While there have been many paleoclimate studies on the precessional control of climate, typically only the orbital phase where perihelion occurs close to the solstices has received attention. Here, we explore how precession affects the seasonal evolution of the Asian summer monsoon in the transitional seasons of boreal spring and autumn. With perihelion occurring in boreal spring, the Hadley circulation weakens over the northern Indian Ocean, linked to precession-enhanced sensible heating over the Tibetan Plateau. There is an early northward migration of the midlatitude westerly jet stream, and the advancement of the pre-monsoon along the Asian–Australian land bridge. The pre-monsoon response to precession may have had a major role in the early part of the last deglaciation, when perihelion last occurred during boreal spring. A weak continental summer monsoon and autumn aphelion during the early part of the last deglaciation led to a weak Pacific high over the east of coastal East Asia, allowing for a vigorous oceanic western North Pacific monsoon in the late summer. Additionally, the seasonal expansion of oceanic monsoon trough could shed light on the quasi-stationarity of the oceanic monsoon during a precessional cycle.

It is debatable whether external forcing can change the state of the climate. This study explored how the 1990s decadal climate pattern shifts can be volcanically modulated. We investigated simulated decadal changes in the 1990s with and without volcanic aerosols and determined the atmospheric change that occurred in winter over the North Pacific Ocean. The volcanic eruptions contribution can be characterized as a series of rapid changes, including the strengthening and poleward shift of the midlatitude westerly jet stream.

In addition to the short-lived radiative effects primarily induced by the 1991 Mount Pinatubo eruption, the volcanically driven decadal change can be observed in the mid-to-late 1990s, suggesting a time-lagged characteristic of the volcanic climate impact. Compared with the decadal change irrelevant to volcanic eruption, the decadal state more dramatically enters into the next phase when volcanic forcing is included.

In this study, we explored time-varying climatological changes in the East Asian winter monsoon, focusing on the extent to which these changes are regulated by global warming. We examined climatological characteristics by primarily considering the interannual fluctuation (30 years as the unit period) in the upper-level westerly jet stream and the connection of this stream with the underlying monsoon circulation.

Over the last half century, a marked increase in global surface temperature was noted with an overall reduction in the interannual jet stream fluctuation; this occurred concurrently with disconnected interannual variations in the jet stream, midlevel trough, and surface monsoon condition, indicating that the current climatological change is regulated by global warming.

Other climate forces unrelated to the effect of the global temperature increase on the climatological jet stream fluctuation must be considered. When a longer time period in the early 20th century is used in analyses, the suppression of the fluctuation in the climatological jet stream may not always be associated with an increase in temperature. Notably, the jet stream considerably fluctuated only in the negative phase of the PDO, which suggests regulation of PDO-related decadal dynamics.

The climatological changes observed in recent decades may reoccur in a warmer future. The suppression of the fluctuation in the upper-level jet stream and a disconnect among the monsoon vertical circulations may be clearly evident in future projections. The future projections also suggest the influence of additional climate forces on climatological changes, including PDO-modulated pattern shifts.

During the 1990s, pronounced changes occurred in major climatological fields worldwide. In the Northern Hemisphere, regarding the major shifts in the dry and wet patterns, the mid-1990s decadal change occurred more rapidly in winter than in summer. What accompanied by this decadal change were the strengthened lower-tropospheric anticyclones over the subtropical Pacific Ocean and a northward shift of the midlatitude westerly jet stream across the Afro–Asia–Pacific region. Increased trade winds resulted in considerable sea surface cooling over the central and eastern Pacific regions. These circulation changes induced the poleward expansion of two boreal winter systems: convection over the Maritime Continent and dry conditions in North Africa and West Asia.

Moreover, the weakening of the Arctic upper-tropospheric circulation further modulated the seasonal shift of the midlatitude jet stream. In boreal summer, the more gradual decadal change was associated with the westward shift of Asian monsoon and northward penetration of African monsoon. What used to be a substantial vertical coupling of the westerly flows above these two monsoons was weakened during the 1990s. Judging from the effects of these circulation changes, it appears that the different rate of change in the jet streams may have exerted a dynamic effect on the pace of the decadal change in different seasons.

