| 1. |
- Hieronymus, Magnus, et al.
(författare)
-
A Novel Machine Learning Based Bias Correction Method and Its Application to Sea Level in an Ensemble of Downscaled Climate Projections
- 2023
-
Ingår i: Tellus, Series A: Dynamic Meteorology and Oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 75:1, s. 129-144
-
Tidskriftsartikel (refereegranskat)abstract
- A new machine learning based bias correction method is presented and applied to sea level in a regional climate model. The bias corrections derived using this method depend on the state of the model it corrects. This contrasts with conventional bias correction methods that operate on distributions of output variables. The dependence on model states allows for better performance on classical skill scores, but it also limits the applicability of the method to models that can perform hindcasts. A very large dataset of corrected hourly sea levels from many different emission scenarios is created. In total the dataset contains over 2600 model years and exists for seven different tide-gauge stations on the Swedish Baltic Sea coast. The prevalence of significant trends in yearly sea level maximum is found to be independent of emission scenario, suggesting that anthropogenic climate change is no significant driver of storm surge variability in the area. Lastly, the dataset is used to estimate return levels for very long return periods, and the block length used in the return level computation is found to affect the result at some stations. This suggests that the commonly used annual maximum approach is not always applicable for determining return levels for sea level.
|
|
| 2. |
|
|
| 3. |
- Hieronymus, Magnus, 1982-
(författare)
-
A note on the influence of spatially varying diffusivities on the evolution of buoyancy with a nonlinear equation of state
- 2014
-
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 44:12, s. 3255-3261
-
Tidskriftsartikel (refereegranskat)abstract
- The oceanic buoyancy sinks from thermobaricity and cabbeling are quantified using hydrography and parameterized diffusive fluxes of heat and salt. A novelty in this study is that the effect of spatially varying diffusivities is considered and quantified. The spatial distribution as well as averages from different ocean basins of the cabbeling and thermobaricity sinks are shown and discussed. The contributions from isoneutral and dianeutral diffusion to the cabbeling and thermobaricity sinks are calculated separately, and their relative importance is seen to vary in the different ocean basins.
|
|
| 4. |
- Hieronymus, Magnus, 1982-
(författare)
-
An investigation into ocean thermodynamics and water-mass transformation
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents oceanic budgets of potential temperature, salinity and buoyancy as well as a novel way of diagnosing water-mass transformation in salinity-temperature space. The buoyancy of seawater is given by a nonlinear function of temperature, salinity and pressure and much of the work in this thesis revolves around how fluxes of heat and salinity influence the buoyancy of seawater through these nonlinearities.Another large part of the material in this thesis is aimed at quantifying the relative importance of different processes for the vertical transport of heat and salinity in the ocean. Careful analysis of those fluxes in an ocean model reveal the different effects of e.g. advection, diffusion and penetrative shortwave radiation. An interesting finding is that the diffusive fluxes due to isoneutral diffusion (diffusion along density surfaces) and dianeutral diffusion (diffusion across density surfaces) have opposing effects on the oceanic heat and salinity budgets.The final major topic of this thesis is water-mass transformation. A quantitative framework for the study of water-mass transformation in salinity-temperature space is introduced. A continuity equation is also derived for salinity-temperature space, which can be used to calculate the time rate of change of volume in a small salinity-temperature interval. The water-mass transformation framework is applied in an ocean general circulation model, and it is shown how the volume distribution in salinity-temperature space is affected by the different tracer fluxes in the model. It is also shown how the transformation framework is related to earlier work on thermohaline streamfunctions.
|
|
| 5. |
- Hieronymus, Magnus, et al.
(författare)
-
Finding the Minimum Potential Energy State by Adiabatic Parcel Rearrangements with a Nonlinear Equation of State : An Exact Solution in Polynomial Time
- 2015
-
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 45:7, s. 1843-1857
-
Tidskriftsartikel (refereegranskat)abstract
- The problem of finding the state of minimum potential energy through the rearrangement of water parcels with a nonlinear equation of state is discussed in the context of a combinatorial optimization problem. It is found that the problem is identical to a classical optimization problem called the linear assignment problem. This problem belongs to a problem class known as P, a class of problems that have known efficient solutions. This is very fortunate since this study's problem has been suggested to be an asymmetric traveling salesman problem. A problem that belongs to a class called NP-hard, for which no efficient solutions are known. The difference between the linear assignment problem and the traveling salesman problem is discussed and made clear by looking at the different constraints used for the two problems. It is also shown how the rearrangement of water parcels that minimizes the potential energy can be found in polynomial time using the so-called Hungarian algorithm. The Hungarian algorithm is then applied to a simplified ocean stratification, and the result is compared to a few different approximate solutions to the minimization problem. It is found that the improved accuracy over the approximate methods comes at a high computational cost. Last, the algorithm is applied to a realistic ocean stratification using a technique that splits the minimization problem into smaller bits.
|
|
| 6. |
- Hieronymus, Magnus, et al.
