Chih-Pei Chang

How to Measure the Strength of the East Asian Summer Monsoon

Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979–2006 period. The existing indices can be classified into five categories: the east–west thermal contrast, north–south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Nino, while the first category reflects the second mode that corresponds to the developing El Nino. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.

East Asian Monsoon

East Asian Summer and Winter Monsoon: Seasonal March of East Asian Monsoon (Y Ding) The Eastern Asian Winter Monsoon (J Chan & C Li) The Maritime Continent Monsoon (C-P Chang & P A Harr) Interannual Variations: Interannual Variability, Global Teleconnection, and Potential Predictability Associated with the Asian Summer Monsoon (W K M Lau et al.) East Asian Monsoon -- ENSO Interactions (B Wang & T Li) Climate Variations of Summer Monsoon Over China (R Huang) General Circulation Modeling: Model Simulations of ENSO-Related Variability of the Coupled Atmosphere-Ocean System in the East Asian Monsoon Region (N-C Lau et al.) Current Status of AGCM Monsoon Simulations (I-S Kang) Simulation Studies for the Asian Monsoon by Using the CCSR/NIES AGCM (A Sumi et al.) Synoptic and Mesoscale Processes: Research on the Phenomena of Meiyu during the Past Quarter Century: An Overview (G T-J Chen et al.) Large and Mesoscale Features of Baiu Front Associated with Intense Rainfalls (K Ninomiya) Oceanic Asian Monsoon Convection: Results from the 1998 SCSMEX (R Johnson & P Ciesielski) Interactions with Other Circulations: Monsoon-Related Tropical Cyclones in East Asia (R Elsberry) Progress in the Study of Subtropical Anticyclone Dynamics (G Wu et al.) Maintenance and Seasonal Transitions of the Stationary Waves During East Asian Monsoon (M Ting & R Joseph)

Another Look at Interannual to Interdecadal Variations of the East Asian Winter Monsoon: The Northern and Southern Temperature Modes

Abstract This study investigates the causes of interannual-to-interdecadal variability of the East Asian (EA; 0°–60°N, 100°–140°E) winter monsoon (EAWM) over the past 50 yr (1957–2006). The winter mean surface air temperature variations are dominated by two distinct principal modes that together account for 74% of the total temperature variance. The two modes have notably different circulation structures and sources of variability. The northern mode, characterized by a westward shift of the EA major trough and enhanced surface pressure over central Siberia, represents a cold winter in the northern EA resulting from cold-air intrusion from central Siberia. The southern mode, on the other hand, features a deepening EA trough and increased surface pressure over Mongolia, representing a cold winter south of 40°N resulting from cold-air intrusion from western Mongolia. The cold northern mode is preceded by excessive autumn snow covers over southern Siberia–Mongolia, whereas the cold southern mode is preceded b...

Kelvin Wave-CISK: A Possible Mechanism for the 30–50 Day Oscillations

Abstract Two categories of theories have been proposed to explain the observed tropical intraseasonal oscillations whose main periodicity is between 30–50 days: (i) those based on eastward propagating Kelvin waves maintained by cumulus heating; and (ii) those based on interactions with stationary oscillations of the basic state. Recent numerical modeling studies have simulated certain important aspects of the oscillation particularly the slower propagation speed as compared with the normal Kelvin waves. Motivated by these results which lend support to the first category, a linear theoretical analysis of the equatorial β-plane wave-CISK was carried out with a focus on the Kelvin modes. Our results show that two types of CISK modes may arise from an interaction of vertical modes. For heating with a maximum in the lower troposphere, the instability is due to the lowest internal mode which gives a stationary, east-west symmetrical structure. When heating is maximum in the midtroposphere, eastward propagating ...

The East Asian Winter Monsoon

The midlatitude component ofthe East Asian winter monsoon (EA WM) is characterized by the cold-core Siberian-Mongolian High (SMH) at the surface whose variability affects all scales of the extratropical circulations. The SMH has been weakening in recent decades, which appears to correlate with the negative phase ofNAOIAO due to increased warm air advection over the Eurasian continent and the resultant reduction in snow cover. However, it is not clear that this recent decrease in the EA WM intensity is unique in the most recent 400 years. Periodical cold air outbreaks that cause high-impact weather are associated with the intra seasonal and synoptic variation of the SMH, and they often continue as cold monsoonal surges into the tropics and affect the tropical component of the EA WM. There is some evidence that intraseasonal variability has decreased in 1990s although extreme weather events in the past few years counter this trend, An important mechanism for the intraseasonal and higher frequency enhancement of the SMH comes from upper level blocking ridges over the Atlantic and the Pacific. The Atlantic blocking triggers a Rossby wave train that has a downstream effect of enhancing the SMH, The Pacific blocking forces the SMH through slow retrogression of the blocking center.

