295 research outputs found
Physical and biogeochemical forcing of oxygen and nitrate changes during El Niño/El Viejo and La Niña/La Vieja upper-ocean phases in the tropical eastern South Pacific along 86° W
Temporal changes in the water mass distribution and biogeochemical signals in the tropical eastern South Pacific are investigated with the help of an extended optimum multi-parameter (OMP) analysis, a technique for inverse modeling of mixing and biogeochemical processes through a multidimensional least-square fit. Two ship occupations of a meridional section along 85°50' W from 14° S to 1° N are analysed during relatively warm (El Niño/El Viejo, March 1993) and cold (La Niña/La Vieja, February 2009) upper-ocean phases. The largest El Niño–Southern Oscillation (ENSO) impact was found in the water properties and water mass distribution in the upper 200 m north of 10° S. ENSO promotes the vertical motion of the oxygen minimum zone (OMZ) associated with the hypoxic equatorial subsurface water (ESSW). During a cold phase the core of the ESSW is found at shallower layers, replacing shallow (top 200 m) subtropical surface water (STW). The heave of isopycnals due to ENSO partially explains the intrusion of oxygen-rich and nutrient-poor antarctic intermediate water (AAIW) into the depth range of 150–500 m. The other cause of the AAIW increase at shallower depths is that this water mass flowed along shallower isopycnals in 2009. The shift in the vertical location of AAIW reaching the OMZ induces changes in the amount of oxygen advected and respired inside the OMZ: the larger the oxygen supply, the greater the respiration and the lower the nitrate loss through denitrification. Variations in the intensity of the zonal currents in the equatorial current system, which ventilates the OMZ from the west, are used to explain the patchy latitudinal changes of seawater properties observed along the repeated section. Significant changes reach down to 800 m, suggesting that decadal variability (Pacific decadal oscillation) is also a potential driver in the observed variability
From interannual to decadal: 17 years of boundary current transports at the exit of the Labrador Sea
Over the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53°N observatory (moorings and shipboard station data) measuring the top-to-bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 ± 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 ± 3.9 Sv) and Lower North Atlantic Deep Water (LNADW, 15.3 ± 3.8 Sv). The transport variability ranges from days to decades, with the most prominent multiyear fluctuations at interannual to near decadal time scales (±5 Sv) in the LNADW overflow water mass. These long-term fluctuations appear to be in phase with the NAO-modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely, the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom-intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the deep water export by 10–15%
Multidisciplinary Observing in the World Ocean\u27s Oxygen Minimum Zone Regions: From Climate to Fish - The VOICE Initiative
Multidisciplinary ocean observing activities provide critical ocean information to satisfy ever-changing socioeconomic needs and require coordinated implementation. The upper oxycline (transition between high and low oxygen waters) is fundamentally important for the ecosystem structure and can be a useful proxy for multiple observing objectives connected to eastern boundary systems (EBSs) that neighbor oxygen minimum zones (OMZs). The variability of the oxycline and its impact on the ecosystem (VOICE) initiative demonstrates how societal benefits drive the need for integration and optimization of biological, biogeochemical, and physical components of regional ocean observing related to EBS. In liaison with the Global Ocean Oxygen Network, VOICE creates a roadmap toward observation-model syntheses for a comprehensive understanding of selected oxycline-dependent objectives. Local to global effects, such as habitat compression or deoxygenation trends, prompt for comprehensive observing of the oxycline on various space and time scales, and for an increased awareness of its impact on ecosystem services. Building on the Framework for Ocean Observing (FOO), we present a first readiness level assessment for ocean observing of the oxycline in EBS. This was to determine current ocean observing design and future needs in EBS regions (e.g., the California Current System, the Equatorial Eastern Pacific off Ecuador, the Peru-Chile Current system, the Northern Benguela off Namibia, etc.) building on the FOO strategy. We choose regional champions to assess the ocean observing design elements proposed in the FOO, namely, requirement processes, coordination of observational elements, and data management and information products and the related best practices. The readiness level for the FOO elements was derived for each EBS through a similar and very general ad hoc questionnaire. Despite some weaknesses in the questionnaire design and its completion, an assessment was achievable. We found that fisheries and ecosystem management are a societal requirement for all regions, but maturity levels of observational elements and data management and information products differ substantially. Identification of relevant stakeholders, developing strategies for readiness level improvements, and building and sustaining infrastructure capacity to implement these strategies are fundamental milestones for the VOICE initiative over the next 2-5 years and beyond
Rapid cooling and increased storminess triggered by freshwater in the North Atlantic
Recent winters have been unique due to the rapid and extreme cooling of the subpolar North Atlantic. Here, we present a novel view on its causes and consequences. Combining in‐situ observations with remote sensing and atmospheric reanalysis data, we show that increased freshening of the subpolar region gives rise to a faster surface cooling in fall and winter. Large freshwater events, in particular, result in pronounced cold anomalies with sharp temperature gradients that promote an enhanced storminess. The storms reinforce the cooling by driving stronger heat losses and modulating the surface flow. Consistent with this mechanism, past freshwater events have been followed by cold anomalies in winter of ~‐2°C and increases in the North Atlantic Oscillation index of up to~0.6 within 3 years. We expect that future freshwater discharges into the North Atlantic will amplify the cold anomaly and trigger an enhanced wintertime storminess with far‐reaching climatic implications
