The ocean takes up 93
The ocean absorbs more than 90
Ventilation involves exchange between the surface mixed layer and the ocean
interior
It is not only the rate of exchange between the ocean surface mixed layer and the interior that is important but also the length of time that subducted waters remain in the interior before coming up again and being re-entrained/obducted into the surface mixed layer. For example, as a consequence of anthropogenic warming, the subducted waters warm with time as the mixed layer warms; hence, where these warmer waters re-entrain into the surface mixed layer rapidly within a few years of their subduction, the surface mixed layer will warm further. Conversely, waters that spend longer in the interior before they obduct will have subducted earlier when they are still relatively cool; thus, these waters warm the mixed layer less when they re-entrain.
The fidelity of future climate projections relies on an accurate
representation of ocean ventilation and on the ability of the next generation
of numerical models to predict transient and regional climate change
The high-latitude oceans (e.g. the subpolar North Atlantic and Southern Ocean),
where the densest waters are formed, play a prominent role in global
ventilation, as up to two-thirds of the volume of the ocean interior and
more than three-quarters of the deep ocean are thought to be ventilated at
these locations
In contrast to the more established view, recent studies using new
observations in the subpolar North Atlantic indicate the prevailing
importance of the deep waters formed in the Irminger and Iceland basins over
the Labrador Sea as largely responsible for the observed variability in the
overturning
Processes occurring at lower latitudes and in upwelling layers also affect the
renewal of water masses in the ocean's interior, indicating a decoupling of
ventilation from the overturning circulation. Subduction into the subtropical
thermoclines is a major driver of ventilation. Additionally, diapycnal and
isopycnal diffusion play an important role in ventilation at both low and high
latitudes, e.g. in the upwelling regions of the Southern Ocean
Age tracers that track the length of time since waters have been ventilated
have previously been used in numerical models to understand ventilation
timescales
We carried out numerical simulations where we resolved the interannual variability in both forcing and circulation, and discerned the impact that this variability has on the inventory and distribution of a passive tracer. To achieve this, we analysed changes in ocean ventilation using simulated interannually varying dye tracers, which represent distinct annual seawater vintages. These allowed us to explore the links between subduction, ocean circulation, and surface forcing on a range of different timescales and globally, by following the pathways of the passive tracers. With our approach, we aimed to separate the roles played by deep water formation, ventilation, overturning, and how these are driven by surface forcing on interannual to interdecadal timescales.
Our simulations can be used to inform the interpretation of
anthropogenically sourced transient tracers such as CFCs
The model, its spin-up, and the experimental design for the injection of the dye tracers are described in Sect. 2. Changes in the dye distribution are shown globally and as a function of latitude and depth in Sect. 3, highlighting the role of interannual variability in the evolution of the different dyes and the differences and similarities between the two hemispheres. The dominant role of surface forcing in setting these patterns and its implications for the interpretation of observational data, as well as potential drivers of longer-term changes and larger-scale signals, are discussed in Sects. 4 and 5.
Numerical simulations are run using a global configuration of the Nucleus for
European Modelling of the Ocean
This model configuration involves a vertical mixing scheme based on the
turbulent kinetic energy model of
Winter (i.e. temporal maximum) mixed-layer depth (m) between 2004 and 2017 in NEMO
Our ocean–ice simulation is forced with the JRA55-do surface atmospheric
dataset
Our model configuration gives an average AMOC (maximum in the overturning
streamfunction at 1000
The representation of the mixed layer in the model is of particular interest
for our analysis, given its key role in driving ocean ventilation
The model reproduces the deep winter mixed layers both in the North Atlantic
subpolar gyre and in the Sub-Antarctic Mode Water formation regions, as well
as along the Antarctic coast (Fig. 1a); the latter is not captured in the EN4 and
Argo datasets (Fig. 1b–d) due to limited observational capability under
ice. However, it is known that the NEMO model is characterised by excessively
strong convection in the Labrador and Irminger seas, down to depths that are
much higher than what is measured in observations (Fig. 1c), even at higher
horizontal resolutions
CFC-11 distribution in the model along a mid-Atlantic section at
25
Anthropogenic transient tracers such as CFCs are often used for model
validation and to evaluate ocean ventilation
It is also worth noting that there are large differences in the simulation of
the magnitude and variability of the (A)MOC in numerical models, both at lower
and higher resolutions
In this study, we aim to resolve the pathways and the inventories of ventilated waters after they leave the mixed layer. This is achieved by using a set of annually distinct dye tracers that are introduced in each year of the last (fifth) cycle through the surface forcing, meaning that the effective model spin-up is 240 years and the simulation with passive tracers is 60 years, which is the length of the available atmospheric forcing for the version of JRA-55 used (from 1958 to 2017).
