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Tsing-Chang Chen

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Tsing-Chang Chen

Abstract

Several interesting characteristics of African easterly waves (AEWs) were observed and investigated by previous studies: two separate propagation paths, genesis mechanisms, restriction of vertical development, and the interaction with the African easterly jet (AEJ). However, some aspects of these characteristics have been neglected: the contrast of the AEW population along the two propagation paths, the AEW genesis mechanism over the Saharan thermal low and the role played by the low-level North African circulation in this mechanism, the dynamical mechanism restricting the vertical development of AEWs, and the synoptic relationship and interaction between the AEJ and the AEWs along the two propagation paths. The ECMWF reanalyses for the 1991–2000 period supplemented with those of 1979 were analyzed to explore these AEW features. Major findings of this effort are the following:

  1. The population of AEWs along the propagation path north of the AEJ (AEWn) is approximately 2.5 times of that along the propagation path south of the AEJ (AEWs).
  2. The AEWn geneses primarily occur over the three convergent centers and the southwestward extension of the Saharan thermal low. Underneath the midtropospheric Saharan high, the baroclinic instability of a shallow, low static stability environment, which may be triggered by the intrusion of dry northerlies over central North Africa, leads to the AEW genesis.
  3. Continental-scale upward motion along the Saharan thermal low and the cyclonic-shear side of the AEJ maintains positive vortex stretching below the Saharan high and the western part of the Asian monsoon high. These two regions thus form a favorable environment for the development of AEWs within the near-surface troposphere along the Saharan thermal low and the midtroposphere south of the AEJ.
  4. The passage of AEWn (AEWs) across the coastal zone of West Africa is accompanied by a weak (strong) AEJ and weak (strong) Saharan high. The westward propagation and development/maintenance of the two types of AEWs are achieved through vorticity advection by the AEJ, which is the major AEW–AEJ interaction.
These findings will facilitate the search for AEW dynamics and aid in assessing the impact of AEW activity on North African climate change.

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Tsing-Chang Chen

Abstract

The kinetic energy of divergent and rotational flows, and the conversion between these two energy reservoirs, have been computed using the 200 mb fields in the tropics (15°S–20°N) and the subtropics (20–42.5°N)during the summers of 1967 and 1972. The computations are performed in the spectral domain.

The numerical results of the conversion between the kinetic energy of divergent and rotational flows and the spectral distribution in the tropics are close to the estimate of generation of kinetic energy by Krishnarmurti and his colleagues. It is suggested that the computation of conversion between the kinetic energy of divergent and rotational flows using the horizontal winds may provide an alternative method of evaluating the generation of kinetic energy in the tropics.

The comparison of the kinetic energy of divergent and rotational flows, and conversion between these two energy reservoirs in both the tropics and the subtropics, are also discussed. Effort is also made, to compare the contributions of the standing and transient modes to these energy variables.

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Tsing-Chang Chen

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The conspicuous feature of the midtropospheric North African summer circulation is the Saharan high surrounded on its southern rim by the African easterly jet (AEJ). Like a major monsoon circulation, the Saharan high is juxtaposed with the North African divergent center to the east and the eastern Atlantic convergent center to the west. Different from a major monsoon circulation, these pronounced midtropospheric circulation components are overlaid by the western part of the Tibetan high. Because of the unique roles played by the Saharan high and the African easterly jet in the North African summer circulation, an effort is made to explore maintenance mechanisms of these two midtropospheric circulation elements. Major findings of this effort are summarized as follows:

  1. In terms of the velocity potential maintenance equation, it is shown that the North African divergent center over the Chad–Sudan region is maintained by the vertical differential heating established by the Saharan thermal-low heating in the lower troposphere and the Saharan radiative cooling in the upper troposphere.
  2. The Saharan high is spatially in quadrature with the North African divergent center. It is inferred from the streamfunction budget analysis that the Saharan high is maintained by the east–west circulation, which is formed by the east–west differential heating between the Saharan thermal-low heating and the eastern North Atlantic cooling. This inference is further substantiated by forced barotropic model simulations.
  3. The AEJ around the tropical periphery of the Saharan high is almost perpendicular to the equatorward divergent northerlies spilling out of the North African divergent center. The energetic interaction between divergent and rotational flows reveals that this jet is maintained by the Coriolis acceleration associated with these divergent winds.

