Easterly wind - Hentofu

Although it has a northerly or southerly component, it is a wind that blows steadily from east to west along the latitude circle. It prevails in the lower layers of the polar regions and in the tropics, and is called the polar easterly wind and the tropical easterly wind (or trade wind), respectively. In the theory of atmospheric circulation, the tropical easterly wind is often referred to as the tropical easterly wind. Compared to the polar easterly wind, the tropical easterly wind is thicker vertically and reaches a height of 8 to 10 kilometers or more. Just as there are westerly wind waves in the mid-latitude westerly winds, the tropical easterly wind also meanders north and south, and this wave is called the Easterly wave. The part of the westerly wind wave that meanders toward the low latitudes is the trough, but the trough of the Easterly wave is the part that undulates toward the high latitudes. Easterly waves move westward at about 20 kilometers per hour, and on the west side of the valley (the front of the wave), the air diverges, creating a downdraft, which creates a clear area, and then converges on the east side of the valley, creating an updraft, which creates a bad weather area. If this wave becomes unstable, it can become a vortex and grow into a tropical cyclone (typhoon or hurricane).

[Atsushi Kurashima and Takashi Aoki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

南や北の成分をもちながらも、おおむね緯度圏に沿って東から西に定常的に吹く風。極地方の下層と、熱帯に卓越し、それぞれ極偏東風、熱帯偏東風（または貿易風）とよばれる。大気大循環論では単に偏東風といえば、熱帯偏東風をさす場合が多い。極偏東風に比べると、熱帯偏東風は鉛直方向に厚く、高さは8～10キロメートルまたはそれ以上に及ぶ。中緯度偏西風に偏西風波動があるように、熱帯偏東風も南北に蛇行して流れ、この波動はイースタリ・ウェーブ（偏東風波動）とよばれている。偏西風波動は低緯度側に蛇行する部分が谷であるが、イースタリ・ウェーブの谷は高緯度側に波打つ部分にあたる。イースタリ・ウェーブは時速20キロメートル程度で西進し、谷の西側（進行前面）で大気が発散して下降気流がおこり晴天域となるため、谷の近傍と東側で収束し上昇気流がおこって悪天域となる。この波動が不安定化すると、渦巻になって、熱帯低気圧（台風やハリケーン）にまで成長することがある。

［倉嶋　厚・青木　孝］

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