Observed changes in significant wave heights derived from long-term homogenized measurements offshore mainland Portugal

• Homogenization methodology is applied to 40-yr wind-wave dataset collected offshore mainland Portugal at 100 m water depth. • Long-term trend analysis of significant wave heights indicates non-climatic step changes smaller than 0.16 m. • Monthly mean value of significant wave height increases at a rate of +10.0 mm/yr for the October, November and December quarter on the northwest coast. • Monthly 90-percentile value of significant wave height decreases at a rate of −22.2 mm/yr for the October, November and December quarter on the south coast. • ERA5 accurately reflects wave trend direction but significantly underestimates trend rates. Trends in wind-wave climates across the globe have been primarily addressed using numerical models. Availability of long-term data collected by wave buoys is often scarce and they present inhomogeneities associated with wave buoy size and hardware over time. Here, a trend analysis was conducted on approximately 40 years of homogeneous wind-wave data collected by wave buoys offshore mainland Portugal. For that, a homogenization methodology based on RHTestsV4, with ERA5 wave hindcast as reference time series was used. Results indicate that along the north-western coastline facing the North Atlantic, an increasing trend of monthly mean significant wave height of +10 mm/yr was observed at FigLei record for the months between October and December. Along the south-western coastline, no statistically significant trends were observed. Along the southern coastline, which is also exposed to wind-waves generated in the Mediterranean Sea results at Faro record show a decreasing trend of monthly 90th percentile of -22.2 mm/yr between October and December. A further comparison between the wind-wave trends obtained with local wave buoys and those from the global ERA5 wave hindcast highlights that the trends of the later can be opposite, or they can vary by up to a factor of 10 which emphasizes the importance of long-term wave buoy observations networks for a more accurate understanding of local wave climates.