Working Group Members:Barney Balch, Michael Bender, Ron Benner,
Ken Brink, Robert Byrne, Deborah Bronk, Ken Buesseler, Mary-Elena
Carr , Jon Cole, Mick Follows, Niki Gruber, Rick Jahnke, Mercedes
Pasqual, Mike Roman, Jorge Sarmiento, Deborah Steinberg
The North Atlantic: A Natural Ocean Laboratory for Climate Change and Carbon Cycle Science
Improved quantification of the zonal distribution of terrestrial, fossil-fuel carbon sinks depends critically on continued and improved observation and modeling of the North Atlantic ocean carbon sink (Sarmiento and Wofsy, 1999). This need has been identified as a high priority in the the US Carbon Cycle Science Plan (CCSP). We believe that OCTET can, and should, address this issue with process studies to identify, and quantify more accurately the mechanisms responsible for carbon fluxes and their variability in the North Atlantic basin.
The North Atlantic is a relatively small ocean basin with a confined geometry and high biogeochemical and physical variability. The physical regime is dominated by vigorous meridional mass and heat transports in the Gulf Stream and Deep Western Boundary Current. Water mass transformations, associated with air-sea interactions, lead to ventilation of the main thermocline by subduction at mid-latitudes and deep water formation in the subpolar regions. A large fraction of the ocean's deep waters and their properties originate in the North Atlantic. The vigorous surface heat exchanges drive the strong regional net uptake of carbon dioxide from the atmosphere. Indeed, the North Atlantic is the most intense carbon sink among the major basins on an areal basis. The basin is rich in mesoscale activity, particularly in the western margin, associated with baroclinic instability of the Gulf Stream system. The basin is biologically very active: The high latitude spring phytoplankton bloom is one of the most conspicuous seasonal planetary features seen from space and is an icon of the unique biogeochemical features of this basin. The basin has a rich iron source from Saharan and Sahel dust, and geochemical evidence indicates that the North Atlantic subtropics are a region of active nitrogen fixation (Michaels et al., 1996; Gruber and Sarmiento, 1997).
There is significant physical and biogeochemical variability on interannual and decadal timescales (e.g., Deser and Blackmon, 1993; Bates, 2000). A significant fraction of the physical variability is associated with the North Atlantic Oscillation (NAO). The NAO is a characterization of regional climate variability, regimes of which are indicated by the difference in sea level pressure between Iceland and Portugal (e.g., Hurrell, 1995). Clear relationships between biogeochemical and physical variability associated with the NAO have been documented, but the underlying mechanistic connections are not yet understood (e.g., Bates, 2000; Taylor and Stephens, 1980). The natural variability of the basin, as characterized by the NAO, provides a context in which to conduct natural experiments on changes in ocean biogeochemistry that will occur with change in ocean climate. Variability in aeolian dust transport, the balance of limiting nutrients, or community structure may also influence the biological carbon pump either in connection with, or independent of, changes in the physical environment. By observing and understanding the interannual and decadal connections, and revealing the underlying mechanisms of the physical and biogeochemical environment of the North Atlantic, we will proceed towards a better understanding of broader issues of longer term, global change.
Further details and overviews on scientific advances and research programs in the North Atlantic can be found in the recent JGOFS literature (Ducklow and Harris, 1993; Ducklow et al., 1997; Hansell, 1999; Doney et al., 1999; Karl and Michaels, 1996; Siegel et al., 2000).
We recommend a North Atlantic strategy for OCTET, encompassing a decadal-scale research program, emphasizing a focus on observing and understanding variability in bloom dynamics, margin fluxes and the relationship to regional climate change (including changes characterized by the NAO). The ongoing time-series station at Bermuda, along with required studies of the biogeochemical responses to mesoscale eddies, will provide a basis for comparative studies with the subpolar gyre. The small size of the North Atlantic makes basin scale study relatively tractable, both observationally and for numerical simulations. Patterns of physical variability associated with the NAO suggest "action centers" where local observations may reveal wider scale variability. Strong contrasts with the North Pacific system in bloom amplitude, iron limitation and physical regime, all within regions at the same latitudes and with similar meteorological forcing, will provide the basis for unique interbasin comparisons. As one of the smallest of ocean basins, the North Atlantic also has large areas of continental margin relative to the open ocean, affording the potential for regional studies of the importance of shelf processes and exchanges.
Here we outline the unique features, uncertainties and priorities identified by the OCTET North Atlantic Working Group and provide recommendations to OCTET for specific studies in the North Atlantic basin. We identify four areas of more specific focus which are expanded upon in the following sections. Most of these themes have wider oceanic relevance, but we identify them here as particularly significant for the North Atlantic:
Research Questions
1. North Atlantic Carbon Sink:
What is the magnitude of the carbon sink (natural and perturbed) in the North Atlantic? How significant is this on the global scale?How accurately must we quantify the North Atlantic sink to sufficiently define the distribution of fossil fuel carbon sinks (oceanic and terrestrial)?
