1,721,087 research outputs found
Supplementary material for manuscript: "Evaluating the physical and biogeochemical state of the global ocean component of UKESM1 in CMIP6 Historical simulation"
No full textThe attached Matlab scripts were used to produce the figures that appear in a submission to Geoscientific Model Development entitled "Evaluating the physical and biogeochemical state of the global ocean component of UKESM1 in CMIP6 Historical simulation" by Yool et al.
The reference number for the GMD manuscript is gmd-2020-333.
The scripts are provided as is, and make use of local files that are not included here. The intention is to record the output processing and plotting methods used in the production of the manuscript
Modeling the Role of Nitrification in Open Ocean Productivity and the Nitrogen Cycle
No full textThe ocean is an important component of the global carbon cycle, and currently serves as the principal sink for anthropogenic CO2 from the atmosphere. A key role in the natural oceanic carbon cycle is played by the plankton ecosystem, which acts to elevate the storage capacity of the ocean, but it is believed that this will experience change in the future in response to anthropogenic forcing. One of the approaches used to understand and forecast the oceanic carbon cycle is ecosystem modeling, and this is typically grounded on the nitrogen cycle because of the strong regulatory role this element plays in biological productivity. Nitrification is one of the central processes in the oceanic nitrogen cycle, one whose role may change in the future, but also one with a particular relevance to observational efforts to quantify the biological carbon cycle. Here, we describe and summarize current efforts to model nitrification in pelagic open ocean ecosystems, and look forward to future avenues for progress
On the future navigability of Arctic sea routes: high-resolution projections of the Arctic Ocean and sea ice decline
Full text linkThe rapid Arctic summer sea ice reduction in the last decade has lead to debates in the maritime industries on the possibility of an increase in cargo transportation in the region. Average sailing times on the North Sea Route along the Siberian Coast have fallen from 20 days in the 1990s to 11 days in 2012–2013, attributed to easing sea ice conditions along the Siberian coast. However, the economic risk of exploiting the Arctic shipping routes is substantial. Here a detailed high-resolution projection of ocean and sea ice to the end of the 21st century forced with the RCP8.5 IPCC emission scenario is used to examine navigability of the Arctic sea routes. In summer, opening of large areas of the Arctic Ocean previously covered by pack ice to the wind and surface waves leads to Arctic pack ice cover evolving into the Marginal Ice Zone. The emerging state of the Arctic Ocean features more fragmented thinner sea ice, stronger winds, ocean currents and waves. By the mid 21st century, summer season sailing times along the route via the North Pole are estimated to be 13–17 days, which could make this route as fast as the North Sea Route
Mind the gap: The impact of missing data on the calculation of phytoplankton phenology metrics
No full textAnnual phytoplankton blooms are key events in marine ecosystems and interannual variability in bloom timing has important implications for carbon export and the marine food web. The degree of match or mismatch between the timing of phytoplankton and zooplankton annual cycles may impact larval survival with knock-on effects at higher trophic levels. Interannual variability in phytoplankton bloom timing may also be used to monitor changes in the pelagic ecosystem that are either naturally or anthropogenically forced. Seasonality metrics that use satellite ocean color data have been developed to quantify the timing of phenological events which allow for objective comparisons between different regions and over long periods of time. However, satellite data sets are subject to frequent gaps due to clouds and atmospheric aerosols, or persistent data gaps in winter due to low sun angle. Here we quantify the impact of these gaps on determining the start and peak timing of phytoplankton blooms. We use the NASA Ocean Biogeochemical Model that assimilates SeaWiFS data as a gap-free time series and derive an empirical relationship between the percentage of missing data and error in the phenology metric. Applied globally, we find that the majority of subpolar regions have typical errors of 30 days for the bloom initiation date and 15 days for the peak date. The errors introduced by intermittent data must be taken into account in phenological studies
An examination of the 'continental shelf pump' in an open ocean general circulation model
No full textIn a recent study of the shelf region of the East China Sea, Tsunogai et al. [1999] estimated that a combination of air-sea exchange and biological and physical transport processes could transfer carbon from the shelf region into the open ocean at a rate of 35 g Cm-2
yr-1. Contrasting with the solubility and biological pumps of the open ocean, they described this collective activity as the ‘‘continental shelf pump’’ and suggested that if this pump operated throughout the world’s shelf regions, it could be responsible for ocean uptake of ~1 Gt C yr-1
(~50% current ocean uptake of anthropogenic CO2). In this work a general circulation model (GCM) is used to explore the potential strength of this pump across the world’s shelves. Since the
GCM does not represent the continental shelf regions explicitly, a parameterization of the pump has been used. Results of simulations find modeled pump activity very variable between shelf regions, with the East China Sea shelf behaving very similarly to the global average. Storage of pump carbon is particularly high in the Atlantic Ocean and other regions where deep water is formed. A considerable reservoir of pump carbon becomes trapped under the Arctic ice sheet. Simple extrapolations from the results suggest that should shelf regions absorb CO2 at the rate of the East China Sea, the pump would account for a net oceanic uptake of 0.6 Gt C yr-1
Basin-wide mechanisms for spring bloom initiation; how typical is the North Atlantic?
