The Ocean Atlas of Hawai'i
The Ocean Atlas of Hawai'i
Text: P. Flament+, S. Kennan+, R. Lumpkin+, M. Sawyer+, E. D. Stroup+ Data compilation: K. Bigelow#, P. Caldwell#, J. Deshayes, J. Firing+, B. Kilonsky+, C. Motell+, J. Potemra+, F. Santiago-Mandujano+, M. Seki# + School of Ocean and Earth Science and Technology University of Hawai'i # National Oceanographic and Atmospheric Administration Copyright by P. Flament (1996)
This is a description of the ocean around Hawai'i - marine climate, water properties, currents, tides, waves. We are hoping to provide a primary reference for the general public - whether your interest is surfing, sport fishing, yachting, swimming, or simple curiosity, and for the professionals - fishermen, beach guards, civil defense, search and rescue, and ship operators. Please help us by sending your comments: what you liked, what you did not like, what you wish we had explained better, what we forgot to talk about.
Click on the plate numbers anchored in the text, or on the small gif images at the end of each paragraph, to display full size gif images of the plates . Or click here for a list of the plates.
Table of contents
Introduction
Water motions occur over a wide range of time and space scales. At the largest scales are basin-wide currents, the average ocean circulation, slow interannual changes such as the El Nino-Southern Oscillation (ENSO), and seasonal variations. At intermediate scales (weeks to months and tens to hundreds of kilometers) are ocean eddies and fronts. Tides have periods from a few hours to a day. Surface waves occur at periods of seconds to minutes. Finally, at the smallest scales of centimeters and seconds, is ocean turbulence - small eddies that eventually mix water properties, much like stirring coffee in a cup.
All other water motions, except tides and tsunamis, result directly or indirectly from interactions with the atmosphere at the surface, through the horizontal force of the wind, heating or cooling by the air and by radiation, and precipitation and evaporation. Global variations of these processes determine the large scale ocean circulation; local variations shape regional characteristics.
Marine climate of the central Pacific
Northeasterly trade winds dominate over the Hawaiian islands chain and further south, while westerlies are found further north. This circulation follows the North Pacific anticyclone, which shifts northward in summer, when trade winds intensify and reach on average 35 N, and southward in winter, when westerlies extend as far south as 28 N (Plates 1 and 2). The annual average trade wind speed is 6 m/s (13.5 mph) at 20 N. Evaporation exceeds precipitation between 15 N and 36 N (except near the islands); further south and north, precipitation exceeds evaporation. There is a net cooling of the ocean by the atmosphere over the entire region.
Surface water temperature and salinity
Surface water temperatures have a strong north to south gradient, and a small annual cycle, being lowest around March 15, and highest around September 15 (Plate 3). The average surface water temperature around O'ahu is 24 C (75 F) in winter and 27 C (81 F) in summer. The variations of temperature tend to parallel the island chain, i.e. surface waters are in general warmer to the west at a given latitude.
Surface salinities reflect the balance between precipitation and evaporation. The average salinity is maximum (35.2 ppt) at 26 N, and decreases southward to 34.3 ppt at 10 N, precipitation being stronger in the vicinity of the Inter-Tropical Convergence Zone (Plate 4).
Vertical profiles of water properties
The average vertical profiles of temperature, salinity, and nutrients (nitrate+nitrite) were computed from series of monthly surface-to-bottom measurements made at 22 45 N 158 W (Plates 5 and 6). Near the surface, the water column is mixed by the wind and has uniform properties; the depth of this turbulent layer varies from nearly 120 m (400 ft) in winter to less than 30 m (100 ft) in summer. Below the mixed layer there is a sharp decrease in temperature (called a thermocline), from 25 C (77 F) at the surface to 5 C (41 F) at 700 m (2300 ft) depth, then a gradual decrease to 1.5 C (36 F) at the bottom.
The salinity distribution reflects the sinking of water from the north: higher salinity water of 35.2 ppt at 150 m (500 ft) depth, traceable to the high surface salinity water north of Hawai'i (see Plate 4); low salinity water of 34.1 ppt at 500 m (1670 ft) depth, traceable to low surface salinity water further to the north west. Below this depth, salinity increases gradually to 34.7 ppt for abyssal waters. The concentration of nutrients (nitrate NO3 and nitrite NO2) is small at the surface (less than 1 umole/kg), but increases steadily to the bottom, reaching 40 umole/kg or more; similar vertical distributions are found for phosphate and silicate.
The distribution of nutrients illustrates how small vertical motions generally are in the ocean. The upper 100 m (330 ft) or so is illuminated by sunlight (the euphotic layer), but lack of nutrients limits growth of phytoplankton (microscopic plants), much like lack of fertilizers would limit growth of vegetation over land. Deeper in the water, nutrients are abundant, but there is no light to support photosynthesis.
Where and when upward vertical motions exist, nutrients are brought into the euphotic layer, resulting in increased biological productivity. Upward nutrient transport occurs when strong winds increase the turbulence in the mixed layer (for example, in winter, or during a storm or hurricane), or when surface currents diverge, causing upwelling of deeper water.
