The formation mechanism of the Bonin high in August

Transcription

1 Q. J. R. Meteorol. Soc. (2003), 129, pp doi: /qj The formation mechanism of the Bonin high in August By TAKESHI ENOMOTO 1, BRIAN J. HOSKINS 2 and YOSHIHISA MATSUDA 3 1 Frontier Research System for Global Change, Japan 2 University of Reading, UK 3 University of Tokyo, Japan (Received 25 December 2001; revised 31 August 2002) SUMMARY The Bonin high is a subtropical anticyclone that is predominant near Japan in the summer. This anticyclone is associated with an equivalent-barotropic structure, often extending throughout the entire troposphere. Although the equivalent-barotropic structure of the Bonin high has been known for years among synopticians because of its importance to the summer climate in east Asia, there are few dynamical explanations for such a structure. The present paper attempts to provide a formation mechanism for the deep ridge near Japan. We propose a new hypothesis that this equivalent-barotropic ridge near Japan is formed as a result of the propagation of stationary Rossby waves along the Asian jet in the upper troposphere ( the Silk Road pattern ). First, the monthly mean climatology is examined in order to demonstrate this hypothesis. It is shown that the enhanced Asian jet in August is favourable for the propagation of stationary Rossby waves and that the regions of descent over the eastern Mediterranean Sea and the Aral Sea act as two major wave sources. Second, a primitive-equation model is used to simulate the climatology of August. The model successfully simulates the Bonin high with an equivalentbarotropic structure. The upper-tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hpa, when a wave packet arrives. Sensitivity experiments are conducted to show that the removal of the diabatic cooling over the Asian jet suppresses the Silk Road pattern and formation of an equivalent-barotropic ridge near Japan, while the removal of the diabatic heating in the western Paci c does not. KEYWORDS: Asian jet Desert descent Indian summer monsoon Rossby waves Silk Road 1. INTRODUCTION The Bonin (Ogasawara) high is a subtropical anticyclone observed near Japan in the summer. In the early summer (June and early July), its centre at sea level is usually located near the Bonin (Ogasawara) islands (142 ± E, 27 ± N) and there is an associated moisture supply to the Baiu frontal zone (Ninomiya and Akiyama 1992). The Bonin high in this period typically extends zonally in the mid and lower troposphere. After the Baiu (late July and August), the Bonin high, by contrast, has a deep vertical structure: the ridge near Japan extends throughout the troposphere with only a slight westward tilt. It covers Japan and brings to it a dry and hot late summer climate. Although it is a stationary ridge, its intensity modulates with a typical time-scale of days. Figure 1 shows charts produced from the Japan Meteorological Agency (JMA) data for 31 July 2000 at different levels when this ridge had just developed and the Baiu front had disappeared. As seen in these charts, the horizontal scale is km. The equivalent-barotropic structure of this synoptic ridge is apparent: the midtropospheric anticyclone exists approximately beneath the one in the upper troposphere. It appears that the north Paci c anticyclone near the sea surface has extended westwards in association with the intensi cation of the anticyclone above. Such an equivalentbarotropic structure has been known for many years among Japanese forecasters as a typical ow pattern for a hot spell. Since the Bonin high is stationary and deep, it affects the path of tropical cyclones: they do not tend to cross the anticyclone but are advected around it. Severe phenomena such as thunder storms may occur on the south-western fringe of the anticyclone associated with a cut-off low generated from a streak of large potential vorticity (PV) (Tsuboki and Ogura 1999) in the upper troposphere and with Corresponding author: Frontier Research System for Global Change, , Showamachi, Kanazawa-ku, Yokohama, Kanagawa , Nippon (Japan). enomoto.takeshi@nasda.go.jp c Royal Meteorological Society,

2 158 T. ENOMOTO et al. (a) contour interval = 100 (m) (b) contour interval = 40 (m) (c) contour interval = 4 (hpa) Figure 1. Distribution of the geopotential height Z (m) at (a) 200 hpa, (b) 500 hpa and (c) sea-level pressure (hpa) at 12 UTC on 31 July The regions with (a) Z > m and (b) Z > 5880 m near Japan are shaded. Produced from Japan Meteorological Agency analyses.

