2. Wochenbericht POS350

Die 350. Forschungsfahrt des FS POSEIDON fand vom 26. April bis 10. Mai 2007 unter der Fahrtleitung von Dr. Gerd Krahmann stat

Variability of the Northern Annular Mode's signature in winter sea ice concentration

Historical winter sea ice concentration data are used to examine the relation between the Northern Annular Mode (NAM) and the sea ice concentration in the Nordic seas over the past 50 years. The well known basic response pattern of a seesaw between the Labrador Sea and the Greenland, Iceland and Barents seas is being reproduced. However, the response is not robust in the Greenland and Iceland seas. There the observed variability has a more complex relationship with surface temperatures and winds. We divide the sea ice response into three spectral bands: high (P 15 year) filtered NAM indices. This division is motivated by the expected slow response of the ocean circulation which might play a significant role in the Greenland and Iceland seas. The response to the NAM is also examined separately for the periods before and after 1976 to identify variations due to the relocation of the northern centre of the North Atlantic Oscillatio

Arctic Ocean sea ice response to Northern Annular Mode-like wind forcing

The response of the Arctic Ocean sea ice system to Northern Annular Mode-like wind forcing has been investigated using an ocean/sea ice general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the Arctic Ocean's response to idealized winter wind anomalies on interannual to multi-decadal time scales. The sea ice response of the model consists of a rapid change of ice movements leading to widespread variation in sea ice thickness and concentration. In most areas the response is largely independent of the forcing frequency with only a slight increase towards longer periods. Only the Greenland Sea exhibited a change in sign of sea ice concentration anomalies at about 20 years period which appears to be caused by slow adjustment of the oceanic circulation

internal wave and mixing processes studied by contemporaneous hydrographic, current, and seismic measurements ; Funchal - Lissabon, 26.04.-10.05.2007

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Evaluation of seismic reflector slopes with a Yoyo-CTD

In spring 2007 a dedicated seismic and oceanographic experiment was conducted in the Gulf of Cadiz. Employing two research vessels seismic and hydrographic observations were made contemporaneously and in close proximity. At a 12 hour long station inside a Meddy a conductivity-temperature-depth (CTD) probe was lowered and raised repeatedly while the seismic vessel conducted a repetitive survey on tracks nearby. Over the period 17 CTD profiles were collected covering the depth interval from 500 to 1700 m. The CTD data show the Meddy's elevated temperatures and salinities as well as varying intrusive features. When converted into reflection coefficients and convoluted with the seismic source signal the CTD data agrees well with nearby seismic data. The comparison of the temporal/spatial slopes of CTD-derived reflectors with those of isopycnals shows a good agreement when the slopes are determined over intervals shorter than 4 hours

Deep echoes: an interdisciplinary approach to investigate the oceans

Seismic oceanography is a new field in marine research combining expertise of proven geophysical methods with classical physical oceanography to get more insight into physical processes in the water column. An interdisciplinary working group at IFM-GEOMAR has sucessfully used the in-house expertise for deep ocean investigations

Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500m is estimated to be 1×10−5 m2 s−1, with lower and upper 95%confidence limits of 0.8×10−5 m2 s−1 and 1.4×10−5 m2 s−1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 μmol kg−1 yr−1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper CentralWater layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways