2. Topics longwave radiation anomaly in December 2006 and December 2005 respectively. In December 2006 (Fig. 2.1 Factors that caused extremely warm weather 2.1.8a), the contrast between the enhanced convections in from southern Siberia to Japan in winter 2006/07 the western Indian Ocean and the suppressed convections in the Maritime Continent were quite remarkable. In the 2.1.1 Introduction upper troposphere, anticyclonic circulation anomalies In winter 2006/07 (i.e. from December 2006 to and wave activity flux associated with Rossby wave February 2007, referred to below as DJF06/07), seasonal packet propagation were clearly seen over northern India mean temperatures were extremely high from southern (Fig. 2.1.8a). Weak cyclonic circulation anomalies in Siberia to Japan (Fig. 2.1.2). In this report, the features southern and strong anticyclonic circulation and factors of atmospheric circulation mainly over the anomalies to the east of Japan were also observed. These eastern Asian region are summarized and compared with indicate that, in the region south of Japan, the those in winter 2005/06 (DJF05/06), when northeastern northwesterly flow had been weaker than its normal. Asia experienced extremely cold weather in December. Moreover, cyclonic circulation anomalies around the Aleutian Islands were clearly seen. This is consistent 2.1.2 Features of the atmospheric circulation and with an eastwardly shift of the Aleutian low. comparison with winter 2005/06 Conversely, in December 2005 when a La Niña event The center of the Aleutian low shifted eastward from occurred, convective activities were enhanced over a its normal position and the Siberian high had been wide area centered near Indonesia (Fig. 2.1.8b). Strong weaker than normal from its center to eastern Siberia in anticyclonic circulation anomalies associated with DJF06/07 (Fig. 2.1.4). This indicates that the northeastern enhanced convections were seen in southern China, and Asian winter monsoon had been weak and that cold cyclonic circulation anomalies to the east of Japan were surges occurred less frequently. The seasonal mean strengthened by Rossby wave propagation along the polar-front jet from Europe to Lake Baikal, which is Asian jet. This indicates that northwesterly flow was unclear in the climatological conditions (Fig. 2.1.5a), was dominant in the lower troposphere and cold surges significantly clearer and stronger than normal (Fig. frequently occurred. As mentioned above, the contrasts 2.1.5b). This condition persisted throughout the season. between DJF06/07 and DJF05/06 are quite clear. Fig. 2.1.6a shows the first leading eigenvector of winter mean sea-level pressure (SLP) derived from EOF 2.1.3 Consideration of the Siberian high’s analysis (referred to below as SLP_EOF1) as an indicator development of Arctic Oscillation (AO). The time series scores (Fig. Takaya and Nakamura (2005) analyzed that Rossby 2.1.6b) were calculated by projecting the five-day mean wave breaking is one of the most important processes for SLP data onto SLP_EOF1. These scores in DJF06/07 had the Siberian high’s development. As an example, from 1 been positive from December to January. This is to 3 December 2005, the strongest Siberian high of approximately consistent with the features of the DJF05/06 developed, and the SLP near its center polar-front jet mentioned above. Conversely, the scores exceeded 1060 hPa (not shown). In association with the in DJF05/06 had been negative. Moreover, the features of Rossby wave breaking that propagated along the Asian the zonal mean wind speed from Eurasia to the northern jet and the vorticity flux by the transient eddy, the Pacific were compared between DJF06/07 and DJF05/06 distributions of potential vorticity on 320 K isentropic (Fig. 2.1.7). In DJF05/06 (Fig. 2.1.7b), the strong jet surface (Fig. 2.1.9b) show that sudden penetration of low stream from 30 to 40°N and extremely weak westerlies potential vorticity from the Kamchatka Peninsula to from 40 to 60°N had persisted during the first half of the eastern Siberia occurred and an anticyclonic isolated winter. In contrast, in DJF06/07 (Fig. 2.1.7a), while the eddy was formed on 1 December 2005. This isolated jet stream had gradually shifted equatorward without eddy slowly moved westward and persisted over central strengthening, the westerlies from 50 to 60°N Siberia with the supply of low potential vorticity from its corresponding to the polar-front jet had intensified upstream. As for DJF06/07, on 12 and 13 December 2006 gradually and had then been stronger than normal (Fig. 2.1.9a), a large penetration of low potential vorticity approximately during DJF06/07 and after. from Europe to western Russia was observed and an Figs. 2.1.8a and b show the monthly mean 200 hPa anticyclonic isolated eddy was formed. However, this wave activity flux, stream function and outgoing eddy flowed smoothly eastward and merged with the low

6

Fig. 2.1.1 Winter mean temperature anomaly in Fig. 2.1.2 Winter 2006/07 mean normalized Japan from 1899 to 2007 temperature anomaly The bars show anomalies from climatological mean (1971-2000 average). The blue line shows five-year running mean anomalies and the red line shows the long-term linear trend. Unit is Celsius.

（a）Winter 2006/07

（b）Winter 2005/06

Fig. 2.1.4 Winter 2006/07 mean sea level pressure and anomaly in the northern hemisphere Contours show sea level pressure in an interval of 4-hPa. Shadings show SLP anomalies.

Fig. 2.1.3 Time series of 5-day mean temperature anomaly for subdivisions (a) and (b) represent Winter 2006/07 and Winter 2005/06, respectively.

7 （a）Normal winter mean （b）Winter 2006/07 mean

Fig. 2.1.5 Normal winter mean (a) and winter 2006/07 mean (b) 300 hPa wind speed and vectors Black lines show wind speed in an interval of 20 m/s. Light blue and blue shading shows values greater

than 20 m/s and 40 m/s respectively. Green lines in (b) show normal wind speed in an interval of 40 m/s.

（a）SLP_EOF1 (b) Time series of SLP_EOF1 scores by the 5-day mean data.

Fig. 2.1.6 Distribution of the first eigenvector calculated from EOF analysis of winter mean sea level pressure in the northern hemisphere (the north of 20°N) (a) and time series of scores calculated from the 5-day mean data (b) EOF analysis is conducted with covariance matrix for 26 samples of seasonal mean sea level pressure from winter

1979/1980 to winter 2004/2005. The scores are calculated by projecting the 5-day mean sea level pressure anomaly onto the 1st leading EOF vector.

8

（ ） a Winter 2006/07

（b）Winter 2005/06

Fig. 2.1.7 Latitude time cross section of 5-day mean 300 hPa zonal wind (zonal mean)

Black lines show wind speed in an interval of 20 m/s. Shadings show wind speed anomalies in an interval of 4 m/s. (a) and (b) represent Winter 2006/07 and Winter 2005/06, respectively .

（a）December 2006

（b）December 2005

Fig. 2.1.8 Monthly mean 200 hPa wave activity flux, stream function and outgoing longwave radiation (OLR) anomaly 2 2 Vectors show horizontal component of wave activity flux. Unit is m /s . Contours show stream function anomalies in an interval of 4x106m2/s. Shadings show OLR anomalies (W/m2). (a) and (b) represent Dec. 2006

and Dec. 2005, respectively.

