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= Typhoon =
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Introduction
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A typhoon is a mature tropical cyclone that develops between 180° and
100°E in the Northern Hemisphere. This region is referred to as the
Northwestern Pacific Basin, and is the most active tropical cyclone
basin on Earth, accounting for almost one-third of the world's annual
tropical cyclones. For organizational purposes, the northern Pacific
Ocean is divided into three regions: the eastern (North America to
140°W), central (140°W to 180°), and western (180° to 100°E). The
Regional Specialized Meteorological Center (RSMC) for tropical cyclone
forecasts is in Japan, with other tropical cyclone warning centers for
the northwest Pacific in Hawaii (the Joint Typhoon Warning Center),
the Philippines and Hong Kong. While the RSMC names each system, the
main name list itself is coordinated among 18 countries that have
territories threatened by typhoons each year.
Within the northwestern Pacific, there are no official typhoon seasons
as tropical cyclones form throughout the year. Like any tropical
cyclone, there are few main requirements for typhoon formation and
development: (1) sufficiently warm sea surface temperatures, (2)
atmospheric instability, (3) high humidity in the lower to middle
levels of the troposphere, (4) enough Coriolis effect to develop a low
pressure center, (5) a pre-existing low level focus or disturbance,
and (6) a low vertical wind shear. While the majority of storms form
between June and November, a few storms do occur between December and
May (although tropical cyclone formation is at a minimum during that
time). On average, the northwestern Pacific features the most numerous
and intense tropical cyclones globally. Like other basins, they are
steered by the subtropical ridge towards the west or northwest, with
some systems recurving near and east of Japan. The Philippines receive
the brunt of the landfalls, with China and Japan being impacted
slightly less. Some of the deadliest typhoons in history have struck
China. Southern China has the longest record of typhoon impacts for
the region, with a thousand-year sample via documents within their
archives. Taiwan has received the wettest known typhoon on record for
the northwest Pacific tropical cyclone basins.
Etymology and usage
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The term 'typhoon' is the regional name in the northwest Pacific for a
severe (or mature) tropical cyclone, whereas 'hurricane' is the
regional term in the northeast Pacific and northern Atlantic.
Elsewhere this is called a 'tropical cyclone', 'severe tropical
cyclone', or 'severe cyclonic storm'.
The French 'typhon' is attested with the meaning of whirlwind or storm
since 1504. The 'Oxford English Dictionary' cites Hindustani 'ṭūfān'
and Chinese 'tai fung' giving rise to several early forms in English.
The earliest forms in English—"touffon", later "tufan", "tuffon", and
others—derive from Hindustani 'ṭūfān', with citations as early as
1588. From 1699 appears "tuffoon", later "tiffoon", derived from
Chinese with spelling influenced by the older Hindustani-derived
forms. The modern spelling "typhoon" dates to 1820, preceded by
"tay-fun" in 1771 and "ty-foong", all derived from the Chinese 'tai
fung'.
The Hindustani source word 'ṭūfān' ("violent storm"; Perso-Arabic: ,
Devanagari: ) comes from the Persian 'tūfān' () meaning "storm" which
comes from the verb 'tūfīdan' (), "to roar, to blow furiously". The
word طوفان ('ṭūfān') is also derived from Arabic as coming from
'ṭāfa', to turn round.
The Chinese source is the word 'tai fung' or 'taifeng' (). The modern
Japanese word, 台風 (たいふう, 'taifuu'); as well as the modern Korean word
태풍 (taepung) are also derived from Chinese. The first character is
normally used to mean "pedestal" or "stand", but is actually a
simplification of the older Chinese character 颱, which means
"typhoon"; thus the word originally meant "typhoon wind".
The Ancient Greek ('Typhôn', "Typhon") is related and has secondarily
contaminated the word. The Persian term may originally have been
influenced by the Greek word.
Intensity classifications
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A 'tropical depression' is the lowest category that the Japan
Meteorological Agency uses and is the term used for a tropical system
that has wind speeds not exceeding 33 kn. A tropical depression is
upgraded to a 'tropical storm' should its sustained wind speeds exceed
34 kn. Tropical storms also receive official names from RSMC Tokyo.
Should the storm intensify further and reach sustained wind speeds of
48 kn then it will be classified as a 'severe tropical storm'. Once
the system's maximum sustained winds reach wind speeds of 64 kn, the
JMA will designate the tropical cyclone as a 'typhoon'—the highest
category on its scale.
Since 2009 the Hong Kong Observatory has divided typhoons into three
different classifications: 'typhoon', 'severe typhoon' and 'super
typhoon'. A 'typhoon' has wind speed of 64-79 knots (73-91 mph;
118-149 km/h), a severe typhoon has winds of at least 80 kn, and a
super typhoon has winds of at least 100 kn. The United States' Joint
Typhoon Warning Center (JTWC) unofficially classifies typhoons with
wind speeds of at least 130 knots (67 m/s; 150 mph; 241 km/h)—the
equivalent of a strong Category 4 storm in the Saffir-Simpson scale—as
'super typhoons'. However, the maximum sustained wind speed
measurements that the JTWC uses are based on a 1-minute averaging
period, akin to the U.S.' National Hurricane Center and Central
Pacific Hurricane Center. As a result, the JTWC's wind reports are
higher than JMA's measurements, as the latter is based on a 10-minute
averaging interval.