In these simulations, the dye tracers are released uniformly across the global
ocean, with a 6-month hemispheric offset, as tracer injection starts during
the summer in each hemisphere (i.e. starts in January and ends in December in
the Southern Hemisphere and starts in July and ends in June in the Northern
Hemisphere). A transition zone is introduced between 20
Each dye tracer is injected throughout its release year (its “vintage”) by
relaxing the tracer's concentration to a value of 1 throughout the top seven
vertical levels (down to
Schematic representation of the injection of dye tracers in the model.
Dye inventory for each ocean basin and globally 22 years after injection (in 1980 for the 1958 vintage; blue bars, left
For the analysis presented in Sect. 3, we use a subset of dyes, injected in 1958 to 1993, so that we can follow their evolution from the year of injection and for 25 years of simulation for each one of the 36 vintages, allowing us to investigate the role of interannual variability and pathways from annual to multidecadal timescales. Note that the comparison of these simulated dye tracers to observed CFCs distributions (see Fig. 2) is not straightforward. While the CFCs' source behaves like a step function, our vintage tracers are best represented by a top-hat/delta function, so it is the sum of the concentration of all of the different vintage tracers that could be compared with CFCs.
The dye concentration,
Our set-up allows us to identify when and where water masses were last ventilated, to investigate the role of interannual variability in determining the tracers' distribution, and to unpick the role of surface forcing and ocean circulation in setting these pathways and timescales.
The dye inventory evolves differently over time in each basin, and most of it can be found in the Atlantic on the timescales that we are examining, despite the substantially smaller volume of the Atlantic than the Indo-Pacific (Fig. 4). Over 60 years, the percentage of the global inventory held in the Atlantic increases. This is because the dye penetrates more deeply in the Atlantic, reaching depths where it is unable to re-enter the surface mixed layer, whereas the penetration into the deep Southern and Pacific oceans is less marked (Fig. 5a, b). This may not be fully realistic; as previously discussed, models are known to struggle with the representation of dense-water formation in the Southern Ocean, especially at the coarse resolution of this simulation, which may explain the low concentrations (and relatively small inventory) of tracer in the deep Southern Ocean (Figs. 4, 5).
Mid-Atlantic cross section (25
Within 60 years (the end of the simulation), the core of the dye in the
Atlantic has reached the deep ocean (down to
In our set-up (see Fig. 3), we can also analyse the evolution of dyes injected in each year (vintages) separately. This can be effectively displayed using the “ventilation thickness” metric, as defined in Sect. 2.4 (Fig. 6). For each year, the dye concentration peaks during the winter in each hemisphere (where it is injected), and the ventilation thickness in the year of injection is always higher in the Southern Hemisphere (Fig. 6c). In each hemisphere, the interannual differences between each of the dyes largely derives from changes in the background ocean circulation and the effect of surface forcing on the mixed-layer depth at the time of injection, as the dyes are independent of each other and identify different seawater vintages for each year. After the first year of dye injection, the interannual variability in the peak in ventilation thickness between the different vintages mostly reflects the variability in mixed-layer depth in different years, driven by the surface forcing (Fig. 6).
