Millennial-scale versus long-term dynamics in the surface and subsurface of the western North Atlantic Subtropical Gyre during Marine Isotope Stage 5


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a.bahr [ at ] em.uni-frankfurt.de

Abstract

Subtropical Gyres are an important constituent of the ocean–atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~ 140 and 70 ka, Marine Isotope Stage (MIS) 5. Using (isotope) geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean. Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress. During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of this warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events.



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Scopus/ISI peer-reviewed
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Published
Eprint ID
34350
DOI 10.1016/j.gloplacha.2013.08.013

Cite as
Bahr, A. , Nürnberg, D. , Karas, C. and Gruetzner, J. (2013): Millennial-scale versus long-term dynamics in the surface and subsurface of the western North Atlantic Subtropical Gyre during Marine Isotope Stage 5 , Global and Planetary Change, 111 , pp. 77-87 . doi: 10.1016/j.gloplacha.2013.08.013


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