Search Results

You are looking at 1 - 10 of 66 items for

  • Author or Editor: C. P. Chang x
  • All content x
Clear All Modify Search
C-P. Chang

Abstract

No abstract available.

Full access
C-P. Chang

Abstract

No Abstract.

Full access
Zhuo Wang and C-P. Chang

Abstract

Atmospheric general circulation model (AGCM) simulations are carried out to test a hypothesis (Chang et al.) for the asymmetric monsoon transition in which the maximum convection marches gradually from the Asian summer monsoon to the Asian winter monsoon during boreal fall but experiences a sudden transition in the reverse during boreal spring. In the control run, the AGCM is driven by the climatological mean sea surface temperature (SST) with a realistic annual cycle, and it reproduces the observed asymmetric monsoon transition. In the sensitivity test, the model is driven by a similarly realistic SST but whose annual cycle is symmetric. The northwestward march of the maximum convection in boreal spring becomes more gradual, resulting in an overall near-symmetric pattern for the monsoon seasonal transition. The AGCM simulations confirm the hypothesis that the atmospheric mass redistribution due to the different land–ocean thermal memories leads to a seasonally different horizontal convergence field and it facilitates the southeastward monsoon march in boreal fall, while it hinders the northwestward monsoon march in boreal spring, contributing to the asymmetric monsoon transition.

Full access
H. Lim and C-P. Chang

Abstract

Tropical heating due to latent heat release has been proposed as a source that may influence midlatitude weather through teleconnection patterns. However, such heating is usually-internal (with the maximum in the midtroposphere) and, by itself, inefficient in exciting external, barotropic-type responses, which are necessary for the teleconnection mechanism. To study this problem, a simple two-level model is used to investigate the effect of vertical shear, differential damping and the planetary boundary layer on the characteristics of the atmospheric response to internal heating.

All three effects are found to enable a transfer of energy from the internal-mode motions, which are directly forced by the internal heating, to external-mode motions. To generate external-mode divergent motions, it is necessary to have a planetary boundary layer or other equivalent effects that force vertical motion at the bottom of the atmosphere. The efficiency of generation increases with the horizontal scale. On the other hand, vertical shear is normally the main effect for generating external-mode rotational motions, and the efficiency of generation decreases with the horizontal scale. In addition, this efficiency depends strongly on the relative vertical shear (vertical shear divided by vertical-mean wind). When the relative vertical shear is greater than unity, the external mode finally attains an amplitude larger than that of the internal mode.

The solution of an initial-value problem shows that in tropical regions, the process of energy transfer from internal to external-mode motions takes about two weeks to complete. This rather slow rate of energy transfer to external-mode motions implies that in a given vertical wind shear, the responses to a transient heating lasting for only a few days will have stronger internal mode (more baroclinic) structure than those to a steady-state heating.

Full access
H. Lim and C-P. Chang

Abstract

Using a linearized model with constant mean wind on an equatorial β-plane, two types of atmospheric response to steady tropical forcing are studied by an eigenmode analysis. The first type is the less rotationally trapped “barotropic” motions which show some of the characteristics of the teleconnection patterns observed by Bjerknes (1966) and Horel and Wallace (1981). The second type is the more rotationally trapped, warm-core, deep baroclinic motions which resemble the Walker circulations. The analysis reveals several important inadequacies in previous modeling studies and provides a relatively comprehensive explanation of the dynamics of both types of motions.

A vertical normal mode consideration shows that the barotropic-type motions are difficult to excite by internal beating. Surface heating, which has been neglected in many previous numerical modeling studies, is found to be important in directly forcing this type of motion. Furthermore, the possible contribution by large equivalent depth internal modes to the teleconnection response was not considered in these studies. A ray tracing approach is used to demonstrate the importance of the rotational trapping effect on the propagation of these deep internal modes.

