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

2014 ◽  
Vol 44 (5) ◽  
pp. 1354-1371 ◽  
Author(s):  
S. F. Gary ◽  
M. S. Lozier ◽  
Y.-O. Kwon ◽  
J. J. Park
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Water Mass ◽  
Turnover Time ◽  
Formation Processes ◽  
Mean Flow ◽  
Vorticity Field ◽  
Mode Water ◽  
Lagrangian Particles ◽  
Subtropical Mode Water

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.

scholarly journals Tracking water masses using passive-tracer transport in NEMO v3.4 with NEMOTAM: application to North Atlantic Deep Water and North Atlantic Subtropical Mode Water

2019 ◽  
Author(s):  
Dafydd Stephenson ◽  
Simon Müller ◽  
Florian Sévellec
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
North Atlantic Deep Water ◽  
The Arctic ◽  
Global Ocean ◽  
Passive Tracer ◽  
Tracer Transport ◽  
Mode Water ◽  
Subtropical Mode Water

Abstract. Water mass ventilation provides an important link between the atmosphere and the global ocean circulation. In this study, we present a newly developed, probabilistic tool for offline water mass tracking. In particular, NEMOTAM, the tangent-linear and adjoint counterpart to the NEMO ocean general circulation model, is modified to allow passive-tracer transport. By terminating dynamic feedbacks in NEMOTAM, tagged water can be tracked forward and backwards in time as a passive dye, producing a probability distribution of pathways and origins, respectively. Upon contact with the surface, the tracer is removed from the system, and a record of ventilation is produced. Two test cases are detailed, examining the creation and fate of North Atlantic Subtropical Mode Water (NASMW) and North Atlantic Deep Water (NADW) in a 2&degree; configuration of NEMO run with repeated annual forcing for up to 400 years. NASMW is shown to have an expected age of 4.5 years, and is predominantly eradicated by internal processes. A bed of more persistent NASMW is detected below the mixed layer with an expected age of 8.7 years It is shown that while model NADW has two distinct outcrops (in the Arctic and North Atlantic), its formation primarily takes place in the subpolar Labrador and Irminger seas. Its expected age is 112 years.

scholarly journals Tracking water masses using passive-tracer transport in NEMO v3.4 with NEMOTAM: application to North Atlantic Deep Water and North Atlantic Subtropical Mode Water

2020 ◽  
Vol 13 (4) ◽  
pp. 2031-2050
Author(s):  
Dafydd Stephenson ◽  
Simon A. Müller ◽  
Florian Sévellec
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
North Atlantic Deep Water ◽  
The Arctic ◽  
Global Ocean ◽  
Passive Tracer ◽  
Tracer Transport ◽  
Mode Water ◽  
Subtropical Mode Water

Abstract. Water mass ventilation provides an important link between the atmosphere and the global ocean circulation. In this study, we present a newly developed, probabilistic tool for offline water mass tracking. In particular, NEMOTAM, the tangent-linear and adjoint counterpart to the NEMO ocean general circulation model, is modified to allow passive-tracer transport. By terminating dynamic feedbacks in NEMOTAM, tagged water can be tracked forward and backward in time as a passive dye, producing a probability distribution of pathways and origins, respectively. To represent surface (re-)ventilation, we optionally decrease the tracer concentration in the surface layer and track this concentration removal to produce a ventilation record. Two test cases are detailed, examining the creation and fate of North Atlantic Subtropical Mode Water (NASMW) and North Atlantic Deep Water (NADW) in a 2∘ configuration of NEMO run with repeated annual forcing for up to 400 years. Model NASMW is shown to have an expected age of 4.5 years and is predominantly eradicated by internal processes. A bed of more persistent NASMW is detected below the mixed layer with an expected age of 8.7 years. It is shown that while model NADW has two distinct outcrops (in the Arctic and North Atlantic), its formation primarily takes place in the subpolar Labrador and Irminger seas. Its expected age is 112 years.

scholarly journals Effects of Mesoscale Eddies on Subduction and Distribution of Subtropical Mode Water in an Eddy-Resolving OGCM of the Western North Pacific

