Tracer Fields Research Articles

Global surface eddy diffusivities derived from satellite altimetry

Velocities derived from AVISO sea‐surface height observations, adjusted to be nondivergent, are used to simulate the evolution of passive tracers at the ocean surface. Eddy mixing rates are derived from the tracer fields in two ways. First, the method of Nakamura is applied to a sector in the East Pacific. Second, the Osborn‐Cox diffusivity is calculated globally to yield estimates of diffusivity in two dimensions. The results from the East Pacific show weak meridional mixing at the surface in the Southern Ocean (<1000 m2 s−1, consistent with previous results) but higher mixing rates (~3000–5000 m2 s−1) in the tropical ocean. The Osborn‐Cox diagnostic provides a global picture of mixing rates and agrees reasonably well with the results from the East Pacific. It also shows extremely high mixing rates (~104 m2 s−1) in western boundary current regions. The Osborn‐Cox diffusivity is sensitive to the tracer initialization, which we attribute to the presence of anisotropic mixing processes. The mixing rates are strongly influenced by the presence of a mean flow nearly everywhere, as shown by comparison with an eddy‐only calculation, with the mean flow absent. Finally, results are compared with other recent estimates of mixing rates using Lagrangian and inverse methods.

An Ensemble Kalman Filter multi-tracer assimilation: Determining uncertain ocean model parameters for improved climate-carbon cycle projections

An Ensemble Kalman Filter is applied to assimilate observed tracer fields in various combinations in the Bern3D ocean model. Each tracer combination yields a set of optimal transport parameter values that are used in projections with prescribed CO2 stabilization pathways. The assimilation of temperature and salinity fields yields a too vigorous ventilation of the thermocline and the deep ocean, whereas the inclusion of CFC-11 and radiocarbon improves the representation of physical and biogeochemical tracers and of ventilation time scales. Projected peak uptake rates and cumulative uptake of CO2 by the ocean are around 20% lower for the parameters determined with CFC-11 and radiocarbon as additional target compared to those with salinity and temperature only. Higher surface temperature changes are simulated in the Greenland–Norwegian–Iceland Sea and in the Southern Ocean when CFC-11 is included in the Ensemble Kalman model tuning. These findings highlights the importance of ocean transport calibration for the design of near-term and long-term CO2 emission mitigation strategies and for climate projections.

Towards high resolution mapping of 3-D mesoscale dynamics from observations

Abstract. The MyOcean R&D project MESCLA (MEsoSCaLe dynamical Analysis through combined model, satellite and in situ data) was devoted to the high resolution 3-D retrieval of tracer and velocity fields in the oceans, based on the combination of in situ and satellite observations and quasi-geostrophic dynamical models. The retrieval techniques were also tested and compared with the output of a primitive equation model, with particular attention to the accuracy of the vertical velocity field as estimated through the Q vector formulation of the omega equation. The project focused on a test case, covering the region where the Gulf Stream separates from the US East Coast. This work demonstrated that innovative methods for the high resolution mapping of 3-D mesoscale dynamics from observations can be used to build the next generations of operational observation-based products.

A baroclinic discontinuous Galerkin finite element model for coastal flows

Numerical modelling of coastal flows is a challenging topic due to complex topography of the coastal zone, rapid flow dynamics and large density variations. Such phenomena are best simulated with unstructured grid models due to their highly flexible spatial discretisation. This article presents a three-dimensional discontinuous Galerkin finite element marine model. Discontinuous Galerkin spatial discretisation is combined with an explicit mode splitting time integration scheme. Slope limiters are introduced to guarantee high quality of the tracer fields in the presence of strong gradients. Free surface movement is accounted for by means of an Arbitrary Lagrangian Eulerian (ALE) moving mesh method. Water volume and tracers are conserved. The conservation properties and baroclinic adjustment under gravity are tested with numerical benchmarks. Finally, the model is applied to the Rhine river plume in an idealised setting.