Large scale currents
The depth of the 10 C (50 F) isotherm (roughly in the middle of the thermocline) varies markedly, from more than 450 m (1500 ft) northwest of the islands, to less than 240 m (800 ft) to the northeast and southeast (Plate 7). Variations of temperature result in variations of density, and therefore of weight and pressure of water columns. Ocean currents are set up by such variations of pressure. Owing to the earth rotation, these so-called geostrophic currents follow lines of constant isotherm depth.
The average currents form a large (basin-scale clockwise circulation (called gyre) centered at about 32 N. At the latitude of Hawai'i, the circulation is roughly from east to west and intensifies southward. The strength of geostrophic currents decreases with depth, by a factor of two every few hundred meters; below 1000 m (3300 ft), their average is generally less than 5 cm/s (0.1 knot), and their patterns are not entirely known.
In the surface layer, however, currents driven by the wind combine with geostrophic currents to yield more complicated flow patterns. Surface currents must be measured directly, or computed from the drift of ships or oceanographic buoys (Plate 8).
South of Hawai'i, the surface North Equatorial Current (NEC) reaches an average westward speed of 17 cm/s (0.35 knot) at 13 N, and gradually decreases towards the islands. Between 18 N and 22 N, the currents are strongly influenced by the islands. The NEC forks at Hawai'i island; the northern branch becomes the North Hawaiian Ridge Current (NHRC), and intensifies near the islands with a typical width of 100 km (54 NM) and speed of 25 cm/s (0.5 knot). West of the islands, two elongated circulations appear. A clockwise circulation is centered at 19 N, merging to the south with the southern branch of the NEC. A counter-clockwise circulation is centered at 20 30 N. Between them is the narrow Hawaiian Lee Counter Current (HLCC), extending in longitude from 170 W to 158 W. Surface currents over the western islands and north east of the NHRC are variable and their average is smaller than can be estimated from existing data.
The current arrow shown at each grid point is the average of all observations falling within one degree of that point, and is thus the most probable current that one would experience. However, because ocean currents vary in time, freely drifting objects would never follow exactly the flow patterns in Plate 8. The map of current variability, a measure of how much actual currents would likely depart from the average, indicates that the lee of the islands is populated by vigorous eddies or swirls, which obliterate the slower average circulation (Plate 9).
Regional currents
The island chain affects the ocean by two important mechanisms: interactions of the islands with the large scale ocean currents, and wind speed variations in the lee of the islands, as sketched in plate 10.
At the northern and southern boundaries of each island, the trade winds with speeds of 10-20 m/s (22-44 mph) are separated from the calmer lee by narrow wind shear lines. The northern shear line of Hawai'i island, bordering rough seas in the Alenuihaha Channel, is a spectacular sight for passengers flying to Kona. Locally, the depth of the surface mixed layer depends on wind speed: in the channels, deep mixed layers are observed; in the lee, stirring by the wind is not sufficient to mix down solar heating and intense day time warming of the ocean surface results. Sharp surface temperature changes (called fronts), sometimes reaching a difference of 4 C (7 F), are often associated with these wind shear lines.
Variations of wind have subtle effects on current patterns. In the northern hemisphere, when wind blows for many days over a surface mixed layer, the water moves to the right of the wind, due to the earth's rotation. Therefore, water moves away from the northern shear line; to compensate for this divergent surface motion, water moves up (upwells) from greater depths, appearing as a cold spot at the surface. Similarly, water moves towards the southern shear line, resulting in a deepening of the thermocline there.
Geostrophic currents result from these variations of thermocline depth, in the form of intense counter-clockwise eddies under northern shear lines, and (somewhat less intense) clockwise eddies under southern shear lines (Plate 10). This process is quite dramatic: the depth of the mixed layer in the lee of the island of Hawai'i can vary from less than 20 m (65 ft) in the counter-clockwise eddy, to more than 120 m (400 ft) in the clockwise eddy. The large counter-clockwise average circulation (Plate 8) is believed to result from the repeated occurrence of eddies spun up by the shear lines of the islands of Maui and Hawai'i.
Eddies can also be generated by intense currents such as the NEC impinging on the islands, much like swirls found in a swift river downstream of a bridge pile. The large clockwise circulation south west of the Hawai'i island (Plate 8) appears to be caused by many such clockwise eddies repeatedly formed near South Point.
Plate 11 shows a typical satellite image of sea surface temperature in the lee of Hawai'i, with a large cold eddy to the west of Kona. However, because the generation of eddies is essentially random, this example is not a permanent representation of the location of eddies and fronts.
Tides and other oscillations
Forced like clockwork by the moon and the sun, the tides are the most predictable oceanic motions. The gravitational pull of the moon (and to a lesser extent of the sun) creates "bulges" of water on opposite sides of the earth. A point on the earth passes through these bulges twice a day, resulting in semi-diurnal (half daily) components to the tide. Because the moon and the sun do not generally lie over the equator, one of the bulges is larger than the other, leading to diurnal (daily) components to the tides.