9 potential vorticities in its downstream. In addition, the seen at 50 to 60°N in the lower troposphere. This penetration of low potential vorticity from the northern indicates, in higher latitudes, that the baroclinic waves Pacific to Siberia had never been observed (not shown). were more active than normal and that eddies associated From this point of view, the distribution of wind speed with them transported eastward momentum from the anomalies in DJF06/07 can be considered as follows. It upper troposphere to the lower. In the upper troposphere, can be assumed that the westerly from Europe to Siberia equatorward anomalies and weak divergence anomalies and the Bering Sea was so strong (Fig. 2.1.5b) that at 60°N are seen. This indicates that the eddies Rossby wave breaking rarely occurred in the North transported eastward momentum poleward by deflecting Pacific, and when it did occur in Europe, the isolated and propagating equatorward. eddy flowed smoothly eastward and did not persist. Conversely, in December 2005 (Fig. 2.1.10b) in the lower troposphere, upward anomalies at 40°N and 2.1.4 Consideration of the maintenance of the downward anomalies at 60°N are seen. This indicates polar-front jet that the eddies’ active areas were located in an One of the important processes that maintained the equatorward-shifted position and that they were more anomalous westerly is considered from the point of view inactive at 60°N than their normal. This suggests that of eddy forcing on the zonal flow. Eliassen-Palm flux eastward eddy momentum transport from the upper (EP-flux) is used to diagnose the eddy mean flow troposphere to the lower was less than normal. In the interaction. The vector of EP-flux represents the upper troposphere, poleward anomalies of EP-flux and direction of the eddy’s energy propagation, and its convergence anomalies in the higher latitudes were horizontal and meridional components represent clearly seen. This indicates that eddies transported westward momentum and heat transport by eddy eastward momentum equatorward and decelerated the respectively. This means that eastward momentum polar-front jet. In both cases, it is suggested that eddies represents transport in the opposite direction to EP-flux. acted to maintain anomalous westerly winds. In addition, the total eddy forcing can be represented by the divergence (westerly acceleration) and convergence 2.1.5 Consideration of the active convections over (westerly deceleration) of EP-flux. In order to express the Indian Ocean in December 2006 eastward eddy momentum transport accurately, EP-flux As shown in Section 2.1.2, the contrast between the is calculated based on the mass-weighted isentropic zonal enhanced convections in the western Indian Ocean and mean field (Tanaka et al. (2004), Iwasaki (1989)). In the the suppressed convections in the Maritime Continent extra-tropics, the zonal mean momentum equation can be were quite remarkable (Fig. 2.1.8a). In order to consider represented approximately as follows: the effects of active convection over the Indian Ocean on atmospheric circulation, the linear regression between the reversed OLR anomalies averaged for the area of ∂u * 1 ⎛ 1 ∂ ∂Fz † ⎞ = f v* + ⎜ Fφ cos φ + ⎟ (1) 0-15°N, 60-90°E and 200 hPa stream function anomalies ∂t aρ 0 cosφ ⎝ a cosφ ∂φ ∂z † ⎠ in December are shown in Fig. 2.1.12. The result of this linear regression indicates that if convections are ⎛ ⎞ ⎜ 1 ⎛ ∂Φ ⎞ ⎟ F = ρ 0a cosφ −(u'v')*,−(u'w†')* + p ⎜ ⎟ (2) enhanced in the Northern Hemisphere side of the Indian ⎜ 0 ⎟ ρ ga cosφ ⎝ ∂λ ⎠ † ⎝ p ⎠ Ocean, anticyclonic circulation anomalies from India to the Middle East and cyclonic circulation anomalies from where u, v and w are zonal, meridional and vertical winds China to the south of Japan are likely to appear. In respectively. F is EP-flux, and Fφ and Fz† are meridional addition, anticyclonic anomalies to the east of Japan are and vertical components of EP-flux respectively. The seen, although they are not statistically significant. In fact, values f, a, φ, p, Φ, λ, ρ。and g are the Coriolis parameter, Rossby wave packets, which propagated from Europe radius of the Earth, latitude, pressure, geopotential, and Africa, were observed three times in December 2006 longitude, density and gravitational acceleration (not shown). In order to confirm whether the tropical respectively. The symbols †, *, overline and prime convections work as part of the source of Rossby waves, indicate isentropic zonal mean, mass-weighted mean, the Rossby wave source (S’) was calculated (Lu and Kim, zonal mean and anomaly from zonal mean respectively. 2004). In December 2006 (Fig. 2.1.10a), upward anomalies of EP-flux and divergence anomalies of EP-flux are clearly

10

（a）Dec. 14 00UTC (left), Dec. 15 00UTC (center), Dec. 16 00UTC (right) in 2006

（b）Dec. 1 00UTC (left), Dec. 2 00UTC (center), Dec. 3 00UTC (right) in 2005

Fig. 2.1.9 Potential vorticity on 320 K isentropic surface in December 2006 (a) and Decmber 2005 (b)

Unit is PVU(1PVU=10-6 m2/s K/kg). (a) and (b) represent 14-16 Dec. 2006 and 1-3 Dec. 2005, respectively. Red arrows indicate cut-off low potential vorticity.

(hPa) (hPa )

Fig. 2.1.10 Eliassen-Palm flux (EP-flux) and its divergence anomaly based on mass weighted isentropic zonal means in the northern hemispheric troposphere Vectors show EP-flux anomalies (m2/s2). Shadings show divergence of EP-flux anomalies (m/s/day). (a) and (b) represent Dec. 2006 and Dec. 2005, respectively.

11 References S’= −∇ ⋅{( f +ζ )Vχ’+ζ’Vχ +ζ’Vχ’)} Iwasaki, T., 1989: A diagnostic formulation for

= −( f +ζ )D’−Vχ’⋅∇( f +ζ ) wave-mean flow interactions and Lagrangian mean circulation with a hybrid vertical coordinate of

−ζ’D −Vχ ⋅∇ζ’ pressure and isentropes. J. Meteorol. Soc. Jpn., 67, 293– 312. −ζ’D’−Vχ’⋅∇ζ’ (3) James, I. N., 1994: Introduction to Circulating Atmospheres, Cambridge University Press, 171-184. where f, ζ, Vχ and D are the Coriolis parameter, relative Lu, R., and B. J. Kim, 2004: The climatological Rossby vorticity, divergent wind and divergence respectively. wave source over the STCZs in the summer Northern Overline and prime indicate climatological mean and Hemisphere. J. Meteorol. Soc. Jpn., 82, 657–669. anomaly respectively. The vorticity advection is the total Takaya, K., and H. Nakamura, 2005: Mechanisms of of the second, fourth and sixth terms of Equation (3), and intraseasonal amplification of the cold Siberian high. the vorticity forcing is the total of the rest. Each term was J. Atmos. Sci., 62, 4423-4440. calculated using daily data. From 16 to 25 December Tanaka, D., T. Iwasaki, S. Uno, M. Ujiie and K. Miyazaki, 2006, the convections in the Indian Ocean were the most 2004: Eliassen-Palm flux diagnosis based on isentropic active, and the strongest Rossby wave propagation over representation. J. Atmos. Sci., 61, 2370-2383. Eurasia during December 2006 was observed. Focusing on the vorticity advection by divergent wind (Fig. 2.1.13a), negative vorticity generation was clearly seen from the Arabian Peninsula to northwestern India. In this region, the absolute values of negative vorticity generation by advection terms were larger than those by vorticity forcing. This indicates that the tropical convections in the Indian Ocean strengthened the Rossby wave propagation from Europe.