How is the sink related to fluxes of freshwater and materials from the Arctic Ocean?
A primary motivation for OCTET will be to determine the distribution, magnitude and interannual variability (see next subsection) of oceanic carbon sources and sinks on a regional basis. North Atlantic CO2 uptake is the most intense, per unit area, of the major basins. Current estimates infer a North Atlantic carbon sink (natural and perturbation) of between 0.23 and 0.48 Gt C per year (Lefevre et al., 1999). Approximately one third of this flux may be anthropogenic carbon. While the North Atlantic is the best resolved basin in terms of our knowledge of the spatial distribution and temporal variability of air-sea CO2 flux, better constrained estimates are crucial to improve the identification and quantification of the regional distribution of fossil fuel carbon sinks (Fan et al., 1999).
The magnitude of the biological export of carbon to depth in the basin is uncertain, with estimates ranging from <10% to ~40% of the global total. The supply of nutrients by alternative processes, such as nitrogen fixation and atmospheric deposition, may take on added significance in the North Atlantic. The North Atlantic is relatively iron rich, so the extreme phosphorus deficits in the surface Sargasso Sea will play a central role in limiting nitrogen fixation there.
2. Variability of the North Atlantic Carbon Sink:
What is the interannual and decadal variability in the uptake of carbon in the North Atlantic basin?What are the mechanisms (physical and biological) which give rise to this variability? Is this variability significant in global terms?
What uncertainty does this impose on our time mean estimates?
Can we relate the variability in the carbon pumps to shifts in climate regimes such as the NAO?
What are the links between large-scale, low-frequency variations like NAO and higher frequency phenomena which seem to exert strong control on regional to local fluctuations in biogeochemical cycling?
Coupled atmosphere-ocean model simulations have predicted a significant warming of the surface waters of the ocean (e.g., +2.5°C in 100-150 y) (e.g., Sarmiento et al., 1998). Such a temperature change would probably result in increased surface ocean stratification in the low to mid-latitudes and increased precipitation at high latitudes. An overarching goal of carbon cycle science is to understand how such changes in ocean stratification and mixing would affect the carbon pumps, through modulation of solubility effects, total and export production, plankton community structure, and their biogeochemical consequences. These questions are particularly significant for the North Atlantic, where much of the oceanic deep water is formed and its properties set.
We may use the natural variability of the system to address such questions in a natural laboratory. Increased stratification reduces the input of nutrients to the euphotic zone by vertical mixing, an effect already seen at the BATS site as reduced primary and new production during positive phases of the North Atlaintic Oscillation (see below). Long term changes in such conditions may drive lasting changes in the taxonomic composition of both the primary producers and consumers, thereby changing the efficiency of export of biogenic particles.
Biological Variability.
To some extent, biological variability of the North Atlantic ocean is controlled by the variations of the physical environment. Indeed, it is the response to climate changes that is the major question of interest. In the next sub-section, we outline the nature of physical variability in the basin, and how the observed natural patterns of variability may provide a focus for OCTET studies. Here, first, we identify some open questions regarding biological variability:
Would changes in vertical mixing result in changes in primary and export production via changes in N and P delivery or in light supply (Dutkiewicz et al., 2000)? What fraction of the total export is delivered from the spring bloom and will it change?
How will changes in total and export production be reflected in partitioning among DOC, DON, DOP and their particulate counterparts?
How is export production related to the balance of various biological processes (nitrogen fixation, denitrification, and calcification), and how will the relationship change?
How do changes in mixing and stratification result in changes in plankton community structure during and following the spring bloom (e.g., dominance shift from diatoms to picoplankton and from large crustacean grazers to microzooplankton)?
Variations in primary and new production at higher latitudes due to changes in stratification, whether forced by thermal or freshwater inputs, remains subject to conjecture. The absence of a long running data set of the appropriate biogeochemical variables precludes a more deterministic view of the effects. The partitioning of the biological pump between dissolved and particulate material appears to be strongly impacted by stratification/nutrient availability, though the mechanisms are unknown. Net DOM production in nutrient impoverished provinces is a much larger fraction of net production than in systems rich in nutrients (Hansell and Carlson, 1998). The export of net community production, whether as DOM or as sinking biogenic particles, too will be controlled by the ecosystem structure and the rate of overturning circulation, all of which are modulated by physical variability. Increased concentrations of CO2 in the surface North Atlantic will lower the pH and, therefore, the carbonate concentrations. These changes may affect the potential rates of calcification both by free living calcifiers and hard corals.
Physical Variability and the North Atlantic Oscillation.
