From microbes to large predators, there is increasing evidence that marine life is shaped by short-lived submesoscales currents that are difficult to observe, model, and explain theoretically. ...Whether and how these intense three-dimensional currents structure the productivity and diversity of marine ecosystems is a subject of active debate. Our synthesis of observations and models suggests that the shallow penetration of submesoscale vertical currents might limit their impact on productivity, though ecological interactions at the submesoscale may be important in structuring oceanic biodiversity.
Estimates of the ocean biological carbon pump are limited by uncertainties in the magnitude of the physical injection of particulate and dissolved organic carbon to the ocean interior. A major ...challenge is to evaluate the contribution of these physical pumps at small spatial and temporal scales (<100 km and <1 month). Here, we use a submesoscale permitting biophysical model covering a large domain representative of a subpolar and a subtropical gyre to quantify the impact of small‐scale physical carbon pumps.The model successfully simulates intense eddy‐driven subduction hot spots with a magnitude comparable to what has been observed in nature (1,000–6,000 mg C·m−2·day−1). These eddy‐driven subduction events are able to transfer carbon below the mixed‐layer, down to 500‐ to 1,000‐m depth. However, they contribute <5% to the annual flux at the scale of the basin, due to strong compensation between upward and downward fluxes. The model also simulates hot spots of export associated with small‐scale heterogeneity of the mixed layer, which intermittently export large amounts of suspended particulate and dissolved organic carbon. The mixed‐layer pump contributes ∼20% to the annual flux. High‐resolution measurements of export flux are needed to test models such as this one and to improve our mechanistic understanding of the biological pump and how it will respond to climate change.
Key Points
A high‐resolution model simulates realistic hot spots of organic carbon export by physical subduction
Eddy‐driven subduction contributes little to annual export due to compensation between upward and downward fluxes
Eddy‐driven spatiotemporal variations in the mixed‐layer efficiently export dissolved and particulate organic carbon
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Accurately representing the ocean carbon cycle in Earth System Models (ESMs) is essential to understanding the oceanic CO2 sink evolution under CO2 emissions and global warming. A key uncertainty ...arises from the ESM's inability to explicitly represent mesoscale eddies. To address this limitation, we conduct eddy‐resolving experiments of CO2 uptake under global warming in an idealized mid‐latitude ocean model. In comparison with similar experiments at coarser resolution, we show that the CO2 sink is 34% larger in the eddy‐resolving experiments. 80% of the increase stems from a more efficient anthropogenic CO2 uptake due to a stronger Meridional Overturning circulation (MOC). The remainder results from a weaker reduction in CO2 uptake associated to a weaker MOC decline under global warming. Although being only a fraction of the overall response to climate change, these results emphasize the importance of an accurate representation of small‐scale ocean processes to better constrain the CO2 sink.
Plain Language Summary
Today, the ocean absorbs ∼25% of the CO2 emissions caused by human activities. This CO2 sink is primarily driven by the increase of CO2 in the atmosphere, but it is also influenced by physical changes in the ocean's properties. Earth System Models are used to project the future of the ocean CO2 sink. Due to limited computational capacity, ESMs need to parameterize flows occurring at scales smaller than ∼100 km, their typical horizontal grid resolution. To overcome the computational limitations, we use an ocean biogeochemical model representing an idealized North Atlantic ocean of reduced dimensions. We conduct simulations of global warming using increasingly finer horizontal resolutions (from ∼100 km to ∼4 km). Our findings demonstrate that the ocean CO2 uptake is highly influenced by resolution. This sensitivity primarily stems from how the overturning circulation's mean state depends on resolution, as well as how it responds to global warming. Although our results capture only a fraction of the overall oceanic response to climate change, they emphasize the significance of accurately representing the role of small‐scale ocean processes to better constrain the future evolution of ocean carbon uptake.
