Eighteen Degree Water Research Articles

Ekman Transport as the Driver of Extreme Interannual Formation Rates of Eighteen Degree Water

AbstractIn the North Atlantic subtropical gyre, the Eighteen Degree Water (EDW) is a voluminous heat reservoir, submerged under a seasonal pycnocline that can be progressively removed through the winter, allowing EDW ventilation in the early spring. We target the EDW formation extremes, namely 2004–2005, 2009–2010, and 2012–2013 for the strong years, and 2007–2008, 2008–2009, 2011–2012, and 2013–2014 for the weak years. We employ gridded hydrographic datasets mainly measured by Argo floats over the last 20 years, and provide a synthetic study on the extreme events of strong and weak EDW formation of this time period. We found that the Ekman transport is the indicator and driving mechanism explaining these extremes. Strong (Weak) EDW formation years correspond with atmospheric patterns resembling NAO− (NAO+), attributed to a strong (weak) winter air‐sea surface heat loss, and a strong (weak) winter heat loss due to Ekman transport. Further, we show that such extreme Ekman advection patterns can be linked to mid‐latitude storms, of which both intensity and duration have an impact on the extreme of EDW ventilation in the western subtropical North Atlantic. To yield a strong EDW formation, it requires a large winter heat deficit due to Ekman divergence, which can be sufficiently represented by numbers of strong winter storms, most notably, remnants of hurricanes and US east coast snowstorms. Meanwhile, to yield a weak EDW formation, apart from weak atmospheric forcings, a remnant positive heat content anomaly carried through from previous years would serve as an unfavorable preconditioning, hindering the EDW formation.

Oxygen Seasonality, Utilization Rate, and Impacts of Vertical Mixing in the Eighteen Degree Water Region of the Sargasso Sea as Observed by Profiling Biogeochemical Floats

AbstractSeasonal oxygen structure and utilization in the Sargasso Sea are characterized using nine profiling floats with oxygen 2021 sensors (years 2005–2008), deployed in an Eighteen Degree Water (EDW) experiment (CLIMODE). During autumn‐winter when the mixed layer is deepening, oxygen increases from the surface to the base of the EDW at 400 m. During spring‐summer, oxygen decreases except between the seasonal pycnocline and compensation depth, creating the seasonal shallow oxygen maximum layer (SOMax) with oxygen production of 0.04 μmol kg−1·day−1. In the underlying seasonal oxygen minimum (SOMin), the oxygen utilization rate (OUR) is 0.10 μmol kg−1·day−1, decreasing with depth to 0.04 μmol kg−1·day−1 in the EDW. Remineralization in May to August is double that of August to November. The Sargasso Sea is a net carbon producer; estimated annual export production from the top 100–250 m is 2.9 mol C m−2 and from the top 400 m is 4.2 mol C m−2. Below the EDW, oxygen decreases seasonally at the same time as in the EDW, indicating remineralization down to 700 m. However, on isopycnals in this deeper layer, oxygen increases during May to September, likely due to lateral advection from nonlocal surface outcrops. Summer shoaling of these isopycnals creates this paradox. The complex vertical oxygen structure in the upper 200 m enables important vertical diffusive flux that modifies the OUR calculated from oxygen change. Ignoring mixing underestimates maximum remineralization by 19% and underestimates maximum net production by 88%. However, vertical mixing is negligible in the deeper layers, so the associated total integrated remineralization error is 5%–9%.

Source waters contribution to the tropical Atlantic central layer: New insights on the Indo-Atlantic exchanges

