Oahu Surf Climatology

This is an educational outreach product intended for a general audience. Feedback is welcome.

Average surf conditions provide a benchmark for judging day-to-day variations. Statistics based on visual surf observations and buoy measurements have been calculated for the north and south shores of Oahu. For the east side, buoy data are used to define the average trade windswell characteristics. The west side of Oahu receives swell from sources that influence both the north and the south shores-- or a combination of both. Quick link to primary products.

Winter surf, North Shore, Oahu. Photo: Steve Businger, Dept. of Atmospheric Sciences, University of Hawaii

Data Sources

North Shore, Oahu

Weather makes the wind; wind makes the waves. The north to west exposures of Oahu receive swell generated primarily in the mid latitudes of the North Pacific, and on rare occasions from tropical sources. Ocean surface winds are strongest near intense surface low pressure areas, which reach a maximum of intensity in the northern hemisphere winter. Subsequently, the North Shore annual cycle of surf follows suit. The large range between the minimum, average, and maximum on any given day draws attention to the need for up-to-date, accurate surf forecasts for safe planning.

In the general discussion of the NOAA Oahu Collaborative Surf Forecast, the surf for a given forecast period is compared to averages for context. North Shore surf statistics are defined for select time periods. The mean is the most common statistic to denote average conditions. The median and mode are other ways to define the most common heights that occur over a given period. These parameters tend to be lower than the mean for North Shore surf, which leans higher due to the influence in the calculation of the mean from select extreme days being well beyond 2 standard deviations above the mean. The difference between the median or mode and the mean gets larger for peak season. Most commonly in the Collaborative Forecast, the heights for a given day are referred to the average of the active season (September to May), which if averaged over the mean, median and mode is nominally 12' peak face for the H1/10 breakers in zones of high refraction.

The standard deviation (St. Dev.) denotes how much variation from the mean can occur during the given time period. As seen in the annual cycle between the maximum and minimum for any calendar day, the variability in range of heights is greatest during the peak month (January) and season (winter). This helps fine tune the interpretation of the active season average (September to May), which is 12' +/- 8' peak face. In other words, during the active season, the heights fall mostly within 4-20' peak face for the H1/10 breakers.

The Waimea Buoy provides an additional perspective on defining the averages. Monthly means of significant wave height (Hs) and peak wave period (Tp) show an annual cycle centered on winter months. This calculation was performed on all available data using the "Averages: monthly" option in the CDIP Summaries: Min Max Mean. The average Hs and Tp over the active season (September to May) are 6' at 12 second intervals.

From the Hs and Tp, estimates of surf heights, referred to as the surf compute, can be made based on an empirical method. This is the same formula used daily in the Collaborative Surf Forecast, which predicts a deep water Hs and Tp, from which breaker heights are computed.

The mean H1/10 breaker heights computed from the monthly mean Hs and Tp compare well with the independently calculated monthly means from the visual surf observations (Goddard-Caldwell Database). The buoy surf compute is slightly lower than the visual observations for select months. This is due to the influence of shorter period wind waves (trades and konas) that are included in the monthly mean Tp calculation, giving a low bias to Tp compared to if the wind waves were filtered out. For the active season average, the buoy average (6'@12s) results in a surf compute 11' peak face, which also has a slight low bias relative to the estimate from the visual observations, for reasons described above.

As an additional test, the monthly means of the visual observations were performed over differing time periods. It shows very little deviation, giving confidence to the statistics.

Wave direction from the Goddard-Caldwell data shows how the dominant direction varies through the active season (September to May). More northerly components are most common during fall and spring. During winter, the west to northwest components happen more often. This is related to the jet stream track, which shifts equatorward in winter. In turn, the lower-latitude, surface low pressure systems aim more westerly swell component at Hawaii.

South Shore, Oahu

Southerly exposures of Oahu typically receive swell that is generated primarily in the mid latitudes of the South Pacific. On rare occasions, swell reaches Oahu from north Pacific tropical cyclones south of Hawaii or from more distant tropical cyclones in the southern hemisphere. The mid latitudes have the strongest, broadest surface low pressure systems within fall through spring of the respective hemisphere, which centers the South Shore annual cycle of surf around the austral winter, or summer in Hawaii. Surf is much lower relative to the North Shore, because the South Shore sources are remote. The southerly swell loses size as it traverses the vast distance beyond 3000 nm to Hawaii.

South Shore surf statistics show June as the peak month and place spring closely behind summer for the peak season. The southerly surf maximum in late spring to early summer is likely related to less Antarctic ice sheet during this time (austral fall to early winter), which allows more open ocean for wave generation.

The median and mode share insight into most common conditions. Most days during the active season (March to October) were reported in H1/10 as either 2 or 3 Hawaii scale, or 4' or 6' after converting to peak face. The coarse resolution hinders accuracy. The mean is nearer to the median and mode for the South Shore relative to the North Shore, since the percentage of days of large events is less on the South Shore.

In the Collaborative Forecast discussion, heights on southerly exposures are often referred to the average of the active season (March to October). Averaging over the mean, median and mode gives 5.5' peak face H1/10 breakers. Keep in mind this refers to high refractive zones, such as Ala Moana and Diamond Head. Elsewhere, the average for less refractive zones, such as the Waikiki area, would be lower (~ 4').