(författare)
-
Interannual Variability of the Overturning and Energy Transport in the Atmosphere and Ocean During the Late Twentieth Century with Implications for Precipitation and Sea Level
- 2020
-
Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 33:1, s. 317-338
-
Tidskriftsartikel (refereegranskat)abstract
- The overturning circulations in the atmosphere and ocean transport energy from the tropics to higher latitudes and thereby modulate Earth's climate. The interannual variability in the overturning over the last 40 years is found to be dominated by two coupled atmosphere-ocean modes. The first is related to the meridional motion of the intertropical convergence zone and the second to El Nino. Both modes have a strong influence on the sea level variability in the tropical Indo-Pacific Ocean. The interannual variability of the cross-equatorial energy transport is dominated by the first mode, and the variability is larger in the Indo-Pacific Ocean than in the Atlantic Ocean or the atmosphere. Our results suggest an important role of oceanic energy transport in setting precipitation patterns in the tropics and a key role of the Indo-Pacific Ocean as a climate modulator.
|
|
| 7. |
- Hieronymus, Magnus, et al.
(författare)
-
Oceanic Overturning and Heat Transport : The Role of Background Diffusivity
- 2019
-
Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 32:3, s. 701-716
-
Tidskriftsartikel (refereegranskat)abstract
- The role of oceanic background diapycnal diffusion for the equilibrium climate state is investigated in the global coupled climate model CM2G. Special emphasis is put on the oceanic meridional overturning and heat transport. Six runs with the model, differing only by their value of the background diffusivity, are run to steady state and the statistically steady integrations are compared. The diffusivity changes have large-scale impacts on many aspects of the climate system. Two examples are the volume-mean potential temperature, which increases by 3.6 degrees C between the least and most diffusive runs, and the Antarctic sea ice extent, which decreases rapidly as the diffusivity increases. The overturning scaling with diffusivity is found to agree rather well with classical theoretical results for the upper but not for the lower cell. An alternative empirical scaling with the mixing energy is found to give good results for both cells. The oceanic meridional heat transport increases strongly with the diffusivity, an increase that can only partly be explained by increases in the meridional overturning. The increasing poleward oceanic heat transport is accompanied by a decrease in its atmospheric counterpart, which keeps the increase in the planetary energy transport small compared to that in the ocean.
|
|
| 8. |
- Hieronymus, Magnus, 1982-, et al.
(författare)
-
The budgets of heat and salinity in NEMO
- 2013
-
Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 67, s. 28-38
-
Tidskriftsartikel (refereegranskat)abstract
- The near steady state heat and salinity budgets under surfaces of constant depth are examined in the Nucleus for European Modelling of the Ocean (NEMO) model. It is seen that the heat fluxes in NEMO are difficult to reconcile with the idea of a deep ocean in advection-diffusion balance. Some reasons for this are that the resolved heat advection is downward above 2000 m, and that geothermal heating is, in fact, a major heat source in the deeper parts of the domain. It is also seen that isoneutral diffusion gives a very large contribution to the budgets and that the fluxes from isoneutral diffusion is in general upward. It is explained how the sign of these fluxes depends on the stratification. The heat budget for the upper 100 m of the ocean is seen to be dominated by penetrative shortwave radiation, which is so influential that we would have a mixed layer of considerable thickness even in the absence of other sources of turbulent mixing. Penetrative shortwave radiation is therefore a considerable source of potential energy.
|
|
| 9. |
- Hieronymus, Magnus, 1982-, et al.
(författare)
-
The buoyancy budget with a nonlinear equation of state
- 2013
-
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 43:1, s. 176-186
-
Tidskriftsartikel (refereegranskat)abstract
- The nonlinear equation of state of seawater introduces a sink or source of buoyancy when water parcels of unequal salinities and temperatures are mixed. This article contains quantitative estimates of these nonlinear effects on the buoyancy budget of the global ocean. It is shown that the interior buoyancy sink can be determined from surface buoyancy fluxes. These surface buoyancy fluxes are calculated using two surface heat flux climatologies, one based on in situ measurements and the other on a reanalysis, in both cases using a nonlinear equation of state. It is also found that the buoyancy budget in the ocean general circulation model Nucleus for European Modeling of the Ocean (NEMO) is in good agreement with the buoyancy budgets based on the heat flux climatologies. Moreover, an examination of the vertically resolved buoyancy budget in NEMO shows that in large parts of the ocean the nonlinear buoyancy sink gives the largest contribution to this budget.
|
|
| 10. |
- Hieronymus, Magnus, 1982-, et al.
