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WP 1.1: Role of the tropical Atlantic for climate variations in the Atlantic sector

Peter Brandt, GEOMAR Kiel, pbrandt(at)geomar.de
Marcus Dengler, GEOMAR Kiel, mdengler(at)geomar.de
Rebecca Hummels, GEOMAR Kiel, rhummels(at)geomar.de

Josefine Herrford, GEOMAR Kiel, jherrford(at)geomar.de
Moacyr Araujo, UFPE Recife, Brasilien, moa.ufpe(at)gmail.com
Doris Veleda, UFPE Recife, Brasilien, doris.veleda(at)ufpe.br
Jonathan Durgadoo, GEOMAR Kiel, jdurgadoo(at)geomar.de
The variability of the circulation in the tropical Atlantic is closely related to rainfall variability over Africa and South America and thereby is of important socio-economic impact. The western boundary current system serves as a chokepoint for the circulation, where the variability of both, the Atlantic Meridional Overturning Circulation (AMOC) as well as of the Subtropical Cell (STC) is imprinted.
During the first phase of RACE a mooring array was deployed within the western boundary current system off Brazil. A first analysis of the data sets retrieved so far showed that essential integral characteristics of the western boundary current such as e.g. strength and the water mass characteristics of the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) as well as their intraseasonal variability can be determined with the obtained data. In comparison with similar observations from 2000-2004 first results about the variability on longer timescales could be drawn.
The continuation of the current observations off Brazil will enable further insights into the variability of the boundary current system on longer timescales. Together with bottom pressure measurements (PIES) on both sides of the Atlantic, transport time series of the NBUC, DWBC and the AMOC at 11°S will be generated and analyzed along with other time series compiled within the RACE II community as well as other national and international projects. This project aims at better understanding the role of the tropical Atlantic Ocean for climate variability on interannual to multidecadal timescales in order to assess risks and dangers, validate numerical models and ultimately improve their skill in reliable climate predictions.
The scientific issues, which will be investigated in the framework of RACE II, include the analysis of the
  • transport variability of the NBUC, DWBC and the AMOC at 11°S on intraseasonal to decadal time scales
  • relation of transport variability of the western boundary current in the tropical South Atlantic (warm and cold water route) and the variability within the subpolar North Atlantic with respect to the meridional propagation of AMOC signals
  • spreading of water mass anomalies within the AMOC, which can e.g. originate from the variability in the inflow of saline water masses from the Indian into the Atlantic Ocean
  • relation between NBUC variability at 11°S and EUC variability at 23°W and its impact on the heat and freshwater balance of the mixed layer, the ocean-atmosphere interactions and rainfall variability over West Africa
velocity section along 11°S

Fig. 1 a) Average alongshore velocity at 11°S from 8 ship sections based on ADCP observations.

transport time series for the NBUC and the DWBC at 11°S

Transport time series based on moored observations for the NBUC (b) and the DWBC (c) for the period 2000-2004 and 2013-2014 (from Hummels et al., 2015).