Winds, sea levels and North Atlantic Oscillation (NAO) influences: An evaluation

This work presents the analysis of fifteen years (1993 to 2007 inclusive) MetOffice wind data for the Bristol Channel/Severn Estuary. Data from the Turbot Bank Buoy, located within the open waters of the Bristol Channel, showed significant reducing trends in monthly mean and maximum extreme wind speeds. Coupled with significant falling extreme sea level trends over the same time period, and direct correlation between wind speed and sea level, the increased storminess experienced during the latter half of the twentieth century seems not to have continued into the first decade of the twenty-first century. Wind speeds and directions between 1993 and 2002 inclusive at Turbot Bank Buoy were found to be significantly higher than those measured at the relatively nearby Rhoose land based station (Paired t test: tcalc = 87.74, df = 9, p < 0.01 and tcalc = 46.6, df = 9, p < 0.01 respectively). This highlights problems in using land based wind records to represent sea conditions, as well as using one location as representative of conditions along a coastline. Analysis further showed a small range of annual mean wind angles (circa 180° to 210° true). There was a significant regression between wind speed and NAO Index, given by 0.33 ± 0.06 knots/NAO unit (p = 0.000). This was more important during positive NAO phases, as wind speeds reduced during negative phases. Regression also indicated a direct positive relationship between NAO Index and wind direction (15.7 ± 7.5°/NAO unit; p = 0.05), where higher wind angles are linked to a rising NAO. Therefore, results have major implications for morphological response to wind (speed and direction), sea level (mean and extreme) and NAO Index (positive and negative). Conditions are unlikely to be unique to this coastline and it is suggested that similar analyses are replicated elsewhere to improve understanding and characterisation of uncertainty.