3 THE FORMATION MECHANISM OF THE BONIN HIGH 159 the moistened south-easterly ow near the surface. Large interannual variability of the Bonin high results in anomalies in the summer climate in east Asia. In spite of its meteorological, climatological and socio-economic importance, the cause of the equivalent-barotropic ridge has not been clear. Gill (1980) obtained a ow pattern similar to the one observed in the northern-hemisphere summer by superposing the responses to heating which are symmetric and anti-symmetric about the equator. His solutions, however, do not include anything similar to the Bonin high. An observational study by Nitta (1987) suggested that a stationary Rossby-wave propagation from the western Paci c heating (the PJ pattern) appears when the active convective region in the western Paci c is shifted anomalously northward. Kurihara and Tsuyuki (1987) and Nikaidou (1989) argue that the equivalent-barotropic structure can be intensi ed by the PJ pattern. However, the PJ pattern is not always observed when the equivalentbarotropic structure of the Bonin high is enhanced. In fact, the external mode can propagate to the midlatitudes only when it is generated through vertical shear of the zonal wind or boundary-layer damping (Hoskins and Karoly 1981; Kasahara 1984; Lim and Chang 1986; Kato and Matsuda 1992). Moreover, the forcing of a stationary Rossby wave from south of the critical latitude (c D U D 0, where c is the phase speed of the stationary Rossby wave and U the basic zonal wind speed) is possible only through advection by the divergent wind (Sardeshmukh and Hoskins 1988). The above considerations suggest that the PJ pattern does not explain the existence of the Bonin high, although it may contribute to the variations of the high. Recently, Rodwell and Hoskins (1996, hereafter RH), Rodwell and Hoskins (2001) and Hoskins (1996) suggested a mechanism for the intensi cation of summer subtropical anticyclones. Their mechanism appears to be applicable to the eastern part of the north Paci c anticyclone. The descent enhanced by the monsoon heating to its east, however, produces a baroclinic structure as observed in the north-eastern Paci c. The present study will show evidence that suggests that the equivalent-barotropic structure of the Bonin high is not caused by western Paci c heating, but by stationary waves propagating from the west. In section 2, we examine the climatology for August to explore features of the Bonin high and the general circulation in the northern hemisphere. The design of numerical experiments is described in section 3. First we show that August climatology is reproduced with a primitive-equation model using observed diabatic heating as a forcing. Second, the heating/cooling of speci c regions are removed to examine responses. In section 4, these results are examined. In section 5, we summarize the proposed formation mechanism for the equivalent-barotropic structure of the Bonin high. 2. CLIMATOLOGY In this section data from the European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis for (ERA-15) is used to describe the equivalentbarotropic structure of the Bonin high in August. The climatological ow is also examined to show that there is evidence for the existence of stationary waves on the Asian jet. (a) The equivalent-barotropic structure Figure 2 shows the distribution of the zonally asymmetric 200 hpa stream function and mean-sea-level pressure in August in relevant, but slightly shifted, regions. The Tibetan high in the upper troposphere (Fig. 2(a)) and the north Paci c anticyclone near the surface (Fig. 2(b)) are the two most notable features. The Tibetan high extends to the

4 160 T. ENOMOTO et al. (a) contour interval = 2.5e6 (m 2 s 1 ) (b) contour interval = 1 (hpa) Figure 2. (a) The observed zonally asymmetric stream function (m 2 s 1 ) at 200 hpa and (b) mean-sea-level pressure (hpa) in August. Negative contours are dashed. Both plots show latitudes 0 ± 60 ± N but the longitudes are (a) 0 ± 180 ± E and (b) 60 ± 240 ± E. (Data from ECMWF re-analysis for (ERA-15)). west of the Indian heating, which can be interpreted as a Rossby-wave response (Gill 1980). The Tibetan high has a signi cant eastward extension near 140 ± E, which is not simply attributable to an atmospheric response to the Indian heating. This anticyclone is sometimes so intensi ed that it can be seen even in the total eld as was the case for the time shown in Fig. 1(a). In spite of its amplitude reduction, Fig. 2(a) shows that the anticyclonic anomaly near Japan remains in the climatology. Therefore, it can be regarded as a quasi-stationary ridge. It is interesting to note that on average the Bonin high at sea level is not as clear (Fig. 2(b)), although there is a ridge near 160 ± E, 40 ± N extending north-westward. A centre of the Bonin high is, however, often observed in daily charts. The lack of an independent centre in the sea-level climatology is probably due to large variation in the location and intensity of this surface anticyclone within a month and from year to year. The monsoon trough near Taiwan is another feature of the post-baiu summer in east Asia. This trough extends from China to the east of the Philippines. The overlapping of the upper-tropospheric and sea-level anticyclones near Japan in Fig. 2 indicates the depth of this ridge. This can be better shown by the longitude height cross-section of the zonally asymmetric stream function at 40 ± N in August (Fig. 3). The ridge over Japan extends from the tropopause to the mid troposphere, therefore, it is distinct from the Tibetan high and the north Paci c high (Neyama 1968). In the lower troposphere it merges into the broad anticyclone region in the Paci c. The purpose of the present study is to provide a formation mechanism for this deep ridge. The north Paci c anticyclone near 210 ± E, by contrast, is so shallow that it is intense only at levels close to the surface, decays rapidly in the vertical, and disappears above 500 hpa.

