Die Arktische Oszillation im Kieler Klimamodell

The Arctic Oscillation (AO) is the dominant mode of variability of the mean sea level pressure (MSLP) in the Northern hemisphere. It is a result of an EOF-analysis for SLP3 anomalies during winter. Because of a decisive influence of the AO on atmospheric processes in the troposphere and stratosphere (Randall et al., 2007), it is of interest how the AO behaves during changing climatological conditions. Therefore three runs of the Kiel Climate Model (KCM) will be analyzed considering the AO: a control run with pre-industrial CO2-level, an A1B scenario and a scenario in which the CO2-level will change by one percent per year until a doubling of the pre-industrial CO2-level is reached ongoing with a stabilization period. Comparing the AO of the control run with the AO of the CO2-forced run the result reaches an equal anomaly in the Arctic centre. By contrast the variability in the western North Atlantic Ocean is sinking while it is clearly rising in the North Pacific Ocean. Additionally the second EOF indicates an increasing variability in the Pacific Ocean, while it is also presenting a higher degree regarding the overall variability. As a result of the temporal evolution of the AO, calculated for the control run, in the forced scenario there is a distinctive positive trend in the AO-Index. This implies a lower SLP than the average in the Arctic. In order to classify the results of the KCM, the AO of the A1B scenario in the period of 1950 to 1999 of the KCM is compared to results simulated by the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) ensembles of 14 coupled atmosphere-ocean-models, the resulting Multi-Model and observed data with regard to Miller et al. (2006). The observed data indicate as well as the A1B scenario of the KCM and the Multi-Model a positive trend in the AO-Index but the magnitude is varying. The results of the spatial distribution of the variability are showing a strong dispersion in between the models. The variability in the North Pacific Ocean is overestimated in the KCM as it is in most of the other models as well. Furthermore the AO indicates, with regard to the other IPCC models, a high agreement with the observed data in the Arctic centre and the Atlantic area

Ocean impact on the 79 North Glacier, Northeast Greenland

The retreat and acceleration of marine-terminating glaciers around the coast of Greenland observed over the last two decades have partly been attributed to a warming of Atlantic Water (AW) circulating around the subpolar North Atlantic. This thesis investigates the impact of the ocean circulation on the 79 North Glacier (79NG), which has Greenland's largest floating ice tongue. One overall hypothesis tested in this thesis is whether a long-term warming of AW in Fram Strait has spread across the continental shelf off Northeast Greenland (NEG) toward the 79NG, which may explain the recent thinning observed at the floating ice tongue. A detailed bathymetry is crucial for studying pathways of AW across the NEG continental shelf. Thus, the first part of this thesis is devoted to an accurate representation of the continental shelf bathymetry and 79NG cavity geometry. Based on a collection of hydrographic data obtained between 1979 and 2016 it is shown that the Norske Trough, the southern branch of the characteristic C-shaped trough system on the continental shelf, is filled up by warm Atlantic Intermediate Water (AIW) exceeding 1 degree Celsius at depths below 200-250 m. Current velocities from moored and lowered ADCPs inside Norske Trough indicate that a boundary current transports warm AIW toward the 79NG. The results show that Norske Trough provides the main pathway for warm AIW from the continental shelf break toward the 79NG. Anomalies in AW temperatures in Fram Strait could reach the 79NG within 1.5 years. A unique data set comprising bathymetric, hydrographic, and current velocity observations obtained during the R/V Polarstern cruise PS100 in front of the 79NG calving front emphasizes the importance of the complex bathymetry for the heat transport into the cavity below the floating ice tongue. A density-driven gravity plume, steered by the bathymetry outside the cavity, transports warm AIW into the subglacial cavity. These findings imply that the AIW layer thickness on the continental shelf plays an important role in determining the strength of the overturning in the cavity and thereby melting at the base of the floating ice tongue. For the first time, the major exchange flow across the 79NG calving front (i.e., between the continental shelf and the 79NG cavity) was captured by synoptic hydrographic and velocity observations. A bottom-intensified flow transports 140 plus/minus 20 GW of heat into the cavity via a 500 m deep and 2 km wide depression. A heat supply of this magnitude causes an average melt rate of 8.3 plus/minus 2.1 m/yr at the ice base of the 79NG. By mixing of AIW and glacial meltwater (i.e., both basal meltwater and subglacial runoff), a shallow outflow of glacially modified AIW is generated. The observed ocean exchange flow across the 79NG calving front suggests that the overturning in the cavity has a strength of 36 plus/minus 17 mSv. Combining historic and recent hydrographic data from the 79NG cavity and the NEG continental shelf reveals a warming of AIW by 0.4 plus/minus 0.1 degree Celsius between the late 1990s and recent years. Numerical experiments performed with a one-dimensional ice-shelf plume model suggest that the observed changes go along with a 44 plus/minus 13% increase in average basal melt rates. The results verify the hypothesis that a long-term warming of AW in Fram Strait has spread across the continental shelf into the 79NG cavity and emphasize that the ocean is the main driver of the ice thickness loss at the 79NG. Warm AIW is also present at the Zachariae Isstrom (ZI) located 50 km south of the 79NG, which was shown for the first time from temperature measurements. The warming in AIW presumably has driven the disintegration of the floating ice tongue of ZI in 2012/14. A further warming of waters below the 79NG may cause a similar rapid loss of the entire floating ice tongue as observed at ZI

Report on Mooring processing of PS109/PS114 recoveries (NE Greenland continental shelf)

This report documents the processing applied to the physical oceanography sensors on moorings recovered during the R/V Polarstern expeditions PS109 and PS114 on the Northeast Greenland continental shelf