Genesis
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There are six main requirements for tropical cyclogenesis:
sufficiently warm sea surface temperatures, atmospheric instability,
high humidity in the lower to middle levels of the troposphere, enough
Coriolis force to develop a low pressure center, a pre-existing low
level focus or disturbance, and low vertical wind shear. While these
conditions are necessary for tropical cyclone formation, they do not
guarantee that a tropical cyclone will form. Normally, an ocean
temperature of 26.5 °C (79.7 °F) spanning through a depth of at least
50 m is considered the minimum to maintain the special mesocyclone
that is the tropical cyclone. These warm waters are needed to maintain
the warm core that fuels tropical systems. A minimum distance of 500
km (300 mi) from the equator is normally needed for tropical
cyclogenesis.
Whether it be a depression in the Intertropical Convergence Zone
(ITCZ) or monsoon trough, a broad surface front, or an outflow
boundary, a low level feature with sufficient vorticity and
convergence is required to begin tropical cyclogenesis. About 85 to 90
percent of Pacific typhoons form within the monsoon trough. Even with
perfect upper-level conditions and the required atmospheric
instability, the lack of a surface focus will prevent the development
of organized convection and a surface low. Vertical wind shear of less
than 10 m/s (20 kn, 33 ft/s) between the ocean surface and the
tropopause is required for tropical cyclone development. Typically
with Pacific typhoons, there are two outflow jets: one to the north
ahead of an upper trough in the Westerlies, and a second towards the
equator.
In general, westerly wind increases associated with the Madden-Julian
oscillation lead to increased tropical cyclogenesis in all tropical
cyclone basins. As the oscillation propagates from west to east, it
leads to an eastward march in tropical cyclogenesis with time during
that hemisphere's summer season. On average, twice per year twin
tropical cyclones will form in the western Pacific Ocean, near the 5th
parallel north and the 5th parallel south, along the same meridian, or
line of longitude. There is an inverse relationship between tropical
cyclone activity in the western Pacific basin and the north Atlantic
basin, however. When one basin is active, the other is normally quiet,
and vice versa. The main reason for this appears to be the phase of
the Madden-Julian oscillation, or MJO, which is normally in opposite
modes between the two basins at any given time.
Frequency
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Storm Frequency Tropical storms and Typhoons by month, for the period
1959-2015 (Northwest Pacific)
Month Count Average
Jan 28 0.5
Feb 14 0.2
Mar 26 0.5
Apr 37 0.6
May 66 1.2
Jun 100 1.8
Jul 221 3.9
Aug 310 5.4
Sep 280 4.9
Oct 228 4.0
Nov 139 2.4
Dec 69 1.2
Annual 1518 26.6
|Source: **JTWC**
Nearly one-third of the world's tropical cyclones form within the
western Pacific. This makes this basin the most active on Earth.
Pacific typhoons have formed year round, with peak months from August
to October. The peak months correspond to that of the Atlantic
hurricane seasons. Along with a high storm frequency, this basin also
features the most globally intense storms on record. One of the most
recent busy seasons was 2013. Tropical cyclones form in any month of
the year across the northwest Pacific Ocean, and concentrate around
June and November in the northern Indian Ocean. The area just
northeast of the Philippines is the most active place on Earth for
tropical cyclones to exist.
Across the Philippines themselves, activity reaches a minimum in
February, before increasing steadily through June, and spiking from
July through October, with September being the most active month for
tropical cyclones across the archipelago. Activity falls off
significantly in November, although Typhoon Haiyan, the strongest
Philippine typhoon on record, was a November typhoon. The most
frequently impacted areas of the Philippines by tropical cyclones are
northern and central Luzon and eastern Visayas. A ten-year average of
satellite determined precipitation showed that at least 30 percent of
the annual rainfall in the northern Philippines could be traced to
tropical cyclones, while the southern islands receive less than 10
percent of their annual rainfall from tropical cyclones. The genesis
and intensity of typhoons are also modulated by slow variation of the
sea surface temperature and circulation features following a
near-10-year frequency.
Paths
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Most tropical cyclones form on the side of the subtropical ridge
closer to the equator, then move poleward past the ridge axis before
recurving north and northeast into the main belt of the Westerlies.
Most typhoons form in a region in the northwest Pacific known as
typhoon alley, where the planet's most powerful tropical cyclones most
frequently develop. When the subtropical ridge shifts due to El Niño,
so will the preferred tropical cyclone tracks. Areas west of Japan and
Korea tend to experience many fewer September-November tropical
cyclone impacts during El Niño and neutral years. During El Niño
years, the break in the subtropical ridge tends to lie near 130°E,
which would favor the Japanese archipelago. During La Niña years, the
formation of tropical cyclones, and the subtropical ridge position,
shift westward across the western Pacific Ocean, which increases the
landfall threat to China and greater intensity to Philippines. Those
that form near the Marshall Islands find their way to Jeju Island,
Korea. Typhoon paths follow three general directions.