ALBERT

Description: 07.08.2020 – 10-08.2020 Kiel (Germany) – Kiel (Germany) MNF-Pher-110 The main purpose of the ALKOR cruise AL541 was the training of students in observational techniques used in physical oceanography. The students who participated in the trip attend the module "Measurement Methods of Oceanography" which is offered in the Bachelor program "Physics of the Earth System" at CAU Kiel. During the AL541 the students were instructed in instrument calibration and in the interpretation of measurement data at sea. In addition, the students had the opportunity to learn about working and living at sea and to explore and study the impact of physical processes on the western Baltic Sea, the sea at their doorstep. In addition, the students had the opportunity to learn about working and living at sea and to explore and investigate the effects of physical processes in the western Baltic Sea, the sea on their doorstep. Due to the COVID situation, only day trips could be made, to the Fehmarn Belt and to the time series station Boknis Eck. In the Fehmarn Belt, two sections were made (on 07 & 10.08.) and the tripod mooring could be recovered and deployed again. Measurements were taken at the Boknis Eck time series station and a section was acquired in the deep channel west of Kiel Bay.

Description: The warming climate is causing an increasing ocean stratification as a consequence of intensified ocean surface warming. Changes in ocean stratification affect e.g., ocean ventilation, mixed layer entrainment, heat exchange and air-sea interactions. In turn, these changes play a role for carbon uptake, oxygen distribution and nutrient fluxes that are crucial for marine ecosystems. How exactly these physical and biological processes will be affected remains uncertain and the effects of stratification changes probably vary among regions. Taking advantage of the excessive Argo observation dataset from 2006-2022, this thesis examines the decadal variability and recent changes of the upper-ocean stratification and surface mixed layer for the period of the most intense warming to-date. In particular, the southeastern tropical Atlantic Ocean (10°S-20°S, 5°W-15°E) shows continuous warming and freshening of the mixed layer for the period of 2006-2020. The resulting surface density reduction affects the upper-ocean stratification that strengthened by 30%. A procedure is developed to determine the vertical stratification maximum from the Argo array on global scales. Globally, the summer and winter vertical stratification maxima intensify by 7-8% from 2006-2021 while the mixed layer deepens by 4 m, proposing a de-coupling of the upper-ocean from the ocean interior. Changes in the upper-ocean structure impact the potential vorticity (PV) of the water column. This thesis establishes a relation between the decadal variability of North Atlantic Oscillation (NAO) and a zone of high PV in the northeastern tropical Atlantic, which plays a role for the subtropical-tropical pathways. During positive NAO phases, intensified trade winds result in isopycnal heave and squeezing of density layers that strengthen the PV. Analyses of the Sverdrup streamfunction and geostrophic velocities indicate enhanced thermocline flow via the western boundary toward the equator.

Description: In this paper, we review observational and modelling results on the upwelling in the tropical Atlantic between 10∘ N and 20∘ S. We focus on the physical processes that drive the seasonal variability of surface cooling and the upward nutrient flux required to explain the seasonality of biological productivity. We separately consider the equatorial upwelling system, the coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. All three tropical Atlantic upwelling systems have in common a strong seasonal cycle, with peak biological productivity during boreal summer. However, the physical processes driving the upwelling vary between the three systems. For the equatorial regime, we discuss the wind forcing of upwelling velocity and turbulent mixing, as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is located along its northern boundary and is driven by both local and remote forcing. Particular emphasis is placed on the Guinea Current, its separation from the coast and the shape of the coastline. For the tropical Angolan upwelling, we show that this system is not driven by local winds but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios.

Description: A warming and freshening trend of the mixed layer in the upper southeastern tropical Atlantic Ocean (SETA) is observed by the Argo float array during the time period of 2006–2020. The associated ocean surface density reduction impacts upper-ocean stratification that intensified by more than 30% in the SETA region since 2006. The initial typical subtropical stratification with a surface salinity maximum is shifting to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. During the same period isopycnal surfaces in the upper 200 m are shoaling continuously. Observed wind stress changes reveal that open ocean wind curl-driven upwelling increased, however, partly counteracted by reduced coastal upwelling due to weakened alongshore southerly winds. Weakening southerly winds might be a reason why tropical surface waters spread more southward reaching further into the SETA region. The mixed layer warming and freshening and associated stratification changes might impact the marine ecosystem and pelagic fisheries in the Angolan and northern Namibian upwelling region.