5 THE FORMATION MECHANISM OF THE BONIN HIGH 161 pressure (hpa) contour interval = 15 K contour interval = 2.5e6 m 2 s 1 Figure 3. Longitude height cross-section of the stream function (m 2 s 1, zonal average removed) superimposed on the potential temperature (K) distribution at 40 ± N in August. Negative contours are dashed. (ECMWF reanalysis for (ERA-15) data). Nikaidou (1987) pointed out that the intensi cation of the Bonin high and the associated hot spell is caused by a negative PV anomaly near the tropopause. A negative PV anomaly is also found in the climatology. Figure 4(a) shows the longitude height cross-section at 40 ± N of the zonally asymmetric PV. There is an independent centre near 140 ± E and 200 hpa (360 K). Anticyclonic ow exists below this anomaly as in the idealized case (Hoskins et al. 1985). On the 360 K isentropic surface, the Bonin high is characterized by PV meridional overturning (PV D 2 and 3 PV units (1 PVU D 10 6 K kg 1 m 2 s 1 ) in Fig. 4(b)). Postel and Hitchman (1999) reported that such PV overturning is frequent in this region in summer. In view of the differences in their vertical structures and PV distributions, the Bonin high with an equivalent-barotropic structure should be distinguished from the Tibetan and north Paci c subtropical anticyclones and interpreted as a phenomenon directly associated with the negative PV anomaly near the tropopause. Then the question as to what is the formation mechanism of the Bonin high with an equivalent-barotropic structure is reduced to determining the cause of the upper-tropospheric negative PV anomaly. (b) The Silk Road pattern In August, the Asian jet develops in the upper troposphere as shown in Fig. 5(a). The jet core with zonal winds exceeding 30 m s 1 exists on the northern ank of the Tibetan anticyclone. The jet core can be considered to be intensi ed by the anticyclonic ow of the Tibetan high. The meridional wind distribution given in Fig. 5(b) implies the existence of stationary waves on the jet: southerlies and northerlies alternate every km along the Asian jet. The southerly and northerly maxima near 130 ± E and 150 ± E are associated with the upper-tropospheric anticyclone over Japan. The power spectral density of meridional wind along the Asian jet has peaks at 14 days, which agrees with the typical time-scale of the Bonin high, and days (Terao 1998). A stationary wave train appears to be initiated near the jet entrance and to propagate along

6 162 T. ENOMOTO et al. (a) pressure (hpa) contour interval = 15 K contour interval = 0.25 PVU (b) contour interval = 1 (PVU) Figure 4. (a) Longitude height cross-section of the potential vorticity (PV, 1 PV unit D 10 6 K kg 1 m 2 s 1 ) at 40 ± N. (b) The PV distribution on the 360 K potential-temperature surface. Both panels are for August and negative contours are dashed. (ECMWF re-analysis for (ERA-15) data). the jet. A similar pattern can be found when the zonal average and wave number k D 1 and 2 components that represent the Tibetan anticyclone are removed from the stream function (not shown). The Asian jet is strong enough to be a waveguide. Figure 6(a) shows the local effective beta eff 2 U=@y 2 in local Cartesian coordinates, where is the meridional gradient of the Coriolis parameter), derived using a Mercator projection (Hoskins and Karoly 1981). Large values of eff are found along the Asian jet. Regions with negative values of eff which extend zonally on both sides of the jet act to prevent the stationary wave from meridional propagation. As in Hoskins and Ambrizzi (1993), the maximum in the distribution of eff implies that the Asian jet acts as a waveguide. A study of the climatology of the midlatitude waveguide conducted by Newman and Sardeshmukh (1998) showed that the waveguide associated with the Asian jet is strongest in August. Figure 6(b) shows the stationary Rossby wave number K s (D p eff =U in local Cartesian coordinates) at 200 hpa. The value of K s varies between 7 and 9 along the Asian jet. Assuming the width of the waveguide to be 30 ±, the zonal wave number is estimated to be 6.5. These values are similar to those obtained for the June, July and August ow at 300 hpa by Ambrizzi et al. (1995).

7 THE FORMATION MECHANISM OF THE BONIN HIGH 163 (a) contour interval = 5 (ms 1 ) (b) contour interval = 1 (ms 1 ) Figure 5. (a) The zonal and (b) meridional wind speeds (m s 1 ) at 200 hpa in August. Negative contours are dashed. (ECMWF re-analysis for (ERA-15) data). Figure 6. The distributions of (a) the effective beta eff ( U yy in local Cartesian coordinates, m 1 s 1 ) and (b) the stationary Rossby wave number K s (D p eff =U in local Cartesian coordinates) at 200 hpa. Negative contours are dashed. (ECMWF re-analysis for (ERA-15) data).

8 164 T. ENOMOTO et al. (a) (b) contour interval = 0.5 (hpah 1 ) 2 contour interval = 5e-11 (s 2 ) Figure 7. The distributions of (a) the vertical pressure velocity! (hpa h 1 ) at 500 hpa and (b) the vorticity forcing f D 0 (s 2 ) at 200 hpa. Negative contours are dashed. (ECMWF re-analysis for (ERA-15) data). As pointed out in RH and seen in Fig. 7(a), there are localized regions of descent over the eastern Mediterranean Sea and the Aral Sea. RH showed (1) that the locations of descent are determined by the orography, (2) that about half of the descent is caused by the westerlies moving down the sloping isentropes, and (3) that the descent is enhanced and localized further by radiative cooling over the arid regions. Associated with such a descent, there is a signi cant convergence in the upper troposphere. This convergence acts as a vorticity source, f D 0, where D 0 is the asymmetric divergence, as shown in Fig. 7(b). The latitudinal variation of the Coriolis parameter f gives a poleward bias so that the divergence in the low latitudes is less important as a vorticity forcing than that in the midlatitudes. The vorticity forcing is located near the jet entrance and anomalies induced by it can be easily communicated to the east by the propagation of quasistationary Rossby-wave activity along the jet waveguide. The representation of f D 0 as the Rossby wave source ignores, for example, the effect of advection by the divergent wind, highlighted by Sardeshmukh and Hoskins (1988). However, the neglected terms are found to be insigni cant in the present case. In particular, it is worthwhile to mention that the effect of advection by the divergent wind discussed in Sardeshmukh and Hoskins (1988) is negligible. 3. EXPERIMENTAL DESIGN Hoskins and Rodwell (1995) simulated the summer climatology quite accurately using time integration of a simple three-dimensional model forced by speci ed heating. We adopt their model with slightly improved resolution. They concentrated on the

