ALBERT

Description: The Kuroshio Current System in the North Pacific displays path transitions on a decadal timescale. It is known that both internal variability involving barotropic and baroclinic instabilities and remote Rossby waves induced by North Pacific wind stress anomalies are involved in these path transitions. However, the precise coupling of both processes and its consequences for the dominant decadal transition timescale are still under discussion. Here, we analyse the output of a multi-centennial high-resolution global climate model simulation and study phase synchronisation between Pacific zonal wind stress anomalies and Kuroshio Current System path variability. We apply the Hilbert transform technique to determine the phase and find epochs where such phase synchronisation appears. The physics of this synchronisation are shown to occur through the effect of the vertical motion of isopycnals, as induced by the propagating Rossby waves, on the instabilities of the Kuroshio Current System.

Description: Previous studies have indicated that most of the net sinking associated with the downward branch of the Atlantic Meridional Overturning Circulation (AMOC) must occur near the subpolar North Atlantic boundaries. In this work we have used monthly mean fields of a high-resolution ocean model (0.1 deg at the equator) to quantify this sinking. To this end we have calculated the Eulerian net vertical transport (WΣ) from the modelled vertical velocities, its seasonal variability and its spatial distribution under repeated climatological atmospheric forcing conditions. Based on this simulation, we find that for the whole subpolar North Atlantic WΣ peaks at about −14 Sv at a depth of 1139 m, matching both the mean depth and the magnitude of the meridional transport of the AMOC at 45° N. It displays a seasonal variability of around 10 Sv. Three sinking regimes are identified according to the characteristics of the accumulated W with respect to the distance to the coast: one within the first 110 km and onto the bathymetric slope at around the peak of the boundary current speed (regime I), the second between 110 km and 290 km covering the remainder of the shelf where mesoscale eddies exchange properties (momentum, heat, mass) between the interior and the boundary (regime II), and the third sinking regime at larger distances from the coast where WΣ is mostly driven by the ocean's interior eddies (regime III). Regimes I and II accumulate ∼ 90 % of the total sinking and display smaller seasonal changes and spatial variability than regime III. We find that such a distinction in regimes is also useful to describe the characteristics of WΣ in marginal seas located far from the overflow areas, although the regime boundaries can shift a few tens of km inshore or offshore depending on the bathymetric slope and shelf width of each marginal sea. The largest contributions to the sinking come from the Labrador Sea, the Newfoundland region and the overflow regions. The magnitude, the seasonal variability and the depth at which WΣ peaks vary for each region, thus revealing a complex picture of sinking in the subpolar North Atlantic.

Description: The mesoscale variability in the Caribbean Sea is dominated by anticyclonic eddies that are formed in the eastern part of the basin. These anticyclones intensify on their path westward while they pass the coastal upwelling region along the Venezuelan and Colombian coast. In this study, we used a regional model to show that this westward intensification of Caribbean anticyclones is driven by the advection of cold upwelling filaments. These dense filaments are advected by the anticyclones, leading to an increase of the horizontal density gradients at the western side of the anticyclones. Following the thermal wind balance, the increased density gradients result in an increase of the vertical shear of the anticyclones and to their westward intensification. To assess the impact of variations in upwelling on the anticyclones, several simulations were performed in which the northward Ekman transport (and thus the upwelling strength) is altered. As expected, stronger (weaker) upwelling is associated with more stronger (weaker) offshore cooling and a more (less) westward intensification of the anticyclones. The simulations with weaker upwelling show farther advection of the Amazon and Orinoco River plumes into the basin. The dispersion of the river plumes affects the formation process of the anticyclones, where the horizontal density gradients were mainly determined by the salinity gradients of the river plume and not by temperature gradients that were associated with upwelling.

Description: Earth's global mean surface temperature has increased by about 1.0 °C over the period 1880–2015. One of the main causes is thought to be the increase in atmospheric greenhouse gases. If greenhouse gas emissions are not substantially decreased, several studies indicate that there will be a dangerous anthropogenic interference with climate by the end of this century. However, there is no good quantitative measure to determine when it is too late to start reducing greenhouse gas emissions in order to avoid such dangerous interference. In this study, we develop a method for determining a so-called point of no return for several greenhouse gas emission scenarios. The method is based on a combination of aspects of stochastic viability theory and linear response theory; the latter is used to estimate the probability density function of the global mean surface temperature. The innovative element in this approach is the applicability to high-dimensional climate models as demonstrated by the results obtained with the PlaSim model.

Description: In this paper we present the Oceanographic Multipurpose Software Environment (OMUSE). OMUSE aims to provide a homogeneous environment for existing or newly developed numerical ocean simulation codes, simplifying their use and deployment. In this way, numerical experiments that combine ocean models representing different physics or spanning different ranges of physical scales can be easily designed. Rapid development of simulation models is made possible through the creation of simple high-level scripts. The low-level core of the abstraction in OMUSE is designed to deploy these simulations efficiently on heterogeneous high-performance computing resources. Cross-verification of simulation models with different codes and numerical methods is facilitated by the unified interface that OMUSE provides. Reproducibility in numerical experiments is fostered by allowing complex numerical experiments to be expressed in portable scripts that conform to a common OMUSE interface. Here, we present the design of OMUSE as well as the modules and model components currently included, which range from a simple conceptual quasi-geostrophic solver to the global circulation model POP (Parallel Ocean Program). The uniform access to the codes' simulation state and the extensive automation of data transfer and conversion operations aids the implementation of model couplings. We discuss the types of couplings that can be implemented using OMUSE. We also present example applications that demonstrate the straightforward model initialization and the concurrent use of data analysis tools on a running model. We give examples of multiscale and multiphysics simulations by embedding a regional ocean model into a global ocean model and by coupling a surface wave propagation model with a coastal circulation model.