2.1.6 Summary The factors that brought the strong westerly and weak East Asian winter monsoon in DJF 06/07 are summarized below.

(1) Active eddies in the higher latitudes developed and transported eastward momentum from the upper troposphere to the lower. They deflected and propagated equatorward in the upper troposphere and maintained the anomalous westerlies. (2) Rossby wave propagation along the Asian jet brought anticyclonic anomalies to the south-east of Japan and cyclonic anomalies around the Aleutian Islands. This is one of the factors that weakened the Asian winter monsoon. It is considered that the tropical convections in the Indian Ocean strengthened the energy of the Rossby waves along the Asian jet. Comparison of the westerlies and EP-flux between DJF05/06 and DJF06/07 shows clear contrasts between DJF06/07 and DJF05/06 in the first half of the season.

12

Fig. 2.1.12 Linear regression between monthly OLR-ION and monthly mean 200 hPa stream function anomalies in December Contours show stream function anomalies in an 6 2 interval of 1x10 m /s. Shadings show anomalies are significant at the 95% level. The period for calculation is 1979-2004. Red square shows the area of OLR-ION (The reversed OLR anomalies averaged for the area of 0-15°N, 60-90°E). (a)vorticity advection term by divergent wind

(b)vorticity forcing

(c)total Fig. 2.1.11 Latitude time cross section of 5-day integrated each term of zonal averaged momentum equation based on mass weighted isentropic mean at 305hPa (a) Actual acceleration of zonal wind. (b) Total acceleration of (c), (d), (e). (c) Acceleration by Corioli forcing. (d) Acceleration by meridional divergence of EP-flux. (e) Acceleration by vertical divergence of EP-flux. Units are m/s/5days. Lines represent zonal wind anomalies.

Fig. 2.1.13 200 hPa Rossby wave source calculated by divergent wind and relative vorticity averaged on 16-25 December 2006 (a), (b) and (c) represent vorticity advection by divergent wind, vorticity forcing and total -6 respectively. Unit is 1x10 /s/day.

13 2.2 Factors that caused delayed onset/end of rainy weaker than normal around Japan, and the subtropical jet season and extremely hot midsummer and early shifted southward from its normal position (Fig. 2.2.8). autumn 2007 in Japan Consequently, the Baiu front was located over Japan, and the end of the rainy season was delayed. One of the 2.2.1 Introduction causes for the weaker subtropical high around Japan was Summer 2007 was characterized by the delayed onset the suppressed convection near the (Fig. and end of the rainy season and an extremely hot 2.2.9). In terms of monthly mean, below-normal midsummer and early autumn in Japan (Fig. 2.2.1). In temperatures were observed in most of Japan except for June, the onset of the rainy season was delayed over most Okinawa/Amami, which were covered by the subtropical of Japan except for southern Kyushu. This resulted in high. extremely below-normal precipitation amounts in Fig. 2.2.10 shows the distribution of eigenvectors Western Japan (Fig. 2.2.2). In July, the end of the rainy calculated from rotated EOF analysis of monthly mean season was also delayed, with below-normal 200 hPa zonal wind in July and a time series of its temperatures and above-normal precipitation amounts monthly mean EOF score. The eigenvector distribution observed over most of Japan. In mid-August, extremely indicates that the subtropical jet over the Far East shifted high temperature anomalies were observed over wide southward or northward from its normal position. The areas of Japan, and a record-high daily maximum time series of the EOF score suggests that Japan tends to temperature was set in Kumagaya and Tajimi on 16 experience a colder-than-normal climate in the case of August. These extremely high temperature anomalies remarkable positive scores (e.g. 1993, 2003). In July continued to September. This section presents analysis of 2007, the Asian jet over Japan shifted southward from its the convective activities and atmospheric circulation normal position and indicated a large positive score. This associated with these extreme climate events in summer is consistent with the low-temperature anomalies around 2007. Japan.

2.2.2 Delayed onset of rainy season in Japan 2.2.4 Extremely high temperatures from midsummer In the tropics, a La Niña event had continued since to early autumn in Japan spring 2007, and negative (SST) In early August, the active convection area associated anomalies were observed in the equatorial eastern Pacific. with the active phase of the MJO moved northward from Suppressed convection from the western to the eastern near the Maritime Continent to the north of the Pacific (Fig. 2.2.3) was consistent with the linear Philippines (Fig. 2.2.11), and the in the south regression between NINO.3 SST and OLR in June (Fig. of Japan was enhanced in the lower troposphere(Fig. 2.2.4) except near the Philippines. In the Asian monsoon 2.2.12). region, convective activities were significantly enhanced In mid-August, quasi-stationary Rossby wave around the Arabian Sea and suppressed from the Bay of propagation along the Asian jet and the polar-front jet Bengal to the Philippines. The subtropical high was stream over Eurasia became clear, and enhanced the weaker than normal in the south of Japan (Fig. 2.2.6) due trough over eastern China and the ridge near Japan (Fig. to the suppressed convection around the Philippines. It is 2.2.13). At 200 hPa height (Fig. 2.2.14), the northern pointed out that the subtropical high near Japan is edge of the Tibetan high was remarkably waved, and the strengthened when convective activities near the high itself was stronger than normal in its northeast Philippines are enhanced (Nitta, 1987). region. A barotropic anticyclone centered in the upper At 100 hPa (Fig. 2.2.7), the Tibetan high shifted troposphere therefore covered Japan and caused westward from its normal position, and was weaker than extremely high temperature anomalies. Fig. 2.2.15 shows normal near Japan. The subtropical jet (i.e. the Asian jet) the distribution of 360 K isentropic potential vorticity shifted southward from its normal position over Japan over the North Pacific. The potential vorticity map is a (Fig. 2.2.8). Consequently, the Baiu front was located far natural diagnostic tool well suited to making dynamical south of Japan, and the onset of the rainy season was processes directly visible to the human (Hoskins et al, delayed. 1985). Fig. 2.2.15 shows that the contours over Eurasia clearly meandered, and low potential vorticity entered 2.2.3 Delayed end of rainy season in Japan the area near Japan and caused wave breaking. These are From early to mid-July, the subtropical high was consistent with the meandering Asian jet and

14

Fig. 2.2.1 Time series of 5-day running mean Fig. 2.2.2 Monthly precipitation ratios (%), (Top) temperature anomaly for subdivisions Jun. 2007, (Bottom) Jul. 2007

Fig. 2.2.3 Monthly mean Outgoing Longwave Radiation (OLR) anomaly (Jun. 2007) 2 Contour interval is 10 W/m . Original data are provided by courtesy of CDC/NOAA.

