long NBUC transport time series from 11°S yields a seasonal cycle of 2.5 Sv amplitude with its northward maximum in July. Interannual NBUC transport variations are small, varying only by ±1.2 Sv during the four years, with no detectable trend. The southward flow of NADW within the deep western boundary current at 5°S is 25.5 ± 8.3 Sv with an offshore northward recirculation, yielding a nine-section mean of 20.3 ± 10.1 Sv west of 31.5°W. For Antarctic Bottom Water, a net northward flow of 4.4 ± 3.0 Sv is determined at 5°S. For the 11°S section, the moored array data show a pronounced energy maximum at 60–70-day period in the NADW depth range, which was identified in related work as deep eddies translating southward along the boundary. Based on a kinematic eddy model fit to the first half of the moored time series, the mean NADW transfer by the deep eddies at 11°S was estimated to be about 17 Sv. Given the large interannual variability of the deep near-boundary transport time series, which ranged from 14 to 24 Sv, the 11°S mean was considered to be not distinguishable from the mean at 5°S

Florida current: low-frequency variability as observed with moored current meters during april 1982 to june 1983.

A 1-year time series of volume transport through the Florida Straits near 27 �N was derived from an array of five subsurface current meter moorings. The transport estimates, determined on the basis of constant shear extrapolation of the subsurface velocities to the surface, are in good agreement with transports derived from submarine cable and Pegasus measurements. The annual transport cycle in 1982-1983 is complicated by large-amplitude fluctuations on time scales of 1 to 3 weeks, but it does exhibit a transport maximum in summer and a minimum in fall-winter, consistent with historical results and of similar magnitude. The energy density spectrum of transports is continuous with a slope of about -1.5 and does not show a gap between the periods of weeks and seasonal. Evidence was found for atmospheric forcing of transport fluctuations, with highest coherence between transport and the local meridional wind stress at periods of 10 and 15 days during the summer and 5 and 40 days during the winter.