NAO and extreme ocean states in the Northeast Atlantic Ocean

Abstract

Abstract. Large scale atmospheric oscillations are known to have an influence on waves in the North Atlantic. In quantifying how the wave and wind climate of this region may change towards the end of the century due to climate change, it is useful to investigate the influence of large scale oscillations using indices such as the North Atlantic Oscillation (NAO: fluctuations in the difference between the Icelandic low pressure system and the Azore high pressure system). In this study a statistical analysis of the station-based NAO index was carried out using an ensemble of EC-Earth global climate simulations, where EC-Earth is a European-developed atmosphere ocean sea-ice coupled climate model. The NAO index was compared to observations and to projected changes in the index by the end of the century under the RCP4.5 and RCP8.5 forcing scenarios. In addition, an ensemble of EC-Earth driven WAVEWATCH III wave model projections over the North Atlantic was analysed to determine the correlations between the NAO and significant wave height (H s ) and the NAO and extreme ocean states. For the most part, no statistically significant differences were found between the distributions of observed and modelled station-based NAO or in projected distributions of the NAO.Means and extremes of H s are projected to decrease on average by the end of this century. The 95th percentile of H s is strongly positively correlated to the NAO. Projections of H s extremes are location dependent and in fact, under the influence of positive NAO the 20-year return levels of H s were found to be amplified in some regions. However, it is important to note that the projected decreases in the 95th percentile of H s off the west coast of Ireland are not statistically significant in one of the RCP4.5 and one of the RCP8.5 simulations (me41, me83) which indicates that there is still uncertainty in the projections of higher percentiles.

Catalogue of extreme wave events in Ireland: revised and updated for 14 680 BP to 2017

“…Further work is required to increase confidence in these results since uncertainty remains in the future position of the jet stream, in particular in the North Atlantic (IPCC, 2013;Gallagher et al, 2016), and subsequently the projected changes in wave heights. Additionally, recent research suggests that the effect of the North Atlantic Oscillation (NAO) should be taken into account when considering future extremes in the North Atlantic, as a positive phase may in fact enhance extremes of significant wave height (Gleeson et al, 2017).…”

Case study of the winter 2013/2014 extreme wave events off the west coast of Ireland

“…In an additional study, focusing on extreme wave events off Ireland, Gleeson et al (2017) found evidence of some extreme wave enhancement under the influence of a strongly positive North Atlantic Oscillation, off the west coast of Ireland. Further studies are required to try to address the current uncertainties in North Atlantic wave and storm projections and their affect on Ireland.…”

Skilful Seasonal Prediction of Ocean Surface Waves in the Atlantic Ocean

“…However, a clear dependency between the storm's frequency and the low-frequency atmospheric variability such as the North Atlantic Oscillation (NAO) has been previously found (Gleeson et al, 2017;Rogers, 1997;Semedo et al, 2011;Wang & Swail, 2001). However, a clear dependency between the storm's frequency and the low-frequency atmospheric variability such as the North Atlantic Oscillation (NAO) has been previously found (Gleeson et al, 2017;Rogers, 1997;Semedo et al, 2011;Wang & Swail, 2001).…”

“…Storm dynamics in the North Atlantic has a typical time scale of days. However, a clear dependency between the storm's frequency and the low-frequency atmospheric variability such as the North Atlantic Oscillation (NAO) has been previously found (Gleeson et al, 2017;Rogers, 1997;Semedo et al, 2011;Wang & Swail, 2001). Depending on the NAO phase, different weather regimes (e.g., wind regimes; Cassou et al, 2011), mean, and extreme wind and wave conditions develop over the North Atlantic.…”