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Abstract

Visual observations from Voluntary Observing Ships (VOS) represent the longest available records of global wind waves going back to the end of 18^th century. Besides the longest continuity compared to the other datasets, VOS reports are consistent in terms of observational practice, i.e. these were always visual reports performed by marine officers and mates. Moreover, VOS data are the only providing separate estimates of wind sea and swell. We present the analysis of changes in wind wave characteristics over the global ocean for the period 1880 – 2020 available from ICOADS collection. Visual reports form ICOADS were the subject of quality control and multiple corrections to minimize or totally cancel known biases in visual wave observations. Also for minimizing temporally inhomogeneous sampling errors we applied the procedure of sub-sampling aiming on homogenization of sampling over 140 yr period. Thus, we present first comprehensive assessment of long-term changes in wind wave characteristics over the global ocean. Linear trends were estimated separately for wind sea, swell and significant wave height which was derived form sea and swell estimates. Statistically significant upward trends in significant wave height are observed in the North Pacific ranging from 0.2 to about 1 cm per year in some regions. These trends are contrasting primarily downward changes in the North Atlantic. We also consider long-term changes in extreme waves which regionally demonstrate tendencies opposite to the mean wave characteristics. Special consideration is also given to the coastal areas characterized by rich sampling.