We perform the first analysis of tropospheric transport using the global boundary propagator Green function, 𝒢, which partitions air at every point and time according to both the transit time since ...last surface contact and the location of that contact. We compute 𝒢 for a 3‐year period with the MATCH model driven by NCEP reanalyses. Last contact time is resolved in 3‐d intervals, and last‐contact location is resolved with a global tiling of 41 patches concentrated in the Northern Hemisphere. The transport climate is quantified for four midlatitude receptor regions in terms of the seasonal mean surface‐origin and transit‐time partitioning of the column burden, the surface flux of newly arriving air, and the distribution of air mass in transit from source to receptor surface. At long transit times a nearly receptor‐independent pattern of last‐contact location is governed by where air is injected into the upper troposphere by deep convection and the high terrain of Tibet. The receptor origin composition of the column burden changes only slowly after ∼40 d for winter and fall, while the composition of the flux onto the receptor continues to change at ∼60 d. European and SE Asian air contribute comparably to the flux onto eastern North America, in spite of SE Asian air having the dominant burden. The flux of European air onto SE Asia in winter and fall is larger than the flux of SE Asian air onto Europe. The surface‐to‐surface transport mass distribution, ℛ, is used to identify transit‐time‐dependent source‐receptor teleconnections.
39Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of 39Ar using a steady ocean circulation inverse model. Our ...estimates match available 39Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that 39Ar traces out the world ocean's ventilation pathways and that the 39Ar age ΓAr and the ideal mean age have broadly similar large‐scale patterns. At the surface, 39Ar is close to saturated except at high latitudes. Undersaturation imparts a finite 39Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating ΓAr by ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of 39Ar, thus providing guidance for future measurements.
Key Points
We estimate the global three‐dimensional 39Ar distribution in the ocean using a data‐assimilated circulation model
The 39Ar age correlates with the ideal mean age, encodes TTD‐shape information, and traces out the world ocean's ventilation pathways
High‐latitude surface waters are undersaturated in 39Ar imparting a reservoir age of order 100 years to Antarctic waters
Abstract
39
Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of
39
Ar using a steady ocean circulation inverse ...model. Our estimates match available
39
Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that
39
Ar traces out the world ocean's ventilation pathways and that the
39
Ar age Γ
Ar
and the ideal mean age have broadly similar large‐scale patterns. At the surface,
39
Ar is close to saturated except at high latitudes. Undersaturation imparts a finite
39
Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating Γ
Ar
by ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of
39
Ar, thus providing guidance for future measurements.
Key Points
We estimate the global three‐dimensional
39
Ar distribution in the ocean using a data‐assimilated circulation model
The
39
Ar age correlates with the ideal mean age, encodes TTD‐shape information, and traces out the world ocean's ventilation pathways
High‐latitude surface waters are undersaturated in
39
Ar imparting a reservoir age of order 100 years to Antarctic waters
We develop novel locally defined diagnostics for the efficiency of the ocean's biological pump by tracing carbon throughout its lifetime in the ocean from gas injection to outgassing and counting the ...number of passages through the soft‐tissue and carbonate pumps. These diagnostics reveal that the biological pump's key controls on atmospheric pCO2 are the mean number of lifetime pump passages per dissolved inorganic carbon (DIC) molecule at the surface and the mean aphotic sequestration time of regenerated DIC. We apply our diagnostics to an observationally constrained carbon‐cycle model that features spatially varying stoichiometric ratios and is embedded in a data‐assimilated global ocean circulation. We find that for the present‐day ocean an average of 44 ± 4% of DIC in a given water parcel makes at least one lifetime passage through the soft tissue pump, and about 4% makes at least one passage through the carbonate pump. The global mean number of lifetime pump passages per molecule, including the fraction with zero passages, is N¯soft=0.65±0.08 and N¯carb∼0.04 for the soft‐tissue and carbonate pumps. Using idealized perturbations to sweep out a sequence of states ranging from zero biological activity (pCO2atm=493±1 ppmv) to complete surface nutrient depletion (pCO2atm=207±1 ppmv), we find that fractional changes in pCO2atm are dominated by fractional changes in the number of soft‐tissue pump passages. At complete surface nutrient depletion, the mean fraction of DIC that has at least one lifetime passage through the soft‐tissue pump increases to 69 ± 5% with N¯soft=1.6±0.3.