Ocean Variability at Greenland's Largest Glacier Tongue Linked to Continental Shelf Circulation

Increased ocean‐to‐ice heat fluxes play a key role in the accelerated mass loss of Greenland’s marine‐terminating glaciers. Ocean current variability leads to variations in this heat flux. A year‐long time series of ocean currents at all gateways to the ocean cavity under Greenland’s largest remaining floating ice tongue at the Nioghalvfjerdsfjorden Glacier (79NG) was analyzed. The variability of the exchange flow at intra‐annual to near‐daily timescales was characterized. The currents exhibit considerable variability with standard deviations exceeding the time mean flow strength by a factor of 2. The inflow of warm Atlantic Intermediate Water into the cavity and the outflow via the northernmost calving front were directly coupled on intra‐annual timescales (periods, T > 30 days) with enhanced fluctuations in the winter months. A strong correlation between the variability of the deep inflow and currents in the subsurface boundary current on the continental shelf suggests a link between cavity and continental shelf circulation. Variability on higher frequencies (T < 30 days) in the outflow was only partly induced by the inflow variability. Two export branches of the cavity circulation were identified, which were potentially constrained by subglacial meltwater channels. The relative importance of the two export branches varies on monthly time scales. This research has provided evidence that the large intra‐annual ocean current variability at the 79NG is strongly influenced by the continental shelf circulation. Temporally varying preferred export routes increase the complexity of the cavity circulation

A full year of turbulence measurements from a drift campaign in the Arctic Ocean 2019-2020

Ocean turbulent mixing is a key process in the global climate system, regulating ocean circulation and the uptake and redistribution of heat, carbon, nutrients, oxygen and other tracers. In polar oceans, turbulent heat transport additionally affects the sea ice mass balance. Due to the inaccessibility of polar regions, direct observations of turbulent mixing are sparse in the Arctic Ocean. During the year-long drift expedition “Multidisciplinary drifting Observatory for the Study of Arctic Climate” (MOSAiC) from September 2019 to September 2020, we obtained an unprecedented data set of vertical profiles of turbulent dissipation rate and water column properties, including oxygen concentration and fluorescence. Nearly 1,700 profiles, covering the upper ocean down to approximately 400 m, were collected in sets of 3 or more consecutive profiles every day, and complemented with several intensive sampling periods. This data set allows for the systematic assessment of upper ocean mixing in the Arctic, and the quantification of turbulent heat and nutrient fluxes, and can help to better constrain turbulence parameterizations in ocean circulation models.publishedVersio

Enhanced turbulence driven by mesoscale motions and flow-topography interaction in the Denmark Strait Overflow plume

The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012, a multiplatform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study, we report on two high-dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O( math formula) W kg−1 was associated with enhanced small-scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast-response thermistor. Isotherm displacement slope spectra reveal a wave number-dependence characteristic of turbulence in the inertial-convective subrange ( math formula) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom-intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy-driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow-topography interaction is suggested to drive the intense mixing at this site

Pathways and sources of the warm Atlantic IntermediateWater in the trough system leading to the 79-North Glacier in a high resolution model

More than 25% of mean global sea level rise is caused by mass loss of Greenland Ice Sheet (GrIS). A significant part of this melt is attributed to the interaction between marine terminating glaciers of the GrIS and the surrounding warm ocean waters. However, the sources and pathways of the warm waters on the shelf, their variability and mechanisms of the heat transfer involved are variable regionally and yet largely unknown. In this work, we focus on the 79-North Glacier (79-NG), a major glacier in North-East Greenland that was subject to an increased melt in the last years. Recent observations show that Atlantic Intermediate Water (AIW) warmer than 1°C reaches the 79NG via the trough system on the East Greenland continental shelf. In particular, these observations indicate that AIW reaches the glacier rather through the southern Norske Trough than through the northern Westwind Trough. Here we employ Lagrangian modelling and analysis using a high resolution FESOM (Finite Element Sea Ice-Ocean Model) simulation. Particle trajectories representing warm AIW mass are calculated to determine the pathways of this water mass on the adjacent shelf in the Norske Trough, and we analyze the water property changes along the trajectories. Moreover, to identify the sources of the AIW in the vicinity of the 79-NG, we compute backward particle trajectories

Pathways and sources of the warm Atlantic IntermediateWater in the trough system leading to the 79-North Glacier in a high resolution model

More than 25% of mean global sea level rise is caused by mass loss of Greenland Ice Sheet (GrIS). A significant part of this melt is attributed to the interaction between marine terminating glaciers of the GrIS and the surrounding warm ocean waters. However, the sources and pathways of the warm waters on the shelf, their variability and mechanisms of the heat transfer involved are variable regionally and yet largely unknown. In this work, we focus on the 79-North Glacier (79-NG), a major glacier in North-East Greenland that was subject to an increased melt in the last years. Recent observations show that Atlantic Intermediate Water (AIW) warmer than 1°C reaches the 79NG via the trough system on the East Greenland continental shelf. In particular, these observations indicate that AIW reaches the glacier rather through the southern Norske Trough than through the northern Westwind Trough. Here we employ Lagrangian modelling and analysis using a high resolution FESOM (Finite Element Sea Ice-Ocean Model) simulation. Particle trajectories representing warm AIW mass are calculated to determine the pathways of this water mass on the adjacent shelf in the Norske Trough, and we analyze the water property changes along the trajectories. Moreover, to identify the sources of the AIW in the vicinity of the 79-NG, we compute backward particle trajectories