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ラージスケールの大気循環場の観点から,平成18年豪雪の実態を調べ豪雪発生の要因を考察した.2005年12月が過去50年間で最も冬季東アジアモンスーンが強く,豪雪発生の主要因であると考えられる.冬季モンスーンの強化と関連する特異な大気循環偏差は主に低緯度域で観測された.具体的には,南シナ海・フィリピン海周辺で活発な積雲対流活動がみられ,その対流加熱により中国南部の対流圏上層に非常に強い高気圧が形成された.大規模な高気圧性渦度が定在ロスビー波の波源となり,東方へのエネルギー伝播を通して冬季東アジアモンスーンの強化に寄与した可能性が示唆される.過去の豪雪年と較べて,熱帯からの遠隔強制が強くみられたのが平成18年豪雪の特徴である.

The extratropical response to typhoon-related convective forcing over the western North Pacific in late summer is examined based on ECMWF global reanalysis (ERA-40) data during the 1958-2001 period. Typhoon activity is intimately associated with most of the major events in which an extratropical wavetrain structure prevails from the north of the Philippines through the central North Pacific. The vertical structure of the wavetrain pattern changes from baroclinic to a barotropic along the great circle. The analysis of the wave activity flux indicates that the extratropical wavetrain is stimulated by stationary Rossby waves. It was found that one or two typhoons, which are a synoptic-scale convective heat source over the western North Pacific, can induce the barotropic Rossby wavetrain and significantly influence the summer weather in the vicinity of Japan as remote forcing.

A remarkable change in the relationship between the El Nino-Southern Oscillation (ENSO) and Indian summer monsoon (ISM) during the last five decades is examined using a fine resolution rainfall data set, ENSO indices and a large-scale monsoon circulation index. Although the all-India monsoon rainfall (IMR) index represents rainfall variations particularly over central and western parts of India, it does not represent rainfall variability in northeast India, which leads to the weakening correlation between IMR and the large-scale monsoon index in recent decades. The relationship between ISM and ENSO was strong in late summer over central and northwest India before the late 1970s but, after the late 1970s, the strongest relationship between ISM and ENSO shifted to early summer and also moved to over northeast India. In effect, the recent weakening relationship between ISM and ENSO based on the use of IMR represents a change in dominance of the spatial correlation pattern, from northwest-central parts to northeast part after the late 1970s.

An anomalously warm sea-surface temperature (SST) developed to the east of Japan in the midlatitude western North Pacific (WNP) during three consecutive years, from 1999 through to 2001. The anomalies in those years simultaneously reached a maximum deviation (>2.5 °C) during the boreal summer. Prior to this seasonal peaking, convective activity was significantly enhanced around the Philippine Sea, while Japan and the adjacent ocean were characterized by weak convection. Almost concurrently with this, the lower troposphere in the subtropical WNP exhibited an anomalous cyclonic flow slightly to the northwest of the Philippine convection. In contrast, anticyclonic anomalies prevailed over the midlatitude WNP in the area centred around 35 °N, 150 °E. The many changes observed in the convection and circulation fields can be explained by wave trains related to the stationary Rossby wave. In situ heat budget analysis at the ocean surface indicates that, in association with the midlatitude anticyclone, the combined effect of reduced evaporative cooling and intensified downward shortwave radiation flux can be responsible for the salient increase in SST. Furthermore, analysis of the subsurface water reveals that the anomalous state in the midlatitude WNP was limited in the near-surface layer, which is consistent with the result of the heat exchange at the ocean surface. Finally, these anomalous years correspond to a cold episode of El Nino, suggesting the potential predictability of the Asian summer monsoon. In view of this, we also examine precursor signals in the pre-monsoon season over the tropical Indian Ocean and their possible linkage to summertime climate conditions in the midlatitude WNP. Copyright © 2004 Royal Meteorological Society

Four teleconnection patterns that are possibly associated with the anomalous summer climate in Japan and the surrounding regions, were extracted by applying empirical orthogonal function and regression analyses to stream function anomalies. The two teleconnection patterns prevailing over northern Eurasia, especially in early summer, called the Europe-Japan (EJ) 1 and EJ2, are linked with the variability of the Okhotsk high. The third teleconnection pattern, called the West Asia-Japan (WJ), is a stationary wave-train pattern along the upper-level subtropical jet from West Asia to the central North Pacific, which is possibly excited by the anomalous convective heating of the Indian summer monsoon. The final teleconnection pattern is identified with the Pacific-Japan (PJ), found by Nitta. Teleconnection indices that account for the variability of those patterns are also defined on a monthly basis. The PJ and WJ patterns, which are more influential teleconnection patterns than the others, are closely related to the summer temperature anomalies, especially in northern and western Japan, respectively. EJ1 and EJ2 were amplified in several extreme summers, and they played a vital role in the cool summer of 2003, along with PJ. A combination of two or three teleconnection patterns was also responsible for the occurrence of the recent extreme summers. Monitoring the major teleconnection patterns is very useful for understanding and forecasting the anomalous summer climate in East Asia.

