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: 14.09.2021 – 17.09.2021 Kiel (Germany) – Kiel (Germany) MNF-Pher-110 The main purpose of the ALKOR cruise AL564 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 AL564 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. During AL564 the students spend two nights at sea (September 14th to 16th 2021) and also visited the Boknis Eck time series site (September 17th 2021) where the students have been introduced in testing of equipments and yoyo CTD profiling.

Description: The Atlantic Subtropical Cells (STCs) are shallow wind-driven overturning circulations that consist of poleward Ekman transport from the tropics to the subtropics in the surface layer, subduction in the subtropics and equatorward geostrophic flow at thermocline level. They are eventually closed by equatorial and eastern boundary upwelling. To date, the Atlantic STCs have mainly been investigated in general circulation or data assimilation models while the only observational study has been conducted before the Argo float era. In this thesis, for the first time, the mean state of the horizontal branches of the Atlantic STCs is characterized along 10°N and 10°S based on 13 years of hydrographic data from Argo floats. The interface depth between the surface layer and the thermocline layer is defined by the seasonally varying depth at which the meridional velocity reverses sign from poleward to equatorward (30 - 70m). The lower boundary of the thermocline layer is characterized by another flow reversal at the 26.0 kg m-3 isopycnal. Within the thermocline layer, a mean equatorward transport of about 9 Sv (5 Sv at the western boundary west of 32°W, 4 Sv in the interior) along 10°S is observed. Here, the western boundary transport at 10°S is derived from ship section data because of a transport underestimation by Argo float data. In contrast, only about 3 Sv of equatorward transport are entering the equatorial region at thermocline level along 10°N of which the majority is concentrated at the western boundary west of 50°W. This asymmetric thermocline layer transport convergence is largely balanced by a rather symmetric net transport divergence in the surface layer, where the Ekman divergence exceeds the geostrophic convergence. Overall, the resulting residual of about 3 Sv is attributed to western boundary transport uncertainty at 10°N and diapycnal transport equatorward of 10°N/S associated with the northward return flow of the Atlantic Meridional Overturning Circulation that partly upwells in the tropics. With this newly available Argo float data set, another part of this thesis focuses on the observed transport variability of the individual horizontal branches of the Atlantic STCs with an emphasis on their connection to tropical sea surface temperature (SST) anomalies. For the first time, surface layer and thermocline layer transport time series associated with the Atlantic STCs are derived from Argo float observations. Both layers are dominated by seasonal variability. The thermocline layer convergence varies from 7-14 Sv whereas the surface layer divergence varies from 0 to more than 20 Sv between boreal summer and winter, respectively. Although interannual transport fluctuations at thermocline level are relatively weak, they are suggested to modulate equatorial SST anomalies at time scales of ~5 years and longer. Remarkably, at these time scales, only positive anomalies of the interior part of the thermocline layer transport convergence are leading negative equatorial SST anomalies. At shorter time scales, both SST and interior thermocline layer transport convergence anomalies are rather forced in parallel by local changes of zonal wind stress. Moreover, at thermocline level, the western boundary transport component is anti-correlated with interior transport anomalies on all time scales and in both hemispheres. Due to the small time lags between both transport components, it is suggested that locally forced westward propagating Rossby waves cause the anti-correlation. However, Rossby waves that are forced poleward of the zonal sections at 10°N/S and reach the western boundary to propagate equatorward are expected to distort the initial time lag due to the locally forced Rossby waves. Overall, the results of this thesis provide a new detailed description of the different characteristics of the Atlantic STCs in terms of both their mean state and temporal variability while shedding light on processes that so far have only been observed in the Pacific Ocean or in model studies. It further proves the relevance and capability of Argo float observations within the scope of the Atlantic STC circulation while also addressing their limitations. Eventually, the results aim to provide a benchmark against which general circulation models should be validated.