The effect of precession on paleoclimate changes depends on eccentricity. However, whether and to what degree eccentricity relates to millennial-scale monsoonal changes remain unclear. By investigating climate simulations with a fixed precession condition of 9 ka before the present, we explored the potential influence of eccentricity on early-Holocene changes in the Afro–Asian summer monsoons. Compared with the lower eccentricity of the present day, higher eccentricity in the early Holocene strengthened the continental summer monsoons, Pacific anticyclone, and Hadley circulation, particularly over the ocean. Over Africa, the eccentricity-induced “dry-gets-wetter” condition could be related to the Green Sahara, suggesting a superimposed effect of precession. Over the western Pacific, the tropical response to eccentricity may have been competitive in terms of what an extremely high obliquity may have caused.

A downscaled modulation of eccentricity in relation to precession and obliquity cannot be ignored when paleomonsoon records are studied. Regarding early-Holocene monsoonal changes in South Asia, however, a high eccentricity may have had only a secondary effect on enhancing the monsoonal precipitation in the southern edge of the Tibetan Plateau, exhibiting the weak power of candle-like heating. This suggested that sizable monsoonal changes over the northern Indian Ocean and India–Pakistan region are unrelated to early-Holocene eccentricity.

Seasonally, the East Asian particulate matter (PM) level is higher in the winter–spring period than in summer, at which time the level rapidly decreases due to the summer monsoon migration. Attempting to attribute the East Asian PM pollution to a source without considering such natural factors is challenging. However, to what degree the effect of season on an attribution bias remains open; the bias may even be implicated in PM-related health effects.

This study examined seasonal dynamics including the unusual precipitation evolution during 2020—a year in which coronavirus-related lockdowns occurred frequently worldwide—and suggested a large-scale effect from the removal of PM pollutants from most of the coastal cities in East Asia. In winter–spring 2020, compared with that of previous years, a deeper and farther southward intrusion of the East Asian coastal trough and a stronger surface monsoon flow acted jointly to transport air pollutants over the Korea–Japan region. In summer 2020, the strength and migration of the western North Pacific (WNP) high increased precipitation and removed air pollutants in the mid-latitude East Asia, whereas it reduced precipitation in the subtropical WNP. Consequently, the reduced PM level in the subtropical region (including Taiwan) may be irrelevant to the anomalous seasonal pattern.

Although an artificial effect is conceivable and may be primarily responsible for the marked decrease in 2020 East Asian PM pollutants in some subtropical cities, the modulation of a large-scale and precipitating effect also deserves consideration.

Understanding what drives a shift of the Afro–Asian summer monsoons from the continents to oceanic regions provides valuable insight into climate dynamics, changes, and modeling. Presently, the Afro–Asian summer monsoons have distinct timing onsets with a large range in late spring to mid-summer, following seasonal evolution of solar insolation and the mechanistic connection of the monsoons. In late July, the atmospheric circulation and precipitation enter another monsoon phase across West Africa, South Asia, and East Asia–WNP, corresponding to the development of the subtropical and oceanic monsoons. To explore what drives a shift of the Afro–Asian summer monsoons from the continents to oceanic regions, we investigated the asynchronous Holocene evolution of the monsoons and focused on whether the seasonal differences in insolation between early and late summer would cause shifting patterns of the monsoons.

We use data–model synthesis to focus on the differential seasonal responses of solar insolation and monsoons to orbital changes in the Holocene. We observe coordinated and stepwise seasonal evolution of summer monsoons across the mid-Holocene, suggesting the strengthening of the midlatitude jet stream as a bridge in the upper troposphere. Prior to the mid-Holocene, insolation had decreased considerably in early summer; the continental monsoons migrated southeastward, which corresponded to a more pronounced rainy season in coastal East Asia. In late summer, insolation did not decrease until the mid-Holocene. The continued weakening of the continental monsoons, combined with weakened insolation, rapidly enhanced the intrinsic dynamics over East Asia–Western North Pacific and accelerated a large-scale migration of the monsoon, suggesting orbital control of seasonal diversity.