2010 ◽  
Vol 40 (8) ◽  
pp. 1748-1765 ◽  
Author(s):  
Shiro Nishikawa ◽  
Hiroyuki Tsujino ◽  
Kei Sakamoto ◽  
Hideyuki Nakano
Keyword(s):  
North Pacific ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Mesoscale Eddies ◽  
Mean Flow ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
Eddy Transport ◽  
Pv Gradient

Abstract The effects of mesoscale eddies on the subduction and distribution of the North Pacific Subtropical Mode Water (STMW) are investigated using an eddy-resolving ocean general circulation model (OGCM). First, the subduction rate is calculated and the contribution of eddies to the subduction of STMW is estimated. It is found that eddy subduction significantly contributes to the total subduction of STMW. Second, eddy thickness transport and diapycnal flux are directly diagnosed to investigate the large-scale eddy-induced transport process of STMW. The large southward eddy thickness transport in the STMW core density is consistent with eddy subduction. The eddy transport on the isopycnal surface of STMW is directed down the thickness gradient and traverses the mean flow. The meridional eddy transport streamfunction indicates two eddy circulation cells south of 30°N, associated with the circulation of STMW. These cells flatten density surfaces, similar to the effect of the Gent and McWilliams (GM) scheme. The subducted STMW is gradually dissipated to lower or higher densities in the main thermocline, basically by vertical diffusion. Finally, local processes of eddy subduction and transport of STMW are explored using an anticyclonic eddy. Results imply two possible local processes of the eddy subduction of STMW. One is the destruction of a potential vorticity (PV) gradient by eddy mixing, where the PV gradient is due to winter deep mixed layer formation. The other is the southward translation of anticyclonic eddies that accompany low PV.

Enhanced warming of the subtropical mode water in the North Pacific and North Atlantic

2017 ◽  
Vol 7 (9) ◽  
pp. 656-658 ◽  
Author(s):  
Shusaku Sugimoto ◽  
Kimio Hanawa ◽  
Tomowo Watanabe ◽  
Toshio Suga ◽  
Shang-Ping Xie
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
The North ◽  
The North Pacific

scholarly journals Multi-decadal uptake of carbon dioxide into subtropical mode water of the North Atlantic Ocean

2011 ◽  
Vol 8 (6) ◽  
pp. 12451-12476 ◽  
Author(s):  
N. R. Bates
Keyword(s):  
Carbon Dioxide ◽  
Atlantic Ocean ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Dissolved Inorganic Carbon ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract. Natural climate variability impacts the multi-decadal uptake of anthropogenic carbon dioxide (Cant) into the North Atlantic Ocean subpolar and subtropical gyres. Previous studies have shown that there is significant uptake of CO2 into the subtropical mode water (STMW) that forms south of the Gulf Stream in winter and constitutes the dominant upper-ocean water mass in the subtropical gyre of the North Atlantic Ocean. Observations at the Bermuda Atlantic Time-series Study (BATS) site near Bermuda show an increase in dissolved inorganic carbon (DIC) of +1.51 ± 0.08 μmol kg−1 yr−1 between 1988 and 2011. It is estimated that the sink of CO2 into STMW was 0.985 ± 0.018 Pg C (Pg = 1015 g C) between 1988 and 2011 (~70 % of which is due to uptake of Cant). However, the STMW sink of CO2 was strongly coupled to the North Atlantic Oscillation (NAO) with large uptake of CO2 into STMW during the 1990s (NAO positive phase). In contrast, uptake of CO2 into STMW was much reduced in the 2000s during the NAO neutral/negative phase. Thus, NAO induced variability of the STMW CO2 sink is important when evaluating multi-decadal changes in North Atlantic Ocean CO2 sinks.

scholarly journals A recent decline in North Atlantic subtropical mode water formation

