Dynamics of the Canadian Arctic Archipelago throughflow: A numerical study with a finite element sea ice and ocean model

The Canadian Arctic Archipelago (CAA) connects the Arctic Ocean and Baffin Bay through narrow channels and is one of the key gateways where freshwater leaves the Arctic. It has therefore the potential to affect the deep convection in the northern North Atlantic. Representing the CAA in traditional global models still poses a challenge due to the small scale nature of the narrow passages. In this study we apply a global, multi-resolution sea ice ocean model (the Finite Element Sea ice Ocean Model, FESOM) with refinement in the CAA up to 5 km, while keeping a coarse resolution setup otherwise. With this model setup, a hindcast simulation for the period 1968-2007 was performed. The first goal of this thesis is to assess the model behavior in the CAA region and in the Arctic Ocean. The model assessment revealed good agreement with sea ice conditions in the Arctic Ocean and with fluxes through the main gates of the Arctic Ocean. During the period 1968-2007 the mean volume transports through Lancaster Sound and Nares Strait amount to 0.86 Sv (1 Sv = 10^6 m^3/s) and 0.91 Sv, respectively. The monthly mean volume transport through western Lancaster Sound is highly correlated with the observational estimate (r=0.81). A comparison of simulated sectionally averaged velocities in Nares Strait with observational estimates reveals good agreement (r=0.57). The simulated mean CAA freshwater export rate is 123 mSv, slightly higher than the observational estimate (101 -10 mSv). The local refinement of 5 km allows to investigate the freshwater contribution of individual narrow straits to the Parry Channel. In the second part of the thesis, the mechanisms driving the interannual variability of freshwater transports through the CAA are analyzed. The interannual variability is determined by sea surface height (SSH) gradients between the Arctic Ocean and northern Baffin Bay. The variability of fluxes through Lancaster Sound and Nares Strait is mainly determined by that of the SSH on the shelf along the Beaufort Sea coast and in the northeastern Baffin Bay, respectively. Sea level variations north of the CAA are explained by changes in the wind regimes (cyclonic vs. anticyclonic) associated to release or accumulation of freshwater from the Beaufort Gyre, whereas sea level in the northeastern Baffin Bay can be attributed to ocean-atmosphere heat fluxes over the Labrador Sea. Both processes are linked with the North Atlantic Oscillation type of atmospheric variability. In the last part of the thesis, the effect of mesh resolution in the CAA area is evaluated by performing experiments with and without highly resolved archipelago (5 km vs. 24 km resolution). Increased resolution in the CAA leads to higher freshwater transports through the CAA; at the same time transports on the eastern side of Greenland are reduced. The `redirection' of Arctic freshwater affects convection in the Labrador Sea and thus the Atlantic meridional overturning circulation. We conclude that multi-resolution models like FESOM are promising tools for global climate modeling, as they are able to present small scale processes in a global setup

Credible Worst Case Tsunami Scenario Simulation for Padang

Padang in West-Sumatra is one of the priority regions of the German-Indonesian Tsunami Early Warning System project (GITEWS). From 2006 onwards, Alfred Wegener Institute (AWI) as the lead organization within GITEWS for simulation products, has contributed simulation results and inundation map information to the community of Padang. In this memorandum, we intend to communicate latest results of our simulations

The Pan-Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015)

Abstract Primary production in the Central Arctic Ocean (CAO) is limited by light and bioavailable nutrients. With the decline of the sea-ice cover in recent decades, and the resulting increase in light availability, nitrate limitation has been speculated to become more prominent. We used an eddy-permitting biogeochemical model simulation to estimate nitrate advective fluxes at different spatio-temporal scales (synoptic, mesoscale and sub-mesoscale) over the 1985–2015 period. We found that the pan-Arctic continental slope contributes disproportionately to the Dissolved Inorganic Nitrogen supply and that this supply is intensifying through two main processes: lateral eddy transport and upwelling. Despite this increasing supply in nitrate and an intensification of ocean dynamics, the nutrient supply is decreasing everywhere else in the central basins and the simulation indicates that the CAO is still shifting from light to nutrient limitation

A 4.5 km resolution Arctic Ocean simulation with the global multi-resolution model FESOM1.4