A modulation of the tidal range results from the relative position of the moon and the sun: when the moon is new or full, the moon and the sun act together to produce larger "spring" tides; when the moon is in its first or last quarter, smaller "neap" tides occur. The cycle of spring to neap tides and back is half the 27-day period of the moon's revolution around the earth, and is known as the fortnightly cycle. The combination of diurnal, semi-diurnal and fortnightly cycles dominates variations in sea level throughout the islands, as illustrated by tidal curves for Honolulu and Hilo (Plate 12).
On scales of oceanic basins, tides exist as very long waves propagating in patterns determined by their period and the geometry of the basin. Plate 13 shows the response of the North Pacific to the tidal period of 23 h 56 min, the largest diurnal component. Lines along which high tide occurs at the same time (called phase lines), converge to a point west of Hawai'i where the tidal range is zero (called amphidrome). Phase lines rotate counter-clockwise around this amphidrome, so that the offshore diurnal tide reaches the Hawai'i island first, then sweeps across Maui, O'ahu and finally Kauai.
Local bathymetry affects the ranges and phases of the tides along the shore, as the tidal waves wrap around the islands. For example, high tide at Haleiwa on the north shore of O'ahu occurs over an hour before high tide at Honolulu Harbor. The ranges and phases of the semi-diurnal component of 12 h 25 min period (called M2), and of the diurnal component of 23 h 56 min period (called K1) at several coastal sea level stations are given in the table below:
| Harbor | M2 range | M2 phase * | K1 range | K1 phase * |
|---|---|---|---|---|
| Hilo | 43.6 cm (17.2") | -1h 2m | 34.8 cm (13.7") | +0h 18m |
| Kawaihae | 39.4 cm (15.5") | -0h 11m | 32.6 cm (12.8") | -0h 34m |
| Honolulu | 33.0 cm (13.0") | 0 | 31.4 cm (12.4") | 0 |
| Kaneohe | 31.0 cm (12.2") | -1h 28m | 37.0 cm (14.6") | +1h 19m |
| Kahului | 33.0 cm (13.0") | -1h 40m | 35.4 cm (13.9") | +0h 15m |
| Nawiliwili | 29.8 cm (11.7") | -0h 28m | 31.6 cm (12.4") | +0h 6m |
| Port Allen | 29.8 cm (11.7") | -0h 36m | 31.8 cm (12.5") | -0h 31m |
* phase given in hours and minutes before (-) or after (+) Honolulu harbor
Tidal currents result from tidal variations of sea level, and near shore are often stronger than the large scale circulation. Current meter records collected off O'ahu, Maui and Hawai'i (Plate 14) show that semi-diurnal and diurnal tidal currents tend to be aligned with the shoreline. Due to high variability of tidal currents around the islands, however, this statistical picture may not correspond to the flow at a particular time: tidal currents cannot be predicted as precisely as sea level. Strong swirls often result from tidal currents flowing around points and headlands, and present hazards to divers.
Variations of sea level and currents at periods of 1.5 to 3 days are also observed around the Hawaiian islands. Although they manifest themselves as oscillations, just like tides, they are not forced by gravitation, but by time-varying winds and possibly swells. They displace the sea surface by only a few centimeters, but the depth of isotherms by tens of meters. Such oscillations, usually occurring during the winter, may be associated with currents up to 50 cm/s (1 knot), and horizontal water displacements of 8 km (5 miles).
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 (Plates 15 - 17). 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.
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.
Approximate conversion factors
Current speeds: 10-50 cm/s = 0.2-1 knots = 0.36-1.8 km/h = 0.22-1.1 mph Depths: 10-100-1000 m = 30-300-3000 feet = 5-50-500 fathoms Distances: 100 km = 62 miles = 54 nautical miles (NM) Pressure: 1000 mbar = 14.5 psi = 29.5 inches mercury Temperatures: 5-15-25 degrees Celsius = 41-59-77 degrees Farenheit Wave heights: 0.3-1-3-10 m = 1-3-10-30 feet Wind speeds: 5-10 m/s = 10-20 knots = 18-36 km/h = 11-22 mph
On-line references
Additional on-line references, plots, maps and data relevant to the meteorology and oceanography of the central Pacific is available on the World-Wide-Web. We list here the URLs of major servers of interest; further pointers can be found in those servers:
//lumahai.soest.hawaii.edu/ current Hawai'i weather //www.eis.noaa.gov/ NOAA environmental information services //gcmd.gsfc.nasa.gov/ NASA global change directory //topex-www.jpl.nasa.gov/ satellite surface height, wind, waves //podaac-www.jpl.nasa.gov/ sea surface temperature archives //gri.sourceforge.net tools for displaying oceanographic data
Plates
Click on the plate numbers anchored in the text, on the thumbnail gif images at the end of each paragraph, or on the list below to display full size gif images of the plates
. Or click here to download EPS files of the plates at full resolution.Last revision: Wed Sep 9 12:40:19 HST 1998