Meteorological and hydrological data were analysed, over 37 years during the period from 1965 to 2001, recorded by the Kurobe Dam meteorological observatory, which is located in a nature-reserved mountainous area within a heavy-snowfall region of central Japan. The analysis demonstrated that both winter runoff and air temperature increased significantly during this period. The interannual variation of winter runoff was governed not only by precipitation, but also by air temperature. The significant increasing trends in air temperature were also found at the adjacent JMA meteorological stations, showing a high correlation with air temperature observed by the Kurobe Dam.

A dominant impact of the El Nino-Southern Oscillation (ENSO) on the South Asian summer monsoon interannual variability is identified using the National Centers for Environmental Prediction /National Center for Atmospheric Research reanalysis aided by an ocean general circulation model. It has an equatorially symmetric structure and is most pronounced at the growth phase of ENSO in late summer during the period from the 1960s to mid-1970s when tropospheric biennial oscillation (TBO)-like ENSO dominates. As ENSO develops, an anomalous Walker circulation system over the tropical Indian and Pacific oceans changes from a single-cell regime in summer to a double-cell regime in fall. Meanwhile, rainfall anomalies of an equatorially symmetric structure are induced over the tropical Indian Ocean, accompanied by north-south twin circulation anomalies in the lower troposphere. The northern circulation is dynamically linked with anomalous monsoon rainfall over India especially in late summer. This research suggests that a combination of the wind-evaporation feedback in the Indian Ocean and ocean dynamics in the tropical Pacific is crucial for the regime transition of the anomalous Walker circulation system associated with the TBO-like ENSO. As the seasonality of the ENSO cycle changes before and after the late 1970s, the difference in ENSO impacts between its growth and decay phases may have influenced the ENSO-monsoon relationship and caused its interdecadal change.

Using the European Centre for Medium-Range Weather Forecasts reanalysis data, we examine whether an onset mechanism of the Australian summer monsoon proposed by Kawamura et al., which incorporates possible air-sea feedback processes, can apply to the South Asian summer monsoon system as well. In their mechanism, a combination of the increase in sea surface temperatures and dry intrusion into the layer at 600-850 hPa level over the ocean off, the equatorial side of a continent, plays a crucial role in enhancing potentially convective instability prior to the onset. It is found that the onset mechanism is able to apply to the abrupt onset of the Indian summer monsoon at the beginning of June, rather than the earliest onset over the Indochina peninsula in middle May. This is consistent with the observational fact that abruptness of the monsoon onset is most evident over the coastal regions of the Indian subcontinent. The Indochina peninsula is characterized by a relatively slow onset, although its date is earliest. The asymmetry of the transition speed between the onset and retreat regimes also has similar regional features. If the mechanism operates efficiently on the Indian subcontinent and adjacent oceans, it is anticipated that the onset of the Indian summer monsoon is delayed, as compared to that over Southeast Asia because subsidence in the periphery of a sub-continental scale thermal low, resulting from intensification of land-ocean thermal contrast, inhibits convection. This may be one of the possible reasons why the two major onsets seen in the South Asian summer monsoon system are clearly distinguished from each other.

The onset processes of the zonally asymmetric anomalies of convection and sea surface temperature (SST) over the tropical Indian Ocean are investigated with considering seasonal evolution, and interannual variability, of the large-scale convection anomalies in the Asian summer monsoon, using outgoing longwave radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern of the convection anomalies is particularly dominated in boreal autumn. Some recent studies have noted that these anomalies, based on the atmosphere-ocean coupling phenomenon, can be developed and maintained by itself. The time evolution shows that the eastern part of the zonally asymmetric anomalies over the Indian Ocean lead the western part of those. In July, the negative SST anomalies and positive OLR anomalies first appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatological southeasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SST cooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetric anomalies in the following autumn. It is suggested that this southeasterly wind acceleration over the southeastern Indian Ocean is closely linked to the meridionally asymmetric anomalies of convection, between the maritime continent and the South China Sea/Philippine Sea (SCS/PS). That is, the intensification of the local Hadley circulation over the western Pacific associated with the enhanced convection over the SCS/PS, and suppressed convection over the maritime continent, is found to be a clear precursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been noted that the convection anomalies over the southern and northern parts of the meridionally asymmetric anomalies over the western Pacific are not always the opposite sign, and seem to have different interannual variability respectively. It is likely that the former might be strongly influenced by the ENSO, through the Walker circulation anomalies and the latter might be affected by the modulation of the intraseasonal variation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomalies is also suggested from the occurrence of the intensification of the local Hadley circulation in boreal summer.

