Drivers of freshwater distribution in the Arctic and Atlantic oceans

Oceanic circulation plays an important role in setting the climate of the Arctic and the Northern Atlantic regions. Currents conveying large volumes of water masses at various depths transport heat and salt to great distances, forming a global circulation system. In the Atlantic, the Meridional Overturning Circulation (MOC) is driven by exchanges of heat, freshwater and momentum with the atmosphere. Previous modeling studies suggest that the stability of the MOC is sensitive to different climate scenarios due to the sensitivity of the deepwater formation, a crucial component of the circulation to perturbations of freshwater content. Global climate models predict significant temperature rise in the future with larger trends at higher latitudes, and an enhanced hydrological cycle. Both of these trends act against the MOC, decreasing its strength by reducing meridional air temperature differences and freshening of ocean waters in key high latitude areas. The observed increase of the strength of the North Atlantic Oscillation (NAO) in recent decades, a trend that is predicted by many climate models to persist in the future, however, acts as a driver of the MOC. This duel of the evolution of fresh water fluxes and the development of the NAO is most likely going to define the strength of the MOC in the future. We examine the effects of different NAO scenarios using the Modini-system, a partially coupled spin-up that allows prescription of wind stresses for the ocean in the otherwise fully coupled Earth System Model of the Max Planck Institute. In our work we describe the processes affecting the circulation in more detail. We present our first results by investigating the role different wind stress patterns play in shaping fresh water reservoirs and exchanges between different subregions of the Arctic and the Atlantic Ocean

Freshwater variability in the Arctic Ocean and subarctic North Atlantic

In the past decades, observations in the upper Arctic Ocean and subpolar North Atlantic have shown signicant freshwater changes that were in each region mainly attributed to independent processes. Both regions are sensitive to changes in the density stratication with possible implications for the ocean/atmsophere heat exchange and the deep convection. Thus changes in the freshwater content of the Arctic Ocean and subpolar North Atlantic have the potential to impact the climate locally and globally. The objectives of the present study are to investigate the freshwater content (co)variability of the upper Arctic Ocean and subpolar North Atlantic, to identify the processes causing the observed changes in freshwater content and to analyse possible drivers of these processes. To investigate the freshwater content variability I used objectively mapped salinity fields for the subpolar North Atlantic and Nordic Seas, and objectively mapped liquid freshwater inventories and sea ice volume estimates from the Pan-Arctic Ice Ocean Modeling and Assimilation product for the upper Arctic Ocean. To explore possible links, I compared the liquid freshwater content of the subarctic North Atlantic (SANA; combination of subpolar North Atlantic and Nordic Seas) with the sum of liquid and solid freshwater content of the upper Arctic Ocean from the observational and assimilation products. I found a distinct anti-correlation of the freshwater anomalies in these two regions between 1992 and 2013 with anomalies of the same magnitude. An analysis of freshwater fluxes from the global Finite Element Sea ice Ocean Model and the Common Ocean-ice Reference Experiment version 2 atmospheric forcing data set suggested that the observed freshwater variations resulted from changing freshwater transports. Variations in the Arctic freshwater export to the North Atlantic are found to be most important for the total freshwater content variability of the upper Arctic Ocean and for the liquid freshwater content variability of the western SANA. The eastern SANA freshwater content seems to be mainly influenced by the exchange with the subtropical North Atlantic. Furthermore, this study reveals that the observed freshwater changes are correlated with the Arctic and North Atlantic Oscillation indices. Therefore I suggest that a changing freshwater export from the Arctic Ocean to the SANA responds to decadal alternations of the dominant large-scale atmospheric variability. Thereby the export through the Canadian Arctic Archipelago is associated to different patterns of the atmospheric and oceanic pressure and circulation than the export through the Fram Strait and Barents Sea Opening. I propose, that the recently observed rapid changes in the SANA and upper Arctic Ocean freshwater content resulted from an interplay of these different driving patterns causing parallel changes in the freshwater export on both sides of Greenland. According to the present phase of the decadal alternations of the atmospheric variability and the final years in my freshwater content time series, the fresh water accumulated in the Arctic Ocean during the previous decades started to be released into the SANA. This release might continue in the following years and could have the potential to impact the Atlantic Meridional Overturning Circulation and the oceanic heat release to the Arctic atmosphere and sea ice

Link between multidecadal freshwater anomalies in the Arctic Ocean and subpolar North Atlantic

A significant increase in liquid freshwater content has been observed in the Arctic Ocean over the last 20 years, whereas the Arctic sea ice volume shrank significantly. In contrast, the North Atlantic became more saline in recent years. Both regions are of great importance for the global ocean circulation and climate, and salinity changes may have a profound impact on the global climate. We found that for the period between 1992 and 2013, the liquid freshwater content of the subpolar North Atlantic, calculated from objectively mapped in-situ salinity measurements, and the total freshwater content of the Arctic Ocean, i.e. the liquid freshwater content and freshwater stored in sea ice, are significantly negative correlated (r=-0.77). Moreover, the amount of the anomalies are of the same size. Furthermore, the time series hint at multi-decadal oscillations. The highest negative correlation with the total freshwater content of the Arctic Ocean can be found in the Irminger and Labrador Seas, while we observed a positive correlation east of the Mid-Atlantic Ridge at the path of the North Atlantic Current, which is the source of Atlantic Water entering the Arctic Ocean through the Nordic Seas. We suggest a redistribution of freshwater as a response to frequent changes in atmospheric pressure patterns. Under certain conditions the freshwater is re-routed and kept in the Arctic Ocean, while it is released under other conditions. We conclude that decadal scale changes of the freshwater content in the North Atlantic, particularly those in the deep water formation sites like the Labrador Sea, are originating in the Arctic Ocean