FIG. (a, JAS) Major components of the West African summer monsoon index (zonal winds at 200 hPa and wind speeds at 925 hPa), the South Asian monsoon Hadley index (meridional winds at 200 and 850 hPa), and the WNP summer monsoon index (zonal winds at 850 hPa) are marked with blue for high obliquity and orange for low obliquity. Oval denotes overall precipitation regions in high (blue) vs. low (orange) obliquity.

FIG. (b, DJF) The sea level pressure (contours in lower panel) and the 500-hPa winds (vectors in upper panels) are shown. The East Asian midlevel coastal trough and Hadley circulation across East Asia are marked with blue for high obliquity and orange for low obliquity.

Obliquity, as a factor controlling seasonality, has been considered a modulator of paleomonsoon evolution. However, in contrast to the clearly identified contribution of precession, East Asian proxy records rarely provide a robust signal of obliquity. By investigating climate simulations with extremely low versus high obliquity, this study examined the seasonal contribution of obliquity, finding that obliquity has substantial effects on the diversity of precipitation stages over East Asia–Western North Pacific (WNP). The sensitivity of greenhouse gases (GHG) to the effects of obliquity also suggests that the contribution of obliquity could depend on GHG concentrations.

When GHG forcing is weak, summer precipitation migrates following the obliquity-driven meridional insolation changes over Africa, South Asia, and East Asia. In contrast to a condition of high obliquity with a continental origin of the monsoon, summer monsoon precipitation is confined to the subtropical area including the existence of the WNP monsoon trough in low obliquity. The intrinsic dynamic mode over East Asia–WNP, characterized by a meridional dipole pattern in circulation and precipitation, disappears when obliquity is extremely high. In winter, the strength of synoptic scale (the midlevel trough, Siberian high, and Aleutian low over and around East Asia) and larger scale (midlatitude westerly jet stream and Hadley circulation) circulation is coincidently weaker when compared with that in high obliquity. Obliquity-driven changes over East Asia–WNP are also partly sensitive to GHG concentrations. Furthermore, some major obliquity-driven changes weaken and even disappear when GHG concentrations increase.

FIG. 3 TYPES Precipitation (shaded, mm day−1) and geopotential heights in 100–200 hPa (contour lines, the value 14300 gpm was subtracted and only results higher than 0 are shown) for the ICM, TM, and IM during 1979–1994.

FIG. DECADAL CHANGES Locations in longitudes (pentad average) of the South Asian high center within 100–200 hPa in May–June for each year (100% is equivalent to 12 pentads); contour lines denote that a long-term average (1979–2010) is subtracted over each longitude position; a 5-year running mean was applied.

The seasonal evolution of the upper tropospheric South Asian high follows and influences underlying summer monsoon advancement. A strong connection between the South Asian high and westerly perturbation to the north suggests further planetary-scale dynamical control of the monsoon. In the mid-1990s, a clear location shift of the South Asian high in May–June was noted and was observed in fewer (more) frequencies of the high centers over the Indochina Peninsula (Iranian Plateau). Continental confinement of monsoonal circulation and precipitation was observed during 1995–2010, as opposed to larger-scale development in the Asia–Pacific region during 1979–1994. In view of early-summer monsoon evolution, a westward shifting and faster migration of the South Asian high may imply increased control of the midlatitude dynamics. By contrast, the convection over the tropical Western North Pacific (WNP) has an opposite and delayed contribution to monsoon advancement. After the mid-1990s than it had been previously, the midlatitude jet stream largely weakened over northern Africa and the East Asia–Pacific region, corresponding to an increase in the upper tropospheric geopotential heights north of the jet stream. Climate model experiments further reveal that the warming over Europe–Asia and temperature change in the North Atlantic can result in the change in midlatitude perturbations and the monsoon evolution in the mid-1990s, suggesting large-scale and dynamic impact on monsoon climatology.