9 THE FORMATION MECHANISM OF THE BONIN HIGH 165 contour interval = 25 (Wm 2 ) Figure 8. The column-integrated diabatic heating (W m 2 ) calculated from ECMWF re-analysis for (ERA-15) data. The rectangles in the upper left and lower right designate the Silk Road cooling (0 ± 140 ± E, 30 ± 50 ± N) and the western Paci c heating (110 ± 180 ± E, 0 ± 30 ± N), respectively. Regions with cooling below 25 W m 2 are shaded. near-surface ow, which was well reproduced. Here the concentration is on the uppertropospheric ow which is also represented reasonably well. The results are capable of both qualitative and quantitative comparisons with the climatology. The dynamical core of our model is a variant of the nonlinear spectral primitive model on the sphere developed at the University of Reading (the Reading Spectral Model, Hoskins and Simmons 1975), a model that has been used in numerous studies. This spectral model is used with triangular truncation at wave number 42 with 15 vertical levels in ¾ coordinates. Our model does not include any parametrized physical processes other than linear damping. Instead, the climatological diabatic heating is used as a forcing. This diabatic heating eld is computed from analysis data as the residual from the adiabatic terms in the thermodynamic equation. Therefore, it includes the results of all the physical processes such as condensational heating, radiative heating and surface uxes. The advantage of this method is simplicity. Since the examination of each diabatic process is out of the scope of the present research, the use of the residual heating is suf cient for our goal. The August climatology is taken from the ECMWF re-analysis for the period of (ERA-15). Some variables, such as the diabatic heating, were diagnosed by the Centre for Global Atmospheric Modelling (P. Berrisford, personal communication). The three-dimensional diabatic heating for August derived from ERA-15 shown in Fig. 8 is used as a constant forcing of the model. The model is initialized with the zonally averaged elds derived from climatology and integrated in time for 30 days. The climatological wind and temperature elds for August represented in 17 pressure levels are extrapolated to sea level in order to initialize the integration without orography. Then the zonally symmetric components are evaluated at 15 model levels in ¾ coordinates. The earth orography is incorporated into the model in the following way. The orographic geopotential at the surface taken from the ERA dataset is smoothed with a spectral lter of form exp[ Kfn.n C 1/g 2 ] (Hoskins 1980). Here the value of K is chosen so that exp[ KfN.N C 1/g 2 ] D 0:1 where N is the truncation wave number. Antarctica is removed as such polar forcing with a speci ed zonal ow makes little sense and causes problems in the model. Before the heating is turned on, the orography is elevated linearly in ve days to minimize undesired waves. As the model surface

10 166 T. ENOMOTO et al. is lifted, air mass is excluded and the temperature at each ¾ level is reduced. Thus, a hydrostatic adjustment is made with respect to the pressure and temperature (Hoskins and Rodwell 1995): the term n m.t/ (1) RT is added to the surface pressure tendency and the term dt 0 0.¾ / n m.t/ d ln ¾ RT is added to the temperature tendency during the orography growth. Here R is the gas constant, T C D 286 K is the temperature of the standard atmosphere at the mean height of world orography, Á n m is the surface geopotential height at zonal wave number m and total wave number n, and T0 0.¾ / designates the layer average temperature. The dynamical effect of the boundary layer is represented by Rayleigh damping in the lowest two layers. The Rayleigh damping coef cients for these two layers are chosen to have time-scales of (5 d, 1 d) over the ocean and (1.25 d, 0.25 d) over the land, respectively. Hoskins and Rodwell (1995) showed that the land/sea contrast in drag is important for reproducing the low-level ows, in particular the Somali jet. The model also has a sponge layer at the top three levels represented by Rayleigh damping with time-scales of 8, 12, and 16 days, respectively. Newtonian cooling with a time-scale of 25 days is applied at most levels. The timescale for the Newtonian cooling decreases to 10 and 5 days at the lowest two levels. The model has a r 6 hyper-diffusion with a time-scale of 6 hours for the largest wave number. First the global heating is imposed to simulate the August ow. We refer to this experiment as the control run. Then we conduct sensitivity experiments by removing various parts of the heating. In order to test if the western Paci c heating creates the equivalent-barotropic structure of the Bonin high, we remove the western Paci c heating (110 ± 180 ± E, 0 ± 30 ± N, Fig. 8). Then the cooling over the Silk Road (0 ± 140 ± E, 30 ± 50 ± N) is removed by setting negative values to zero in order to reduce the descent in this region. Any heating in this region is retained. Without the Silk Road cooling, the generation of the stationary wave is expected to be suppressed so that we can test if the propagation of the stationary Rossby wave along the Asian jet is responsible for the equivalent-barotropic structure of the Bonin high. The northern-hemisphere summer ow produced by this model was set up by day 10 and changes are relatively small until about day 30. After that baroclinic instability becomes important and more complexity in the model would be required for realism. Consequently we use an average from days 10 to 30 to illustrate the overall results. (2) 4. RESULTS In this section the numerical results are examined to test the ideas described in section 2. First we compare the climatology and the control run in which the global diabatic heating is used as forcing. Then we examine the results from the sensitivity experiments to clarify the origin of the equivalent-barotropic structure near Japan. Finally, we demonstrate the propagation of stationary waves along the Asian jet in the model.

