Category: theses

Coupled data assimilation for climate prediction: a focus on ocean-atmosphere coupling (PhD thesis)

Lilian Carolina Garcia Oliva (2024-10-17): Coupled data assimilation for climate prediction: a focus on ocean-atmosphere coupling. PhD thesis, University of Bergen, Bergen, Norway. https://hdl.handle.net/11250/3157446

Summary: Seasonal-to-Decadal (S2D) climate predictions can provide decision-making information for diverse sectors, such as food security, energy and climate adaptation. The initial condition of the ocean is fundamental for providing skilful S2D predictions. A method to estimate the ocean’s initial condition is by merging the model and observations through a process called Coupled Data Assimilation (CDA). Ocean observations have demonstrated their potential to achieve skilful prediction. The Norwegian Climate Prediction Model (NorCPM) features an advanced Ocean Data Assimilation (ODA) scheme based on an ensemble method. This thesis outlines our efforts to improve S2D predictions within the NorCPM using atmospheric observations.

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The Role of Ocean Heat Content on the Madden–Julian Oscillation (PhD thesis)

Ashneel Chandra (2024-03-19): The Role of Ocean Heat Content on the Madden–Julian Oscillation. PhD thesis, University of Bergen, Bergen, Norway. https://hdl.handle.net/11250/3124162

Summary: The overall goal of this dissertation is to understand the role of upper ocean heat content (OHC) and equatorial ocean dynamics on the Madden-Julian Oscillation (MJO). While the response of the ocean to atmospheric forcing on intraseasonal timescales has been studied extensively, the feedback of OHC on the MJO has not been systematically investigated. Recently, a new line of research has emerged that highlights the interaction between ocean dynamics, OHC, and the MJO in the Indian Ocean (IO) basin. In the IO, synchronization between oceanic equatorial waves and the MJO is possible because of the basin-scale, the propagation speed of oceanic equatorial waves, and the timescale of MJO variability. In a series of three papers, this thesis aims to contribute to understanding the variability and interactions between the MJO, equatorial ocean dynamics, and OHC in the IO basin.

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Present and future drivers of Arctic sea ice variability (PhD thesis)

Jacob Dörr (2024-03-07): Present and future drivers of Arctic sea ice variability. PhD thesis, University of Bergen, Bergen, Norway. https://hdl.handle.net/11250/3124162

Summary: The long-term decline of the Arctic sea-ice cover is overlaid by substantial interannual to decadal internal variability. This variability is a major source of uncertainty in projections over the next decades, including the timing of a seasonally ice-free Arctic. Understanding the mechanisms of internal variability and how they modify the evolution of the sea-ice cover will enable better predictions, and help to constrain future projections of the sea-ice cover. As the Arctic becomes ice-free in summer, future sea-ice loss and variability will be largest in winter. Winter sea-ice variability is currently strongest in the Barents Sea, but as the ice edge retreats, more central regions of the Arctic Ocean will see increased sea-ice variability, where the mechanisms and drivers might be different. This thesis advances our understanding of the present and future atmospheric and oceanic drivers of winter sea-ice variability, and how internal variability has modified the observed changes in the summer and winter sea-ice cover.

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Prediction of Harmful Algae Blooms Impacting Shellfish Farms in Norway (PhD thesis)

Silva, Edson (2023-11-30). Prediction of Harmful Algae Blooms Impacting Shellfish Farms in Norway. PhD thesis, University of Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3104786

Summary: Harmful algae blooms (HABs) cause severe damage to the ecosystem and human health, and have significant economic impacts on shellfish farms. HAB prediction models have become increasingly popular because they can help stakeholder to take mitigation actions and reduce economic loss. Few studies have attempted to predict toxic algae species related to shellfish contamination because the time extent of data is limited and modeling the environmental response of specific taxa is complex. However, toxic algae monitoring programs have now been running for several years and have produced large datasets of toxic algae. Combined with long-time series observations by satellites and model reanalysis, we can now calibrate prediction models for toxic algae affecting shellfish farms. This thesis calibrates machine learning models to predict toxic algae impacting shellfish farms in Norwegian coastal waters for the first time. It is conducted by combining toxic algae data from the Norwegian Food Safety Authority with satellite observations of Chla concentration, Suspended Particulate Matter (SPM), Sea Surface Temperature (SST), Photosynthetically Active Radiation (PAR), and wind speed, as well as model reanalysis data of Mixed Layer Depth (MLD) and Sea Surface Salinity (SSS). Paper I demonstrates that the blooms phenology has a strong interannual variability in the North, Norwegian, and Barents Seas, which is related to the variability of the environmental ocean and atmospheric factors (SST, MLD, SPM, and winds). It implies that these variables are potential predictors for blooms in the region. Paper II exhibit that a Support Vector Machine (SVM) model can predict the presence probability of eight toxic algae on the Norwegian coast using SST, PAR, SSS, and MLD. The models can also predict the probability of harmful levels for Alexandrium spp., Alexandrium tamarense, Dinophysis acuta, and Azadinium spinosum. It can produce a climatological overview of the HABs along the Norwegian coast and provide monitoring and prediction applications. Paper III extends the SVM application to the prediction of D. acuminata abundance in a sub-seasonal range (7 -28 days) when fed with the current and past D. acuminata abundance, SST, PAR, and wind speed. The sub-seasonal forecast model is developed for the Lyngenfjord in northern Norway as a proof of concept. The probability estimates in Paper II and the sub-seasonal forecast of D. acuminata abundance in Paper III are two complementary approaches. The first is employable in the entire coast even where algae monitoring is unavailable, while the latter requires tuning to specific aquaculture farms and can achieve refined prediction. Since the SVM models are fed with data commonly available worldwide, they are portable to other regions where data from harmful algae monitoring programs are available.