FIG. Annual variation of 5-day air quality (percentages of pollution days) during 2004–2015 in northern (red circles) and southern (blue circles) Taiwan. Results applied by 3-pentad running mean are shown by solid lines. Locations of the observed air quality in Taiwan are marked in (g); gray shadings denote topography (m).

By investigating data during 2004–2015, this study explored the dynamic mechanisms of seasonal control of air quality over southern and northern Taiwan. Overall, higher (lower) pollution frequencies were observed in dry (wet) stages. Analysis of seasonality further suggested that pollution over southern Taiwan exhibited a symmetric characteristic, and that tropical modulation was a driving factor, whereas pollution over northern Taiwan displayed an asymmetric seasonality and likely followed the asymmetric East Asian seasonality. With the retreat of summer monsoon, air quality in southern Taiwan entered a poor phase rapidly; by contrast, good air quality was observed over northern Taiwan, which was due to the gradual southward migration of the western North pacific (WNP) high.

Detailed dynamic processes contributing to atmospheric conditions and large-scale circulation were further compared between pollution and nonpollution cases. During dry stages, the midlatitude perturbations observed as wavy patterns had downstream effects on the atmospheric condition in Taiwan. The downstream effect of the midlatitude wave activity on air pollution was obviously found in the Dry Northerly stage in northern Taiwan and the Dry Southerly stage in southern Taiwan. In contrast to poor air quality during dry stages, air pollution was rarely observed during wet stages. The strength and location of the WNP high, which can be involved in the formation of nearby tropical cyclones, may mainly influence air quality in Taiwan.

FIG. Schematic of broadscale monsoon evolution from July to August. Combined with the heating enhancement of the equatorial Indian Ocean (EIO), peak West African (WF) monsoon rainfall induces an anomalous anticyclone in the lower troposphere of the India–Pakistan region, weakens the South Asian upper-level anticyclone, and leads to a net reduction in the heating over the Arabian Sea (gray shading arrows). The enhanced heating over the subtropical WNP (SWNP) is associated with the monsoon gyre formation, which enhances the Bay of Bengal convection whereas suppresses the ascending southerlies in the Himalayan foothills and northern India–Pakistan region. The monsoon inversion of the Arabian Sea results from the competition among the Africa–Asia–WNP monsoon subsystems, in August as the peak season.

The summer monsoon inversion in the Arabian Sea is characterized by a large amount of low clouds and August as the peak season. Atmospheric stratification associated with the monsoon inversion has been considered a local system influenced by the advancement of the India–Pakistan monsoon. Empirical and numerical evidence from this study suggests that the Arabian Sea monsoon inversion is linked to a broader-scale monsoon evolution across the African Sahel, South Asia, and East Asia–Western North Pacific (WNP), rather than being a mere byproduct of the India–Pakistan monsoon progression.

In August, the upper-tropospheric anticyclone in South Asia extends sideways corresponding with the enhanced precipitation in the subtropical WNP, equatorial Indian Ocean, and African Sahel while the middle part of this anticyclone weakens over the Arabian Sea. The increased heating in the adjacent monsoon systems creates a suppression effect on the Arabian Sea, suggesting an apparent competition among the Africa–Asia–WNP monsoon subsystems. The peak Sahel rainfall in August, together with enhanced heating in the equatorial Indian Ocean, produces a critical effect on strengthening the Arabian Sea thermal inversion. By contrast, the WNP monsoon onset which signifies the eastward expansion of the subtropical Asian monsoon heating might play a secondary or opposite role in the Arabian Sea monsoon inversion.

FIG. Schematic of seasonal evolution (spring to autumn) of the subtropical circulation in the Asian–Pacific monsoon region. Color shadings denote elevation (unit: km).