Fig. 2.2.4 Regression coefficients between NINO.3-SST and OLR (Jun.) Gray shadings show 95% confidence level. Red (Blue) lines show positive (negative) anomalies.

Fig. 2.2.5 Time-Longitude cross section of 5-day mean 200 hPa velocity potential (5S-5N) (Apr. -Sep. 2007) 3-pentad running mean velocity potential

anomalies. Contour interval is 2x106 m2/s. Blue shadings show stronger-than-normal divergence.

15

Fig. 2.2.6 Monthly mean 500 hPa height and anomaly in the Northern Hemisphere (Jun. 2007) Fig. 2.2.7 Monthly mean 100 hPa height and Contours show heights in an interval of 60 m. anomaly in the Northern Hemisphere Shaded patterns show height anomalies . (Jun. 2007) Contours show heights in an interval of

120 m. Shaded patterns show height anomalies.

Fig. 2.2.8 Latitude-Time cross section of 5-day mean 200 hPa zonal wind (90°E-150°E mean) (Jun. – Sep. 2007) Contours show normal zonal wind in an interval

of 10 m/s. Shaded patterns show zonal wind anomalies. Fig. 2.2.9 Monthly mean OLR anomaly (Jul. 2007) Contour interval is 10 W/m2.

16

Fig. 2.2.10 (Left) Distribution of eigen vector of the first component calculated from rotated EOF analysis of monthly mean 200 hPa zonal wind in July (1979-2004, the north of 20°N, unit: m/s). (Right) Time

series of monthly mean EOF score of the first component in every July (red-line: normalized monthly mean, green-line: 5-year running mean, blue circles: the years when Eastern Japan was colder than normal in July).

Fig. 2.2.11 Latitude-Time cross section of 5-day mean OLR (120°E-140°E mean) Contour interval is 20 W/m2. Contour above 240 W/m2 are suppressed. Green lines show 2 200 and 240 W/m for normal OLR.

Fig. 2.2.13 5-day mean wave activity flux (Vectors) and stream function anomalies (Contours) at 300hPa (9-13Aug. 2007) Contour interval is 4x106 m2/s. Negative

anomalies are shaded blue.

Fig. 2.2.12 5-day mean 850 hPa stream function anomalies (Contours) and OLR anomalies (Shadings), 850hPa wave activity flux (Vectors) (6-10Aug.

2007) Contour interval is 2.5x106 m2/s.

17 strengthened Tibetan high over Japan. Enomoto (2004) analysis was executed for the period 11-15 September pointed out that in midsummer, stationary Rossby wave when extremely high temperatures were observed as in packets sometimes propagate along the Asian jet and August. Fig. 2.2.21c suggests that the horizontal strengthen the anticyclone centered in the upper advection was the main factor contributing to the heating troposphere over Japan, causing extremely high from 11-15 September. temperature anomalies there. The extreme values observed in mid-August 2007 were caused by this 2.2.5 Comparison with other years mechanism. 1994 and 2004 were the most typical years in which In mid-August, the westerly jet meandered remarkably Japan experienced extremely high temperatures in over the Northern Hemisphere. In late August, however, summer since 1990. The end of the rainy season in these the zonal flow became dominant. At the same time, the years was about one week earlier than normal in both Tibetan high near Japan weakened, and the extremely years. In July and August 1994, enhanced convection high temperatures in Northern and Eastern Japan near the Philippines and the propagation of subsided. However, Western Japan continued to be quasi-stationary Rossby wave packets along the Asian jet covered with the barotropic anticyclone, and the strengthened the barotropic anticyclone over Japan, and remarkably high temperatures persisted. caused extremely high temperature anomalies similar to To estimate the contribution of advection, adiabatic those of August 2007. In July 2004, a barotropic heating and diabatic heating to the temperature change, anticyclone developed over Japan, though convective heat budget analysis was executed for the periods 8-12 activities were suppressed near the Philippines. August and 12-16 August at 925 hPa (Figs. 2.2.16 and 2.2.17). Fig. 2.2.16a shows that the total heating was 2.2.6 Discussion strong around Northern Japan on 8-12 August, while Fig. (1) Suppressed convection near the Philippines in June 2.2.16c suggests that horizontal advection was the main and July factor contributing to the heating. On the other hand, In June 2007, the enhanced convection around the strong heating was observed around Eastern and Western Arabian Sea might have suppressed the convection near Japan on 12-16 August (Fig. 2.2.17a). Fig. 2.2.17d the Philippines because of the statistically significant suggests that the adiabatic heating associated with the relationship between them (not shown). It is possible that descent in the barotropic anticyclone was the main factor the strengthened local Hadley circulation suppressed contributing to the heating. convection near the Philippines in July 2007. Next, vorticity budget analysis was executed to estimate the factors contributing to the enhanced (2) Momentum budget analysis using EP-flux anticyclone at 850 hPa near Japan. In Fig. 2.2.18, the Eliassen-Palm flux (EP-flux) is used to diagnose the horizontal divergence was the main factor contributing to eddy mean flow interaction. The divergence and the enhanced negative vorticty (i.e. the anticyclone) for convergence of EP-flux represents the acceleration and the period 11-15August. deceleration of westerlies respectively. EP-flux based on In September, enhanced convection was observed near mass-weighted isentropic zonal means was used here the Philippines (Fig. 2.2.20), and the subtropical high (Tanaka et al. 2004). was stronger than normal near Japan. Quasi-stationary In September 2007, the jet stream shifted northward in Rossby wave packets clearly propagated along the Asian the Northern Hemisphere. Divergence anomalies of jet (Fig. 2.2.19), which meandered remarkably and EP-flux were clearly seen at 40 to 70°N in the upper shifted northward from its normal position near Japan troposphere, and the transient eddies contributed to the (Fig. 2.2.8). These characteristics resembled those seen northward shift of the jet stream more than the stationary in August, and extremely high temperatures were also eddies (Fig. 2.2.22). In addition to this, high frequency observed over Japan in September as well as August. disturbances that were active on the northern side of both In September, the Asian monsoon was extremely storm tracks (Fig. 2.2.23) contributed to the maintenance active, and its indicator SAMOI-A was +2.2, making it of zonal wind anomalies in the Northern Hemisphere. the most active Asian monsoon in September since 1979. Consistent with the Asian monsoon activity, the Tibetan high was stronger than normal in its northern region. To estimate the contribution of advection, heat budget

18

Fig. 2.2.14 5-day mean 200 hPa height and wind vector (9-13Aug. 2007) Contour interval is 30m (>=12300m) and 300m (<12300m) for heights.

