Causes for Atlantic Freshwater Content Variability in the GECCO3 Ocean Synthesis

Regional freshwater content (FWC) changes are studied over the period 1961–2018 using the GECCO3 ocean synthesis. In four dynamically distinct regions of the Atlantic, the study identifies causes for FWC variability with a focus on interannual and decadal time‐scale changes. Results show that in each region, it is a combination of the surface freshwater flux and the net freshwater transport across the region's boundaries that act jointly in changing the respective FWC. Surface flux mainly contributes to the FWC variability on multi‐decadal time scales. The impact of surface flux also increases toward the tropics. On shorter time scales, it is especially horizontal transport fluctuations, leading to FWC changes in mid and high latitudes. Going from north to the south, the transport across a single meridional boundary becomes less correlated with the FWC changes but the net transport across both boundaries plays an increasingly important role. Moreover, the subpolar box is mainly gyre driven, which differs from the other two, essentially overturning driven, North Atlantic boxes. In the tropical Atlantic, the shallow overturning cell and the deep overturning contribute about equal amounts to the freshwater variations.

Plain Language Summary: Causes for freshwater content (FWC) variability in the Atlantic Ocean are analyzed for four study areas over the period 1961–2018 based on a model simulation (GECCO3 ocean synthesis). Targeting relatively long time scales, interannual, decadal to multi‐decadal FWC changes are separated into the contributions from variations of the freshwater input/output through the ocean surface and from freshwater transport (FWT) variations related to the ocean circulation changes. Surface freshwater flux is more influential on multi‐decadal time scales, and its impact increases toward the tropics. On shorter time scales, the oceanic FWT across the boundaries of the region dominates the FWC changes in mid and high latitudes. The transport variability in the subpolar region is mainly driven by the horizontal circulation, while transports resulting from vertical salinity differences are more important at lower latitudes. Moreover, in the tropics transports related to shallow salinity differences are not negligible on interannual time scales.

Key Points: The net freshwater transport across the meridional boundaries dominates the freshwater content variations in mid and high latitudes. The importance of surface freshwater flux variations increases toward the tropics and on multi‐decadal time scales. Subpolar changes are mainly gyre driven, while overturning and especially the shallow overturning cells contribute more at lower latitudes.

Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

https://icdc.cen.uni-hamburg.de/en/gecco3.html

http://www.metoffice.gov.uk/hadobs/en4/download-en4-2-2.html

https://www.cen.uni-hamburg.de/en/icdc/data/atmosphere/hoaps.html