This study reveals the influence of the Subtropical Mode Water (STMW) varieties and their spatiotemporal variability on the upper thermocline of the tropical Atlantic Ocean. Optimum Multiparameter water mass analysis was applied using the hydrographic and chemical datasets available from the World Ocean Database 2018 and The Global Ocean Ship-Based Hydrographic Investigations Program. Eighteen Degree Water (EDW) and Madeira Mode Water (MMW), sourced in the northwest and northeast North Atlantic, respectively, contribute to the North Atlantic Central Water (NACW) layer, which follows the North Equatorial Current and setting up the limit with the South Atlantic Central Water (SACW) at about 10°N. The contribution of the EDW to the NACW spreads at the surface layer (excluding the first 100 m) until the 26.6 kg m−3 isopycnal (~250 m). MMW contributes to denser levels of the upper layer, mainly between 26.8 and 27.0 kg m−3 (300–700 m). In the southern hemisphere, mode waters formed in the eastern South Atlantic (STMW18) and along the edge of the subtropical front (STMW12) were the main contributors to the surface and denser levels, respectively. The variety formed in the Brazil-Malvinas confluence (STMW14) was restricted to the southwestern side of the South Atlantic gyre, while the Subtropical Indian Mode Water (STIMW; mode water advected from the Indian Ocean into the South Atlantic Ocean) influenced the SACW structure between 20°S and 30°S with an averaged mixing fraction of ~34% ± 20%, mainly at isopycnals of 26.2 and 26.4 kg m−3 (100–400 m). The main source of the STIMW, the Agulhas eddies, act as interocean exchange structures that transport an average of 5.5 ± 3.2 Sv (1 Sv = 106 m3 s−1) of STIMW to the Atlantic, even after they have been altered by splitting and/or merging processes. In addition, a smaller contribution of ~20% of STIMW was associated with the North Brazil Current at ~350 m, showing the interhemispheric connection between the Indian Ocean and the upper branch of the Atlantic Meridional Overturning Circulation. Therefore, due to the previously reported increase of the Agulhas leakage and the dynamic of the southern Equatorial Current bifurcation, the heat and salt fluxes to the northern hemisphere may likely increase in the following years.

Observations of cross-frontal exchange associated with submesoscale features along the North Wall of the Gulf Stream

Using high-resolution measurements of the Gulf Stream North Wall (GSNW), we investigated whether detachments from the warm current at the edge of the front, known as streamers, affect the overall heat and salt content of the region. Temperature, salinity, and velocity data were collected across the front from towed CTDs, shipboard ADCPs, and gliders following a Lagrangian drifter that was deployed at the GSNW during winter 2012. Four streamers were identified, all of which expanded laterally 10–15 km, with vertical salinity interleaving down to a 200 m depth. We observed that temperature and salinity (T/S) increased along the trajectory of the Lagrangian float. These trends were density compensated and ranged from the surface down to a 200 m depth and across a 5 km band. The heat and salt budget analysis showed that surface fluxes, advection due to large-scale circulation, and diapycnal mixing could not explain the observed increase in T/S in the mixed layer and in the subsurface area. The only possible source that could explained the increase in T/S was along-isopycnal mixing. Estimates of the Reynolds transport supported this conclusion, although the low number of realizations meant these estimates were not statistically significant. From the heat and salt budgets, we observed that an along-isopycnal diffusivity, κi, of 110±30m2s−1 accounted for the estimated residual and matched the Reynolds transport estimates. This value of κi is consistent with other studies that assert that lateral mixing is required for the production of Eighteen Degree Water (EDW) subtropical mode water.

Year-to-Year Reoutcropping of Eighteen Degree Water in an Eddy-Resolving Ocean Simulation

AbstractWinter outcropping of the Eighteen Degree Water (EDW) and its subsequent dispersion are studied using a ° eddy-resolving simulation of the Family of Linked Atlantic Modeling Experiments (FLAME). Outcropped EDW columns in the model simulations are detected in each winter from 1990 to 1999, and particles are deployed in the center of each outcropped EDW column. Subsequently, the trajectories of these particles are calculated for the following 5 yr. The particles slowly spread away from the outcropping region into the nonoutcropping/subducted EDW region south of ~30°N and eventually to the non-EDW region in the greater subtropical gyre. Approximately 30% of the particles are found in non-EDW waters 1 yr after deployment; after 5 yr, only 25% of the particles are found within EDW. The reoutcropping time is defined as the number of years between when a particle is originally deployed in an outcropping EDW column and when that particle is next found in an outcropping EDW column. Of the particles, 66% are found to reoutcrop as EDW in 1 yr, and less than 5% of the particles outcrop in each of the subsequent 4 yr. While the individual trajectories exhibit significant eddy-like motions, the time scale of reoutcropping is primarily set by the mean circulation. The dominance of reoutcropping in 1 yr suggests that EDW outcropping contributes considerably to the persistence of surface temperature anomalies from one winter to the next, that is, the reemergence of winter sea surface temperature anomalies.

