A few years ago, Ocean Voyager featured an article about how winds generate ocean waves. Recall that ocean waves in any given area and at any given time exist as a spectrum, meaning there are waves with many different characteristics present. To quantify the wave spectrum, a parameter called “significant wave height” (SWH) is used, and all sea state analyses and forecasts make use of this parameter. By definition, there will always be some waves higher than SWH. It is generally accepted that the highest waves seen within a wave spectrum will be about twice the SWH, and these larger waves will show up approximately once every 1,000 waves under normal circumstances. This works out to be once about every three hours or so for deep-water ocean waves.
This provides our first possibility of identifying where and when big waves can occur, and this is simply to look for areas of high SWH on wind/wave charts and double the analyzed or forecast SWH in a given area. This provides the master of a vessel with some knowledge of areas to avoid if the vessel is particularly susceptible to occasional larger waves. With wave forecasts of up to 120 hours available from NOAA’s Ocean Prediction Center, areas of high SWH can be identified, and decisions can be made about routing based on the areas of higher SWH.
An important delineation needs to be made regarding SWH, and that is to determine whether an area of large SWH is occurring, or is expected to occur, within the fetch area that is generating the waves, or if the waves have propagated out of their generation area and are no longer being impacted by the local winds. While within the fetch area, the waves will still be growing until a “fully developed sea” has been reached, providing there is sufficient time. When the Beaufort wind force and fetch length are known, these can be used with tables available in navigation references, such as Bowditch, which will provide the SWH and period of a fully developed sea along with the duration that is required for those parameters to be reached. Once SWH has grown to a fully developed sea, waves will not grow any larger, provided the fetch, length and wind force do not change. The attainment of a fully developed sea requires the fetch area to persist in the same location for the duration given in the reference table. Within a fetch area that features strong winds, the sea state is likely to be dominated by waves generated by the wind in the fetch area, and in most cases the seas will be fairly regular.
Once waves propagate out of a fetch area, they will no longer be affected by the local wind and are termed “swells.” They will no longer grow, and in fact they will begin to subside slowly. However, the periods of the waves will remain constant as they move through the water, and in the case of a fetch area with strong winds, the waves will remain quite prominent and will keep moving until they run into shallow water or land. The propagation speed of the swell is directly related to the period as long as the waves remain in deep water.
When swells move into water less than one half of the wavelength in depth, the character of waves will begin to change as they “feel the bottom.” This process begins as the waves are slowed by friction with the bottom, but the wave periods will remain the same. This means the wave lengths become shorter to keep the time between successive crests the same as the waves slow down, and the waves become steeper, leading to greater SWH. Thus we have identified another situation where big waves can occur, and that is when a large swell set from a distant storm moves into shallow waters, generally over a continental shelf or in coastal waters. A similar effect can occur when swell sets pass over seamounts or other shallow areas like banks. If the waves get steep enough, they break.
Large waves that impacted the California coast in December 2023 can be explained by this phenomenon. Prominent on a surface analysis chart is a hurricane force low, and there is a large fetch area to its south, between about 33N and 40N from about 145W west to about 155W. The corresponding wind/wave analysis indicates a maximum SWH of 38 feet in this fetch area. SWH along most of the central California coast at this time is less than 12 feet. By 1200 UTC 28 Dec 2023, when the hurricane force low became weaker and moved northeast to the eastern Gulf of Alaska, another very strong low has moved into the region with its center near 38N/146W.
Six hours later, the SWH was 26 feet along the central California coast. These waves were generated from the fetch area and propagated east to the coast. There were very few isobars in the California coastal waters at this time, suggesting the winds were relatively light in this area despite the big waves. The 24-hour wave period forecast valid at 1200 UTC 28 Dec 2023 clearly showed the swells moving toward the west coast of the U.S. with very long periods (17-19 seconds). This means these waves were moving fast (more than 50 knots) and had very long wave lengths (more than 1,500 feet). The continental shelf is not very wide along the west coast of North America, so the growth of these waves as they move into shallow water will not be shown very well on the scale of these wind/wave charts. However, they were quite large and in fact caused significant damage along the coast, particularly where wave energy was able to get into smaller bays and harbors.