Description: Key Points: • Observational transport time series of the Atlantic Subtropical Cells reveals dominant seasonal variability for horizontal branches • On time scales longer than ~5 years, interior thermocline layer transport convergence modulates equatorial sea surface temperature anomalies • Western boundary current and interior transport anomalies are partly compensating each other at thermocline level on all time scales The shallow meridional overturning cells of the Atlantic Ocean, the subtropical cells (STCs), consist of poleward Ekman transport at the surface, subduction in the subtropics, equatorward flow at thermocline level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational estimate of transport variability associated with the horizontal branches of the Atlantic STCs in both hemispheres based on Argo float data and supplemented by reanalysis products. Thermocline layer transport convergence and surface layer transport divergence between 10°N and 10°S are dominated by seasonal variability. Meridional thermocline layer transport anomalies at the western boundary and in the interior basin are anti‐correlated and partially compensate each other at all resolved time scales. It is suggested that the seesaw‐like relation is forced by the large‐scale off‐equatorial wind stress changes through low‐baroclinic‐mode Rossby wave adjustment. We further show that anomalies of the thermocline layer interior transport convergence modulate sea surface temperature (SST) variability in the upwelling regions along the equator and at the eastern boundary at time scales longer than 5 years. Phases of weaker (stronger) interior transport are associated with phases of higher (lower) equatorial SST. At these time scales, STC transport variability is forced by off‐equatorial wind stress changes, especially by those in the southern hemisphere. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by local changes of zonal wind stress.

Description: In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐ a ) was observed during the Atlantic Niño, due to pronounced meridional Chl‐ a gradients that partly compensated for the weak TIWs. Plain Language Summary Every few years the eastern equatorial Atlantic Ocean is significantly warmer than usual during boreal summer. Such warm events are referred to as Atlantic Niño events, and share similarities with El Niño events in the Pacific. In 2021, the strongest Atlantic Niño in at least four decades was observed in the equatorial Atlantic. This study is the first that investigates the complex interaction between Atlantic Niño, tropical Atlantic upper‐ocean currents, and equatorial waves based on various observational data sets. We show that the developing 2021 Atlantic Niño weakened both the background flow and the variability of near‐surface currents in May, which in turn largely reduced the strength of intraseasonal (20–50 days) waves that are usually generated by instability of the upper‐ocean zonal currents. As a consequence, the cooling effect that these waves usually have north of the equator and the warming effect along the equator vanished from May to July 2021. Interestingly, variability of chlorophyll concentration was enhanced, suggesting that enhanced meridional chlorophyll gradients compensated for reduced wave activity. Key Points The developing 2021 Atlantic Niño led to weaker equatorial surface currents and reduced vertical shear of upper‐ocean horizontal velocity Strong reduction of the surface flow, eddy flux, and meridional temperature gradient in May caused extremely weak and delayed tropical instability wave (TIW) season Reduced meridional TIW advection contributed to sharpen the north equatorial Chl‐ a front resulting in modest intraseasonal Chl‐ a variability

Description: Based on velocity data from a long-term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper-ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind-driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south-easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases. Key Points: - Composites of Tropical Instability Waves at 0°N, 23°W show a surface (subsurface) velocity maximum during northward (southward) phases - Meridional wind stress forces a seasonally-varying, shallow cross-equatorial overturning cell-the Equatorial Roll - The superposition of Tropical Instability Waves and Equatorial Roll causes asymmetric mixing during north- and southward phases

Description: The Atlantic Subtropical Cells (STCs) consist of poleward Ekman transport in the surface layer, subduction in the subtropics, and equatorward transport in the thermocline layer that largely compensates the surface Ekman divergence and closes the STCs via equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients exchange. However, their representation in state-of-the-art coupled models has not been systematically evaluated. In this study, we investigate the performance of the Coupled Model Intercomparison Project Phase 6 climate models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, that is, the meridional Ekman and geostrophic flow across 10°N and 10°S, and the Equatorial Undercurrent (EUC) at 23°W. We find that the model ensemble reveals biases toward weak Southern Hemisphere Ekman transport and interior geostrophic transports, as well as a weak EUC. We then investigate the large inter-model spread of these key components and find that models with strong Ekman divergence between 10°N and 10°S tend to have strong mixed layer and thermocline interior convergence and strong EUC. The inter-model spread of the EUC strength is primarily associated with the intensity of the southeasterly trade winds in the models. Since the trade-wind-induced poleward Ekman transports are regarded as the drivers of the STCs, our results highlight the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing.