Time Series Of Volume Transport Research Articles

The heat storage variability in the Brazil Current

We investigated the variability of the oceanic heat storage on the western boundary of the South Atlantic Ocean. We aimed to understand the specific contribution of the Brazil Current as it travels southwards. The heat storage is evaluated near three latitudes, namely at 15◦S, 24◦S and 34.5◦S. Numerical outputs of the oceanic potential temperature, salinity, and meridional components of the current’s velocity resulting from the Estimating the Circulation and Climate of the Ocean (ECCO) model were used. We examined the time series of the heat storage from 1992 to 2015 for the three latitudes. The mean heat storage increased towards the south with a maximum at 24◦S. At 34.5◦S, there is a decrease in this property possibly due to its proximity to the Brazil-Malvinas Confluence. The Brazil Current’s volume transport time series was correlated with the corresponding heat storage. The latitude that presented the highest correlation at interannual scale was 15◦S with a value of 0.84 at a 95% of confidence level. The model outputs were also compared with in situ expendable bathythermograph measurements. Despite the presence of gaps and peaks indicating extreme events in the in situ data, both time series were statistically close and they showed similar mean annual cycles. No significant long-term HS trend was found at any of the three latitudes.

Volume Transport Time Series and Variability of the North Atlantic Eastern Boundary Current at Goban Spur

AbstractThe Eastern Boundary Current is an essential part of the water mass exchange between the subtropical and subpolar North Atlantic. Here, we study the offshore branch of the European Shelf Current (ESC) over the Goban Spur slope area off Ireland. Our analysis is based on current measurements obtained from a multiyear mooring effort, complemented by ship‐board observations along a hydrographic section, satellite‐derived estimates of absolute dynamic topography with geostrophic currents, and float trajectories. These data serve to quantify the offshore branch of the ESC on intraannual to interannual timescales. From the moored observations, we derive a mean poleward along‐slope volume flux of 3.7 0.7 Sv for the period 2017–2019. Using a multilinear regression model and geostrophic surface velocities, we extend the time series to the period 1993–2019 and obtain a long‐term mean transport of 3.2 0.4 Sv. Both time series show strong variability ranging from −7.5 to 15.7 Sv. The variability is linked to a dynamic eddy field, especially a stationary cyclonic circulation pattern near the mooring array, and meandering of current branches originating from the North Atlantic Current.We find no evidence of a consistent deep boundary current extending from the shelf break to the position of the offshore mooring (4,500 m depth), but confirm a persistent along‐slope flow at the shallower slope (1,500 m depth). Geostrophic surface velocities and float trajectories reveal that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions but rather indicate the intermittent character of the flow.

A three-dimensional gravest empirical mode determined from hydrographic observations in the western equatorial Pacific Ocean

A spatially varying non-stationary gravest empirical mode (GEM) field is constructed by incorporating the calendar month as an extra parameter to infer the subsurface thermohaline structure from the Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean. This monthly GEM (MGEM) overcomes the application limitation of traditional GEM in the equatorial ocean due to the poor vertical coherence thermohaline structure there. The MGEM can capture 77% of the thermohaline variance for the upper 500 dbar, which is only 62% by the traditional GEM method. The most significant improvement of MGEM is for the deep ocean between 500 and 1900 dbar, where only 12% of the thermohaline variance can be captured by the GEM, but improved to 44% by the MGEM method. Based on the MGEM, a 24-month volume transport time series is estimated from observations by three PIESs deployed across the nascent North Equatorial Countercurrent (NECC) at 4°N, 6°N, and 8°N along the 130°E transect during 2014–2016. By establishing an empirical relationship between the 24 month-long volume transport and the synchronous altimeter data, a 24 year-long time series of NECC volume transport during 1993–2016 are constructed, and the seasonal and interannual variations of the NECC are investigated. The nascent NECC between 4°N-6°N was found to have quite strong seasonal variations with the strongest transport of 23 Sv occurring in February and the weakest transport of 11 Sv occurring in September. It is strengthened during the developing phase of El Niños, and is weakened during La Niñas. The 2015/16 El Niño exerts a significant impact on the nascent NECC with the amplitude comparable to that of the 1997/98 event, which even obscures the NECC's seasonality. • Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean for the first time. • Development of a 3-dimensional GEM field with the third axis being time-of-year (MGEM field). • Significant improvement of MGEM in capturing thermohaline variance of deep ocean between 500 and 1900 dbar. • The nascent NECC has strong seasonal variations with its transport being doubled in winter compared with that in fall.

