Abstract's details

Toward a probabilistic assessment of the global ocean response to fully-varying river runoffs.

Thierry Penduff (CNRS - IGE, France)


Stephanie Leroux (DATLAS, France); William Llovel (CNRS - LOPS, France); Jean-Marc Molines (CNRS - IGE, France); Jerome Vialard (IRD - LOCEAN, France); Matthieu Lengaigne (IRD - MARBEC, France); Soumaia Tajouri (LOPS, France); Florian Sevellec (CNRS - LOPS, France); Julien Jouanno (IRD - LEGOS, France); Fabrice Papa (IRD - LEGOS, France); Clement de Boyer Montegut (IFREMER - LOPS, France)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Science II: Large Scale Ocean Circulation Variability and Change

Presentation type: Type Oral

The main goal of the IMHOTEP OST-ST project is to investigate the response of the global ocean to fully-varying freshwater discharges from rivers and from the Greenland ice sheet between 1980 and 2018. This response concerns sea surface salinity (SSS), regional sea surface height (SSH) and its manometric/steric components, heat and freshwater contents, water mass properties, regional to basin-scale circulation, etc. We are currently analyzing satellite/in-situ observations and the outputs from a recent series of NEMO-based global ocean/sea-ice/iceberg 1/4° simulations where runoffs are individually set to climatological and fully-variable in various source regions.

The JRA55 fully-variable atmospheric forcing is applied in all simulations without any SSS relaxation, which is replaced by a precipitation correction term that lets salinity free to evolve. Daily river discharge forcing data were derived from the ISBA-CTRIP hydrological reanalysis, and monthly (solid and liquid) freshwater discharges from Greenland are derived from satellite altimetry data (SAR mode) combined with regional climate simulations (Mouginot et al., 2019). Special attention was dedicated to the bathymetry and the partitionning between solid and liquid discharges all around Greenland, in order to realistically feed icebergs and freshwater plumes.

This presentation is focused on the interannual variability and 40-year trends of SSS in response to fluctuating runoffs at global scale, based on 5 simulations and a comparison to an ESA L4 product merging SMOS, Aquarius and SMAP SSS data (Boutin et al., 2021). Results show that the simulated SSS interannual variability compares well with observations. River runoffs directly force interannual SSS variability which can reach 0.5 to 0.75 g/kg in specific regions (Amazon plume, Siberian coast, Maritime Continent, China river mouths, etc) and about 0.2 g/kg along many coasts, with substantial penetration into the open ocean in several regions. Substantial (positive or negative) 40-year trends in the runoffs of key rivers also have a direct impact on regional SSS trends, which exhibit substantial freshening trends off Siberian coasts and throughout the Maritime Continent, and salinization off the equatorial African west coast in particular.

These first results also suggest that SSS interannual variability is primarily driven by atmospheric and runoff fluctuations; this is quite different for the interannual variability of SSH and of many other oceanic climate indices, which are strongly impacted by chaotic intrinsic variability (CIV) in addition to the former two external drivers. Assessing the interannual response of these latter oceanic fields to fluctuating runoffs thus require an attenuation of this low-frequency CIV, which in turn requires ensemble ocean simulations with slightly different initial states (as done during the OCCIPUT project, https://meom-group.github.io/projects/occiput/). The IMHOTEP project team will perform in 2022 two 10-member ensembles of global ocean/sea-ice 1/4° simulations driven by the same JRA55 forcing, one ensemble with climatological and one without interannually-varying runoffs: comparing the fluctuations of ensemble means of key variables will allow a more complete analysis of the multivariate and multi-scale response of the ocean to fluctuating runoffs, regardless of CIV. These analyses will provide direct insight for the interpretation of signals observed by satellite and in situ platforms, and their attribution to distinct drivers.

Oral presentation show times:

Room Start Date End Date
Sala Grande Wed, Nov 02 2022,15:15 Wed, Nov 02 2022,15:30
Thierry Penduff