Rising sea level poses significant challenges to infrastructure and populations, particularly for coastal and island communities in the North Pacific where population density at or near coastal waters is high. A significant portion of the United States' security, commerce, and ecosystem assets are located at or near the coast, making them vulnerable to sea level rise. Although global mean sea level (MSL) rise is a fundamental consideration, regional mean sea level (RSL) height variability within ocean basins and along their boundaries can be more critical, particularly in the North Pacific where the amplitude of interannual RSL variability is high and wave activity is intense. The main causes for global MSL rise are added water from the melting of ice sheets and glaciers and thermal expansion of the oceans, both driven by global warming.
Regional sea level variations appear to fluctuate about the globally averaged trend, which has increased from the tide gauge estimate of about 1.7 - 0.5 millimeters per year over the twentieth century to the satellite altimetry estimate of about 3.1 - 0.7 millimeters per year since 1993 [e.g., Bromirski et al., 2011]. However, recent studies show that regional sea level trends are affected by local and remote wind forcing (Figure 1), which can cause sustained changes in ocean circulation and sea level height. Along the U.S. Pacific coast, tide gauges suggest that regional sea level rise is approximately equal to global MSL rise over most of the twentieth century, but altimetry (Figure 2) and tide gauges (Figure 3) both indicate that RSL rise is significantly less than global MSL rise since about 1980 [Bromirski et al., 2011].
Wind stress curl-related Ekman pumping and alongshore wind stress-related Ekman transport (Figure 1) mainly drive these regional departures from the global trend. These processes alter the thermocline depth, with a deeper thermocline associated with raised sea level height. Persistent regional wind stress patterns spanning a few decades, as well as basin-wide wind-driven circulation changes and strong El Niño-related fluctuations on shorter time scales, strongly affect sea level trends along the Pacific coast of North America, exemplified by the San Francisco record (Figure 3). The difference between pre-1930 and post-1980 mean RSL levels (red dashed lines in Figure 2) at San Francisco is about 15.8 centimeters, giving an RSL rise of about 3.2 millimeters per year over the 1930-1980 epoch, similar to recent altimetry global MSL rise estimates.
RSL provides the base level on which tides, storm surge, and waves are superimposed. Meteorologically-driven storm surge raises coastal sea levels [Bromirski et al., 2003], allowing more wave energy to reach farther shoreward. In the vulnerable San Francisco Bay/Delta region, storm surge is generated outside the Golden Gate, and propagates into San Francisco Bay and throughout the Delta [Bromirski and Flick, 2008].