These findings not only reveal the maintenance mechanism of the Saharan high and the associated AEJ, but also facilitate the search for answers to some problems of the North African summer weather/climate system.

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Tsing-Chang Chen

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Summer stationary waves in the Northern Hemisphere are separated by a midlatitude transition zone into the subtropical monsoon regime with a vertical phase reversal and the subarctic regime with a vertically uniform structure. The dynamics and maintenance mechanism of the subtropical stationary waves have been investigated in the context of monsoon circulation. Depicted in terms of streamfunction with 40-yr ECMWF Re-Analysis (ERA-40), the dynamic characteristics of stationary waves in the transition zone and the subarctic region are thus the focus of this study. The dynamics and maintenance mechanism of these waves were explored with the streamfunction budget and the velocity potential maintenance equations.

Stationary waves across the transition region consist of anticyclonic shear zones over the North Pacific and North Atlantic and a cyclonic shear zone in east Eurasia. These transition elements are linked to subtropical oceanic anticyclones and continental thermal lows. At high latitudes, a three-wave structure emerges with a weak central Eurasian trough aligned with two deep oceanic troughs. A longitudinal phase change occurs across the transition zone, but the direction of the east–west circulation associated with the transitional anticyclonic (cyclonic) zone is the same as that of the subtropical trough (high). This phase change is caused by the dynamics transition from the Sverdrup regime to the Rossby regime because of the increasing importance of relative vorticity advection. At high latitudes, relative vorticity advection becomes the dominant dynamic process in the upper atmosphere, but is negligible in the lower troposphere. This subarctic dynamic regime results in the vertically uniform structure of stationary waves. These waves are maintained by in situ diabatic heating (cooling) ahead of three subarctic troughs (ridges). Thus, the structure of the east–west circulation of subarctic stationary waves is opposite to that of subtropical stationary waves. These findings not only disclose more detailed structure and dynamics of summer stationary waves, but also provide a more complete basis to validate summer climate simulations and to search for the cause of interannual variation in summer climate.

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Tsing-Chang Chen

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The wind fields in the tropics (15°S–15°N) generated by the FGGE IIIb analyses of the European Center for Medium-Range Weather Forecasts are used to compute the kinetic energy of baroclinic (vertical shear) and barotropic (vertical mean) flows, and the conversion between these two energy reservoirs for the FGGE (1979) summer. The computations are carried out in the longitudinal spectral domain.

The computational results show that the baroclinic kinetic energy of every wave component is larger than its corresponding barotropic kinetic energy. The kinetic energy is converted from the baroclinic flow to the barotropic flow, and in the tropics the magnitude of this energy conversion is an order smaller than that in middle latitudes as studies by Wiin-Nielsen and his colleague have shown. This energy conversion is dominated by wavenumber 1, like other spectral energy studies of the tropical upper troposphere. Since the computations of the conversion between baroclinic and barotropic kinetic energy only involves the horizontal wind fields, the analysis of this energy conversion may offer an alternative in exploring the atmospheric energetics in the tropics. Further effort is made to compare the contributions of the standing (time-mean) and transient (departure from the time-mean) modes to the aforementioned energy variables.

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Tsing-Chang Chen

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The contributions of standing (time-mean) and transient (time-departure) waves to the atmospheric spectral energetics are analyzed using the NMC (National Meteorological Center) data of winter 1976–1977. It is found that the standing long waves are responsible for the major horizontal sensible heat transport and also for the significant horizontal momentum transport. Furthermore, the major contents of AE (eddy available energy) and KE (eddy kinetic energy) of standing waves are in the long-wave regime. However, the spectral energetics analysis indicates that the standing long waves are energetically less efficient than the transient long and short waves. It is suggested that the lower efficiency of the standing long waves in the atmospheric energetics may be one of the physical factors causing the underforecast of the standing long waves in the numerical weather prediction models.

