Transports and variability-driving mechanisms in Flemish Pass at the western boundary of the subpolar North Atlantic (FLEPVAR)
Zusammenfassung der Projektergebnisse
The DFG-funded project FLEPVAR dealt with the importance of Flemish Pass at the western edge of the subpolar North Atlantic for the southward flow of Labrador Sea Water (LSW). This water mass contributes to the North Atlantic Deep Water (NADW) and is part of the climate-relevant, cold, and deep limb of the Atlantic Meridional Overturning Circulation (AMOC). LSW is carried out of its formation region, the Labrador Sea (located between Canada and Greenland), by the Deep Western Boundary Current (DWBC). The presence of Flemish Cap, a shallow topographic obstacle located downstream at 47°N, causes the DWBC to split into two separate branches. The major flow follows the continental slope south around Flemish Cap. Lighter and shallower NADW components like the LSW, however, also pass through Flemish Pass, a shallow underwater channel with a sill depth of about 1200 m located to the west of Flemish Cap. Knowledge on the strength of LSW transports in Flemish Pass as well as on processes governing the variability was so far limited but is of high interest for assessing probable changes in the strength of the AMOC and resulting impacts on the local climate. During FLEPVAR, characteristics of two modes of LSW (upper LSW and deep LSW) including volume transports, hydrographic variability and variability-generating mechanisms have been inferred for Flemish Pass and investigated on short-term to decadal time scales based on shipand mooring-based observations at 47°N as well as a high-resolution (1/12°) ocean model. Focus was on the upper LSW, as only minor parts of the deep LSW can pass through the shallow Flemish Pass. The average flow through Flemish Pass was quantified and found to be weaker compared to the DWBC pathway. It amounts to about 20% of the southward flow of upper LSW in the model and in the observations, showing that Flemish Pass as a significant transport pathway for LSW is not negligible. It rather avoids potential stirring and deflection into the interior North Atlantic. South of Flemish Cap the two current branches merge again and form a strong southwestward flow around the slopes of the southern Grand Bank. The model showed that the average flow is constrained to a narrow current of 150 km width here, and the transport is significantly less than at 53°N. Regions located to the northeast and southeast of Flemish Cap were identified as important for diverting volume transport away from the boundary into the interior Newfoundland Basin following interior pathways. Transport time series (observations: 2012-2014; model: 1948-2009) showed high short-term variability on time scales of days to weeks that was explained by the presence of topographic Rossby waves or coastal trapped waves. As the model also showed strong variability on seasonal to interannual time scales, further driving mechanisms of the LSW transport variability were analyzed in the model by investigating running correlations of the transport time series with possible forcing factors. Several causes were found to impact on variability observed in Flemish Pass: during the 1960s and 1970s, atmospheric forcing dominated the variance, whereas the position of the warm and saline North Atlantic Current strongly influenced the variability of upper LSW in Flemish Pass in the 1980s and 1990s. Hydrographic data derived from two decades of ship-based observations (1993-2013) was further complemented with Argo data. The resulting time series for the central Labrador Sea, the Flemish Pass, and the DWBC (both at 47°N) revealed a significant warming and salinification of upper LSW and almost identical trends at all three locations. This suggest that changes in the formation region, which are advected downstream, dominate the long-term hydrographic trends of upper LSW observed at 47°N between 1993 and 2013. The numerical model simulations for the central Labrador Sea and Flemish Pass provided evidence that these trends are part of a multi-decadal cycle.
Projektbezogene Publikationen (Auswahl)
- Export of Labrador Sea Water through Flemish Pass, 3rd Young Scientists Conference: Interdisciplinary Approaches to Global Change, Kiel, Germany, 01./02.10.2012
Schneider, L., D. Kieke, K. Jochumsen, M. Rhein, D. Quadfasel, E. Varotsou, N. Serra, E. Colbourne, and I. Yashayaev
- Transport variability of Labrador Sea Water through Flemish Pass in relation to changes at 53°N, IAHS-IAPSO-IASPEI Joint Assembly 2013, Gothenburg, Sweden, 07/2013
Varotsou, E., K. Jochumsen, N. Serra, D. Kieke, and L. Schneider
- Variability of Labrador Sea Water exported through Flemish Pass, IAHS-IAPSO- IASPEI Joint Assembly 2013, Gothenburg, Sweden, 26.07.2013
Schneider, L., D. Kieke, K. Jochumsen, M. Rhein, E. Varotsou, N. Serra, I. Yashayaev, and E. Colbourne
- The Bremen NOAC observation system in the subpolar North Atlantic, US AMOC Meeting 2014, Seattle, USA, 09.09.2014
Kieke, D., M. Rhein, A. Roessler, C. Mertens, R. Steinfeldt and L. Schneider
- Transport variability of Labrador Sea Water through Flemish Pass in relation to changes at 53°N, Ocean Sciences Meeting 2014, Honolulu, USA, 26.02.2014
Varotsou, E., K. Jochumsen, N. Serra, D. Kieke, and L. Schneider
- Variability of Labrador Sea Water exported through Flemish Pass, Ocean Sciences Meeting 2014, Honolulu, USA, 27.02.2014
Schneider, L., D. Kieke, K. Jochumsen, M. Rhein, I. Yashayaev, and E. Varotsou
- (2015), Interannual transport variability of Upper Labrador Sea Water at Flemish Cap, J. Geophys. Res., 120, 5074-5089
Varotsou, E., K. Jochumsen, N. Serra, D. Kieke, and L. Schneider
(Siehe online unter https://doi.org/10.1002/2015JC010705) - (2015), Variability of Labrador Sea Water transported through Flemish Pass during 1993 - 2013, J. Geophys. Res., 120, 5514-5533
Schneider, L., D. Kieke, K. Jochumsen, E. Colbourne, I. Yashayaev, R. Steinfeldt, E. Varotsou, N. Serra, and M. Rhein
(Siehe online unter https://doi.org/10.1002/2015JC010939) - (2016), Transport variability-driving mechanisms in Flemish Pass at the western boundary of the Subpolar North Atlantic, Dissertation, University of Hamburg
Varotsou, E.
