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Edmund K. M. Chang

Abstract

In this paper, ECMWF 40-yr reanalysis data have been examined to study the influence of upper-level wave packets propagating across Asia into the Pacific on surface cyclone development over the Pacific. Previous studies have shown that in winter, wave packets propagate across Asia over two branches—a northern branch over Siberia and a southern branch along the subtropical jet across southern Asia. Results presented here show that subsequent to the presence of wave packets on either branch, the frequency of occurrence of deep cyclones (defined as cyclones with central pressure below 960 hPa), as well as explosively deepening cyclones (those with a deepening rate of 1 Bergeron or more), are significantly enhanced. This enhancement also clearly follows the wave packet eastward as it propagates across the Pacific.

Wave packets from the two branches are found to interfere with each other, such that if wave packets of the appropriate configuration are present on both the northern and southern branch, subsequent surface cyclone development over the western Pacific is further enhanced. Examination of the evolution of the anomalies suggests that these interferences can largely be explained by linear superposition of wave packets from the two branches.

Examination of the evolution of the composite structure of wave packets that are followed by the development of a significant surface cyclone indicates that cyclones that develop as the northern packet propagates into the Pacific are phase locked with the upper-level trough and maintain a favorable westward tilt with height throughout their development, consistent with the hypothesis that cyclogenesis is triggered by the approach of the wave packet. In contrast, significant cyclones whose development are influenced by the southern packets initially develop west of the upper-level trough, and propagate eastward with a phase speed that is much faster than that of the upper-level trough, attaining a westward phase tilt with height only at the mature stage, suggesting that cyclogenesis for these cases is probably not triggered by the wave packet.

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Edmund K. M. Chang
and
Isidoro Orlanski

Abstract

An idealized primitive equation model is used to determine the factors controlling the dynamics and maintenance of eddy activity in a storm track. The results show that localized regions of enhanced baroclinicity do not necessarily lead to localization of eddy activity. By studying the energetics of the storm track, it is shown that while baroclinic conversion does indeed correlate with the region of maximum baroclinicity, it is the downstream radiation of energy through the ageostrophic geopotential fluxes which acts as a trigger for the development and maintenance of eddy activity over less baroclinic regions, extending the region of eddy activity much further downstream from the region of high baroclinicity. Examples of eddy life cycles are given that show that convergence and divergence of ageostrophic fluxes can dominate baroclinic and barotropic conversion, especially in regions with weak baroclinicity. Factors that may limit the zonal extent of a storm track are discussed. Evidence of downstream development over the wintertime Pacific storm track based on analyses of ECMWF data is also shown.

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Edmund K. M. Chang

Abstract

The resonance between a neutral Eady mode and a continuous mode with the same phase speed is used to interpret the growth of neutral modes discussed by Farrell. It is shown that the existence of such a resonance may lead to spurious linear growth of error in a discretized quasigeostrophic (QG) model if one of the levels lies very close to the steering level of the neutral Eady normal mode. For a primitive equation model, this spurious resonance manifests itself as an exponentially growing-decaying pair of short waves. Such waves are similar to those found by Arakawa and Moorthi for the Lorenz grid for the QG equations. However, here it is found that for the primitive equation model, these spurious short waves exist both for the Lorenz grid and the Charney–Phillips (CP) grid, although the growth rate is somewhat smaller for the CP grid.

It is also shown that the growth rate of the nongeostrophic short waves discussed by Stone is erroneously overestimated in vertically discretized models.

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Edmund K. M. Chang

Abstract

The structure and evolution of transient disturbances in the Northern Hemisphere winter season are examined using one-point regression maps and longitude-height sections derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses for seven winter seasons. With the use of unfiltered time series of normalized 300-mb meridional wind perturbations at a grid point in the Pacific storm track as the reference time series, regression statistics for perturbations in the horizontal wind, geopotential height, temperature, and vertical velocity are derived. The resulting perturbation fields exhibit characteristics of midlatitude baroclinic waves, such as a westward tilt with height in the velocity and height fields and eastward tilt in the temperature field, with typical wavelengths of 4000 km and periods of around 4 days.

The main difference between the results of this work and previous similar analyses is in the propagation characteristics of the baroclinic wave trains. The wave trains found here exhibit characteristics of downstream development, with successive perturbations developing toward the downstream side of existing perturbations. An analysis of the eddy kinetic energy budget of the wave train indicates that downstream radiation of ageostrophic geopotential fluxes by existing perturbations triggers the development of new eddies downwind, with baroclinic conversion becoming important only during the later part of the life cycle of a downstream developed wave. The Appendix suggests that the difference between these results and those of previous works is mainly due to time filtering obscuring the delicate signal of downstream development.

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Isidoro Orlanski
and
Edmund K. M. Chang

Abstract

With the use of a simple primitive equation model, it is demonstrated that the convergence/divergence of ageostrophic geopotential fluxes can be a major source/sink of kinetic energy for both downstream and upstream development of baroclinic waves, and can play a dominant role during the early stages of wave development. It is also shown that both surface friction and β effects lead to an asymmetry in the upstream versus downstream development, with downstream development much stronger. A total group velocity is defined based on ageostrophic fluxes, and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed.

