【This article is an English version of [My article in Japanese which appeared in this blog on 2018-07-11].】
【This article may be further updated. I do not always specify what parts are updated and when. 】
- 1 -
I talked with foreign students who stay in Japan but do not read or write Japanese. I found that they have little knowledge about the weather patterns which often appear in each of the seasons in Japan, even though they do have basic knowledge of meteorology. The lack may become an obstacle to handle cases of weather around Japan during their research. (Those people who grow in Japan usually learn it at middle schools, or get informed during everyday life from television, newspapers etc.)
"Weather patterns" may mean spatial distributions of "weather" such as "fine", "cloudy", etc. It may mean spatial distributions of "pressure systems" such as "lows" ("cyclones") and "highs" ("anticyclones") in what is customarily called "weather charts", which are basically maps with iso-lines of sea level pressure. I take the latter meaning in the context of this article.
As for literature which document weather patterns in each of the seasons in Japan, I find Fukui (1977), but no newer one. (My search was limited to academic literature in climatology and the official web site of Japan Meteorological Agency. Travel guides sometimes have information of normal values of temperature and precipitation, but usually no more. There may be some documents in other genres which were out of my scope.) I think that some documents in English should be produced to help resident foreign students and to help those who want to make inter-regional studies. (I am not sure whether we should document climate of Japan or climate of East Asia which includes Japan as a part.)
【[Note added 2020-04-22] Japan Meteorological Agency's web site does have a page of "General Information on Climate of Japan". http://www.data.jma.go.jp/gmd/cpd/longfcst/en/tourist.html It is helpful. But I wish more information. 】
Anyway, I tried to explain to a foreign student in English, showing figures of books written in Japanese (e.g. Kusaka, 2013; Nakamura et al. 1986, 1996). Afterwards, I quickly wrote the story in Japanese in my previous blog post (and now I rewrite it in English). The discussion may be sloppy. I intentionally took months as basic time intervals for simplicity even though I know that it is inappropriate for precise documentation of the seasonal march.
- 2 -
Japan has a considerably large north-south extent. It is not easy to describe climate of the whole area. It seems practical to limit the target to the zone between 31 deg. and 37 deg. North latitude, where the majority of the population reside. (I do not mean to ignore livelihood of people living outside of the zone.) I tentatively call it "the central zone of Japan". Note that it covers, not only Chûbu region which is often translated as "Central Japan", but also Kyûshû, Chûgoku-Shikoku, Kinki and Kantô regions.
- 3 -
The central zone of Japan is covered by the mid-latitude westerly wind belt for the most of the year.
Extra-tropical cyclones (which accompany clouds and precipitation) and moving anticyclones (which usually accompany fine weather) both travel from west to east. The time interval between arrival of an extra-tropical cyclone to another is around 7 days. Extra-tropical cyclones and moving anticyclones are related to troughs and ridges, respectively, of the westerlies in the middle troposphere (typically 500 hPa level). The weather pattern of extra-tropical cyclones and moving anticyclones is dominant during spring (March-April-May) and autumn (October-November).
- 4 -
In winter (December-January-February), the weather pattern of winter monsoon becomes dominant. In winter, surface temperature is lower on land of Eurasian continent to the west of Japan than on the Pacific Ocean to the east. Then, in the lower troposphere, an anticyclone over the continent and a cyclone over the ocean will appear persistently, and wind blows from the continent to the ocean. Because of the rotation of the Earth, the outflow becomes flow circulating around the continental anticyclone, clockwise when viewed from above. It becomes NW'ly wind over the central zone of Japan, and NE'ly in the subtropical zone to the south of Japan. (There must be some inflow to the continent, probably in the middle troposphere, to compensate for the outflow in the lower troposphere, but it is difficult to directly recognize it.)