Westward Propagating Cloud Patterns in the Tropical Pacific as seen from Time-Composite Satellite Photographs

Abstract A new method of using satellite photographs in the study of large-scale cloud motions in the tropics is presented. Cloud clusters are seen to be propagating westward with a phase speed ∼9 m sec−1 in the summer of 1967, and can sometimes be followed all the way across the Pacific. It is suggested that this method can be used in the study of tropical wave disturbances.

Viscous Internal Gravity Waves and Low–Frequency Oscillations in the Tropics

Abstract In this paper we deal with the interpretation of observed oscillations in the tropical troposphere and stratosphere within the framework of the equatorial wave theory. A difficulty with this problem arises when one compares the short vertical wavelength (or equivalent depth) predicted by the classical theory and the observed large vertical scales associated with the low Doppler-shifted frequencies of the tropospheric oscillations. In this analysis it is shown that the inclusion of simple linear damping, justified by budget studies which revealed the important role of cumulus momentum transport, has a strong influence at low frequencies on the forced equatorial waves and results in two types of dispersive relationships. The first type is characteristic of the regular internal gravity waves which have fast phase speeds and weak vertical attenuation. The second type is dominated by the viscous damping time scale and has slow phase speeds and strong vertical trapping. The theory predicts that the str...

On the Relationship between Western Maritime Continent Monsoon Rainfall and ENSO during Northern Winter

Abstract Several studies have reported that Indonesian rainfall is poorly correlated with El Nino–Southern Oscillation (ENSO) events during the northern winter wet monsoon season. This work studies the relationship between the Nino-3 (5°S–5°N, 150°–90°W) sea surface temperature (SST) and the Maritime Continent monsoon rainfall during 1979–2002. The study indicates that the correlations are mostly negative except in the vicinity of Sumatra and Malay Peninsula (SMP, including the western sections of Java and Borneo), where the correlations range from zero to weakly positive. The monsoon rainfall during ENSO events is influenced by a pair of anomalous Walker cells and a low-level closed circulation centered near the Philippines. East of SMP, the rainfall is negatively correlated with Nino-3 SST. The anomalous low-level wind over the Indian Ocean west of SMP causes rainfall to also be correlated negatively with Nino-3 SST, but rainfall over SMP is sheltered from this effect because of the high mountains along...

Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea

Abstract The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981–86 are used to study the tropical circulations associated with the development of low-level westerlies during both events. Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough–front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is t...

The global monsoon system : research and forecast

The “Vertical Structure of Diabatic Processes of the Madden-Julian Oscillation” global-model evaluation project developed a novel experimental framework, which produces a complete characterization of models’ abilities to simulate the Madden-Julian oscillation (MJO). The three components of the project comprise 2-day and 20-day hindcasts and 20-year simulations; each obtained heating, moistening and momentum tendencies from the models’ sub-grid parameterizations. Thirty-five centers provided output for at least one component; nine centers provided data for all three. The models vary greatly in MJO fidelity in climate and hindcast experiments, yet fidelity in one was not correlated with fidelity in the other. In 20-year simulations, strong MJO models demonstrated heating, vertical-velocity and zonal-wind profiles that tilted westward with height, as in reanalysis data. The 20-day hindcasts showed no correspondence between the shape of the heating profile and hindcast skill. Low-to-mid-level moistening at moderate rain rates was a consistent feature of high-skill models and absent from low-skill models, suggesting a role for boundary-layer and congestus clouds in the MJO transition, which was confirmed by timestep data from the 2-day hindcasts. These hindcasts revealed a poor simulation of the MJO transition phase, even at short leads, with large mid-tropospheric dry biases and discrepancies in radiative-heating profiles.Global Monsoon Regional Monsoons Synoptic and Mesoscale Weather Intraseasonal Prediction Numerical Modeling Ocean and Air-Sea Interaction Land and Aerosol Processes Climate Change.