In April, the larger temperature gradients between the two regions (Indochina Peninsula minus Philippine Sea) cause larger southerlies and support spring rains. However, the midlevel high band splits until the summer monsoon onset in after mid-to-late May; this suggests that only an increase in the zonal temperature contrast alone cannot drive the splitting of the subtropical high in March–April (Figure 13a). For early summer (Figure 13b), including the complex topography of the Indochina Peninsula strengthens the cyclonic streamfunction in South and East Asia, whereas it strengthens the anticyclonic streamfunction in the subtropical Western North Pacific (WNP). The continental monsoon (southwesterly) is primarily intensified. For late summer (Figure 13c), including the Indochina Peninsula topography strengthens the cyclonic streamfunction over the Peninsula, deepening the monsoon trough. The subtropical high belt east–west splits in late April over the Bay of Bengal, while it unites in late October over India (Figure 13d), indicating asymmetric seasonal migration of the monsoon circulation.

FIG. Schematic of the relative effects of precession and obliquity (the shaded color refers to Fig. 3; arrows with full color denote PminO24 minus PmaxO23; arrows with grid points or lines denote PminO24 minus PminO22); only different responses between PminO24 minus PmaxO23 and PminO24 minus PminO22 are marked in PmaxO23 (middle panels). In the early summer (a), the minimum precession and high obliquity are attributable to the northwestward shift and strengthening of the continental monsoon in Asia–Africa. In the tropics, precession enhances heating primarily over the Maritime Continent, whereas obliquity enhances heating mostly over the equatorial Indian Ocean, suggesting a competing role. By August (b), the Asian monsoon has shifted southeastward. Obliquity critically enhances the peak value of precipitation and convection in the equatorial Indian Ocean. A competing effect of precession and obliquity exists for reducing zonal heating contrast in the tropics.

On orbital timescales, higher summer insolation is thought to strengthen the continental monsoon while weakening the maritime monsoon in the Northern hemisphere. Through simulations using the Community Earth System Model, we evaluated the relative influence of perihelion precession and high obliquity in the early Holocene during the Asian summer monsoon. The major finding was that precession dominates the atmospheric heating change over the Tibetan Plateau–Himalayas and Maritime Continent, whereas obliquity is responsible for the heating change over the equatorial Indian Ocean. Thus, precession and obliquity can play contrasting roles in driving the monsoons on orbital timescales.

FIG. Meridional wind speeds (shadings, m s-1) and vertical motion (contour lines, Pa min-1) along 90°E: (a–d) Phase 1 of T1, T0, T0H, and T0NH; (e–h) Phase 2 of T1, T0, T0H, and T0NH. Black bars denote topography, and positive contour lines for vertical motion are not displayed.

The subseasonal migrations of the East Asian summer monsoon are nearly identical to that of the South Asian summer monsoon. In mid-May to mid-June (Phase 1), the South Asian westerly strengthens, the center of the South Asian high moves northwestward, and the East Asian frontal system coupled to and located north of the Western North Pacific (WNP) high moves northward, with all of these actions occurring rapidly. In mid-June to late July (Phase 2), the strength of the South Asian westerly reaches a maximum, the center of the South Asian high remains at approximately 30°N, and the northward propagation of the WNP high becomes stagnant. The speed of the northward movement of the WNP high in Phase 2 is only half the speed of that in Phase 1. By late June, the South Asian monsoon has reached northern India and been blocked by the Himalayas. This indicates that the Himalayas have an effect of constraining the speed of the northward movement of the South Asian high and, in turn, the WNP high.

Climate model experiments further reveal that the near-stationary nature of the East Asian frontal system in Phase 2 is related to the blocking of the South Asian summer monsoon by the Himalayas. We suggest that the evolution of the WNP high is constrained by the evolution of the South Asian high, and the Meiyu–Baiu is connected to the South Asian summer monsoon through the impact of the South Asian monsoon heating on the upper tropospheric circulation.