Freshwater variability in the AO and SPNA: a Comparison from the 1990s to Present References

A significant increase in liquid freshwater content has been observed in the Arctic Ocean over the last 20 years, whereas the Arctic sea ice volume shrank significantly. In contrast, the North Atlantic became more saline in recent years. Both regions are of great importance for the global ocean circulation and climate, and salinity changes may have a profound impact on the global climate. We found that for the period between 1992 and 2013, the liquid freshwater content of the subpolar North Atlantic, calculated from objectively mapped in-situ salinity measurements, and the total freshwater content of the Arctic Ocean, i.e. the liquid freshwater content and freshwater stored in sea ice, are significantly negative correlated (r=-0.77). Moreover, the amount of the anomalies are of the same size. Furthermore, the time series hint at multi-decadal oscillations. The highest negative correlation with the total freshwater content of the Arctic Ocean can be found in the Irminger and Labrador Seas, while we observed a positive correlation east of the Mid-Atlantic Ridge at the path of the North Atlantic Current, which is the source of Atlantic Water entering the Arctic Ocean through the Nordic Seas. We suggest a redistribution of freshwater as a response to frequent changes in atmospheric pressure patterns. Under certain conditions the freshwater is re-routed and kept in the Arctic Ocean, while it is released under other conditions. We conclude that decadal scale changes of the freshwater content in the North Atlantic, particularly those in the deep water formation sites like the Labrador Sea, are originating in the Arctic Ocean

Süßwasserschwankungen im arktisch-nordatlantischen Raum

In the past decades, observations in the upper Arctic Ocean and subpolar North Atlantic have shown signicant freshwater changes that were in each region mainly attributed to independent processes. Both regions are sensitive to changes in the density stratication with possible implications for the ocean/atmsophere heat exchange and the deep convection. Thus changes in the freshwater content of the Arctic Ocean and subpolar North Atlantic have the potential to impact the climate locally and globally. The objectives of the present study are to investigate the freshwater content (co)variability of the upper Arctic Ocean and subpolar North Atlantic, to identify the processes causing the observed changes in freshwater content and to analyse possible drivers of these processes. To investigate the freshwater content variability I used objectively mapped salinity fields for the subpolar North Atlantic and Nordic Seas, and objectively mapped liquid freshwater inventories and sea ice volume estimates from the Pan-Arctic Ice Ocean Modeling and Assimilation product for the upper Arctic Ocean. To explore possible links, I compared the liquid freshwater content of the subarctic North Atlantic (SANA; combination of subpolar North Atlantic and Nordic Seas) with the sum of liquid and solid freshwater content of the upper Arctic Ocean from the observational and assimilation products. I found a distinct anti-correlation of the freshwater anomalies in these two regions between 1992 and 2013 with anomalies of the same magnitude. An analysis of freshwater fluxes from the global Finite Element Sea ice Ocean Model and the Common Ocean-ice Reference Experiment version 2 atmospheric forcing data set suggested that the observed freshwater variations resulted from changing freshwater transports. Variations in the Arctic freshwater export to the North Atlantic are found to be most important for the total freshwater content variability of the upper Arctic Ocean and for the liquid freshwater content variability of the western SANA. The eastern SANA freshwater content seems to be mainly influenced by the exchange with the subtropical North Atlantic. Furthermore, this study reveals that the observed freshwater changes are correlated with the Arctic and North Atlantic Oscillation indices. Therefore I suggest that a changing freshwater export from the Arctic Ocean to the SANA responds to decadal alternations of the dominant large-scale atmospheric variability. Thereby the export through the Canadian Arctic Archipelago is associated to different patterns of the atmospheric and oceanic pressure and circulation than the export through the Fram Strait and Barents Sea Opening. I propose, that the recently observed rapid changes in the SANA and upper Arctic Ocean freshwater content resulted from an interplay of these different driving patterns causing parallel changes in the freshwater export on both sides of Greenland. According to the present phase of the decadal alternations of the atmospheric variability and the final years in my freshwater content time series, the fresh water accumulated in the Arctic Ocean during the previous decades started to be released into the SANA. This release might continue in the following years and could have the potential to impact the Atlantic Meridional Overturning Circulation and the oceanic heat release to the Arctic atmosphere and sea ice

Advective pathways of nutrients and key ecological substances in the Arctic

The Arctic Ocean is undergoing remarkable environmental changes due to global warming. The rise in the Arctic near-surface air temperature during the past decades is twice as high as the global average, a phenomenon known as the “Arctic Amplification”. As a consequence the Arctic summer sea ice extent has decreased by more than 40 % in recent decades, and moreover a year-round sea ice loss in extent and thickness was recorded. By opening up of large areas formerly covered by sea ice, the exchange of heat, moisture and momentum between the ocean and atmosphere intensified. This resulted in changes in the ocean circulation and the water masses impacting the marine ecosystem. We investigate these changes by using a large set of hydrographic and biogeochemical data of the entire Arctic Ocean. To better quantify the current changes in the Arctic ecosystem we will combine our observational data analysis with model simulations using a very high resolution (1/12°) biogeochemical atmosphere-sea ice-ocean model from our partners at the National Oceanographic Center in the UK (Yevgeni Aksenov and Stefanie Rynders)