The Fate of North Atlantic Subtropical Mode Water in the FLAME Model

Abstract North Atlantic Subtropical Mode Water, also known as Eighteen Degree Water (EDW), has the potential to store heat anomalies through its seasonal cycle: the water mass is in contact with the atmosphere in winter, isolated from the surface for the rest of the year, and reexposed the following winter. Though there has been recent progress in understanding EDW formation processes, an understanding of the fate of EDW following formation remains nascent. Here, particles are launched within the EDW of an eddy-resolving model, and their fate is tracked as they move away from the formation region. Particles in EDW have an average residence time of ~10 months, they follow the large-scale circulation around the subtropical gyre, and stratification is the dominant criteria governing the exit of particles from EDW. After sinking into the layers beneath EDW, particles are eventually exported to the subpolar gyre. The spreading of particles is consistent with the large-scale potential vorticity field, and there are signs of a possible eddy-driven mean flow in the southern portion of the EDW domain. The authors also show that property anomalies along particle trajectories have an average integral time scale of ~3 months for particles that are in EDW and ~2 months for particles out of EDW. Finally, it is shown that the EDW turnover time for the model in an Eulerian frame (~3 yr) is consistent with the turnover time computed from the Lagrangian particles provided that the effects of exchange between EDW and the surrounding waters are included.

How Well Do Climate Models Reproduce North Atlantic Subtropical Mode Water?

Abstract Formation and the subsequent evolution of the subtropical mode water (STMW) involve various dynamic and thermodynamic processes. Proper representation of mode water variability and contributions from various processes in climate models is important in order to predict future climate change under changing forcings. The North Atlantic STMW, often referred to as Eighteen Degree Water (EDW), in three coupled models, both with data assimilation [GFDL coupled data assimilation (GFDL CDA)] and without data assimilation [GFDL Climate Model, version 2.1 (GFDL CM2.1), and NCAR Community Climate System Model, version 3 (CCSM3)], is analyzed to evaluate how well EDW processes are simulated in those models and to examine whether data assimilation alters the model response to forcing. In comparison with estimates from observations, the data-assimilating model gives a better representation of the formation rate, the spatial distribution of EDW, and its thickness, with the largest EDW variability along the Gulf Stream (GS) path. The EDW formation rate in GFDL CM2.1 is very weak because of weak heat loss from the ocean in the model. Unlike the observed dominant southward movement of the EDW, the EDW in GFDL CM2.1 and CCSM3 moves eastward after formation in the excessively wide GS in the models. However, the GFDL CDA does not capture the observed thermal response of the overlying atmosphere to the ocean. Observations show a robust anticorrelation between the upper-ocean heat content and air–sea heat flux, with upper-ocean heat content leading air–sea heat flux by a few months. This anticorrelation is well captured by GFDL CM2.1 and CCSM3 but not by GFDL CDA. Only GFDL CM2.1 captures the observed anticorrelation between the upper-ocean heat content and EDW volume. This suggests that, although data assimilation corrects the readily observed variables, it degrades the model thermodynamic response to forcing.

The contributions of atmosphere and ocean to North Atlantic Subtropical Mode Water volume anomalies

Subtropical Mode Water (STMW) in the western North Atlantic Ocean, or Eighteen Degree Water (EDW), as it is commonly known, is formed near the Gulf Stream in the wintertime, is dissipated by mixing and is removed by subduction. The ability of EDW to store and discharge large quantities of heat over periods of several years contributes to the memory of the climate system. To complement the 2-year field and modeling program of the CLIvar MOde Water Dynamics Experiment (CLIMODE) with a perspective of interannual-to-decadal variability (1985–2007), we used a simple box model to hindcast observed EDW volume anomalies in two regions: one in which EDW is formed and an adjacent region of subducted EDW. Estimates of the relative contributions of heat flux anomalies, vertical mixing from Ekman advection, mixing, and circulation are examined using proxy variables derived from winds, sea surface temperature, hydrographic data and altimetric sea level. The importance of each process is evaluated by its contribution to observed EDW volume anomalies in two regions. The study produced some robust conclusions: (1) anomalies of formation by surface heat fluxes are clearly reflected in EDW volume anomalies with some contributions by Ekman advection; (2) of the newly formed EDW about 65% is lost by mixing and about 35% is transferred to the subducted region; (3) mixing losses are well parameterized by the meandering of the nearby GS and (4) transfer and losses from the subducted region can be parameterized by the geostrophic surface flow.

