Application of continuation methods in physical oceanography

A specific example will be considered in which continuation methods are used to study fundamental problems in physical oceanography.The separation be- havior of the Gulf Stream in the North Atlantic is a long standing problem in dynamical oceanography,with state-of-the-art ocean models still having trouble to simulate to correct mean path.Bifurcation analysis in simplified models,as used here,points to one of difficulties involved:there may be two separation paths at the same forcing conditions.By studying the stability of steady North Atlantic ocean circulation patterns,also transitions to time-dependence are found which are linked to specific low frequency variability in the Gulf Stream region

Internal variability of the wind-driven ocean circulation

The ocean circulation is known to vary on a multitude of time and spatial scales. Due to the large heat capacity of the oceans, variations in its circulation have a profound impact on climate. Therefore, understanding the origin of this variability and its sensitivity to physical parameters is an important issue in climate research. Part of the variability of the earth's climate on intermonthly to interannual time scales is caused by variations of the wind-driven ocean circulation. In particular, the mid-ocean jets associated with the large-scale gyre circulation are highly variable.This type of variability may arise as a result of internal processes in the ocean, external processes like for example changes in the surface wind forcing, and through coupled ocean-atmosphere interactions. In this thesis, focus is on the internal variability of the wind-driven ocean circulation, and on understanding the basic physical processes that give rise to this internal variability. Thereto, the characteristics of an idealized double-gyre circulation, serves as a prototype for major mid-ocean jet systems like the Gulf Stream, are explored in a systematic way. As a consequence of the idealizations in the model configuration, the results of this study have to be discussed qualitatively rather than quantitatively. The approach used in this thesis is based on the notion that the internal variability of the flow is directly linked to its (in-)stability. Therefore, valuable information on the (origin of the) variability can be obtained by determining stationary solutions for the flow, and studying the stability of these flows with respect to small perturbations. This is done here for changing values of a specific model parameter, starting from a parameter regime where the flow is stable. The characteristics of the most unstable perturbations (i.e., those that destabilize the flow when the ratio of forcing to dissipation is still relatively small) can be studied in detail. These modes are expected to dominate the internal variability, both close to the stability boundary as well as far into the unstable regime. The results of the analyses presented in this thesis allow for a detailed study of the origin, sensitivity, and underlying physics of the internal variability, and provides a valuable interpretation framework for the results of time integrations. In the future , this combined dynamical systems and time series analysis can be used to analyze more realistic models, which for example include the thermohaline component of the circulation as well. Such studies are expected to lead to a better understanding of the observed variability of the ocean circulation

Dynamics of downwelling in an eddying marginal sea: Contrasting the eulerian and the isopycnal perspective

In this study, we explore the downward branch of the Atlantic meridional overturning circulation (AMOC) from a perspective in depth space (Eulerian downwelling) as well as from a perspective in density space (diapycnal downwelling). Using an idealized model, we focus on the role of eddying marginal seas, where dense water is formed by deep convection due to an intense surface heat loss. We assess where diapycnal mass fluxes take place, investigate the pathways of dense water masses, and elucidate the role of eddies. We find that there are fundamental differences between the Eulerian and diapycnal downwelling: the strong Eulerian near-boundary downwelling is not associated with substantial diapycnal downwelling; the latter takes place in the interior and elsewhere in the boundary current. We show that the diapycnal downwelling appears to be more appropriate to describe the pathways of water masses. In our model, dense water masses are exported along two routes: those formed in the upper part of the boundary current are exported directly; those formed in the interior move toward the boundary along isopycnals due to eddy stirring and are then exported. This study thus reveals a complex three-dimensional view of the overturning in a marginal sea, with possible implications for our understanding of the AMOC.Environmental Fluid Mechanic

Constructing scenarios of regional sea level change using global temperature pathways

