1,721,230 research outputs found
Multiple timescales of stochastically forced North Atlantic Ocean variability: A model study
Full text linkThe Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) are important elements in mechanisms for multidecadal variability in models in the North Atlantic Ocean. In this study, a 2000-year long global ocean model integration forced with the atmospheric patterns associated with a white noise North Atlantic Oscillation (NAO) index is shown to have three distinct timescales of North Atlantic Ocean variability. First, an interannual timescale with variability shorter than 15 years, that can be related to Ekman dynamics. Second, a multidecadal timescale, on the 15- to 65-year range, that is mainly concentrated in the SPG region and is controlled by constructive interference between density anomalies around the gyre and the changing NAO forcing. Finally, the centennial timescales, with variability longer than 65 years, that can be attributed to the ocean being in a series of quasi-equilibrium states. The relationship between the ocean’s response and the NAO index differs for each timescale; the 15-year and shorter timescales are directly related to the NAO of the same year, 15- to 65-year timescales are dependent on the NAO index in the last 25–30 years in a sinusoidal sense while the 65-year and longer timescales relate to a sum of the last 50–80 years of the NAO index
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Diapycnal and along isopycnal mixing, estimated from the tracer release experiment, at the North Atlantic oxygen minimum zone
Full text linkA deliberate Guinea Upwelling Tracer Release Experiment (GUTRE) in 2008 - 2010 was used to study diapycnal mixing and lateral stirring in the thermocline of tropical northeastern Atlantic. The ultimate goal of this work was to investigate oxygen supply pathways at the upper boundary of North Atlantic oxygen minimum zone. The mean diapycnal mixing coefficients were computed for a tracer patch integrated over 30 months. The strong variation of stratification in the tracer occupied area offered the advantage of using the advection-diffusion equation in isopycnal coordinates with the thickness-weighted averaging, when analyzing the diapycnal tracer spread. Overall, the vertical mixing estimates were found about 30% smaller than estimates in North Atlantic Tracer Release Experiment (NATRE), performed about 15° to the north from our experiment. In general, the internal wave-wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, GUTRE results are larger than parametrized by the low latitude of the site (4°N - 12°N). The mean isopycnal mixing coefficient was estimated by computing the growth of the second moment of a cloud of tracer. The zonal component of lateral mixing was found more than two times larger than the meridional component. The finding reflects the stretching effect of zonal jets prevalent in the tropical Atlantic Ocean. The application of conceptual jet model allowed to evaluate the mean zonal jet velocities which cause an enhancement of mixing in zonal direction. Additionally, the effective mixing coefficient was computed which indicated that zonal jets do not merely stretch the tracer patch in zonal directions, but increase mixing by increasing tracer gradients. The uncertainties of the estimates were inferred from synthetic particle release using a high resolution ocean circulation model. Finally, a large database of vertical oxygen profiles in oxygen minimum region and high precision diapycnal mixing estimates allowed for a good estimate of diapycnal oxygen supply through the upper boundary into the oxygen minimum zone. In comparison to conceptual model of oxygen consumption with depth, the diapycnal oxygen supply was estimated to be as large as about half of the lateral oxygen supply. Furthermore, in the oxygen minimum region, the separate shallow oxygen minimum was found at about 100 m depth. The finding indicates that there is no direct net vertical oxygen flux from the surface layer into the oxygen minimum zone. All of oxygen supply, hence, is associated with remote pathways
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Extratropical Sea Surface Temperature Impacts on Large-scale Atmospheric Circulation
Full text linkThe atmospheric response to large-scale extratropical North Pacific sea surface temperature (SST) anomalies associated with the Pacific Decadal Oscillation (PDO) is assessed using observational data, coupled ocean-atmosphere general circulation model (CGCM) simulations, and forced high-resolution atmospheric general circulation model (AGCM) experiments. The characteristics of the atmospheric response as well as its sensitivity to daily to decadal variability in the background SST, on which the SST anomalies are superimposed, are inspected. Here, only the boreal winter (DJF) season is considered. The analysis of the observational data reveals that the PDO exhibits significant variability on interannual, decadal and multidecadal timescales, and is highly correlated with the El Niño/Southern Oscillation (ENSO). In the atmosphere, significant spectral peaks at similar frequencies as those identified in the PDO are found in the observed sea level pressure over the North Pacific, suggesting potential links between the ocean and the atmosphere. This link is further supported by a number of regression and correlation analyses. It is proposed that SST anomalies associated with the PDO can persist for several winters through the “reemergence” mechanism, repeatedly exerting anomalous SST forcing on the atmosphere and causing the latter to respond. The atmospheric response is of the “cold SST-low pressure”-type, also known as the “cold-trough” (or “warm-ridge”) response. Exactly the same analyses as those described above are repeated on the output of a coarse-resolution CGCM, the Kiel Climate Model (KCM). Results show that the KCM depicts the reemergence mechanism but does not simulate an atmospheric response consistent with that suggested by the observations. It is conjectured that too coarse horizontal resolution