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Edmund K. M. Chang

Abstract

In this paper, the mean meridional circulation (MMC) forced by eddy fluxes of heat and momentum is examined using a simple quasigeostrophic, two-level model of the zonal-mean atmosphere. Analytic solutions have been obtained, which show that analyses of the eddy-induced MMC using the Kuo-Eliassen equation are most appropriate for high-frequency fluctuations. For steady-state or low-frequency fluctuations, the eddy fluxes will lead to changes in the zonal-mean zonal wind and temperature. These changes in the zonal-mean state will induce changes in frictional dissipation and diabatic heating, which (together with the eddy fluxes) are constrained to satisfy a generalization of the Eliassen-Palm theorem and will also act as source terms to the Kuo-Eliassen equation. The inclusion of these induced terms usually leads to a significant enhancement in the diagnosed intensity of the MMC. This can explain why previous studies of the MMC found a much weaker eddy-induced Ferrel cell than that observed when the induced frictional and diabatic heating terms were left out and the eddy fluxes only were used as source terms. The relevant timescale separating the high- and low-frequency limits is found to be the radiative timescale in the model.

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Edmund K. M. Chang

Abstract

Experiments have been performed using a simple global model with idealized physics and zonally symmertrical forcings to investigate the influence of Hadley circulation intensity changes on extratropical climate. The heating within the Tropics is latitudinally concentrated, while the heating in the extratropics is kept unchanged. This loads to an increase in the intensity of the Hadley circulation. As found earlier by Hou, along with the increase in the intensity of the Hadley circulation, there is a statistically significant temperature increase in the winter high latitudes.

Zonal-mean diagnostics have been performed in order to identify the link between the changes in the Tropics and the extratropics. Detailed diagnosis of the heat budget shows that warming in the winter high latitudes is induced by changes in the mean meridional circulation, over the opposing cooling effects caused by changes in the eddy heat fluxes. Such a change is consistent with an equatorward shift of the jet stream and its associated heated pattern. It is suggested that the equatorward shift in jet position is caused by an increase in westerly acceleration within the Tropics associated with the enhancement in the intensity of the Hadley circulation. Limitations of the model are also discussed.

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Edmund K. M. Chang
and
Isidoro Orlanski

Abstract

A modified energy flux is defined by adding a nondivergent term that involves β to the traditional energy flux. The resultant flux, when normalized by the total eddy energy, is exactly equal to the group velocity of Rossby waves on a β plane with constant zonal flow. In this study, we computed the normalized energy flux for linear wave packets in baroclinic basic states with different vertical profiles. The results show that the normalized energy flux is a good approximation to the group velocity of all parts of the wave packet for the basic states examined.

The extension to the nonlinear case is briefly discussed. The magnitude of the fluxes of a downstream developing wave group over the wintertime northern Pacific storm track defined by a regression analysis is computed, and the group velocity defined by the energy fluxes is found to be comparable to the group velocity of propagation of the observed wave packet. The results indicate a very strong component of downstream energy radiation, suggesting that downstream energy dispersion is very important in the evolution of waves in the storm track.

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Edmund K. M. Chang

Abstract

A series of experiments has been performed using an idealized model of the global atmosphere to study the role eddies play in communicating changes in the zonal mean state between the Tropics and extratropics. When an oscillatory heating perturbation centered about the equator is imposed, the author found a poleward-propagating zonal wind anomaly emanating from the Tropics into the midlatitudes when the heat source oscillates with a period of around 25–100 days. At higher frequency, most of the zonal wind perturbation is confined within the Tropics, while at lower frequency, the main signal occurs in the midlatitudes.

The angular momentum budget and Eliassen–Palm cross sections have been examined. The results suggest that eddies act to communicate changes in the Tropics into the midlatitudes in at least two ways. First, changes in zonal mean zonal wind in the Tropics lead to a shift in the eddy angular momentum divergence pattern. Second, heating in the Tropics changes the temperature gradients between the Tropics and midlatitudes, giving rise to changes in the amplitude of eddy fluxes and hence eddy momentum divergence. Both effects act to damp the perturbation in the Tropics, as well as to transmit the tropical perturbation poleward into the midlatitudes. A simple three-component analytical model has been developed based on these ideas, and the model reproduces the main features observed from the numerical model experiments.

Low-frequency (period 200 days and longer) variability excited by tropical heating has been examined further. When the perturbation is a single heat source centered on the equator, the author found that the main response appears to be a standing oscillation in the midlatitudes, with very weak poleward-propagating signal. However, when the author added a heating source at 15° latitude with the opposite phase, an apparently significant poleward-propagating signal from the Tropics into the extratropics was obtained. Analyses suggest that this poleward-propagating signal may just be an illusory superposition of two largely standing oscillations located side by side, each with relatively weak poleward propagating tendency of its own.

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Xiaosong Yang
and
Edmund K. M. Chang

Abstract

The eddy–zonal flow feedback in the Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence time scale of the leading principal components (PCs) of the zonal-mean zonal flow shows substantial seasonal variation. In the SH summer, the persistence time scale of PC1 is significantly longer than that of PC2, while the persistence time scales of the two PCs are quite similar in the SH winter. A storm-track modeling approach is applied to demonstrate that seasonal variations of eddy–zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence time scale. The eddy feedback time scale estimated from a storm-track model simulation and a wave-response model diagnostic shows that PC1 in June–August (JJA) and December–February (DJF), and PC2 in JJA, have significant positive eddy–mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback time scales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence time scale.

Eliassen–Palm flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF and PC2 in JJA, is as follows: an enhanced baroclinic wave source (heat fluxes) at a low level in the region of positive wind anomalies propagates upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of the climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of the jet flow. The “barotropic governor” plays an important role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy–zonal flow feedback.

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