An unusually prolonged foehn was observed at Toyama in the Hokuriku district, located on the coast of the Sea of Japan of central Japan, from 30 July to 3 August 1999. A 5-day foehn is extraordinarily persistent and was not observed during the 24-year period 1975–1998, which makes the 1999 summer quite exceptional. While an anticyclone east of Japan is intensified over the 5-day period, a combination of the anticyclone with a rapidly developing cyclone over southeastern Siberia and a typhoon migrating northward into the East China Sea induces a definite intensification of southerly geostrophic winds over central Japan, a favorable condition for the occurrence of unusually prolonged foehn. Observational and model results show that on a monthly mean basis, a noticeable east-west pressure gradient around Japan is present due to the prominence of the Pacific and Japan (PJ) teleconnection pattern in response to tropical convective heating, resulting in reinforcing southerly geostrophic winds across central Japan. Once large-scale circulation anomalies are initiated and sustained as a result of such an extratropical response, the development and movement of adjacent synoptic-scale disturbances are largely regulated by those anomalies. A combined effect of the excitation of the PJ pattern and associated synoptic-scale disturbances is crucial for the extraordinary persistence of foehn along the coast of the Sea of Japan. The PJ pattern that appeared in July 1999 has an unusual geographical location and configuration because enhanced cumulus convection over the warm pool region of the western North Pacific is significantly displaced about 20 � westward and 2–3 � northward, compared to the typical hot summer that Japan experiences. This displacement is presumably attributed to a similar shift of distinctive warm SST anomalies over the warm pool region.

Using an ocean general circulation model forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis, we examined possible roles of the tropical Indian Ocean in the Asian summer monsoon and El Nino-Southern Oscillation (ENSO) coupling. A dominant precursory signal of anomalous monsoon circulation relevant to ENSO is the presence of latitudinally asymmetric anomalies of tropical convection and sea surface temperature (SST) near the equator in the preceding spring. The model results show that the equatorial asymmetry of surface latent heat flux plays a vital role in generating the equatorially asymmetric SST in the preceding spring, which implies that a positive wind-evaporation-SST (WES) feedback proposed by Xie and Philander [1994] is crucially responsible for generation and maintenance of those equatorial asymmetries. The westward extension of anomalous convection from the vicinity of the Philippines into the northern Indian Ocean from the preceding winter to spring, which has a signal of ENSO, can be viewed as a developing process of WES. Persistence of the WES feedback regime affects land surface hydrologic processes in the Asian continent through a Rossby wave response to anomalous convective heating. It is anticipated that the WES mode in the tropical Indian Ocean serves as a bridge between the ENSO prevailing in the preceding winter and the anomalous summer monsoon.

Using an ocean general circulation model (OGCM) forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the NCEP/NCAR reanalysis of 1973-1995, we examined interannual atmosphere-ocean variations in the tropical western North Pacific related to the Asian summer monsoon-ENSO coupling. The OGCM simulation was successful in reproducing the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines that is linked with anomalous tropical convection in that vicinity. A heat budget analysis shows that a longitudinal asymmetry of surface latent heat flux anomalies is crucially responsible for the reinforcement and persistence of the east-west gradient of SST anomalies between the two regions. It is also found that the transition from premonsoon regime to monsoon regime in the tropical Indian Ocean affects the interannual atmosphere-ocean variations in the tropical western North Pacific during the period from the late 1970s to the early 1990s. In the spring before strong Asian summer monsoon, an equatorially asymmetric air-sea coupled mode tends to appear in the tropical Indian Ocean. Simultaneously with the beginning of the strong monsoon regime, the northern Indian Ocean and South China Sea are covered by cool SST anomalies due to enhanced wind speed and evaporation, whereas warm anomalies relevant to a cold episode of ENSO are still maintained in the warm pool region east of the Philippines. The latitudinally asymmetric anomalies of tropical convection and SST become dissipated and convection over the warm pool region of the western North Pacific becomes localized and enhanced with the establishment of east-west gradient of SST anomalies. Due to enhanced convective heating, equatorially asymmetric atmospheric Rossby waves are excited to the west of anomalous convection, which induce low-level westerly anomalies. This dynamic process further facilitates the localization of intense convection through the change in surface latent heat flux and SST. While such a positive atmosphere-ocean feedback system persists in boreal summer, the Pacific and Japan (PJ) teleconnection pattern in response to enhanced convection prevails and brings about unusually hot summers in recent years especially in the vicinity of Japan.

Axial and equatorial atmospheric angular momentum (AAM) functions for the rotational dynamics of the Earth are calculated monthly from ensemble mean data of three independent 40-year simulations during 1955-1994 by the global model of the Japan Meteorological Agency (JMA) forced by observed near-global sea surface temperature (SST) conditions. The model results are compared with those from the reanalysis data of the National Centers for Environmental Prediction (NCEP) and the operational objective analysis data of JMA and with the functions inferred from the observed length of day (LOD) and polar motion. The annual term of the simulated axial wind AAM function (dimensionless relative angular momentum of atmosphere due to zonal wind) during 1984-1994 agrees well with those from the two analysis data sets and roughly with the inferred function from LOD, while the semi-annual term is considerably over-estimated, suggesting an incompleteness in the simulated subtropical zonal winds. The annual term of the simulated equatorial pressure AAM function (dimensionless atmospheric inertia products due to atmospheric mass redistribution) is considerably over-estimated with respect to those from the two analysis data sets, presumably due to the large simulated redistribution of atmospheric mass between the Eurasian continent and the North Pacific Ocean. For interannual Variations during 1955-1994, only the axial wind AAM function is reasonably simulated and shows good agreement with that from NCEP data as well as the Southern Oscillation Index. The above results lead to an understanding that the SST-forced AGCM simulates reasonably the atmospheric axial modes exciting LOD change but not the equatorial (non-axial) modes exciting the polar motion.