in the atmospheric component of the KCM inhibits a realistic representation of the atmospheric response to extratropical North Pacific SST anomalies. To further investigate the mechanism of the atmospheric response and to examine the reason for the failure of the KCM to simulate such a response, a high-(horizontal) resolution version of the ECHAM5 AGCM is integrated in stand-alone mode forced by prescribed SSTs. The SST forcing consists of the observed daily SSTs of 1981–1990, which serve as the background SSTs, on which a fixed PDO-like SST anomaly, which as been derived from observations, is superimposed. Results show that the winter-mean atmospheric response to the SST anomaly (averaged over all 10 winters) is equivalent barotropic, with a significant low pressure anomaly over the eastern North Pacific and a northeast-southwest shift in the storm track. It is further pointed out that the response is established and maintained by anomalous eddy vorticity and momentum flux convergence. Noticeable changes in zonal and vertical winds are also found. In terms of surface pressure, the model results compare reasonably well with the observations. In order to further investigate the role of the background SSTs in the atmospheric response, a set of sensitivity experiments with the ECHAM5 AGCM is designed employing identical SST anomalies but different background SSTs. It is found that daily background SST variability plays a key role in the atmospheric response, whereas interannual variability of the background SST is only of minor importance. It is thus proposed that sufficient daily extratropical SST variability must be simulated by the ocean components and resolved by the atmospheric components of CGCMs to enable realistic simulation of decadal climate variability in the North Pacific sector. Identical AGCM integrations were conducted for the subsequent 10 winters of 1991–2000. Together, the two sets of integrations reveal a remarkable decadal transition with sign reversal in terms of the area-averaged 500 hPa height anomalies over the North Pacific. Although the response is characterized by an equivalent barotropic, circumglobal Rossby wave train in both periods, the primary wave source switches from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981–1990 to mean-flow divergence over the western North Pacific in 1991–2000. By examining the decadal changes of the background SSTs, it is then proposed that the wave source transition can be linked to the decadal reduction of daily SST variability in the eastern North Pacific and the decadal strengthening of the Oyashio Extension front over the western North Pacific. In this view, the decadal variations of small-scale features of the background SSTs, temporal and spatial, are emphasized
The importance of the representation of stratospheric dynamics and chemistry for surface climate variability
Full text linkThe stratosphere is attracting more and more attention for its potential to improve northern hemisphere (NH) seasonal weather forecasts, by modulating the North Atlantic Oscillation, or for its importance to reproduce observed southern hemisphere (SH) tropospheric jet trends, shaped by stratospheric ozone depletion. However, open questions remain regarding, for example, the stratospheric impact onto the ocean or the extent to which the complexity of stratospheric chemistry is important for surface climate. Within this thesis these two questions shall be answered. The importance of including a full representation of the stratosphere for North Atlantic and North Pacific Ocean variability is investigated for the first time. Furthermore, this thesis investigates the impact of interactive chemistry onto surface climate variability in an unprecedented model study, systematically reducing the complexity of stratospheric chemistry from an interactive chemistry scheme to a specified one prescribing zonal mean as well as zonally asymmetric ozone concentrations. [...] This thesis advances the understanding of the impact of stratospheric dynamics and feedbacks between chemistry and dynamics on surface variability. It concludes that the dynamical representation of the stratosphere is crucial for a more realistic representation of surface variability in the North Atlantic and Pacific Ocean. It underlines the importance of feedbacks betweenchemistry and dynamics for the characteristics of the stratospheric mean state, variability and stratosphere-troposphere-coupling. Especially under a strong ozone forcing, such as the observed Antarctic ozone depletion, incorporating interactive chemistry or at least a zonally asymmetric ozone forcing in a climate model is necessary for a proper representation of surface climate variability
Oxygen Variability and Eddy-driven Meridional Oxygen Supply in the Tropical North East Atlantic Oxygen Minimum Zone
Full text linkThe distribution of the mean oceanic oxygen concentration results from a balance between ventilation and consumption and, in particular, reveals extended oxygen minimum zones (OMZ) in the eastern tropical Pacific and Atlantic at intermediate depth (300m - 700m). It has been recently shown that OMZs expand in size and are subject to a significant oxygen decrease, where the OMZ in the Tropical North East Atlantic (TNEA) holds the most significant and largest oxygen trend. This study analyzes hydrographic and velocity data from shipboard and moored observations which were acquired along the 23°W section cutting meridionally through the TNEA OMZ, in order to (i) quantify regional differences in the oxygen variability, (ii) identify the role of two mixing processes (mesoscale stirring and diapycnal mixing) in the production of oxygen variance based on the extended Osborn-Cox model and (iii) estimate the role of mesoscale eddies in the meridional ventilation of the TNEA OMZ. Enhanced oxygen variability is found at the boundaries of the OMZ which is predominantly generated by mesoscale stirring along isopycnals and diapycnal mixing across isopycnals. South of the OMZ core (which is located at about 11°N), mesoscale stirring dominates the