11 THE FORMATION MECHANISM OF THE BONIN HIGH 167 (a) contour interval = 2.5e6 (m 2 s 1 ) Figure 9. The distributions of (a) 200 hpa asymmetric stream function and (b) surface pressure as in Fig. 2 but for the control experiment (averaged between days 10 and 30). The subjectively drawn solid line to the east of Japan designates a high-pressure ridge that corresponds to the upper- and mid-tropospheric anticyclone. (a) The simulation of the August climatology Figure 9 shows the zonally asymmetric stream function and the mean-sea-level pressure averaged between days 10 and 30 in the control experiment. Since the residual heating for August is used in the control experiment, the ow obtained in the model is expected to be similar to the climatology for August, not only qualitatively but also quantitatively. In fact, most of the contours in Fig. 9(a) coincide closely with their counterparts in Fig. 2(a). Exceptions are the values of stream function around the Tibetan Plateau. The values obtained in the control experiment are too large. This discrepancy may be due to the relatively short interval for averaging of the model result and a lack of gravity-wave drag parametrization in the model. As we shall see in subsection 4(c), a wave packet propagates along the Asian jet and introduces an amplitude modulation of the Bonin high with a period of days. Moreover, the instantaneous Tibetan high in the upper-tropospheric daily chart is often so deformed that its pattern is far from the oval seen in the climatology (Fig. 2(a)). Therefore, the model contours may not be as smooth as those in the climatology, which is an average for 15 years. We do not use a gravity-wave drag in order to keep the model as simple as possible. The anticyclone near Japan (135 ± E, 40 ± N) is quite successfully reproduced in the control run. The strength of the model anticyclone is such that it exhibits closed contours from upper to mid troposphere, which is consistent with the climatology. In fact, this anticyclone was already simulated in Hoskins and Rodwell (1995) but considered to be a possible model defect. A cyclone over the eastern Mediterranean Sea is also present in the control run.

12 168 T. ENOMOTO et al. (a) contour interval = 2.5e6 (m 2 s 1 ) (b) contour interval = 2.5e6 (m 2 s 1 ) Figure 10. The distribution of the 200 hpa stream function (m 2 s 1, averaged between days 10 and 30) in the sensitivity experiments: (a) the western Paci c heating removed and (b) the Silk Road cooling removed. Negative contours are dashed. The surface anticyclonic centre near the tip of the Kamchatska Peninsula (155 ± E, 47 ± N) seen in Fig. 9(b) appears to re ect the upper-level ridge in addition to the nearsurface Okhotsk high caused by local diabatic cooling. The signature of the uppertropospheric ridge can be seen in the surface high-pressure ridge toward Japan (marked by a solid line) from the high-pressure centre. Other features in the observed surface pressure eld are reproduced in the control experiment reasonably well. Most of the contours of the north Paci c anticyclone coincide with those observed except for ones near its centre. The ow around the monsoon trough near Taiwan is very similar to that observed. General similarities between the result from the control experiment and climatology indicate that our model successfully reproduces the observed surface pressure and the equivalent-barotropic structure near Japan. This enables us to perform experiments to relate the ow features of interest to components of the heating eld. We compare the results from such sensitivity experiments with the control experiment in the next section. (b) Sensitivity experiments In this subsection, a comparison is made between the control experiment and the experiments without western Paci c heating and Silk Road cooling. Figures 10(a) and (b) show the zonally asymmetric stream functions in the upper troposphere in the experiment without western Paci c heating and the one without Silk Road cooling, respectively. From Fig. 10(a) it is seen that, even if the western Paci c heating is not present, the anticyclone over Japan (135 ± E, 40 ± N) still exists. In the experiment

13 THE FORMATION MECHANISM OF THE BONIN HIGH 169 (a) contour interval = 1 (hpa) (b) contour interval = 1 (hpa) Figure 11. As in Fig. 10 but for the mean-sea-level pressure (hpa). The subjectively drawn solid line near Japan in (a) designates a high-pressure ridge that corresponds to the upper- and mid-tropospheric anticyclone. without Silk Road cooling, by contrast, this anticyclone is absent (Fig. 10(b)). Instead, there is a broad ridge across northern China. The ridge is much less conspicuous in the control run (Fig. 9(a)) and in the experiment without the western Paci c heating (Fig. 10(a)) since it is weakened by the trough near 120 ± E associated with the stationary Rossby waves induced by the Silk Road cooling. Over the eastern Mediterranean Sea, a cyclone is formed in Fig. 10(a), but this cyclone is not present in Fig. 10(b). Clearly the diabatic cooling in the jet entrance region creates the cyclonic anomaly there. The role of the western Paci c heating in the upper-tropospheric ow is to extend the Tibetan high south-eastward by inducing the anticyclone over the Philippines, but it does not signi cantly affect midlatitudes. Figure 11 shows the distribution of the mean-sea-level pressure (a) in the experiment without western Paci c heating and (b) in the experiment without Silk Road cooling. In the experiment without western Paci c heating, the monsoon trough near Taiwan is almost completely absent (Fig. 11(a)). It appears that the pressure pattern east of the dateline is unaffected by the removal of the western Paci c heating. However, the poleward ow near (170 ± E, 25 ± N) in Fig. 9(b) is weakened in Fig. 11(a). The above differences may be interpreted in terms of the classical Gill pattern. The western Paci c heating induces a surface low and an upper-tropospheric high to its west. The surface westward ow which can be interpreted as a Kelvin wave turns north-westward near the heating. Both the Gill pattern and the model response to the western Paci c heating share these features. To the north of 40 ± N, the lack of western Paci c heating does not appear to in uence the surface pressure distribution. In particular, the isolated