Two noticeable abrupt changes in Northern Hemispheric winter circulation have occured in recent decades, one around 1977 and the other around 1989. In the 1970s occurrence, changes in equatorial sea surface temperatures (SSTs) occurred concurrently with those in mid-latitude atmospheric circulation. In the case of the late 1980s, however, no important variation in SSTs was seen in the equatorial regions. In order to identify the differences in the characteristics of the changes in circulation between the two cases, observed 500 hPa heights were compared to simulations with an atmospheric general circulation model (GCM) that used observed SSTs as surface boundary conditions. The results of simulation suggest that the change in circulation in the late 1980s was not the direct response to the changes in SSTs, but can rather be characterized as a triggered planetary scale internal mode of variability in the winter atmosphere.

The slowly evolving annual cycle (signified as L-mode) was defined by the sum of the first three sinusoids in a series of climatological pentad mean data of infrared equivalent black body temperature, 850 hPa wind, temperature, geopotential height and specific humidity, while the rapidly varying annual cycle (denoted by S-mode) was determined by adding up the remaining sinusoids. During early summer, the L-mode exhibited a blocking-type configuration with a ridge over the Kamchatka-Okhots. region, whereas a trough stretched eastward from northern China (center of a continental heat low) to Japan and beyond. The establishment of a locally independent L-mode anticyclonic cell over the Sea of Okhotsk enhanced low-level easterly anomalies from the vicinity of the Aleutian islands through northern Japan. The L-mode easterly anomalies merged with southwesterly anomalies along the southeastern periphery of a continental-scale heat low, thus generating a pronounced cyclonic shear zone around Japan accompanied by low-level moisture convergence. This L-mode lower tropospheric trough in intrinsically association wit h the east-west temperature gradient between East Asia and the western North Pacific plays a vital role in the formation of Baiu system in early summer. The L-mode southwesterlies along the east coast of China, which are attributed to the prominence of the continental-scale heat low, served as a bridge that links the monsoon westerlies of tropical origin with the westerly jet of mid-latitude origin. This resultec in producing a low-level westerly duct extending from the South China Sea to the central North h Pacific. At the Baiu onset phase (mid-June), the S-mode onset cyclone of convective origin developed over the South China Sea, and concurrently the S-mode onset anticyclone organized to the northeast of the onset cyclone. The consolidated effect of these onset vortexes amplified in the vicinity of the low-level westerly duct was to cause the northward advection of warm and moist air from the tropics to southern Japan. By mid-July, the Asian continental heat low reached its peak and the summer monsoon over Southeast Asia became fully established. At the Baiu withdrawal phase (late July). the continental heat low began to decay because of land surface cooling; nevertheless, the L-mode Pacific High still developed northward anti was most intense at the beginning of August As the east-west temperature gradient between the ocean and continent decreased, the L- mode lower tropospheric trough near Japan dissipated, whereas L-mode WNPM (summer monsoon over the western North Pacific) trough prevailed in the subtropics. The withdrawal of Baiu was also characterized by the dominance of S-mode disturbances as well as the Baiu onset, It is thus anticipated that the seasonal evolution of L-mode circulation field, associated with continent-ocean thermal contrast, strongly regulates the activities of S-mode perturbations in and around the low-level westerly duct. It seems likely that the westerly duct served as a wave guide for the barotropic Rossby wave dispersion of the S-mode disturbances.

An ensemble of three 40-year parallel simulations was performed using a T42 AGCM version of the Japan Meteorological Agency global model to answer the question why extraordinary cool anti hot summers in East Asia, especially Japan and Korea, tend to occur very frequently in recent years from the late 1970s to the early 1990s. Three independent long-term integrations from January 1955 to December 1994 were forced by the same SST boundary condition observed on the global scale. Our AGCM simulations employing prescribed observed SSTs were successful in reproducing extratropical circulation anomalies that bring about the decadal-scale amplitude modulation of interannual variations of summer mean temperatures in the vicinity of Japan. During the period from the beginning of 1980s to the early 1990s, the interannual variability of the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines became appreciably large, was accompanied by anomalous cumulus convection around the Philippines, and its phases coincided quite well with those of model-simulated lower-tropospheric geopotential height variations near Japan. The anomalous convective heating substantially affected summertime lower tropospheric circulation anomalies in East Asia through the dynamic process of the excitation of PJ teleconnection pattern (Nitta, 1987). The anomalous SST forcing from the tropics is crucially responsible for the frequent occurrence of extreme cool and hot summers in Japan and Korea from the late 1970s to the early 1990s. The presence of strong east-west gradient of SST anomalies across the Philippines is primarily attributed to the significant coupling of weak (strong) South Asian summer monsoon and the warm (cold) episode of ENSO. The warm episodes that occurred during the period from the late 1970s to the early 1990s are appreciably different from a typical model of El Nino event exemplified by Rasmusson and Carpenter (1982) in terms of seasonal evolution. It is anticipated that both unusually persistent ENSO signals from the preceding winter until summer and the associated South Asian summer monsoon activity strongly regulate the formation of the east-west SST gradient near the Philippines in boreal summer.

