ALBERT

Description: The climate in the Atlantic region is essentially influenced by the Atlantic meridional overturning circulation (AMOC) which carries warm waters into northern latitudes and returns cold deep water southward across the equator. In the Labrador Sea basin a major component of the cold limb of the Atlantic meridional overturning circulation (AMOC) is formed. The intermediate water mass that is part of this deep convection process is the Labrador Sea Water (LSW) which can be separated into two different classes: the deep LSW (dLSW) and the less dense upper LSW (uLSW). Both LSW modes are formed by convection, accompanied by a strong surface cooling during winter conditions, which leads to an increase in the near-surface density and to an unstable stratification and a homogenization of the water column. In this study we simulated the deep-water formation in the Labrador Sea using the Finite-Element Sea-Ice Ocean Model (FESOM) in a global model setup with regional focus on the Labrador Sea and Greenland Sea. We evaluated the capability of the model setup to reproduce a realistic deep water formation in the Labrador Sea by analyzing the modeled Labrador Sea hydrography and we compared the modeled and observational derived dLSW and uLSW layer thicknesses for the time interval 1958-2007. It is shown that the model is able to reproduce different phases in the temporal evolution of the potential density, temperature and salinity, which are known in observational data. Based on composite maps of the thermal and haline contributions to the surface density flux we can prove that the central Labrador Sea in the model is dominated by the thermal contributions of the surface density flux, while the haline contributions are limited to the branch of the Labrador Sea Boundary Current system, where they are dominated from the haline contributions of sea ice melting and formation. Our model results feature a shielding of the central Labrador Sea from the haline contributions by the Labrador Sea Boundary Current system. Furthermore we investigated modes of interannual to decadal variability for the period 1958-2004 and attributed the general variability in the model to the atmospheric forcing and to internal modes of the ocean system. Based on a North Atlantic Deep Water (NADW) index defined for a normal and random forced FESOM run, where the interannual to decadal atmospheric variability in the random forced run is replaced by white noise, we identify modes of interannual to quasi-decadal variability of 7yr and 14yr, respectively. The origin of the 14yr variability is attributed to the atmospheric forcing, while the 7yr variability is linked to internal modes of the ocean. To further isolate the horizontal, but also the vertical variability in the model, we apply a principal oscillation pattern analysis in a three dimensional context. Two exceptional stable interannual modes are captured by the POP analysis and their variability is attributed to a propagating Rossby wave structure.

Description: Deep-water formation in the Labrador Sea is simulated with the Finite-Element Sea-Ice Ocean Model (FESOM) in a regionally focused, but global covered model setup. The model has a regional resolution of up to 7km. Our simulations cover the time period 1958-2007. We evaluate the capability of the model setup to reproduce a realistic deep water formation in the Labrador Sea. Two classes of Labrador Sea Water (LSW) are analysed and compared to LSW layer thicknesses derived from observations in the formation region for the time interval 1988-2007. It is shown that the model is able to reproduce four phases in the temporal evolution of the potential density, temperature and salinity, since the late 1980s, which are known in observational data. These four phases are characterized by a significantly different LSW formation. The first phase is characterized in the model by a fast increase in the the convection depth of up to 2000m, accompanied by an increased Spring production of deep Labrador Sea Water (dLSW). In the second phase, the dLSW layer thickness remains on a high level for several years, while the third phase features a gradual decrease in the deep ventilation and the renewal of the deep ocean layers. The fourth phase features an almost constant dLSW layer thickness on a reduced level. By applying a Composite Map Analysis between an index of dLSW and sea level pressure over the entire simulation period from 1958-2007, it is shown that a pattern which resembles the structure of the North Atlantic Oscillation (NAO) is one of the main triggers for the variability of LSW formation. Our model results indicate that the process of dLSW formation can act as a low-pass filter to the atmospheric forcing, so that only persistent NAO events correlate with the dLSW index. Based on composite maps of the thermal and haline contributions to the surface density flux we can prove that the central Labrador Sea in the model is dominated by the thermal contributions of the surface density flux, while the haline contributions are limited to the branch of the Labrador Sea boundary current system (LSBCS), where they are dominated from the haline contributions of sea ice melting and formation. Our model results feature a shielding of the central Labrador Sea from the haline contributions by the LSBCS, which only allows a minor haline interaction with the central Labrador Sea by lateral mixing. Based on the comparison of the simulated and measured LSW layer thicknesses as well as vertical profiles of potential density, temperature and salinity we show that the FESOM model is a suitable tool to reproduce the regional dynamics of the LSW formation in a global covered context.

