Gravest Empirical Mode Research Articles

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.

The North Atlantic Current and its Volume and Freshwater Transports in the Subpolar North Atlantic, Time Period 1993–2016

AbstractThe North Atlantic Current (NAC) supplies the subpolar gyre with warm and saline water from the subtropics as part of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). In the context of climate changes, the North Atlantic Ocean is one of the key regions to investigate the variability of the overturning circulation in which salinity and freshwater variability are playing a central role. Through the gravest empirical mode (GEM) method, we reconstructed salinity and velocity fields in the same spatiotemporal resolution as the sea surface height (SSH) product. The time series of salinity, freshwater, and volume transport are characterized by strong interannual variability related to most of the recently developed subpolar gyre indices. The variability of the freshwater transport in the study area is as high or even higher than the freshwater fluxes from the Arctic Ocean and thus needs to be considered for the impact of freshwater on the subpolar North Atlantic. Our analysis also revealed that the NAC import into the subpolar gyre is not only occurring in the western Atlantic close to the western boundary current. One o the NAC branches that forms at the Mann Eddy contributes about 15% of the volume transport and 28% of the freshwater transport crossing from the western into the eastern Atlantic north of 45°N. This branch has not been subject to the strong lateral mixing with freshwater at the western boundary, and thus, the salinity of this NAC pathway is higher than the one at the western boundary.

Sea level anomaly intelligent inversion model based on LSTM-RBF network

Sea level anomaly (SLA) can be decomposed into two contributions: one from changes in mass in the water column (barotropic part) and the other from purely steric changes (baroclinic part) obtained by gravest empirical mode (GEM) lookup table. The establishment of GEM lookup table requires a large amount of data preprocessing (eliminating abnormal data, removing seasonal signals). The relationship between hydrological data (temperature, salinity and depth), vertical acoustic travel time and geopotential height is complex and requires human guidance. In view of the above shortcomings, this paper proposes the neural network-based SLA Intelligent Inversion model to fit the SLA model based on the traditional GEM field. The historical hydrological data is measured by conductivity temperature depth casts in the Eastern Taiwan and the northeastern part of Luzon. During the modeling process, firstly, the long short-term memory (LSTM) network is employed to detect and replace the abnormal data of the hydrological data. Secondly, the radial basis function (RBF) model based on the traditional GEM is constructed. The model can remove the seasonal signals. Besides, it can build the relationship between vertical acoustic travel time and geopotential height. The performance of the model is verified by the data set. It proves that compared with the traditional GEM, the relative error of network based on LSTM-RBF is less than 0.15%. Moreover, it can process abnormal data and remove seasonal signals. This study provides a novel method for preprocessing of hydrological data and SLA inversion based on Pressure-sensor-equipped inverted echo sounders.

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.

Estimating thermohaline variability of the equatorial Pacific Ocean from satellite altimetry

The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode (GEM) diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.

A high resolution salinity time series 1993–2012 in the North Atlantic from Argo and Altimeter data

AbstractThe study of salinity changes has been hampered by the lack of temporal and spatial resolution of the observations. In order to improve the spatial and temporal distribution of salinity observations, we used the Gravest Empirical Mode (GEM) technique to calculate high‐resolution salinity distributions as a function of dynamic height for the period 1993–2012. This technique combined Argo and altimeter data to exploit the relationship between T/S profiles and dynamic height in the North Atlantic. The method was valid in the upper 700 m mainly at and near the pathways of the North Atlantic Current (NAC), but failed in regions with weak stratification or with ambiguities in the T/S relationships. Coherent, multiannual large‐scale variability was observed, with many features present in all regions, albeit with weaker amplitudes in the eastern basins. Some of the interannual features in the northeastern Atlantic basins were unrelated to the variability further south and west, pointing to an occasional advection of subtropical water in the eastern Atlantic. Origin and advection of salinity anomalies with the NAC from the North American Basin into the western subpolar North Atlantic are correlated with the state of the North Atlantic Oscillation (NAO) and dampened by the surface freshwater fluxes. Other mechanisms influencing the salinity pattern are the changing location of the subpolar front, also related to the NAO. The large multiyear variability in the 20 year time series obscured any potential trends caused by global warming. Only the Rockall Trough showed a salinity increase of 0.03 per decade.