14 170 T. ENOMOTO et al. anticyclone represented by the 1014 hpa contour at the tip of the Kamchatska Peninsula remains also in Fig. 11(a). Although the amplitude of the upper- and mid-tropospheric ridge across Japan decreases toward the surface, it is still possible to discern its signature in sea-level pressure. This surface anticyclone re ects the upper-tropospheric ridge somewhat more strongly than that in the climatology and constitutes the equivalent-barotropic structure of the Bonin high (Fig. 10(a) and Fig. 11(a)) as in the control run (Figs. 9(a) and (b)). Without Silk Road cooling this anticyclone is weakened by 2 hpa (Fig. 11(b)). More importantly it loses its separate structure and the extension toward northern Japan is no longer apparent. The weakening of this high can be associated with the absence of the deep equivalent-barotropic anticyclone. Although the distribution of pressure contours in this region is not greatly different from climatology (Fig. 2(b)), the corresponding ridge does not exist in the upper troposphere without Silk Road cooling (Fig. 10(b)). Local diabatic cooling over the Sea of Okhotsk appears to be responsible for the shallow structure seen there. Thus, it is concluded that the equivalent-barotropic structure of the Bonin high is reproduced even when the western Paci c heating is absent and is not simulated when the Silk Road cooling is removed in our model. We have conducted many other sensitivity experiments. Two of them will be mentioned here. The descent over the eastern Mediterranean Sea can be reduced by removing the cooling over the Sahara desert and the north African topography (RH). In this case, the ridge over Japan is signi cantly weakened: the values of perturbation stream function for zonal wave numbers greater than 2 are less than half of those in the control experiment (not shown). Similarly the descent to the south of the Aral Sea can be reduced by removing the cooling over the middle east and the Iranian orography. Again, the Bonin high becomes much weaker (not shown). (c) Wave propagation in the model In this subsection, we examine the time evolution of the upper-tropospheric eld and demonstrate that the Silk Road pattern is caused by the Silk Road cooling. Figure 12 shows a longitude time section (Hovmüller plot) of the perturbation stream function à 0 in the upper-troposphere, which is calculated by removing the low-wave-number components k 6 2. It is assumed that these low wave components represent the planetaryscale structure, such as the Tibetan high. In both the control experiment and the one without western Paci c heating, a wave train originating in the desert region near 45 ± E at day 21 is found. The phase is not propagating, but the maximum amplitude of the wave train moves eastward with time. The wave train originates in the jet entrance region, propagates along the jet and reaches the jet exit region near Japan. There are about two wavelengths in 100 degrees, which corresponds to an approximate average zonal wave number k D 7. This value agrees with stationary wave number K s calculated from the climatological zonal wind eld (Fig. 6(b)). In Fig. 12 the subjectively drawn diagonal line following the amplitude maxima gives an estimated group velocity c gx D 18 m s 1, or 55 degrees in 3 days at the jet core. The group velocity is reduced near the jet exit, where the zonal wind speed is smaller. This suggests that large amplitudes may be achieved in the region of Japan because of what Nakamura (1994) has referred to as Rossby-wave blocking and Swanson (2000) has referred to as accumulation. The Hovmüller plot for the experiment without Silk Road cooling exhibits a completely different pattern, as shown in Fig. 12(c). After day 15, there is no organized propagation such as that found in the control experiment. This result clearly demonstrates that the Silk Road pattern is caused by the diabatic cooling in the desert region.

15 THE FORMATION MECHANISM OF THE BONIN HIGH 171 (a) (b) day day contour interval = 1e6 (m 2 s 1 ) contour interval = 1e6 (m 2 s 1 ) (c) day contour interval = 1e6 (m 2 s 1 ) Figure 12. Longitude time plots of the perturbation stream function (m 2 s 1, zonal wave number k > 3) at 34.9 ± N, ¾ D 0:23 for (a) the control experiment, (b) the experiment without western Paci c heating, and (c) the experiment without Silk Road cooling. The solid line in (a) connects the peaks near the jet core (60 ± 120 ± E).