2020 ◽  
Vol 10 (4) ◽  
pp. 335-341
Author(s):  
Samuel W. Stevens ◽  
Rodney J. Johnson ◽  
Guillaume Maze ◽  
Nicholas R. Bates
Keyword(s):  
North Atlantic ◽  
Mode Water ◽  
Subtropical Mode Water ◽  
Water Formation

scholarly journals Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling

2016 ◽  
Author(s):  
Johannes Karstensen ◽  
Florian Schütte ◽  
Alice Pietri ◽  
Gerd Krahmann ◽  
Björn Fiedler ◽  
...  
Keyword(s):  
High Resolution ◽  
Large Scale ◽  
Cold Water ◽  
Phase Speed ◽  
Ekman Transport ◽  
Buoyancy Frequency ◽  
Low Oxygen ◽  
Mean Flow ◽  
Mode Water ◽  
The Core

Abstract. The physical (temperature, salinity, velocity) and biogeochemical (oxygen, nitrate) structure of an oxygen depleted coherent, baroclinic, anticyclonic mode-water eddy (ACME) is investigated using high-resolution autonomous glider and ship data. A distinct core with a diameter of about 70 km is found in the eddy, extending from about 60 to 200 m depth and. The core is occupied by fresh and cold water with low oxygen and high nitrate concentrations, and bordered by local maxima in buoyancy frequency. Velocity and property gradient sections show vertical layering at the flanks and underneath the eddy characteristic for vertical propagation (to several hundred-meters depth) of near inertial internal waves (NIW) and confirmed by direct current measurements. A narrow region exists at the outer edge of the eddy where NIW can propagate downward. NIW phase speed and mean flow are of similar magnitude and critical layer formation is expected to occur. An asymmetry in the NIW pattern is seen that possible relates to the large-scale Ekman transport interacting with ACME dynamics. NIW/mean flow induced mixing occurs close to the euphotic zone/mixed layer and upward nutrient flux is expected and supported by the observations. Combing high resolution nitrate (NO3−) data with the apparent oxygen utilization (AOU) reveals AOU:NO3− ratios of 16 which are much higher than in the surrounding waters (8.1). A maximum NO3− deficit of 4 to 6 µmol kg−1 is estimated for the low oxygen core. Denitrification would be a possible explanation. This study provides evidence that the recycling of NO3−, extracted from the eddy core and replenished into the core via the particle export, may quantitatively be more important. In this case, the particulate phase is of keys importance in decoupling the nitrogen from the oxygen cycling.

scholarly journals North Atlantic Subtropical Mode Waters and Ocean Memory in HadCM3

2006 ◽  
Vol 19 (7) ◽  
pp. 1126-1148 ◽  
Author(s):  
Chris Old ◽  
Keith Haines
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Large Scale ◽  
Feedback Mechanism ◽  
Heat Fluxes ◽  
Climate Feedback ◽  
Trade Wind ◽  
Mode Water ◽  
Mode Waters ◽  
Long Term Storage

Abstract A study of the formation and propagation of volume anomalies in North Atlantic Mode Waters is presented, based on 100 yr of monthly mean fields taken from the control run of the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3). Analysis of the temporal and spatial variability in the thickness between pairs of isothermal surfaces bounding the central temperature of the three main North Atlantic subtropical mode waters shows that large-scale variability in formation occurs over time scales ranging from 5 to 20 yr. The largest formation anomalies are associated with a southward shift in the mixed layer isothermal distribution, possibly due to changes in the gyre dynamics and/or changes in the overlying wind field and air–sea heat fluxes. The persistence of these anomalies is shown to result from their subduction beneath the winter mixed layer base where they recirculate around the subtropical gyre in the background geostrophic flow. Anomalies in the warmest mode (18°C) formed on the western side of the basin persist for up to 5 yr. They are removed by mixing transformation to warmer classes and are returned to the seasonal mixed layer near the Gulf Stream where the stored heat may be released to the atmosphere. Anomalies in the cooler modes (16° and 14°C) formed on the eastern side of the basin persist for up to 10 yr. There is no clear evidence of significant transformation of these cooler mode anomalies to adjacent classes. It has been proposed that the eastern anomalies are removed through a tropical–subtropical water mass exchange mechanism beneath the trade wind belt (south of 20°N). The analysis shows that anomalous mode water formation plays a key role in the long-term storage of heat in the model, and that the release of heat associated with these anomalies suggests a predictable climate feedback mechanism.

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