Summary of THOR publications
This document contains the titles and the abstracts of:
- peer-reviewed articles
- publications in plan, to be submitted and submitted
- thesis (bachelor, master and PhD)
- interventions and reports (presentations)
The latest update is available: download the July 2013 Word file
The full version of the peer-reviewed articles are downloadable for THOR members in internal area (log-in).
Thesis based on THOR data
1 ) Kopte, Robert (2010): Untersuchungen zu Schwankungen der Konvektionsaktivität in Labradorsee und Irminger See --- 1997 – 2009, Bachelor Thesis (903592) at GEOMAR
Interannual to seasonal variability of convective activity is investigated in both Labrador and Irminger Seas between 1997 and 2009. Possible reasons for these variabilities and their direct consequences for the convection basins are discussed. Long-time data, collected by moorings K1 (central Labrador Sea) and CIS (central Irminger Sea), was used for analysis of wintertime Mixed Layer Depths and temperature developments, supported by ARGO data from profiles taken within the convective regions closely matching to the moorings. Furthermore a dataset, provided by the NCEP/NCAR reanalysis project served the examination of net heat fluxes through the air-sea interface to determine the role of atmospheric forcing for the convective process.
There is no indication for an upcoming phase of enhanced convective activity following the 2008 event, when surprisingly deep-reaching convection to depths of approx. 1800m, triggered by massive atmospheric forcing, took place after several winters of rather weak convection. As one consequence the warming trend in the intermediate water layers was disrupted just shortly in 2008, but has been ongoing with similar intensity since. However, it is expected that there is a consistent horizontal input of heat into the central Labrador Sea, partially compensating massive heat losses during the convective season. Compared to the Labrador Sea, convection in the Irminger Sea generally is less deep-reaching, because of differences in atmospheric excitation. It did not exceed 1000m during the investigated period. Hence it can be concluded that there possibly was no contribution in the formation of Labrador Sea Water by convective activities in the Irminger Sea during that time.
2 ) Voet, G. (2010): Dissertation “On the Nordic Overturning Circulation“,Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften im Department Geowissenschaften der Universitaet Hamburg
The oceanic Meridional Overturning Circulation is an important element of the world’s climate system that strongly influences the European climate. Projections from computer models simulating ocean and atmosphere for a future climate affected by increased anthropogenic greenhouse gas emissions predict a weakening of the Meridional Overturning Circulation of the North Atlantic, especially of its branch into the Nordic Seas. As global computer
simulations still have coarse grids compared to the processes that affect the Meridional Overturning Circulation, a better understanding of these processes is crucial for improved climate predictions and the validation of these. This work aims at closing gaps in the understanding of processes associated with the Nordic Overturning Circulation. The Nordic Seas are a site for the transformation of the water masses of the surface branch to the deep branch of the Meridional Overturning Circulation. The identification of important areas for this transformation within the Nordic Seas is currently an area of intense research efforts. Using position data of autonomous profiling floats, the mid-depth circulation of the Nordic Seas, its variability and forcing mechanisms are analyzed in this thesis. This helps to understand the redistribution of different water masses within the Nordic Seas. The export of dense water from the Nordic Seas is restricted by the shallow Greenland-Scotland Ridge. The overflows of dense water across the deeper passages in this ridge are well understood, monitored and modeled. In contrast, the part of the overflow across the shallow and wide opening between Iceland and the Faroe Islands is less investigated so far. Using recently recorded current data and historical hydrographic data, an estimate for the mean overflow across the Iceland-Faroe Ridge and its seasonal variability is obtained in this thesis. This aims at closing a gap in the budget of volume transport exchanges between the Nordic Seas and the North Atlantic, leading to better understanding and detectability of changes in the strength of the Nordic branch of the Meridional Overturning Circulation. The dense water that has overflowed the Greenland-Scotland Ridge sinks to greater depths of the North Atlantic due to its high density compared to the ambient water masses. During this descent, the overflow plumes are subject to vigorous entrainment that changes the water mass properties and increases the volume transport of the dense current. Two studies within this work are dedicated to analyzing the importance of turbulent vertical mixing and horizontal stirring by meso-scale eddies for the entrainment into the dense plumes. A better understanding of these processes may help parameterising them into coarse computer models, allowing for an improved representation of the Meridional Overturning Circulation in the models.