Water mass characteristics in the western North Pacific based on a streamfunction projection

A streamfunction projection method called gravest empirical mode (GEM) is applied to the hydrographic section at 137°E to filter out eddy noises in the western North Pacific and derive quantitative ensemble-average water mass properties in the North Equatorial Current region. The GEM fields capture more than 80% of total property variances in the thermocline layer. The core layer structures of key water masses, including the North Pacific Tropical Water (NPTW) and the North Pacific Intermediate Water (NPIW), are examined with a definition of water mass boundary based on property gradient. It shows that a tongue of maximal root-mean-square (RMS) residual exists in the upper half of NPIW for all water properties. These subsurface RMS tongues appear to be close to sharp property gradients. It is the first time a GEM diagnosis is applied to nutrient data, which reveals a drastic difference of N/P reaction rate ratio above and below the maximal-nutrient core at 1250 m. Additionally, a GEM velocity reconstruction successfully produces the North Equatorial Undercurrent (NEUC), demonstrating the stable thermal-wind nature of this newly-discovered current.

The bimodality of the Luzon Strait deep water

Combined conductivity-temperature-depth (CTD) casts and Argo profiles, 3 086 historical hydrocasts were used to quantify the water column characteristics in the northern South China Sea (SCS) and its adjacent waters. Based on a two-dimensional “gravest empirical mode” (GEM), a gravitational potential (ϕ, a vertically integrated variable) was used as proxy for the vertical temperature profiles T G(p,ϕ).ϕ integrated from 8 MPa to the surface shows a close relationship with the temperature, except in the deep layer greater than 15 MPa, which was caused by the bimodal deep water in the region. The GEM temperature profiles successfully revealed the bimodality of the Luzon Strait deep water, that disparate hydrophic vertical profiles can produce distinct specific volume anomaly (δ) in the SCS and the western Philippine Sea (WPS), but failed in the Luzon Strait, where different temperature profiles may produce a same δ. A significant temperature divergence between the SCS water and the WPS water confirmed that the bimodal structure is strong. The deepwater bifurcation starts at about 15 MPa, and gets stronger with increasing depth. As the only deep channel connecting the bimodal-structure waters, water column characteristics in the Luzon Strait is in between, but much closer to the SCS water because of its better connectivity with the SCS. A bimodal temperature structure below 15 MPa reveals that there was a persistent baroclinic pressure gradient driving flow through the Luzon Strait. A volume flux predicted through the Bashi Channel with the hydraulic theory yields a value of 5.62×106 m3/s using all available profiles upstream and downstream of the overflow region, and 4.03×106 and 2.70×106 m3/s by exclusively using the profiles collected during spring and summer, respectively. No volume flux was calculated during autumn and winter because profiles are only available for the upstream of the Bashi Channel during the corresponding period.

Frontal movements and property fluxes: Contributions to heat and freshwater trends in the Southern Ocean

[1] We examine the synoptic variability of temperature and salinity in the Southern Ocean using a static gravest empirical mode (GEM) mapping based on historical hydrography and a time-evolving projection of the GEM created using satellite altimetry (satGEM). The GEM and satGEM projections allow the separation of observed trends into a component due to a shift of the circumpolar fronts (adiabatic) using the satGEM and a component due to changes in water masses (diabatic) expressed as a temporal trend in residual between historical hydrography and the static GEM. The mean southward movement of the Antarctic Circumpolar Current (ACC) fronts drives an adiabatic warming of 1.196 ± 0.090 W m−2 distributed over most of the ACC and at all depths. This is strongest where the meridional temperature gradient is largest, such as in the upper 1000 dbar of the Subantarctic Front (SAF). There is a weak adiabatic freshening of 6.57 ± 0.18 mm yr−1 m−2, concentrated mainly below the Antarctic Intermediate Water and south of the SAF. Residuals between historical hydrography and the static GEM field, driven by diabatic changes in the water mass structure, have temporal trends in temperature and salinity. When these trends are integrated over the whole ACC there is a net cooling of −0.628 ± 0.129 W m−2 and strong freshening of 30.27 ± 0.70 mm yr−1 m−2. By combining the diabatic and adiabatic components and integrating over the ACC the net increase in heat and freshwater in the region is 0.570 ± 0.099 W m−2 and 36.91 ± 0.72 mm yr−1 m−2, respectively. The sum of the adiabatic and diabatic changes are consistent with previous trends inferred from observations.

