Subtropical - Anetta

Japanese: 亜熱帯 - あねったい

The northern and southern hemispheres of the Earth are roughly divided into three zones: tropical, temperate, and cold. The transitional region between the tropical and temperate zones is called the subtropics. It has almost the same meaning as the warm temperate zone. This name is not included in the Köppen climate classification, but it is defined as a range of 4 to 11 months with an average temperature of 20°C or higher, and 1 to 8 months with a temperature of less than 20°C. According to this definition, about 60% (50 locations) of the 80 locations in Japan listed in the "Nenpyo (Annual Science Table)" (compiled by the National Astronomical Observatory) are subtropical. As such, it is not necessarily a clear regional division in climatology, and it is difficult to specify a strict geographical range, so it is considered to be an area influenced by the mid-latitude high pressure zone (subtropical high pressure zone) centered around 25 to 30 degrees latitude. In terms of the global wind system, it is located between the westerly winds heading toward high latitudes and the trade winds heading toward the equator, and is a so-called divergence zone, where descending air currents occur to compensate for this and high pressure develops. As a result, arid regions are widely distributed, with steppe and desert climates. In the Northern Hemisphere, desert regions include the Sahara, Arabia, Pakistan, the plains of Central and Southern China, and southern North America, while in the Southern Hemisphere, the Kalahari Desert in Africa and the Great Desert of Australia occupy one-quarter of the total land area. It is the hottest region in the world, with more than four months in which the temperature exceeds 20°C, but unlike the tropics, it has rather cold winters. Forests are only scattered in the subtropical monsoon region, and they occupy a small area. In terms of flora alone, the subtropical region in East Asia is limited to the area south of the southern tip of Yakushima, including Taiwan.

In addition, the jet stream that flows in the mid-low latitudes is called the subtropical jet, but this does not necessarily correspond to the subtropical climate. Also, among tropical lakes, lakes with large annual temperature ranges and vertical changes are called subtropical lakes. Regur soil in India, Tyre in Morocco, and prairie soil in southern North America are all examples of subtropical soils.

[Yoshitaka Fukuoka]

Vegetation

In humid areas such as Yunnan, China, tropical rainforests are followed to the north by forests of Chestnut Oak and other evergreen oak species, which show the same trend as in northern Taiwan, and the species that make up the forests are essentially the same as those in Japan's evergreen broadleaf forests. In East Asia, the evergreen broadleaf forest zone corresponds to the subtropics. The sclerophyll forest zone in the Mediterranean and California, and the evergreen broadleaf forest zone in Florida also correspond to the subtropics, and there are vegetation zones in the Southern Hemisphere that roughly correspond to these.

[Tatsuyuki Ohba]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地球の北半球も南半球も、熱帯、温帯、寒帯の三つに大別されるが、このうち、熱帯と温帯の中間にある漸移的な地域を亜熱帯と称する。暖温帯とほぼ同じ意味である。この名称はケッペンの気候区分にはないが、平均気温20℃以上の月が4～11か月まで、また20℃以下の月が1～8か月の範囲とされている。この定義でいうと、『理科年表』（国立天文台編）における日本国内80地点中約60％（50地点）が亜熱帯に該当する。このように、気候学的にはかならずしも明確な地域区分ではなく、厳密に地理的範囲を示すことはむずかしいことから、緯度25～30度あたりに中心をもつ中緯度高圧帯（亜熱帯高圧帯）の影響を受ける地域とされている。世界の風系からいえば、高緯度へ向かう偏西風と赤道へ向かう貿易風の間に位置し、いわゆる発散域で、これを補うように下降気流が生じ高気圧が発達する。そのため、広く乾燥地域が分布し、ステップ気候（草原気候）や砂漠気候がみられる。北半球ではサハラ、アラビア、パキスタン、中国の華中・華南の平原、北米南部などの砂漠地域、南半球ではアフリカのカラハリ砂漠、オーストラリアの大砂漠などがあり、全陸地の4分の1を占めている。世界でもっとも高温な地域で、気温が20℃を超える月が4か月以上あるが、熱帯と違って、やや寒冷な冬を有する。森林は亜熱帯季節風（亜熱帯モンスーン）地域に点在するだけで、その占める面積は少ない。植物相だけからいえば、東アジアにおける亜熱帯地域は、屋久島(やくしま)南端あたりより南の台湾を含めた地域に限られる。

　なお、ジェット気流のうち、中低緯度あたりのものを亜熱帯ジェットというが、かならずしも亜熱帯気候と地域的に一致していない。また、熱帯湖のうち、水温年較差や垂直方向の変化が大きい湖を亜熱帯湖と称する。インドのレグール土や、モロッコのティルス、北米南部のプレーリー土などを亜熱帯土という。

［福岡義隆］

植生

多湿な中国の雲南などでは熱帯降雨林に引き続いてその北方に現れるのはクリガシ属、常緑のカシ類などの林で、台湾の北部などと同じ傾向を示し、森林の構成種属は本質的に日本の照葉樹林と変わらない。東アジアでは照葉林帯（照葉樹林帯）が亜熱帯に相当する。地中海やカリフォルニアの硬葉林帯（硬葉樹林帯）、フロリダの常緑広葉林帯（広葉樹林帯）も亜熱帯に対応し、南半球でもほぼこれらに対応する植生帯がある。

［大場達之］