Malvinas Current Volume Transport at 41°S: A 24 Yearlong Time Series Consistent With Mooring Data From 3 Decades and Satellite Altimetry

AbstractWe combined altimetric data and the in situ data sets from three 10 years apart mooring deployments to compute a coherent and accurate volume transport time series of the Malvinas Current (MC) at 41°S. We used a method developed in Koenig et al. (2014) and explored three types of geostrophic shear to estimate the uncertainty derived from the lack of velocity data in the upper 300 m. The mean MC transport over 24 years in the upper 1,500 m is 37.1 ± 2.6 Sv and the standard deviation 6.6 ± 1 Sv. Since 1993, annual mean transports have varied from 32 to 41 Sv and the three in situ records corresponded to low annual mean transports. The MC transport time series is not stationary, its spectral content evolves with time showing significant energy at the 30–110 days, semiannual and annual period. The distribution of the MC volume transport anomalies is asymmetric, negatively skewed with larger negative anomalies than positive anomalies. Several transport maxima appear to result from cyclonic eddies that propagate northwestward following the 4,000–5,000 m isobaths and locally reinforce the circulation on the slope when they reach 41°S. During transport maxima, the northernmost extension of the Subantarctic Front (SAF) remains at its mean location (39.5°S). During minima, the SAF migrates southward of 41°S as positive anomalies shed by the Brazil Current overshoot moves westward onto the slope. Apart from continental trapped waves, changes in the MC volume transport at 41°S show no correlation with upstream conditions on the continental slope.

A stable Faroe Bank Channel overflow 1995–2015

Abstract. The Faroe Bank Channel (FBC) is the deepest passage across the Greenland–Scotland Ridge (GSR) and there is a continuous deep flow of cold and dense water passing through it from the Arctic Mediterranean into the North Atlantic and further to the rest of the world ocean. This FBC overflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which has recently been suggested to have weakened. From November 1995 to May 2015, the FBC overflow has been monitored by a continuous ADCP (acoustic Doppler current profiler) mooring, which has been deployed in the middle of this narrow channel. Combined with regular hydrography cruises and several short-term mooring experiments, this allowed us to construct time series of volume transport and to follow changes in the hydrographic properties and density of the FBC overflow. The mean kinematic overflow, derived solely from the velocity field, was found to be (2.2 ± 0.2) Sv (1 Sv = 106 m3 s−1) with a slight, but not statistically significant, positive trend. The coldest part, and probably the bulk, of the FBC overflow warmed by a bit more than 0.1 °C, especially after 2002, increasing the transport of heat into the deep ocean. This warming was, however, accompanied by increasing salinities, which seem to have compensated for the temperature-induced density decrease. Thus, the FBC overflow has remained stable in volume transport as well as density during the 2 decades from 1995 to 2015. After crossing the GSR, the overflow is modified by mixing and entrainment, but the associated change in volume (and heat) transport is still not well known. Whatever effect this has on the AMOC and the global energy balance, our observed stability of the FBC overflow is consistent with reported observations from the other main overflow branch, the Denmark Strait overflow, and the three Atlantic inflow branches to the Arctic Mediterranean that feed the overflows. If the AMOC has weakened during the last 2 decades, it is not likely to have been due to its northernmost extension – the exchanges across the Greenland–Scotland Ridge.

