East Asian Monsoon

The following sections are included:

• PREFACE

• CONTENTS

This chapter presents a comprehensive review of the seasonal march of the East Asian summer monsoon. First, the earliest onset of the summer monsoon over the South China Sea (SCS) and the Indo-China Peninsula occurring around mid-May or even earlier, is discussed in terms of low-level and upper-level wind, OLR and rainfall fields. A sudden change in these fields before and after the onset is revealed. This is characterized by a wind switch from low-level easterlies and high-level westerlies to low-level westerlies and high-level easterlies, the rapid growth of convection and increase in rainfall, and the eastward retreat of the subtropical high. The onset of the summer monsoon over the Indo-China Peninsula and the SCS is preceded by the development of circulation features and convective activity in the tropical East Indian Ocean and the Bay of Bengal. These are characterized by the development of a twin cyclone crossing the equator, the rapid acceleration of low-level westerlies, and a significant increase in both the areal extent and the intensity of convection.

Climatologically, the seasonal march of the summer monsoon displays distinct stepwise northward and northeastward advances. Over East Asia, two abrupt northward jumps and three stationary periods have been identified while over the western North Pacific three stages in the onset and advance of the summer monsoon have been identified. The noted East Asian monsoon rainy seasons, including the pre-summer rainy season over South China, Meiyu/Baiu over eastern China and Japan, and Changma over Korea occur normally during the stationary periods that are imbedded in the northward advance of the summer monsoon.

Major physical processes and mechanisms responsible for the onset and seasonal progress are discussed with special emphasis on three controlling factors and related physical mechanisms. The land-sea thermal contrast and the effect of the elevated heat source of the Tibetan Plateau are identified as pre-conditions for the abrupt onset of the Asian summer monsoon over the SCS and Indo-China Peninsula through the rapid reversal of the meridional temperature gradient. The arrival of the intra-seasonal oscillations (ISO) provides a triggering mechanism, along with the several phase-locking wet ISO phases. The intrusion of mid-latitude troughs into the northern SCS and central and northern Indo-China Peninsula is also seen to be another triggering mechanism that induces the convective activity through the release of potential instability, thus enhancing the monsoon trough there through the feedback process of meso-scale convective systems.

This chapter mainly focuses on the characteristics of the East Asia winter monsoon (EAWM). An examination of the climatology of the boreal winter in Asia shows that the EAWM results from the development of a cold-core high over the Siberia-Mongolia region. The movement of this cold air southward produces pressure surges and temperature drops across the Asian continent. Two types of such surges can be identified: the northerly surge (NS) and the easterly surge (ES). The initiation of the NS begins with the eastward passage of a polar jet streak west of Lake Balkhash. The eastward migration of this jet streak over the Siberia-Mongolia region intensifies a cold high there, which eventually leads to a southward outpour of the cold air in the lower troposphere. Such a push of the cold air then excites gravity waves that propagate across the South China Sea, which results in convection over the maritime continent. On the other hand, an ES is apparently the consequence of an initially eastward and then southeastward migration of a cold pool that splits off from a quasi-stationary high-pressure system over the Siberia-Mongolia region due to the passage of a 500-hPa ridge over the region. As the low-level anticyclone moves to the east coast of China, it initiates a southward surge of cool air and strong winds along the coast, resembling a coastal Kelvin wave. Its strength is usually much less than that of the NS. Other than these surges, a significant effect of the EAWM is the explosive development of low-pressure systems over the East China Sea as the cold air moves off the continent and over the warm water, which results from the strong baroclinity between the cold air from the continent and warm air over the ocean, and the subsequent potential instability, rising motion and latent heat release. The last section of the chapter discusses intraseasonal, interannual and interdecadal variations of the EAWM, which can be related to similar oscillations in other planetary-scale circulation features. These include the 10-20-day oscillation, the Madden-Julian Oscillation, the polar vortex, the El Niño/Southern Oscillation, sea-surface temperature anomalies in the North Pacific, the North Atlantic Oscillation, and the East Asia summer monsoon. Furthermore, “two-way” interactions between the EAWM and some of these oscillations have also been found.

The Maritime Continent and northern Australia is a region of strong seasonal variation in wind and rainfall regimes, which consist of a prevailing easterly wind and dry conditions during the boreal summer and prevailing westerly winds and wet conditions during the boreal winter. During the boreal winter, latent heat release associated with the wet season over the Maritime Continent and northern Australia contributes to a major component of the global circulation, which has been linked to significant tropical-extratropical interactions. Although substantial research has been conducted with respect to the northern Australia component of the boreal winter monsoon, the variations associated with the Maritime Continent component of the large-scale monsoon system have received less attention. In this Chapter, the annual cycle of rainfall and the interannual, sub-seasonal, and synoptic variabilities associated with the boreal winter monsoon over the Maritime Continent are described.

