Global oceanic heat and fresh water forcing datasets based on ERA-40 and ERA-15

A global heat and fresh water dataset based on the data of the second ECMWF Re-Analysis project (ERA-40) is presented that is constructed in the same way as the dataset based on the data of the first project (ERA-15) and can be used as surface boundary conditions for ocean models with sea ice components. The definition of these conditions is based on bulk formulae. A mean annual cycle on a daily basis was constructed from ERA-40 for all relevant parameters including wind stress. Continental runoff is considered by using a runoff model. To estimate implied meridional oceanic heat transports and to avoid temporal drifts of globally averaged deep ocean temperature and salinity in ocean model simulations, the heat and fresh water budgets have been closed by applying an inverse procedure to fine-tune the fluxes towards observed transports. Winds and short wave radiation at Southern higher latitudes and short and long wave radiation in the subsidence zones are corrected. Applied to any ocean/sea-ice model, the forcing dataset would induce only a relative small net sea-surface buoyancy loss. A comparison of both datasets shows that the effects of the changes in the assimilation system of ERA are larger than the effects due to the different periods. The latter effects reveal a southward shift of the Intertropical Convergence Zone (ITCZ) in time and a pattern in the difference of the net heat flux corresponding to a low phase of the North Atlantic Oscillation (NAO). This pattern is essentially determined by a similar pattern in the latent heat flux with a gradient across the North American basin of about 15 to 20 Wm-2 for the annual mean. That confirms that the first dataset is biased towards a NAO high phase whereas the second one covers a whole NAO period