A significant fraction of meteorological variability in the North Atlantic region, on interannual to decadal timescales, can be characterized in terms of the NAO (e.g., Hurrell, 1995). Patterns of North Atlantic ocean variability reflect those in the atmospheric forcing (e.g., Dickson et al., 1996) and may feed back on the atmosphere (e.g., Cjaza and Marshall, 2000). The NAO is a unique mode of climate variability, distinct from ENSO in its non-equatorial in origin, and characterized by a dipole meridional oscillation in atmospheric pressure between the Iceland Low and the Azores High. Weather systems track the westerly jet from North America to Europe. During negative phases of the NAO, storm tracks shift southward enhancing heat loss from the surface of the subtropical ocean, thereby enhancing mode water formation and deepening winter-time mixed layers (Dickson et al., 1996). In the positive NAO phase the opposite effect occurs, increasing temperature and stability in the subtropical gyre and deep convection in the subpolar regions shifts from the Greenland to the Labrador Sea. These interannual and decadal changes in the physical system have been observed to cause strong anomalies in the biological and solubility pumps in the Sargasso Sea (Bates et al., 2000). At higher latitudes, relative changes in copepod abundances have been shown to correlate with the NAO and other indicators of regional climate change such as the position of the north wall of the Gulf Stream (Taylor and Stephens, 1980).
These observed correlations between regional climate indices and biogeochemical variables are suggestive, but the underlying mechanistic connections, and larger scale quantification of the impact on carbon fluxes remain open questions, the answers to which will provide insight into more general climate-biogeochemical connections. Understanding the system within the NAO framework will provide a structure for ongoing and future studies of climate and carbon cycle interactions. The classical tripole pattern of the NAO, in sea level pressure or SST variations, suggests centers of action, where a judicious, yet limited, observational framework (perhaps a small number of time-series stations supplemented with regional transects) may provide key data which can be leveraged to understand and quantify the large scale variability. Coordination with the climate and modeling communities from the outset of such observational programs will enhance the likelihood of a successful observational program in this regard.
Although these questions are focused on large-scale, low-frequency phenomena, it is recognized that some of the underlying controls may reside in processes which operate on much smaller spatial scales and shorter temporal scales. For example, it has been suggested that mesoscale flows are responsible for supplying the nutrients required to sustain high levels of new production observed in oligotrophic regions of the main subtropical gyres (e.g., McGillicuddy and Robinson, 1997). Understanding interannual to decadal scale fluctuations of the biological pump requires knowledge of the links between the mesoscale and basin-scale phenomena such as NAO. In order for future programs to develop sufficient mechanistic understanding to allow skillful quantification and prediction, they must address the issues of physical-biogeochemical interactions across the variety of scales on which they operate.
3. Remineralization of Sinking Particles:
What is the variability in remineralization length scales for the spring bloom and what factors in the upper ocean and in the mesopelagic control these scales in time and space?Why is the strong north-south gradient in ocean color and annual production observed in the euphotic zone not observed in export and seafloor metabolism?
How does the recycling efficiency of the mesopelagic layer differ between low vs. high latitudes?
How will changes in vertical mixing affect the annual cycle of mesopelagic community structure and function?
It is enigmatic that a very strong meridional gradient exists in ocean color and rates of primary production, with higher plant biomass and rates of growth in the subpolar waters than in the subtropics, yet only a weak gradient is found in the rates of catabolism at the ocean floor. This finding suggests that the remineralization length scale, that is the depth to which a fraction of the sinking particles reaches prior to mineralization, may be shallower in the higher latitudes. With shallow remineralization length scales, the effect would be to put relatively more of the mineralization into the shallow water column than into deep water or the sediments. A new research initiative aimed at the mesopelagic "twilight zone" of net remineralization below the euphotic layer is needed to uncover the biogeochemical mechanisms and ecological processes underlying length scale variability.
4. Coastal and Open Ocean Carbon Uptake:
What is the proportion of carbon sequestration over multi-decadal time scales contributed by ocean margin vs. deep ocean processes and how does the proportion respond to climate change?
The spring phytoplankton bloom in the North Atlantic is intense because of deep vertical mixing and the iron-replete nature of the basin, releasing phytoplankton from nutrient limitation each spring. The subpolar bloom contributes about half the annual organic particle flux to the deep sea in a brief interval in the late spring. The bloom was studied in the US JGOFS North Atlantic Bloom Experiment Pilot Study in 1989 at a single site for a brief period and somewhat more comprehensively by European JGOFS and related programs. However, even this conspicuous phenomenon may not dominate carbon fluxes in the North Atlantic. Due to the North Atlantic basin's small size, and the generally broad extent and shallow depth of continental shelves surrounding the basin, continental margin-derived fluxes may be especially important. The flux from the subpolar bloom area is less than 1/4 of that needed to balance deep AOU requirements for the total North Atlantic (from the equator northward). Margins and lower latitude areas must contribute significantly, as must export of DOM with deep and intermediate water formation. How the contributions of the open ocean and the ocean margins to carbon fluxes vary over interannual or decadal time scales is unknown. The volume transport and patterns of thermohaline circulation will certainly control export as DOM and, therefore, its contribution to AOU.