3 ) Mjell, T.L., U. S. Ninnemann and H. F. Kleiven (2008): Variability in ISOW Vigor Over the Last Millennium and its Relationship to Climate. 2008 Eos. Trans. AGU, 89 (53), Fall Meet. Suppl., Abstract PP11A-1373. (Master Thesis)
Resolving the scale and origins of recent low frequency (decadal-centennial) climate variations, such as the Little Ice Age (LIA), is crucial for predicting how natural variability and anthropogenic forcing will interact to affect future climate evolution. These multi-decadal to centennial variations are often postulated to involve, or even be driven by, changes in the Atlantic Meridional Overturning Circulation (AMOC). Assessing this hypothesis for events such as the LIA and the Atlantic Multidecadal Oscillation (AMO) requires records capable of depicting multidecadal changes in past ocean circulation. Here we use well dated (210Pb and AMS 14C), high sedimentation rate, multi and gravity cores taken on the Gardar Sediment Drift (60.19oN, 23.58oW, 2081m) to reconstruct decadal to centennial variability in the properties and vigor of the eastern branch of the Nordic Seas overflows over the past millennium. The Gardar drift accumulates on the eastern flank of the Reykjanes ridge due the supply of sediments provided by the overlying Iceland Scotland Overflow Water (ISOW), an important component of NADW. We reconstruct the bottom water physical and chemical properties of ISOW using the oxygen and carbon isotopes of benthic foraminifera (C. wuellerstorfi), while changes in the vigor of near bottom flow are inferred from size variations in the sediment mean sortable silt. Taken together, the records provide a sub-decadally sampled history of ISOW variability spanning from ~900-2002 AD. In addition, changes in surface hydrography are reconstructed using δ18O of the planktonic foraminifera N. pachyderma (d) and G. inflata. Initial results show significant multi-decadal and centennial variability in near surface temperature and/or salinity throughout the last millennium. Although there is a clear warming/freshening trend in our surface water proxies since 1850, similarly large events have occurred repeatedly throughout the record, with the warmest/freshest period in the record occurring in the seventeenth century. Considering these surface water records in the context of regional climate records we discuss the link between decadal-centennial scale climate change and ISOW variability.
4a ) Nummelin A. (2010): Denmark Strait Overflow. Department of Physics, University of Helsinki, 30 pp. (Bachelor thesis)
5a ) Korhonen, M. (2011) Time and space variability of freshwater content, heat content and seasonal ice melt in the Arctic Ocean 1991-2008. Master Thesis at the University of Helsinki, 87 pp.
The Arctic Ocean gains freshwater mainly through precipitation, river discharge and the Bering Strait inflow. In addition the recent reduction in sea ice volume is suggested to decrease the surface salinity and thus contribute to the freshwater content in the upper ocean. The large freshwater flux adds to the stability of the upper ocean, restricting vertical mixing above the halocline and insulating the sea ice from the heat stored in the Atlantic layer. Therefore the low salinity surface has been considered crucial for the maintenance of the ice cover. The recent freshening has not, despite the established strong stratification, been able to restrain the accelerating ice loss and other heat sources besides the Atlantic water are investigated. Possible sources are the waters advecting from the Pacific Ocean and the solar insolation warming the Polar Mixed Layer. Since the ongoing freshening, oceanic heat sources and the sea ice melt are closely related, this study attempts to associate the ongoing variability regarding these three properties.
Using data on conductivity, temperature and depth obtained during various icebreaker expeditions conducted in late summer between 1991 and 2008 time and space variability in the upper 1000 m water column is studied. For examining the vertical distribution of freshwater and heat, the water column is further divided into the Polar Mixed Layer, upper and lower haloclines, upper and lower Atlantic layers as well as upper Polar Deep Water. Furthermore, to distinguish meltwater from freshwater content in the Polar Mixed Layer, the salinity marked by the depth of winter convection is used. The meltwater content estimated from the hydrographic observations is compared to the available atmospheric heat input obtained from NCEP/NCAR Reanalysis. For documenting the space variability, resulting largely from the different properties of waters derived from the Atlantic and Pacific oceans, the Arctic Ocean is subdivided into eight basins and sub-surface ridges.
The largest time and space variability of freshwater content occurs in the Polar Mixed Layer and the upper halocline. The freshening of the upper ocean during the 2000s is ubiquitous in the Arctic Ocean although the most persistent increase occurs in the Canada Basin where the intensified Ekman convergence accumulates freshwater to the upper halocline. Whereas the upper halocline gains freshwater through the increasing volume, the salinity of the Polar Mixed Layer is plausibly decreasing due to the increased sea ice melt. In the Eurasian Basin the freshening is attributed mainly to the distribution of the waters deriving from the continental shelves while the seasonal ice melt remains rather modest. Particularly the melt in the Nansen Basin appears independent of the perpetually increasing heat content in the Atlantic layer. While the warming of the Atlantic inflow was ubiquitous in the Arctic Ocean during the 1990s, no obvious signal of the more recent warming is captured beyond the Nansen Basin. Although no evident warming is observed in the upper ocean, it is suggested that the presumably increasing absorption of solar energy has been used in the ice melt prior to the observations.