Plain Language Summary
Tiny plants floating near the ocean surface use sunlight to convert nutrients and dissolved CO2 into organic matter. A fraction of this organic matter sinks, transferring carbon from the surface to the deep ocean, a process known as the biological pump. The biological pump reduces atmospheric CO2 and is thus important for moderating climate. The organic matter is oxidized by microbes, which regenerates inorganic carbon that can then pass through the biological pump again or return to the atmosphere. Here, we calculate, for the first time, the number of passages through the biological pump that a typical carbon molecule makes during its lifetime in the ocean. This carbon‐based measure of pump efficiency is useful for quantifying the biological pump's control on atmospheric CO2. We find that in the current state of the ocean 44 ± 4% of the dissolved inorganic carbon makes at least one lifetime passage through the biological pump, with a mean number of 0.65 ± 0.08 passages per molecule. In numerical experiments that stimulate the biological pump to utilize all available nutrients, the fraction with at least one passage increases to 69 ± 5% with 1.6 ± 0.3 passages per molecule, which draws down atmospheric CO2 by about 70 ppmv.
Key Points
The mean number of passages through the biological pump per carbon molecule is a natural metric of pump efficiency
Globally averaged 44 ± 4% of the dissolved carbon in a given water parcel is biologically pumped with 0.65 ± 0.08 lifetime passages per molecule
Changes in atmospheric CO2 are related to changes in the mean number of pump passages and the mean injection‐tagged sequestration time
Abstract
Interhemispheric transport from the earth’s surface north of 32.4°N (region ΩN) to the surface south of 32.4°S (region ΩS) is quantified using the path-density diagnostic developed in Part I ...of this study. The path density is computed using the Model of Atmospheric Transport and Chemistry (MATCH) driven by NCEP reanalyses. The structure of both the ΩN → ΩS and ΩN → ΩN zonally averaged path densities is examined in detail for air that had last ΩN contact (“ΩN air”) during January and July. The path density provides the joint probability that ΩN air will make its next surface contact with either ΩN or ΩS, that it will have surface-to-surface transit time τ ∈ (τ, τ + dτ), and that it can be found in volume element d 3r during its surface-to-surface journey. The distribution of surface-to-surface transit times, the probability of ΩN air making next contact with ΩS, and the probability of finding ΩN air destined for ΩS in the stratosphere are computed from suitable integrations of the path density. Approximately one-third of the ΩN air undergoes interhemispheric transport to ΩS, with a ∼20% probability of being found in the stratosphere during its surface-to-surface journey. The stratospheric fraction has about equal contributions from the part of the stratosphere that is isentropically isolated from the troposphere and from the part that is isentropically connected to the troposphere (i.e., from the overworld and the stratospheric middleworld in the terminology of Hoskins). The flow rate through the stratospheric middleworld is about twice as large as the flow rate through the overworld.
Abstract
A new path-density diagnostic for atmospheric surface-to-surface transport is formulated. The path density η gives the joint probability that air whose last surface contact occurred on patch ...Ωi at time ti will make its next surface contact with patch Ωf after a residence time τ ∈ (τ, τ + dτ) and that it can be found in d3r during its surface-to-surface journey. The dependence on τ allows the average surface-to-surface flow rate carried by the paths to be computed. A simple algorithm for using passive tracers to determine η is developed. A key advantage of the diagnostic is that it can be computed efficiently without an adjoint model and using only a moderately large number of tracers. The nature of the path density is illustrated with a one-dimensional advection–diffusion model. In Part II of this study, the path density diagnostic is applied to quantify interhemispheric transport through the troposphere and stratosphere.
The stoichiometric carbon to phosphorus ratios (rC:P) in suspended particulate organic matter (POM) are generally inversely correlated with surface phosphate (PO4) concentration. However, it is ...uncertain if previously suggested relationships between rC:P and PO4 are appropriate for the vertical export flux of organic matter. Using a global steady‐state inverse ocean biogeochemistry model and annual‐mean observed tracers, we estimate optimal parameters for both linear and power law representations of rP:C (= 1/rC:P), and find rP:C = (0.0066 ± 0.0018) × PO4 + (0.0053 ± 0.0010) and rP:C = (0.0112 ± 0.0018) × PO4(0.36±0.07), respectively, where PO4 is in μM. Both parameterizations allow us to fit global tracer observations equally well, but the power law model implies an up to 50% larger uptake rC:P in oligotrophic gyres. For both formulations, the POM export rC:P from the euphotic zone is nearly equal to the uptake rC:P, while the dissolved organic matter export rC:P is up to two times larger than the uptake rC:P. Although weakly constrained, our model suggests that in eutrophic regions the vertical organic P fluxes are attenuated faster with depth than the organic C fluxes. By contrast, in oligotrophic regions there are no discernible differences between the organic P and C flux‐attenuation profiles. As a result, the large spatial range of rC:P spanning 50–200 at the base of the euphotic zone is diminished to 110–160 at 2000 m depth. In oligotrophic regions at 150–500 m depths, our estimated export rC:P values are significantly lower than those measured with sediment traps, implying a potentially large modulation of export rC:P by migrating zooplankton within the twilight zone.