Other situations where waves greater than SWH are more likely to occur include areas where more than one swell set is present. When swells from two different fetch areas arrive in the same region, the waves will interfere with one another, and this can lead to constructive (or additive) interference between the wave sets, leading to a few larger waves, perhaps even greater than twice the SWH of either swell set. This can occur when swells from two distant storms propagate into the same region. It also occurs within one storm system, particularly around frontal boundaries. Typically, abrupt wind shifts occur around fronts, and this means fetch areas can be adjacent to one another on either side of a front, and the waves from one fetch area can propagate out of one fetch and directly into another, again leading to the possibility of waves much greater than the SWH of either fetch. Constructive interference can also occur when a faster moving swell set overtakes slower swells, even when they are moving in the same direction.
Another situation where waves greater than SWH are more likely to occur is an areas where wind and/or waves are opposing relatively strong ocean currents. This will result in the shortening and steepening of the waves. In currents like the Gulf Stream in the Atlantic and the Agulhas Current east of South Africa, it is generally accepted wave heights can be up to 50 percent greater than expected. When doing this calculation, it is important to remember in areas without strong currents, the extreme wave is generally twice the SWH, so within strong currents, the largest waves can be 50 percent greater than twice the SWH.
In April 2005, a cruise ship was sailing in the Gulf Stream off the Carolina coast in an area where SWH was forecast to be 7.5 meters. Twice the SWH would have been 15 meters, and another 50 percent more would have been 22.5 meters, or nearly 74 feet. This cruise ship encountered a wave of about 70 feet in this area, causing significant damage to the ship and injuries to the passengers. Looking at the chart and seeing expected SWH of 7.5 meters (close to 25 feet) might not suggest a wave greater than 70 feet could occur. But by recalling waves up to twice the SWH can occur, we see a wave could be up to 50 percent greater, a “freak wave.”
A “freak wave,” or “rogue wave,” is often heard when large ocean waves are discussed, and these waves are defined in the Glossary of Meteorology, published by the American Meteorological Society, as an “unexpectedly occurring wave of great height (and also steepness).”
A recent report of a rogue wave occurred at a U.S. military base on Kwajalein Atoll, Marshall Islands, in the tropical latitudes of the western Pacific Ocean. There were several very large waves that swept over the low-lying island in January 2024 and led to a great deal of damage to buildings and other infrastructure. An online search will yield some incredible video of this occurrence. Examining the charts for this event helps us see what contributed to this event.
A surface analysis chart valid at 0000 UTC 20 Jan 2024 for the tropical Pacific shows a large, strong low-pressure system well to the northeast of the atoll. Its center was more than 1,500 miles away at the time of this chart. This system had been present for a couple of days leading up to the time of this chart and had been tracking generally east through the western Pacific. Its large circulation contained several fetch areas, which generated large waves that propagated away from the fetch areas. On the west side of this system, winds would have been coming generally from the north, so the waves generated would have been moving south. This can be seen clearly by examining the wave period forecast chart valid at the same time as the surface analysis. While this chart does not extend as far south as the Kwajalein Atoll, there is a large area of very long period swells shown on the chart moving south to the west of the International Dateline, heading in the direction of Kwajalein.
A wind/wave analysis chart valid at 1200 UTC 20 Jan 2024, 12 hours after the surface analysis chart, shows SWH of around 15 feet in the vicinity of Kwajalein. However, the waves that impacted the base were likely larger than this. There were likely several contributing factors. First, the overall SWH field was large, and waves of up to twice the SWH could have been present. As these waves encountered the shallower water surrounding the atoll, they would have become steeper, though they also likely would have broken. It is also worth noting the winds were moderately strong from the northeast. This would have generated another wave set from that direction, and given that the larger swells generated by the big storm a couple of days earlier were coming from the north, these two wave sets likely interfered with one another, leading to additive effects and the possibility of some very large waves.
To summarize, when assessing the possibility for very big waves, consider the following:
1.
SWH present or forecast? Remember that waves of up to twice SWH will occur at times within the wave spectrum.
2.
Are larger swells propagating into shallower water? This will lead to higher and steeper waves as the waves slow down.
3.
Is there more than one swell set present? Wave period forecast charts show only the dominant wave period, which represents the swell set with the most energy. Look for other upwind fetch areas in different directions or strong winds that will build waves in the local area.
4.
Is the area of interest near a strong ocean current? Expect up to 50 percent greater wave heights than would normally be found. Pay attention to locally strong tidal currents for the same reason. Even though these currents are of short duration, they can still enhance wave activity.
As always, being vigilant about all weather and ocean conditions will make voyaging safer.