Ventilation thickness for all dyes/vintages (1958–2017) in the Northern Hemisphere
Ventilation thickness 1 year
Globally, during the first year when the dye is injected, the highest tracer
concentrations can be seen in regions that correspond to the deepest mixed
layers (see Fig. 1a), such as the subpolar North Atlantic near the locations
of deep-water formation from the Labrador and Irminger seas to the Nordic
Seas, as well as along the paths of the Gulf Stream and Kuroshio and in the
Mediterranean Sea (Fig. 7a). Strong dye uptake can also be seen in the
Sub-Antarctic Mode Water (SAMW) formation regions in the Southern Hemisphere
and along the Antarctic coast and in the Weddell Sea, where dense water is
formed (Fig. 7a). For the rest of the analysis, the main focus will remain on
two chosen timescales, of 3 and 25 years, by using a subset of the dye tracers
(as introduced in Sect. 2.3). Three years after injection, the vintages
represent seawater that has moved below the base of the mixed later and been
subducted, whereas after 25 years, they correspond to water that has flowed out
of the upper ocean. Three years after injection, the dye is spreading
prominently in the North Atlantic subpolar gyre (Fig. 7b); after 25 years,
it has reached the entire basin and has also starting to spread southward
along the western boundary (Fig. 7c). It is, however, worth remembering that
boundary currents are not particularly well-represented at 1
By using a subset of dyes, from 1958 to 1993, we can follow their evolution
from the year of dye injection and for 25 years of simulation for each of
the 36 vintages. Looking at the evolution of the dyes as the anomaly from
the mean ventilation thickness (for the 1958–1993 time period) highlights the
differences between the independent vintages that are due to interannual
variability (Fig. 8). The strongest differences from the mean (up to
Ventilation thickness anomaly (from the 1958–1993 mean) for 36 of the 60 dyes (1958 to 1993 vintages) from injection to 25 years later.
Ventilation thickness for both hemispheres for the same 36 dyes shown in Fig. 8 for 3 years
For all vintages, the absolute ventilation thickness is higher in the Southern
Hemisphere after 3 years than in the Northern Hemisphere
(Fig. 9). Vintages with a ventilation thickness that is higher than the mean
are not necessarily the same in both hemispheres, representing the different
responses to the surface forcing and the background circulation
(e.g. stratification) and the local processes at play. On this timescale, the
vintages represent seawater that has been subducted after injection and
has then moved below the base of the mixed later. In the Northern Hemisphere, the
vintages from the early 1990s have relatively high values (Fig. 9a), which
correspond to years of observed strong convection in the Labrador Sea
Correlation between ventilation thickness after 3 years and after 25 years in the Northern Hemisphere
To further investigate this apparent link between the processes affecting
subduction close to the time of dye injection and over the following 2 decades, we correlate the ventilation thickness for the two timescales of 3
and 25 years (Fig. 10). These show a high correlation (
Ventilation in the Northern Hemisphere appears to be twice as persistent as in the Southern Hemisphere (see the slope for the correlations in Fig. 10a and c), which could be thought of as a rate of erosion of the ventilated water masses. This means that subducted waters are exported (and isolated) away from deep mixed-layer regions faster in the Northern Hemisphere than the Southern Hemisphere. The residuals of the correlations (Fig. 10a, c) also highlight some of the longer-term variability, particularly in the Northern Hemisphere, where the residuals for the “later” vintages corresponding to the early 1990s appear to be rising (Fig. 10b).
Dyes that were injected in years characterised by stronger convection result in higher ventilation thickness after 3 and 25 years (Fig. 9), but this also impacts the evolution of the tracer's vertical distribution over time. This is shown for seven of the vintages (highlighted in Fig. 10) analysed so far, covering a range of higher and lower (than the respective hemispheric mean) ventilation thicknesses (Figs. 11, 12).