FIG. Schematic of the atmospheric response to a cooling Sea of Okhotsk (topography is marked with green).

Westerly perturbation is enlarged over the Far East–Okhotsk region in late June and early July, and is associated with the largest land-sea heating contrast surrounding the Sea of Okhotsk. The corresponding characteristics in the lower troposphere are southward deepening of the cold low over northeastern China, and intensification of the Okhotsk high. Coincidentally, the Meiyu–Baiu coupled with the western North Pacific (WNP) subtropical high is nearly stagnant during this period.

The enhanced land-sea thermal contrast over the pan–Okhotsk region is proposed as an obstacle influencing Meiyu–Baiu migration in late June and early July. During this period, the Meiyu–Baiu is nearly stagnant along the northwestern flank of the WNP subtropical high, and to the north of the Meiyu–Baiu the vortex over northeastern China and the Okhotsk high strengthens (Fig. a, streamlines at 850 hPa, blue). The meridional circulation characterized by two cyclonic mass streamfunctions (Fig. a, gray shading and white streamlines) exhibits a sandwich-like structure (i.e. the WNP subtropical and Okhotsk highs with the Meiyu–Baiu in between). The upper-tropospheric westerly jet stream correspondingly relates to the Meiyu–Baiu position (Fig. b, gray shading).

An experiment with a cooling Sea of Okhotsk suggests that the accompanying intensified Okhotsk high relates to the strengthened WNP subtropical high (Fig. b, increase of sea level pressure shown by blue contour lines, unit is hPa, also refer to blue shading for mean state; a, enhancement of downward motion shown by solid gray vectors), and the confined westerly jet toward the subtropics (Fig. b, orange contour lines and solid vectors, unit is m/s). The consequent enhancement of westerly-associated midtropospheric warm temperature advection fixes the position of the Meiyu–Baiu.

FIG. Topography (km) of (a) 0.16∘ resolution, (b) 1∘ resolution, and (c) 3∘ resolution.

Inevitable drawbacks occur when the Asian monsoon is simulated at a horizontal resolution that is too low to resolve topographic effects and sub-grid processes. By comparing the Coupled Model Intercomparison Project (CMIP) phase 5 simulations conducted at higher (∼1∘) and lower (∼3∘) horizontal resolutions, this study attempted to extend understanding of potential resolution limits in simulating the East Asian presummer climate. April–May is found to have considerable resolution-dependent contrasts in position and strength of precipitation (spring rains and pre–Meiyu) among the simulations. The low-resolution models cannot simulate ascending vertical motion over the mountainous regions in the southern Tibetan Plateau and subtropical East Asia, which coincides with the overestimation of the strength of the lower-tropospheric westerly in 90∘ –105∘E and of the downstream southerly in 110∘ –130∘E.

Because of the southerly wind bias, the East Asian presummer precipitation simulated in the low-resolution models is lower in amount and located farther north compared with the observed and high-resolution models. Despite distinct large-scale climatology between April and late May, the East Asian precipitation contrasts in these two periods are similar between the high- and low-resolutionmodels, which could be possibly explained as a systematic response to resolution. Systematically less precipitation in the high-resolution model compared with the low-resolution model is also observable over the Maritime Continent, whereas a further connection to the East Asian precipitation simulation remains unclear. Regarding projected changes, the resolution-dependent contrasts are also considerable; the contrasts further suggest potential regional interactions such as between the subtropical South and East Asia (10∘–20∘N).

FIG. Schematic diagram of the heating changes (relative to 0K) in mid-June.

The early-Holocene climatic optimum is associated with perihelion precession and high obliquity, though most studies emphasize the former over the latter. Asian monsoon proxy records only decisively show the precessional impact. To explore the obliquity effect, four climate simulations are conducted by fixing orbital parameters of present-day (0K), early Holocene (11K), the lowest obliquity (31K), and 11K's precession and eccentricity with 31K's obliquity (11Kp31Ko). We show that high obliquity significantly augments the precessional impact by shifting the Asian monsoon farther north than present. By contrast, the present-day monsoon seasonality can still be identified in the simulations with low obliquity.

