June 15, 2001
An international team of scientists from the US, China and Japan, including Drs. Shang-Ping Xie and Masami Nonaka of the International Pacific Research Center (IPRC), has discovered a long shadow cast by the Hawaiian Islands in the wind and cloud fields that extends westward for several thousand kilometers (Fig. 1 and 2). In this island shadow, the northeasterly trade winds are weaker and a cloud band forms. They made this discovery by analyzing satellite data.
The IPRC is a joint research unit co-managed by the Frontier Research System for Global Change (FRSGC) and the University of Hawaii (UH), and FRSGC is a joint project of the National Space Development Agency (NASDA) of Japan and the Japan Marine Science and Technology Center (JAMSTEC).
Using a state-of-the-art ocean model, the researchers have also inferred the influence of Hawaii on the ocean, which includes an eastward current flowing from the Asian coast to Hawaii for an astonishing distance of 8,000 km. This eastward Subtropical Countercurrent is a long-standing mystery of oceanography, and now these researchers attributed its cause to the wind-shielding effects of high mountains far away on the Hawaiian Islands (Fig. 3). These results were published in the June 15, 2001 issue of Science.
1. Previous studies
1.1. Wind wake
Elevated islands block the airflow, forming a zone of weak wind on the leeward side called wind wakes. Previous observational and theoretical studies found that the wind-shielding effect of islands can extend only up to several hundred kilometers (Fig. 4).
1.2. Subtropical countercurrent
In the late 1960s, Michitaka Uda and Keiichi Hasunuma discovered the Subtropical Countercurrent which flows eastward in the subtropical Pacific to the south of Japan and to the east of Taiwan. Mechanism of the current, however, remains poorly understood.
2. New results
By analyzing TRMM (Tropical Rainfall Measuring Mission) satellite data, Drs. Xie, Nonaka and their colleagues have discovered that the wind wake of the Hawaiian Islands lasts for an extraordinarily long distance of 3,000 km, many times longer than observed anywhere else on Earth.
The effect on the ocean is also clear, causing a narrow eastward current that extends from Asia towards Hawaii for 8,000 km, a distance between Mt Fuji and Mongolia. (Fig. 5). This oceanic wake of the Hawaiian Islands is successfully reproduced by physical model simulations.
This discovery of the long shadows cast by the Hawaiian Islands on the atmosphere and ocean overturns previous established theories. It provides clues to how strong the atmosphere and the ocean interacts, with important implications for climate research.
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3. Mechanisms for the long wake
The long wake of the Hawaiian Islands forms by the following mechanisms.
(a) The easterly trade winds blow almost year-round near the Hawaiian Islands, and high mountains like Mt Mauna Loa (4,169 m) obstruct these trade winds. As a result, a zone of weak wind develops on the lee side of the island and strong winds are found on the flanks, producing a strong contrast in wind speed. (Fig. 6-1)
(b) The meridional variations in the trade wind speed cause the surface ocean current to converge in the southern portion of the wind wake and to diverge in the northern portion, causing a high- and a low-pressure anomaly, respectively. (Fig. 6-2)
(c) With such high- and low-pressure regions in the ocean, an eastward current is generated in between, with a width of about 100 km. (In the Northern Hemisphere, a person facing the direction of the flow sees high pressure to his/her right and low pressure to the left. The best example of such geostrophic flow is the counter-clockwise flow around a low-pressure system on weather maps.) The wind-driven eastward current carries relatively warm water of the western Pacific Ocean towards the east, a narrow belt of warmer water from the western Pacific south of Japan directing toward Hawaii. (Fig. 6-3)
(d) The air above the warm ocean belt is warmed, creating ascending motion and a band of cloud. At the sea surface, wind flows from the low-temperature into the high-temperature area. With the influence of the Coriolis force, the wind blowing into the warm belt from the north is turned westward and the wind from the south is turned eastward, sustaining a meridional wind contrast mentioned above (Fig.6-4).
(e) The trade winds exist almost all year round near Hawaii. Therefore, the above air-sea interaction causes the continuous development of a system of wind contrast and eastward current for a distance that far exceeds the limit based on theories that assume a uniform ocean.
4. Additional information
In March 1997, the then Japanese Prime Minister, Ryutaro Hashimoto, and US Vice-President, Al Gore, held a meeting on the "Common Agenda for Cooperation on Global Perspective" (Common Agenda) and agreed on the importance of bilateral cooperation in promoting research in the fields of global change and its prediction. The International Pacific Research Center (IPRC, Director: Julian McCreary) is a research institute that was established at the University of Hawaii, based on this Japan-US Common Agenda. The Center is jointly operated by the Frontier Research System for Global Change (FRSGC) and the University of Hawaii. Since October 1997, the IPRC has been conducting research on climate variations in the Asia-Pacific region.
Several eastward flowing currents have been found between 19°N to 25°N in the western Pacific Ocean south of Japan. As the currents are present in the part of the Subtropical Gyre where westward currents prevail, they have been collectively named the Subtropical Countercurrent. (Fig. 7)
The Subtropical Countercurrent was discovered by Michitaka Uda and Keiichi Hasunuma and published in the Journal of the Oceanographical Society of Japan in 1969.
Several mechanisms have been proposed for the Subtropical Countercurrent, but lack of adequate observations have so far prevented them from being confirmed. Drs. Xie and Nonaka's study concerns only the southern branch of the Subtropical Countercurrent near 19°N, revealing an expected driving force for it: mountains thousands kilometers away on Hawaii.
4.3.Tropical Rainfall Measuring Mission (TRMM) (Fig. 8)
The Tropical Rainfall Measuring Mission (TRMM) satellite was launched on November 28, 1997. The satellite orbits at an altitude of 350 km with the aim of gathering data regarding the distribution and intensity of precipitation in the tropical region, which is essential for elucidating climate change on a global scale and monitoring changes in the environment. The plan for the satellite was initiated as a US-Japan joint project in 1986. The National Space Development Agency (NASDA) was responsible for launching the H-II Rocket and developing the Precipitation Radar, which is the key instrument of the satellite. The USA (NASA) developed the satellite body, all the other monitoring instruments, and the satellite operating system that uses a data relay satellite. After being launched from NASDA's Tanegashima Space Center, the TRMM satellite has been operated via the Tracking Data Relay Satellite System (TDRSS) from NASAs Goddard Space Flight Center.
4.4.Areas for Further Research
The discovery of this astounding phenomenon--small islands of Hawaii exert an influence over half the Pacific Ocean--was made possible by new satellite technology that enables regular high-resolution observations over the remote Pacific Ocean, where ocean-atmospheric measurements had to rely on a small number of ships. This research suggests an interaction of the atmosphere and ocean that is stronger than previously thought; a change in sea surface temperature as small as a few tenths of degree in the wake of Hawaii can significantly alter both the wind speed and direction.
By conducting further model experiments and analyses of satellite data, the researchers plan to investigate further the ocean-atmosphere interaction triggered by Hawaii. A better understanding of this interaction can eventually contribute to improved climate models and their predictions.