16 172 T. ENOMOTO et al. (a) Control (b) No western Pacific (c) No Silk Road Day 20 Day 25 Day 27 60E 120E Day 29 Figure 13. The time evolution of potential vorticity (PV) on the µ D 350 K isentrope in (a) the control experiment (left column), (b) the experiment without western Paci c heating (middle column), and (c) the experiments without Silk Road cooling (right column) at day 20, 25, 27 and 29 (from the top to the bottom). The contours are drawn from 1 to 5 PV units (1 PVU D 10 6 K kg 1 m 2 s 1 ) with an interval of 1 PVU. The light and dark shades indicate the regions under 0.4 PVU and over 1 PVU, respectively. Longitudes between 0 ± and 180 ± E are shown with meridians drawn every 30 degrees. (d) PV behaviour Figure 4 implies that the equivalent-barotropic structure of the Bonin high can be considered to be induced by a negative PV anomaly over Japan. Our model results show that the development of such a PV anomaly is associated with the stationarywave propagation. Figure 13 shows the time evolution of PV at µ D 350 K in our experiments. On day 20 in the control experiment (Fig. 13(a)), a trough with large PV begins to develop near 70 ± E. By day 25, a ridge near 90 ± E and a trough downstream become conspicuous. The ridge near 90 ± E is associated with very low PV, less than 0.4 PVU (in light shade). On day 27, a PV roll-up is found over Japan. Finally, on day 29, low PV covers Japan, leading to a higher tropopause in this region (NB the dynamical tropopause may be de ned as 2 PVU). The fact that the PV contours between 60 ± and 90 ± E are rather zonal at this time indicates that the wave packet has already passed. In the experiment without western Paci c heating (Fig. 13(b)), the contours in the midlatitudes evolve in a similar manner to those in the control experiment. The wave breaking (irreversible distortion of PV contours) is more conspicuous over Japan. In the experiment without Silk Road cooling, by contrast, Fig. 13(c) shows that there is little variation in the PV contours in midlatitudes. At 350 K Japan is always covered with PV larger than 1 PVU (dark shading). This almost unvarying PV distribution in midlatitudes indicates that the propagation of the stationary Rossby waves along the jet is very weak. Wave breaking is limited to near the critical latitudes around 30 ± N and does not occur near the jet exit.

17 THE FORMATION MECHANISM OF THE BONIN HIGH 173 North Descent The Asian jet Q H East Figure 14. A schematic diagram illustrating the proposed formation mechanism for the Silk Road pattern and the Bonin high. Q designates the centre of heating, H the centre of the upper-tropospheric anticyclone near Japan, the oval the Tibetan high, the thick arrow the Asian jet, the light arrows the regions of strong descent, and the wavy line a potential-vorticity contour. 5. CONCLUSIONS AND DISCUSSION In this section, we summarize the formation mechanism for the equivalentbarotropic Bonin high. Then, some observational evidence is given. Finally, we discuss the implications of our hypothesis. (a) A new formation mechanism We have proposed a new hypothesis for the formation mechanism for the Bonin high in August based on an examination of the climatology and the results of simple model experiments. The formation mechanism for the equivalent-barotropic Bonin high is schematically illustrated in Fig. 14 and can be summarized as follows: 1. Indian convective heating creates the Tibetan high to its west. Associated with this anticyclone, the westerly zonal wind on the poleward ank is strengthened to form the Asian jet. This provides a basic waveguide favourable for the propagation of quasi-stationary waves. 2. Localized mid-tropospheric descent to the west of the Indian convection is enhanced through the monsoon desert mechanism. The associated upper-tropospheric convergence acts as a wave source near the jet entrance. 3. Quasi-stationary Rossby waves propagate along the Asian jet and accumulate in the jet exit region, creating a ridge near Japan. This upper-tropospheric ridge near Japan is associated with low PV (PV 6 1 PVU, at 350 K). Thus, tropical tropospheric air is brought northward and the tropopause is elevated. 4. The low PV anomaly near Japan induces an anticyclonic ow below. Sometimes the anomaly is so strong that the anticyclonic circulation reaches the surface. The anticyclone formed by the above mechanism is consistent with the observed Bonin high in August in many respects. Zonal scale: The zonal scale of the Bonin high is about half the wavelength of the stationary Rossby wave along the jet. This correspondence is consistent with the

18 174 T. ENOMOTO et al. relationship between the two and with the transition from the zonally elongated structure in the early summer. Equivalent-barotropic structure: It is more natural to associate the deep structure with the stationary Rossby waves propagating along a waveguide than with the external mode generated remotely by tropical heating. Since f=n O.10 2 /, where N is the Brunt Väisälä frequency, and the Rossby depth H f=nl, where L is the horizontal scale of the anomaly, is estimated to be large enough for the anomaly near the tropopause to affect ows in the lower troposphere. Lifetime: The intensity of the Bonin high is modulated with a period of days as mentioned in section 1. The stationary ridge near Japan can be modulated by wave packets propagating from the west. The wave packet simulated in our experiments gives a time-scale similar to the typical lifetime of the Bonin high. Northward migration: The Bonin high migrates northwards during the summer. In particular, it jumps by 1Á 10 ± after the end of the Baiu. The stationary-wave propagation along the jet is consistent with the latitudinal position of the Bonin high in August. (b) The Silk Road pattern in observations The Silk Road pattern is found in observational analyses in terms of both height anomalies and PV. Figure 15 shows height anomalies composed of zonal wave numbers 3 or greater at 200 hpa in August The Bonin high was abnormally developed in this month and brought hot weather to Japan. It is seen that wave trains occur several times within the month. The value of the anticyclonic height anomaly near Japan exceeds 120 m. Assuming quasi-geostrophy and sub-planetary scales, 1Z D 120 m corresponds to 1Ã D 1: m 2 s 1, which is about 50% larger than the value obtained in the model. A typical group velocity is estimated to be 24 m s 1 or 60 degrees in 2.5 days, larger than in the model. The stationary waves also have a somewhat smaller wave number (k D 6) than in the model. Both these features are consistent with a stronger zonal jet in this month exceeding 40 m s 1 at its core in the monthly average (not shown). The Silk Road pattern is also seen in PV. Koide (1996) pointed out the low PV anomaly over Japan as a cause for an extraordinarily hot climate in the summer of A part of his gure is reproduced in Fig. 16, which uses the same code of shading as in Fig. 13. He mentioned that the PV anomaly was of unknown cause and propagated eastward. He noticed that the trough near the Caspian Sea preceded the one over China in the 10-day averaged plots for June (not shown). However, these 10-day averaged plots are not able to show the time evolution of the PV: the equivalent-barotropic structure of the Bonin high has a lifetime of days and the wave packet from the jet entrance region reaches Japan in less than a week. Another related example is found in Dethof et al. (1999) who pointed out that the breaking of the Tibetan anticyclone acts to transport water vapour into the lower stratosphere. The Asian jet acts as a barrier to water-vapour transport into the middle world, and the cut-off anticyclone near Japan is found to play a signi cant role in the water-vapour transport. Our study provides a formation mechanism for such a cut-off anticyclone.