We argue that the upper-tropospheric (South Asian) and lower-tropospheric (North Pacific) high-pressure systems are affected by the subtropical atmospheric heating changes responding to obliquity. As a consequence, associated with the changes in meridional gradients of geopotential height and temperature made by the heating, midlatitude transient eddies and monsoon-midlatitude interactions are modulated.

FIG. Schematic diagram of the atmospheric response (200hPa, 500hPa, and 850hPa) to the mountains for (a) LATE MAY and (b) LATE JULY. Orange color denotes general feature in each period and blue color denotes mountain-induced changes. Abbreviations: H for high pressure and AC/C for anticyclonic/cyclonic circulation anomaly.

Presence of the Arakan and Annamese Mountains helps simulate the vertical coupling of atmospheric circulation over the mountain regions. In late May, the existence of the Arakan Mountains enhances the vertically deep southwesterly flow originating from the trough over the Bay of Bengal. The ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the Tibetan Plateau and transports moisture across the Indochina Peninsula to East Asia. The existence of the Annamese Mountains helps the northward lower-tropospheric moisture transport over the South China Sea into the Meiyu-Baiu system, and the leeside troughing effect of the mountains likely contributes to the enhancement of the subtropical high to the east.

Moreover, the eastward propagation of wave energy from Central Asia to EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and Meiyu-Baiu. Including the Annamese Mountains also induces a Pacific-Japan (PJ) pattern-like perturbation in late July, by enhancing the cyclonic circulation (i.e. monsoon trough) in the lower-tropospheric western North Pacific. This suggests the contribution of the mountain effects to intrinsic variability of the summer monsoon in EAWNP.

FIG. Schematic diagram of the atmospheric response to the 11 ka BP solar forcing. Results are based on the 11K (light colors and dashed contours) and present-day (dark colors and solid contours) simulations from 25 July to 13 August. Contours denote zonal wind speeds (m/s). Blue shadings denote area in 11 ka BP where precipitation was enhanced relative to the present-day condition.

Corresponding to the enhanced land-sea thermal contrast, the upper-tropospheric South Asian high and lower-tropospheric North Pacific high were much stronger, whereas the upper-level jet stream weakened, shifted farther north, and was confined over the Asian continent and precipitation in South Asia and East Asia markedly shifted northward relative to its present-day location. The northward shift of East Asian precipitation was related to the northward shift of the jet stream and expansion of the WNP high.

Furthermore, the upper-level Pacific trough was deeper, corresponding to the weaker upper-level jet stream; this upper-level circulation change was consistent with the stronger triggering mechanism for the present-day WNP summer-monsoon onset. These atmospheric changes occurred despite suppression of the WNP monsoon, apparently as a consequence of the stronger land-sea thermal contrast by the more extreme, stronger WNP high, thereby modulating topography and other potential factors, especially enhanced precipitation over the Himalayas and southern Tibetan Plateau.

FIG. Schematic diagram of the major thermal and dynamical structure during the late May Asian summer monsoon transition. Shadings in the bottom level denote the precipitation pattern in late May.

At the start of the Asian summer monsoon, convection and precipitation develop rapidly in the BoB, along themountainous west coast of the Indochina Peninsula. Corresponding to the BoB monsoon onset in late May, the midtropospheric monsoon trough deepens, combined with the ascending southwesterly in the lower middle levels and the strengthening of the upper tropospheric anticyclone atop the trough. The formation of this atmospheric circulation structure, as well as the topographically anchored precipitation, is crucial during the Asian monsoon transition in May. By comparing the atmospheric thermal and dynamical structure of two global climate model simulations, with and without the Arakan Mountains (the major mountain range in the northwestern Indochina Peninsula), we demonstrated that these mesoscale mountains have a substantial impact on both local and large-scale circulation at the beginning of the Asian summer monsoon.

