Fifteen years of global wave hindcasts using winds from the European Centre for Medium‐Range Weather Forecasts reanalysis: Validating the reanalyzed winds and assessing the wave climate

Abstract

Abstract. The ERA (European Centre for Medium-Range Weather Forecasts Reanalysis) project resulted in a homogeneous data set describing the atmosphere over a time span of 15 years, from 1979 to 1993. To validate (part of) these data against independent observations we use the ERA surface winds to drive the WAM wave model. The modeled significant wave heights are then compared with observations. From this comparison the quality of the fbrcing winds is assessed. The patterns of computed wave heights agree well with observed patterns, and they are of the right magnitude. This confirms the realistic nature of the ERA winds. If one looks in detail, it appears that the significant wave heights resulting from the model are systematically lower than the observed ones in areas of high winds and waves and higher in areas of low winds and waves. It is argued that underestimation at high winds speeds is •nost likely a resolution effect, as wind and thus wave peaks are missed by finite resolution in space and time, while overestimation at low wind speeds most likely results from internal WAM errors. It is concluded that the monthly mean ERA winds are slightly (less than 5%) too low in areas of high winds, while from this study it is not, possible to draw a decisive conclusion on the quality of ERA winds at low wind speeds. At the same time, the hindcast data form a 15-year climatology of global waves. This climatology is analyzed in terms of annual cycle and trends. The largest trends in significant wave height occur in the North Atlantic with an increase of more than 12 cm/yr in January, and south of Africa where the increasing trend exceeds 7 cm/yr in July. These trends, however, are only marginally significant. Furthermore, they exhibit a large month-to-month variability, so that on a seasonal basis the trends are significant only in small parts of the ocean. In conclusion, we are unable to confirm a significant change in wave height during the ERA period.

Changes in significant and maximum wave heights in the Norwegian Sea

“…This section briefly describes statistical distributions in theories for wave fields which have been verified against data obtained from bottom--mounted sensors, buoys and altimeters (Bretschneider, 1959;Dobson et al, 1987;Sterl et al, 1998;Tucker and Pitt, 2001;Stansell, 2004;Vandever et al, 2008;Casas--Prat and Holthuijsen, 2010 Individual wave heights with a narrow--band spectrum are found to follow a Rayleigh distribution in the deep sea (Longuet--Higgins, 1952). Within this narrow--band spectrum of wave heights, the average height of the highest one--third of the waves over an observational period, H1/3, is practically equal to the significant wave height, Hs, that can be derived from the spectrum (Phillips, 1977).…”

Periodicity and patterns of ocean wind and wave climate

“…There is some energy in the inter-annual range, but it is on the same scale as the 40-100 day oscillations. The broad-band signals of the EOFs support the dynamic nature of the North Atlantic Ocean [Sterl et al, 1998]. However, the climate cycles are often described by their inter-annual variability in the literature [e.g., Wang and Swail, 2001;Sterl and Caires, 2005;Fan et al, 2012;Stopa et al, 2013a;Bromirski et al, 2013].…”

Wind waves in the Black Sea: results of a hindcast study

“…This research is carried out on a variety of both temporal and spatial scales. Thus, a part of the research is dedicated to global wind and waves climate (Peterson and Hasse, 1987;Sterl et al, 1998). However, more or less regional hindcast studies prevail in this field, e.g.…”