A significant coupling of the Asian summer monsoon and ENSO was examined using the NCEP/NCAR reanalysis for the period 1973-1995. Results show that a monsoon index, which is defined as meridional gradient of summertime upper-tropospheric thickness (200-500 hPa) anomalies across 20°N over the Indian subcontinent, is highly correlated with Nino-3 SST anomalies in the preceding spring. This is strongly suggestive of the presence of the indirect impact of anomalous SST forcing associated with ENSO on the Asian summer monsoon. Due to attenuated Walker circulation in response to a warm episode, convection is suppressed over the northern tropical Indian Ocean and the maritime continent from the preceding winter to spring. The suppressed tropical convection in the preceding spring generates anomalous cyclonic circulation to the west of the Tibetan Plateau as a result of the Rossby-type response to convective heating off the equator. The convection-induced anomalous cyclonic circulation accompanied by large-scale ascending atmospheric motion contributes substantially to increased rainfall and greater soil moisture, thus resulting in decreased land-surface temperature over central Asia to the northwest of the Indian subcontinent. On the other hand, warm SST anomalies are initially introduced over the tropical Indian Ocean in late spring prior to the onset of the monsoon due to the changes in the surface heat flux and/or dynamic response of the ocean to wind forcing, in intimately association with pronounced in situ low-level northeasterly wind anomalies and less cloud cover. Both these different physical processes in the land and ocean areas are crucially responsible for reduced land-ocean thermal contrast (or reduced meridional tropospheric temperature gradient), eventually bringing about the weakening of the Asian summer monsoon. The reverse situation is quite true for strong monsoon years. Once the summer monsoon becomes weak (strong) at its early stage due to these processes, the initially induced warm (cool) SST anomalies over the tropical Indian Ocean are further intensified. The mechanism proposed here is valid during the period from the late 1970s to the early 1990s when weak and strong monsoon years are categorized. During that period, the unusual Nino-3 SST anomalies tend to persist from the preceding winter until summer, hence serving as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon. However, regardless of when the monsoon-ENSO coupling is prominent, both the springtime outgoing longwave radiation and low-level wind anomalies dominating over the tropical Indian Ocean, associated with anomalous Walker circulation, are still crucial factors in terms of the potential predictability of the Asian summer monsoon.

A set of three climate experiments is performed using a T42 GCM version of the Japan Meteorological Agency global model to examine extratropical interdecadal and interannual variations over the North Pacific region associated with the anomalous SST forcing in the Tropics. Three independent 34-yr integrations from January 1955 to December 1988 are forced by the same SST boundary condition observed on the global scale. The set of these integrations provides clear evidence that the tropical SST impact upon the wintertime extratropical model atmosphere in the North Pacific is very significant. It is also concluded that the abrupt change of midlatitude circulation regime that occurred in the winter of 1976/77 was primarily caused by very localized tropical heating in the central Pacific. This anomalous SST forcing was most likely responsible for persistent negative height anomalies over the central North Pacific during at least the period from 1977 to 1983, which formed a part of the extratropical wav...

An ensemble of climate experiments has been performed using a T42 GCM version of the Japan Meteorological Agency global model to examine abnormal changes in the wintertime extratropical atmospheric response to anomalous sea surface temperature (SST) forcing in recent decades. Three independent 39-year integrations for the period 1955–1993 are forced by the same observed SST boundary condition. Although the tropical Pacific SST anomalies since the late 1980s indicate El Nino-like patterns, the Pacific/North American teleconnection (PNA) pattern is found to be hardly simulated, which stands in sharp contrast with that for the period 1977–1983. The SSTs in the vicinity of the maritime continent (the Indonesian region), including the tropical Indian Ocean, are very high during the period 1987–1993 and associated model atmosphere does undergo pronounced zonal heating over almost the entire tropics. Such tropical heating does not necessarily favor the barotropic Rossby wave dispersion into the extratropics emanating out of convective forcing since weakened Walker circulation is not capable of producing localized strong divergence anomalies, hence inhibiting generation of apparent vorticity sources in the subtropics. If the prevailing tendency of high SSTs in the vicinity of the maritime continent persists from now on, it is anticipated that a good correlation between El Nino-Southern Oscillation (ENSO) and PNA, which has been pointed out by previous numerous studies, becomes insignificant. This anomalous feature may be viewed as a signature of global warming.

