The summary of the atmospheric circulation in summer 2007
From early to mid July, the subtropical high was
weaker than normal around Japan, and the subtropicaljet shifted more southward than normal. Consequently,
the Bai-u front was located over Japan. One of thecauses for the weaker subtropical high around Japanwas suppressed convection near the Philippines. It is
pointed out that the subtropical high near Japan isstrengthened when the convective activities near thePhilippines are enhanced (Nitta, 1987). In early July,
the active phase of the Madden-Julian Oscillation(MJO) propagated from the Maritime Continent to thewestern Pacific and Typhoon “Man-yi” formed near
the Mariana Islands. It passed over the eastward off
the Philippines but the convective activities near thePhilippines were generally suppressed during July.
Fig.2 shows the distribution of eigen vector
calculated from rotated EOF analysis of monthly
mean 200hPa zonal wind in July and the time seriesof its monthly mean EOF score, respectively. The
distribution of eigen vector indicates that thesubtropical jet over the Far East shifts more
southward or northward than normal. The time seriesof EOF score suggests that Japan tends to experiencecolder-than- normal climate in the case of remarkableplus score (e.g. 1993, 2003). In the first half of July,
Asian jet near Japan shifted more southward thannormal. Consequently, the EOF score in July marked
around +0.9 and the monthly mean temperatures werelower than normal over the wide area of Japan.
In late July, quasi-stationary Rossby wave packets
clearly propagated along the Asian jet (Fig.1(c)). Thepackets reached near the eastern region of Japan inthe beginning of August and enhanced theanti-cyclone over the region in the upper troposphere.
In early August, the active convection area associatedwith the active phase of the MJO moved northwardfrom near the Maritime Continent to the north of thePhilippines and the anti-cyclone in the south of Japanwas enhanced.
In mid-August, the quasi-stationary Rossby wave
propagation along the Asian jet and polar front jetover Eurasia became clear (Fig.1(a) and (c)) andenhanced the trough over the eastern China and theridge near Japan. At 200hPa height, the northern edgeof Tibetan high was remarkably waved and Tibetan
high was stronger than normal to its northeast region.
The anti-cyclone centered in the upper troposphere
covered over Japan and caused extremely hightemperature anomalies. Fig.3 shows the distributionof 360K isentropic potential vorticity over the North

Pacific. The potential vorticity map is a natural
diagnostic tool well suited to making dynamical
processes directly visible to the human eye (Hoskinset al, 1985). In Fig.3, the contours over Eurasia
clearly meandered, the low potential vorticity broke
into near Japan and caused the wave breaking. Theseare consistent with the meandered Asian jet andstrengthened Tibetan high over Japan. In midsummer,
thus, stationary Rossby wave packets sometimes
propagate along the Asian jet and strengthen theanti-cyclone centered in the upper troposphere overJapan and cause extremely high temperature
anomalies there (Enomoto, 2004).
To estimate the contribution of the advection, theadiabatic and diabatic heating to the temperaturechange, the heat budget analysis was executed for theperiod of 8-12 Aug. and 12-16 Aug. at 925hPa (Fig.4and Fig.5). Fig.4 (a) shows the total heating wasstrong around northern Japan on 8-12 Aug. Fig.4 (c)suggests the horizontal advection mainly contributed
to the heating. On the other hand, the strong heating
was observed around eastern and western Japan on
12-16 Aug. (Fig.5 (a)), to which the adiabatic heating
associated with the descent in the barotropic
anti-cyclone mainly contributed (Fig.5 (d)).
In mid-August, the westerly jet remarkably
meandered over the northern hemisphere. In lateAugust, the zonal flow became dominant. At the sametime, Tibetan high near Japan were weakened and theremarkably high temperature in northern and easternJapan subsided. However, the western Japan
continued to be covered with the barotropic
anti-cyclone and the remarkably high temperature
persisted.

References
Enomoto, T., 2004: Interannual variability of thebonin high associated with the propagation of
Rossby waves along the Asian jet. J. Met. Soc. Japan,
82, 1019-1034.
Nitta, T., 1987: Convective activities in the tropicalwestern Pacific and their impact on the Northern
Hemisphere summer circulation. J. Met. Soc. Japan,65, 373-390.
Hoskins, B.J., M. E. McIntire, and A. W. Robertson,1985: On the use and significance of isentropic
potential vorticity maps. Quart. J. Roy. Meteor. Soc.,
111, 877-946.
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Fig.1 (a) 200hPa zonal wind(m/s, 130-150°°E mean）, (b)Contours: 500hPa height (m,130-150°°E mean), Shadings:
latitude-time cross section of2OLR (W/m , 110-140°E mean,drawn for south region of45°N), (c) Time series of thezonal component of 200hPa
wave activity flux (m2/s2,60-120°E, 40-50°N mean).

Fig.2 (Left) Distribution of eigen vector calculated from rotated EOF analysis of monthly mean 200hPa zonal
wind in July (m/s, 1979-2004). (Right) Time series of its monthly mean EOF score in every July (red-line: valuesof every years, green-line: 5-year running mean, blue circles show the years when eastern Japan was colder
than normal).

Fig.3 360K isentropic distribution of potential vorticity (PUV).
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Fig.4 (a) Total heating at 925hPa for 8-12 Aug.
(K/day), (b) Diabatic heating (K/day), (c) Horizontal
advection (K/day), and averaged wind at 925hPa forthe period (m/s), (d) Adiabatic heating (K/day).

Fig.5 Same as in Fig.4 but for 12-16 Aug.

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