A horizontal normal mode analysis shows that the most prominent responses in all cases are band disturbances of Kelvin and Rossby modes. In addition, in a westerly mean wind with a speed within the range of the Rossby wave phase speed, Rossby mode lee waves are also excited. These lee waves are replaced by Rossby mode evanescent disturbances in other mean winds. For large equivalent depths in westerlies, the band disturbances combine to produce the teleconnection pattern. For all equivalent depths, the Rossby mode is very important in the equatorial zonal wind band response on both the cast and the west sides of the forcing. For small equivalent depths this makes the Rossby mode an important component of both the eastern and the western Walker circulations.

Several previous models obtained only the Rossby and Kelvin band solutions due to their use of a long- wave approximation. This approximation distorts the responses, with the most serious distortions occurring in mean westerlies. Although the tropical mean wind is often easterly, it is argued that the westerly mean wind solution is probably more relevant to the Walker circulations due to their observed transient nature and the dynamics of transient forcing. In the westerly mean wind solution the Rossby lee waves east of the forcing substantially reduce the longitudinal extent of the zonal wind band to a value comparable to that observed.

Full access
C-P. Chang and H. Lim

Abstract

This work studies the effects of viscosity on the gravest Rossby mode on an equatorial beta plane, using linear damping and cooling coefficients. The results show that the vertical wavelength increased for a given forcing phase speed and that the phase speed is reduced for a given vertical wavelength. In addition, the, viscous damping causes a horizontal tilt of the wave axes resulting in a poleward phase propagation. These results are similar to those obtained by Chang (1977) on Kelvin waves. On the other hand, the importance of viscous effects for Rossby waves increases as the wavelength becomes shorter, while for Kelvin waves it increases as the wavelength becomes longer.

Full access
Hock Lim and C-P. Chang

Abstract

In order to understand the northeasterly monsoon surges and associated tropical motions over Southeast Asia during northern winter, the dynamic response of the tropical atmosphere to midlatitude pressure surges is studied using the linearized shallow-water equations on an equatorial β plane. The forcing is specified to have a Gaussian spatial distribution with a zonal scale corresponding to approximately wavenumber 7 and a meridional scale of approximately 11°. It rises rapidly from zero to maximum within one day or less and then decays slowly over 2–4 days. The main results are as follows:

1) After an initial period of gravity-wave type motions with strong northerly winds, the main tropical response takes the form of a Rossby wave group.

2) A pronounced northeast-southwest tilt in this Rossby wave group develops due to the faster westward group velocity of the lower meridional modes relative to the higher meridional modes.

3) Several conspicuous features of the Rossby response closely resemble the observed flow pattern of the northeast monsoon region, notably the northeasterly wind streak over the South China Sea during cold surges, the mean winter condition of a cyclonic shear trough extending from Borneo to the Philippines, and the enhancement of cyclonic circulation near the northern Borneo coast after surges.

4) The pressure surge forcing also gives rise to eastward moving wave groups of the Kelvin, mixed Rossby-gravity, and inertia-gravity (mainly n=0) modes. The Kelvin wave response, as in the case of thermally forced Kelvin waves, has a preference for longer wavelengths. These wave groups offer a possible interpretation for the eastward moving cloud patterns observed during Winter MONEX by Williams (1981).

Our results suggest that the gross features of the synoptic-scale tropical motions in the northeast monsoon region can be explained in terms of simple equatorial β-plane dynamics without taking into account other physical factors such as orography or boundary-layer friction.

Full access
C-P. Chang and Liana Zambresky

Abstract

This study compares the spectral characteristics of tropical synoptic (5-day) disturbances over the western Pacific contained in radiosonde observations and the initial analysis and 48-h forecast of the navy's operational global model during the northern winter of 1991/92. The power spectra of the analysis agree closely with observations, but the forecast shows larger differences. The westward-propagating horizontal structure is well represented in both the analysis and the forecast, although they tend to produce a higher spatial coherency than that determined from the observations.