Using the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data aided by a coupled ocean-atmosphere model, we investigated two different regimes of anomalous Walker circulation system over the Pacific and Indian Oceans before and after a climate shift, which occurred in the late 1970s. During the period before the climate shift, an upper-level velocity potential anomaly systematically moves eastward from the tropical Indian Ocean to the warm pool region of the western Pacific during the growth phase of El Nino-Southern Oscillation (ENSO). In the meantime, the activities of South Asian and Australian summer monsoon systems are directly affected by the evolution of the anomalous Walker circulation. During the period after the climate shift, in contrast, an upper-level velocity potential anomaly in the vicinity of the Philippine Sea and maritime continent is observed to expand westward into the northern Indian Ocean and South Asia during the decay phase of ENSO. This feature is identified with a major precursory signal of an anomalous South Asian summer monsoon in the preceding spring. The model captures a systematic eastward propagation similar to that observed prior to the late 1970s, but fails to reproduce the westward extension of the velocity potential anomaly observed to prevail after the late 1970s. The model results suggest that the cross-basin connection between the two oceans is a prerequisite for the turnabout of ENSO prior to the climate shift, in terms of the occurrence of westerly wind bursts.

The tropical Madden-Julian oscillation (MJO), which is of convective origin, systematically propagates northward in the Western North Pacific summer monsoon region (5°-20°N, 110°-160°E). The northward propagating MJO, which is statistically significant, displays strong horizontally and vertically asymmetric circulation. The low-level cyclonic response is pronounced due to the presence of convergent, cyclonically sheared mean monsoon flow, while the upper-level anticyclonic counterpart is weak due to the influence of the divergent, anticyclonically sheared upper-level mean monsoon flow. The upper tropospheric MJO response is largely associated with divergent winds, as manifested by strong equatorward outflows that export anticyclonic vorticity into the upper troposphere of the Southern Hemisphere. The summer mean monsoon flow is responsible for reinforcing the vertical asymmetry of the MJO circulation induced by convective heating, and for generating a barotropic component in the baroclinic MJO through the mean monsoon flow-MJO interaction. The mid-latitude intraseasonal oscillation (ISO) displays a barotropic wave train structure along a great circle traversing the North Pacific, and exhibits the largest variability in the westerly jet exit region of the North Pacific (40°-50°N, 180°-150°W). This phenomenon is attributed to the combination of two different dynamic processes. While the MJO is propagating northward, the barotropic MJO component generated by the mean monsoon flow-MJO interaction produces a significant tropical-extratropical interaction on the intraseasonal time scale. Namely, when the 45-day convective activity becomes strongest near the Philippines, the barotropic MJO component acts as an origin of the barotropic Rossby wave dispersion emanating out of the convective forcing, eventually contributing to the development of an extratropical ISO. The enstrophy budget analysis shows, on the other hand, that the largest contribution to ISO is the in situ barotropic mean flow-ISO interaction in and around the jet exit.

Interdecadal and interannual atmospheric variability in the extratropical Northern Hemisphere is investigated using an atmospheric GCM. The model used for this research is a T42 GCM version of the Japan Meteorological Agency (JMA-GSM89) global model. The 34-yr integration from January 1955 to December 1988 has been performed employing the real observed near-global SST condition. To estimate internal variability of the tropical and extratropical atmospheres, another 34-yr integration was conducted using the seasonally varying, climatological SST without interannual variability. Using the rotated EOF analysis, the authors made an intercomparison of the Pacific/North American (PNA) wintertime teleconnection patterns prevailing in the observed and simulated extratropical atmospheres in the two experiments. The polarity of PNA derived from the real SST experiment is indicative of definite interdecal variability. particularly an abrupt change of the midlatitude circulation regime over the North Pacific...

Seasonal variations of large-scale convective activity and wind over the western Pacific are examined using Geostationary Meteorological Satellite infrared equivalent blackbody temperature (T BB ) and European Center for Medium range Weather Forecast (ECMWF) global analyses over a 10-year period from 1980 to 1989. In particular, this study describes an abrupt northward shift of large-scale convective activity over the western Pacific around 20°N, 150°E in late July. The enhanced convective activity is coincident with strong cyclonic circulation there which induces westerlies to the south of the cyclone and easterlies to the north of it. It is emphasized that this strong cyclonic circulation appears suddenly over the subtropical western Pacific region. Monsoon westerlies to the west of 110°E are not similary accelerated at the same time, indicating that this abrupt change is independent of the Asian monsoon system. To the north, an anticyclonic circulation is generated, which corresponds to the withdrawal of the Baiu season over Japan. Furthermore, this abrupt northward shift of large-scale convective activity is shown to be associated with tropical cyclone activity. In the mid latitudes, geopotential height pattern between pre- and post-northward shifts of the large-scale convective activity in late July exhibit equivalent barotropic vertical structure, suggesting the Rossbywave propagation emanating northeastward from the enhanced convective region around 20°N, 140°E (western Pacific) to as far north as 60°N, 180° (Bering Sea). Another feature is that the seasonal increase of sea surface temperature (SST) over the key area (20°N, 150°E) precedes abrupt convective enhancement by about 20 days, exceeding 29°C in early July. It is inferred that the northeastward extension of the warm SST tongue is intimately associated with the enhanced convection in late July. This result suggests that SST warming is not a sufficient condition but certainly one important ingredient for the abrupt northward shift of convections.