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Arctic-Atlantic Climate Variability and Predictability in Observations and in a Dynamical Prediction System (PhD thesis)

Goncalves Dos Passos, Leilane (2023-11-03): Arctic-Atlantic Climate Variability and Predictability in Observations and in a Dynamical Prediction System. PhD thesis, University of Bergen, Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3099594

Summary: The major focus of this thesis is on understanding decadal climate predictability to improve climate models and their predictions. Climate predictions show promising results but are still facing challenges, especially in connecting the ocean and atmosphere. The ocean is the main source of predictability. The ocean’s capacity to store and release heat over long periods of time makes it a thermal memory of the climate system. In the Arctic-Atlantic region, ocean currents transport heat to polar areas, and along this path, the ocean releases the heat to the atmosphere through surface fluxes. From this interaction, both the ocean and the atmosphere change. On the one hand, as the ocean releases heat into the atmosphere, it cools down, increasing its density. The denser water eventually flows southward as part of the Atlantic Meridional Overturning Circulation (AMOC). On the other hand, the atmosphere being warmed by the ocean affects nearby land areas through the winds, influencing the climate variability of Western Europe.
This dynamic ocean-atmosphere interaction is a source of predictability in the Arctic-Atlantic region and is investigated here using observations and a dynamical prediction system, the Norwegian Climate Prediction Model (NorCPM). Dynamical prediction systems are useful tools for investigating and predicting climate variability on decadal timescales. Beginning their development in the early 2000s, these systems are currently the focus of significant efforts by the scientific community to provide operational decadal forecasts with reliable and accurate information. The research of this thesis is aligned with the development of NorCPM while also focusing on investigating key mechanisms that give rise to predictability in the Arctic-Atlantic region.
Climate predictions are initialized in different ways, which affects their performance. The first study of the thesis investigates the best initialization method for the Arctic-Atlantic region using NorCPM. Paper I finds that employing a more complex data assimilation method leads to the improved predictive skill of temperature and salinity in the Subpolar North Atlantic (SPNA) but not in the Norwegian Sea. The loss of skill in the Norwegian Sea is found in regions characterized by intense surface heat fluxes and eddy activity, such as the Norwegian and Lofoten Basins. The warm Atlantic water moving northwards from the SPNA to the Norwegian Sea carries thermohaline anomalies, and it is transformed from light-to-dense waters by surface forcing along the path. These two mechanisms are investigated in observation-based data in Paper II. Their relationship is analyzed, focusing on the decadal timescale in the eastern SPNA. Paper II finds that warm anomalies are associated with surface-forced water mass transformation in the light-density classes, while during cold anomalies, more transformation happens in denser classes. This relationship was disrupted during the Great Salinity Anomaly events of the 70s and 90s. Furthermore, the study highlights a faster propagation of thermohaline anomalies in the SPNA compared to the Norwegian Sea, particularly regarding temperature.
The influence of the ocean on the climate of Europe is investigated in Paper III. This study advances the understanding of how constrained ocean variability impacts the atmosphere of NorCPM. The results show a more realistic thermodynamic component of surface air temperature (SAT) over the ocean and some European regions. Paper III shows that there is potential to improve multi-annual to decadal predictions over Europe, which is currently challenging in prediction systems. The research presented in this Thesis enhances the understanding of climate predictability in the Arctic-Atlantic region. It provides insights into the interactions between the atmosphere and ocean and adds to the development of the Norwegian Climate Prediction Model, contributing to making this prediction system operational in the coming years. Following similar approaches as presented in this thesis for other dynamical prediction systems would be highly recommended.