generation of oxygen variance, whereas above the OMZ core within the deep oxycline (located at about 300m depth) both processes, mesoscale stirring and diapycnal mixing, are found to be of similar importance. Meridional eddy fluxes of oxygen are estimated by using (i) a diffusive flux parameterization based on a lateral eddy diffusion coefficient and the mean isopycnal oxygen gradient, and (ii) a correlation method based on velocity and oxygen time series from moored observations. At the mooring positions 5°N, 23°W and 8°N, 23°W, the results of both methods are in good agreement in the depth range of the OMZ core, yielding a northward oxygen flux from the well-ventilated equatorial region toward the OMZ core. The divergence of the meridional oxygen flux, as obtained from the diffusive flux parameterization, yields an eddy-driven meridional oxygen supply from the south into the OMZ of about 2.4 μmol kg−1 yr−1 at its core depth. Above the OMZ core, mesoscale eddies act to redistribute low-oxygen and high-oxygen waters associated with latitudinally alternating westward and eastward currents. Locally, absolute values of the eddy-driven meridional oxygen supply are greater than 10 μmol kg−1 yr−1 which is likely balanced by mean zonal advection. Combining the above results with recent studies about oxygen consumption, diapycnal oxygen supply and oxygen tendency, a refined oxygen budget for the TNEA OMZ is derived. The eddy-driven meridional oxygen supply accounts for more than 50% of the supply required to balance the estimated oxygen consumption. The oxygen tendency in the TNEA OMZ, as given by the multidecadal oxygen decline, is at maximum slightly above the OMZ core and represents a substantial imbalance of the oxygen budget reaching up to 20% of the magnitude of the eddy-driven oxygen supply. Oxygen data from moored observations was acquired with optode oxygen sensors. To achieve a high quality of these measurements, an in situ calibration procedure is proposed which yields an average measurement error of 4.6 μmol kg−1 with 95% confidence (evaluated for water masses in the upper 1000m of the Tropical Atlantic in combination with onboard lab calibrations against zero oxygen). On the one hand, this calibration procedure yields calibration errors that are worse by roughly a factor of 2 in comparison to sophisticated laboratory calibrations, but on the other hand this simple method is not affected by severe sensor drifts that are frequently observed at some time before or after the field deployment. In case of profiling systems (shown here for a CTD/O2 cast), a time constant correction improves the quality of the measured oxygen profile which is of particular interest for the application in gliders or floats
North Atlantic multidecadal to centennial variability in a model and a marine proxy dataset
Full text linkVariability on decadal and longer timescales is of great interest in climate research due to it’s socio-economic impacts, potential for predictability and masking of anthroprogenic global warming. Observational evidence of multidecadal variability in the North Atlantic exists in the sea surface temperature (SST), often referred to as the Atlantic Multidecadal Variability (AMV), and also in the atmosphere, for example seen in sea level pressure variations associated with the North Atlantic Os- cillation (NAO). Observational oceanic data on these timescales is mainly restricted to the surface, does not extend past the last 145 years and becomes quite sparsely sampled in the higher latitudes in the earlier years. Hence, to increase our understanding of climate variability on these timescales it is essential to turn to both proxy and model data. The first part of this thesis focuses on an annually-resolved proxy record (1818- 1967) of Mg/Ca variations from a North Pacific/ Bering Sea coralline alga. Not only does the algal Mg/Ca have a very strong connection to the local winter SST and a lagged relation to the Aleutian Low it also it has a correlation of -0.87 with the winter NAO and 0.60 with the AMV index on decadal timescales. The link can explain the coherence of decadal North Pacific and AMV, as suggested by earlier studies using climate models and in the limited observational data. The second part of the thesis focuses on the ocean general circulation model, NEMO to better understand AMV. For this purpose the model was forced only with the atmospheric patterns associated with the NAO, both from the observed NAO index and from a 2000 year long white NAO index. Focusing on key ocean circulation patterns, we show that the Atlantic Meridional Overturning Circula- tion (AMOC) and sub-polar gyre (SPG) strength have a red noise response but no dominant timescale, providing no evidence for a oscillatory ocean-only mode of variability. The time derivative of both the AMOC at 30◦N and SPG strength show a strong, almost linear relation to the NAO for timescales longer than 86 and 15 years, respectively. The different response characteristics are confirmed by constructing simple statistical models that show AMOC and SPG variability can be reconstructed by integrating the NAO index by the previous 53 and 10 winters, respectively. Alternatively, the AMOC and the SPG strength can be reconstructed with auto-regressive (AR) models of order seven and five, respectively. A closer look at the ocean model response of the 2000 year long ocean model integration shows three distinct timescales of variability. The first, an interannual timescale with variability shorter than 15 years, can be mainly related to Ekman dynamics. Secondly, the multidecadal timescale, 15-65 years, is mainly concentrated in the SPG and is controlled by temperature variability. Finally, the centennial timescales, with variability longer than 65 years, can be attributed to the ocean being in a series of quasi-equilibrium with the forcing. The statistical models presented in this thesis to reconstruct the AMOC and SPG strength on multidecadal and longer timescales can be useful for prediction and model inter-comparision
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