110 research outputs found

    Increased risk of a shutdown of ocean convection posed by warm North Atlantic summers

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    A shutdown of ocean convection in the subpolar North Atlantic, triggered by enhanced melting over Greenland, is regarded as a potential transition point into a fundamentally different climate regime1,2,3. Noting that a key uncertainty for future convection resides in the relative importance of melting in summer and atmospheric forcing in winter, we investigate the extent to which summer conditions constrain convection with a comprehensive dataset, including hydrographic records that are over a decade in length from the convection regions. We find that warm and fresh summers, characterized by increased sea surface temperatures, freshwater concentrations and melting, are accompanied by reduced heat and buoyancy losses in winter, which entail a longer persistence of the freshwater near the surface and contribute to delaying convection. By shortening the time span for the convective freshwater export, the identified seasonal dynamics introduce a potentially critical threshold that is crossed when substantial amounts of freshwater from one summer are carried over into the next and accumulate. Warm and fresh summers in the Irminger Sea are followed by particularly short convection periods. We estimate that in the winter 2010–2011, after the warmest and freshest Irminger Sea summer on our record, ~40% of the surface freshwater was retained

    EUREC⁎A

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    The science guiding the EURECA campaign and its measurements is presented. EURECA comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EURECA explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EURECA's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

    EUREC⁎A

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    The science guiding the EURECA campaign and its measurements is presented. EURECA comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EURECA explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EURECA's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

    Influence of the oceanic cool skin layer on global air–sea CO2 flux estimates

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    The global oceans are a major sink for atmospheric CO2, but the magnitude of this sink is still under question since there are many uncertainties inherent in determining global CO2 fluxes across the air–sea interface. The sign and magnitude of the air–sea fluxes show significant regional and seasonal variation. The gas transfer variables necessary to determine air–sea CO2 fluxes are temperature dependent and studies of global CO2 fluxes commonly rely on measurements of the sub-surface oceanic mixed layer temperature, rather than the cooler skin temperature for these calculations. This surface skin temperature is, on average, about 0.2K cooler than that of the mixed layer, leading to underestimates of oceanic CO2 uptake when the mixed layer temperature is used for calculations. This study explores the impact, upon both the global annual mean, and as seasonal global distributions, of replacing a mixed layer temperature measurement with a skin temperature measurement to improve global estimates of air–sea CO2 exchange, making use of extensive satellite and in situ measurements. Resulting estimates show, contrary to previous studies, that the contribution of the cool skin is relatively minor on a global scale, suggesting that calculations can confidently continue to move forward in refining estimates and monitoring air–sea CO2 exchange from remotely sensed parameters, providing better resolution both in time and space in future studies. ‱We improve estimates of global CO2 fluxes by replacing Tdepth with SSTskin.‱We utilize improved parameterizations of the SSTskin–Tdepth temperature difference.‱We use more realistic probabilistic representation of the global wind field.‱Satellite derived temperature can be confidently used over ship based observations

    Photophysical consequences of porphyrin tautomerization. Steady-state and time-resolved spectral investigations of a zinc isoporphyrin

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    Isoporphyrins are porphyrin tautomers with a saturated meso carbon and thus an interrupted π system. We report here steady-state optical absorption, fluorescence, and fluorescence polarization data as well as time-resolved results that detail the significant effects of porphyrine tautomerization on the photophysical properties of a metallo-isoporphyrin, zinc 2,3,5,5â€Č,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl) isoporphyrin perchlorate (2). Besides the red-shifted, low-energy absorption bands diagnostic of metallo-isoporphyrins, 2 exhibits a large Stokes shift of its fluorescence emission (approximately 600 cm-1) and an unusually short singlet excited-state lifetime at room temperature (130 ± 15 ps), photophysical properties distinctly different from those of the canonical prophyrin tautomers. The only porphyrins to exhibit marginally similar perturbations of their photophysical properties are those with severely nonplanar macrocyles whose π systems are significantly destabilized by the conformational distortions and thus approach the interrupted π systems of isoporphyrins (Gentemann et al: J. Am. Chem. Soc. 1994, 116, 7363). In addition to providing the first insights into the photophysical consequences of porphyrin tautomerization, the results for the isoporphyrin further document the sensitivity of the fundamental electronic and excited-state properties of porphyrinic chromophores to modulation of their π systems in vitro and, by extrapolation, in vivo as well
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