1,721,782 research outputs found
Interaction of SST Modes in the North Atlantic Ocean
No full textThe spectral origin of the recently discovered multidecadal modes (MMs) and centennial modes (CMs) is explained. These modes appear in the linear stability analysis of thermohaline-driven flows in a singlehemispheric ocean basin. It is shown that both classes of modes arise through interaction of so-called sea surface temperature (SST) modes. These SST modes are damped and nonoscillatory for the unforced (motionless) flow. They become oscillatory under small thermal forcing through mode merging that is induced by the meridional overturning circulation. The type of merger responsible for each class of modes explains many features—for example, why CMs can be found in two-dimensional models whereas MMs cannot—of the patterns of the modes at realistic forcing strength
Par@Graph - a parallel toolbox for the construction and analysis of large complex climate networks
No full textIn this paper, we present Par@Graph, a software toolbox to reconstruct and analyze complex climate networks having a large number of nodes (up to at least 106) and edges (up to at least 1012). The key innovation is an efficient set of parallel software tools designed to leverage the inherited hybrid parallelism in distributed-memory clusters of multicore machines. The performance of the toolbox is illustrated through networks derived from sea surface height (SSH) data of a global high-resolution ocean model. Less than 8 min are needed on 90 Intel Xeon E5-4650 processors to reconstruct a climate network including the preprocessing and the correlation of 3×105 SSH time series, resulting in a weighted graph with the same number of vertices and about 3.2×108 edges. In less than 14 min on 30 processors, the resulted graph’s degree centrality, strength, connected components, eigenvector centrality, entropy and clustering coefficient metrics were obtained. These results indicate that a complete cycle to construct and analyze a large-scale climate network is available under 22 min Par@Graph therefore facilitates the application of climate network analysis on high-resolution observations and model results, by enabling fast network reconstruct from the calculation of statistical similarities between climate time series. It also enables network analysis at un- precedented scales on a variety of different sizes of input data sets
Characterization of the multiple equilibria regime in a global ocean model
No full textLow-resolution global ocean models display hysteresis behaviour when forced with an anomalous freshwater input into the northern North Atlantic. Of central importance in this hysteresis behaviour is the existence of so-called saddle-node bifurcations. In this paper, focus is on the physical characterization of the multiple equilibrium regime using a fully implicit global ocean model for which bifurcation diagrams can be explicitly computed. The physics of the position of the relevant saddle-node bifurcation in parameter space is clarified and a modification of an earlier suggested diagnostic of the presence of the multiple equilibrium regime is proposed. The relevance of this indicator for coupled climate models is shown by studying the sensitivity of the multiple equilibria regime to changes in the horizontal and vertical diffusivities of the model
Phytoplankton bloom dynamics in turbid, well-mixed estuaries: A model study
Full text linkTo gain insight into mechanisms underlying phytoplankton bloom dynamics in turbid, well-mixed estuaries, experiments were conducted with an exploratory model that couples physical and biological processes. The motivation for choosing exploratory models is that they allow investigation of individual processes in isolation, therefore yielding fundamental insight into the mechanisms of the system. The Ems estuary (between the Netherlands and Germany) was selected as the prototype estuary, in which a zone of high turbidity is observed in the middle and upper reach. Results show that the model is capable of capturing the main features of the observed phytoplankton population density (P) patterns, that is, in the lower reach a spring bloom occurs, followed by a secondary bloom in autumn. Results of sensitivity studies reveal that the along-estuary distribution of suspended particulate matter (SPM) is a determining factor for the along-estuary location of blooms and it largely affects the intensity of blooms. The along-estuary advection of nutrients by the subtidal current is important for obtaining blooms with high intensities. In this model, the seasonally varying water temperature has a larger impact on the timing of spring blooms than the seasonally varying incident light intensity. The occurrence of an autumn bloom is due to the fact that during the summer season, the net specific growth rate of phytoplankton decreases. The latter is likely to result from an optimum water temperature (smaller than the maximum water temperature) for phytoplankton growth. Nevertheless, the occurrence and characteristics of autumn blooms are also influenced by seasonal variations in other aspects, for instance, loss of phytoplankton due to grazing
A nonlinear theory of the bimodality of the Kuroshio extension
No full textThe Kuroshio Extension (KE) flow in the North Pacific Ocean displays a very distinctive decadal variability of bimodal character involving two completely different states (a large-meander “elongated” state and a small-meander “contracted” state) connected by very asymmetric temporal transitions. Although such a flow has been widely studied by means of a suite of mathematical models and by using several observational platforms, a satisfactory theoretical framework answering quite elementary questions is still lacking, the main question being whether such variability is induced by a time-varying wind forcing or, rather, by intrinsic oceanic mechanisms. In this context, the chaotic relaxation oscillation produced by a process-oriented model of the KE low-frequency variability, with steady climatological wind forcing, was recently recognized to be in substantial agreement with altimeter data. Here those model results are further compared with a comprehensive altimeter dataset. The positive result of such a comparison allows the conclusion that a minimal model for the KE bimodality has been identified and that, consequently, nonlinear intrinsic oceanic mechanisms are likely to be the main cause of the observed variability. By applying the methods of nonlinear dynamical systems theory, relevant dynamical features of the modeled flow are then explained, such as the origin of the relaxation oscillation as a consequence of a homoclinic bifurcation, the spatiotemporal character of the bimodal behavior, and the degree of predictability of the flow in the different stages of the oscillation (evaluated through a field of finite-time Lyapunov exponents and the corresponding Lagrangian time series
