Amplitude, mechanism, and dynamical significance of salinity variability in the Atlantic and Nordic Seas, analyzed from satellite data and ocean syntheses

by Armin Köhl and Detlef Stammer

Assimilation 

Variations in salinity and how they relate to globally interconnected changing transports, local or non-local surface forcing or mixing processes in the ocean can be investigated by combining all available ocean observations utilizing an adjoint model to obtain a dynamically consistent estimate of the changing ocean circulation and its transport properties. GECCO2 Ocean Synthesis has been completed and has recently been extended until end of 2014. We have extended the assimilated data to include surface salinity from the SMOS satellite mission, which has been reprocessed and analyzed in terms of a quality assessment in the North Atlantic and is available via ICDC. Due to the large biases, limitations of the SMOS salinity became obvious when assimilated (Köhl et al. 2014). Enhancement of the correspondence of the estimated freshwater fluxes to independent satellite based estimates from HOAPS is visible when compared to the recently released HOAPS 3.3 (Fig.1)

Circulations driven by volume fluxes associated with freshwater fluxes Using the configuration of the GECCO2 set up, the dynamical impact of surface volume flux forcing on the global oceans circulation was investigated for the period 1949-2011. We studied the impact of the volume flux on the large scale circulation by comparing a respective model solution to the standard virtual salt flux version (VSF).

Time scales Space-time variability of SSS in the Atlantic Ocean is analyzed using near surface salinity observations from the period 1980-2013 jointly with the output from an eddy-resolving numerical ocean simulation. Results (Sena Martins et al., 2015) show a good agreement between in situ and model results in terms of spatial and temporal mean SSS patterns, geographically-varying SSS variability and spatial and temporal scales of SSS variability.

SMOS and Aquarius Data Cal/Val activities for testing and improving SMOS and Aquarius salinity data. Fig. 2 shows the RMS salinity differences to in situ values as it results from one year worth of SMOS salinity retrievals after applying various corrections and filters. Space-borne SSS measurements provided by the ESA (SMOS) and the NASA "Aquarius/SAC-D" missions, covering the period from May 2012 to April 2013, are compared against in situ salinity measurements obtained in the northern North Atlantic between 20°N and 80°N. Over cold water in the northern North Atlantic, SMOS BEC L4 SSS fields show a temperature-dependent negative SSS bias of up to -2 for temperatures < 5°C.


Goals for the second phase of this project are to analyze the amplitude, mechanism, and dynamical significance of salinity variability in the Atlantic and Nordic Seas, analyzed from satellite data, ocean syntheses and climate models.

  • Continue to improve the quality of SMOS and Aquarius surface salinity data and improve the estimation of the respective error information for analysis and assimilation.
  • Improvement of the GECCO data assimilation system in its ability to represent salinity and fresh water changes.
  • Providing the best possible description of freshwater content changes and the underlying processes in the Atlantic Ocean.
  • Comparisons of estimated freshwater fluxes from the synthesis with results from HOAPS
  • Analysis of salinity changes in the Atlantic and Nordic Seas, based on satellite data, ocean syntheses. Test of the GECCO results, but also interpretation of the observational evidences.
  • Close freshwater budget of the Atlantic Ocean in different regions of the Atlantic. Specific phenomenological studies in selected regions in the tropical and subtropical Atlantic and the impact of mixing in cooperation with WP 2.2.
  • Dynamics and kinematics involved in Atlantic salinity changes. Identification of causes, mechanism and dynamical significance of those changes in surface and subsurface salinity
  • Comparisons with GECCO2 estimation on changes in the tropical Atlantic with observational results obtained in WP 1.3.