The equator serves as a general demarcation between the boreal summer (to its north) and boreal winter (to its south) monsoon rainfall regimes. However, locally the annual cycle is dominated by interactions between the complex terrain and an annual reversal of the surface winds. These interactions cause the summer and winter monsoon regimes to intertwine across the equator. In particular, the boreal winter regime extends far northward along the eastern flanks of the major island groups and landmasses. There is no complementary extension of the boreal summer regime into southern latitudes. The seasonal march is asymmetric during the transitional seasons, with the maximum convection following a gradual southeastward progression from the Asian summer monsoon to the Asian winter monsoon but a sudden transition in the reverse. This asymmetric march is explained by a hypothesis based on the redistribution of mass between land and ocean areas during spring and fall that results from different land-ocean thermal memories. This mass redistribution produces sea-level pressure patterns that lead to asymmetric wind-terrain interactions throughout the region, and a low-level divergence asymmetry in the region that promotes the southward march during boreal fall but opposes the northward march during boreal spring.

Interannual variability in boreal winter Maritime Continent monsoon rainfall is examined with respect to the El Nino-Southern Oscillation (ENSO). Significant variability in the ENSO-monsoon relationship exists across the Maritime Continent and its vicinity, particularly between the Sumatra-Malay Peninsula-western Borneo region and regions to its east and west. A significant part of this variability is linked to the influence of the Walker Circulation and its variations between warm and cold ENSO events. There is also evidence of interdecadal changes in the ENSO-monsoon relationship and its variability.

Both boreal summer and boreal winter rainfall in the Maritime Continent rainfall exhibit signals of the Tropical Biennial Oscillation. Atmosphere-ocean interaction processes appear to play an important role in the production of these signals.

Sub-seasonal and synoptic variability in boreal winter Maritime Continent rainfall is examined with respect to the Madden-Julian Oscillation (MJO), northeasterly cold surges, and the Borneo Vortex. Relationships among all three circulation systems are found to have significant impacts on large-scale atmospheric conditions that impact the spatial distribution of rainfall throughout the region. Convection over the southern South China Sea is strongest during the combination of a northeasterly surge and Borneo vortex. However, the frequency of surges is reduced when the MJO is present.

In this Chapter, aspects of global teleconnection associated with the interannual variability of the Asian summer monsoon (ASM) are discussed. The differences in the basic dynamics of the South Asian Monsoon and the East Asian monsoon, and their implications on global linkages are discussed. Two teleconnection patterns linking ASM variability to summertime precipitation over the continental North America were identified. These patterns link regional circulation and precipitation anomalies over East Asia to continental North America, via coupled atmosphere-ocean variations over the North Pacific. The first pattern, nicknamed as the “Tokyo-Chicago Express”, is associated with the fluctuations of the East Asian jet stream, and downstream amplification linking precipitation anomalies of regions of northern China, Japan, and Korea (the Baiu rain belt) to those in the Pacific Northwest and northern Great Plains of the US. The second pattern, dubbed the “Shanghai-Kansas Express” reflects features of Rossby wave dispersion linking precipitation anomalies in central East Asia to the US Midwest. Both patterns possess strong sea surface temperature (SST) expressions in the North Pacific. Results suggest that the two teleconnection patterns are associated with intrinsic modes of sea surface temperature variability in the extratropical oceans, which are forced in part by atmospheric variability and sustained by air-sea interaction. The first pattern may also be related to, while the second is independent of, tropical SST forcing. The potential predictability of the ASM associated with SST variability in different ocean basins is explored using a new canonical ensemble correlation prediction scheme. It is found that SST anomalies in tropical Pacific, i.e., El Niño, is the most dominant forcing for the ASM, especially over the maritime continent and eastern Australia. SST anomalies in the Indian Ocean may overshadow the influence from El Niño in western Australia and western maritime continent. Both El Niño and North Pacific SSTs contribute to monsoon precipitation anomalies over Japan, southern Korea, and northern and central China. By optimizing SST variability signals from different world ocean basins, the overall predictability of ASM can be substantially enhanced.

