During 2020, a major project got under way to fundamentally change the way that the temperature of the sea surface is derived within ECMWF’s data assimilation system. The new scheme will use satellite radiances directly to capture important variations of the ocean surface.

The sea surface is where the ocean and atmosphere interact, and it sits at the very heart of ECMWF’s Earth system approach to forecasting. Coupling between the atmosphere and ocean provides a key source of predictability in medium- and extended-range forecasts.

The new scheme will use infrared and microwave satellite radiances directly within the four-dimensional variational data assimilation scheme (4D-Var) to produce an analysis of ocean skin temperature – a thin interface surface layer to which the satellites are extremely sensitive. This will replace an imported analysis of daily mean, bulk sea-surface temperature (SST), representative of the top several metres of the ocean.

It is anticipated that the new approach will improve the accuracy of the sea-surface temperature and produce an ocean skin in balance with the atmosphere and ocean state, leading to improved weather forecasts. Early tests have shown significant sensitivity of large-scale atmospheric circulation to changes in ocean skin temperature of just a few tenths of a degree.

Using half-hourly observations from geostationary satellites and frequent overpasses from a constellation of polar-orbiting satellites, the diurnal cycle and other rapid variations in ocean skin temperature will be captured within the new analysis. Heating of the ocean surface through the day is an important source of energy transfer to the atmosphere. Other rapid changes in ocean temperature should also be better represented – such as cold ocean wakes that develop in response to the strong winds in tropical cyclones. In addition to the satellites already being used in 4D-Var, highly accurate data from the Copernicus Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) will be used for the first time at ECMWF.

Ocean skin temperature is a completely new variable that has been incorporated within ECMWF’s data assimilation system and successfully coupled between the atmosphere and the ocean. The thin ocean skin can behave quite differently to the deeper surface layers and must be treated appropriately. ECMWF scientists now have a better understanding of the physics of how the skin interacts with the ocean below and atmosphere above.

There has also been a major upgrade of the infrared radiative transfer modelling to use a consistent state-of-the-art spectroscopy for all satellites. This has benefits across the ECMWF Integrated Forecasting System, as well as being an important prerequisite for obtaining accurate ocean skin temperatures from the available satellite radiance observations.  

Proof-of-concept experiments have already been performed that successfully demonstrate the technical feasibility of the new approach, and the next step will be a full end-to-end science test of the system. Only then will the teams see the full effects of the new scheme. There are still unknowns and more work to do, but if all goes well, the new scheme may be ready for operational use by the end of 2022.

A proof-of-concept testFor the first time, ECMWF scientists saw the potential impact of satellite radiances on surface ocean temperatures within the coupled 4D-Var/NEMOVAR data assimilation system. The plot shows the underlying prior SST field and corrections to this (contours) in response to radiance observation forcings (circles) from the infrared atmospheric sounding interferometer (IASI) onboard EUMETSAT’s MetOp-C satellite.
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For many years sea-surface temperature (SST) has been just an imported lower boundary condition for the atmosphere or an upper boundary condition for the ocean, but it is now at the centre of our coupled prediction system and will be treated as such.

Tony McNally, Principal Scientist in Earth System Assimilation Section at ECMWF