Comparison of HIPOCAS and ERA wind and wave reanalyses in the North Atlantic Ocean

Two pairs of comparisons between HIPOCAS and ERA reanalysis are performed in this paper. HIPOCAS is a hindcast developed under a European project and it is the result of the wave model WAM forced by REMO surface winds in the North Atlantic. ERA-40 and ERA-Interim were developed by the European Centre for Medium-Range Weather Forecasts, ECMWF, adopting the wave model WAM, covering the whole globe. A quick introductory comparison of HIPOCAS against ERA-40 is initially provided, which are hindcasts produced almost at the same time with common features. The main analysis covers a detailed comparison of HIPOCAS and ERA-Interim, which can be considered an update of ERA-40, followed by a comparison of both datasets against satellite data. Wind speed and significant wave height are the two variables considered. In order to compare the described wind and wave datasets, a common period and area were defined, with latitudes between 20° and 68° North and longitudes between 50° and 0° West, from 1979 till 2001. The differences for the whole dataset and for extreme events (upper percentiles) are calculated. A spatial and time integration is applied to determine where and when the main differences between hindcasts occur. The results show a relatively good agreement between HIPOCAS and both ERA-40 and ERA-Interim reanalysis for calm and moderate weather conditions. There is a large increase of the differences under extreme conditions, occurring mainly during the winter at mid-high latitudes. ERA-Interim presents the best results against measurements but suggests some underestimation under extreme events at mid-high latitudes. HIPOCAS tends to overestimate the measurements and over-predict both ERA reanalyses in the range of extreme values.