Description: The warming climate is causing a strengthening of ocean stratification. Ocean stratification, in turn, has significant impacts on physical, biogeochemical and ecological processes, such as ocean circulation, ventilation, air-sea interactions, nutrient fluxes, primary productivity and fisheries. How these processes are affected in detail by changing stratification still remains uncertain and are likely to vary locally. Here, we investigate the state and trend of different parameters characterizing the stratification of the global upper-ocean which can be derived from Argo profiles for the period 2006-2021. Among those parameters are mixed layer depth, magnitude and depth of the vertical stratification maximum. The summertime stratification maximum has increased in both hemispheres, respectively. During wintertime, the stratification maximum has intensified in the Northern Hemisphere, while changes in the Southern Hemisphere have been relatively small. Comparisons to mixed layer characteristics show that a strengthening stratification is mainly accompanied by a warming and freshening of the mixed layer. In agreement with previous observational studies, we find a large-scale mixed layer deepening that regionally contributes to the increasing stratification. Globally, the vertical stratification maximum strengthens by 7-8% and the mixed layer deepens by 4 m during 2006-2021. This hints to an ongoing de-coupling of the surface ocean from the ocean interior. The investigated changes can help determine the origin of existing model-observation discrepancies and improve predictions on climate change impact on upper-ocean ecology and biogeochemistry.

Description: The southeastern tropical Atlantic hosts a coastal upwelling system characterized by high biological productivity. Three subregions can be distinguished based on differences in the physical climate: the tropical Angolan and the northern and southern Benguela upwelling systems (tAUS, nBUS, sBUS). The tAUS, which is remotely forced via equatorial and coastal trapped waves, can be characterized as a mixing-driven system, where the wind forcing plays only a secondary role. The nBUS and sBUS are both forced by alongshore winds and offshore cyclonic wind stress curl. While the nBUS is a permanent upwelling system, the sBUS is impacted by the seasonal cycle of alongshore winds. Interannual variability in the region is dominated by Benguela Niños and Niñas that are warm and cold events observed every few years in the tAUS and nBUS. Decadal and multidecadal variations are reported for sea surface temperature and salinity, stratification and subsurface oxygen. Future climate warming is likely associated with a southward shift of the South Atlantic wind system. While the mixing-driven tAUS will most likely be affected by warming and increasing stratification, the nBUS and sBUS will be mostly affected by wind changes with increasing winds in the sBUS and weakening winds in the northern nBUS.

Description: In the northeastern tropical Atlantic, a region of high potential vorticity (PV) determines the size of the exchange window for the interior thermocline flow of the subtropical cell via its variations in strength and extent. Variability of this PV barrier has the potential to impact the ventilation of the tropical Atlantic on decadal timescales. Here, the impact of the North Atlantic Oscillation (NAO) on the PV barrier related to isopycnals within the thermocline of the subtropical-tropical Atlantic Ocean is assessed from Argo observations for the time period of 2006-2022. Relative to the negative NAO phase (2009-2010), during the positive NAO phase (2014-2019), the North Atlantic subtropical high and the northeast trades are intensified. Satellite-derived wind stress curl shows increased upwelling/downwelling on the equatorward/poleward side of the trade wind zone, respectively. In the subtropical-tropical Atlantic, a symmetric pattern of isopycnal heave is observed: rising isopycnals within 20 degrees N and 20 degrees S and sinking poleward of that. With rising isopycnals, the PV barrier in the northeastern tropical Atlantic becomes stronger. Analyses of geostrophic velocities and the Sverdrup streamfunction show that during the positive NAO phase there are increased equatorward velocities at thermocline level along the western boundary and reduced velocities through the interior as a result of intensified northeast trades and therefore a strengthened PV barrier. Intensified trades lead to enhanced subduction of thermocline waters and, independent of that, to a strengthened Equatorial Undercurrent transport as observed at the mooring site at 0 degrees, 23 degrees W, likely via the pulling effect of the subtropical cells. In the North Atlantic Ocean, subducted water from the subtropics has two possible pathways within the thermocline toward the equatorial region: the interior pathway and the pathway along the western boundary. The size of the exchange window between subtropics and tropics depends on the extent of a barrier zone in the eastern part of the basin that is associated with wind-driven upwelling of density surfaces. The North Atlantic Oscillation (NAO) is the dominant atmospheric climate mode in the North Atlantic and in this study, we show how the NAO impacts the barrier for the equatorward thermocline flow in the tropical Atlantic Ocean. During positive NAO phases (e.g., 2014-2019), density surfaces become shallower and strengthen the barrier, while during negative NAO phases (e.g., 2009-2010) the barrier weakens. Geostrophic velocity analysis reveals that during positive NAO phases more thermocline water is transported equatorward via the western boundary and less via the interior pathway. Additionally, observations from a mooring site at 0 degrees, 23 degrees W show stronger Equatorial Undercurrent transport as a result of intensified trade winds during positive NAO phases. Trade winds in the northeastern tropical Atlantic strengthen during positive phases of the North Atlantic Oscillation (NAO+) Potential vorticity barrier for the interior equatorward thermocline flow of the North Atlantic Subtropical Cell strengthens during NAO+ Annual subduction of thermocline water and Equatorial Undercurrent transport increase simultaneously from 2008 to 2018