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Uncertainties in the rate and magnitude of sea-level rise (SLR) complicate decision making on coastal adaptation. Large uncertainty arises from potential ice mass-loss from Antarctica that could rapidly increase SLR in the second half of this century. To deal with uncertain SLR, the Netherlands has adopted an adaptive pathways strategy. Monitoring of new insights on climatic risks and societal developments is an essential ingredient of this strategy. The implications of SLR may be existential for a low-lying country as the Netherlands, and warrant exploration of high- impact low-likelihood scenarios. Therefore, a study was initiated to analyse the implications of storylines leading to extreme SLR for the current adaptive plan. This paper reports on the process and findings of uncertain high SLR consequences on flood, drought, and coastal management in the Netherlands, and discusses implications for coastal adaptation considering timescales of adaptation including lifetime of decisions and their lead-in time for preparation and implementation. We find that for extreme SLR transformative measures are needed, not only because of the unanticipated sea level values that are inconsistent with current strategy boundary conditions, but also because of high SLR rates that challenge adaptation. Anticipating deeply uncertain high SLR helps to enable timely adaptation and to appreciate the value of emission reduction and monitoring of the Antarctica contribution to SLR.

The potential for break-up of Antarctic ice shelves by hydrofracturing and following ice cliff instability might be important for future ice dynamics. One recent study suggests that the Antarctic ice sheet could lose a lot more mass during the 21st century than previously thought. This increased mass-loss is found to strongly depend on the emission scenario and thereby on global temperature change. We investigate the impact of this new information on high-end global sea level rise projections by developing a probabilistic process-based method. It is shown that uncertainties in the projections increase when including the temperature dependence of Antarctic mass loss and the uncertainty in the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble. Including these new uncertainties we provide probability density functions for the high-end distribution of total global mean sea level in 2100 conditional on emission scenario. These projections provide a probabilistic context to previous extreme sea level scenarios developed for adaptation purposes.

East of Madagascar, the shallow “South Indian Ocean Counter Current (SICC)” flows from west to east across the Indian Ocean against the direction of the wind-driven circulation. The SICC impinges on west Australia and enhances the sea level slope, strengthening the alongshore coastal jet: the Leeuwin Current (LC), which flows poleward along Australia. An observed transport maximum of the LC around 22°S can likely be attributed to this impingement of the SICC. The LC is often described as a regional coastal current that is forced by an offshore meridional density gradient or sea surface slope. However, little is known about the controls of these open-ocean gradients. The regional circulation system is embedded in the subtropical “super gyre” that connects the Indo-Pacific via the Tasman Gateway and the Indonesian passages. The Indonesian Throughflow (ITF) circulates through the Indian Ocean back into the Pacific south of Australia. This return pathway appears to be partly trapped in the upper layer north of an outcrop line. It is redirected along this outcrop line and joins the eastward flow of the SICC. To study the connection of the basin-scale and the inter-ocean-scale dynamics, we apply both an ocean general circulation model and a conceptual two-layer model. Shutdown of the ITF in the models leads to a large decrease in Leeuwin Current transport. Most of the SICC was found to then reconnect to the internal gyre circulation in the Indian Ocean. ITF, SICC and LC thus appear to be dynamically connected.

The climate impact of ocean gateway openings during the Eocene-Oligocene transition is still under debate. Previous model studies employed grid resolutions at which the impact of mesoscale eddies has to be parameterized. We present results of a state-of-the-art eddy-resolving global ocean model with a closed Drake Passage and compare with results of the same model at noneddying resolution. An analysis of the pathways of heat by decomposing the meridional heat transport into eddy, horizontal, and overturning circulation components indicates that the model behavior on the large scale is qualitatively similar at both resolutions. Closing Drake Passage induces (i) sea surface warming around Antarctica due to equatorward expansion of the subpolar gyres, (ii) the collapse of the overturning circulation related to North Atlantic Deep Water formation leading to surface cooling in the North Atlantic, and (iii) significant equatorward eddy heat transport near Antarctica. However, quantitative details significantly depend on the chosen resolution. The warming around Antarctica is substantially larger for the noneddying configuration (∼5.5°C) than for the eddying configuration (∼2.5°C). This is a consequence of the subpolar mean flow which partitions differently into gyres and circumpolar current at different resolutions. We conclude that for a deciphering of the different mechanisms active in Eocene-Oligocene climate change detailed analyses of the pathways of heat in the different climate subsystems are crucial in order to clearly identify the physical processes actually at work.