6 ) Stewart Hall (Oct. 2012),”The Variability of the Denmark Strait overflow”, A thesis submitted to the School of Environmental Sciences of the University of East Anglia in partial fulfilment of the requirements for the degree of Doctor of Philosophy
The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. Therefore changes to the properties of Denmark Strait Overflow Water (DSOW) may have the potential to influence the global climate. This thesis has used observations from current meters, output from an eddy resolving Ocean General Circulation Model and oxygen isotope ratio data to investigate the mechanisms responsible for variability of DSOW salinity. Three main conclusions have been drawn:
- DSOW salinity anomalies are probably caused by changes in the proportions of source waters feeding the overflow, rather than changes in the salinity of the source waters. This is caused by an increase in southward wind stress off the East Greenland coast and a subsequent increase in volume flux of fresh intermediate water arriving at the sill. This results in a freshening of DSOW, with a lag of 4 ½ months from the wind stress change.
- Eddy variability at the moorings may be linked to different proportions of source waters feeding the overflow at the sill, and therefore may be indirectly linked to the wind forcing off the East Greenland coast. This means that short term variability in salinity of a few days at the moorings, could be driven by processes occurring upstream of the sill on much longer interannual time scales.
- DSOW freshwater input derives from meteoric water (i.e. no sea-ice melt input) for the period of June 2009, and a proportion of DSOW source water has also formed sea-ice upstream of the sill. Oxygen isotope ratio analysis is a useful tool to provide an insight into the nature of the regions where DSOW source waters may last have been in contact with the surface.
7) Köllner, Manuela (October 2012): Signal propagation and entrainment of Denmark Strait Overflow Water as measured at Ammassalik, 2007-2012, Master Thesis at the University of Hamburg
Masterarbeit im Fach Physikalische Ozeanographie, verfasst an der Fakultät für Mathematik, Informatik und Naturwissenschaften, im Fachbereich Geowissenschaften, Centrum für Erdsystemforschung und Nachhaltigkeit, Institut für Meereskunde der Universität Hamburg
Changes in the water mass characteristics of the Denmark Strait Overow Water (DSOW) can inuence the North Atlantic DeepWater which is an important component of the Atlantic Meridional Overturning Circulation. Temperature and salinity data from mooring arrays at the Denmark Strait sill and near Ammassalik between 2007 and 2012 were analyzed to quantify changes in hydrographic properties and to identify their sources. No trends in the potential temperature and salinity time series were detected. High variability on short time scales is present at the moorings, but in the power spectra no dominant frequency is evident. A seasonal cycle can be detected in all time series, but can only explain up to 10% of the variance.
Two freshening events can be identi_ed in April to June 2011 and in January 2012.
Both reach about half the magnitude as the ones detected in 1999 and 2004. For potential temperature no outstanding events were recognized. The DSOW potential temperature time series of the Denmark Strait and Ammassalik array show good correlation. Temperature signals are thus advected from the Denmark Strait to the Ammassalik array. Salinity signals at Ammassalik cannot be traced to the sill.
Hence salinity signals are not advected from the Denmark Strait. Entrainment of warm and salty Atlantic water and fresh East Greenland Current (EGC) water can explain the whole range of temperature and salinity changes in the DSOW. Changes in the EGC can hence strongly inuence the salinity variability, as was suggested in earlier studies. Salinity minima can therefore result from higher entrainment rates of EGC water.
8) Andrea Diana Tegzes, PhD Thesis to be defended Jan/Feb 2013: “The main branch of the Atlantic Inflow into the Nordic Seas over the Holocene: an assessment of its long-term natural variability and its potential role in initiating the ‘8.2ka event’ and the Little Ice Age”
9) Nummelin, A. (2012), Convection in the Greenland Sea – Model and Observations –. Master Thesis, Department of Physics, University of Helsinki, Finland, 83 pp.
10) Katrin Latarius (2013), "Über die Wassermassentransformation im Europäischen Nordmeer, Prozess-Studien und Budgets", PhD thesis at the University of Hamburg