Tide and sea level fluctuations from days to decades

Factors contributing to the water level along the coast of southern California on time scales of days to decades are reviewed. The authors compare their amplitudes to that of "mean" sea level (MSL) rise. The tide is the largest influence on coastal water levels in California on these time scales, and the only component that is predictable in detail. Other factors include storm surges, large-scale changes in water temperature, wind-driven circulation, and climate-related El Nino events. All of these raise or lower sea level for characteristic lengths of time from hours to years, and with amplitudes ranging from 10 cm to several m. On time scales shorter than about 100 yrs, they are all comparable to MSL rise, and in some cases much larger. Because of the astronomical motions, tidal oscillations appear in bands around 1 and 2 cycles per day. Along the California coast, tides are mixed with a mean range of about 1.5 m, and an extreme range of up to 3 m. The nearly equal measures of diurnal and semidiurnal components gives rise to characteristic patterns of timing and height of high tide, low tide, and tide range with periods ranging from a day to 18.6 yrs, and including important variations twice per month, twice per year, and every 4.4 yrs. Further, tides along most of California\´s open coast show a secular increase in range, meaning that mean high water is actually rising faster than MSL. Compared with the East and Gulf coasts of the United States, storm surges in southern California are small, ranging up to at most 30 cm. As a consequence of seasonal heating and cooling, and the effects of changing large-scale wind and barometric pressure patterns over the eastern Pacific Ocean, the average water levels in southern California fluctuate by about 15 cm during an average year. The lowest levels occur during spring, when the surface waters are coldest and therefore occupy less volume, while the highest levels occur in autumn, when temperatures peak. El Nino is a state - > - > of the ocean and atmosphere in the tropical Pacific Ocean appearing every 2 to 7 years with various strengths, and lasting 6 to 18 months. Annual MSL records from San Francisco available since 1854 suggest that the rate of MSL rise on the California coast may be greater after about 1920 than it was before. No mechanism for this apparent acceleration has been defined, let alone determined if it has any relation to global warming. It may even be confined to the San Francisco Bay region, although Seattle shows a similar pattern.