but the interpretation of these changes is controversial. Analyses of global ocean winds from ship observations (Tokinaga and Xie, 2011), satellite microwave sensors (Wentz et al., 2007), and satellite altimeters (Young et al., 2011a) indicate that wind speeds have risen over the global oceans, although the trends found by Young et al. (2011a) are greater than those derived from Tokinaga and Xie (2011) and Wentz et al. (2007) by approximately a factor of two (Wentz and Ricciardulli, 2011; Young et al., 2011b). The Young et al. (2011a) analysis also found that wind speeds within the highest 1 percent of events have risen over much of the extra-tropical oceans over the past two decades, including an increase of about 1 percent per year in the northeast Pacific, and that this increase is accompanied by increases in the extreme wave heights. The latter occurs in particular in the northeast Pacific Ocean, which is consistent with increasing extreme wave heights (by as much as 2 m over the record period) during big storms recorded in near coastal deep-water buoy records from northern California to Washington (Allan and Komar, 2006; Menéndez et al., 2008; Ruggiero et al., 2010). However, further analysis by Gemmrich et al. (2011) suggests that much of this change is spurious, caused by changes in buoy hardware and data processing. All of these estimates were made from records that are only a few decades long, and thus partly reflect changes in wind forcing associated with natural climate variability such as the Pacific Decadal Oscillation and other interannual-interdecadal fluctuations. However, the global extra-tropical pattern of extreme wave increase found by Young et al. (2011a) is atypically widespread for most decadal natural variability, and thus might indicate a longer trend. As yet there is no good explanation for why such a trend would occur.

Summary

Periods of anomalously high sea levels and wave heights along the west coast of the United States exhibit considerable variability on synoptic, interannual, and decadal timescales, in association with ENSO and other climate patterns. Some evidence suggests that wave heights have increased along the west coast from northern California to Washington during the past few decades. However, it is likely that much of this increase is associated with interannual- to decadal-scale natural variability of the Pacific atmosphere-ocean system. Some global climate models predict that the North Pacific storm track will shift northward as global climate warms during the next several decades, which would generate extreme wave heights and storm surges along the Oregon and Washington coasts. However, a northward shift in the North Pacific storm track has not yet been confirmed.

All climate models project ample winter storm activity in the North Pacific in future decades, suggesting that periods of anomalously high sea level and high waves will continue to occur along the west coast. Storm-generated bursts of high sea levels and waves are expected to vary from year to year and decade to decade. Over the next few decades, these anomalies will likely eclipse the secular rise in sea level (few to several mm per year). Short-period fluctuations of sea level may sometimes exceed 20 cm, and storm-driven wave heights of 1 m or even higher amplitudes than are seen in the historical record could easily occur. These variations will have greatest impact when they occur on days with high tides.

SEA-LEVEL FINGERPRINTS OF MODERN LAND ICE CHANGE

As glaciers and ice sheets melt and lose mass and the melt water is transferred from the continents to the ocean, the solid earth deforms and the gravitational field of the planet is perturbed. The addition of new water to the ocean basins and the associated gravitational and deformational effects create regional patterns of sea level change. Both modern melting and deglaciation of the ancient ice sheets affect sea-level change along the west coast of the United States. Melting of the ancient ice sheets caused the solid earth to rebound (glacial isostatic adjustment), resulting in significant vertical land motions in the vicinity of the California, Oregon, and Washington coasts. In contrast, modern melting affects land motions at the ice masses, which are far from the U.S. west coast, but the gravitational effect influences the height of the sea surface in the northeast Pacific Ocean. This section describes the effects of modern land ice melt on sea-level rise off the coasts of California, Oregon, and Washington. The effects of ancient ice melt are discussed in the following section (see “Glacial Isostatic Adjustment” below).



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