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Trends and internal variability in Brazilian hydropower catchments (Master’s thesis)

Byermoen, Emilie. 2023: Trends and internal variability in Brazilian hydropower catchments. Master’s thesis, University of Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3071878

Summary: Hydropower is a major energy source in Brazil, and long-term hydropower production planning is crucial both for maintaining energy and water security in the country. The amount of water that is available to electricity production in the reservoirs have changed in the recent years, and there is an urgent need to understand the cause(s) of these changes, and whether observed stream flow trends will persist, reverse or amplify in the future. In this thesis, I therefore separate externally forced precipitation and evaporation trends and variability from internal variations originating in the ocean for three hydrographic catchments in Brazil: Óbidos catchment in Amazon, Propria catchment in São Francisco and Porto Murtinho catchment in Paraguay. I compare an ocean anomaly assimilation experiment of Norwegian Climate Prediction Model (NorCPM) to an externally forced historical experiment and observed stream flow, precipitation and evaporation in the catchments. The results indicate that the multi-decadal increasing stream flow trend in Amazon is (partly) externally forced, and might therefore persist, but that the SON stream flow is tightly connected to JJA precipitation variation which is shown to be driven by ocean variation, and may therefore reverse in the future. The long-term decrease of precipitation in São francisco is likely to be caused by internal variability, and is therefore likely to (partly) restore in the future, but results indicate that decadal stream flow variations in the basin is substantially impacted by other factors than precipitation as well. São Francisco catchment is found to be strongly connected to DJF precipitation variations that the model is unable to replicate. In Paraguay, I find that the austral summer stream flow is tightly connected to inter-annual precipitation variability that originates in the ocean in austral winter and spring. The steep significant decrease in stream flow over the last decades in Paraguay catchment is likely to have additional causes than precipitation, according to the results. All the results have implications for hydropower and water management planning in the three catchments in Brazil.

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Ocean–atmosphere interaction at the Gulf Stream sea surface temperature front: variability and impacts on midlatitude atmospheric circulation (PhD thesis)

Luca Famooss-Paolini (2023-05-26): Ocean–atmosphere interaction at the Gulf Stream sea surface temperature front: variability and impacts on midlatitude atmospheric circulation. PhD thesis, Ca’ Foscari University, Italy. http://hdl.handle.net/10579/25044 .

Summary: Recent studies show that the Gulf Stream Front (GSF) is an essential ingredient of the Northern Hemisphere climate. However, the nature of the air-ocean interaction associated with the GSF variability is not understood. This thesis first analyses the atmospheric response to the meridional slip of the GSF and its dependence on model resolution, using multi-model atmospheric simulations and the ERA5 reanalysis. Finally, the thesis analyses the spectral features of the NAO-GSF interaction and the mechanisms through which the NAO forces the GSF slip, using atmospheric and oceanic reanalyses. Regarding the first point, the results show that the GSF slip induces local diabatic heat anomalies that are mainly balanced by the vertical motion and meridional transport of transient eddy streams. On the large scale, the GSF slip is associated with the homo-directional slip of the eddy-driven jet and the storm-track. However, the atmospheric response is dependent on model resolution. Only those with a resolution higher than 50 km reproduce a response similar to the observed anomalies. Regarding the second point, the results show that the NAO and the meridional position of the GSF covary on the decadal scale, but only during 1972-2018. The non-stationarity of this decadal covariability is also shown by the time dependence of their lead-lag relationship. The lag between the NAO and the GSF response on the decadal scale can be interpreted as the effect of several mechanisms. However, not all of them are stationary. There is evidence of Rossby wave propagation only before 1990, which can explain the time dependence of the NAO-GSF lead-lag relationship..

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Factors influencing interannual variability of Belg rain in Ethiopia (Master’s thesis)

Knudsen, Carina. 2023: Factors influencing interannual variability of Belg rain in Ethiopia. Master’s thesis, University of Bergen, Norway. https://bora.uib.no/bora-xmlui/handle/11250/3059081

Summary: The aim of this thesis is to investigate the factors affecting the interannual variability of the Belg rain in Ethiopia, in addition to see in which degree the NorESM can capture these factors. A significant connection was found between Belg rain and five ocean regions: Agulhas current, the northern and southern patch of the PMM, Benguela Niño, and the Indian Ocean. There was also found a connection between Belg rainfall in Ethiopia and a negative NAO index and La Niña events. The results showed that the wind pattern over the Indian Ocean is a large contributor, in addition to the Subtropical Westerly Jet. The weather in Ethiopia is highly variable, and capturing this variability has been a major challenge. Investigating the factors causing interannual variability is an important step in improving seasonal predictions and climate services. These predictions can contribute to warning systems in case of extreme events, which is important due to Ethiopia’s dependence on agriculture.