Eighteen Degree Water formation within the Gulf Stream during CLIMODE

Analysis of wintertime CLIMODE data for 2007 indicates that a substantial portion of new Eighteen Degree Water (EDW) is likely ventilated within the eastward flowing Gulf Stream (GS) between 67°W and 52°W longitudes, possibly exceeding that formed elsewhere in the northern Sargasso Sea. Use of some global air–sea interaction data sets applied to the study region for Feb/Mar of 2007 indicate that this winter may have been anomalously energetic in air–sea exchange compared to the mean of the prior 19yr. The largest heat and freshwater fluxes found directly over the meandering warm core of the Gulf Stream are capable of removing most of the subtropical heat anomaly of the GS, but cross-frontal fluxes of salinity are required to account for the observed regional salinity structure. An isopycnal diffusivity of ∼100m2s−1 is inferred from the salinity balance. This mixing would also account for the observation that EDW formed in the GS is slightly fresher than that formed in northern Sargasso Sea. The lateral flux of heat across the GS north wall also acts to cool the resulting EDW water, but the heat balance for EDW production is largely determined from GS advection and air–sea fluxes, in contrast to salinity. Based on oxygen saturation data, we estimate that 1.8–3.0Sv-yr of new EDW is formed in the GS for the winter of 2007. EDW originating from the GS is generated in a separate location from where it is accumulated in the northern Sargasso Sea. This manner of EDW formation will produce unique characteristics of EDW found in the northern Sargasso Sea: ones that differ in T/S properties from that formed south of the GS under the more traditional 1D, cooling-driven convection process.

A comparison of North Pacific and North Atlantic subtropical mode waters in a climatologically-forced model

Subtropical mode water (STMW), a water mass with homogeneous temperature and density and low potential vorticity, is formed in the subtropical gyres of both the North Pacific and North Atlantic Oceans. In the North Atlantic, this water mass is known as Eighteen Degree Water (EDW), while the corresponding water mass in the Pacific is identified as North Pacific Subtropical Mode Water (NPSTMW). This analysis compares properties of EDW with NPSTMW as well as the intrinsic oceanic variability of both, within the framework of a high resolution model with climatological atmospheric forcing. Interannual variability is evident in both volume and characteristics of EDW and NPSTMW, but the magnitude of variability is small, indicating that observed variability is largely forced by atmospheric changes. Volume of EDW is greater than volume of NPSTMW, but volume variability is similar in the two basins, and is dominated by the seasonal cycle in both cases. The most significant differences are found in the average ages of EDW compared to NPSTMW. Although the volume of STMW formed in each basin is similar, different circulation patterns lead to a higher average age in the North Atlantic, as EDW is more likely to be advected away from the formation region and remain subducted in the North Atlantic, while NPSTMW is more likely to be reventilated the following winter in the North Pacific.

Direct observation of subtropical mode water circulation in the western North Atlantic Ocean

Eighteen Degree Water (EDW) is the dominant subtropical mode water of the North Atlantic subtropical gyre and is hypothesized as an interannual reservoir of anomalous heat, nutrients and CO2. Although isolated beneath the stratified upper-ocean at the end of each winter, EDW may re-emerge in subsequent years to influence mixed layer properties and consequently air–sea interaction and primary productivity. Here we report on recent quasi-Lagrangian measurements of EDW circulation and stratification in the western subtropical gyre using an array of acoustically-tracked, isotherm-following, bobbing profiling floats programmed to track and intensively sample the vertically homogenized EDW layer and directly measure velocity on the 18.5°C isothermal surface.The majority of the CLIVAR Mode Water Dynamics Experiment (CLIMODE) bobbers drifted within the subtropical gyre for 2.5–3.5 years, many exhibiting complex looping patterns indicative of an energetic eddy field. Bobber-derived Lagrangian integral time and length scales (3 days, 68km) associated with motion on 18.5°C were consistent with previous measurements in the Gulf Stream extension region and fall between previous estimates at the ocean surface and thermocline depth. Several bobbers provided evidence of long-lived submesoscale coherent vortices associated with substantial EDW thickness. While the relative importance of such vortices remains to be determined, our observations indicate that these features can have a profound effect on EDW distribution. EDW thickness (defined using a vertical temperature gradient criterion) exhibits seasonal changes in opposition to a layer bounded by the 17°C and 19°C isotherms. In particular, EDW thickness is generally greatest in winter (as a result of buoyancy-forced convection), while the 17°–19°C layer is thickest in summer consistent with seasonal Ekman pumping. Contrary to previous hypotheses, the bobber data suggest that a substantial fraction of subducted EDW is isolated from the atmosphere for periods of less than 24 months. Seasonal-to-biennial re-emergence (principally within the recirculation region south of the Gulf Stream) appears to be a common scenario which should be considered when assessing the climatic and biogeochemical consequences of EDW.

