- 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
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 below.
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.
Plate 10. Conceptual diagram showing the modulation of sea surface temperature by variations of wind speed in the lee of the islands (graphics by Brooks Bays). The yellow arrows represent intensified winds in the channels, yielding cooler surface temperatures (light blue); in the calm lee, warmer surface temperatures are observed (pink). These variations of wind speed induce divergent and convergent surface currents (horizontal blue arrows), which in turn lift or depress the thermocline (vertical blue arrows), eventually leading to the formation of clockwise (anticyclonic) and counter-clockwise (cyclonic) eddies (gray curved arrows).
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.
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 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 appears to be caused by many such clockwise eddies repeatedly formed near South Point.
The image below 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.
Plate 11. A typical afternoon satellite image of surface temperature west of Hawai'i, from 10 December 1991. The signature of an intense counter-clockwise cold eddy can be seen west of Kona. Source: Satellite Oceanography Laboratory, University of Hawaii. Units: degrees Celsius.
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