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  • Other Sources  (6)
  • Articles (OceanRep)  (6)
  • AtlantOS
  • Copernicus Publications (EGU)
  • American Meteorological Society
  • Springer Nature
  • 2000-2004  (6)
  • 1
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    Cross wavelet analysis: significance testing and pitfalls (2004)
    Copernicus Publications (EGU)
    In:  Nonlinear Processes in Geophysics, 11 (4). pp. 505-514.
    Details
    Publication Date: 2017-02-15
    Description: In this paper, we present a detailed evaluation of cross wavelet analysis of bivariate time series. We develop a statistical test for zero wavelet coherency based on Monte Carlo simulations. If at least one of the two processes considered is Gaussian white noise, an approximative formula for the critical value can be utilized. In a second part, typical pitfalls of wavelet cross spectra and wavelet coherency are discussed. The wavelet cross spectrum appears to be not suitable for significance testing the interrelation between two processes. Instead, one should rather apply wavelet coherency. Furthermore we investigate problems due to multiple testing. Based on these results, we show that coherency between ENSO and NAO is an artefact for most of the time from 1900 to 1995. However, during a distinct period from around 1920 to 1940, significant coherency between the two phenomena occurs.
    Type: Article , PeerReviewed
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  • 2
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    Tempting long-memory - on the interpretation of DFA results (2004)
    Copernicus Publications (EGU)
    In:  Nonlinear Processes in Geophysics, 11 (4). pp. 495-503.
    Details
    Publication Date: 2017-02-15
    Description: We study the inference of long-range correlations by means of Detrended Fluctuation Analysis (DFA) and argue that power-law scaling of the fluctuation function and thus long-memory may not be assumed a priori but have to be established. This requires the investigation of the local slopes. We account for the variability characteristic for stochastic processes by calculating empirical confidence regions. Comparing a long-memory with a short-memory model shows that the inference of long-range correlations from a finite amount of data by means of DFA is not specific. We remark that scaling cannot be concluded from a straight line fit to the fluctuation function in a log-log representation. Furthermore, we show that a local slope larger than α=0.5 for large scales does not necessarily imply long-memory. We also demonstrate, that it is not valid to conclude from a finite scaling region of the fluctuation function to an equivalent scaling region of the autocorrelation function. Finally, we review DFA results for the Prague temperature data set and show that long-range correlations cannot not be concluded unambiguously.
    Type: Article , PeerReviewed
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  • 3
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    Nonlinear dimensionality reduction in climate data (2004)
    Copernicus Publications (EGU)
    In:  Nonlinear Processes in Geophysics, 11 . pp. 393-398.
    Details
    Publication Date: 2017-02-15
    Description: Linear methods of dimensionality reduction are useful tools for handling and interpreting high dimensional data. However, the cumulative variance explained by each of the subspaces in which the data space is decomposed may show a slow convergence that makes the selection of a proper minimum number of subspaces for successfully representing the variability of the process ambiguous. The use of nonlinear methods can improve the embedding of multivariate data into lower dimensional manifolds. In this article, a nonlinear method for dimensionality reduction, Isomap, is applied to the sea surface temperature and thermocline data in the tropical Pacific Ocean, where the El Niño-Southern Oscillation (ENSO) phenomenon and the annual cycle phenomena interact. Isomap gives a more accurate description of the manifold dimensionality of the physical system. The knowledge of the minimum number of dimensions is expected to improve the development of low dimensional models for understanding and predicting ENSO.
    Type: Article , PeerReviewed
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  • 4
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    Cyclic Markov chains with an application to an intermediate ENSO model (2003)
    Copernicus Publications (EGU)
    In:  Nonlinear Processes in Geophysics, 10 (3). pp. 197-210.
    Details
    Publication Date: 2017-05-18
    Description: We develop the theory of cyclic Markov chains and apply it to the El Niño-Southern Oscillation (ENSO) predictability problem. At the core of Markov chain modelling is a partition of the state space such that the transition rates between different state space cells can be computed and used most efficiently. We apply a partition technique, which divides the state space into multidimensional cells containing an equal number of data points. This partition leads to mathematical properties of the transition matrices which can be exploited further such as to establish connections with the dynamical theory of unstable periodic orbits. We introduce the concept of most and least predictable states. The data basis of our analysis consists of a multicentury-long data set obtained from an intermediate coupled atmosphere-ocean model of the tropical Pacific. This cyclostationary Markov chain approach captures the spring barrier in ENSO predictability and gives insight also into the dependence of ENSO predictability on the climatic state.
    Type: Article , PeerReviewed
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  • 5
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    Nitrous oxide emissions from the Arabian Sea: A synthesis (2001)
    Copernicus Publications (EGU)
    In:  Atmospheric Chemistry and Physics, 1 . pp. 61-71.
    Details
    Publication Date: 2018-09-26
    Description: We computed high-resolution (1º latitude x 1º longitude) seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N2O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N2O emissions from the Arabian Sea was estimated to be at least 65%.
    Type: Article , PeerReviewed
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  • 6
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    Cephalopod Hox genes and the origin of morphological novelties (2003)
    Springer Nature
    In:  Nature, 424 (6952). pp. 1061-1065.
    Details
    Publication Date: 2021-07-14
    Description: Cephalopods are a diverse group of highly derived molluscs, including nautiluses, squids, octopuses and cuttlefish. Evolution of the cephalopod body plan from a monoplacophoran-like ancestor1 entailed the origin of several key morphological innovations contributing to their impressive evolutionary success2. Recruitment of regulatory genes3, or even pre-existing regulatory networks4, may be a common genetic mechanism for generating new structures. Hox genes encode a family of transcriptional regulatory proteins with a highly conserved role in axial patterning in bilaterians5; however, examples highlighting the importance of Hox gene recruitment for new developmental functions are also known6,7. Here we examined developmental expression patterns for eight out of nine Hox genes8 in the Hawaiian bobtail squid Euprymna scolopes, by whole-mount in situ hybridization. Our data show that Hox orthologues have been recruited multiple times and in many ways in the origin of new cephalopod structures. The manner in which these genes have been co-opted during cephalopod evolution provides insight to the nature of the molecular mechanisms driving morphological change in the Lophotrochozoa, a clade exhibiting the greatest diversity of body plans in the Metazoa.
    Type: Article , PeerReviewed
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