Combining Observations from Multiple Platforms across the Kuroshio Northeast of Luzon: A Highlight on PIES Data

AbstractThis study presents amended procedures to process and map data collected by pressure-sensor-equipped inverted echo sounders (PIESs) in western boundary current regions. The modifications to the existing methodology, applied to observations of the Kuroshio from a PIES array deployed northeast of Luzon, Philippines, consist of substituting a hydrography-based mean travel time field for the PIES-based mean field and using two distinct gravest empirical mode (GEM) lookup tables across the front that separate water masses of South China Sea and North Pacific origin. In addition, this study presents a method to use time-mean velocities from acoustic Doppler current profilers (ADCPs) to reference (or “level”) the PIES-recorded pressures in order to obtain time series of absolute geostrophic velocity. Results derived from the PIES observations processed with the hydrography-based mean field and two GEMs are compared with hydrographic profiles sampled by Seagliders during the PIES observation period and with current velocity measured concurrently by a collocated ADCP array. The updated processing scheme leads to a 41% error decrease in the determination of the thermocline depth across the current, a 22% error decrease in baroclinic current velocity shear, and a 61% error decrease in baroclinic volume transports. The absolute volume transport time series derived from the leveled PIES array compares well with that obtained directly from the ADCPs with a root-mean-square difference of 3.0 Sv (1 Sv ≡ 106 m3 s–1), which is mainly attributed to the influence of ageostrophic processes on the ADCP-measured velocities that cannot be calculated from the PIES observations.

Warming and Weakening of the Abyssal Flow through Samoan Passage

AbstractThe abyssal flow of water through the Samoan Passage accounts for the majority of the bottom water renewal in the North Pacific, thereby making it an important element of the meridional overturning circulation. Here the authors report recent measurements of the flow of dense waters of Antarctic and North Atlantic origin through the Samoan Passage. A 15-month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013. This allows for an update of the only prior volume transport time series from the Samoan Passage from WOCE moored measurements between 1992 and 1994. While highly variable on multiple time scales, the overall pattern of the abyssal flow through the Samoan Passage was remarkably steady. The time-mean northward volume transport of about 5.4 Sv (1 Sv ≡ 106 m3 s−1) in 2012/13 was reduced compared to 6.0 Sv measured between 1992 and 1994. This volume transport reduction is significant within 68% confidence limits (±0.4 Sv) but not at 95% confidence limits (±0.6 Sv). In agreement with recent studies of the abyssal Pacific, the bottom flow through the Samoan Passage warmed significantly on average by 1 × 10−3°C yr−1 over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed increasing role of the deep oceans for heat uptake, decreasing flow through Samoan Passage may indicate a future weakening of this trend for the abyssal North Pacific.

Anatomy of the Antarctic Circumpolar Current volume transports through Drake Passage

AbstractThe 20 year (October 1992 to August 2013) observation‐based volume transport time series of the Antarctic Circumpolar Current (ACC) through Drake Passage (DP) across the Jason altimeter track #104 is analyzed to better understand the ACC transport variability and its potential causes. The time series of three transport components (total (TT), barotropic (BT), and baroclinic (BC)) referenced to 3000 m present energetic intraseasonal fluctuations, with a salient spectral peak at 50 and 36 days, with the largest (least) variance being associated with the BT (BC) component. Low‐frequency variations are much less energetic with a significant variance limited to the annual and biannual timescales and show a nonstationary intermittent link with the Southern Annular Mode and the Nino 3.4 index for interannual timescales. The region around 57°S in the Yaghan Basin appears to be a strategic point for a practical monitoring of the ACC transport, as the whole‐track TT is significantly correlated with the local TT (r = 0.53) and BT (r = 0.69) around 57°S. These local BT (and TT) variations are associated with a well‐defined tripole pattern in altimetric sea level anomaly (SLA). There is evidence that the tripole pattern associated with BT is locally generated when the BC‐associated mesoscale SLAs, which have propagated eastward from an upstream area of DP, cross the Shackleton Fracture Zone to penetrate into the Yaghan Basin. Barotropic basin modes excited within the Yaghan Basin are discussed as a plausible mechanism for the observed energy‐containing intraseasonal spectral peaks found in the transport variability.

