Über die Wassermassentransformation im Europäischen Nordmeer: Prozess-Studien und Budgets.

Latarius, Katrin (2013) Über die Wassermassentransformation im Europäischen Nordmeer: Prozess-Studien und Budgets. (PhD/ Doctoral thesis), Universität Hamburg, Hamburg, Germany, 170 pp.

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Within the Arctic Mediterranean the inflowing warm and saline Atlantic Waters are transformed into cold and relatively fresh, dense overflow waters, which flow back into the North Atlantic at depth. The major part of the transformation takes place in the ice-free areas of the Nordic Seas. This investigation examines the contribution of the deep basins in the Nordic Seas to the overall transformation.

Since 2001 hydrographic measurements with profiling floats were conducted in the Nordic Seas. This data set provides insight into the temporal development of the hydrography in the four deep basins (Norwegian Basin, Lofoten Basin, Greenland Sea and Icelandic Plateau). In the study on hand the data are used to describe the variability of the hydrography on seasonal and inter-annual to decadal time scales. In combination with sea surface flux data heat and freshwater budgets for the basins are established. Based on the budgets the relative importance of the deep basins for the water mass transformation is estimated.

Time series of temperature and salinity for the basins are dominated by the seasonal signal in the upper 500 to 750 m. Amplitudes are up to 2.5°C. A harmonic analysis is used to separate it from the signals on longer time scales. The detailed knowledge of the seasonal signal can be used for the analysis of longer-term variability on the basis of historical data. Without a seasonal correction there is a risk of misinterpretation due to aliasing, because data were mainly taken in the summer month.

With a maximum total length of the time series of 11 years the separation of inter-annual and decadal variability is mathematically not possible. However the temperature and salinity development in the Greenland Sea and the Norwegian Basin is interpreted in relation to the propagation of decadal to multi-decadal anomalies in the properties of the inflowing Atlantic water around the Nordic Seas, which was analyzed in other studies. Furthermore the time series from the Greenland Sea demonstrate the decay of the intermediate temperature maximum in the time span of float-observations and the inter-annual variability of the near-surface (0-50m) freshwater content, which in turn influences the convection depth in the following winter.

Heat and freshwater budgets give insight into the interplay of lateral exchange between the basins and the boundary currents, the sea-surface heat and freshwater fluxes and the development of heat and freshwater content inside the basins. The development inside the basins is derived from the float-observations. Surface fluxes are taken from a corrected NCEP data set. The data set was selected in a detailed examination as the most realistic one. The lateral exchange is given by the residuum of the two other components and is identified with the contribution of the basins to the total transformation.

The most striking result is that more heat is imported laterally into the basins below 50 m than the basins release back to the atmosphere, because they additionally release heat laterally in the near-surface layer. Furthermore the basins gain more freshwater laterally in the near-surface layer than from the atmosphere. That means the water mass transformation is underestimated when taking only surface fluxes into account.

The four basins contribute 17% to the total water mass transformation of the Arctic Mediterranean in terms of heat and 7% in terms of freshwater although they account for only 4% of the total area of the Arctic Mediterranean. Within the Nordic Seas the contribution from the eastern basins is most important. Increasing input of freshwater, caused by climate change, will foremost influence the transformation in the west. Therefore a future decrease, but no break-down, of the transformation seems realistic, which may result in a slow-down of the Atlantic Meridional Overturning Circulation.

In relation to examinations of the transformation in the whole Nordic Seas it becomes obvious, that the deep basins are particularly important. Although they account for only one fifth of the total area they contribute 40% to the total transformation in the Nordic Seas.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Quadfasel, Detlef and Backhaus, Jan
Keywords: POSEIDON; POS437; Nordic Seas; water mass transformation; hydrography; heat and freshwater budgets; seasonal to decadal variability
Open Access Journal?: Yes
Date Deposited: 01 Oct 2015 10:41
Last Modified: 01 Oct 2015 10:41
URI: http://oceanrep.geomar.de/id/eprint/29865

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