The observed vertical structure, with a very small low-level tilt in the eastern region (east of 140°E) and an eastward tilt throughout the troposphere in the western region (west of 140°E), contrasts sharply with the typical summer structure observed previously. This contrast, when compared with the seasonal difference in vertical mean wind shear due to the Fast Asian monsoons, is consistent with the theoretical effect of the vertical shear on tropical wave structure. The east-west variation of the vertical structure is reproduced by the analysis, but the forecast structure has a weaker vertical organization over areas where a well-organized structure is observed, and a stronger organization to the west where it is not observed. It appears that the model's dynamics produces a downstream propagation that results in a wave structure that is partially artificial.

Full access
C-P. Chang and Tim Li

Abstract

The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase from northern summer in South Asia to southern summer in Australia, and how the reversed phase can last through three locally inactive seasons to the next monsoon, are studied by a simple tropical atmosphere–ocean–land model. The model has five boxes representing the South Asian and Australian monsoon regions and the equatorial Indian and western and eastern Pacific Oceans. The five regions interact with each other through the SST–monsoon, evaporation–wind, monsoon–Walker circulation, and wind stress–ocean thermocline feedbacks.

A biennial oscillation emerges in a reasonable parameter regime, with model SST and wind variations resembling many aspects of the observed TBO. Warm SST anomalies (SSTA) in July in the equatorial Indian Ocean cause an increase of surface moisture convergence into South Asia, leading to a stronger monsoon. The monsoon heating on one hand induces a westerly wind anomaly in the Indian Ocean, and on the other hand intensifies a planetary-scale east–west circulation leading to anomalous easterlies over the western and central Pacific. The westerly anomaly over the Indian Ocean decreases the local SST, primarily by evaporation–wind feedback. The easterly anomaly in the central Pacific causes a deepening of the ocean thermocline in the western Pacific therefore increasing the subsurface and surface temperatures. In addition, a modest easterly anomaly in the western Pacific opposes the seasonal mean westerlies so evaporation is reduced. These effects overwhelm those of the cold zonal advection and anomalous upwelling. The net result is warm SSTA persisting in the western Pacific through northern fall, leading to a stronger Australian monsoon.

Meanwhile, the warming in the western Pacific also induces a stronger local Walker cell and thus a surface westerly anomaly over the Indian Ocean. This westerly anomaly helps the cold SSTA to persist through the succeeding seasons, leading to a weaker Asian monsoon in the following summer. During northern winter the westerly anomaly associated with the stronger Australian monsoon, through anomalous ocean downwelling and reduction of evaporation (when the seasonal mean wind is easterly), reinvigorates the warm SSTA in the western Pacific, which has been weakened by the slow cold advection from the eastern Pacific. This further intensifies the eastern Walker cell and helps to keep the eastern Pacific cold.

The authors’ theory indicates that the TBO is an inherent result of the interactions between northern summer and winter monsoon and the tropical Indian and Pacific Oceans. Thus, it is an important component of the tropical ocean–atmosphere interaction system, separate from the El Niño–Southern Oscillation. While the eastern Pacific plays only a passive role in this mechanism, the western Pacific–Maritime Continent region is crucially important. It serves as a bridge in space and time, both in connecting the convection anomaly from the northern summer to the northern winter monsoon and in channeling the feedback of the northern winter monsoon to the Indian Ocean.

Full access
Kshudiram Saha and C-P. Chang

Abstract

Two cases of summer monsoon depressions in the vicinity of the Bay of Bengal are analyzed to study the importance of the baroclinic mechanism. Both cases show a baroclinic structure with well-defined warm and cold sectors, the latter being situated to the east of the former in a region where the thermal wind is easterly throughout the troposphere. In a developing depression, the geopotential and the temperature fields differ in phase such that warm advection from the north occurs to the west of the depression center and cold advection from the south to the east. There is also strong convergence to the west and divergence to the east of the depression center in the lower troposphere, and vice versa in the upper troposphere. Thus a divergent secondary circulation exists in the zonal-vertical plane with warm air rising to the west and cold air sinking to the east. Marked increases in upward motion and relative divergence (divergence at 200 mb minus divergence at 850 mb) in the southwest sector occur during the period of development. Intensity changes of the depressions are found to be strongly influenced by thermal advection from mid-latitude disturbances to the north, giving further evidence of the importance of baroclinic processes.

Full access