Estimating the Four-Dimensional Structure of the Southern Ocean Using Satellite Altimetry

Abstract A gravest empirical mode (GEM) projection of temperature and salinity fields over the circumpolar Southern Ocean is presented and is used in combination with satellite altimetry to produce gridded, full-depth, time-evolving temperature, salinity, and velocity fields. Optimal interpolation of historical hydrography, including Argo floats, is used to produce GEM projections of the circumpolar temperature and salinity fields. Parameterizing these fields by dynamic height allows the use of altimetric SSH values from 1992–2006 to create synoptic temperature and salinity fields at weekly intervals on a ⅓° grid at 36 depth levels. The satellite-derived temperature and salinity fields generally capture over 90% of the property variance below the thermocline, with RMS residuals of 1.16°C and 0.132 in salinity at the surface, decreasing to less than 0.45°C and 0.05 below 500 dbar. The combination of altimetry with the GEM fields allows the resolution of the subsurface structure of the filamentary fronts and eddy features. Velocity fields derived from the time-evolving temperature and salinity fields reproduce the Antarctic Circumpolar Current (ACC) velocity structure well, and are strongly correlated (r > 0.7) with in situ measurements from current meters and drifters, with RMS velocity residuals of 4.8–14.8 cm−1 in the Subantarctic Front.

Stream-coordinate structure of oceanic jets based on merged altimeter data

The jet structure of the Southern Ocean front south of Australia is studied in stream-coordinate with a new altimeter product—Absolute Dynamic Topography (ADT) from AVISO. The accuracy of the ADT data is validated with the mooring data from a two-year subantarctic-front experiment. It is demonstrated that the ADT is consistent with in-situ measurements and captures the meso-scale activity of the Antarctic Circumpolar Current (ACC). Stream-coordinate analysis of ADT surface geostrophic flows finds that ACC jets exhibit large spatio-temporal variability and do not correspond to particular streamfunction values. In the circumpolar scope ACC jets display a transient fragmented pattern controlled by topographic features. The poleward shift of jet in streamfunction space, as revealed by a streamwise correlation method, indicates the presence of meridional fluxes of zonal momentum. Such cross-stream eddy fluxes concentrate the broad ACC baroclinic flow into narrow jets. Combined with a recent discovery of gravest empirical mode (GEM) in the thermohaline fields, the study clarifies the interrelationship among front, jet and streamfunction in the Southern Ocean.

South Atlantic mass transports obtained from subsurface float and hydrographic data

Mean total (barotropic + baroclinic) mass transports of the oceanic top 1000 dbar are estimated for two regions of the South Atlantic between 18°S and 47°S. These transports are obtained by using Gravest Empirical Mode (GEM) fields calculated from historical hydrography with temperature and position data from quasi-isobaric subsurface floats deployed from 1992 through 2001. The float-GEM- estimated total mass transports reveal a Brazil Current with a southward flow of 20.9 Sv at 30°S and 46 Sv at 35°S (1 Sverdrup, Sv = 10 6 m

An altimetry‐based gravest empirical mode south of Africa: 1. Development and validation

Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a two‐dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root‐mean‐square (RMS) error is 0.15°C, 0.02, and 0.02 kg m−3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM‐based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM‐produced cross‐sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s−1. The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010).

Comparison of sea surface dynamic heights estimated from inverted echo sounder data and satellite altimeter data

Sea surface dynamic height (SSDH) anomalies estimated from long-time inverted echo sounder (IES) data using gravest empirical mode method are compared with satellite altimeter data. The IES-derived SSDH anomalies agree well with the altimetric SSDH anomalies, although the IES-derived SSDH variation is slightly underestimated. We calculate power spectra of these SSDH anomalies to discuss time-scale dependency of their differences. The differences between two SSDH anomalies are found mainly caused by components with the periods from 50 days to 100 days, which correspond to meso-scale eddies or small meanders of the Kuroshio. These differences result in smaller variation of the IES-derived SSDH.