Evolution and formation of North Atlantic Eighteen Degree Water in the Sargasso Sea from moored data

Two profiling subsurface moorings were deployed as part of the CLIvar MOde Water Dynamics Experiment (CLIMODE) to study the formation and evolution of Eighteen Degree Water (EDW) from November 2006 to November 2007. Both moorings were deployed south of the Gulf Stream in the EDW outcrop region, the northwestern part of the subtropical gyre of the North Atlantic. The two moorings captured the seasonal evolution of EDW characterized by gradual mixed layer deepening and wintertime outcrop, rapid restratification from May to June and slower dissipation during the rest of the year. Superimposed on this seasonal cycle, the moored records are characterized by high frequency passing of eddies with a characteristic time scale of ∼10 days, i.e. it took about 10 days for eddies to pass the mooring sites. The net impact of these eddy fluxes is evaluated by analyzing one-dimensional heat and salt budgets of the upper ocean at the moorings and comparing them to the local air–sea fluxes. It is shown that oceanic lateral fluxes converge heat and salt into the formation region during winter thus offsetting the heat loss to the atmosphere and influencing the formation of EDW. A comparison with results from a one-dimensional model shows that without the lateral fluxes EDW would outcrop earlier and it would be colder and fresher. The warm, salty waters transported into the region originate from the Gulf Stream and this suggests that frontal processes likely play a fundamental role in EDW formation and its evolution.

Sea–air CO2 flux in the North Atlantic subtropical gyre: Role and influence of Sub-Tropical Mode Water formation

The uptake of atmospheric carbon dioxide (CO2) into the mid-latitudes of the North Atlantic Ocean through the production of wintertime Sub-Tropical Mode Water (STMW) also known as Eighteen Degree Water (EDW) is poorly quantified and constrained. Nonetheless, it has been proposed that the EDW could serve as an important short-term sink of anthropogenic CO2. The objective of the present investigation was to determine sea–air CO2 gas exchange rates and seawater CO2 dynamics during wintertime formation of EDW in the North Atlantic Ocean. During 2006 and 2007, several research cruises were undertaken as part of the CLIMODE project across the northwest Atlantic Ocean with the intent to study the pre-conditioning, formation, and the evolution of EDW. Sea–air CO2 exchange rates were calculated based on measurements of atmospheric pCO2, surface seawater pCO2 and wind speed with positive values denoting a net flux from the surface ocean to the atmosphere. Average sea–air CO2 flux calculated along cruise tracks in the formation region equaled −18±6mmol CO2m−2d−1 and −14±9mmol CO2m−2d−1 in January of 2006 and March of 2007, respectively. Average sea–air CO2 flux in newly formed outcropping EDW in February and March of 2007 equaled −28±10mmol CO2m−2d−1. These estimates exceeded previous flux estimates in this region by 40–185%. The magnitude of CO2 flux was mainly controlled by the observed variability in wind speed and ΔpCO2 with smaller changes owing to variability in sea surface temperature. Small but statistically significant difference (4.1±2.6μmolkg−1) in dissolved inorganic carbon (DIC) was observed in two occurrences of newly formed EDW in February and March of 2007. This difference was explained either by differences in the relative contribution from different water masses involved in the initial formation process of EDW or temporal changes owing to sea–air CO2 exchange (∼25%) and vertical and/or lateral mixing (∼75%) with water masses high in DIC from the cold side of the Gulf Stream and/or from below the permanent thermocline. Based on the present estimate of sea–air CO2 flux in newly formed EDW and a formation rate of 9.3Svy (Sverdrup year=106m3s−1 flow sustained for 1 year), CO2 uptake by newly formed EDW may constitute 3–6% of the total North Atlantic CO2 sink. However, advection of surface waters that carry an elevated burden of anthropogenic CO2 that are transported to the formation region and transformed to mode water may contribute additional CO2 to the total net uptake and sequestration of anthropogenic CO2 to the ocean interior.