Key Points
We conducted idealized ocean simulations under global warming and rising atmospheric CO2 at coarse and eddy‐resolving resolutions
CO2 sink is larger by 34% at eddy resolution, due to larger anthropogenic CO2 uptake combined with weaker climate feedback
This ensues from the model's overturning circulation sensitivity to resolution in both historical and future state
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The ocean's ability to sequester carbon away from the atmosphere exerts an important control on global climate. The biological pump drives carbon storage in the deep ocean and is thought to function ...via gravitational settling of organic particles from surface waters. However, the settling flux alone is often insufficient to balance mesopelagic carbon budgets or to meet the demands of subsurface biota. Here we review additional biological and physical mechanisms that inject suspended and sinking particles to depth. We propose that these 'particle injection pumps' probably sequester as much carbon as the gravitational pump, helping to close the carbon budget and motivating further investigation into their environmental control.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The critical depth hypothesis (CDH) is a predictive criteria for the onset of phytoplankton blooms that comes from the steady-state analytical solution of a simple mathematical model for ...phytoplankton growth presented by Sverdrup in 1953. Sverdrup's phytoplankton-only model is very elementary compared with state-of-the-art ecosystem models whose numerical solution in a time-varying environment do not systematically conform to the CDH. To highlight which model ingredients make the bloom onset deviate from the CDH, the complexity of Sverdrup's model is incrementally increased, and the impact that each new level of complexity introduced is analysed. Complexity is added both to the ecosystem model and to the parameterization of physical forcing. In the most complete experiment, the model is a one-dimensional Nutrient-Phytoplankton-Zooplankton model that includes seasonally varying mixed layer depth and surface irradiance, light and nutrient limitation, variable grazing, self-shading, export, and remineralization. When complexity is added to the ecosystem model, it is found that the model solution only marginally deviates from the CDH. But when the physical forcing is also changed, the model solution can conform to two competing theories for the onset of phytoplankton blooms-the critical turbulence hypothesis and the disturbance recovery hypothesis. The key roles of three physical ingredients on the bloom onset are highlighted: the intensity of vertical mixing at the end of winter, the seasonal evolution of the mixed-layer depth from the previous summer, and the seasonal evolution of surface irradiance.
Among the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming ...trend in sea surface temperatures in the tropics during the past century—although the contribution of such a large warming to productivity changes has remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.
Key Points
Reduction of up to 20% in marine phytoplankton in the Indian Ocean during the past six decades
Reduction in marine productivity is attributed to the rapid warming in the Indian Ocean
Future climate projections indicate further warming and subsequent reduction in marine productivity
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In Arabian Sea (AS), land-locked northern boundary and strong seasonal productivity lead to the formation of one of the most intense open ocean Oxygen Minimum Zones (OMZs). Presence of this perennial ...OMZ has important consequences on adjacent coastal fisheries and ecosystem. Simulations from CMIP5 suggest significant weakening of both monsoonal winds and productivity under high emission scenario. But the fate of AS OMZ in this scenario - whether it will expand or shrink - still remains elusive, mainly due to poor representation of extent and strength of AS OMZ in CMIP5 present-day simulations. Here we analyze the distribution of O2 in AS from a subset of three contrasted CMIP5 simulations, complemented with a set of regional downscaled model experiments which we forced at surface and open boundaries using information from those three CMIP5 models. We test two regional downscaling approaches - with and without correction of CMIP5 biases with respect to observations. Using a set of sensitivity experiments, we disentangle the contributions of local (atmospheric) forcing vs. remote (at the lateral boundaries) forcing in driving the future projected O2 changes. While CMIP5 projects either shrinking or expansion of the AS OMZ depending on the model, our downscaling experiments consistently project a shrinking of AS OMZ. We show that projected O2 changes in OMZ layer are affected by both local and remote processes, in contrast with surface deoxygenation, essentially driven by surface warming. In the southern AS, the main response to climate change is oxygenation that originates from the boundaries, and hence downscalled and CMIP5 model responses are similar. In contrast, in northern AS, downscaling yields a substantial reduction in O2 projection discrepancies because of a minimal influence of remote forcing there leading to a stronger sensitivity to improved local physics and improved model representation of present-day conditions. We find that when corrected for present-day biases, projected deoxygenation in the northern AS is shallower. Our findings indicate the importance of downscaling of global models in regions where local forcing is dominant, and the need for correcting global model biases with respect to observations to reduce uncertainties in future O2 projections.