The winter monsoon brings somewhat complex distribution of precipitation to Japan. The air of the monsoon wind is originally dry when it comes out of the continent. It gets water vapor from the sea surface (of the Sea of Japan or East China Sea). Then, the moistened air comes to the windward (NW side) of the mountains of the islands of Japan, and it is lifted by the slope. Clouds and precipitation occur. Because of low temperature, the precipitation is usually snowfall. In the leeward (SE side) of the mountains, air parcels which have lost water vapor are forced to go down, which bring fine weather. (This contrast of weather between the windward and the leeward is, meteorologically speaking, a "meso-scale" feature, and it is still difficult to represent it in global weather forecast models or global climate models.)
In winter, the weather pattern of extra-tropical cyclones and moving anticyclones may appear, besides the pattern of winter monsoon. Snowfall in the SE side of the mountains of the islands of Japan is brought by combination of extra-tropical cyclones and low temperature.
- 5 -
In summer (around August), the central zone of Japan is usually covered by the subtropical high. Temperature is high and humidity is rather high, but cloudiness is low. (Being in the west side of an ocean, occurrence of cumulus clouds is not always suppressed, even though being covered by the subtropical high.)
In this season, meso-scale precipitation often occurs over land (of the central zone of Japan), usually related to the diurnal cycle (e.g. fine weather in the morning and thunderstorms in the afternoon).
- 6 -
In June and July, the central zone of Japan is usually rainy. The situation is called Baiu. It is a feature contiguous with Meiyu in Central China. The structure in the atmosphere which brings Baiu is called the Baiu Front. The Baiu Front may be characterized by the contrast of amount of water vapor across the front. (cf. A front accompanying extra-tropical cyclones is characterized by the contrast of temperature across it.)
In the scope of global atmospheric circulation, the Baiu Front can be categorized as one of "sub-tropical precipitation zones", together with the South Pacific Convergence Zone and the South Atlantic Convergence Zone (Kodama, 1992). The Baiu Front is more spatially fixed than the SPCZ and the SACZ.
In the Baiu Front, sometimes meso-scale clusters of cumulus clouds develop and bring heavy rain. Rainfall in the Baiu Front often continues day-and-night. (Cf. Rainfall in the tropics usually has strong diurnal variability, so there are usually sunny hours even in the rainy season there.)
If we use the term "monsoon" in a broad sense, Baiu is an important part of the Asian Monsoon. On the other hand, Baiu is not monsoon in its narrow sense.
- 7 -
In early autumn (around September), the central zone of Japan is often rainy. The situation (called Akisame) is somewhat similar to Baiu, but it has less persistence than Baiu.
The amount of precipitation in September is large, partly because of Akisame, but more importantly because of tropical storms, to be mentioned below.
- 8 -
Tropical storms are generated over the tropical ocean to the south of Japan, and comes to Japan at the irregular timing generally between June and November. Strong tropical storms in the Northwest Pacific are called "typhoons". They bring heavy rain and strong winds.
- Eiichiro Fukui [福井 英一郎] ed. 1977: The Climate of Japan. Kodansha & Elsevier Scientific Pub. Co.
- Yasumasa Kodama [児玉 安正], 1992: Large-scale common features of subtropical precipitation zones (the Baiu Frontal Zone, the SPCZ, and the SACZ). Part I: Characteristics of subtropical frontal zones. Journal of the Meteorological Society of Japan, 70: 813-836. https://doi.org/10.2151/jmsj1965.70.4_813
- Kusaka Hiroyuki [日下 博幸], 2013: Manande miru to Kikôgaku wa Omoshiroi (Learning Climatology is Fun). Bere Shuppan (Beret Publishing Co.) [My memo about the book, in Japanese]
- Nakamura Kazuo [中村 和郎], Kimura Ryûji [木村 龍治], Uchijima Zenbei [内嶋 善兵衛], 1986, revised edition 1996: Nihon no Kikô (Climate in Japan) (Nihon no Shizen (Nature in Japan) 5). Iwanami Shoten [My memo about the book, in Japanese]