A long-term volume transport time series estimated by combining in situ observation and satellite altimeter data in the northern South China Sea

We deployed five pressure-recording inverted echo sounders (PIES) along a satellite altimeter track across the continental slope of the northern South China Sea (NSCS) from October 2012 to July 2014, and obtained a time series of volume transport (VT PIES ) across the sec- tion from their records. Applying the empirical relation- ship between VT PIES and the satellite altimeter sea surface height anomaly difference across the section, we obtained a time series of volume transport (VT NSCS ) over 22 years from 1992 to 2014. The VTNSCS shows a small mean value of −1.6 Sv (1 Sv = 10 6 m 3 s −1 ) (i.e., toward the south- west), but a significant seasonal reversal and mesoscale eddy induced fluctuations ( −11.8 to 19.7 Sv). The monthly mean of VTNSCS over 22 years shows a maximum (3.6 Sv) in July and a minimum (−7.3 Sv) in December. This is the first long time series of volume transport for the NSCS based on in situ data.

Volume transport of the Antarctic Circumpolar Current: Production and validation of a 20 year long time series obtained from in situ and satellite observations

AbstractA 20 year long volume transport time series of the Antarctic Circumpolar Current across the Drake Passage is estimated from the combination of information from in situ current meter data (2006–2009) and satellite altimetry data (1992–2012). A new method for transport estimates had to be designed. It accounts for the dependence of the vertical velocity structure on surface velocity and latitude. Yet unpublished velocity profile time series from Acoustic Doppler Current Profilers are used to provide accurate vertical structure estimates in the upper 350 m. The mean cross‐track surface geostrophic velocities are estimated using an iterative error/correction scheme to the mean velocities deduced from two recent mean dynamic topographies. The internal consistency and the robustness of the method are carefully assessed. Comparisons with independent data demonstrate the accuracy of the method. The full‐depth volume transport has a mean of 141 Sv (standard error of the mean 2.7 Sv), a standard deviation (std) of 13 Sv, and a range of 110 Sv. Yearly means vary from 133.6 Sv in 2011 to 150 Sv in 1993 and standard deviations from 8.8 Sv in 2009 to 17.9 Sv in 1995. The canonical ISOS values (mean 133.8 Sv, std 11.2 Sv) obtained from a year‐long record in 1979 are very similar to those found here for year 2011 (133.6 Sv and 12 Sv). Full‐depth transports and transports over 3000 m barely differ as in that particular region of Drake Passage the deep recirculations in two semiclosed basins have a close to zero net transport.

Combining in situ measurements and altimetry to estimate volume, heat and salt transport variability through the Faroe–Shetland Channel

Abstract. From 1994 to 2011, instruments measuring ocean currents (Acoustic Doppler Current Profilers; ADCPs) have been moored on a section crossing the Faroe–Shetland Channel. Together with CTD (Conductivity Temperature Depth) measurements from regular research vessel occupations, they describe the flow field and water mass structure in the channel. Here, we use these data to calculate the average volume transport and properties of the flow of warm water through the channel from the Atlantic towards the Arctic, termed the Atlantic inflow. We find the average volume transport of this flow to be 2.7 ± 0.5 Sv (1 Sv = 106 m3 s–1) between the shelf edge on the Faroe side and the 150 m isobath on the Shetland side. The average heat transport (relative to 0 °C) was estimated to be 107 ± 21 TW (1 TW = 1012 W) and the average salt import to be 98 ± 20 × 106 kg s−1. Transport values for individual months, based on the ADCP data, include a large level of variability, but can be used to calibrate sea level height data from satellite altimetry. In this way, a time series of volume transport has been generated back to the beginning of satellite altimetry in December 1992. The Atlantic inflow has a seasonal variation in volume transport that peaks around the turn of the year and has an amplitude of 0.7 Sv. The Atlantic inflow has become warmer and more saline since 1994, but no equivalent trend in volume transport was observed.