Full text linkThe annual phytoplankton bloom is a key event in pelagic ecosystems. Variability in the timing, or phenology, of these blooms affects ecosystem dynamics with implications for carbon export efficiency and food availability for higher trophic levels. Furthermore, interannual variability in phytoplankton bloom timing may be used to monitor changes in the pelagic ecosystem that are either naturally or anthropogenically forced. The onset of the spring bloom has traditionally been thought to be controlled by the restratification of the water column and shoaling of the mixed layer, as encapsulated in Sverdrup's critical depth hypothesis. However, this has been challenged by recent studies which have put forward different mechanisms. For example, the critical turbulence hypothesis attributes bloom initiation to a reduction in turbulent mixing associated with the onset of positive net heat fluxes (NHFs). To date, the majority of studies on bloom initiation mechanisms have concentrated on North Atlantic datasets leaving their validity in other subpolar regions unknown. Here, we use chlorophyll output from a model that assimilates satellite ocean colour data to calculate bloom initiation timing and examine the basin-wide drivers of spatial and interannual variability. We find that the date that the NHF turns positive is a stronger predictor for the date of bloom initiation, both spatially and interannually, across the North Atlantic than changes in the mixed layer depth. However, results obtained from the North Pacific and Southern Ocean show no such basin-wide coherency. The lack of consistency in the response of the subpolar basins indicates that other drivers are likely responsible for variability in bloom initiation. This disparity between basins suggests that the North Atlantic bloom initiation processes are unique and therefore that this region may not be a suitable model for a global, theoretical understanding of the mechanisms leading to the onset of the spring bloom
Variability in efficiency of particulate organic carbon export: A model study
Full text linkThe flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e-ratio). Limited observations of the e-ratio are available and there is thus considerable interest in using remotely-sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e-ratio and the subsequent effect on export estimates using a high resolution global biogeochemical model. NEMO-MEDUSA represents export as separate slow and fast sinking detrital material whose remineralisation is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow sinking particles are factors in determining e-ratio, suggesting that current empirical algorithms for e-ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e-ratio variability. Extrapolating global export estimates from instantaneous measurements of e-ratio is strongly affected by seasonal variability, and can result in errors in estimated export of up to ±60%
Simulated impact of double-diffusive mixing on physical and biogeochemical upper ocean properties
No full textA global ocean circulation model coupled with a simple marine ecosystem model including the biogeochemical cycles and air–sea fluxes of oxygen and carbon dioxide is used to investigate the impact of double-diffusive mixing on upper ocean physical and biogeochemical properties. By comparing results for two different parameterizations of double-diffusive mixing, we also examine the sensitivity of our estimates to the particular representation of this process in general circulation models. Differences between the two parameterizations considered turned out to be much smaller than the difference with respect to a model run without double-diffusive mixing. For both parameterizations, the impact on upper ocean temperatures and salinities is relatively small (±1°C, ±0.25 psu regionally and 0.04°C, 0.01 psu as global rms difference over the top 50 m) and changes in surface heat flux amount to 0.05 W m?2 globally. However primary production and export production in the oligotrophic subtropics are found to increase by up to 80% and 120%, respectively, when double diffusion is switched on in the model. Double-diffusive nutrient supply generates an additional oceanic carbon uptake of about 0.4 g C m?2 year?1, amounting to 0.14 Gt C year?1 globally. <br/
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