Fig. 2.2.16 (a) Total heating at 925 hPa for 8-12 Aug. 2007 (K/day), (b) Diabatic heating (K/day), (c) Horizontal advection (K/day), and Fig. 2.2.15 360 K isentropic distribution of potential averaged wind at 925 hPa for the period vorticity (PVU) (00UTC15Aug. 2007) (m/s), (d) Adiabatic heating (K/day)

Fig. 2.2.17 Same as in Fig. 2.2.16 but for 12-16 Aug. 2007

Fig. 2.2.18 (a) Total variation of vorticity at 850 hPa for 11-15Aug. 2007, (b) Tilting, (c) Beta effect, (d) Horizontal divergence, (e) Horizontal advection, (f) Vertical 5 advection (10 /s/day)

19

Fig. 2.2.21 Same as in Fig. 2.2.16 but for 11-15 Sep. 2007

Fig. 2.2.19 5-day mean wave activity flux (Vectors) and stream function anomalies

(Contours) at 300 hPa (13-17Sep. 2007) Contour interval is 4x106 m2/s. Negative anomalies are shaded blue.

Fig. 2.2.20 Monthly mean OLR anomaly (Sep. 2007) Contour interval is 10W/m2.

20

Fig. 2.2.23 Kinetic energy of high-frequency variation at 500 hPa in the Northern Hemisphere (Sep. 2007) Black lines show kinetic energy per

unit mass of high-frequency variation in an interval of 5m2/s2. Green lines show normal kinetic energy per unit mass of high-frequency variation in an 2 2 interval of 10 m /s .

Fig. 2.2.22 Eliassern-Palm flux (EP-flux) and divergence anomaly based on mass weighted isentropic zonal means in the Northern Hemispheric troposphere, (Top) total, (Middle) stationary eddy, (Bottom) transient eddy (Sep. 2007)

Vectors show EP-flux anomaly (m2/s2). Shadings show divergence of EP-flux anomalies (m/s/day).

21 2.2.7 Summary 2.3 The 2007 La Niña Event ― an early report ― In the summer of 2007, the delayed onset and end of In relation to the La Niña event that began in spring the rainy season in Japan were both caused by the (March-May) 2007 (referred to below as “the 2007 La suppressed convection near the Philippines and the Niña Event”), this section describes: weaker-than-normal subtropical high near Japan. From August, convective activities were enhanced near the ・ the variation of sea surface temperatures in the El Philippines and the anticyclone near Japan was enhanced Niño monitoring region, in the lower troposphere. Around the same time, ・ the conditions of the ocean and atmosphere in the quasi-stationary Rossby wave packets clearly propagated equatorial Pacific, along the Asian jet and enhanced the ridge near Japan. In ・ the characteristics compared with past La Niña consequence, barotropic anticyclone centered in the events, upper troposphere covered Japan and caused extremely ・ the influence on atmospheric circulation, and high temperature anomalies in Japan. It is not clear, ・ the impact on the climate in Japan and throughout however, why convection and circulation varied in the the world. patterns that caused the hot summer in Japan in August. The 2007 La Niña Event was ongoing as of January 2008. The El Niño outlook issued in January 2008 by References JMA stated that the event was likely to continue until at Enomoto, T., 2004: Interannual variability of the Bonin least spring 2008. high associated with the propagation of Rossby waves along the Asian jet. J. Met. Soc. Japan, 82, 1019-1034. 2.3.1 Variation of sea surface temperatures in the Nitta, T., 1987: Convective activities in the tropical El Niño monitoring region western Pacific and their impact on the Northern Fig. 2.3.2 shows the SST deviation from a sliding Hemisphere summer circulation. J. Met. Soc. Japan, 65, 30-year mean SST averaged over the NINO.3 region in 373-390. 2006 and 2007, and for past La Niña events after 1949. Hoskins, B. J., M. E. McIntire, and A. W. Robertson, NINO.3 SST was above normal from autumn (September 1985: On the use and significance of isentropic - November) 2006 to winter (December - February) potential vorticity maps. Quart. J. Roy. Meteor. Soc., 2006/07, and then dropped to 0.5°C below normal in 111, 877-946. March 2007. The NINO.3 SST deviation remained Tanaka, D., T. Iwasaki, S. Uno, M. Ujiie and K. Miyazaki, roughly flat from March to June, and thereafter dropped 2004: Eliassen-Palm flux diagnosis based on isentropic monotonically to -1.7°C in December. This value representation. J. Atmos. Sci., 61, 2370-2383. (-1.7°C) is the third largest of the maximum NINO.3 SST Terao, T., T. Kubota, 2005: East-west SST contrast over deviations in past La Niña events (the largest is -2.0°C the tropical oceans and the post El Niño western North for the 1988-1989 event). Pacific summer monsoon. Geophys. Res. Letters, 32, L15706. 2.3.2 Conditions of the ocean and atmosphere in the equatorial Pacific Fig. 2.3.4 indicates the time-longitude cross section of SST and ocean heat content (OHC) anomalies along the equatorial Pacific in 2006 and 2007. At the beginning of 2007, SSTs were above normal in the eastern equatorial Pacific, while OHCs were below normal in the central part. Negative OHC anomalies then migrated eastward. When they reached the South American coast in spring, SSTs turned to below normal in the eastern equatorial Pacific. Negative SST anomalies persisted in the eastern equatorial Pacific thereafter, while SSTs were near normal in the central part until summer (June-August). In September, negative SST anomalies appeared in the central part in response to easterly wind anomalies

22

Fig. 2.3.1 The El Niño monitoring region

Fig. 2.3.2 The SST deviation from a sliding 30-year mean SST averaged over the NINO.3 region for the 2007 La Niña event and the past La Niña events since 1949 “YEAR 0” means the year that the La Niña event began. The La Niña years are 1949, 1954, 1964, 1967, 1970, 1973, 1975, 1984, 1988, 1995, 1998, 2005, and 2007. “YEAR-1” and “YEAR+1” means the previous and the following year, respectively.

JAN.2007 2007年 １月(a) (a)

MAR.2007 2007 年 ３月(b) (b)

MAY2007 2007 年 ５月(c) (c)

JUL.2007 2007 年 ７月(d) (d)

SEP.2007 2007 年 ９月(e) (e)

NOV.2007 2007 年 11(f) 月 (f)

Fig. 2.3.3 Monthly mean SST anomalies (left panels) and dep th-longitude cross sections of temperature anomalies along the equator in the Pacific Ocean (right panels) from January to November 2007

