ALBERT

Description: Highlights: • Analysis of hydrographic and current observations (1989–2014) in the western equatorial Atlantic. • Lower NADW and lighter AABW form an interactive transition layer in the northern Brazil Basin. • Proof of long-term abyssal warming on isobars in the western tropical Atlantic. • Warming of densest AABW is mainly caused by descent of isopycnal surfaces and volume loss of dense water masses. • Changes on isopycnal surfaces show warming in the 1990s and cooling in the 2000s. Abstract: The flow of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) contributes to the Atlantic meridional overturning circulation. Changes in the associated water mass formation might impact the deep ocean's capacity to take up anthropogenic CO2 while a warming of the deep ocean significantly contributes to global sea level rise. Here we compile historic and recent shipboard measurements of hydrography and velocity to provide a comprehensive view of water mass distribution, pathways, along-path transformation and long-term temperature changes of NADW and AABW in the western South and Equatorial Atlantic. We confirm previous results which show that the northwest corner of the Brazil Basin represents a splitting point for the southward/northward flow of NADW/AABW. The available measurements sample water mass transformation along the two major routes for deep and bottom waters in the tropical to South Atlantic – along the deep western boundary and eastward, parallel to the equator - as well as the hot-spots of extensive mixing. We find lower NADW and lighter AABW to form a highly interactive transition layer in the northern Brazil Basin. The AABW north of 5 °S is relatively homogeneous with only lighter AABW being able to pass through the Equatorial Channel (EQCH) into the North Atlantic. Spanning a period of 26 years, our data also allow an estimation of long-term temperature trends in abyssal waters. We find a warming of 2.5 ± 0.7∙10−3 °C yr−1 of the waters in the northern Brazil Basin at temperatures colder than 0.6 °C throughout the period 1989–2014 and can relate this warming to a thinning of the dense AABW layer. Whereas isopycnal heave is the dominant effect which defines the vertical distribution of temperature trends on isobars, we also find temperature changes on isopycnals in the lower NADW and AABW layers. There temperatures on isopycnals exhibit decadal variations with warming in the 1990s and cooling in the 2000s - the contributions to the trends on isobars range from about 50% in the lighter AABW layers in the EQCH up to a maximum of 80% in the transition layer the lower NADW and lighter AABW form in the northern Brazil Basin.

Description: This study is based on hydrographic data and direct velocity measurements from all available ship sections conducted within the region 37°W-19°W/15°S-10°N, spanning the period 1989 - 2014 and going to full depth to cover abyssal waters.

Description: This data collection contains RAFOS float trajectories from the Northeast Atlantic, in and around the Iceland Basin, collected at depths of 1500 m and 2600 m. The data are from the time period 1997-2003 and were collected as part of the project "Sonderforschungsbereich 460 (SFB460)", funded by the German "Deutsche Forschungsgemeinschaft". The data are presented in two versions: one version contains the full time series with possible gaps in the trajectories whenever the RAFOS tracking system was unavailable (see "Download data"), and the other version has gaps removed/interpolated and has been subjected to a smoothing filter (see "Other version"). Further details are available in a report that is included with this data collection as well (see "Further details").