In this chapter, we describe characteristic spatial-temporal structures of the East Asian monsoon anomalies associated with the ENSO. A Pacific-East Asian teleconnection hypothesis is put forth to explain how the El Niño affects the “upstream” climate in East Asia. The key circulation system that conveys the impact of El Niño to East Asia is the anomalous anticyclone in the western North Pacific (WNP). This anomalous anticyclone is maintained through a positive thermodynamic air-sea feedback in the WNP. The wind fluctuations associated with the monsoon anomaly may further remotely impact the El Niño evolution through exciting consecutive equatorial oceanic Kelvin waves. The rainfall variability over the Meiyu region exhibits a strong biennial signal in the correlation with the equatorial eastern Pacific and Indian Ocean SST. This interannual monsoon-ENSO relationship is subject to an interdecadal variation. For 1951-77 an El Niño-like condition precedes both enhanced Meiyu and southeast China monsoon seasons, whereas for 1978-96 an El Niño warming precedes an enhanced Meiyu but a deficient rainfall season in southeast China. It is hypothesized that this results from the interdecadal change of the basic state. The possible impacts of East Asian winter monsoon on the onset of El Niño is also reviewed and discussed.

The progress in recent studies on climate variations of the summer monsoon in China is reviewed in this paper. Especially, the characteristics of the intraseasonal, interannual and interdecadal variations of the summer monsoon in China and its surrounding regions, which can cause severe drought and flooding disasters there, are discussed from the analyses of observed data. Moreover, some causes and physical processes of these variations are also sought from the components of the East Asian climate system. Besides, the scientific problems which need to be studied in the near future are suggested.

The impact of El Niño-Southern Oscillation (ENSO) on the East Asian Monsoon (EAM) has been examined using a general circulation model (GCM). The observed monthly changes in sea surface temperature (SST) in the equatorial Pacific east of 172°E during 1950-99 were inserted as the lower boundary condition of the model. For all oceanic grid points lying outside of the region of SST prescription, the atmosphere was coupled to an oceanic mixed layer model.

The typical evolution of the atmosphere-ocean system during ENSO was analyzed using composite charts. These patterns show that the key changes in the EAM sector are related to a prominent sea level pressure anomaly simulated over the South China Sea and subtropical northwestern Pacific. During warm ENSO events, the circulation anomalies associated with this anomalous anticyclone correspond to weaker winter monsoon flows along the East Asian coast, as well as above-normal precipitation over southern China. These signals move systematically eastward during the following spring and summer. Stationary wave modeling indicates that the atmospheric anomaly in the EAM region is essentially a Rossby-wave response to the ENSO-related diabatic heating pattern over the equatorial western Pacific.

The wintertime atmospheric circulation anomalies over the western Pacific generate strong SST anomalies in the following spring. Further model diagnoses indicate that the feedback of these SST changes on the atmosphere leads to eastward propagation of the pressure anomaly in the EAM region, and to amplification of rainfall anomalies along the Meiyu-Baiu front.

The atmospheric and oceanic changes in the EAM sector described in this chapter are discussed in the broader context of ENSO influences on the entire Asian-Australian monsoon system.

This chapter assesses the overall performance of current GCMs in simulating the summer monsoon rainfall, particularly its seasonal evolution and the anomalies during the 1997-98 El Niño period. Eleven GCM data used in the present study are from the CLIVAR/Monsoon GCM intercomparison project.

Several numerical simulations of the Asian monsoon have been conducted at CCSR (Center for Climate System Research) since 1991. The results are summarized from a viewpoint of sensitivity to spatial resolution and the boundary conditions such as SST and land surface conditions.

T21, T42 and T106 results are compared to demonstrate the sensitivity to spatial resolution. Simulation performance is generally better, the higher the resolution, although it should be noted that increase in resolution does not automatically contribute to improvement in simulation. Care must be taken to match the spatial resolution to the actual physical processes involved.

Regarding the surface boundary conditions, it is concluded that the SST anomaly contributes to the inter-annual variability, as defined by the Webster-Yang Index, more than the land surface conditions. At the same time, contribution of sea surface temperature anomalies over the several regions is estimated. The contribution of both the tropical eastern Pacific and the Indian Ocean is noted. It is emphasized that an anti-cyclonic anomaly of 850 hPa over the tropical Western Pacific plays an important role in controlling the East Asian monsoon activity.

Meiyu is a unique feature of East Asia and is one of the most important weather phenomena in China and vicinity. Numerous studies have been focused in the last quarter century on various aspects of the Meiyu system. The main purpose of this paper is to present an overview of the current understanding of the climatological characteristics of the Meiyu systems, as well as the structure and dynamics of the related features such as the Meiyu fronts, monsoon disturbances, low-level jets, mesoscale convective systems (MCSs), and orographically related disturbances during the Meiyu season.