Global tracer concentration 25 years after injection for seven different vintages as a function of depth
Tracer concentration 25 years after injection for two of the vintages: one that was injected in a year of strong Northern Hemisphere convection
The vertical distribution of the tracer (Fig. 11a) is characterised by two
prominent peaks, where the shallower one at around 500
Note that Fig. 11 a and b are equivalent to Fig. 5b and c respectively, but the different dyes are shown 25 years after injection (to allow the comparison between different vintages), whereas Fig. 5 shows the dye distribution at the end of the simulation, 60 years after dye injection.
Differences in the Atlantic are also shown along a north–south section in the
centre of the basin (same as Figs. 2a and 5a) for two of the vintages 25 years
after their injection (Fig. 12): one from a strong Northern Hemisphere
ventilation year (1992) and one from a weak Northern Hemisphere
ventilation year (1987). The tracer reaches deeper and with higher concentrations when it is injected in years characterised by
strong convection in the subpolar North Atlantic (Fig. 12a), and the
distribution is lower in the top
When considering ocean ventilation and how it regulates the dynamics of
tracers and their connection to the surface ocean, it is important to remember
that it is driven by processes occurring at lower latitudes as well as the
overturning circulation and dense-water formation in the high-latitude oceans
Despite the known biases for NEMO, such as the overly deep mixing in the Labrador Sea (as discussed in Sect. 2.2) and the fact that not all processes can be simulated accurately at resolutions that are not eddy-resolving, this does not affect our results correlating ventilation thickness on different timescales and highlighting the dominant role of surface forcing in setting the evolution of the tracers' distribution and inventory.
The dye uptake in the first (injection) year largely reflects the state and
variability of the mixed layer during the season of active convection, which
is driven by the surface forcing in that year (Figs. 1a, 7a) and the
stratification in that year (“preconditioning”). Three years after
injection, the amount of dye that has reached below the base of the mixed
layer is also dependent on other factors, linked to both lateral mixing and
the surface forcing in the years immediately following injection
The correlation between dye retention after 25 years and the background conditions close to the time of injection (just after the strongest interannual differences have decreased; see Fig. 8) highlights the key role of the surface atmospheric forcing in driving long-term ocean ventilation and, more broadly, in determining the distribution of passive tracers over time (Fig. 10). As the variability in ventilation near the time of dye injection sets the long-term variability for the dye inventory, there is potential for forecasting how the distribution of a tracer in the ocean will evolve in the future, from a prior knowledge of the surface air–sea fluxes and mixed-layer properties.
Finally, the strong correlations in ventilation thickness between 3 and 25 years after injection in both the Northern and Southern hemispheres (Fig. 10; see slope of correlations) imply that, given the strong interannual variability in the initial surface forcing, it is this variability that will continue to dominate on longer timescales, largely overriding the different processes that drive how passive tracers are removed or taken up in the two hemispheres. The Northern Hemisphere is characterised by more persistent anomalies, as the ventilated waters penetrate more deeply where they are better isolated from surface influence, whereas the anomalies are initially stronger in the Southern Hemisphere (Fig. 8), but then dissipate faster than in the Northern Hemisphere, partly due to the more effective mixing along sloping isopycnals in the Southern Ocean. This means that the tracer eventually gets mixed back and ventilated even when the initial amount that is subducted is substantially higher than the mean, while the dye remains in the interior for longer in the subpolar North Atlantic once it has reached a deeper horizon (Figs. 11, 12). In other words, subducted waters are exported (and isolated) away from deep mixed-layer regions faster in the Northern Hemisphere than in the Southern Hemisphere.
