No abstract

Abstract. Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near-coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land-based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site-specific, empirically derived seismic-to-wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near-coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.

[1] Long-term changes in global mean sea level (MSL) rise have important practical implications for shoreline and beach erosion, coastal wetlands inundation, storm surge flooding, and coastal development. Altimetry since 1993 indicates that global MSL rise has increased about 50% above the 20th century rise rate, from 2 to 3 mm yr −1 . At the same time, both tide gauge measurements and altimetry indicate virtually no increase along the Pacific coast of North America during the satellite epoch. Here we show that the dynamical steric response of North Pacific eastern boundary ocean circulation to a dramatic change in wind stress curl, t xy , which occurred after the mid-1970s regime shift, can account for the suppression of regional sea level rise along this coast since 1980. Alarmingly, mean t xy over the North Pacific recently reached levels not observed since before the mid-1970s regime shift. This change in wind stress patterns may be foreshadowing a Pacific Decadal Oscillation regime shift, causing an associated persistent change in basin-scale t xy that may result in a concomitant resumption of sea level rise along the U.S. West Coast to global or even higher rates.

The longest available hourly tide gauge record along the West Coast (U.S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in ''storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.