Performance of Wireless Chemical Sensor Network With Dynamic Collaboration

The chemical tracers dispersed by turbulent motion in the environment display rather complex and even chaotic properties. Meanwhile, chemical tracer detecting sensors with air sampling consume significant energy. Hazardous chemical releases are rare events. If all sensors in a wireless chemical sensor network (WCSN) are left in the active state continuously, it would result in significant power consumption. Therefore, dynamic sensor activation is crucial for the longevity of WCSNs. Moreover, the statistical characteristics of chemical tracers to be detected (temporal and spatial correlations, etc.) and placement of chemical sensors can also become the key parameters that influence the WCSN design and performance. In this paper, we investigate the effect of spatial correlation of a chemical tracer field, and also the effect of network topology, on the performance of a WCSN that employs an epidemiology-based dynamic sensor activation protocol. We present a simulation framework that comprises models of the spatially correlated tracer field, individual chemical sensor nodes, and the sensor network. After validating this simulation framework against an analytical model, we perform simulation experiments to evaluate the effect of spatial correlation and network topology on selected performance metrics: response time, level of sensor activation, and network scalability. Our simulations show that spatial correlation of chemical tracer field has a detrimental effect on the performance of a WCSN that uses an epidemiological activation protocol. The results also suggest that a WCSN with random network topology has poorer performance compared to one with a regular grid topology in this application.

An evaluation of tracer fields and anthropogenic carbon in the equatorial and the tropical North Atlantic

The transit time distribution method was applied to dichlorodifluoromethane and sulfur hexafluoride measurements from four cruises to the tropical North Atlantic between 2006 and 2009 in order to estimate anthropogenic carbon (Cant) concentrations. By assuming an Inverse Gaussian distribution of the transit time distribution the best fit to the data was achieved with the ratio of mean age to width equals 1. Significant differences in the mean age and Cant concentrations between the equatorial belt (5°S–5°N) and the Guinea dome area (5°–15°N) was found. Mean ages are higher and Cant concentrations are lower in the Guinea dome area than at same depths, or densities, in the equatorial belt. The mean column inventories in the upper 1200m are higher by about 3molm−2 in the equatorial belt compared to the Guinea dome area. The mean column inventory of Cant, for the whole water column, in the tropical Atlantic is 32.2molm−2 (error range: 30.6–45.2molm−2), which is significantly lower than the previous estimates. The total Cant inventory in the eastern tropical Atlantic is 2.5Pg (error range: 2.3–3.5Pg) for an area of 6×106km2, comprising the Guinea dome region and the equatorial belt. The equatorial belt has 40% higher storage of Cant compared to the Guinea dome area which reflects the occurrence of relatively young deep waters at the equator, being high in anthropogenic carbon. Our tracer based Cant estimates were compared to Cant concentrations calculated with the TrOCA method applied to measurements conducted in 1999. The TrOCA based estimates are significantly higher than our tracer based Cant estimates. Comparison between tracer measurements in 1999 and the 2006–2009 time-frame revealed possible speed-up of ventilation in the upper water column, increasing the Cant concentration in this depth range at a faster rate and a Cant increase of 12.1μmolkg−1 in the tropical surface water was found.

Standing Eddies in the Meridional Overturning Circulation

Abstract The role of standing eddies for the meridional overturning circulation (MOC) is discussed. The time-mean isopycnal meridional streamfunction is decomposed into a time- and zonal-mean part, a standing-eddy part, and a transient-eddy part. It turns out that the construction of an isopycnal MOC with an exactly vanishing standing-eddy part has to be performed by zonal integration along depth-dependent horizontal isolines of time-mean density. In contrast, zonal integration along time-mean geostrophic streamlines generally only leads to an isopycnal MOC with a reduced standing-eddy part. A generalized approach of constructing meridional transport streamfunctions by two tracer fields and the generalized way to neutralize the corresponding standing-eddy part is given to summarize the discussion. Using the results of an idealized Southern Ocean model, it is demonstrated that neglecting the depth dependence of the zonal integration paths by integrating along density contours or geostrophic streamlines of a fixed depth (“contour depth”) may represent an acceptable approximation: although the standing-eddy part then exactly vanishes only at the contour depth (except for the ageostrophic surface layer using geostrophic streamlines), the overall standing-eddy part is significantly reduced for adequate contour depths. In the idealized Southern Ocean model, density contours at middepth and surface geostrophic streamlines represent the most adequate approximations. Moreover, it is found that the effect of changing the zonal integration paths from latitude circles to curvilinear paths on the zonally averaged density is of the same order as changing from Eulerian to isopycnal averaging.

