Investigating the 15 years air-sea flux climatology from the ECMWF re-analysis project as a surface boundary condition for ocean models

Abstract

A 15‐year climatology of surface fluxes from the 6‐h forecasts provided by the ECMWF re‐analysis (ERA15) is assessed in the context of global ocean circulation modelling. The long‐term mean forcing function defined by heat, freshwater and momentum surface fluxes is examined over the global ocean, and compared with other available data (satellite, in situ). This reveals significant global and regional biases in the ERA15 surface fluxes. The short‐wave radiative flux is generally underestimated within the intertropical band. However, a misrepresentation of marine stratus in the regions of subsidence of the Walker cell leads to an excess of short‐wave radiation there. The meridional oceanic heat and freshwater transports are computed and compared with other estimates. It is found that some deficiencies in heat transport are related to the underestimate of the short‐wave radiation within the tropics. The largest discrepancies between transport estimates generally appear in the Indian and the South Pacific oceans. The contribution of Ekman heat transport in the net meridional heat transport is also diagnosed using ERA15 wind stress. This highlights the major role of the Ekman processes in transporting heat, mainly in tropical regions. Finally, a dynamic insight into the thermohaline forcing of ocean models with ERA15 is provided by examining the haline and thermal surface density fluxes. The global biases in heat and freshwater fluxes are found to have a non‐negligible impact on the spatial distribution of the density fluxes. It is concluded that the climatological ERA15 surface fluxes need to be properly adjusted, especially in the tropics, in order to use them to force ocean models. Copyright © 2000 Royal Meteorological Society