The standard deviation for the South Shore varies much less than the North Shore, since the range of breakers is much lower on the South Shore. The active season (March to October) can better be described as most commonly 5.5' +/- 2.3', in other words, most of the time H1/10 surf heights fall within 3'-8' peak face.

Since the available PacIOOS/CDIP nearshore buoys off the southern shores of the Hawaiian islands can receive swell from north Pacific sources, and the magnitude of the southerly swell is typically low, a similar estimate of monthly mean Hs and Tp as done for the Waimea Buoy off the North Shore was not calculated for southerly surf.

Boreal winter months (December to March) provide infrequent surf for south shores from north Pacific sources. Extra-tropical cyclones of lower latitude in the western Pacific can generate westerly-component, long-period swell (Hs~2-4', Tp~14-20s, wave direction less than 280 degrees). H1/10 surf heights at exposed locations typically range from ~5' to as high as 12' for peak face value. In addition, Kona weather patterns occur within fall through spring when sharp upper atmospheric troughs or cut off lows and their associated surface low pressure features unfold to the immediate west of Hawaii. The Kona winds (from within SE to W) generate windswell (Hs~6-12', Tp~6-10s) with rough, choppy H1/10 breaker heights within 4-10'. There is great interannual variability in both of these winter sources.

Trade Windswell

Surf observations in the Goddard-Caldwell Database for Windward Oahu began in 1987. North Beach was used into the 1990s, then a combination of Makapuu or a "windward index" based on Caldwell observations in the Kailua-Kaneohe area, which would include windswell as well as north-component longer period swell. Ideally, only Makapuu would be the best choice for an exclusive trade windswell breaker estimate. However, it is not digitally available. Thus, available wind and buoy data are studied.

Mean winds provided by the NOAA/NESDIS National Centers for Environmental Information from land-based observations in Hawaii show variability from place to place due to local topography. The mean wind at Kaneohe Bay MCAS is biased low due to shadowing by Mokapu crater. A more optimal location would be wind measurements offshore. Buoy 51001 is too far northwest and buoy 51002 and 51004 are too far south and southeast, respectively. The best located buoy, NOAA Buoy 51026 was positioned north of Molokai from 1993 to 1996. However, the short time-series limits statistical confidence. The buoy 51026 monthly wind speed statistics reflect a complicated annual cycle with three maximas (Nov-Dec, Apr, Jun-Jul). The annual mean wind speed is ~14 knots (16 mph). The agreement with the Kahului, Maui wind averages increases confidence in this statistic. Such a moderate breeze would make seas of 5 feet with 6 second peak wave periods, and in turn, breakers from windswell of H1/10 peak face ~3 feet. This estimate for a mean breaker height from trade windswell seems low based on Caldwell's 30 years of wind- and kite-surfing in the Kailua area surf and cross-comparisons with nearshore buoy readings.

A better estimate can be made from the offshore buoy data. Buoy 51026 is not used due to exposure to north-component swell. The Mokapu buoy off Kailua, Oahu has less north swell exposure and is better suited for analysis of the trade windswell. From the 30-minute samples over a 16-year period (2000-2016), these data were filtered to remove the long period north Pacific swell and the windswell beyond the range of the trades. Only readings within peak wave direction 30 to 100 degrees and peak wave period less than 13 second were analyzed. No effort was made to remove any tropical cyclone swell of wave periods less than 13 seconds, since such sources rarely exceed levels generated by strong to near gale trades. The Mokapu buoy trade windswell sample statistics give a mean annual significant wave height (Hs) = 6.4 feet at peak wave period (Tp) of 8.3 seconds. This is larger than one would expect given the mean local wind. The waves are higher and longer since the fetch area to the east to northeast of Hawaii is vast, meaning windswell is propogating into the area, not just being generated locally.

The empirical method, or surf compute, was created matching long-period swell at the Waimea Buoy to the North Shore visual surf observations. The method is unrealistic for wave periods less than ~10 seconds. Based on Caldwell's unofficial comparison under trade windswell conditions between buoy readings and breaker size for high refraction reefs in the Kailua-Kaneohe area, Hs=6.4' at Tp=8.4 seconds should give an H1/10 breaker of approximately 5 feet, which is the assumed trade windswell average surf height.

Associated Links

Surf Research NOAA/NESDIS/NCEI Hawaii Liaison/Pacific Region Science Officer

Aucan Directional Wave Climatology Hawaii

Wave Climate SOEST Coastal Hazards Analysis Report, Coastal Geology Group, University of Hawaii

Hawaii Wave Climatology from Hindcast Department of Ocean and Resources Engineering, University of Hawaii

NOAA/NESDIS National Centers for Environmental Information (NCEI) Historic Weather, Climate and Oceanographic Data

Central North Pacific Climatology Overview NOAA/NESDIS/NCEI

Wind-wave Climate of the Pacific Ocean CSIRO, Australia

Questions or comments: Mr. Patrick Caldwell, patrick.caldwell@noaa.gov

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