Auxialiary material for Paper 2015JCXXXXXX
Wave Power Variability and Trends Across the North Atlantic
Peter D. Bromirski, Scripps Inst. of Oceanography
Daniel R. Cayan, Scripps Inst. of Oceanography & USGS
J. Geophys. Res., XXX (BX), doi:10.1029/2015 JCXXXXXX, 2015
Introduction
Model Performance Validation
Performance of WAVEWATCH III (WW3) wave model significant wave height (Hs)
estimates was carried out in part by comparisons with all available NOAA buoy Hs
through 2008 (Figures V2a-V2d). Also shown are comparisons between WW3 Hs and
buoy Hs for 1998 and 2008, examining individual annual wave cycles in detail. WW3 –
buoy comparisons during 2008 compare WW3 hindcast Hs with more recent buoy data.
The WW3 model forced by NCEP NRA-1 winds generally underestimate NOAA buoy
Hs peak event amplitudes. However, the arrival times of the events are generally wellcorrelated, particularly at deep-ocean locations, as indicated by both the time series
comparisons (Figures V2a, V2c, V3a, V3c, V4a, V4c) and the relatively high R2 for
scatter plots of corresponding WW3 and buoy Hs estimates (V2b, V2d, V3b, V3d, V4b,
V4d).
Together with Hs inconsistencies identified at individual buoys [Gimmerich et al., 2011],
the data gaps in buoy Hs time series and the sparse spatial coverage both emphasize the
advantage of WW3 model data for determining long-term trends.
Comparisons of WW3 and western boundary buoy Hs at both nearshore and offshore
NOAA buoy locations (Figure V1) during winter (Nov.-Mar., Figures V5a and V5b) and
summer (May-Sep., Figures V5c and V5d) seasons over the 1981-2008 epoch, during
which at least 50% of corresponding buoy observations were available, indicate that there
are generally no statistically significant trends in the regression of the correlation slopes
between model and buoy Hs.
Similar validation methodology was employed for Hs data from five buoys in the
Northeast Atlantic obtained from the UK Met Office (see Figure V6 for locations). These
buoy Hs amplitudes were much better correlated with WW3 model Hs (Figures V7-V9),
likely reflecting better resolution of wind fields across the northern North Atlantic for
more mature storm systems than the wind fields along the western boundary of the North
Atlantic that are affected by storm systems coming off the North American continent.
Comparison of WW3 and buoy wave power, Pw, estimates for in the northeast Atlantic
(Figure 11b) with Pw estimates along the western boundary of the North Atlantic (Figure
11a) shows a significantly greater underestimation along the western boundary.
Climatological means (Figure 12) demonstrate that the Nov.-Mar. winter season
generally have the highest wave activity, with generally May-Sep. the lowest.
Spatial Patterns
The distribution of wave activity across the North Atlantic basin is demonstrated by
spatial percentile distributions (Figure S1) and spatial and temporal Empirical Orthogonal
Function (EOF) / Principal Component (PC) analyses (Figures S2-S6).
The dominant mode of variability across the North Atlantic is associated with
atmospheric circulation patterns associated with the NAO, shown in Figure S7 from
1950-2013. The 5-yr running mean (green curve) shows the winter NAO was generally
negative prior to 1980 and generally more positive after 1980, motivating epoch selected
in this study to examine the influences of the phase of the NAO on wave activity across
the North Atlantic.
References
Gemmrich J., B. Thomas, and R. Bouchard (2011). Observational changes and trends in
northeast Pacific wave records. Geophys. Res. Lett., 38,
doi:10.1029/2011GL049518.