23 persisting around the date line in the lower troposphere, Event on global atmospheric circulation is described by and persisted thereafter. On the other hand, both SSTs comparing the features of monthly and seasonal and OHCs were above normal in the western equatorial circulation in summer (June - August) and autumn Pacific as of spring. (September - November) 2007 with the statistical ones. The Southern Oscillation Index (SOI) had been To investigate the influence of the 2007 La Niña Event, negative since summer 2006, but moved to near zero at distribution maps of the atmospheric circulation the beginning of 2007. The SOI oscillated from month to anomalies (200 hPa and 850 hPa stream function month around zero until summer 2007, and positive anomalies) regressed on the SST anomalies in the values then dominated from autumn (Fig. 2.3.5). NINO.3 region (referred to below as the NINO.3 index) Equatorial zonal wind indices in the central Pacific were produced. The linear regression coefficient was showed persistent westerly and easterly wind anomalies calculated between the NINO.3 index normalized by its in the upper and lower troposphere respectively from standard deviation from 1979 to 2004 and the stream spring 2007 (Fig. 2.3.6). function. The features shown below correspond to the Spring and summer of 2007 saw a weak pattern in atmospheric circulation anomalies when the NINO.3 which convective activities were below normal around index (normalized) is equal to -1.0. the date line and above normal around Indonesia (Fig. It is suggested that the atmospheric response 2.3.7). This pattern is usually seen during La Niña events. associated with the ENSO is not always symmetric with On the other hand, convective activities near the respect to El Niño and La Niña events (Hoerling et al., Philippines were near normal, in contrast to the usual 2001). In spite of this limitation, the circulation patterns pattern of La Niña events. In autumn, similar patterns shown in the regression maps when the NINO.3 index is were strengthened, and convective activities were also -1.0 could be generally considered as an atmospheric enhanced near the Philippines. response to La Niña events. Fig. 2.3.10 shows maps of the three-month mean 2.3.3 Characteristics compared with past La Niña stream function and its anomaly in summer and autumn events 2007, and the three-month mean linear regression Fig. 2.3.8 shows SST anomalies in summer 2007 (top) coefficient between the stream function and NINO.3 in and those of the summers during the last five La Niña the same season. The gray shading in the maps on the events (bottom). Negative SST anomalies were more right represents a 95% confidence level based on localized in the eastern equatorial Pacific in summer F-testing. In summer and autumn 2007, equatorial 2007 compared with past events, while positive SST symmetric cyclonic and anticyclonic circulation anomalies in the western equatorial Pacific were larger in anomalies were observed in the upper and lower summer 2007 than in the past. troposphere of the central Pacific in response to the Comparing the time series of the NINO.4 SST convective activities in the equatorial Pacific described anomaly during the 2007 La Niña Event with the past La in Section 2.3.2. In addition to these anomalies, October Niña years (Fig. 2.3.9), the time at which it turned and November saw equatorial symmetric cyclonic negative was the latest of the past La Niña years except circulation anomalies in the lower troposphere of the 1995 and 2005. Thus, the period between the beginning Indian Ocean (Fig. 2.3.11b). These are consistent with of the La Niña event and the expansion of negative SST the features of La Niña events. anomalies into the central equatorial Pacific was In the South Pacific, wave train type anomalies are exceptionally long. found in the upper troposphere (Fig. 2.3.10e). This is One reason why the SST anomalies in the central called the Pacific-South American (PSA) pattern, and is a equatorial Pacific did not decrease until summer was the major atmospheric response to ENSO in the Southern eastward migration of warm waters in May and July in Hemisphere. Summer and autumn 2007 saw the features response to the westerly wind anomalies in the lower of cyclonic and anticyclonic anomalies in the central troposphere in the western equatorial Pacific associated tropical and eastern South Pacific respectively (Fig. with the Madden-Julian Oscillation (MJO). 2.3.10a, c). In the lower troposphere, a barotropic anticyclonic circulation anomaly formed in the eastern 2.3.4 The influence of the La Niña event on South Pacific, bringing a continuous cold outbreak over atmospheric circulation southern South America (Fig. 2.3.10b, d). In this subsection, the influence of the 2007 La Niña In summer, one of the features of La Niña events

24

Fig. 2.3.4 Time-longitude cross sections of SST (left) and OHC (right) anomalies along the equator in the Pacific Ocean in 2006 and 2007

Fig. 2.3.5 Monthly value (thin lines) and 5-month running

mean (thick lines) of the El Niño monitoring Fig. 2.3.6 Time series of the OLR index near the date line index (top; NINO.3 SST deviation from a (OLR-DL), the equatorial zonal wind index at sliding 30-year mean) and the southern the upper troposphere in the central Pacific oscillation index (bottom) (U200-CP), and the equatorial zonal wind Shades indicate the periods of the El Niño and La index at the lower troposphere in the central Niña events. Pacific (U850-CP) Thin lines indicate monthly value, and thick lines

indicate 5-month running mean. Shades indicate the periods of the El Niño and La Niña events.

Fig. 2.3.7 3-month mean outgoing long wave radiation (OLR) anomalies in spring (top), summer (middle), and autumn (bottom) of 2007 Negative (positive) anomalies indicate more (less) active convections than normal.

25

Fig. 2.3.8 3-month mean SST anomalies for summer The top panel shows the condition in summer of 2007, and the bottom panel is a composite Fig. 2.3.9 The SST anomaly averaged over the NINO.4 map for the past La Niña events (1973, 1975, region for the 2007 La Niña event and the 1985, 1988, and 1999). past La Niña events since 1949 The contour intervals are 0.5. “YEAR 0” means the year that the La Niña event began. The La Niña years are 1949, 1954, 1964, 1967, 1970, 1973, 1975, 1984, 1988, 1995, 1998, 2005, and 2007. “YEAR-1” and “YEAR+1” means the previous and the following year, respectively.

Fig. 2.3.10 3-month mean stream function and anomaly (left) and 3-month mean linear regression coefficient between stream function and NINO.3 Contours and shadings show stream function and its anomaly respectively for a-d. Contour interval is 8x106m2/s for a, c and 4x106m2/s for b, d. Contours and shadings show liner regression coefficient and 95% confidence level based on F-testing, respectively for e-h. Contour interval is 1x106m2/s for e, g and 0.5x106m2/s for f, h.