Description: The mesoscale variability in the Caribbean Sea is dominated by anticyclonic eddies that are formed in the eastern part of the basin. These anticyclones intensify on their path westward while they pass the coastal upwelling region along the Venezuelan and Colombian coast. In this study, we used a regional model to show that this westward intensification of Caribbean anticyclones is steered by the advection of cold upwelling filaments. Following the thermal wind balance, the increased horizontal density gradients result in an increase in the vertical shear of the anticyclones and in their westward intensification. To assess the impact of variations in upwelling on the anticyclones, several simulations were performed in which the northward Ekman transport (and thus the upwelling strength) is altered. As expected, stronger (weaker) upwelling is associated with stronger (weaker) offshore cooling and a stronger (weaker) westward intensification of the anticyclones. Moreover, the simulations with weaker upwelling show farther advection of the Amazon and Orinoco River plumes into the basin. As a result, in these simulations the horizontal density gradients were predominantly set by horizontal salinity gradients. The importance of the horizontal density gradients driven by temperature, which are associated with the upwelling, increased with increasing upwelling strength. The results of this study highlight that both upwelling and the advection of the river plumes affect the life cycle of mesoscale eddies in the Caribbean Sea.

Description: Earth's Global Mean Surface Temperature (GMST) has increased by about 1.0 °C over the period 1880–2015. One of the main causes is thought to be the increase in atmospheric greenhouse gases (GHGs). If GHG emissions are not substantially decreased, several studies indicate there will be a dangerous anthropogenic interference (DAI) with climate by the end of this century. However, there is no good quantitative measure to determine when it is "too late" to start reducing GHGs in order to avoid DAI. In this study, we develop a method for determining a so-called Point of No Return (PNR) for several GHG emission scenarios. The method is based on a combination of stochastic viability theory and uses linear response theory to estimate the probability density function of the GMST. The innovative element in this approach is the applicability to high-dimensional climate models as is demonstrated by results obtained with the PLASIM climate model.

Description: In this special issue contribution, I provide a personal view on the role of bifurcation analysis of climate models in the development of a theory of climate system variability. The state of the art of the methodology is shortly outlined, and the main part of the paper deals with examples of what has been done and what has been learned. In addressing these issues, I will discuss the role of a hierarchy of climate models, concentrate on results for spatially extended (stochastic) models (having many degrees of freedom) and evaluate the importance of these results for a theory of climate system variability.

Description: The Kuroshio Current System in the North Pacific displays path transitions on a decadal time scale. It is known that both internal variability involving barotropic and baroclinic instabilities and remote Rossby waves induced by North Pacific wind-stress anomalies are involved in these path transitions. However, the precise coupling of both processes and its consequences for the dominant decadal transition time scale are still under discussion. Here, we analyse the output of a multi-centennial long high-resolution global climate model simulation and study phase synchronisation between Pacific zonal wind-stress anomalies and Kuroshio Current System path variability. We apply the Hilbert transform technique to determine the phase and find epochs where such phase synchronisation appears. The physics of this synchronisation is shown to occur through the effect of the vertical motion of isopycnals, as induced by the propagating Rossby waves, on the instabilities of the Kuroshio Current System.

Description: Studies on the palaeoclimate and palaeoceanography using numerical model simulations may be considerably dependent on the implemented geographical reconstruction. Because building the palaeogeographic datasets for these models is often a time-consuming and elaborate exercise, palaeoclimate models frequently use reconstructions in which the latest state-of-the-art plate tectonic reconstructions, palaeotopography and -bathymetry, or vegetation have not yet been incorporated. In this paper, we therefore provide a new method to efficiently generate a global geographical reconstruction for the middle-late Eocene. The generalised procedure is also reusable to create reconstructions for other time slices within the Cenozoic, suitable for palaeoclimate modelling. We use a plate-tectonic model to make global masks containing the distribution of land, continental shelves, shallow basins and deep ocean. The use of depth-age relationships for oceanic crust together with adjusted present-day topography gives a first estimate of the global geography at a chosen time frame. This estimate subsequently needs manual editing of areas where existing geological data indicate that the altimetry has changed significantly over time. Certain generic changes (e.g. lowering mountain ranges) can be made relatively easily by defining a set of masks while other features may require a more specific treatment. Since the discussion regarding many of these regions is still ongoing, it is crucial to make it easy for changes to be incorporated without having to redo the entire procedure. In this manner, a complete reconstruction can be made that suffices as a boundary condition for numerical models with a limited effort. This facilitates the interaction between experts in geology and palaeoclimate modelling, keeping reconstructions up to date and improving the consistency between different studies. Moreover, it facilitates model inter-comparison studies and sensitivity tests regarding certain geographical features as newly generated boundary conditions can more easily be incorporated in different model simulations. The workflow is presented covering a middle-late Eocene reconstruction (38 Ma), using a MatLab script and a complete set of source files that are provided in the supplementary material.