The Agulhas Current is a powerful boundary current that flows southward on the eastern coast of South Africa. At the southern termination of the continental shelf, most of the current turns back to the Indian Ocean in the Agulhas Return Current, this is called the Agulhas retroflection. In this process some eddies are shed from the current and propagate westward in the Atlantic Ocean. The water that originates from the Indian Ocean and enters the South Atlantic Ocean is called the Agulhas leakage. It is believed that this flux of relatively warm and salty water plays an important role in the Atlantic Meridional Overturning Circulation. In this thesis the dynamics of the retroflection process is studied. A new regime of retroflection is found for highly turbulent flows. In this regime the volume of leakage reaches a maximum and stays unchanged while both the wind stress curl and the Agulhas Current transport increase. This is found to be due to an increase of the interactions with the northern branch of the Antarctic Circumpolar Current. The importance of the Indonesian Throughflow, the equatorial flux of water from the Pacific Ocean to the Indian Ocean, is also explored using numerical models. It is shown that it leads to strengthen the leakage but does not change the proportion of retroflection. On the way from the Indonesian Through flow to the Agulhas Current an important passage is the South East Madagascar Current. Using dynamic topography data from satellite altimetry, dipolar structures are found to form continuously at the southern tip of Madagascar. The perturbations induced by these structures contributed to the two early retroflections of the Agulhas Current observed over the last 20 years. Finally, a method is developed and validated to measure the Agulhas leakage volume transport from satellite altimetry data.

From an analysis using almost 20 years of satellite altimetry data and in situ observations, we have shown thatat the southern tip of Madagascar the SEMC breaks up in a series of nearly symmetric dipolar vortex pairs of approximately zero net relative vorticity. They appear at a frequency of about 4–6 per year. Most vortex pairs break up shortly after their formation and interact with previously formed cyclones and anticyclones. The velocity shear across the SEMC is so large that the dynamics of the evolution of the dipole is dominantly nonlinear. A surprising consequence is that there is no retroflection of the SEMC nor does it continue as an unstable westward jet. Instead the lobes develop symmetrically and the dynamics appear more similar to that of modon formation [Flierl et al., 1983]. The EKE south of Madagascar was shown to correlate with the strength of the SEMC prior to separation. Such a relationship could not be found by Palastanga et al. [2006] probably because they used a more upstream satellite transect to determine the surface velocity of the SEMC. Within the SEMC, there is a short decorrelation length scale (also observed in the Gulf Stream [Sturges and Hong, 2001]), and the velocity at our transect does not significantly correlate with the velocity at locations further than 450 km upstream. The very early retroflection of the AC in 2001 was an exceptional one. Only one more appeared in the 18 year period (in 2008). In both cases, it needed a concerted action of cyclonic meanders of the AC and the ARC to shortcut the retroflection loop with an active role of the Madagascar dipoles in generating the Agulhas cyclones. Interannual variability in the separation region appeared related with the ENSO cycle, but not significantly with the IOD. Significant correlations were found between the NINO3.4 index and both the strength of the SEMC and the EKE southwest of Madagascar. Positive and negative lags are between 250 and 500 days. Preceding a positive (negat

Analysis of the along track satellite data near the southern tip of Madagascar shows that the separation in this region occurs in the form of counter rotating pairs – dipoles. The integral of relative vorticity across the dipole's pathway is almost zero which supports the hypothesis that the western boundary current (WBC) is a source for the dipole formation, assuming the validity of the Stewart constraint for the WBC: the net relative vorticity across the WBC is zero. For the numerical experiments three separation regions are chosen: the southern and northern tips of Madagascar as well as South Africa. It is shown that with the use of simplified linear and piecewise linear wind profiles, which are applied only for a 20-degree eastern sector of the domain, it is possible to generate at least three types of separation: shedding eddies from the southern tip of Madagascar, retroflection of the Agulhas Current and a free jet from the northern tip of Madagascar. Furthermore, slip and no-slip boundary conditions as they apply in Hallberg Isopycnal Model (HIM) are discussed here. It is shown that only under no-slip boundary conditions, isolated dipoles might be generated at the separation. Analysis of the integrals of relative vorticity across the western boundary layer and across the dipole’s pathway in southern Madagascar illustrates that the source of dipoles at the separation is the WBC, which is in correspondence with observations.

Analysis of the along track satellite data near the southern tip of Madagascar shows that the separation in this region occurs in the form of counter rotating pairs – dipoles. The integral of relative vorticity across the dipole's pathway is almost zero which supports the hypothesis that the western boundary current (WBC) is a source for the dipole formation, assuming the validity of the Stewart constraint for the WBC: the net relative vorticity across the WBC is zero. For the numerical experiments three separation regions are chosen: the southern and northern tips of Madagascar as well as South Africa. It is shown that with the use of simplified linear and piecewise linear wind profiles, which are applied only for a 20-degree eastern sector of the domain, it is possible to generate at least three types of separation: shedding eddies from the southern tip of Madagascar, retroflection of the Agulhas Current and a free jet from the northern tip of Madagascar. Furthermore, slip and no-slip boundary conditions as they apply in Hallberg Isopycnal Model (HIM) are discussed here. It is shown that only under no-slip boundary conditions, isolated dipoles might be generated at the separation. Analysis of the integrals of relative vorticity across the western boundary layer and across the dipole’s pathway in southern Madagascar illustrates that the source of dipoles at the separation is the WBC, which is in correspondence with observations.