고해상도 해양예보모형 HYCOM에 재현된 쓰시마난류

This study investigates the characteristic of the Tsushima Warm Current from an assimilated high resolution global ocean prediction model, <TEX>$1/12^{\circ}$</TEX> Global HYbrid Coordiate Ocean Model (HYCOM). The model results were verified through a comparison with current measurements obtained by acoustic Doppler current profiler (ADCP) mounted on the passenger ferryboat between Busan, Korea, and Hakata, Japan. The annual mean transport of the Tsushima Warm Current was 2.56 Sverdrup (Sv) (1 Sv = <TEX>$10^6m^3s^{-1}$</TEX>), which is similar to those from previous studies (Takikawa et al. 1999; Teague et al. 2002). The volume transport time series of the Tsushima Warm Current from HYCOM correlates to a high degree with that from the ADCP observation (the correlation coefficient between the two is 0.82). The spatiotemporal structures of the currents as well as temperature and salinity from HYCOM are comparable to the observed ones.

On the mass and salt budgets for a region of the continental shelf in the southern Mid‐Atlantic Bight

Two field studies were conducted across and along the continental shelf, one from February to May 1996 (deployment 1) and the other from July to October 1996 (deployment 2) in part to determine the mass and salt budgets of shelf water from south of Cape Henry to north of Cape Hatteras, the southernmost portion of the Mid‐Atlantic Bight. The temporal means of current meter records indicated that most of the water enters the region across its northern boundary near the shelf break as part of a southward, alongshore current and exits the southeast corner as a southeastward flowing current. Estimates of the volume transports indicated that not all the transport across the northern boundary was accounted for by transport across the southern boundary and that the remainder occurred as a broad, diffusive flow across the eastern boundary at the shelf break. Time series of volume transport across northern and southern boundaries were very similar and associated with variations in the alongshore wind stress and sea level, indicative of a geostrophic balance. Examination of the individual current meter records indicated these fluctuations were very barotropic even during deployment 2, which included the stratified summer season. Time series of the volume transport across the eastern boundary at the shelf break strongly mirrored the volume transport across the northern boundary minus that across the southern boundary, suggesting that the inferred eastern boundary transport was real and accommodated whatever the southern boundary could not. The turbulent salt flux across each boundary contributes very little to the net salt flux. The mean and time‐dependent salt fluxes show nearly identical patterns as the respective mass fluxes because the salt fluxes are almost governed by current velocity fields. The instantaneous and mean salt fluxes across each boundary were very well approximated by the instantaneous and mean volume transports across the boundary times the deployment average salinity across that boundary, respectively. The Ocean Margins Program (OMP) moored current and salinity observations appear sufficient to make estimates of the mean and time‐dependent mass and salt balance.

Volume transport of the Malvinas Current: Can the flow be monitored by TOPEX/POSEIDON?

Current meter measurements were collected between 40° and 41°S in the Malvinas (Falkland) Current from December 1993 to June 1995. Owing to the premature failure of a mooring, a reliable volume transport time series could only be calculated for 254 days, leading to a mean transport of about 41.5 Sv with a standard deviation of 12.2 Sv. This time series is tentatively extended to 386 days. It is also shown that the TOPEX/POSEIDON altimeter, combined with the statistical information on the vertical structure of the current provided by the current meters, can be used to sensitively monitor the flow. A 3‐year‐long time series of transport is derived, which is well correlated (0.8) to the transport estimated from current meters, including about 60% of the variance of the flow at periods beyond 20 days; this series makes it possible to consider a relatively broad spectral range. Dominant periods are 50–80 days and close to 180 days. Interannual variations are large. Comparatively, little energy is found at the annual period, suggesting that the Malvinas Current has only little impact on the annual migrations of the confluence. The predominance of a semiannual cycle is compatible with a remote forcing of the flow suggested by previous numerical studies.

