FIGURE 4.16 Monthly sea level for Seattle, Washington, from 1900 to 2008. Straight-line fits to the data show the relative sea-level rise (blue line), the sea-level rise adjusted for atmospheric pressure (green line), and the sea-level rise adjusted for vertical land motion and atmospheric pressure (red line).
Sea level at any given place and time depends on the global sea level and the net contribution of atmospheric, oceanographic, geologic, and anthropogenic processes operating in the area. Processes that raise relative sea level in the northeastern Pacific Ocean include warm phases of climate oscillations (El Niños, positive phase of the PDO) and land subsidence due to glacial isostatic adjustment, sediment compaction, and the withdrawal of groundwater or hydrocarbons. Processes that lower relative sea level include cool phases of climate oscillations (La Niñas, negative phase of the PDO), gravitational and deformational effects of modern melting of glaciated land masses, and land uplift due to tectonics or fluid recharge.
The highest sea levels recorded along the west coast are usually associated with El Niño events, which can elevate coastal sea level by 10–30 cm for several winter months. Cool climate phases have less influence on local sea level than warm climate phases. Changes between warm and cool climate phases, which occur on seasonal to multidecadal timescales, cause large-amplitude variations in the relative sea-level trend.
Modern melting of glaciers and ice sheets adds new water to the ocean basins and produces gravitational and deformational effects that create regional patterns of relative sea-level change. The glaciated land masses that most effect sea level along the west coast of the United States are Alaska, which is close, and Greenland and Antarctica, which are large. The gravitational and deformational effects reduce the contribution of melting of these three ice sources to relative sea-level rise for 1992–2008 by about 42 percent along the north coast (Neah Bay, Washington), 24 percent along the central coast (Eureka,