Monitoring the Ocean Heat Content and the Earth Energy imbalance from space altimetry and space gravimetry: the MOHeaCAN project.

Florence Marti (Magellium, France)


Michaël Ablain (Magellium, France); Robin Fraudeau (Magellium, France); Rémi Jugier (Magellium, France); Benoit Meyssignac (LEGOS, France); Alejandro Blazquez (LEGOS, France); Jérôme Benveniste (ESA, Italy); Marco Restano (ESA, Italy)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science I: Climate data records for understanding the causes of global and regional sea level variability and change

Presentation type: Type Forum

The Earth is currently emitting less energy towards space than the radiative energy it receives from the sun due to anthropogenic emissions of greenhouse gases. It is essential to estimate and analyse this Earth Energy Imbalance (EEI) at the top of the atmosphere if we want to understand the Earth’s changing climate. The long term mean EEI associated with anthropogenic forcing amounts of ~1 W.m-2 (Trenberth et al. 2017) is small compared to the amount of energy entering and leaving the climate system (of ~340 W.m-2) making it a challenge to estimate with observations. An accuracy of 0.3 W.m−2 at decadal time scales is necessary to evaluate this long term mean in EEI However, ideally an accuracy of 0.1 W.m-2 at decadal time scales is desirable if we want to monitor the small variations of the EEI over the long term that are due to natural climate processes such as El Nino/Southern Oscillation or volcanic eruptions (Meyssignac et al. 2019).
In the frame of the MOHeaCAN project supported by ESA, the EEI indicator is deduced from the global change in Ocean Heat Content (OHC) which is a very good proxy of the EEI since ocean stores 93% of the heat stored by the Earth in response to EEI. The OHC is estimated from space altimetry and gravimetry missions (GRACE and GRACE-FO). This “Altimetry-Gravimetry” approach is promising because it provides consistent spatial and temporal sampling of the ocean, it samples nearly the entire global ocean, except for polar regions, and it provides estimates of the OHC over the ocean’s entire depth. Consequently, it complements the OHC estimation from ARGO.
The MOHeaCAN product contains monthly time series (between August 2002 and June 2017) of several variables, the main ones being the regional OHC (3°x3° spatial resolution grids), the global OHC and the EEI indicator. Uncertainties are provided for variables at global scale, by propagating errors from sea level measurements (altimetry) and ocean mass content (gravimetry). In order to calculate OHC at regional and global scales, a new estimation of the expansion efficiency of heat at global and regional scales have been performed based on the global ARGO network.
The first results indicate a mean EEI of 0.84 W.m-2 over the whole period within an uncertainty of ±0.12 W.m-2 (68% confidence level - 0.20 W.m-2 at the 90% CL). This figure is in agreement (within error bars at the 90% CL) with other EEI indicators based on ARGO data (e.g. OHC-OMI from CMEMS) although the best estimate is slightly higher by about 10-20%. The scientific validation is still on-going to investigate and secure our understanding of these differences, and make thorough comparisons.
Florence Marti