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C-P. Chang

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C-P. Chang

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C-P. Chang
and
Liana Zambresky

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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.

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Jeng-Ming Chen
and
C-P. Chang

Abstract

In Part I a multiple-set canonical correlation analysis (MCCA) was proposed to generalize the conventional two-set canonical correlation analysis. The MCCA seeks the optimal correlation among more than two data fields through a diagonalization of the product or the squared product of the correlation matrices between selected (desired) field pairs. In this study a specific case is used to empirically test the sensitivities of the MCCA technique. The case study uses an MCCA application of the 850-hPa meridional wind data over the tropical western Pacific to study tropical synoptic wave disturbances during summer. Successive 12-h meridional winds are used as the different data fields. The result shows that the method is stable with respect to sampling changes when the data contain significant signals of physical phenomenon and not stable when the data are random. The study also confirms the use of the largest residual correlation, or the largest cross-component correlation, as a preliminary significance test for the technique.

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Kshudiram Saha
and
C-P. Chang

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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.

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C-P. Chang
and
K. M. Lau

Abstract

Objectively analyzed 200 mb winds of four winters are used to study the short-term (several days) teleconnections between planetary-scale circulation components over the monsoon region. The composited structures suggest that during very active northeasterly monsoon (surge) periods, the midlatitude and tropical circulation components vary in a coherent way. The jet streak and local Hadley circulation over East Asia, the divergent flow over the maritime continent, and the equatorial Walker circulations over the Pacific and Indian Oceans all strengthen steadily, while the secondary jet streak over West Asia weakens. During inactive (break) periods the midlatitude circulation components all exhibit reversed changes, while the variations in the tropics are less coherent, although still showing reversed tendencies. The results basically verify the short-term teleconnection model proposed by Chang and Lau (1980), but more complex processes in the acceleration of the midlatitude jet streaks are indicated. In addition, the contrast between the very active and break monsoon periods suggests the relative importance of tropical versus midlatitudinal forcings in different regions of the monsoon circulation.

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C-P. Chang
and
K. G. Lum

Abstract

Previous studies of cold surges during the northern winter monsoon suggested a short-term midlatitude-tropical interaction such that the variations of the midlatitude jet over East Asia correspond to variations in tropical convection. However, because cold surges occur during periods of intensified baroclinicity, it is possible that the strengthening of the jet may be due entirely to midlatitude baroclinic development rather than the enhanced local Hadley circulation forced by the increase in tropical convection. In this study objectively analyzed 200 mb wind data for the 1983/84 winter are examined to address the problem of determining cause and effect in short-term midlatitude-tropical interactions.

Significant positive correlation between the midlatitude jet acceleration and tropical divergence was found in several regions in the Asia-Pacific-Indian Ocean region. Among six major intensifications of the East Asian jet streak maximum during December 1983, three were related to tropical cyclone activity and two to cold surges. The tropical cyclone cases led to the conclusion that the midlatitude jet can be influenced effectively by tropical convective activity on a day-to-day basis. This result has possible implications for midlatitude weather forecasting. The study also confirmed the downstream propagation of the strengthened jet streaks, which is probably due to self-advection, and the existence of thermally indirect circulations at the exit region of the jet, both in the time-mean and in the transient motion fields.

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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.

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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.

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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.

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