The North Atlantic receives large inputs from rivers draining about half the North American continent, and additional inputs from the Arctic Ocean, itself a river-dominated basin. The Arctic Ocean's stock of dissolved organic matter receives strong inputs from the rivers draining the north slopes of the continents; how this contribution to total carbon flux may change with a warming Arctic is uncertain.
Suggested Research Strategies
One of the most valuable insights taken from the many US JGOFS programs is that a long-term, continuous presence in and above the ocean is necessary in order to identify the major gaps in our understanding of processes of interest. Interannual changes that were not predicted led to some of the most important advances and questions in our science. This continuous presence took the form of well-supported time series stations in the North Atlantic and North Pacific during the JGOFS program. Such high cost observatories cannot be reproduced at numerous sites in the ocean, but some form of continuous observing capability will be necessary and valuable. The North Atlantic subtropical gyre is the best resolved in terms of temporal variability, particularly if the existing time series programs (BATS, CARIACO, CaTS, and ESTOC) are better linked. Our biggest need in the context of OCTET is a continuous spring and summer presence in the subpolar waters of the North Atlantic, which will provide the requisite biogeochemical data. The classical tripole pattern of the NAO in sea level pressure or SST variations suggests likely centres of action, where a judicious, yet limited, observational framework may provide a key to understanding the broader regional variability. Strong ties to the physical ocean observing programs of CLIVAR and GOOS could help provide the data required for observing winter time air-sea exchange processes and export with thermohaline circulation. We must continue to exploit space observing programs to the fullest extent possible (e.g., ocean color and derivable products relevant to ocean biogeochemistry). Preliminary modeling studies will help to determine the optimal observation sites. Models will also provide a framework for interpretation of the data in the wider context.
We must make continuous observations of the major physical systems that impact directly on the major biogeochemical processes. Such distinct systems include the equatorial upwelling zone, the zone of high nitrogen fixation in the tropical Atlantic, the strong meridional flow in the Florida Strait, the primary sites of shelf/basin exchange, the regions of mode and deep water formation, the oligotrophic zones in which biogeochemical cycles are strongly impacted by intermittent events such as eddies, the areas of deep winter mixing experiencing strong spring blooms, and the equatorward flow at mesopelagic depths.
Focus on the mesopelagic zone, employing gradients in the biogeochemical signals in this zone to gather integrative information on the surface ocean processes, will prove a particularly powerful tool. Data on bioactive and time tracers in the mesopelagic have provided valuable information on fluxes from the surface ocean. In the North Atlantic, gradients in subsurface layers such as the subtropical mode water provide mass balance constraints on biogeochemical processes that are highly variable in time and space, and therefore difficult to constrain with surface-only measurements. The integrated contributions of the ocean margins too may be resolvable via study of geochemical gradients on subsurface isopycnal layers replenished by margin exchange.
North Atlantic Ocean: Unique Features
Prominent Physical Features:
1. Confined geometry
2. Strong meridional overturn over all depths into one small basin
3. Strong transport from tropical/subtropical to ventilation regions
4. Arctic ocean exchanges
5. Large ratio margin area: basin area
6. Broad shelves
7. High riverine inputs
8. High dust inputs
9. Strong surface and thermocline eddy structures
Biogeochemical and Biological Features:
1. Intense high latitude spring phytoplankton bloom & biogeochemical impacts
2. Conspicuous coccolithophorid blooms
3. Excess of N2-fixation over denitrification (positive N* signal)
4. Possible net heterotrophy (in a small basin)
5. Iron replete basin
6. Strong CO2 sink (per unit area)
7. Non-Redfield DIC draw down in some regimes
8. Anthropogenic inputs (atmospheric and river derived)
Resources and Advantages for Study:
1. Relatively small basin surrounded by major seagoing nations and population centers: ease of logistics
2. Valuable archived data resources (e.g., WOCE, JGOFS, TTO etc)
3. Ongoing focus of major biological & physical programs [e.g., several ocean biogeochemistry time-series programs (BATS, ESTOC, CaTS, CARIACO), CPR, Argo, CLIVAR, GOOS]
4. High level effort with sophisticated models (including hurricane forecasting, etc.)
Arctic Oscillation Impacts:
1. AA climate oscillation not Equatorial in origin &endash; distinct phenomenon
2. Understanding primitive and still emerging
3. Effects on: convection, biogeochemistry, ecology
4. Interannual &endash; multi-decadal time scales
References
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