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Understanding the dynamics of recent Norwegian extreme weather events and their influence on energy production (Master’s thesis)

Pecnjak, Martin (2021-08-05). Understanding the dynamics of recent Norwegian extreme weather events and their influence on energy production (Master’s thesis, University of Bergen, Bergen, Norway). https://bora.uib.no/bora-xmlui/handle/11250/2778409 .

Summary: The growing frequency and severity of extreme weather events in the Northern Hemisphere has prompted a lot of research being done on their origin and physical mechanisms. Both simplified and complex approaches have been introduced in defining and understanding these events, where they look into high-amplitude quasi-stationary Rossby waves and their quasi-resonant amplification. However, different approaches exist to investigating extreme events and these were just a motivation for this thesis. Since the resonance method is suit- able mostly for summer events and the events discussed in this thesis have happened in all seasons, a different approach was needed. The events in question were a winter drought, two summer and autumn floods, a winter snowfall and a spring/summer heatwave in the areas of south and southwestern Norway. In order to detect certain features which would help solve this issue, we look into anomalies of different meteorological variables such as geopoten- tial height, surface temperature, precipitation and snowfall rate and zonal and meridional winds. Deep and thorough statistical and dynamical analyses are applied to define the out- comes and the physical origins which would help us obtain a clear picture on the whole case. The finite-amplitude local wave activity (LWA) diagnostic, as a measure of the meandering of the jet stream, has helped to give a clear picture along with the large-scale circulation. This method can be used as a proxy for the strength of the eddy-driven jet and the storm track. It has proven to be the key factor in defining what has exactly caused the events in ques- tion. The results and findings have shown that the LWA is a conclusive tool in determining whether an extreme event was related to a blocking pattern or not, while the LWA budget equation components have shed light on the so far poorly understood dynamical aspects which led to the events. The zonal LWA flux has proven to be a good predictor of blocking with its onset in the early stages of the events, similar to the traffic jam concept introduced by (Nakamura and Huang, 2018). The jet stream has a capacity for the LWA flux similar to how a highway has a capacity for the number of vehicles on it. If the capacity is exceeded, blocking occurs, and this is readily shown in the results and findings of this work. As for the budget equation components, the zonal LWA flux convergence has proven to be the key in maintaining the increase of the LWA as well as also having an early onset in each blocking event in agreement with the LWA flux. On the other hand, the residual in the LWA budget, which represents the non-conservative small-scale processes (diabatic sources and sinks of LWA), dampens the LWA. The LWA method has also proven to be useful in all seasons. The motivation for the thesis also came from the influence of the events on the meteorological variables related to the Norwegian energy production. The results show us clues into possible ways of improving forecasting of such events and minimizing their harmful impacts. They also show possibilities in improving energy management, infrastructure, allocation of resources and preparedness of the society for damages and hazards caused by the events. This was not fully investigated in this thesis and is the next step in the research of this topic.

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Future Abrupt Changes in Winter Barents Sea Ice Area (Master’s thesis)

Rieke, Ole (2021-06-01). Future Abrupt Changes in Winter Barents Sea Ice Area (Master’s thesis, University of Bergen, Bergen, Norway). https://bora.uib.no/bora-xmlui/handle/11250/2762637 .

Summary: The Barents Sea is an area of strong anthropogenic winter sea ice loss that is superimposed by pronounced internal variability on interannual to multidecadal timescales. This internal variability represents a source of large uncertainty in future climate projections in the Barents Sea. This study aims to investigate internal variability of Barents Sea ice area and its driving mechanisms in future climate simulations of the Community Earth System Model Large Ensemble under the RCP8.5 climate scenario. We find that although sea ice area is projected to decline towards ice-free conditions, internal variability remains strong until late in the 21st century. A substantial part of this variability is expressed as events of abrupt change in the sea ice cover. These internally-driven events with a duration of 5-9 years can mask or enhance the anthropogenically-forced sea ice trend and lead to substantial ice growth or ice loss. Abrupt sea ice trends are a common feature of the Barents Sea in the future until the region becomes close to ice-free. Interannual variability in general, and in form of these sub-decadal events specifically, is forced by a combination of ocean heat transport, meridional winds and ice import, with ocean heat transport as the most dominant contributor. Our analysis shows that the influence of these mechanisms remains largely unchanged throughout the simulation. Investigation of a simulation from the same model where global warming is limited to 2°C shows that both mean and variability of sea ice area in the Barents Sea can be sustained at a substantial level in the future, and that abrupt changes can continue to occur frequently and produce sea ice cover of similar extent to present day climate. This highlights that future emissions play an essential role in the further decline of the Barents Sea winter sea ice cover. The results of this thesis contribute to a better understanding of Arctic sea ice variability on different time scales, and especially on the role of internal variability which is important in order to predict future sea ice changes under anthropogenic warming.

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