Trajectory-Tracking Scheme in Lagrangian Form for Solving Linear Advection Problems: Preliminary Tests

A Lagrangian linear advection scheme, which is called the trajectory-tracking scheme, is proposed in this paper. The continuous tracer field has been discretized as finite tracer parcels that are points moving with the velocity field. By using the inverse distance weighted interpolation, the density carried by parcels is mapped onto the fixed Eulerian mesh (e.g., regular latitude–longitude mesh on the sphere) where the result is rendered. A renormalization technique has been adopted to accomplish mass conservation on the grids. The major advantage of this scheme is the ability to preserve discontinuity very well. Several standard tests have been carried out, including 1D and 2D Cartesian cases, and 2D spherical cases. The results show that the spurious numerical diffusion has been eliminated, which is a potential merit for the atmospheric modeling.

Impact of a monotonic advection scheme with low numerical diffusion on transport modeling of emissions from biomass burning

An advection scheme, which maintains the initial monotonic characteristics of a tracer field being transported and at the same time produces low numerical diffusion, is implemented in the Coupled Chemistry‐Aerosol‐Tracer Transport model to the Brazilian developments on the Regional Atmospheric Modeling System (CCATT‐BRAMS). Several comparisons of transport modeling using the new and original (non‐monotonic) CCATT‐BRAMS formulations are performed. Idealized 2‐D non‐divergent or divergent and stationary or time‐dependent wind fields are used to transport sharply localized tracer distributions, as well as to verify if an existent correlation of the mass mixing ratios of two interrelated tracers is kept during the transport simulation. Further comparisons are performed using realistic 3‐D wind fields. We then perform full simulations of real cases using data assimilation and complete atmospheric physics. In these simulations, we address the impacts of both advection schemes on the transport of biomass burning emissions and the formation of secondary species from non‐linear chemical reactions of precursors. The results show that the new scheme produces much more realistic transport patterns, without generating spurious oscillations and under‐ and overshoots or spreading mass away from the local peaks. Increasing the numerical diffusion in the original scheme in order to remove the spurious oscillations and maintain the monotonicity of the transported field causes excessive smoothing in the tracer distribution, reducing the local gradients and maximum values and unrealistically spreading mass away from the local peaks. As a result, huge differences (hundreds of %) for relatively inert tracers (like carbon monoxide) are found in the smoke plume cores. In terms of the secondary chemical species formed by non‐linear reactions (like ozone), we found differences of up to 50% in our simulations.

Popularity framework to process dataset traces and its application on dynamic replica reduction in the ATLAS experiment

The ATLAS experiment's data management system is constantly tracing file movement operations that occur on the Worldwide LHC Computing Grid (WLCG). Due to the large scale of the WLCG, statistical analysis of the traces is infeasible in real-time. Factors that contribute to the scalability problems include the capability for users to initiate on-demand queries, high dimensionality of tracer entries combined with very low cardinality parameters, and the large size of the namespace. These scalability issues are alleviated through the adoption of an incremental model that aggregates data for all combinations occurring in selected tracer fields on a daily basis. Using this model it is possible to query on-demand relevant statistics about system usage. We present an implementation of this popularity model in the experiment's distributed data management system, DQ2, and describe a direct application example of the popularity framework, an automated cleaning system, which uses the statistics to dynamically detect and reduce unpopular replicas from grid sites. This paper describes the architecture employed by the cleaning system and reports on the results collected from a prototype during the first months of the ATLAS collision data taking.