26 involves barotropic anticyclonic anomalies over and to 2007 La Niña Event. the east of Japan (Fig. 2.3.10e, f). In summer 2007, this Fig. 2.3.14 and Fig. 2.3.15 show significant climate was not clearly seen (Fig. 2.3.10a), except in August, anomalies in autumn 2007, and these are associated with when the feature was dominant due to active propagation the La Niña events in autumn. It is likely that (1) cold of quasi-stationary Rossby wave packets along the Asian conditions around northern Thailand, (2) wet conditions jet and enhanced convection around the Philippines, as around the Philippines, and (3) cold conditions in the described in Section 2.2. Pacific coastal area of South America, were related to the Fig. 2.3.11 shows maps of the monthly mean 850 hPa 2007 La Niña Event. stream function and its anomaly in September and The 2007 La Niña Event is considered to have affected November 2007, and the monthly mean 850 hPa linear the climate of Japan by two events; the first is represented regression coefficient between the stream function and by the heat waves in August and September brought by a NINO.3 for the same months. The figure indicates a very strong subtropical high, while the second is the different atmospheric response to ENSO between early minimal rainfall in Western Japan through autumn. These and late autumn over the Far East. Namely, in September are consistent with the statistical features of both climate and October, anticyclonic circulation anomalies are and atmospheric circulations of La Niña events. dominant, whereas in November it is displaced by Although the strong cold spell in the latter half of cyclonic circulation anomalies, which bring chilly November (with record-breaking snowfall around weather to Japan. mountainous areas in the Tohoku and Hokuriku districts) In September 2007, La Niña event features were was also considered to be brought by a typical circulation clearly seen over and to the east of Japan (Fig. 2.3.11a, c), pattern seen during La Niña events, it continued for only although the record-breaking active Asian monsoon may about ten days. have influenced the formation of the atmospheric Though SSTs in the tropical western Pacific were circulation mentioned in Section 2.2. In October 2007, a higher than normal as usually seen during La Niña events, slight anticyclonic circulation anomaly was seen around convective activities were less than normal in both June Japan (Fig. 2.3.11b, d). However, the features were not and July. The Pacific high expanded less than normal clearly seen in November (Fig. 2.3.11c, e) except around around Japan, and the onset and end of the Baiu season 15 - 25 November when a cold outbreak over Japan was was one or two weeks later than normal in most regions observed and convective activities were enhanced around of Japan. The Baiu season in 2007 was in contrast to the the South China Sea and the Philippines. statistical trend whereby La Niña events lead to the Baiu In summary, the features of La Niña events were finishing earlier. clearly dominant in the tropical and southern Pacific in On the other hand, the active phase of the MJO reached summer and autumn 2007. These features were also the western equatorial Pacific where sea surface observed in the Far East in August and September 2007. temperatures were higher than normal associated with the However, it is not easy to describe how much impact the La Niña event, and then remarkably active convection 2007 La Niña Event had on atmospheric circulation in continued in the western tropical Pacific through August this area because other processes (such as propagation of and September. These active convections strengthened quasi-stationary Rossby wave packets) may have the Pacific high around Japan, which brought hot and fine influenced the atmospheric circulation there. days nationwide. Western Japan experienced extremely hot conditions in late summer, and a record-high monthly 2.3.5 Impact on climate in Japan and throughout mean temperature was seen in September. the world Western Japan also experienced record-breaking low Fig. 2.3.12 shows significant climate anomalies in levels of rain in autumn 2007. This was caused by summer 2007. Comparing these anomalies with those continued fine days in the first half of autumn due to a associated with La Niña events in summer (Fig. 2.3.13), strong Pacific high over Japan, by a complete absence of and considering the features of atmospheric circulation approaching Western Japan due to the strong described in Section 2.3.4, it is likely that (1) warm and high, and by minimal influence from cyclones in the dry conditions in the mid-western USA, (2) warm latter half of autumn. The stronger Pacific high is conditions around Argentina, (3) warm conditions from consistent with the statistical features of La Niña events Melanesia to northern New Zealand, and (4) wet in September. Consequently, it can be thought that the La conditions in northeastern Australia, were related to the Niña event is related to the record-breaking low levels of

27

Fig. 2.3.11 Monthly mean 850hPa stream function and anomaly (left) and 3-month mean linear regression coefficient between 850hPa stream function and NINO.3 Contours and shadings are the same as ones in Fig. 2.3.10b and f.

Fig. 2.3.12 Significant climate anomalies in summer 2007 Fig. 2.3.13 Significant climate anomalies associated to It shows warm/cold/wet/dry areas based on 3 La Niña events in summer month mean temperature / 3 month total It shows the 10% significant climate anomaly precipitation. Circled numbers indicate the areas during 1949-2004 La Niña events to climates possibly associated to the La Niña event. neutral years.

Fig. 2.3.14 Significant climate anomalies in autumn Fig. 2.3.15 Significant climate anomalies associated to 2007 La Niña events in autumn The same as Fig. 2.3.12. The same as Fig. 2.3.13.

28 rain in autumn 2007 over Western Japan. This indicates that the 2007 La Niña Event may have affected the world climate, especially in the tropics and 2.3.6 Summary South America. The features of oceanographic and atmospheric conditions over the equatorial Pacific associated with the 2007 La Niña Event and the resulting influence on the References world climate can be summarized as follows: Hoerling, M. P. and A. Kumar, T. Xu, 2001: Robustness - The 2007 La Niña Event began in spring 2007, and is of the nonlinear climate response to ENSO’s extreme projected to continue until spring 2008. phases. J. Climate, 14, 1277-1293. - NINO.3 SST deviation (-1.7°C) in December was the Karoly, D. J. 1989: Southern Hemisphere circulation third largest of all such deviations during past La Niña features associated with El Niño–Southern Oscillation events. events, J. Climate, 2, 1239-1252. - SST anomalies in the eastern equatorial Pacific turned to below normal when OHC anomalies along the equatorial Pacific reached the coast of South America, and persisted thereafter. Negative SST anomalies appeared in the central equatorial Pacific in September and persisted thereafter. - Negative SST anomalies were more localized in the eastern equatorial Pacific in summer than in the composite of SST anomalies for the last five La Niña events. - The SOI oscillated from month to month around zero until summer 2007, and positive values dominated thereafter. - Equatorial zonal wind indices in the central Pacific showed westerly and easterly wind anomalies in the upper and lower tropospheres respectively from spring. - Convective activities were suppressed around the date line and enhanced around Indonesia from spring. This pattern is consistent with the statistical features of La Niña events. - The statistical atmospheric circulation features of La Niña events were clearly dominant in the tropical and southern Pacific in summer and autumn. In the Far East, the features were also observed in August and September. - Corresponding to these atmospheric circulation features, the climate anomalies in temperature and precipitation statistically seen during La Niña events were observed in the USA and from Australia to South America in summer and Southeast Asia and South America in autumn. - Remarkably high temperatures in most regions of Japan from August to September and light precipitation in Western Japan in autumn 2007 were observed. These were consistent with the statistical features of La Niña events. As outlined above, the statistical features of La Niña events were clearly observed in summer and autumn.