The decline in oxygen supply to the ocean associated with global warming is
expected to expand oxygen minimum zones (OMZs). This global trend can be
attenuated or amplified by regional processes. In ...the Arabian Sea, the
world's thickest OMZ is highly vulnerable to changes in the Indian monsoon
wind. Evidence from paleo-records and future climate projections indicates
strong variations of the Indian monsoon wind intensity over climatic
timescales. Yet, the response of the OMZ to these wind changes remains poorly
understood and its amplitude and timescale unexplored. Here, we investigate
the impacts of perturbations in Indian monsoon wind intensity (from −50 to
+50 %) on the size and intensity of the Arabian Sea OMZ, and examine the
biogeochemical and ecological implications of these changes. To this end, we
conducted a series of eddy-resolving simulations of the Arabian Sea using the
Regional Ocean Modeling System (ROMS) coupled to a nitrogen-based
nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model that
includes a representation of the O2 cycle. We show that the Arabian Sea
productivity increases and its OMZ expands and deepens in response to monsoon
wind intensification. These responses are dominated by the perturbation of
the summer monsoon wind, whereas the changes in the winter monsoon wind play
a secondary role. While the productivity responds quickly and nearly linearly
to wind increase (i.e., on a timescale of years), the OMZ response is much
slower (i.e., a timescale of decades). Our analysis reveals that the OMZ
expansion at depth is driven by increased oxygen biological consumption,
whereas its surface weakening is induced by increased ventilation. The
enhanced ventilation favors episodic intrusions of oxic waters in the lower
epipelagic zone (100–200 m) of the western and central Arabian Sea, leading to
intermittent expansions of marine habitats and a more frequent alternation of
hypoxic and oxic conditions there. The increased productivity and deepening
of the OMZ also lead to a strong intensification of denitrification at depth,
resulting in a substantial amplification of fixed nitrogen depletion in the
Arabian Sea. We conclude that changes in the Indian monsoon can affect, on
longer timescales, the large-scale biogeochemical cycles of nitrogen and
carbon, with a positive feedback on climate change in the case of stronger
winds. Additional potential changes in large-scale ocean ventilation and
stratification may affect the sensitivity of the Arabian Sea OMZ to monsoon
intensification.
Bringing physics to life at the submesoscale Lévy, Marina; Ferrari, Raffaele; Franks, Peter J. S. ...
Geophysical research letters,
07/2012, Volume:
39, Issue:
14
Journal Article
Peer reviewed
Open access
A common dynamical paradigm is that turbulence in the upper ocean is dominated by three classes of motion: mesoscale geostrophic eddies, internal waves and microscale three‐dimensional turbulence. ...Close to the ocean surface, however, a fourth class of turbulent motion is important: submesoscale frontal dynamics. These have a horizontal scale of O(1–10) km, a vertical scale of O(100) m, and a time scale of O(1) day. Here we review the physical‐chemical‐biological dynamics of submesoscale features, and discuss strategies for sampling them. Submesoscale fronts arise dynamically through nonlinear instabilities of the mesoscale currents. They are ephemeral, lasting only a few days after they are formed. Strong submesoscale vertical velocities can drive episodic nutrient pulses to the euphotic zone, and subduct organic carbon into the ocean's interior. The reduction of vertical mixing at submesoscale fronts can locally increase the mean time that photosynthetic organisms spend in the well‐lit euphotic layer and promote primary production. Horizontal stirring can create intense patchiness in planktonic species. Submesoscale dynamics therefore can change not only primary and export production, but also the structure and the functioning of the planktonic ecosystem. Because of their short time and space scales, sampling of submesoscale features requires new technologies and approaches. This paper presents a critical overview of current knowledge to focus attention and hopefully interest on the pressing scientific questions concerning these dynamics.
Key Points
Submesoscale physics control ecology locally, but also feedback to basin scales
Strong gradients in community structure are created at the submesoscale
Despite recent innovations, sampling the submesoscale remains a major challenge
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