Spatial Growth of Perturbations in a Turbulent Baroclinic Jet

Abstract The spatial spreading of infinitesimal disturbances superposed on a turbulent baroclinic jet is explored. This configuration is representative of analysis errors in an idealized midlatitude storm track and the insight gained may be helpful to understand the spreading of forecast errors in numerical weather prediction models. This problem is explored through numerical experiments of a turbulent baroclinic jet that is perturbed with spatially localized disturbances. Solutions from a quasigeostrophic model for the disturbance fields are compared with those for a passive tracer to determine whether disturbances propagate faster than the basic-state flow. Results show that the disturbance spreading rate is sensitive to the structure of the initial disturbance. Disturbances that are localized in potential vorticity (PV) have far-field winds that allow the disturbance to travel downstream faster than disturbances that are initially localized in geopotential, which have no far-field wind. Near the jet, the spread of the disturbance field is observed to exceed the tracer field for PV-localized disturbances, but not for the geopotential-localized disturbances. Spreading rates faster than the flow for geopotential-localized disturbances are found to occur only for disturbances located off the jet axis. These results are compared with those for zonal and time-independent jets to qualitatively assess the effects of transience and nonlinearity. This comparison suggests that the average properties of localized perturbations to the turbulent jet can be decomposed into a superposition of dynamics associated with a time-independent parallel flow plus a “diffusion” process.

Composition changes after the "Halloween" solar proton event: the High Energy Particle Precipitation in the Atmosphere (HEPPA) model versus MIPAS data intercomparison study

Abstract. We have compared composition changes of NO, NO2, H2O2, O3, N2O, HNO3, N2O5, HNO4, ClO, HOCl, and ClONO2 as observed by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat in the aftermath of the "Halloween" solar proton event (SPE) in late October 2003 at 25–0.01 hPa in the Northern Hemisphere (40–90° N) and simulations performed by the following atmospheric models: the Bremen 2-D model (B2dM) and Bremen 3-D Chemical Transport Model (B3dCTM), the Central Aerological Observatory (CAO) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, the modeling tool for SOlar Climate Ozone Links studies (SOCOL and SOCOLi), and the Whole Atmosphere Community Climate Model (WACCM4). The large number of participating models allowed for an evaluation of the overall ability of atmospheric models to reproduce observed atmospheric perturbations generated by SPEs, particularly with respect to NOy and ozone changes. We have further assessed the meteorological conditions and their implications for the chemical response to the SPE in both the models and observations by comparing temperature and tracer (CH4 and CO) fields. Simulated SPE-induced ozone losses agree on average within 5 % with the observations. Simulated NOy enhancements around 1 hPa, however, are typically 30 % higher than indicated by the observations which are likely to be related to deficiencies in the used ionization rates, though other error sources related to the models' atmospheric background state and/or transport schemes cannot be excluded. The analysis of the observed and modeled NOy partitioning in the aftermath of the SPE has demonstrated the need to implement additional ion chemistry (HNO3 formation via ion-ion recombination and water cluster ions) into the chemical schemes. An overestimation of observed H2O2 enhancements by all models hints at an underestimation of the OH/HO2 ratio in the upper polar stratosphere during the SPE. The analysis of chlorine species perturbations has shown that the encountered differences between models and observations, particularly the underestimation of observed ClONO2 enhancements, are related to a smaller availability of ClO in the polar night region already before the SPE. In general, the intercomparison has demonstrated that differences in the meteorology and/or initial state of the atmosphere in the simulations cause a relevant variability of the model results, even on a short timescale of only a few days.

Mixed Layer Lateral Eddy Fluxes Mediated by Air–Sea Interaction

Abstract The modulation of air–sea heat fluxes by geostrophic eddies due to the stirring of temperature at the sea surface is discussed and quantified. It is argued that the damping of eddy temperature variance by such air–sea fluxes enhances the dissipation of surface temperature fields. Depending on the time scale of damping relative to that of the eddying motions, surface eddy diffusivities can be significantly enhanced over interior values. The issues are explored and quantified in a controlled setting by driving a tracer field, a proxy for sea surface temperature, with surface altimetric observations in the Antarctic Circumpolar Current (ACC) of the Southern Ocean. A new, tracer-based diagnostic of eddy diffusivity is introduced, which is related to the Nakamura effective diffusivity. Using this, the mixed layer lateral eddy diffusivities associated with (i) eddy stirring and small-scale mixing and (ii) surface damping by air–sea interaction is quantified. In the ACC, a diffusivity associated with surface damping of a comparable magnitude to that associated with eddy stirring (∼500 m2 s−1) is found. In frontal regions prevalent in the ACC, an augmentation of surface lateral eddy diffusivities of this magnitude is equivalent to an air–sea flux of 100 W m−2 acting over a mixed layer depth of 100 m, a very significant effect. Finally, the implications for other tracer fields such as salinity, dissolved gases, and chlorophyll are discussed. Different tracers are found to have surface eddy diffusivities that differ significantly in magnitude.