A Multi-Index GEM Technique and Its Application to the Southwestern Japan/East Sea

Abstract This paper demonstrates a new gravest empirical mode (GEM) technique that constructs multi-index lookup tables of temperature (T) and specific volume anomalies (δ) using historical hydrocasts as a function of three indices: round-trip travel time (τ) from sea floor to the surface, sea surface temperature, and pressure. Moreover, the historical hydrocasts are separated into non-mixed-layer (NML) and mixed-layer (ML) groups, and a single GEM field is constructed for each group. This is called the MI-GEM technique. The appropriate dates for MI-GEM fields are determined by the monthly distribution of the number of NML and ML profiles in the historical hydrocasts, which are also well correlated with the strength of the winds during the 2 yr of observations. The T and δ profiles that are determined by this MI-GEM technique capture 92% and 88% of the T and δ variances in the depth range of 0–200 db. These values reduce by about one-third of the unexplained error variance of the residual GEM, which was recently developed and applied to the optimal interpolated τ data in the southwestern Japan/East Sea (JES) by Mitchell et al. Comparisons with the in situ CTD casts demonstrate that the MI-GEM technique almost always produces improved full water column profiles of T and δ. Whereas the residual GEM estimates had exhibited qualitatively erroneous features like T inversions in the near–surface layer and too thin or thick intermediate water layers in some regions, the MI-GEM estimates avoid those problems, which were inherent to the residual GEM technique in the southwestern JES.

The Residual GEM Technique and Its Application to the Southwestern Japan/East Sea

Abstract The standard gravest empirical mode (GEM) technique for utilizing hydrography in concert with integral ocean measurements performs poorly in the southwestern Japan/East Sea (JES) because of a spatially variable seasonal signal and a shallow thermocline. This paper presents a new method that combines the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) static climatology (which implicitly contains the mean seasonal signal) with historical hydrography to construct a “residual GEM” from which profiles of such parameters as temperature (T) and specific volume anomaly (δ) can be estimated from measurements of an integral quantity such as geopotential height or acoustic echo time (τ). This is called the residual GEM technique. In a further refinement, sea surface temperature (SST) measurements are included in the profile determinations. In the southwestern JES, profiles determined by the standard GEM technique capture 70% of the T variance and 64% of the δ variance, while the residual GEM technique using SST captures 89% of the T variance and 84% of the δ variance. The residual GEM technique was applied to optimally interpolated τ measurements from a two-dimensional array of pressure-gauge-equipped inverted echo sounders moored from June 1999 to July 2001 in the southwestern JES, resulting in daily 3D estimated fields of T and δ throughout the region. These estimates are compared with those from direct measurements and good agreement is found between them.

Decadal Current Variations in the Southwestern Japan/East Sea

Absolute geostrophic velocities were calculated along TOPEX/Poseidon (T/P) groundtracks located in the Ulleung Basin of the southwestern Japan/East Sea (JES) from a combined analysis of nearly a decade of T/P data and two years of pressure-gauge-equipped inverted echo sounder (PIES) data obtained during the United States Office of Naval Research’s JES Program. Geostrophic velocities have been calculated daily for the Ulleung Basin from June 1999 to July 2001 from a three-dimensional mapping of temperature and salinity produced by PIES data interpreted via the Gravest Empirical Mode (GEM) technique combined with the Navy’s Modular Ocean Data Assimilation System (MODAS). These velocities were then used to convert T/P velocity anomalies to absolute velocities for the T/P time period of 1993 to 2002. Current intensities and variabilities associated with the East Korean Warm Current, Ulleung Warm Eddy, and Offshore Branch are examined. Spatial and temporal variations of the sea surface circulation are strong. Intensification of the currents generally occurred during the fall season. The flow pattern in individual years differed greatly from year to year and differed from climatology in important qualitative ways.

Transformation of the Warm Waters of the North Atlantic from a Geostrophic Streamfunction Perspective

To obtain a description of the hydrographic state of the North Atlantic Current‐subpolar front (NAC‐SPF) system, historical hydrographic data from the subpolar North Atlantic are projected into a baroclinic streamfunction space, resulting in three-dimensional Gravest Empirical Mode (GEM) fields for temperature and specific volume anomaly, parameterized by pressure, dynamic height, and day of the year. From the specific volume anomaly GEM, the corresponding potential vorticity field is calculated. These fields are constructed for 12 subregions, chosen to follow the mean path of the NAC‐SPF system. Analysis of the seasonal potential vorticity cycle of the GEM fields shows that the main mechanism for the formation of Subpolar Mode Water is winter convection. The GEM fields are also used to obtain the approximate location of formation sites for the different Subpolar Mode Water classes. The evolution of the mean fields for the waters is studied along baroclinic streamlines of the NAC‐SPF system. This shows that cross-frontal mixing, between the cold and fresh subpolar waters and the salty and warm waters coming north from the subtropics via the Gulf Stream, is the dominant mechanism for the light-to-dense transformation process of the NAC‐SPF waters that enter the western subpolar region. On the other hand, a combination of atmospheric cooling, vertical mixing during wintertime convection, and entrainment of the saltier waters found on the northeastern subtropical gyre is the main factor transforming the NAC‐SPF waters that enter the eastern subpolar gyre. This suggests that an influx along the eastern margin of salty water from the European Basin plays a significant role in the transformation of the NAC‐SPF waters that continue their way toward the Nordic seas.