The strong interannual variability that characterises ventilation in the
Northern Hemisphere on these timescales (Fig. 9) is largely driven by the
Irminger, Nordic, and Labrador seas (Fig. 7a, b), which are the sites of
most active deep convection in the winter months
There is coherent structure in the residuals of the correlation between
Northern Hemisphere ventilation thickness close to the time of dye injection
and 25 years later, and the residuals deviate from the trend (rise) for the
vintages corresponding to the early 1990s (Fig. 10b). We expect that the role
of surface buoyancy forcing through air–sea fluxes, as well as modes of
climate variability that may affect convection in the subpolar North Atlantic
(e.g. North Atlantic Oscillation – NAO – or Atlantic Multidecadal
Variability – AMV) should already be captured by the correlation itself
(Fig. 10a). However, these will, in turn, also affect the background
circulation. We examine whether there is a relationship between the residuals
and the strength of the large-scale circulation (AMOC). The hypothesis is that
a stronger circulation (AMOC) might also be linked with moving water more
effectively away from the ventilation site and reducing the rate at which dye
is returned to the mixed layer and, thus, removed from the system. We consider
the correlation between the residuals in the Northern Hemisphere and the AMOC
at 26
Time series of the AMOC at 26
The mechanism behind the connection that we find between the strength of the
AMOC at 26
There is also some structure in the residuals of the correlation for the
Southern Hemisphere (Fig. 10d), perhaps reflecting how ventilation here
appears to be stronger before the 1980s and consistently lower in the later
period (Fig. 9a). This could highlight background changes in the strength of
the subtropical gyres, driven by changes in the Southern Hemisphere winds,
affecting SAMW formation
Finally, while it would also be desirable to perform longer simulations with passive tracers and assess their uptake on centennial timescales, our results highlight how this would be problematic. In fact, a tracer's pathways, inventory, and distribution will be strongly dependent on the initial surface forcing, when the tracer enters the ocean, especially for the Northern Hemisphere (see Figs. 11, 12). This means that the cumulation of the inventory of a simulated tracer over time will provide an aliased view of its evolution. At the same time, it is currently too computationally expensive to introduce interannually varying tracers to resolve the full variability on such long timescales.
We have used a set of interannually varying passive dye tracers in an ocean–ice model to explore pathways and timescales of ocean ventilation. This is a computationally expensive approach, but it allows us to fully capture the pathways of subducted waters after they leave the mixed layer.
The Southern Hemisphere shows more variability in ventilation thickness just after the tracer's injection, whereas the Northern Hemisphere is characterised by higher variability in the 25-year inventory, highlighting different ventilation “efficiencies” and timescales. Subducted waters are exported faster in the Northern Hemisphere than in the Southern Hemisphere, but the correlation between ventilation thickness after 3 and 25 years is strong in both hemispheres. This means that the strong interannual variability in the initial surface forcing will dominate on longer timescales and will largely override the different processes that drive the uptake and export of passive tracers.
Our results highlight the key role played by surface forcing near the time
when a tracer enters the ocean in setting the long-term variability of its
inventory and determining the pathways and timescales of its uptake by the
ocean. This has important implications for the interpretation of observations
that only capture snapshots of the circulation and also offers potential to
forecast changes in the pathways and uptake of tracers by the ocean, such as
anthropogenic carbon and heat
Given the deficiencies of our coarse-resolution model, it would be desirable
but highly computationally expensive to apply our method to a fully
eddy-resolving set-up. The use of higher-resolution models would prove
challenging even with a smaller subset of interannually varying
tracers. Comparisons between different models would also be insightful, but
once again not easy to achieve due to the computational costs; in this case,
the use of offline Lagrangian trajectories could be a satisfactory
compromise. Finally, it would be feasible, but once again computationally very
expensive, to apply our methodology at a 1
The source code for the NEMO model can be downloaded from
AM, AJGN, and ELM designed the ocean–ice simulations with tracers. AM ran the simulations and analysed the data. LC carried out the analysis for the data shown in Fig. 1. All authors contributed to the interpretation of the data and to writing the paper.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We would like to thank the members of the TICTOC project for useful discussions on the results presented here and their interpretation. We also thank the editor and the two anonymous reviewers for their constructive comments, which improved the paper.
Alice Marzocchi, A. J. George Nurser, Louis Clément, and Elaine L. McDonagh were supported by the Natural Environment Research Council (grant no. NE/P019293/1; TICTOC). Elaine L. McDonagh was also supported by European Union Horizon 2020 grant no. 817578 (TRIATLAS).
This paper was edited by Trevor McDougall and reviewed by two anonymous referees.