Plain Language Summary
The ratio of carbon to phosphorus (C:P) in organic matter is a key measure of food quality for marine ecosystems and the ability of phytoplankton to fix carbon. It has recently been discovered that phytoplankton communities have higher C:P ratios where nutrients are scarce and lower C:P ratios where nutrients are abundant. Yet, it is unclear whether the C:P ratios in marine organic matter would remain unchanged or be modified while organic matter is transported downward through the water column. Here we provide an independent constraint on how C:P ratios covary with phosphate at the sea surface and how C:P ratios are modified in the vertically exported organic matter at different depths. We find that organic P is preferentially respired by marine biota compared to organic C except in subtropical gyres where zooplankton actively transport organic P to deeper waters. Preferential removal of organic P in high latitudes and relatively deeper transport in low latitudes diminish the large spatial variability in the C:P ratios, initially set at the sea surface, as depth increases.
Key Points
Linear and power law relationships between uptake stoichiometric carbon to phosphorus ratios (rC:P) and phosphate are optimized using an inverse model and multiple biogeochemical tracers
Phytoplankton rC:P and preferential remineralization of dissolved organic phosphorus over dissolved organic carbon equally contribute to spatial variations in DOM export ratio of carbon to phosphorus (C:P) ratios
Estimated particulate organic matter export C:P ratios for 150–500 m depths are compared with estimates based on flux measurements at Stations ALOHA and Bermuda Atlantic Timeseries Study
The atmosphere is known to episodically transport dust, aerosols, and gaseous pollutants from industrialized east Asia, the Gobi desert, and Siberian wild fires to western North America. We give a ...novel characterization of the climatological springtime transport from these regions and of the probability of transport “events,” that is, long‐range transport of high concentrations with minimal dispersion. Our primary transport diagnostic is the transit‐time probability density function (pdf), , which is a tracer‐independent measure of the flow that allows us to isolate the role of transport from other factors such as source variability and chemistry. The pdf approach, unlike typical back‐trajectory analyses, captures transport due to all possible paths and accounts for both resolved advection and subgrid processes. We use a numerical model of the global atmosphere (Model of Atmospheric Transport and Chemistry (MATCH)) driven by National Centers for Environmental Prediction reananlysis data to establish the springtime statistics of daily averages of . A suitably defined average of quantifies the climatological mass fraction of air from the source region per interval of transit times, or ages. Over the North American west coast, this fraction peaks at transit times of ∼8 days in the upper troposphere (∼6 days later at the surface) for the dust and pollution regions and at 12–14 days for the Siberian region. An analysis of the variability of at fixed transit time allows us to identify transport events and to estimate their probability of occurrence. This is illustrated for transport events to the “Pacific Northwest” (PNW) region of North America, defined as (43.8°–53.3°N) × (115.3°–124.7°W). Correlations between averaged over the PNW and the winds at any point in the atmosphere identify large‐scale anomaly structures of the flow that correspond to favorable transport to the PNW.
We explore how the iron dependence of the Si:P uptake ratio RSi:P of diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron ...fertilization. We use a data‐constrained model of the coupled Si‐P‐Fe cycles that features a mechanistic representation of nutrient colimitations for three phytoplankton classes and that is embedded in a data‐assimilated global ocean circulation model. We consider three parameterizations of the iron dependence of RSi:P, all of which are consistent with the available field data and allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply RSi:P decreases with increasing iron concentration, iron fertilization can either cause enhanced silicic acid leakage from the Southern Ocean or strengthened Southern Ocean silicon trapping. Enhanced silicic acid leakage occurs if decreases in RSi:P win over increases in diatom growth, while the converse causes strengthened Southern Ocean silicon trapping. Silicic acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low‐latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron‐replete conditions reduces the global opal export. Regardless of RSi:P parameterization, the global response of the biological phosphorus and silicon pumps is dominated by the Southern Ocean. The Si isotope signature of opal flux becomes systematically lighter with increasing iron‐induced silicic acid leakage, consistent with sediment records from iron‐rich glacial periods.
Key Points
Iron fertilization leads to silicic acid leakage if the Si:P uptake ratio decreases sharply enough to overwhelm increased diatom growth
Silicic acid leakage increases low‐latitude opal export, while the Southern Ocean dominates increases in diatom organic matter export
Silicic acid leakage in response to iron fertilization produces isotopically lighter biogenic opal flux
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