FIGURE 4.1 Trend of thermosteric sea level (mm yr-1) for 1993–2009 (left) and 1961–2008 (right), based on an updated version of data from Ishii and Kimoto (2009). SOURCE: Courtesy of Masayoshi Ishii, Japan Meteorological Research Institute.
in sea level of about 0–0.8 mm yr-1 from 1955 to 2003, rather than a fall (Bindoff et al., 2007). This difference suggests that the spatial pattern of sea level varies on decadal and longer timescales.
Changes in wind-driven ocean circulation can play an important role in determining patterns of sea-level change in the northeast Pacific Ocean on seasonal to decadal and longer timescales (e.g., Timmermann et al., 2010; Bromirski et al., 2011; Merrifield, 2011; Sturges and Douglas, 2011). Recent studies show a decrease in the rate of sea-level rise along the west coast of the United States since 1993, which is consistent with IPCC (2007) findings, but no statistically significant trends appear in tide gage records (Bromirski et al., 2011), satellite altimetry data, or in situ temperature observations since 1980. For example, thermosteric sea-level calculations show falling sea level off the U.S. west coast from 1993 to 2009 (Figure 4.1, left) and rising sea level from 1961 to 2008 (Figure 4.1, right). Bromirski et al. (2011) suggested that the flat sea-level trend since 1980 and the decrease since 1993 are associated with PDO phase changes.
Seasonal and Interannual Variability
Among all the climate modes, ENSO is the dominant cause of sea-level variability in the northeast Pacific Ocean on interannual timescales (e.g., Zervas, 2009; Bromirski et al., 2011). Sea level rises off the west coast of the United States during El Niño events and falls during La Niña events. El Niños differ in magnitude and large-scale form (Barnard et al., 2011) but commonly produce an active winter storm season in the northeast Pacific. The associated winds and ocean circulation changes may elevate sea level by 10–30 cm for several months along the west coast (Chelton and Davis, 1982; Flick, 1998; Bromirski et al., 2003; Allan and Komar, 2006; Komar et al., 2011). In fact, the highest sea levels recorded along the west coast were usually associated with El Niño events (e.g., Figure 4.2). For example, on January 27, 1983, during one of the largest El Niños in half a century, seven tide gages along the west coast (San Diego, Los Angeles, Monterey, Crescent City, Charleston, Astoria, and Seattle) recorded their highest water levels.1 Peak sea level was 24 cm above predicted in San Diego (104 years of record), 31 cm above predicted in Los Angeles (87 years of record), and 76 cm above predicted in Seattle (112 years of record).
Large El Niño and La Niña events also can be seen in satellite altimetry data. The top panels of Figure 4.3 show the sea-level rise observed during the El Niño of 1997–1998 and the sea-level fall observed during the 1999 La Niña. The ENSO signal is strongly seasonal and reaches a peak amplitude in the Northern Hemisphere winter. Figure 4.3c shows the ocean seasonal cycle, which is occasionally magnified by ENSO.