Spatiotemporal variability of preferred global-scale sea surface temperature anomaly patterns is documented, applying a varimax-rotated empirical orthogonal function (R-EOF) analysis to monthly mean SST anomalies. The present study focuses especially on the interdecadal variability of leading R-EOF modes. It is first found that temporal variability of R-EOF1 has a quasi periodicity of 2–5 years and coincides quite well with the occurrence of the ENSO event; hence this mode can be identified with the ENSO mode and distinguished from the other modes dominated by interdecadal variability. The authors find that R-EOF2 typically shows the dominance of interdecadal variability and signals of the ENSO phenomenon are removed. This mode is characterized by increasing Indian Ocean SST and decreasing central North Pacific SST around 40°–50°N in the recent ten or more years. A further indication is that both R-EOF3 and R-EOF4, which show the dominance of interdecadal variability, are fundamentally regarded a...

Atmospheric circulation patterns associated with snowfall fluctuations in Japan are examined using a rotated empirical orthogonal function (EOF) analysis. We also compute correlation coefficients between the scores of EOF modes in the 500 hPa geopotential height field of the Northern Hemisphere (NH) and amounts of snowfall in Japan on annual, monthly and pentad time scales. It is found that recent variability of snowfall amount in Japan is closely related to the long-term variations of large-scale circulation patterns. It is suggested that the dominance of teleconnection patterns such as Pacific/North American (PNA) and Northern Asian (NA) are responsible for the increase of snowfall in the coastal regions of the Sea of Japan during the cold period for Japan (1977–86).

[1] The relationships between large-scale wintertime circulation and extratropical cyclones that develop explosively (the so-called bomb cyclones) over the western North Pacific are investigated using Japanese long-term reanalysis project data. On a monthly basis, the East Asian winter monsoon variability strongly modulates the bomb cyclone activity in terms of its geographical distribution. When the monsoon is strong, the bomb cyclone activity tends to concentrate in the vicinity of the Kuroshio Current and the Kuroshio Extension near Japan, while when the monsoon is weak, it disperses over the broader areas. The enhancement of the monsoon increases the heat and moisture supply from warm currents, facilitating unstable conditions within the atmospheric boundary layer and intensifying baroclinicity in the lower troposphere. These factors are believed to play a role in inducing bomb cyclones, particularly along the warm currents. On submonthly timescales, the stationary Rossby wave propagation along the South Asian waveguide serves as a prominent trigger for the rapid reinforcement of synoptic-scale disturbances around Japan. When a pronounced bomb cyclone comes to its mature stage northeast of Japan, it is capable of exciting stationary Rossby waves downstream from the Asian jet exit region as vorticity forcing. The stationary wave packets developing southeastward across the North Pacific Ocean basin induce surface cyclogenesis in the vicinity of the Hawaiian Islands by leading to the equatorward advection of higher potential vorticity from the midlatitudes, bringing about the occurrence of kona storms, which cause weather hazards in Hawaii.

[1] The mechanism of the East Asian winter monsoon variability in response to El Nino−Southern Oscillation (ENSO)-related tropical forcing is investigated using Japanese long-term reanalysis project data, additionally aided by the Japan Meteorological Agency climate data assimilation system. There are at least two different responses, zonally symmetric and asymmetric, of the Asian jet over South Asia to the ENSO-related tropical convective forcing during the Northern Hemisphere winter. The zonally symmetric response, induced by zonally extended anomalous convection from the Philippine Sea through southern India and Sri Lanka, is pronounced at the mature phase of ENSO. The zonally asymmetric response is intimately associated with anomalous convection localized in the vicinity of the Philippine and South China seas, accompanied by an anomalous Walker circulation cell between the Maritime Continent and tropical Indian Ocean. When this asymmetric response is prominent, ENSO-related anomalous convection can give rise to a change in the East Asian winter monsoon system through stationary Rossby wave propagation along the South Asian waveguide. The North Atlantic Oscillation (NAO)-related extratropical forcing is also a crucial factor and contributes not only to the downstream development of subpolar teleconnections across northern Eurasia but also to the reinforcement of the zonally asymmetric pattern of the Asian jet over South Asia, resulting in a significant effect on the East Asian winter monsoon circulation. A combination of the ENSO- and NAO-related forcing plays a vital role in triggering the occurrence of extraordinary anomalous monsoon circulations, such as extremely heavy snowfall in the 2005/2006 winter in Japan.

[1] The lagged relationship between the El Nino-Southern Oscillation (ENSO) and Asian Summer Monsoon (ASM) variability is investigated using the Commonwealth Scientific and Industrial Research Organization (CSIRO) Mark3 coupled model. Composite analyses of 17 warm and 16 cold events from an extended unforced run reveal that the model captures the asymmetric structures of rainfall and SST anomalies over the tropical Indian Ocean during northern hemisphere spring which is one of the major precursory signals of anomalous ASM variability. As a remote forcing mechanism prior to the ASM onset, simulated anomalous convection over the northern Indian Ocean strongly influences the land surface hydrologic conditions over central and southwest Asia. Simulated land-surface processes result in anomalous temperatures over the land, which in turn produce a change in the land-ocean thermal contrast over the domain. The results are consistent with observed features, which show a delayed and indirect impact of ENSO on the ASM variability during the early summer monsoon period, but not during the rest of the monsoon season.