Optimal localized observations for advancing beyond the ENSO predictability barrier
Full text linkThe existing 20-member ensemble of 50 yr ECHAM5/MPI-OM simulations provides a reasonably realistic Monte Carlo sample of the El Niño–Southern Oscillation (ENSO). Localized observations of sea surface temperature (SST), zonal wind speed and thermocline depth are assimilated in the ensemble using sequential importance sampling to adjust the weight of ensemble members. We determine optimal observation locations, for which assimilation yields the minimal ensemble spread. Efficient observation locations for SST lie in the ENSO pattern, with the optimum located in the eastern and western Pacific for minimizing uncertainty in the NINO3 and NINO4 index, respectively. After the assimilation of the observations, we investigate how the weighted ensemble performs as a nine-month probabilistic forecast of the ENSO. Here, we focus on the spring predictability barrier with observation in the January–March (March–May) period and assess the remaining predictive power in June (August) for NINO3 (NINO4). For the ECHAM5/MPI-OM ensemble, this yields that SST observations around 110° W and 140° W provide the best predictive skill for the NINO3 and NINO4 index, respectively. Forecasts can be improved by additionally measuring the thermocline depth at 150° W
Effect of ocean gateways on the global ocean circulation in the late Oligocene and early Miocene
No full textWe investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns. Using a fully coupled climate model in an idealized setup, we compare situations corresponding to the late Oligocene, the early Miocene, and present day. The model results show the existence of a flow reversal through the Panama Seaway between the Oligocene and Miocene. This flow reversal is induced by global tectonic changes related to the widening of the Southern Ocean passages and the closing of the Tethys Seaway. It mainly involves the wind-driven ocean circulation, in agreement with previous model studies. The global thermohaline circulation in the Oligocene and Miocene simulations is significantly different from the present-day conveyor circulation, as there is deepwater formation in both the North Atlantic and the North Pacific oceans. In particular, in the Oligocene simulation the salinity contrast between the Atlantic and Pacific oceans is reduced because of water mass exchange through the low-latitude connections between the two oceans
Long bed waves in tidal seas: an idealized model
No full textAn idealized model is proposed to explain the appearance of the long bed waves that have been recently observed in shallow tidal seas. The model assumes that these bedforms grow due to tide-topography interaction. The water motion is described by means of the depth-averaged shallow water equations and the bottom evolution is governed by conservation of sediment mass. The sediment transport formulation includes a critical bottom stress below which no sediment moves. Also, anisotropic sediment transport, due to local bottom slopes in the longitudinal and transverse directions, is taken into account. A linear stability analysis of the flat bottom configuration reveals that different bottom patterns can emerge. In accordance with previous analyses, for strong tidal currents, the fastest growing modes are sand banks. However, if the tidal currents are elliptical and the maximum bottom stress is just above its threshold value for the initiation of sediment motion, the model shows the presence of further growing modes which resemble the long bed waves observed in the fiel
A new interpretation of the two-step δ18O signal at the Eocene-Oligocene boundary
Full text linkThe most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E–O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ18O record, showing an increase by 1.2–1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ18O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ13C record are indicative of a greenhouse gas control on climate. Previous studies show that a decline in pCO2 beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ18O. Furthermore, they did not address the potential role of changes in ocean circulation in the E–O transition. Here a new interpretation of the δ18O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ18O represents a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. This shift can be initiated by a small density perturbation in the model, although there is also a parameter regime for which the shift occurs spontaneously. The second step in the δ18O profile occurs due to a rapid glaciation of the Antarctic continent. This new interpretation is a robust outcome of our model and is in good agreement with proxy data
Effect of a secondary channel on the non-linear tidal dynamics in a semi-enclosed channel: a simple model
No full textNon-linear tidal dynamics are investigated in a network that consists of a semi-enclosed main channel and a secondary channel at an arbitrary position. The water motion, governed by the one-dimensional shallow water equations, is forced by an incoming tidal wave. Solutions are obtained with the method of characteristics. The overall aim is to quantify and understand the spatial structure of different tidal harmonics (the principal tide and its nonlinear overtides) and of tidal asymmetry for both the vertical and the horizontal tide in the main channel for different locations of the secondary channel. This is of practical interest in the context of possible construction of secondary channels to reduce tidal range in estuaries. Moreover, tidal asymmetry is an important factor in driving net sediment transport. Analysis of the different tidal harmonics shows that their characteristics are similar to those obtained with an earlier linear model. In particular, amplitudes of the harmonics are reduced landward of the secondary channel if the latter is positioned less than a quarter wavelength of the respective tidal wave away from the landward boundary. Thus, the distortions of the tide due to the presence of the secondary channel are generated locally and afterwards propagate through the network. Tidal asymmetry is quantified by examining tidal range, flood-to-ebb ratio and the duration of the falling tide and the duration between maximum flood andmaximum ebb. A spatial non-uniform reduction in tidal range is observed that shows very localised increase and decrease depending on the position of the secondary channel. The changes in the velocity characteristics induce changes in net sediment transport. It turns out that the direction of the peak current, derived from the flood-toebb ratio, is not sensitive to the position of the secondary channel, whereas the duration between flood and ebb can change from more to less than half the tidal cycle. However, the changes in the velocity asymmetries are confined to a small region
- …
