Atmospheric Forcing in Ocean Forecasting

So far the week has focused on ocean data. But the ocean doesn’t move on its own – it’s driven from above. This lecture, with Ebenezer Nyadjro, turns to atmospheric forcing: the atmospheric data that drives ocean models, and why its quality is so decisive for a forecast.

The lecture works through:

• What atmospheric forcing is – the atmosphere supplies the ocean with winds, heat, freshwater, and pressure, and these exchanges are among the primary drivers of ocean variability. A model of the ocean must be told what the atmosphere above it is doing
• The main forcing variables – the typical inputs a model needs, usually all at once: wind speed and direction, surface pressure, air temperature, humidity, precipitation, shortwave and longwave radiation, and latent and sensible heat fluxes
• Surface fluxes and air-sea exchanges – how forcing controls heat exchange, momentum transfer, evaporation and precipitation, and ocean mixing at the surface
• Where forcing comes from – the major operational providers, all of them numerical weather prediction (NWP) systems: ECMWF / ERA5, GFS (NOAA), NAVGEM (US Navy), the UK Met Office, and GEM (Canada)
• Wind forcing and ocean response – how winds generate surface wind stress, drive currents, and trigger upwelling and mixing – especially important along coasts
• Why it matters for Africa – atmospheric forcing is essential for the upwelling systems, coastal circulation, fisheries, and marine safety that many African coastal regions depend on
• Why forcing quality matters – errors in the atmospheric forcing feed straight through into errors in the forecast: in sea surface temperature, currents, mixed-layer depth, and sea level. A good ocean model with poor forcing will still produce a poor forecast
• Reanalysis versus forecast products – the key distinction: reanalysis (like ERA5) is the best estimate of past conditions, used to drive hindcasts and analyses; a forecast product (like an ECMWF forecast) predicts future conditions. Same kind of data, different time direction and purpose

The key takeaways: atmospheric forcing drives ocean forecasts; many variables are needed simultaneously; operational forcing comes from NWP systems; and forcing quality strongly shapes forecast skill. The next lecture turns to the regional and in-situ data that complement these global inputs.