Spurious multiple equilibria introduced by convective adjustment

The application of bifurcation analysis to ocean climate models is substantially hampered by difficulties associated with the use of convective adjustment, i.e. a parameterisation of convection in which the vertical diffusion of heat and salt is greatly enhanced whenever the water column becomes statically unstable. When tracking steady solutions of these models in parameter space, problems arise due to the occurrence of a multitude of saddle-node bifurcations, each of which is related to a minor reorganisation of convection. In this paper, we analyse the origin of the multiple steady states in detail using a one-dimensional column model. By analytical evaluation of the eigenvalue problem conditions are formulated under which bifurcations may occur. Using numerical continuation methods a regime of multistability is identified and it is shown that the number of coexisting steady states increases with increasing resolution, while the extent of parameter space in which they occur decreases. A comparison of the numerical results to corresponding analytical solutions reveals that the multiple equilibria are inherent to the discretisation and hence artificial. Apparently, successful application of convective adjustment requires knowledge of subgrid-scale tracer fields. Two alternative convective adjustment schemes are proposed that (partially) overcome these problems. Results from a fully implicit model based on the two-dimensional primitive equations indicate that the physically relevant phenomena due to the nonlinear character of large-scale oceanic flow, notably the bifurcations related to the salt advection feedback, are preserved under the alternative formulations.

Proposal of a new Lagrangian particle model for the simulation of dense gas dispersion

A Lagrangian particle dispersion model for dense gas dispersion is proposed. It is a new version of MicroSpray oriented to simulate dense gas dispersion in urban or industrial environment, and includes the treatment of obstacles, complex terrain and low wind. The dense gas descent is computed by a 3D dynamical plume rise/descent model, the gravity spreading and bottom boundary condition are accounted for by empirical formulations. The differences between simulations obtained with the new model and with the standard one are presented. The model is validated against tracer field data gathered in the Thorney Island experiment 8.

Issues in the assimilation of the sources of a trace gas by inverse modelling

The problem of reconstructing the (otherwise unknown) source and sink field of a tracer in a fluid is studied by developing and testing a simple tracer transport model of a single-level global atmosphere and a dynamic data assimilation system. The source/sink field (taken to be constant over a 10-day assimilation window) and initial tracer field are analysed together by assimilating imperfect tracer observations over the window. Experiments show that useful information about the source/sink field may be determined from relatively few observations when the initial tracer field is known very accurately a-priori, even when a-priori source/sink information is biased (the source/sink a-priori is set to zero). In this case each observation provides information about the source/sink field at positions upstream and the assimilation of many observations together can reasonably determine the location and strength of a test source.

High-order schemes for 2D unsteady biogeochemical ocean models

Accurate numerical modeling of biogeochemical ocean dynamics is essential for numerous applications, including coastal ecosystem science, environmental management and energy, and climate dynamics. Evaluating computational requirements for such often highly nonlinear and multiscale dynamics is critical. To do so, we complete comprehensive numerical analyses, comparing low- to high-order discretization schemes, both in time and space, employing standard and hybrid discontinuous Galerkin finite element methods, on both straight and new curved elements. Our analyses and syntheses focus on nutrient–phytoplankton–zooplankton dynamics under advection and diffusion within an ocean strait or sill, in an idealized 2D geometry. For the dynamics, we investigate three biological regimes, one with single stable points at all depths and two with stable limit cycles. We also examine interactions that are dominated by the biology, by the advection, or that are balanced. For these regimes and interactions, we study the sensitivity to multiple numerical parameters including quadrature-free and quadrature-based discretizations of the source terms, order of the spatial discretizations of advection and diffusion operators, order of the temporal discretization in explicit schemes, and resolution of the spatial mesh, with and without curved elements. A first finding is that both quadrature-based and quadrature-free discretizations give accurate results in well-resolved regions, but the quadrature-based scheme has smaller errors in under-resolved regions. We show that low-order temporal discretizations allow rapidly growing numerical errors in biological fields. We find that if a spatial discretization (mesh resolution and polynomial degree) does not resolve the solution, oscillations due to discontinuities in tracer fields can be locally significant for both low- and high-order discretizations. When the solution is sufficiently resolved, higher-order schemes on coarser grids perform better (higher accuracy, less dissipative) for the same cost than lower-order scheme on finer grids. This result applies to both passive and reactive tracers and is confirmed by quantitative analyses of truncation errors and smoothness of solution fields. To reduce oscillations in un-resolved regions, we develop a numerical filter that is active only when and where the solution is not smooth locally. Finally, we consider idealized simulations of biological patchiness. Results reveal that higher-order numerical schemes can maintain patches for long-term integrations while lower-order schemes are much too dissipative and cannot, even at very high resolutions. Implications for the use of simulations to better understand biological blooms, patchiness, and other nonlinear reactive dynamics in coastal regions with complex bathymetric features are considerable.