This review paper discusses the multi-scale features, from the large scale to the mesoscale, of the Meiyu-Baiu front observed in 1-10 July 1991 based on the author's recent studies. Main data utilized are ECMWF reanalysis data and Geostationary Meteorological Satellite infrared data. During this period, the Meiyu-Baiu front with intense rainfalls extended from the foot of the eastern Tibetan Plateau to the Japan Islands in association with the westward extension of the North Pacific subtropical anticyclone, development of a blocking ridge and two cold lows in the northern latitudes (45°-60°N). A strong low-level jet stream, nearly moist neutral stratification, and a strong gradient of the specific humidity and that of equivalent potential temperature characterize the Meiyu-Baiu frontal zone. The weak thermal gradient in the Meiyu region of frontal zone is due to the cooling in the frontal precipitation zone. The relatively large thermal gradient in the Baiu region of frontal zone is sustained between the tropical maritime airmass and the polar maritime airmass. The strong meridional convergence of the moisture flux sustains the large precipitation in the frontal zone. The differential advection of the equivalent potential temperature generates convective instability against the stabilizing effect of the cumulus convection, and thus, a moist neutral stratification is sustained during the period of intense rainfalls. The strong low-level convergence in the frontal zone is accompanied by the northward strong acceleration along the northwestern rim of the westward protruding subtropical anticyclone.

A large diurnal variation of the convective clouds is observed over 90°-100°E, whereas the eastward passages of subsynoptic-scale and meso-a-scale cloud systems are evident in the frontal zone to the east of ~110°E. These cloud systems formed in the heavy rainfall area over the Continent, and develop during the passage along the Baiu frontal zone, where the significant baroclinity is present within a nearly moist neutral layer in the lower troposphere. A few meso-a-scale cloud systems develop along the trailing portion of the preceding subsynoptic-scale cloud system. They form a family of cloud systems in the Meiyu-Baiu front.

Observations from the May-June 1998 South China Sea Monsoon Experiment (SCSMEX) have been used to determine the characteristics of the onset of the summer monsoon over the northern South China Sea (SCS). The SST gradually increased over the northern SCS during SCSMEX, interrupted by slight cooling following monsoon onset until early June when the warming resumed. Surface sensible and latent heat fluxes increased after onset, but then decreased in June as a result of warm, moist air advecting over cooler water near the south China coast. The heating and moistening rates and vertical eddy flux of total heat during the early-June active period were greater than those observed during the May monsoon onset active period, indicating more vigorous deep convection as the monsoon ensued, a finding supported by Tropical Rainfall Measuring Mission (TRMM) precipitation radar data. Analysis of ground-based radar data reveals that lower and middle level vertical shears exerted a dominant control over the structure and orientation of mesoscale convective systems over the northern SCS. The findings are consistent with those of LeMone et al. for the equatorial western Pacific, except two new organizational modes have been identified: shear-parallel bands for strong low-level shear and weak midlevel shear when the air is dry aloft, and shear-parallel bands for strong shears in both layers when the shear vectors are in the same direction. Midlatitude influences likely contributed to these two additional modes.

After monsoons, the tropical cyclone is the major economic and social weather event that impacts the people of East Asia. In many aspects, the large-scale circulations established by the monsoon that extend to the adjacent ocean areas control when and where tropical cyclones will form and where the cyclones will move. This chapter will explore recent understandings of these monsoon-related effects on tropical cyclone formation and motion in the East Asia region. Seasonal displacements of the East Asia monsoon circulation are accompanied by latitudinal displacements of monsoon trough-related tropical cyclone formations over the western North Pacific. Intraseasonal variations between active and inactive monsoon troughs lead to favorable and unfavorable environmental conditions for tropical cyclone formation. Some theoretical and observational studies are beginning to provide more insight on specific aspects of tropical cyclone formation in the monsoon environment, but more studies are required. A recent modeling study by H.-C. Kuo and colleagues provides some new insights on the important problem of why tropical cyclone formation is favored near the eastern end of the monsoon trough. The roles of the upstream waves in the easterlies, the convergence effect in the zone between the equatorial westerlies and the easterlies, and the nonlinear effects leading to wave energy accumulation and axisymmetrization of the waves into vortices are clarified by that study. The environmental influence of the monsoon circulation on tropical cyclone motion has been described in terms of synoptic patterns and regions. The typical monsoon trough situation is included in the Standard synoptic patterns, and the reverse-oriented monsoon trough is included in the Poleward synoptic pattern. A monsoon Gyre synoptic pattern is unique to the western North Pacific. In each pattern, synoptic regions are defined that have characteristic tracks, so that a change in pattern/region combination is accompanied by a track change. This pattern/region change may occur via various tropical cyclone-environment interactions, including a sudden poleward track change when a tropical cyclone interacts with a monsoon gyre.