19 THE FORMATION MECHANISM OF THE BONIN HIGH 175 day contour interval = 1e6 (m 2 s 1 ) Figure 15. Longitude time plot as in Fig. 12 but for the perturbational geopotential height in August 1985 at 200 hpa and 37.6 ± N. Figure K potential-vorticity contours as in Fig. 13 but for July 1994 and for 10-day averages (adopted from Koide 1996). (c) Implications The observational evidence given here in terms of upper-tropospheric height and PV variations suggests that the Silk Road pattern is not merely a model phenomenon but is real and supports our proposed mechanism for the formation of the Bonin high. We plan to explore further the existence and variation of the Silk Road pattern with observed datasets, such as the ECMWF re-analysis, and with a model including various physical processes. We also plan to investigate the role of the Silk Road pattern in the disappearance of the Baiu frontal zone. In connection with this, Ueda et al. (1995) described the abrupt northward shift of the convective activity in the western Paci c and argued that the convective jump acts to terminate the Baiu season. It will be interesting to investigate whether the development of the Bonin high is an alternative mechanism for the disappearance of the Baiu frontal zone or is related to the convective jump. The intraseasonal variation of meridional wind along the Asian jet with a day period

20 176 T. ENOMOTO et al. found by Terao (1998) may be relevant to this topic. A recent study by Krishnan and Sugi (2001), conducted independently from ours, provides statistical evidence that the Silk Road pattern (their Asian Continental Pattern ) may appear in June and July in conjunction with the activity in the Indian monsoon and in the Baiu frontal zone. A similar teleconnection pattern in July has been investigated by Lu et al. (2002). Our hypothesis suggests that the enhancement of the Bonin high in the post-baiu period is caused by the propagation of stationary Rossby waves along the Asian jet. As discussed in section 1, the Bonin high already exists in the Baiu season, when it is most conspicuous in the mid to lower troposphere. At this time the Bonin high is probably associated with the descending branch of the Hadley cell, as traditionally understood. Therefore, it would be helpful to be aware of the Silk Road pattern as well as the convective activity in the western Paci c when analysing late-summer enhancement of the Bonin high. Our study is an extension of that by Rodwell and Hoskins (1996). Based on the dynamical link between the monsoon ascent and desert descent to its west, they argue that the Indian monsoon and the eastern Mediterranean Sea with its neighbouring desert regions can be viewed as a closely coupled system, as opposed to the traditional monsoon-only system that excludes the desert. We have shown that the late summer climate in east Asia is linked to the monsoon desert system via the Asian jet waveguide. This implies that a change in this monsoon desert jet system can cause a variation in the east Asian summer climate. Our hypothesis provides a way to approach interannual variability of the summer climate in east Asia. First of all, the variation in convection near the Bay of Bengal may result in a change in the monsoon anticyclone and the strength of the descent. The former affects the Asian jet and the characteristics of stationary waves, such as the preferred wavelength and group velocity. The Asian jet provides a medium to convey the information on meteorological events that occur near the jet entrance. For example, wave activity emitted from decaying blocks in Europe may be communicated downstream. The intensity of the Asian jet in particular affects the location of the Bonin high through the stationary wave number K s. A typical variation j1uj D 5 m s 1 results in j1k s j 1 or j1 j 10 ±. A variation j1uj D 10 m s 1 is less likely, consistent with the lack of occurrence of a trough near Japan, i.e. the Bonin low. These estimates indicate both the robustness of the ridge and its range of variability. It is hoped that further observational and numerical studies will clarify aspects of the variability of summer climate in east Asia. ACKNOWLEDGEMENTS This paper was prepared at the Frontier Research System for Global Change (FRSGC) as an extension to the PhD thesis of one of the authors (T. Enomoto). Comments from anonymous reviewers and the editor were very helpful in improving the manuscript. Valuable comments and suggestions provided by Dr Nakamura (University of Tokyo (UT)), Prof. Yamagata (UT), Dr Niino (Ocean Research Institute/UT), Prof. Kimoto (Center for Climate System Research/UT) are gratefully acknowledged. Discussions with Dr M. Rodwell (Hadley Centre/Met Of ce), Dr Takano (Meteorological Research Institute/JMA), Dr Nikaidou (JMA) and Dr Takaya (FRSGC) were of great help. Part of the study was conducted during the stay of one of the authors (T. Enomoto) at the Department of Meteorology, the University of Reading under the programme for the Japan Society for Promotion of Science research fellows provided

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