(författare)
-
Water Mass Transformation in Salinity–Temperature Space
- 2014
-
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 44:9, s. 2547-2568
-
Tidskriftsartikel (refereegranskat)abstract
- This article presents a new framework for studying water mass transformations in salinity–temperature space that can, with equal ease, be applied to study water mass transformation in spaces defined by any two conservative tracers. It is shown how the flow across isothermal and isohaline surfaces in the ocean can be quantified from knowledge of the nonadvective fluxes of heat and salt. It is also shown how these cross-isothermal and cross-isohaline flows can be used to form a continuity equation in salinity–temperature space. These flows are then quantified in a state-of-the-art ocean model. Two major transformation cells are found: a tropical cell driven primarily by surface fluxes and dianeutral diffusion and a conveyor belt cell where isoneutral diffusion is also important. Both cells are similar to cells found in earlier work on the thermohaline streamfunction. A key benefit with this framework over a streamfunction approach is that transformation due to different diabatic processes can be studied individually. The distributions of volume and surface area in S–T space are found to be useful for determining how transformations due to these different processes affect the water masses in the model. The surface area distribution shows that the water mass transformations due to surface fluxes tend to be directed away from S–T regions that occupy large areas at the sea surface.
|
|
| 11. |
- Hordoir, Robinson, et al.
(författare)
-
Nemo-Nordic 1.0 : a NEMO-based ocean model for the Baltic and North seas - research and operational applications
- 2019
-
Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 12:1, s. 363-386
-
Tidskriftsartikel (refereegranskat)abstract
- We present Nemo-Nordic, a Baltic and North Sea model based on the NEMO ocean engine. Surrounded by highly industrialized countries, the Baltic and North seas and their assets associated with shipping, fishing and tourism are vulnerable to anthropogenic pressure and climate change. Ocean models providing reliable forecasts and enabling climatic studies are important tools for the shipping infrastructure and to get a better understanding of the effects of climate change on the marine ecosystems. Nemo-Nordic is intended to be a tool for both short-term and long-term simulations and to be used for ocean forecasting as well as process and climatic studies. Here, the scientific and technical choices within Nemo-Nordic are introduced, and the reasons behind the design of the model and its domain and the inclusion of the two seas are explained. The model's ability to represent barotropic and baroclinic dynamics, as well as the vertical structure of the water column, is presented. Biases are shown and discussed. The short-term capabilities of the model are presented, especially its capabilities to represent sea level on an hourly timescale with a high degree of accuracy. We also show that the model can represent longer timescales, with a focus on the major Baltic inflows and the variability in deep-water salinity in the Baltic Sea.
|
|
| 12. |
|
|
| 13. |
- Nycander, Jonas, et al.
(författare)
-
The nonlinear equation of state of sea water and the global water mass distribution
- 2015
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:18, s. 7714-7721
-
Tidskriftsartikel (refereegranskat)abstract
- The role of nonlinearities of the equation of state (EOS) of seawater for the distribution of water masses in the global ocean is examined through simulations with an ocean general circulation model with various manipulated versions of the EOS. A simulation with a strongly simplified EOS, which contains only two nonlinear terms, still produces a realistic water mass distribution, demonstrating that these two nonlinearities are indeed the essential ones. Further simulations show that each of these two nonlinear terms affects a specific aspect of the water mass distribution: the cabbeling term is crucial for the formation of Antarctic Intermediate Water and the thermobaric term for the layering of North Atlantic Deep Water and Antarctic Bottom Water.
|
|
| 14. |
- Pemberton, Per, et al.
(författare)
-
Arctic Ocean Water Mass Transformation in S-T Coordinates
- 2015
-
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 45:4, s. 1025-1050
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, watermass transformations in the Arctic Ocean are studied using a recently developed salinity-temperature (S-T) framework. The framework allows the water mass transformations to be succinctly quantified by computing the surface and internal diffusive fluxes in S-T coordinates. This study shows how the method can be applied to a specific oceanic region, in this case the Arctic Ocean, by including the advective exchange of water masses across the boundaries of the region. Based on a simulation with a global ocean circulation model, the authors examine the importance of various parameterized mixing processes and surface fluxes for the transformation of water across isohaline and isothermal surfaces in the ArcticOcean. The model-based results reveal a broadly realistic Arctic Ocean where the inflowing Atlantic and Pacific waters are primarily cooled and freshened before exiting back to the North Atlantic. In the model, the water mass transformation in the T direction is primarily accomplished by the surface heat flux. However, the surface freshwater flux plays a minor role in the transformation of water toward lower salinities, which is mainly driven by a downgradient mixing of salt in the interior ocean. Near the freezing line, the seasonal melt and growth of sea ice influences the transformation pattern.
|
|