The monthly mean data set of the NCEP/NCAR reanalysis is employed to investigate the relation between the subtropical anticyclone and diabatic heating. Criteria for defining the location and intensity of the zonal mean subtropical anticyclone are given to study its characteristics. Comparison between the Hadley circulation and the subtropical anticyclone is made. Results show that the maximum convergence of meridional mass flux occurs in the subtropics, resulting in the formation of the subtropical anticyclone. In the free atmosphere, the ridgeline of the subtropical anticyclone deviates completely from the sinking arm of the Hadley cell, with the former being located equatorward of the latter. Due to friction, the subtropical anticyclone in the planetary boundary layer coincides with the sinking arm of the Hadley cell that extends vertically from the planetary boundary layer to tropopause. It is stressed that either in the free atmosphere or in the planetary boundary layer, descent cannot be considered as a mechanism for the formation of the subtropical anticyclone. The theories of thermal adaptation of the atmosphere to external thermal forcing are employed to understand the formation of subtropical anticyclone in the three dimensional domain. Numerical experiments are designed to verify these theories. Results show that strong land-surface sensible heating in the summer subtropics generates not only the strong surface cyclone and upper layer anticyclone over the continent, but also the strong lower layer equatorward flow along the western coast of the continent, breaking the zonal symmetric anticyclone belt along the subtropics and forming the surface anticyclones over the oceans to the west. On the other hand, strong deep convective latent heating generates upper tropospheric equatorward flow that closes off the upper tropospheric subtropical anticyclone to the west of the heating, and lower tropospheric poleward flow that closes off the lower tropospheric subtropical anticyclone to its east. Radiative cooling along the subtropics in the two hemispheres coincides well with the tropospheric sinking and the location of subtropical anticyclone in the planetary boundary layer. Over the eastern oceans in the subtropics, such radiative cooling is strong with its maximum occurring at the top of the planetary boundary layer. It generates strong surface equatorward flow and shifts the centers of the oceanic subtropical anticyclones, which is forced by the land-surface sensible heating, towards the eastern parts of the oceans. In the boreal summer, the formation of the strong South Asian High (SAH) and the North American anticyclone in the upper troposphere and the subtropical anticyclone over the western Pacific (SAWP) in the middle and lower troposphere is therefore considered partly as a result of the convective latent heating associated with the Asian monsoon, but affected by orography and the surface sensible heating over the continent. On the other hand, the formation of the subtropical anticyclones at the surface over the eastern North Pacific and North America is mainly due to the strong surface sensible heating over continents, but affected by radiative cooling over the eastern North Pacific. It is shown that the different kinds of diabatic heating over each continent and the adjacent oceans along the summer subtropics are organized in a quadruplet pattern LOSECOD that forces a specific zonal asymmetric circulation pattern. The global summer subtropical heating and circulation can then be viewed as “mosaics” of such a quadruplet heating and circulation patterns, respectively. The fundamental importance of the land-sea distribution in forming the summertime subtropical circulations is then proved.

The dominant stationary wave features during the East Asian monsoon season, from April to October, are examined in this study using the NCEP/NCAR reanalysis and the linear and nonlinear stationary wave models. The study focuses on the stationary wave maintenance and seasonal transition and their relations to the monsoon diabatic heating. Monsoon rainfall indices over China, the stationary wave and diabatic heating indices over the monsoon region are constructed for easy comparisons. Four stationary wave features, i.e., the Tibetan anticyclone, the East Asian trough, the subtropical high, and the monsoon trough, are examined in detail. All four stationary wave centers go through distinct seasonal cycle during this seven-month period, with the Tibetan anticyclone, the subtropical high and the monsoon trough intensifying from April to July, and weakening from August to October, while the East Asian trough weakens from April to August and re-establishes itself from August to October. The linear and nonlinear stationary wave models indicate that the effect of the heating is the most dominant in contributing to the stationary wave seasonal cycle. Further decomposition of the heating shows that the monsoon heating is the one that causes the seasonal variation of the stationary wave centers except the East Asian trough.