The effects of sea level change become increasingly relevant for the Dutch coast. Therefore we construct two scenarios for regional sea-level change in the 21st century. They are designed to follow two temperature pathways, in which global mean temperature rises moderately ('G', +1.5 K in 2085) or more substantially ('W', +3.5 K in 2085). Contributions from all major processes leading to sea level rise are included (ocean expansion, glacier melt, ice-sheet changes, and landwater changes), except glacial isostatic adjustment and surface elevation changes. As input we use data from 42 coupled global climate models that contributed to CMIP5. The approach is consistent with the recent fifth assessment Report of IPCC, but provides an alternative viewpoint based on global temperature changes rather than RCPs. This makes them rather accessible and readily applicable to policy makers and the general public. We find a likely range for the G-scenario of +25–60 cm in 2085, and +45–80 cm for the W-scenario. These numbers have been rounded to 5 cm precision, to emphasise to any end-user of these scenarios that estimated lower and upper limits themselves are uncertain

Sinking of dense North Atlantic waters in a global ocean model: location and controls

We 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

Increased Arctic Precipitation Slows Down Sea Ice Melt and Surface Warming

Climate model projections of future climate change exhibit a robust increase in Arctic precipitation, which invokes an array of climate effects. Idealized climate model simulations with artificially increased Arctic precipitation rates exhibit cooling of near-surface atmospheric temperatures and sea ice expansion. We show here that this cooling cannot be attributed to increased surface albedo from fresh snow and less absorption of solar radiation by sea ice, but rather to a reduction in upward oceanic heat flux. This reduction in heat flux is due to increased precipitation that leads to fresher ocean surface waters and, hence, to more stable stratification of the upper Arctic Ocean. This stratification results in cooling of the ocean surface and warming of deeper ocean layers. The simulations show that sea ice expansion and surface cooling peak in the Barents Sea, a region that is very sensitive to changes in mixed layer depth, which decreases considerably there. In the context of a warming Arctic, with concurrent 50% increases in precipitation in 2100, this negative feedback is estimated to slow down projected RCP8.5 Arctic warming by up to 2.0°C in winter and sea ice retreat by a maximum of 11% in autumn, although seasonal variations are considerable.Environmental Fluid Mechanic

The Role of the Mean State of Arctic Sea Ice on Near-Surface Temperature Trends

Century-scale global near-surface temperature trends in response to rising greenhouse gas concentrations in climate models vary by almost a factor of 2, with greatest intermodel spread in the Arctic region where sea ice is a key climate component. Three factors contribute to the intermodel spread: 1) model formulation, 2) control climate state, and 3) internal climate variability. This study focuses on the influence of Arctic sea ice in the control climate on the intermodel spread in warming, using idealized 1% yr(-1) CO2 increase simulations of 33 state-of-the-art global climate models, and combining sea ice-temperature relations on local to large spatial scales. On the Arctic mean scale, the spread in temperature trends is only weakly related to ice volume or area in the control climate, and is probably not dominated by internal variability. This suggests that other processes, such as ocean heat transport and meteorological conditions, play a more important role in the spread of long-term Arctic warming than control sea ice conditions. However, on a local scale, sea ice-warming relations show that in regions with more sea ice, models generally simulate more warming in winter and less warming in summer. The local winter warming is clearly related to control sea ice and universal among models, whereas summer sea ice-warming relations are more diverse, and are probably dominated by differences in model formulation. To obtain a more realistic representation of Arctic warming, it is recommended to simulate control sea ice conditions in climate models so that the spatial pattern is correct

Sea surface height variability in the North East Atlantic from satellite altimetry

Data from 21 years of satellite altimeter measurements are used to identify and understand the major contributing components of sea surface height variability (SSV) on monthly time-scales in the North East Atlantic. A number of SSV drivers is considered, which are categorised into two groups; local (wind and sea surface temperature) and remote (sea level pressure and the North Atlantic oscillation index). A multiple linear regression model is constructed to model the SSV for a specific target area in the North Sea basin. Cross-correlations between candidate regressors potentially lead to ambiguity in the interpretation of the results. We therefore use an objective hierarchical selection method based on variance inflation factors to select the optimal number of regressors for the target area and accept these into the regression model if they can be associated to SSV through a direct underlying physical forcing mechanism. Results show that a region of high SSV exists off the west coast of Denmark and that it can be represented well with a regression model that uses local wind, sea surface temperature and sea level pressure as primary regressors. The regression model developed here helps to understand sea level change in the North East Atlantic. The methodology is generalised and easily applied to other regions.Environmental Fluid Mechanic

North Atlantic Ocean Circulation around Greenland in CESM 2.0

Physical and Space GeodesyEnvironmental Fluid Mechanic