In the framework of developing a global modeling system which can facilitate modeling studies on Arctic Ocean and high- to midlatitude linkage, we evaluate the Arctic Ocean simulated by the multi-resolution Finite Element Sea ice-Ocean Model (FESOM). To explore the value of using high horizontal resolution for Arctic Ocean modeling, we use two global meshes differing in the horizontal resolution only in the Arctic Ocean (24 km vs. 4.5 km). The high resolution significantly improves the model's representation of the Arctic Ocean. The most pronounced improvement is in the Arctic intermediate layer, in terms of both Atlantic Water (AW) mean state and variability. The deepening and thickening bias of the AW layer, a common issue found in coarse-resolution simulations, is significantly alleviated by using higher resolution. The topographic steering of the AW is stronger and the seasonal and interannual temperature variability along the ocean bottom topography is enhanced in the high-resolution simulation. The high resolution also improves the ocean surface circulation, mainly through a better representation of the narrow straits in the Canadian Arctic Archipelago (CAA). The representation of CAA throughflow not only influences the release of water masses through the other gateways but also the circulation pathways inside the Arctic Ocean. However, the mean state and variability of Arctic freshwater content and the variability of freshwater transport through the Arctic gateways appear not to be very sensitive to the increase in resolution employed here. By highlighting the issues that are independent of model resolution, we address that other efforts including the improvement of parameterizations are still required

Impact of Cyclonic Wind Anomalies Caused by Massive Winter Sea Ice Retreat in the Barents Sea on Atlantic Water Transport Toward the Arctic: A Model Study

The Arctic is warming much faster than the global average. This is known as Arctic Amplification and is caused by feedbacks in the local climate system. In this study, we explore a previously proposed hypothesis that an associated wind feedback in the Barents Sea could play an important role by increasing the warm water inflow into the Barents Sea. We find that the strong recent decrease in Barents Sea winter sea ice cover causes enhanced ocean-atmosphere heat flux and a local air temperature increase, thus a reduction in sea level pressure and a local cyclonic wind anomaly with eastward winds in the Barents Sea Opening. By investigating various reanalysis products and performing high-resolution perturbation experiments with the ocean and sea ice model FESOM2.1, we studied the impact of cyclonic atmospheric circulation changes on the warm Atlantic Water import into the Arctic via the Barents Sea and Fram Strait. We found that the observed wind changes do not significantly affect the warm water transport into the Barents Sea, which rejects the wind-feedback hypothesis. At the same time, the cyclonic wind anomalies in the Barents Sea increase the amount of Atlantic Water recirculating westwards in Fram Strait by a downslope shift of the West Spitsbergen Current, and thus reduce Atlantic Water reaching the Arctic basin via Fram Strait. The resulting warm-water anomaly in the Greenland Sea Gyre drives a local anticyclonic circulation anomaly

Properties and dynamics of mesoscale eddies in Fram Strait from a comparison between two high-resolution ocean-sea ice models

Fram Strait, the deepest gateway to the Arctic Ocean, is strongly influenced by eddy dynamics. Here we analyse the output from two eddy-resolving models (ROMS – Regional Ocean Modeling System; FESOM – Finite-Element Sea-ice Ocean Model) with around 1 km mesh resolution in Fram Strait, with a focus on their representation of eddy properties and dynamics. A comparison with mooring observations shows that both models reasonably simulate hydrography and eddy kinetic energy. Despite differences in model formulation, they show relatively similar eddy properties. The eddies have a mean radius of 4.9 and 5.6 km in ROMS and FESOM, respectively, with slightly more cyclones (ROMS: 54 %, FESOM: 55 %) than anticyclones. The mean lifetime of detected eddies is relatively short in both simulations (ROMS: 10 d, FESOM: 11 d), and the mean travel distance is 35 km in both models. More anticyclones are trapped in deep depressions or move toward deep locations. The two models show comparable spatial patterns of baroclinic and barotropic instability. ROMS has relatively stronger eddy intensity and baroclinic instability, possibly due to its smaller grid size, while FESOM has stronger eddy kinetic energy in the West Spitsbergen Current. Overall, the relatively good agreement between the two models strengthens our confidence in their ability to realistically represent the Fram Strait ocean dynamics and also highlights the need for very high mesh resolution

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