Abstract. The Boknis Eck (BE) time series station, initiated in 1957, is one of the longest-operated time series stations worldwide. We present the first statistical evaluation of a data set of nine physical, chemical and biological parameters in the period of 1957-2013. In the past three to five decades, all of the measured parameters underwent significant long-term changes. Most striking is an ongoing decline in bottom water oxygen concentration, despite a significant decrease of nutrient and chlorophyll a concentrations. Temperature-enhanced oxygen consumption in the bottom water and a prolongation of the stratification period are discussed as possible reasons for the ongoing oxygen decline despite declining eutrophication. Observations at the BE station were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM). Reproduced trends were in good agreement with observed trends for temperature and oxygen, but generally the oxygen concentration at the bottom has been overestimated.

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 CO 2 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 Atlanticalong 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 longterm 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 2 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.

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11° S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The observed seasonal cycles of the AMOC, its components as well as the Western Boundary Current as observed with current meter moorings are in general good agreement with results of an ocean general circulation model. The seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, slightly stronger at the western boundary. In the model, seasonal AMOC variability at 11° S is governed by the variability in the eastern basin. Here, long Rossby waves originating from equatorial forcing are known to be radiated from the Angolan continental slope and propagate westward into the basin interior. The contribution of the western basin to AMOC seasonal variability is instead comparably weak as transport variability due to locally forced Rossby waves is mainly compensated by the Western Boundary Current. Our analyses indicate, that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes being not properly represented in the model, uncertainties in the wind forcing are particularly relevant for AMOC estimates at 11° S.

Abstract. Bottom pressure observations on both sides of the Atlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the Atlantic Meridional Overturning Circulation (AMOC) at 11∘ S. Over the period 2013–2018, the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The characteristics of the observed seasonal cycles of the AMOC and its components are compared to results from an ocean general circulation model, which is known to reproduce the variability of the Western Boundary Current on longer timescales. The observed seasonal variability of zonally integrated geostrophic velocity in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The model tends to underestimate the seasonal pressure variability at 300 and 500 m depth, especially at the western boundary, which translates into the estimate of the upper-ocean geostrophic transport. In the model, seasonal AMOC variability at 11∘ S is governed, besides the Ekman transport, by the geostrophic transport variability in the eastern basin. The geostrophic contribution of the western basin to the seasonal cycle of the AMOC is instead comparably weak, as transport variability in the western basin interior related to local wind curl forcing is mainly compensated by the Western Boundary Current. Our analyses indicate that while some of the uncertainties of our estimates result from the technical aspects of the observational strategy or processes not being properly represented in the model, uncertainties in the wind forcing are particularly relevant for the resulting uncertainties of AMOC estimates at 11∘ S.

Abstract. The Time Series Station Boknis Eck (BE), initiated in 1957, is one of the longest operated time series stations worldwide. We present the first statistical evaluation of a dataset of nine physical, chemical and biological parameters in the period of 1957–2013. In the past 3 to 5 decades, all of the measured parameters underwent significant long-term changes. Most striking is an ongoing decline in bottom water oxygen concentration, despite a significant decrease of nutrient and chlorophyll a concentrations. Temperature enhanced oxygen consumption in the bottom water and a prolongation of the stratification period are discussed as possible reasons for the ongoing oxygen decline despite declining eutrophication. Observations at the location BE were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM). Reproduced trends were in good agreement with observed trends for temperature and oxygen, but generally the oxygen concentration at the bottom was overestimated.