Evaluation of operational ECMWF surface freshwater fluxes over oceans during 1991–1997

The distribution of evaporation and precipitation over the ocean is one of the least understood elements of the climate system. Monthly-mean precipitation (P) and evaporation (E) fields from ECMWF operational analyses are studied for the period 1991 to 1997, and computed into the freshwater flux. Monthly-mean time-series of zonally averaged freshwater fluxes highlight several model changes affecting the analyses, in 1997 in particular. The monthly climatology built over the 1991–1996 period exhibits a coherent geophysical behaviour with regard to different sources of data. Global budgets of ECMWF precipitation and evaporation are 10% larger (110 mm/year higher) than climatologies from other sources, but the balance between evaporation and precipitation (E−P) is preserved and is in line with the concept of the World Water Balance. In general, the zonal distribution of the E−P balance correlates qualitatively well with ocean surface salinity. Analysis of the geographical distribution of E and P and meridional transport of freshwater highlights fundamental discrepancies between ECMWF datasets and other sources of data. In particular, ECMWF analyses show significantly higher annual precipitations (700 mm/year more) in intertropical convergence zones, which contribute most to the differences observed in the meridional freshwater transport. For oceanographic applications, the haline and thermal density fluxes are compared. The thermal component generally dominates, except in equatorial regions where both components reach similar amplitudes. The salinity forcing is found to be especially strong in the western equatorial Pacific Ocean.