New perspectives on eighteen-degree water formation in the North Atlantic

In this report, eighteen-degree water (EDW) formation will be discussed, with emphasis on advances in understanding emerging within the past decade. In particular, a recently completed field study of EDW (CLIMODE) is suggesting that EDW formation within a given winter can have at least two different dominant physics and distinct locations: one type formed in the northern Sargasso Sea, largely away from the strong flows of the Gulf Stream where 1D physics may apply, and a second type formed along the southern flank of the Gulf Stream, in a region where the background vorticity of the flow and cross-frontal mixing play key roles in the convective formation process.

Estimated Seasonal Cycle of North Atlantic Eighteen Degree Water Volume

Abstract The seasonal cycle in the volume and formation rate of Eighteen Degree Water (EDW) in the North Atlantic is quantified over the 3-yr period from 2004 to 2006. The EDW layer is defined as all waters that have a temperature between 17° and 19°C. The study is facilitated by a synthesis of various observations—principally Argo profiles of temperature and salinity, sea surface temperature, and altimetry—using a general circulation model as an interpolation tool. The winter increase in EDW volume is most pronounced in February, peaking at about 8.6 Svy, where 1 Svy ≈ 3.15 × 1013 m3 corresponding to a 1 Sv (Sv ≡ 106 m3 s−1) flow sustained for one year. This largely reflects winter EDW formation due to air–sea heat fluxes. Over the remainder of the year, newly created EDW is consumed by air–sea heat fluxes and ocean mixing, which roughly contribute ⅔ and ⅓, respectively. The authors estimate a net annual volume increase of 1.4 Svy, averaged over the 3-yr period. It is small compared to the amplitude of the seasonal cycle (8.6 Svy) and annual formation due to air–sea fluxes (4.6 Svy). The overall EDW layer volume thus appears to fluctuate around a stable point during the study period. An estimate of the full EDW volume budget is provided along with an uncertainty estimate of 1.8 Svy, and largely resolves apparent conflicts between previous estimates.

Mixing rates across the Gulf Stream, Part 1: On the formation of Eighteen Degree Water

Microstructure profiles taken in February 2007 across the Gulf Stream (GS) measured the temporal and spatial variability of the intense mixing that forms Eighteen Degree Water (EDW). Strong winds, gusting to 60 kt, and heat fluxes up to 1000 W m produced moderate-to-strong mixing in the surface mixed layer and the entrainment zone, as well as in the thermocline. In the limit of a vertically balanced heat budget, EDW formation is driven primarily by surface heat loss to the atmosphere across a region extending O(100) km south from the GS core, where entrainment heat fluxes based on dissipation rates were relatively low, O(10~100) W m. Near the GS core, much larger entrainment fluxes, O(100~1000) W m, contribute significantly to cooling the mixed layer, but less so to overall EDW formation due to its smaller volume. Below the mixed layer near the GS, diapycnal diffusivities in the thermocline averaged about O(10) m s , and are approximately 10 times levels previously observed in the GS during other seasons. Banded shear structures in velocity profiles, dominated by shoaling phase coherence and clockwise rotation, indicate that downward-propagating near-inertial waves are responsible for much of this enhanced subsurface mixing.

Subduction on the Northern and Southern Flanks of the Gulf Stream

Abstract Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by ∼90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.

Using Transformation and Formation Maps to Study the Role of Air–Sea Heat Fluxes in North Atlantic Eighteen Degree Water Formation

Abstract The Walin water mass framework quantifies the rate at which water is transformed from one temperature class to another by air–sea heat fluxes (transformation). The divergence of the transformation rate yields the rate at which a given temperature range is created or destroyed by air–sea heat fluxes (formation). Walin’s framework provides a precise integral statement at the expense of losing spatial information. In this study the integrand of Walin’s expression to yield transformation and formation maps is plotted and used to study the role of air–sea heat fluxes in the cycle of formation–destruction of the 18° ± 1°C layer in the North Atlantic. Using remotely sensed sea surface temperatures and air–sea heat flux estimates based on both analyzed meteorological fields and ocean data–model syntheses for the 3-yr period from 2004 to 2006, the authors find that Eighteen Degree Water (EDW) is formed by air–sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The formation rate peaks in February when the EDW layer is thickened by convection owing to buoyancy loss. EDW is destroyed by air–sea heat fluxes from spring to summer over the entire subtropical gyre. In the annual mean there is net EDW formation in the west to the south of the Gulf Stream, and net destruction over the eastern part of the gyre. Results suggest that annual mean formation rates of EDW associated with air–sea fluxes are in the range from 3 to 5 Sv (Sv ≡ 106 m3 s−1). Finally, error estimates are computed from sea surface temperature and heat flux data using an ensemble perturbation method. The transformation/formation patterns are found to be robust and errors mostly affect integral quantities.