On the deep western-boundary current in the Southwest Pacific Basin

The principal system of deep western-boundary currents in the subtropical South Pacific is that in the Southwest Pacific Basin, which transports Circumpolar Deep Water northward and Pacific Deep Water (at mid-depths) southward. The WOCE PCM9 current-meter array was placed across this system at Lat. 32° 30′S in order to measure the mean transports of those components and their variations. The array, consisting of 60 current meters on 20 moorings, extended 1000 km eastward from the Tonga–Kermadec Ridge, and remained in place for 22 months, between February 1991 and December 1992. The instruments were situated approximately at 2500 m, 4000 m, and close to the bottom. CTD sections (including dissolved oxygen and nutrients) were occupied along the array during its deployment and recovery, and, in between, by the WOCE transpacific section P6. Density sections were used to construct objectively-mapped fields of geostrophic velocity, which were adjusted using current-meter data as integration constants to provide snapshots of the full velocity field. The resulting adjusted transports sometimes differed substantially from relative geostrophic transports, but agreed quite well with transports calculated from current records alone. A time series of volume transport was derived from objectively-mapped three-day-averaged currents. The boundary-current system at PCM9 was essentially 700 km wide, with flow most intense on the flank of the Tonga–Kermadec Ridge, where the maximum mean velocity vector had a magnitude of 9.6 cm s −1. The time-averaged transport, integrated horizontally across the array and from 2000 m to the bottom, was 16.0×10 6±11.9×10 6 m 3 s −1 northward. Of this roughly 15.8×10 6±9.2×10 6 m 3 s −1 was northward flow of Circumpolar Deep Water, and 0.2×10 6±5.1×10 6 m 3 s −1 was northward flow of Pacific Deep Water. Even for the 22-month mean, however, the velocity field was strikingly banded vertically, and there was little impression of a zero-velocity surface following the demarcation between Circumpolar Deep Water and Pacific Deep Water across the section; only in the horizontally integrated sense was there a correspondence between water masses and the variation of transport with depth. The very large variability in transport is associated with prominent oscillations of periods near 50 days, 20 days, and 10 days, as well as with strong events distributed irregularly across the array that lead to a concentration of spectral energy in a band between 40 and 200 days. The origins of these disturbances are not known. While unexpectedly large changes in the density field near the Tonga–Kermadec Ridge were observed from one cruise to another, the huge fluctuations in transport seemed to be connected more with velocity signals varying only slowly with depth. No measurable changes in water-mass properties were detected by the cruises during the 22 months of deployment, but the salinity was about 0.01 lower at the salinity maximum in the Circumpolar Deep Water than it had been 25 years earlier. The direct, long-term transport measurement suggests that the total upwelling at 2000 m north of 30° S is 13×10 6 m 3 s −1, corresponding to an areally-averaged vertical velocity of 1.0×10 −5 cm s −1. This is substantially smaller than earlier values, and it helps to reduce estimates of the global deep upwelling closer to those of the global deep downwelling. The small value of Pacific Deep Water transport in the boundary-current system, relative to that of Circumpolar Deep Water, implies, within the framework of the Stommel–Arons dynamics, that little of the deep water entering the Pacific from the Antarctic returns southward at mid-depths. If so, then some present-day circulation schemes and budgetary constructions need to be re-assessed.

Volume Transport of the Antarctic Circumpolar Current from Bottom Pressure Measurements

Meausrements from bottom pressure gauges located at the north and south sides of Drake Passage are used to extend the one-year time series of volume transport of the Antarctic Circumpolar Current (ACC) given previously by Whitworth. A small error in that paper is corrected, and a revised transport time series is presented which shows the importance of including the transport in the northern and southern margins of Drake Passage. Direct measurements of vertical shear averaged across the passage, are in good agreement with geostrophic shear and suggest that the ACC is in geostrophic balance. Although most of the ACC transport is in the baroclinic field, transport fluctuations are mainly barotropic. Transport estimates based on pressure difference alone differ from the estimates of Whitworth by less than 10 × 106 m3 s−1. Fluctuations in transport of almost half of the mean value occur over periods as short as two weeks.

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 degrees 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.