Thanks for your interest. It is my pleasure to answer your questions.
> 1. What is the "Hawaiian wake"?
An island wake is a zone (region) of weak wind behind (on the lee side of ) an isolated island with mountains. The wind is weak there because the montains on the island block the atmospheric flow. Such island wakes are observed very often around the world ocean. In our Science paper, we discovered a very unusual wake west of the Hawaiian Islands.
Near Hawaii, the winds are quite steady, called the northeasterly trade winds since they come from the northeast. There are high moutains with a maximum eleation of 4.2 km that block these trade winds. On the lee side (west of Hawaii), regions of weak winds are observed by NASA's QuikSCAT satellite that measures the wind velocity. What set this Hawaiian wake apart from all other wakes is its extraordinary length--3,000 km, ten times longer than any ever observed elsewhere.
> 2. Why is the wake so long?
We found that the Hawaiian wake has higher sea surface temperature than the surroundings. This high temperature modulates the surface wind and sustains the wind changes for an unusually long distance (3000 km). The high temperature, as it turns out, is a result of an eastward moving ocean current that transports warmer water from the western Pacific. This eastward current is caused by the horizontal wind variations induced by the Hawaiian mountains. In our Science paper, we have described a chain reaction that explains the Hawaiian wind wake, ocean current, its heat transport and effect on water temperature, and the temperature feedback on the wind.
Thus, it is the active interaction between wind, ocean current and temperature that makes this uniquely long wake west of Hawaii. One may ask why this happens only in Hawaii. There are a number of reasons. First, the wind has to be steady to exert a steady forcing on the ocean (which does not react to fast-changing wind). Second, there are high mountains on Hawaii, providing large perturbation to the atmospheric wind. Third, the Hawaiin Islands have a quite large horizontal scale; they extend over four degrees in latitude. >
> 3. What is the importance of the "Hawaiian wake"?
The dicovery of the Hawaiian wake is important for a number of reasons.
a. It is amazing that tiny islands of Hawaii--barely visible in a map of the Pacific Ocean and seemingly insignificant--can leave striking marks on a large ocean like the Pacific. The vast Pacific Ocean is poorly measured until now because only very few ships are out in the sea. We used new satellite observations in our study. The disovery of this far-reaching effect of tiny islands demonstrates a huge potential for observing the oceans from space and for even more surprising discoveries.
b. In fluid physics, people have devised theories of how long a mountain can affect the wind. The maximum wake length is about 500 km in those widely-accepted theories. The Hawaiian wake is six times longer than this theoretical limit, thereby overturning the theories. As stated earlier, the extraordinary length of the Hawaiian wake is due to the strong interaction between the ocean and wind, extra physics not considered in those theories.
c. The eastward ocean current in the far western Pacific south of Japan has been reported by Japanese oceanographers since 1950s, but its physical cause is still in the dark. Our study showed, rather surprisingly, that the cause of a branch of this eastward curernt lies several thousand kilometers away to the east in the Hawaiian Islands, through the wind variations these islands generate. This is an unexpected answer to oceanographers.
d. Scientists do not have a good knowledge of how much wind changes there will be in response to a unit increase in ocean temperature. The strength of ocean-atmosphere interaction revealed in the Hawaiian wake is stronger than scientists have thought. This interaction or coupling strength holds the key to the climate sensitivity, namely how much temperature increase we are going to observe in the global mean temperature as we increase the concentration of carbon dioxide. It is also important for natural climate variations like El Nino.
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Per your request, I am attaching several figures in gif format. All the figures below are based on satellite measurements from space.
Wind velocity vectors and wind speed (color shade) near the Hawaiian Islands.
Far field view of wind velocity vectors and sea surface temperatures over the North Pacific including Hawaii. Note that surface winds converge onto the warm SST band, generating a long band of clouds. Data have been filtered to remove larger-scale background fields.
Surface wind velocity around Hawaii, with the maximum elevation on each major island marked. The inserted picture of a deformed tree is taken at the southern point of Hawaii Island (photo credit: Dr. Tom Schroeder). The tree is bent in the downwind direction by the force of the steady trade winds.
Best regards,
Shang-Ping Xie