1,761 research outputs found
Eddies and algal stoichiometry: Physical and biological impacts on the organic carbon pump
No full textKooijman, S.A.L.M. [Promotor]Dijkstra, H.A. [Copromotor
Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy
No full textSince the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area
Sinking of dense North Atlantic waters in a global ocean model: location and controls
No full textWe investigate the characteristics of the sinking of dense waters in the North Atlantic Ocean that constitute the downwelling limb of the Atlantic Meridional Overturning Circulation (AMOC) as simulated by two global ocean models: an eddy‐permitting model at 1/4° resolution and its coarser 1° counterpart. In line with simple geostrophic considerations, it is shown that the sinking predominantly occurs in a narrow region close to the continental boundary in both model simulations. That is, the regions where convection is deepest do not coincide with regions where most dense waters sink. The amount of near‐boundary sinking that occurs varies regionally. For the 1/4° resolution model, these variations are in quantitative agreement with a relation based on geostrophy and a thermodynamic balance between buoyancy loss and alongshore advection of density, which links the amount of sinking to changes in density along the edge of the North Atlantic Ocean. In the 1° model, the amount and location of sinking appears not to be governed by this simple relation, possibly due to the large impact of overflows and non‐negligible cross‐shore density advection. If this poor representation of the processes governing the sinking of dense waters in the North Atlantic Ocean is a generic feature of such low‐resolution models, the response of the AMOC to changes in climate simulated by this type of models needs to be evaluated with care
TactIQ: Lichte Verstandelijke Beperking bij mensen met een verslaving
No full textItem does not contain fulltex
The Journey That Saved Curious George: The True Wartime Escape of Margret and H.A. Rey
No full textMany years ago, two Jewish artists from Germany met in Brazil, started a business, fell in love, and got married. The Brazilians could not pronounce their German names, so the couple shortened them to H.A. and Margret Rey. While honeymooning in Europe, the Reys decided to stay in Paris so H.A. could pursue a career as a children\u27s book author and illustrator. During this time, H.A. started developing stories about a curious little monkey, influenced by the many monkeys he saw in Brazil. But Hitler\u27s forces delayed H.A.\u27s plans for the little monkey until he and Margret could safely escape Europe
An indicator of the multiple equilibria regime of the Atlantic meridional overturning circulation
No full textRecent model results have suggested that there may be a scalar indicator ? monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity ? is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of ? is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to ?. The sign of ? precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.<br/
Out and All About: Fables for Old and Young
No full textThe title-page is missing in this book, and the only reference on the web is to a reprint of a book before 1923. At least this reference gives us an author. “Routledge” appears on the spine. I read only the first fable, “The Spider and the Ants,” which lasted some 20 pages and seemed to me to lose some of its focus along the way. The book is generously illustrated with partial-page black-and-white illustrations of various sizes along the way. As the beginning T of C shows, there are here some 31 fables on 256 pages before 18 pages of advertisements. Pretty cover and spine, including gold and black on red cloth.H.A. Pag
Dynamics of the Caribbean: Ocean Eddies and Thermohaline Staircases
No full textEnvironmental Fluid Mechanic
The effect of gateways on ocean circulation patterns in the Cenozoic
No full textBoth geological data and climate model studies indicate that substantially different patterns of the global ocean circulation have existed throughout the Cenozoic. In a climate model study of the late Oligocene [von der Heydt, A., Dijkstra, H.A. (2006). Effect of ocean gateways on the global ocean circulation in the late Oligocene and early Miocene. Paleoceanography, 21, PA1011] a “northern sinking” type of circulation was found, with (shallow) deep water formation in both the North Pacific Ocean and the North Atlantic Ocean. This is in contrast to the present-day “conveyor” circulation, where there is deep water formation in the North Atlantic but not in the North Pacific. In order to explain these differences, we use an ocean general circulation model for idealized two-basin flows and study the effect of asymmetries in the continental geometry on the circulation patterns. Two types of asymmetry are considered: (i) the relative northward extent of the Pacific and the Atlantic basin, and (ii) the existence of a circum-global gateway at low latitudes. The more northward extent of the Pacific basin in the Oligocene makes the Conveyor solution less likely and facilitates deep water formation in the North Pacific compared to the North Atlantic. The low-latitude gateway on the other hand, allows salinity and heat exchange between the two main ocean basins and therefore leads to deep water formation in both the North Atlantic and the North Pacific
High-Resolution Modelling of Climate Variability : Using the CESM 1.0.4
No full textMost climate models have a 1◦ (100 km) horizontal resolution. This resolution is too coarse to resolve mesoscale processes in the ocean: ocean eddies. An ocean eddy is characterised by a swirling and turbulent fluid. To resolve ocean eddies within a climate model, a horizontal resolution of 0.1◦ (10 km) is required. Climate models with a horizontal resolution of 1◦ and 0.1◦ are referred to as low-resolution models and high-resolution models, respectively.
Ocean eddies are relevant to the ocean circulation as they stir the (upper) ocean and contribute to the transport of heat and salt. Model biases are reduced in climate simulations in which ocean eddies are explicitly resolved. In low-resolution climate models, eddy-related processes (transport and mixing) are parameterised at the cost of losing eddy characteristics within the model. As a result, the ocean circulation appears to be laminar in low-resolution climate models, sometimes referred to as the ‘honey ocean’.
In this thesis, model output (300 years) of a high-resolution version of the Community Earth System Model (CESM) is analysed. We explore the following research questions in this thesis:
1) Is a high-resolution version of the CESM capable of capturing climate variability (sub-annual - multidecadal) as seen in observations?
2) Are sea-level projections different between the high-resolution CESM and low-resolution CESM?
For the first research question, the analysis is restricted to the Caribbean Sea and the Southern Ocean. The simulated sub-annual ocean variability matches well with observations in the Caribbean Sea. This sub-annual variability is related to ocean eddies. Apart from sub-annual variability, multidecadal variability is found in the Caribbean Sea and surroundings in the high-resolution CESM. This multidecadal variability is induced by ocean eddies in the Southern Ocean and this variability propagates through the entire ocean circulation. From observations, it is known that multidecadal variability exists in the Southern Ocean. However, these observational records are too short (about 30 years) to verify the simulated multidecadal variability in the high-resolution CESM. The low-resolution version of the CESM does not represent ocean eddies, hence, the sub-annual and multidecadal variability is not resolved by this model.
In addition to climate variability, model biases are reduced in the high-resolution CESM compared to the low-resolution CESM. For example, the ocean temperature and sea-ice concentration of the Southern Ocean are realistically resolved in the high-resolution CESM. The ocean temperature distribution of the Southern Ocean controls the amount of mass loss (through basal melt) of the Antarctic ice sheet. In an idealised forcing scenario in which atmospheric CO2 increases over time, the oceanic temperature of the Southern Ocean increases much slower in the high-resolution CESM compared to the low-resolution CESM. This slower temperature increase in the high-resolution CESM is related to ocean eddies. Consequently, the projected global mean sea-level rise is 25% lower in the high-resolution CESM with respect to the low-resolution CESM. Moreover, ocean eddies can affect regional sea-level projections. This demonstrates that both global and regional sea-level projections strongly differ between the high-resolution CESM and low-resolution CESM
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