The formation rate of North Atlantic Deep Water and Eighteen Degree Water calculated from CFC-11 inventories observed during WOCE

The accumulation of man-made chlorofluorocarbons (CFCs) in subsurface water masses is directly related to their formation rate, and the water mass formation rate can be calculated from its CFC inventory. CFC-11 inventories between 65°N and 10°S in the Atlantic Ocean have been calculated for Eighteen Degree Water (EDW) and the components of North Atlantic Deep Water (NADW) from data collected primarily between 1996 and 1998 as part of the World Ocean Circulation Experiment (WOCE). CFC-11 inventories for individual water masses are 5.4 million moles for EDW, 10.5 million moles for Upper Labrador Sea Water (ULSW) (4.6 million moles south of 42°N), 23.4 million moles for Classical Labrador Sea Water (CLSW), 10.4 million moles for Iceland-Scotland Overflow Water (ISOW), and 8.3 million moles for Denmark Strait Overflow Water (DSOW). The estimated error for these inventories is about ±10%. The sum of the NADW components (ULSW, CLSW, ISOW, DSOW) is 53.2 million moles which is about half of the total CFC-11 inventory, 103.8 million moles, in the North Atlantic Ocean. Maps of water column inventories illustrate the formation mechanisms and spreading pathways within these water masses. The inventories directly reflect the input of newly formed water in the North Atlantic over the time scale of the CFC transient, about 3 decades. The interior regions of the North Atlantic contain most (75–80%) of the CFC-11 inventory in NADW indicating strong recirculation and mixing of newly formed NADW from the DWBC into the interior with a time scale of 2–3 decades. Average water mass formation rates between 1970 and 1997 are: 3.3 Sv for EDW, 3.5 Sv for ULSW (2.0 Sv from the central Labrador Sea and 1.5 Sv from the southern Labrador Sea), 8.2 Sv for CLSW, 5.7 Sv for ISOW, and 2.2 Sv for DSOW. Estimated errors are ±20% for CLSW and ±16% for the other water masses. The total for NADW, which forms the deep limb of the North Atlantic Meridional Overturning Circulation, is 19.6 Sv. An extensive test of the effects of temporal variability on the average formation rate calculated from the CFC inventory indicates that the error introduced by the assumption of a constant water mass formation rate is no greater than 15% for CLSW and 10% for the other water masses.

Water mass transformation in the North Atlantic over 1985-2002 simulated in an eddy-permitting model

Abstract. Water mass transformation in the North Atlantic is examined in an eddy-permitting simulation with the OCCAM ocean general circulation model, forced by realistic surface fluxes over the period 1985-2002. Three Atlantic regions are considered - the subtropics, mid-latitudes, the northeast Atlantic - along with the Labrador Sea. The oceanic boundaries of each region coincide with hydrographic sections occupied in recent years. These regions broadly represent the formation sites of Eighteen Degree Water (EDW), Subtropical Mode Water (STMW), Subpolar Mode Water (SPMW) and Labrador Sea Water (LSW). Water mass budgets are obtained for each region and year. Terms in the budget comprise surface-forced transformation rates, boundary exchanges and unsteadiness. Transformation rates due to "total mixing" are obtained as the difference between net and surface transformation rates. Transports at the boundaries are evaluated alongside recent hydrographic section datasets, while surface-driven and mixing-driven transformation rates are compared with estimates based on air-sea flux datasets and inverse analysis of hydrographic data. In general OCCAM compares well with the observations, although two particular discrepancies are identified: deep overflows at high latitudes too light by around 0.2 kg m-3 and spurious heat gain of up to 100 Wm-2 east of the Grand Banks. Over 1985-2002, there is considerable variability on a range of timescales, in the annual surface-driven and mixing-driven formation rates of all four water masses. In the case of EDW and STMW, surface-driven and mixing-driven formation rates largely cancel. This is not so for SPMW and LSW, leading to regional net formation rates of up to 17 Sv and 15 Sv, respectively. In particular, OCCAM successfully simulates the strong LSW formation event of 1989-1994.