FIG. The schematic of years with a strong TP westerly forcing.

When the midlevel cloud amount in SB east of the TP is large, corresponding to a strong TP westerly forcing, the southwest-northeast tilt of the upper level jet stream increases and the ascending southerly east of SB between 110°E and 125°E strengthens. Farther downstream to the region between 125°E and 140°E, a strong TP westerly forcing is related to a weak ascending southerly. Correspondingly, an anomalous anticyclonic circulation between SB and Japan leads to a strong western North Pacific high pressure in the lower troposphere. The monsoons in East Asia and South Asia are both weak, corresponding to a strong TP westerly forcing.

Prior to the rapid transition, the East Asian jet stream (EAJS) weakens and shifts northward, which induces a series of changes in downstream regions; the northeastern stretch of the Asian high weakens, upper-tropospheric divergence in the region southwest of the mid-NP trough increases, and convection is enhanced. At the monsoon transition, upper-level high potential vorticity intrudes southward and westward, convection expand from the mid NP westward to cover the entire subtropical western NP, the lower-tropospheric monsoon trough deepens, surface southwesterly flow strengthens, and the western stretch of the NP high shifts northward 10° latitude to the south of Japan. This series of changes indicates that the EA-WNP late July monsoon transition is initiated from changes in the upper-tropospheric circulation via the weakening of the EAJS.

FIG. The middle- and low-level thick clouds (MLTC, unit: %) in a lunar synodic cycle in the semi-permanent anticyclone (SPA) region (17.5N–27.5N, 115E–135E).

The semi-permanent anticyclone (SPA) in the East Asian winter monsoon (EAWM) exists at lower-tropospheric atmosphere center near Taiwan. Between the first and the last quarter of the moon, the northward movement of the SPA ridge is assessed, to be the cause of the enhancement of windward coastal precipitation in East and Southeast Asia. Corresponding to the SPA variation across the full moon, the geopotential height over the SPA region increased at lower-tropospheric atmosphere and decreased at higher-tropospheric atmosphere. Associated with the squishing atmosphere, the middle- and low-level clouds with thick optical thickness (MLTC) increased significantly, which was highly related to the increased precipitation (there was 1/4 MLTC enhancement in the SPA region calculated by three-fifths of the lunar months between 1985 and 2006). It is suggested that the lunar-synodic cycle might modulate the EAWM precipitation when the atmospheric conditions change and favor development of thick clouds.

FIG. (a) The leading mode of the EOF (EOF1) derived from the outgoing longwave radiation (OLR, unit: Wm-2 ) of 22-summers (JJA). (b) The principal component of the leading EOF mode (PC1) of seven years that have a clear shift in July. The onset dates when PC1 = 0 are given in (b).

At least in seven of the 22 years between 1985 and 2006, the arrival of clouds at the SWNP from the east coincides with a significant change in the upper tropospheric circulation and a rapid northeastward extension of strong convections from the tropical western North Pacific, which essentially is monsoon onset. Before the monsoon onset, the sea surface temperature (SST) increases, but winds remain divergent over the SWNP. Right after the monsoon onset, winds turn convergent and convections enhance, leading to a rapid decrease of surface heating and SST. It is suggested that westward-moving upper-level disturbances might trigger onset of monsoon in July when low-level atmospheric conditions favor development of deep convections.

FIG. The cloud-amount index (cloud amount of DC + CS minus CI, orange shading) and vertical velocity in each level (thick contour). The bar chart at the bottom denotes heavy precipitation and medium to small precipitation respectively.

Based on the cloud classification of the International Satellite Cloud Climatology Project (ISCCP), we found a useful cloud-amount index from cloud amounts of cirrus minus the sum of deep convection and cirrostratus. The index can effectively separate different characteristics of CRF from the APSM time evolution. The cloud-amount index should be more appropriate for APSM studies and model simulations instead of considering only one cloud type in convective systems.