Description: The modeling and understanding of the deep-water formation variability, especially in the North Atlantic sector, is of crucial importance for the common global ocean variability, in particular on interannual to decadal time-scales. The local restriction of the deep water formation areas makes it necessary to follow new model approaches that are able to resolve these areas with a sufficient high resolution without ignoring the global context. This study aims to validate the ability of the Finite-Element Sea-Ice Ocean Model (FESOM) to reproduce a reliable deep water formation in North Atlantic ocean and to analyse its variability on interannual to decadal time-scales. The FESOM approach works on unstructured triangular surface meshes, which allows us to faithfully resolve coastlines and local areas of interest. The first part of the thesis presents the characteristics of a global FESOM setup designed to study the variability in the deep-water formation areas over five decades for the period 1958-2004. The setup features a regionally increased resolution in the deep water formation areas in the Labrador Sea, Greenland Sea, Weddell Sea and Ross Sea as well as in equatorial and coastal areas. Further, this part of the thesis deals with the applied spinup procedure and the general validation of the FESOM model setup with respect to the performance of the sea-ice and ocean model component. Based on the analysis of the Atlantic Meridional Overturning Circulation (AMOC) we demonstrate that the upper ocean is converged within the applied spinup procedure. The sea ice model reproduces realistic sea-ice distributions and variabilities in the sea ice extent on both hemispheres as well as sea ice transport that compares well with observational data. The general ocean circulation model is validated based on a comparison of the model results with Ocean Weather Ship data in the North Atlantic. We can prove that the vertical structure is well captured in areas with improved resolution. Further, we are able to simulate the decadal ocean variability in the Nordic Sea Overflows as well as several salinity anomaly events and corresponding fingerprint in the vertical hydrography. The second part of the thesis focuses on the validation of the model capability to reproduce a realistic deep-water formation in the Labrador Sea. Therefor, we examine two classes of Labrador Sea water (LSW) which are analysed and compared to observed LSW layer thicknesses derived from profile data for the time interval 1988-2007. We show, that the model setup reproduces in the temporal evolution of the potential density, temperature and salinity two different phase since the late 1980s. These two phases are well known in observational data and are characterized by a significantly different LSW formation. Whereas the first phase features a dominant increase in the layer thickness of the deep Labrador Sea water (dLSW), is the second phase characterized by a degeneration of dLSW. To highlight the processes that are responsible for the variability in dLSW layer thickness we apply a Composite Map Analysis (CMA) between an index of dLSW and sea level pressure, as well as the thermal and haline contributions to the surface density flux. The composite maps reveal that a North Atlantic Oscillation like pattern is one of the main triggers for the variability of LSW formation in the model. Our model results indicate that a massive dLSW formation can act as a low-pass filter to the atmospheric forcing, so that only persistent NAO events correlate with the dLSW index. Additionally our results show that the central Labrador Sea in the model is dominated by the thermal contributions of the surface density flux, while the haline contributions are shielded from the central Labrador Sea by the branch of the Labrador Sea Boundary Current system. In our model, this shielding allows only a minor haline interaction with the central Labrador Sea by lateral mixing. Another aim of the thesis is to examine the general model variability on interannual to decadal time scales. Therefore we study the variability in a normal and random forced FESOM run. By definition of a North Atlantic Deep Water (NADW) index for the normal and random forced FESOM run we could identify an interannual and quasi decadal variability of 7.1 yr and 14.2 yr, respectively. It is found that the normal forced run is dominated by the quasi decadal variability and the random forced run by the interannual variability. The quasi decadal variability could be attributed to the atmospheric forcing, while the interannual variability could be linked to internal modes of the ocean. We defined in analogy to the baroclinic mass transport index (BMT) a DGyre from the horizontal barotropic streamfunction. The comparison of the observed BMT index and the modeled DGyre index reveals that the model is able to reproduce the variability of the index comparing to the observed one, although the model tends to overestimate the magnitude of the index. To further isolate the horizontal but also the vertical variability in the model we apply a principal oscillation pattern (POP) analysis in a three dimensional context. We discovered two exceptional strong interannual modes whose variability could be attributed to a propagating Rossby wave structure.

Description: The relationships between the dominant modes of interannual variability of Diurnal Temperature Range (DTR) over Europe and large-scale atmospheric circulation and sea surface temperature anomaly fields are investigated through statistical analysis of observed and reanalysis data. It is shown that the dominant DTR modes as well as their relationship with large-scale atmospheric circulation and sea surface temperature anomaly fields are specific for each season. During winter the first and second modes of interannual DTR variability are strongly related with the North Atlantic Oscillation and the Scandinavian pattern, while the third mode is related with the Atlantic Multidecadal Oscillation. Strong influence of the Atlantic Multidecadal Oscillation and the Arctic Oscillation on spring DTR modes of variability was also detected. During summer the DTR variability is influenced mostly by a blocking-like pattern over Europe, while the autumn DTR variability is associated with a wave-train like pattern, which develops over the Atlantic Ocean and extends up to Siberia. It is also found that the response of DTR to global sea surface temperature is much weaker in spring and summer comparing to winter and autumn. A correlation analysis reveals a strong relationship between DTR modes of variability and the Cloud Cover anomalies during all seasons. The influence of the potential evapotranspiration and precipitation anomalies on DTR modes of variability is strongest during summer, but it is significant also in spring and autumn. It is suggested that a large part of interannual to decadal DTR variability over Europe is induced by the large-scale climate anomaly patterns via modulation of cloud cover, precipitation and potential evapotranspiration anomaly fields.

Description: The characteristics of a global set-up of the Finite-Element Sea-Ice Ocean Model under forcing of the period 1958-2004 are presented. The model set-up is designed to study the variability in the deep-water mass formation areas and was therefore regionally better resolved in the deep-water formation areas in the Labrador Sea, Greenland Sea, Weddell Sea and Ross Sea. The sea-ice model reproduces realistic sea-ice distributions and variabilities in the sea-ice extent of both hemispheres as well as sea-ice transport that compares well with observational data. Based on a comparison between model and ocean weather ship data in the North Atlantic, we observe that the vertical structure is well captured in areas with a high resolution. In our model set-up, we are able to simulate decadal ocean variability including several salinity anomaly events and corresponding fingerprint in the vertical hydrography. The ocean state of the model set-up features pronounced variability in the Atlantic Meridional Overturning Circulation as well as the associated mixed layer depth pattern in the North Atlantic deep-water formation areas.