A Method for Obtaining the Mean Transports of Ocean Currents by Combining Isopycnal Float Data with Historical Hydrography

This article presents a method for obtaining the mean structure of the temperature, specific volume anomaly, and velocity of an ocean current, using isopycnal float data combined with gravest empirical mode (GEM) fields calculated from historical hydrography. A GEM field is a projection on a geostrophic streamfunction space of hydrographic data, which captures most of the vertical structure associated with frontal regions. This study focuses on the North Atlantic Current–subpolar front (NAC–SPF) current system, but the float–GEM method has broad applicability to baroclinic ocean currents in general. The NAC–SPF current system is of climatic interest, being an important conduit of warm salty waters into the northern North Atlantic. It constitutes the upper limb of the thermohaline circulation of the Atlantic Ocean and plays a crucial role in the moderation of European climate, but uncertainties regarding its transport and corresponding heat fluxes remain, mainly because the structure of the system is not well known. This paper shows how isopycnal floats can be used to obtain such estimates. The performance of the float–GEM method is tested in two ways. First, two synoptic hydrographic sections (one across the NAC and the other across the SPF) are reconstructed from simulated isopycnal float pressure measurements. The baroclinic transports of volume and temperature (relative to 1000 dbar) across the sections are well reproduced by the method: the float–GEM transport estimates have an accuracy of ±20% and a precision of ±15% or less, which result in deviations of less than ±10% from the “real” values. In the second test, horizontal maps of pressure and temperature on the δ = −12.7 × 10−8 m3 kg−1 specific volume anomaly surface (σθ ≈ 27.5 kg m−3) are produced, using RAFOS float data from two experiments that sampled the region from 1993 to 2000. These maps compare well with similar maps constructed in previous studies and establish the consistency of the method. The good performance of the float–GEM method gives confidence in this novel way of using isopycnal floats to obtain information on the structure of the ocean. Combined with the velocity measured by the floats, it has the potential to estimate absolute transports and heat fluxes along the NAC–SPF system.

Structure of the North Atlantic current in stream-coordinates and the circulation in the Newfoundland basin

Accurate knowledge of the absolute North Atlantic Current (NAC) transport has been limited in the past by the difficulty of obtaining absolute velocity measurements. For this study, historical hydrography was combined with time series of round trip travel time from inverted echo sounders, bottom pressure sensor measurements, and measured velocities from deep current meters, via a “Gravest Empirical Mode” technique, to yield time series of temperature and absolute velocity sections. These sections were used to determine the mean NAC stream-coordinates structure of temperature and absolute velocity for the period from August 1993 to February 1995. This structure is narrower and stronger than the corresponding Eulerian average, with peak speeds greater than 80 cm s −1 and bottom velocities exceeding 10 cm s −1 . The calculated mean NAC transport was 131±14 (×10 6 m 3 s −1 ); temporal variations resulted in a 41×10 6 m 3 s −1 standard deviation. The stream-coordinates estimate of mean absolute transport was found to be about 10% smaller than the corresponding Eulerian mean transport of 146×10 6 m 3 s −1 as a result of partially summing the north and south transports within the Mann Eddy, a large semi-permanent anticyclonic eddy normally located just offshore of the NAC. Most transport sections across the NAC in this region include the northward transport of the inshore side of the Mann Eddy because it is difficult to discriminate between the two flows. Historical estimates place the Mann Eddy absolute transport around 50–60×10 6 m 3 s −1 ; thus, the transport crossing this transect that can be identified with the NAC is about 90×10 6 m 3 s −1 . The barotropic component of the transport (defined as the bottom velocity multiplied by the water depth) is found to contribute at least 30% of the total absolute transport, depending on the choice of baroclinic reference level. Because a time series of transport measurements was made in this study, the accuracy of the mean transport estimate is better than previous estimates, which have all been based on snapshot measurements. From a combination of these measurements with other published measurements, a scheme for the overall circulation and transport in the Newfoundland Basin is suggested.