29 2.4 Summary of Asian summer monsoon 2007 above normal from southern India to Pakistan, from Observing Asian summer monsoon activity is very Kalimantan Island to western New Guinea and around important since fluctuations in convective activities and the Yellow Sea, and below normal in northern India and atmospheric circulation associated with the event can from Mongolia to northern China (Fig. 2.4.4). influence the summer climate in Asia, including that of According to the India Meteorological Department Japan. In this section, the characteristics of the Asian (IMD), southwest monsoon precipitation over India summer monsoon from June to September 2007 are during the monsoon period was more than normal. The described. onset of the monsoon was later than normal in southeast India and earlier in northwest India. 2.4.1 Monsoon activities and atmospheric The total precipitation during the Baiu season of 2007 circulation was more than normal on the Pacific side of Western and Asian summer monsoon activities inferred from the Eastern Japan, especially in July, and was less in seasonal mean Outgoing Longwave Radiation (OLR) Northern Japan. Both the beginning and the end of the over Southeast Asia and India from June to September Baiu season were later than normal in most parts of (referred to below as “the monsoon period”) were Japan. enhanced in the Arabian Sea and near Indonesia, and During the same period, four-month mean suppressed near Japan (Figs. 2.4.1 and 2.4.2). The most temperatures were higher than normal over most of Asia, active convection area of the Asian monsoon shifted especially from Mongolia to northern China, and were westward from its normal position. slightly lower in some parts of southern China (Fig. In June, convective activities in the Arabian Sea were 2.4.5). extremely enhanced in concurrence with the arrival of the active phase of the Madden Julian Oscillation (MJO), and 2.4.3 Tropical cyclones were suppressed in the Bay of Bengal and Southeast Asia During the monsoon period, 12 tropical cyclones of except in Indonesia (Fig. 4.6.5 in Section 4.6). In the tropical storm (TS) intensity or higher formed over the lower troposphere, the southwest monsoon in the Bay of western North Pacific. This frequency was lower than the Bengal and Southeast Asia was weaker than normal. 1971-2000 average of 16.4 (Fig. 2.4.4). Eleven tropical Meanwhile, in western India and Pakistan, enhanced cyclones approached or made landfall in East and northward water vapor flux and its convergence were Southeast Asia (Table 2.4.1). Man-Yi, which observed, and these areas had extremely heavy hit Japan in mid-July, led to five deaths according to the precipitation (Fig. 2.4.3). Japan Fire and Disaster Management Agency. At the The eastward propagation of the active phase of the beginning of August, typhoon Pabuk and tropical storm MJO was clearly observed in June and July. In early Wutip caused 12 fatalities in the Philippines. In August, the active convection area moved northward mid-August, typhoon Sepat caused more than 50 deaths from near Indonesia to the north of the Philippines (Fig. in China and the Philippines, and in mid-September 3.4.4 in Section 3.4). In the lower troposphere, cyclonic caused 9 fatalities in China. circulation anomalies were observed in southern China, Cyclone Yemyin hit Pakistan at the end of June, and anticyclonic circulation anomalies were seen near causing more than 200 deaths. Japan. The Asian summer monsoon was still very active in 2.4.4 Notable weather-related disasters other than September. Convective activities were remarkably tropical cyclones enhanced in the Asian monsoon region as a whole, and Major weather-related disasters in Asia other than were the most active for September since statistics began those related to tropical cyclones during the monsoon in 1979. The southwest monsoon precipitation in India period are outlined below. was heavier than normal in conjunction with the In India, floods caused over 1,500 fatalities in total. enhancement of the monsoon circulation in early and late Flooding also occurred at the end of July and at the September. beginning of August in Pakistan, Bangladesh and Nepal, resulting in more than 200, 800 and 200 deaths 2.4.2 Precipitation and temperature respectively. During the monsoon period, four-month total In addition, floods led to more than 800 fatalities in precipitation amounts based on CLIMAT reports were China in total. Around Sulawesi Island in Indonesia, over

30 100 people were killed due to floods at the beginning of August. In August and September, drought conditions occurred in northern China according to the Beijing Climate Center of CMA. The fatality data quoted in this section is based on EM-DAT1 other than the case of typhoon Man-yi.

Fig. 2.4.3 OLR anomaly and water vapor flux anomaly in June, 2007 Color shadings indicate OLR anomalies (W/m2) with a contour interval of 10 W/m2,

and vectors indicate water vapor flux anomalies at the 925-hPa level.

Fig. 2.4.1 Four-month mean OLR (Outgoing Longwave Radiation) in June-September, 2007 Solid lines indicate OLR (W/m2) with a

contour interval of 20 W/m2.

Fig. 2.4.4 4-month precipitation ratio (%), and tracks in the northwestern Pacific, from June to September 2007

Fig. 2.4.2 4-month mean OLR and its anomaly in June-September, 2007 2 Solid lines indicate OLR (W/m ) with a contour interval of 20 W/m2, and color

shadings indicate OLR anomalies. Fig. 2.4.5 4-month mean temperature anomaly (˚C) from June to September 2007

1 EM-DAT: The OFDA/CRED Emergency Events Database.

Brussels, Belgium: Center for Research on the Epidemiology of

Disasters. http://www.em-dat.net/

31

Table 2.4.1 Tropical cyclones which affected East and Southeast Asia from June to September 2007 This information is issued by RSMC Tokyo-Typhoon Center, except for “Affected areas”. ID Number Name Duration Category1) Min Pressure2) Max Wind3) Affected areas (UTC) (hPa) (knots) T0703 Toraji 7/4- 7/5 TS 994 35 China T0704 Man-Yi 7/9- 7/15 TY 930 95 Japan T0705 Usagi 7/29- 8/4 TY 945 90 Japan T0706 Pabuk 8/5- 8/9 TY 975 65 China, Korea, The Philippines

T0707 Wutip 8/8 TS 990 35 China, The Philippines

T0708 Sepat 8/12- 8/19 TY 910 110 China, The Philippines T0709 Fitow 8/29- 9/7 TY 965 70 Japan, China T0711 Nari 9/13- 9/16 TY 935 100 Japan, Korea T0712 Wipha 9/16- 9/19 TY 925 100 China T0713 Francisco 9/23- 9/25 TS 990 40 China T0714 Lekima 9/30- 10/4 STS 975 60 Vietnam, The Philippines 1) Intensity classification of tropical cyclones TS: Tropical Storm, STS: Severe Tropical Storm, TY: Typhoon

2) Estimated minimum central pressure 3) Estimated maximum 10-minute mean winds

2.5 Stratosphere of Northern Hemisphere in winter warming. The polar vortex was split into two cells on 28 2006/07 February (Fig. 2.5.3 bottom). Stratospheric sudden warming is generally caused by the upward propagation of planetary waves from the troposphere. Planetary waves deposit westward momentum and create a strong meridional circulation, which produces a large warming effect in the polar stratosphere due to adiabatic heating. Stratospheric sudden warming is identified as minor when the polar temperature increases by 25 degrees or more within a week at any stratospheric level. It is classified as major when the zonal-mean temperature increases in the polar region and the net zonal-mean zonal wind becomes easterly to the north of 60°N at 10 hPa or below. In the winter of 2006/07, two sudden stratospheric warmings occurred. One was a minor warming that occurred from 3 to 9 February, and the other was a major warming from 22 February to 3 March. Temperatures at 30 hPa were approximately below normal except during the terms of the two warmings (Fig. 2.5.1). Fig. 2.5.2 shows a time series of vertical EP-flux at 100 hPa in the Northern Hemisphere. It indicates that planetary waves of zonal wave number 1 were the main factor contributing to the major warming. Fig. 2.5.3 shows the 30 hPa height around the major warming. A polar vortex was distributed from western Russia to northern Canada before the warming (Fig. 2.5.3 top). It then shifted to the eastern side of the hemisphere, and the shift caused the reverse of the zonal wind and the

32

18 Feb. 2007

Fig. 2.5.1 Time series of daily temperature at 12UTC at 30 hPa over the North Pole 28 Feb. 2007 (Sep. 2006-Aug. 2007) Black-line shows daily temperature at 12UTC. Gray line shows normal temperature. (℃)

Fig. 2.5.3 30 hPa height and its anomaly in the Northern Hemisphere before and after the major warming (Top: 18 Feb. 2007, Bottom: 28 Feb. 2007) Contours and shadings show height and its anomaly, respectively. Thin and thick line intervals are 120 m and 600 m, respectively.

Fig. 2.5.2 Time series of vertical component of EP-flux averaged over 30-90N at 100 hPa in Northern Hemisphere (Top: zonal wave number 1, Bottom: zonal wave number 2) Red-bar shows vertical component of EP-flux at 12UTC (left axis). Blue-line shows the previous five consecutive days integration (right axis). Pink shadings show the period of warming. (Pa･m2/s2).

33