Periodic Reporting for period 1 - VERTEXSO (VERTical EXchange in the Southern Ocean)
Reporting period: 2023-01-01 to 2025-06-30
The Southern Ocean plays a unique role in the climate system and for the global society due to its ability to buffer global climatic variations, such as those induced by human activities. However, it can also drive global climatic variations, such as the ones observed during the ice ages. This critical role arises from its ocean circulation that returns century old deep water back to the surface where it exchanges heat and carbon with the atmosphere before the waters return to deeper layers. This so-called overturning circulation has been responsible for removing large amounts of excess heat and carbon dioxide emitted through human activities from the atmosphere and thereby slowing-down global surface warming substantially. Thus, without the Southern Ocean, the global surface climate would have warmed already much more than currently observed.
However, the exact pathways of how deep water comes to the surface and is modified to become new surface water is not yet understood. This lack of understanding is also reflected in global model simulations that struggle to reproduce the upper ocean water column characteristics of this region and its past changes. The main hypothesis of VERTEXSO is that convective vertical plumes induced by surface cooling and salinification during austral winter play an important role in mixing the deep with the surface waters and therefore enable the exchange of heat and carbon between these layers. In ocean models that are used for climate simulations such vertical exchange processes cannot be represented due to simplifications that are made to make them more efficient.
To resolve these issues VERTEXSO aims to develop a thorough understanding of open-ocean convective plumes in the Southern Ocean, including the associated temporal and spatial scales. The project aims to improve how they are represented in climate models and to assess their impact on vertical heat and carbon exchange in these models. We further aim to develop innovative methods to monitor them.
However, the exact pathways of how deep water comes to the surface and is modified to become new surface water is not yet understood. This lack of understanding is also reflected in global model simulations that struggle to reproduce the upper ocean water column characteristics of this region and its past changes. The main hypothesis of VERTEXSO is that convective vertical plumes induced by surface cooling and salinification during austral winter play an important role in mixing the deep with the surface waters and therefore enable the exchange of heat and carbon between these layers. In ocean models that are used for climate simulations such vertical exchange processes cannot be represented due to simplifications that are made to make them more efficient.
To resolve these issues VERTEXSO aims to develop a thorough understanding of open-ocean convective plumes in the Southern Ocean, including the associated temporal and spatial scales. The project aims to improve how they are represented in climate models and to assess their impact on vertical heat and carbon exchange in these models. We further aim to develop innovative methods to monitor them.
In the first part of the project, VERTEXSO investigated observational data from profiling floats that freely drift in the ocean and moorings that are attached to the sea floor. In particular, we investigated data from austral winter to find evidence for the existence of convective plumes in the sea-ice covered Southern Ocean. This analysis revealed a widespread occurrence of sporadic interactions between winter water that is relatively cold, fresh, and rich in dissolved oxygen and the upwelling deep water that is relatively warm, saline, and poor in dissolved oxygen. The largest variations in these properties in time and space, identified by the profiling floats, are found at the base of the winter water, which is marked by the upper limit of the so-called pycnocline, i.e. the main density gradient that separates the surface from the deep ocean. We find that this variability is associated with the upwelling and mixing of deep water into the surface layer, which mostly occurs in the seasonally ice-covered Southern Ocean. In analysis of the temporal evolution of this mixing, we find that this variability is largely induced by the ventilation of the upper pycnocline water during austral winter when the water column is only weakly stratified.
A more detailed analysis of the temporal evolution of the upper ocean water column throughout the winter is performed on a local mooring site in the Weddell Sea. This analysis reveals distinct vertical mixing events that reach the pycnocline in winter and presumably represent convective plumes in the water column. The plumes develop in late winter when the upper ocean salinity has substantially increased due to sea ice formation. They occur together with a decline in the sea ice cover and the direct linkages are still being explored. The plumes are marked by a sharp decrease in the subsurface temperature and salinity during periods of weak density stratification, when presumably cold and fresh waters are entrained into the upwelling deep water.
Another ongoing investigation attempts to reproduce these convective plumes observed on the mooring site with a non-hydrostatic ocean model. In contrast to hydrostatic ocean models, such as the ones used for global climate simulations, this model is able to directly resolve convective processes in the ocean. The model has been successfully set up with the initial conditions at the mooring site and a convective plume could be reproduced for an idealized surface forcing that shows the ventilation of the upwelling deep water and the associated small-scale circulation and mixing processes. Further work is needed to make those simulations more realistic and analyze the model output to better understand the underlying processes.
A more detailed analysis of the temporal evolution of the upper ocean water column throughout the winter is performed on a local mooring site in the Weddell Sea. This analysis reveals distinct vertical mixing events that reach the pycnocline in winter and presumably represent convective plumes in the water column. The plumes develop in late winter when the upper ocean salinity has substantially increased due to sea ice formation. They occur together with a decline in the sea ice cover and the direct linkages are still being explored. The plumes are marked by a sharp decrease in the subsurface temperature and salinity during periods of weak density stratification, when presumably cold and fresh waters are entrained into the upwelling deep water.
Another ongoing investigation attempts to reproduce these convective plumes observed on the mooring site with a non-hydrostatic ocean model. In contrast to hydrostatic ocean models, such as the ones used for global climate simulations, this model is able to directly resolve convective processes in the ocean. The model has been successfully set up with the initial conditions at the mooring site and a convective plume could be reproduced for an idealized surface forcing that shows the ventilation of the upwelling deep water and the associated small-scale circulation and mixing processes. Further work is needed to make those simulations more realistic and analyze the model output to better understand the underlying processes.
After two years of VERTEXSO, we find that convective plumes in the open ocean might be a wide-spread phenomenon that could effectively ventilate the upwelling deep water and could thereby strongly influence the exchange of heat and carbon between the surface and the deep ocean. Past work has indicated that convection only plays a role on the continental shelf or over an area that is called Maud Rise. Our work revises this view, by presenting new evidence for the occurrence of convection elsewhere away from the continental margins. However, this convection likely occurs sporadically and does not reach much deeper than the pycnocline.
Further efforts are required to better understand the influence of these plumes on the overall heat and carbon fluxes of the Southern Ocean and their potential impact on the climate. However, there are challenges involved, due to the lack of representation of these plumes in global climate models, which we will address in the second part of the project.
Further efforts are required to better understand the influence of these plumes on the overall heat and carbon fluxes of the Southern Ocean and their potential impact on the climate. However, there are challenges involved, due to the lack of representation of these plumes in global climate models, which we will address in the second part of the project.