The link between wave height variability in the north Atlantic and the storm track activity in the last four decades

The relationship between storm track activity in the North Atlantic and the recent increase in wave heights observed for parts of this basin is explored by means of statistical analysis. The study is based on numerically reanalysed data of mid‐tropospheric fields and a new numerical wave hindcast. The storm track activity is evaluated in the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis by digital filtering of the 500‐hPa geopotential height fields and latitudinal‐weighed root‐mean‐square average. The wave fields, consisting of a long‐term homogeneous dataset, originate in a 40+ year numerical hindcast of the North Atlantic forced by a wind reanalysis that incorporates kinematically reanalysed storms and re‐assimilated wind observations. The intra‐annual variations in the wave climate are explained by the seasonal changes in the position and intensification of the storm track. Canonical correlation analysis is used to investigate the nature of this relationship for 1958–97. Several patterns emerge that show the most important features of the circulation regimes that determine the wave climate extremes in the North Atlantic. The significant canonical pairs contain most of the variability of the analysed fields and the correlation coefficients of the corresponding components indicate a strong relation between the two. The primary pattern explains the dependence of the wave height extremes (99th percentile and maximum) and the monthly means with the latitudinal shift of the storm track. The worsening wave conditions in the north‐eastern section of the North Atlantic during recent decades appear to be connected to a northward displacement of the storm track. In the central North Atlantic the effect is a reduction in significant wave heights. Subsequent modes are explained in terms of blocking situations and cyclones crossing the Atlantic basin with meridional or wavelike flow. The results also show an increasing trend in the storm activity throughout the period of the hindcast with remarkable correlation with the tendency observed for the North Atlantic Oscillation. Exceptionally calm winters are observed in the late 1960s.