Climate‐Induced Decadal Ocean Wave Height Variability From Microseisms: 1931–2021

Abstract

Long‐term coastal wave climate changes under global warming cannot be reliably assessed from relatively short duration buoy significant wave height (Hs) observations that began about 1980. Alternatively, microseisms at double the wave frequency (DFM) provide proxy records of near‐coastal wave activity. An empirical wave reconstruction methodology transforms DFM from UC Berkeley seismic station BKS to seismic Hs (sHs). Broadband digital seismic observations since 1992 at BKS provide DFM reference spectral levels. Earlier digitized archived BKS seismogram DFM levels are adjusted to give spectral levels that are consistent with current observations, yielding a combined 90‐year sHs record for winters (November–March) spanning the 1931–2021 epoch. Mean winter sHs derived from digitized 1980s seismograms are well‐correlated with nearby buoy Hs. Comparison of deep‐water extreme Hs with sHs demonstrates that sHs is dominated by near‐coastal wave activity. Much greater decadal and interannual variability occurred during the 1931–1970 epoch, including extended periods of exceptionally low sHs during 1939–1947 and 1957–1965 that have not occurred since global warming accelerated near 1970, suggesting that winter extratropical storm intensity has generally increased. Comparisons of winter sHs variability with anomalous sea level pressure (SLPa) patterns across the North Pacific demonstrate the association of coastal sHs with broad‐scale atmospheric variability, with differences between long‐term averages of winter SLPa indicating an Aleutian Low intensification, consistent with both increased mean sHs and occurrence of extreme events since 1970. Recent record global annual temperatures suggest that warming may be accelerating, resulting in stronger storms that intimates further increased winter Hs may be forthcoming.