Simulation niederfrequenter Klimavaribilität im nordatlantischen Ozean und der Arktis

Low-frequency variability in large scale North Atlantic/Arctic properties like Meridional Overturning Circulation, heat transport, deep water formation, overflows, sea ice volume, thickness and extent, as well as the Arctic fresh water budget are studied by means of ensemble simulations with the global coupled ocean/sea ice model MPI-OM forced by realistic daily atmospheric forcing data from the NCEP/NCAR Reanalysis for the period 1948- 2001. Major findings are that wintertime deep convection in the Labrador Sea is dominated by atmospheric forcing, in particular by the North Atlantic Oscillation. Intensified Labrador Sea convection induces substantial changes in the Labrador Sea Water (LSW) properties, in particular colder, fresher and denser LSW. The simulation links these changes to an increase in the Atlantic Meridional Overturning Circulation (MOC) strength. However, Labrador Sea deep convection is also strongly influenced by the presence of surface salinity anomalies, which originate from anomalous Fram Strait sea ice export events. These export events are shown to be mainly wind driven and are the most probable cause of the observed Great Salinity Anomalies of the 70th, 80th and 90th. In contrast to the Labrador Sea deep convection, the Greenland-Island-Norwegian (GIN) Sea deep convection shows a less clear imprint of the North Atlantic Oscillation variability. In the simulation, inter-annual to decadal variability in the Atlantic MOC circulation has its origin in the Labrador Sea, while longer term multi-decadal trends in the MOC are governed by the properties of the overflow waters from the GIN Sea. During the simulation period the strength of both over- flows decreased, while the overflow water density increased. On one hand low-frequency variability of the Arctic sea ice volume is related to sea ice thickness changes, driven in equal parts through variability of atmospheric thermal and fresh water fluxes, and on the other hand through variability of the wind field. While there is a clear decrease of Arctic sea ice volume during the 1990s, there is no such trend present over the full simulation period. Arctic fresh water budget variability in the simulation is dominated by exports of sea ice via Fram Strait, while the sea ice exports are governed by variability of zonal planetary waves. Generally large parts of the observed low frequency variability in the North Atlantic/Arctic can be understood as a passive response of the ocean/sea ice system to variability of the large scale atmospheric forcing

The Deelen infrasound array for recording sonic booms and events of CTBT interest

The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) has build up expertise in infrasound measurements by investigating low frequency events in order to distinguish between seismic and sonic events. KNMI operates, amongst others, a sixteen element microbarometer array with an aperture of 1.5 km, the Deelen Infrasound Array (DIA). Sonic booms and events of Comprehensive Test Ban Treaty (CTBT) interest are recorded within the frequency range of 100 seconds and 40 Hertz. Recently, KNMI and Microflown Technologies B.V. started a collaboration concerning infrasound measurements. This paper reports the use of a novel sensor. The so-called Microflown [1] is an acoustic sensor, sensitive for frequencies from 0Hz up to 1kHz. The Microflown is developed at the University of Twente and commercialised by Microflown Technologies B.V [3]

Control of Volterra systems with scalar kernels

Volterra observations systems with scalar kernels are studied. New sufficient conditions for admissibility of observation operators are developed and some examples are discussed

Revisiting the Meteor 1925-1927 hydrographic dataset reveals centennial full-depth changes in the Atlantic Ocean

The hydrographic data set of the German Atlantic Expedition (GAE) 1925-1927 is compared with the contemporary profiling float and ship-based hydrography to reveal full-depth changes in the Atlantic Ocean between 19°N and 64°S. The volume-mean warming over the last 80 years amounts to 0.119 ± 0.067°C, accompanied by an increase in salinity of 0.014 ± 0.010. A clear vertical structure of these changes is observed: on average, the ocean has warmed by 0.272 ± 0.093°C and became saltier by 0.030 ± 0.014 down to about 2000 m, but cooled and freshened slightly in the deeper layers. These changes can be traced throughout the whole hydrographic survey, indicating the basin-wide character of the observed changes on a centennial timescale. The observed warming is consistent with climate model simulations over the 20th century, suggesting an attribution to anthropogenic forcing. Comparison with the pre-GAE cruises reveals no discernible warming between the 1870s and 1906/1911. © 2013 American Geophysical Union. All Rights Reserved

Arctic-North Atlantic interactions and multidecadal variability of the meridional overturning circulation

Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–oceanmodel ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic.The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively highresolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation

Formation and propagation of great salinity anomalies

North Atlantic/Arctic ocean and sea ice variability for the period 1948–2001 is studied using a global Ocean General Circulation Model coupled to a dynamic/thermodynamic sea ice model forced by daily NCEP/NCAR reanalysis data [Kalnay et al., 1996]. Variability of Arctic sea ice properties is analysed, in particular the formation and propagation of sea ice thickness anomalies that are communicated via Fram Strait into the North Atlantic. These export events led to the Great Salinity Anomalies (GSA) of the 1970s, 1980s and 1990s in the Labrador Sea (LS). All GSAs were found to be remotely excited in the Arctic, rather than by local atmospheric forcing over the LS. Sea ice and fresh water exports through the Canadian Archipelago (CAA) are found to be only of minor importance, except for the 1990s GSA. Part of the anomalies are tracked to the Newfoundland Basin, where they enter the North Atlantic Current. The experiments indicate only a minor impact of a single GSA event on the strength of the North Atlantic Thermohaline Circulation (THC)