Mean circulation and EKE distribution in the Labrador Sea Water level of the subpolar North Atlantic
A long-term mean flow field for the subpolar North Atlantic region
with a horizontal resolution of approximately 25 km is created by gridding
Argo-derived velocity vectors using two different topography-following
interpolation schemes. The 10-day float displacements in the typical drift
depths of 1000 to 1500 m represent the flow in the Labrador Sea Water
density range. Both mapping algorithms separate the flow field into
potential vorticity (PV) conserving, i.e., topography-following contribution
and a deviating part, which we define as the eddy contribution. To verify
the significance of the separation, we compare the mean flow and the eddy
kinetic energy (EKE), derived from both mapping algorithms, with those
obtained from multiyear mooring observations.The PV-conserving mean flow is characterized by stable boundary currents
along all major topographic features including shelf breaks and
basin-interior topographic ridges such as the Reykjanes Ridge or the Rockall
Plateau. Mid-basin northward advection pathways from the northeastern
Labrador Sea into the Irminger Sea and from the Mid-Atlantic Ridge region
into the Iceland Basin are well-resolved. An eastward flow is present across
the southern boundary of the subpolar gyre near 52° N, the
latitude of the Charlie Gibbs Fracture Zone (CGFZ).The mid-depth EKE field resembles most of the satellite-derived surface EKE
field. However, noticeable differences exist along the northward advection
pathways in the Irminger Sea and the Iceland Basin, where the deep EKE
exceeds the surface EKE field. Further, the ratio between mean flow and the
square root of the EKE, the Peclet number, reveals distinct
advection-dominated regions as well as basin-interior regimes in which
mixing is prevailing.</p
On the meridional ageostrophic transport in the tropical Atlantic
The meridional Ekman volume, heat, and salt transport across two
trans-Atlantic sections near 14.5° N and 11° S were
estimated using in situ observations, wind products, and model data. A
meridional ageostrophic velocity was obtained as the difference between the
directly measured total velocity and the geostrophic velocity derived from
observations. Interpreting the section mean ageostrophy to be the result of
an Ekman balance, the meridional Ekman transport of 6. 2 ± 2. 3 Sv
northward at 14.5° N and 11. 7 ± 2. 1 Sv southward at
11° S is estimated. The integration uses the top of the pycnocline
as an approximation for the Ekman depth, which is on average about
20 m deeper than the mixed layer depth. The Ekman transport estimated
based on the velocity observations agrees well with the predictions from
in situ wind stress data of 6. 7 ± 3. 5 Sv at 14.5° N and
13. 6 ± 3. 3 Sv at 11° S. The meridional Ekman heat and
salt fluxes calculated from sea surface temperature and salinity data or from
high-resolution temperature and salinity profile data differ only marginally.
The errors in the Ekman heat and salt flux calculation were dominated by the
uncertainty of the Ekman volume transport estimates
Test Results on the Silicon Pixel Detector for the TTF-FEL Beam Trajectory Monitor
Test measurements on the silicon pixel detector for the beam trajectory
monitor at the free electron laser of the TESLA test facility are presented. To
determine the electronic noise of detector and read-out and to calibrate the
signal amplitude of different pixels the 6 keV photons of the manganese K line
are used. Two different methods determine the spatial accuracy of the detector:
In one setup a laser beam is focused to a straight line and moved across the
pixel structure. In the other the detector is scanned using a low-intensity
electron beam of an electron microscope. Both methods show that the symmetry
axis of the detector defines a straight line within 0.4 microns. The
sensitivity of the detector to low energy X-rays is measured using a vacuum
ultraviolet beam at the synchrotron light source HASYLAB. Additionally, the
electron microscope is used to study the radiation hardness of the detector.Comment: 14 pages (Latex), 13 figures (Postscript), submitted to Nuclear
Instruments and Methods
Uncertainty in temperature response of current consumption-based emissions estimates
Several studies have connected emissions of greenhouse gases to economic and
trade data to quantify the causal chain from consumption to emissions and
climate change. These studies usually combine data and models originating
from different sources, making it difficult to estimate uncertainties along
the entire causal chain. We estimate uncertainties in economic data,
multi-pollutant emission statistics, and metric parameters, and use Monte
Carlo analysis to quantify contributions to uncertainty and to determine how
uncertainty propagates to estimates of global temperature change from
regional and sectoral territorial- and consumption-based emissions for the
year 2007. We find that the uncertainties are sensitive to the emission
allocations, mix of pollutants included, the metric and its time horizon,
and the level of aggregation of the results. Uncertainties in the final
results are largely dominated by the climate sensitivity and the parameters
associated with the warming effects of CO2. Based on our assumptions,
which exclude correlations in the economic data, the uncertainty in the
economic data appears to have a relatively small impact on uncertainty at the
national level in comparison to emissions and metric uncertainty. Much higher
uncertainties are found at the sectoral level. Our results suggest that
consumption-based national emissions are not significantly more uncertain
than the corresponding production-based emissions since the largest
uncertainties are due to metric and emissions which affect both perspectives
equally. The two perspectives exhibit different sectoral uncertainties, due
to changes of pollutant compositions. We find global sectoral consumption
uncertainties in the range of ±10 to ±27 % using the Global
Temperature Potential with a 50-year time horizon, with metric uncertainties
dominating. National-level uncertainties are similar in both perspectives
due to the dominance of CO2 over other pollutants. The consumption
emissions of the top 10 emitting regions have a broad uncertainty range of
±9 to ±25 %, with metric and emission uncertainties
contributing similarly. The absolute global temperature potential (AGTP) with a 50-year time horizon has much higher uncertainties, with considerable
uncertainty overlap for regions and sectors, indicating that the ranking of
countries is uncertain
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