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Tsing-Chang Chen

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Previous studies of extratropical stationary waves in the winter Northern Hemisphere (NH) often focused on effects of orography and land–ocean thermal contrast on the formation, structure, and maintenance of these waves. In contrast, research attention to tropical stationary waves was attracted by the summer monsoon circulations and the ENSO-related climate variability. Consequently, the structure and basic dynamics of tropical stationary waves and the relationship of these waves with those in mid–high latitudes have long been neglected. Thus, the following several distinct features of observed winter NH stationary waves have not been explained: 1) an abrupt change in the longitudinal phase across 30°N; 2) a transition from the vertical phase reversal of tropical stationary waves to the vertically westward tilt of extratropical stationary waves; and 3) a longitudinally quarter-phase relationship between stationary waves and east–west circulations, and a reversal of this relationship across 30°N. It is inferred from a spectral streamfunction budget analysis with the NCEP–NCAR reanalyses that these wave features are caused by the transition of wave dynamics from the Sverdrup regime in the Tropics to the Rossby regime in the mid–high latitudes. Based on the simplified vorticity equations of these two dynamic regimes, analytic solutions obtained with observed velocity potential fields (which were used to portray the global divergent circulation) confirm that the aforementioned distinct features of stationary waves are attributed to the dynamics transition across 30°N. Since east–west circulations are part of the global divergent circulation, it is revealed from a diagnosis of the velocity potential maintenance equation that this circulation component is maintained in the Tropics primarily by diabatic heating and in the mid–high latitudes by both horizontal heat advection and diabatic heating. Evidently, stationary waves are maintained by diabatic heating through the divergent circulation and the dynamics transition of these waves from the Sverdrup regime to the Rossby regime is attributed to strong midlatitude westerlies.

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Tsing-Chang Chen

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The wind fields generated by the ECMWF FGGE III-b analyses at ten mandatory levels were used to examine the tropical enstrophy budget in the spectral domain during the 1979 northern summer (June–August).

The seasonal mean analysis shows that the wave enstrophy has its maximum value in the upper troposphere and its major content in the low wavenumber regime. The wave enstrophy is nonlinearly transferred from the low to the large wavenumber regime. The wave enstrophy in the upper troposphere is supplied through the beta effect, while the upward transport of the wave enstrophy is generated by vortex stretching in the lower troposphere. The zonal enstrophy also has its maximum value in the upper troposphere and is supplied by the beta vortex stretching effects in the upper troposphere.

Both wave and zonal enstrophies in the tropics exhibited a 40–50 day variation in the 1979 northern summer. The time series of various enstrophy variables suggests that the time variation of the wave enstrophy is maintained by the upward transport of this quantity generated in the lower troposphere. The time variation of the zonal enstrophy is supported by this quantity generated in the upper troposphere.

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Tsing-Chang Chen

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A coherent seesaw fluctuation of 200-mb temperature T(200 mb) and 850-mb height Z(850 mb) between the Tibet-India region and the North Pacific is analyzed with the ECMWF FGGE III-b data for the northern summer (May-September 1979). Both T(200 mb) and Z(850 mb) oscillate with an opposite phase between these two regions and with a period of about 30–50 days. Furthermore, T (200 mb) and Z (850 mb) of the same location also oscillate with an opposite phase relationship. It was demonstrated from the empirical orthogonal function (EOF) analysis that this coherent seesaw oscillation of these two variables between the Tibet-India region and the North Pacific can be attributed to the eastward propagation of the 30–50 day oscillation.

The eastward propagation of this low-frequency oscillation may exert a profound impact on the regional atmospheric circulation downstream. The thermal energy pumped up from the ocean by cumulus convections over the moonson regions might be propagated out by this 30–50 day oscillation to warm up the cold center of the North Pacific. Moreover, the oceanic anticyclone over the North Pacific would be intensified periodically by the eastward propagation of this low-frequency oscillation, which would affect the moisture transport over East Asia and even North America. As for the former region, the Mei-yu trough is deepened and the onset of the Mei-yu regime is triggered by the eastward propagation of the low center of the Indian monsoon trough associated with the 30–50 day oscillation.

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