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The Mean Current / Tidal Height Plot shows
color plots of velocities versus depth (vertical axis) and time (horizontal axis). A maximum of three days of data are shown. The lower plot shows tidal amplitude at the site.
Velocity profile data is obtained every one to two seconds. The profiler obtains velocities throughout the water column within vertical bins of 10–50 cm heights. The velocity data is averaged over 20 minute windows to obtain the background current profiles (in order to minimize the effects of waves on the average).
The coastline near KN faces roughly SSW and currents are predominantly along-shore (NNW to SSE or vice-versa). The currents at KN are primarily determined by the surface tides, although large amplitude internal waves are forced in Mamala Bay which also affect nearshore currents significantly, leading to a complex relationship between the surface tidal height and the current velocities. The near-surface currents at KN are also occasionally influenced by winds.
The Current Profile plot shows two views of the currents at KN as a function of depth. The upper plot shows 20-minute averaged velocities in the E–W plane. The lower plot shows the current profile viewed from above.
The Water Quality plot summarizes water characteristics at the KN site. More comprehensive variables will be presented in the future as part of a UH Seagrant-funded study to monitor nearshore water quality. The upper plot shows a profile of raw acoustic intensity obtained from the current profiler measured on a scale of 0–255. Acoustic intensity is a measure of the return signal received by the instrument and represents backscatter due to suspended sediments, density gradients, or biota. At KN, density gradients are generally weak, so acoustic intensity reflects sediment loading. Low intensities generally indicate high visibilities and vice-versa. Note that the acoustic intensity normally drops off with distance (as a flashlight beam loses intensity with distance), so it is generally high near the bottom. More directly useful perhaps for divers and snorkelers is the depth-averaged intensity shown in the second plot. Again, low values indicate high visibility. It’s useful to examine the correlations between currents, tides and intensity.
The third plot shows temperature, representing the value at approximately 10 m depth at KN. The lower plot again shows tidal amplitude for reference.
The Directional Wave Spectra plot includes information on swell height, direction and frequency. The axis indicates the direction waves are traveling towards. The colored regions and contours represent wave energy content. The closer the energy is to the center, the lower the frequency (or higher the period). Energy near the center generally represents swell from more distant sources. Energy farther from the center indicates more locally generated wind swell or storm surf. A tighter peak indicates a clean swell from a single source.
KN is subject to a number of swell sources, including southern hemisphere swells (low-frequency, predominantly in the summer months), trade wind swell (high frequency, generally in the NW direction), and Kona-storm swell (high frequency, N or NE direction). The peak swell direction (highest energy) is indicated by the green line. The dominant swell direction (direction with highest energy including all frequencies) is shown in red. Note that these are not necessarily the same! For example, a good south swell might produce a large, narrow peak in the N direction (green), but still be overwhelmed in total energy by a Kona-storm wind swell traveling towards the NE (red).
Wave height, period and direction for the previous three days are shown in the Wave Characteristics plot. The top plot shows significant wave height determined by two techniques. Significant wave height is a representative swell height, generally described as the average of the largest 1/3 waves. Wave heights are determined using a zero-crossing analysis which identifies individual waves using the point at which the sea surface begins to move downward or upward due to a passing wave. The spectral method for wave height estimates the significant wave height by summing the energy within each frequency band in the spectrum. The two methods should be roughly equal, although varying wave conditions affect the estimation algorithms differently, so that the estimates may not agree.
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