Effect of Tropical Waves on the Tropical Tropopause Transition Layer Upwelling

Abstract An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed “cold trap”) occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.

Total Matrix Intercomparison: A Method for Determining the Geometry of Water-Mass Pathways

AbstractOcean tracer distributions have long been used to decompose the deep ocean into constituent water masses, but previous inverse methods have generally been limited to just a few water masses that have been defined by a subjective choice of static property combinations. Through air–sea interaction and upper-ocean processes, all surface locations are potential sources of distinct tracer properties, and thus it is natural to define a distinct water type for each surface site. Here, a new box inversion method is developed to explore the contributions of all surface locations to the ocean interior, as well as the degree to which the observed tracer fields can be explained by a steady-state circulation with unchanging surface-boundary conditions. The total matrix intercomparison (TMI) method is a novel way to invert observations to solve for the pathways connecting every surface point to every interior point. In the limiting case that the circulation is steady and that five conservative tracers are perfectly observed, the TMI method unambiguously recovers the complete pathways information, owing to the fact that each grid box has, at most, six neighbors. Modern-day climatologies of temperature, salinity, phosphate, nitrate, oxygen, and oxygen-18/oxygen-16 isotope ratios are simultaneously inverted at 4° × 4° grid resolution with 33 vertical levels. Using boundary conditions at the surface and seafloor, the entire interior distribution of the observed tracers is reconstructed using the TMI method. Assuming that seafloor fluxes of tracer properties can be neglected, the method suggests that 25% or less of the water residing in the deep North Pacific originated in the North Atlantic. Integrating over the global ocean, the Southern Ocean is dominant, as the inversion indicates that almost 60% of the ocean volume originates from south of the Southern Hemisphere subtropical front.

Saturation and time dependence of geodynamo models

In this study we address the question under which conditions a saturated velocity field stemming from geodynamo simulations leads to an exponential growth of the magnetic field in a corresponding kinematic calculation. We perform global self-consistent geodynamo simulations and calculate the evolution of a kinematically advanced tracer field. The self-consistent velocity field enters the induction equation in each time step, but the tracer field does not contribute to the Lorentz force. This experiment has been performed by Cattaneo & Tobias (2009) and is closely related to the test field method by Schrinner et al. (2005, 2007). We find two dynamo regimes in which the tracer field either grows exponentially or approaches a state aligned with the actual self-consistent magnetic field after an initial transition period. Both regimes can be distinguished by the Rossby number and coincide with the dipolar and multipolar dynamo regimes identified by Christensen & Aubert (2006). Dipolar dynamos with low Rossby number are kinematically stable whereas the tracer field grows exponentially in the multipolar dynamo regime. This difference in the saturation process for dynamos in both regimes comes along with differences in their time variability. Within our sample of 20 models, solely kinematically unstable dynamos show dipole reversals and large excursions. The complicated time behaviour of these dynamos presumably relates to the alternating growth of several competing dynamo modes. On the other hand, dynamos in the low Rossby number regime exhibit a rather simple time dependence and their saturation merely results in a fluctuation of the fundamental dynamo mode about its critical state.