Impacts of freshwater discharge interannual variability on Ocean heaT-salt contents and rEgional sea level change over the altimetry Period

Author:

William Llovel - (LOPS)

Co-Principle Investigator(s):

Thierry Penduff (IGE)

Co-Investigator(s):

Alice Carret (LEGOS)
Sally Close (LOPS)
Mathieu Lengaigne (LOCEAN)
Stéphanie Leroux (OCEAN NEXT, GRENOBLE)
Jean-Marc Molines (IGE)
Clément Montegut de Boyer (LOPS)
Marie Montero (LOPS)
Olivier Narinc (IGE)
Fabrice Papa (LEGOS)
Florian Sevellec (LOPS)
Pierre Tandeo (IMT-ATLANTIQUE, BREST)
Virginie Thierry (LOPS)
Jérôme Vialard (LOCEAN)

Abstract:

Sea level is being monitored at quasi global scale since the launch of the Topex/Poseidon satellite altimeter and its successors (Jason and ERS constellations, Envisat, Altika, Sentinel-3, etc). These measurements revealed that the global mean sea level experiences a linear increase of 3.3 mm/yr since 1993, with large regional contrasts since certain regions experience sea level rise 3 times larger than this global mean. In addition to global ocean warming and freshwater inputs from continental ice melt (at play at global scale), regional variability and trends respond to changes in salinity, ocean circulation and air-sea flux exchanges. Recent improvements have been made in estimating freshwater discharges from Greenland (based on satellite altimetry, GRACE data and Regional Climate Models) and rivers (from Land Surface Model outputs), and in particular their interannual variability. The main goal of IMHOTEP is to isolate and quantify the response of regional sea level components (thermo-/halo-steric and manometric), oceanic heat and freshwater contents, water mass properties and regional-/basin-scale circulation to fully varying freshwater discharges with a focus on the north/tropical Atlantic and Indian oceans, which receive large amounts of continental freshwater. We will take advantage of synergies between satellite/in situ observations and NEMO simulations. We will first produce five 1-member 1/4° global sensitivity runs where various sources of freshwater are successively set to climatological and fully variable. This will reveal the individual and cumulative impacts of these freshwater sources on the global ocean with a focus on the aforementioned regions, with direct outcomes for the interpretation of satellite and in situ observations.

Eddying ocean simulations have also revealed the existence of another driver of ocean variability and trends (of sea level and many other climate-relevant indices): a substantial low-frequency chaotic intrinsic variability (LF CIV) is spontaneously generated by the ocean and competes with externally-driven sea level fluctuations and long term trends in simulations and observations. The same sensitivity experiments will thus be reconducted in the form of 10-member ensemble simulations to explicitly simulate and separate the forced ocean responses to fluctuating runoffs, to the fluctuating atmosphere, and the LF CIV itself. New statistical techniques and filters calibrated on the ensemble runs will be adapted and applied to increase the size of the ensemble data, and to remove random CIV-related signals from key observational fields. These new data will be made available. IMHOTEP will help interpret the oceanic variability observed by satellites and in situ platforms (altimetry, GRACE, SMOS, Argo, hydrography) in terms of their external (individual runoffs, atmosphere) and internal (LF CIV) drivers, contribute to the detection and attribution of observed signals, assess the representativeness of various observational sources, and provide new types of observation-based data. These outcomes will be of interest for the OST-ST community, for ocean dynamicists, and for climate scientists as the planned analyses will cover several decades.

Supported by CNES