- Introduction
- Marine climate
- Sea surface temperature
- Sea surface salinity
- Vertical structure
- Ocean currents
- Regional processes
- Tides
- Surface waves
- Conversion factors
- On-line references
- Authors
- Copyright
Surface waves
Waves are the most familiar water motion. The surface of the ocean is constantly moving, in response to local seas and to swells from distant storms. Seas result from winds pushing the water into random patterns of ripples and bumps, creating jumbles of many periods and heights interacting and breaking. The stronger the wind and the longer it blows, the greater the height of the sea formed. As waves move away, they are sorted into regularly progressing swells traveling in the same direction, with the longest and fastest in front.
Offshore of the Hawaiian islands, the seas are moderately rough, with significant wave heights of 1-4 m (3-14 ft), varying seasonally with the intensity of the trade winds (see below). Between the islands where the winds are funneled, the seas are intensified. The lee, shielded from the winds, is generally calmer. During winter, however, the winds can shift to the northwest or to the southwest, creating unusual sea conditions.
GEOSAT (87-89) |
TOPEX (92-95) |
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Winter |
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Spring |
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Summer |
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Fall |
Plate 15. Average significant wave height (H1/3) in the Central Pacific for Winter, Spring, Summer and Fall. Period: 1987 to 1989 and 1992 to 1995. Source: GEOSAT Altimeter Mission, US NAVY and TOPEX Altimeter mission, NASA and CNES. Units: meters.
Average annual cycle of wave height at the NOAA buoy 50001, located at 23 25 N 162 20 W. Period: 1981 to 1992. Source: National Data Buoy Center; NOAA. Units: meters.
Along the shores, waves become steeper and break as they enter shallow water; both seas and swells can form breakers. The northeast shores of the islands are exposed to moderate trade wind seas. The northwest shores receive some of these waves, but are primarily exposed to large swells from storms in the northwest Pacific in winter, and are calmer in summer. The north shore of O'ahu is famous for its large surf, facing directly winter swells. Breaking waves with faces over 15 m (50 ft) high have occasionally been observed.
The south shores are usually calm in winter, shielded from northwesterly swells. In summer, however, swells arrive from storms in the southern hemisphere. They take 6-8 days to reach Hawaii and have lost much energy from spreading. They are well sorted and are commonly 1-3 m (3-9 ft) high, rarely approaching the heights seen on the northwest shores in winter. The largest southern waves on record had some faces over 6 m (20 ft) high in June 1995.
Breaking waves transport water towards the shore. This water escapes first alongshore, then back out to sea as narrow rip currents, generally located where the bottom is deepest. Rip currents shorten and steepen incoming waves, causing a confused sea which no swimmer can overcome. Standing on a rocky shoreline can also be dangerous, as one can get washed away by occasional large waves. Coastal areas may also be threatened by breaking waves large enough to flood roads and houses. Although forecasts about general wave conditions can be made, the size or timing of individual waves can never be predicted. All should be aware of these risks, and use extreme caution.
Occurring only a few times a year (usually July-September), hurricanes passing close to the islands are another source of large waves and flooding in Hawai'i. Other waves which may cause flooding are tsunamis, generated not by the wind, but by sudden changes of the seafloor during earthquakes or landslides anywhere in the Pacific basin. Unlike wind waves, they propagate in deep water at speeds depending only on the water depth (typically 220 m/s or 500 mph for a 5000-m deep ocean). When a tsunami hits shore, it breaks much like a fast rising tidal bore, sometimes exceeding heights of 10 m (33ft).
Travel time to Honolulu for tsunami originating at various places in the Pacific Basin. Contours give travel time in hours.
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