[1] An onset mechanism of the Australian summer monsoon that incorporates possible air-sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land-ocean thermal contrast during the premonsoon period results in a well-organized continental-scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600–700 hPa level. Intensified low-level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at � 700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large-scale disturbances with ascending motion (such as the Madden-Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics. INDEX TERMS: 1625 Global Change: Geomorphology and weathering (1824, 1886); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology; KEYWORDS: Australian summer monsoon, onset, convective instability, thermal low, dry intrusion

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Analyse statistique de l'interaction saisonniere entre les figures de circulation en zone temperee et les 2 anomalies dominantes de TSM (SST) dans le Pacifique nord. Le premier mode d'anomalie contribue a la formation et a l'evolution du type Pacifique-Nord Americain de teleconnexion en hiver. Le second mode est partiellement forme par la poussee froide venant de l'Asie de l'Est en hiver, etroitement associee a la predominance d'un indice bas de circulation. Lorsque ce 2eme mode predomine, le type Pacifique ouest predomine a 500 mb. En outre, ce mode est influence par l'invasion d'air polaire; aux latitudes temperees (40N), les anomalies negatives du 1er mode sont formees principalement par un forcage mecanique de l'atmosphere, cependant que celles du 2eme mode, dans le Pacifique ouest, sont formees par un forcage thermique de cette atmosphere

Dominant phase relationships between sea surface temperature (SST), high cloud cover (HCC), and zonal wind at 850 mbar on intraseasonal and interannual time scales in the warm pool region of the western Pacific are examined. Using the complex empirical orthogonal function (CEOF) analysis, two air-sea coupled modes, that is, an intraseasonal mode and an interannual mode, are statistically extracted. It is found that in the intraseasonal mode the area of maximum SST anomaly lies to the east of the center of the active large-scale convection area and therefore positive SST anomalies exist at the easterly anomaly area. The existence of high SST at the easterly anomaly area provides a favorable condition for a large-scale disturbance to propagate eastward, though the evaporation-wind feedback mechanism may also favor its eastward propagation if the mean wind is easterly. It is seen that the intraseasonal mode is very similar to the advective mode proposed by Lau and Shen (1988) with respect to the phase relationship. However, since the time evolution of the intraseasonal mode does not always coincide with the variations of east-west SST gradient over the tropical western Pacific, this mode requires another mechanism other than the east-west SST advection causing the advective mode. On one hand, the interannual mode has a characteristic that the phase relation of 850 mbar zonal wind (u) is different in each region of the tropical western Pacific, whereas the SST is almost in phase with HCC. The predominant growth of the mode defined in the key region (0°–10°N, 130°–160°E) corresponds to the life cycle that occurs when the El Nino/Southern Oscillation (ENSO) begins, develops, and decays. This verifies that the ENSO is one of the prominent phenomena in the air-sea interactive system on the interannual time scale. On the other hand, the mode over the tropical Pacific around the dateline explains the air-sea coupling when large-scale convection is not very active. It is thus clear that evident differences of air-sea coupling exist between the intraseasonal time scale and the interannual time scale. It is also determined that the strength of air-sea coupling varies with year in both time scales. The above results provide a clue for understanding the mechanism of multiscale air-sea coupling in the tropics.

An observational study has been conducted of the southern summer rainfall variability over the semi-arid region of southern Africa (SASA) since 1958, as related to the tropical African rainbelt, tropical atmospheric circulation, and sea surface temperature (SST) patterns. A rotated empirical orthogonal function (R-EOF) analysis was applied to the global monthly SST anomalies. On a year-to-year basis, the first R-EOF mode, related to the El Nino/Southern Oscillation (ENSO) phenomenon, was most strongly correlated with the southern African rainfall and two rainbelt indices ; the latitude of its center of gravity and the total rainfall that occurs over the entire rainbelt zone. The anomalous warming of the central-eastern equatorial Pacific, identified as the first R-EOF mode, was associated with increased 700 hPa heights throughout the tropics except for the eastern Pacific, especially marked over southern Africa and the eastern Indian Ocean-central Pacific region. Simultaneous intensification of the 700 hPa subtropical high over southern Africa was found to cause a northward displacement in the rainbelt's center of gravity and a decrease in total rainfall. These changes in the rainbelt parameters resulted in decreased rainfall for the SASA region, located south of the main rainbelt. On the other hand, the anomalous cooling of the eastern Pacific was related to a weakening of the southern African sub tropical high. This leads to a simultaneous southward advance and enhanced convection of the rainbelt, which ultimately result in increased SASA rainfall. On the decadal time scale, the third R-EOF mode, characterized by a dominant signal over the South Atlantic, is most strongly correlated with trends in the SASA rainfall and rainbelt parameters. The warming of the South Atlantic and simultaneous increases in the 700 hPa heights over southern Africa result in decreased rainfall ; cooling and decreased 700 hPa heights lead to increased rainfall.