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Abstract
The dynamics of equatorially forced climate teleconnections on the sphere is studied using quasi-geostrophic wave theory and numerical models. Using the concept of a refractive index for meridional propagation of energy, it is demonstrated that for zonal mean flow with no horizontal shear, steady-state atmospheric teleconnections are composed of radiating Rossby modes which are forced in weak westerly zonal mean flow and evanescent modes in easterly zonal mean wind. For horizontally sheared zonal mean flow, westerly shear will lead to initial transient growth of wave packets with northwest–southeast tilt. These wave packets will move initially northward from the source region new the equator and subsequently become damped after turning southward at various critical latitudes. In contrast, easterly shear will always cause monotonic decay of all northbound wave packets from the tropics. The results imply that, in the case of a barotropically stable mean flow, kinematic shearing effect will focus or defocus wave energy from tropics to midlatitudes depending on whether the ambient horizontal shear is westerly or easterly. This mechanism also explains why tropical wave energy is naturally drawn toward the exit region of climatological winter jet streams.
Experiments with a nonlinear barotropic spectral model with equatorial forcing shows that wave energy can still propagate away from regions of initially weak tropical easterly mean flow by the shear-induced growth mechanism which modifies the zero-wind line downstream of the source. The interaction of the winter subtropical westerly jet and the wave disturbance generated by diabatic forcing source over the equator produces a quasi-stationary wave pattern reminiscent of the Pacific–North America pattern. The gross features of the tropics and extratropical steady-state response in the radiating mode are in qualitative agreement with that predicted from linear theory.
Abstract
The dynamics of equatorially forced climate teleconnections on the sphere is studied using quasi-geostrophic wave theory and numerical models. Using the concept of a refractive index for meridional propagation of energy, it is demonstrated that for zonal mean flow with no horizontal shear, steady-state atmospheric teleconnections are composed of radiating Rossby modes which are forced in weak westerly zonal mean flow and evanescent modes in easterly zonal mean wind. For horizontally sheared zonal mean flow, westerly shear will lead to initial transient growth of wave packets with northwest–southeast tilt. These wave packets will move initially northward from the source region new the equator and subsequently become damped after turning southward at various critical latitudes. In contrast, easterly shear will always cause monotonic decay of all northbound wave packets from the tropics. The results imply that, in the case of a barotropically stable mean flow, kinematic shearing effect will focus or defocus wave energy from tropics to midlatitudes depending on whether the ambient horizontal shear is westerly or easterly. This mechanism also explains why tropical wave energy is naturally drawn toward the exit region of climatological winter jet streams.
Experiments with a nonlinear barotropic spectral model with equatorial forcing shows that wave energy can still propagate away from regions of initially weak tropical easterly mean flow by the shear-induced growth mechanism which modifies the zero-wind line downstream of the source. The interaction of the winter subtropical westerly jet and the wave disturbance generated by diabatic forcing source over the equator produces a quasi-stationary wave pattern reminiscent of the Pacific–North America pattern. The gross features of the tropics and extratropical steady-state response in the radiating mode are in qualitative agreement with that predicted from linear theory.
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.
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.
Abstract
The linearized shallow-water equatorial β-plane equation was solved for a subset of approximate solutions applicable to thermally driven large-scale tropical circulation. In particular, the heat-induced monsoon circulations during Southeast Asian northeasterly cold surges are investigated. It was demonstrated that the response of the tropical atmosphere to a localized heat source consists of forced Rossby waves propagating westward and Kelvin waves eastward along the equator away from the region of forcing. In general, for any source/sink distribution, the heat-induced motion can have the characteristics of a Walker-type (ν = 0 at the equator) or a Hadley-type (u = 0 at the equator) response or a combination of both, depending on the latitudinal location of the forcing. Away from the equator, a forcing corresponding to the sudden imposition of mass at the lower layer, or equivalently in our model a rapid cooling of the lower troposphere, produces a sudden local surface pressure rise and strong anticyclonic flow to the west of the forcing. Strong NE-SW till in the axis of the anticyclone is observed and can be understood in terms of the dispersion of the various wave modes excited. The low-latitude response is, as expected, dominated by Kelvin and the gravest Rossby wave modes.
Coupled with an equatorial heat source, the sudden cooling of the lower troposphere over a localized area in the subtropics gives rise to a northeasterly wind surge and large-scale Walker and Hadley circulations reminiscent of periods of strong cold surges over East Asia. Finally, the effect of the presence of a mean wind is shown to modify the spatial extent of the equatorial circulation with a mean easterly wind favoring the formation of equatorially trapped Walker cells.
Abstract
The linearized shallow-water equatorial β-plane equation was solved for a subset of approximate solutions applicable to thermally driven large-scale tropical circulation. In particular, the heat-induced monsoon circulations during Southeast Asian northeasterly cold surges are investigated. It was demonstrated that the response of the tropical atmosphere to a localized heat source consists of forced Rossby waves propagating westward and Kelvin waves eastward along the equator away from the region of forcing. In general, for any source/sink distribution, the heat-induced motion can have the characteristics of a Walker-type (ν = 0 at the equator) or a Hadley-type (u = 0 at the equator) response or a combination of both, depending on the latitudinal location of the forcing. Away from the equator, a forcing corresponding to the sudden imposition of mass at the lower layer, or equivalently in our model a rapid cooling of the lower troposphere, produces a sudden local surface pressure rise and strong anticyclonic flow to the west of the forcing. Strong NE-SW till in the axis of the anticyclone is observed and can be understood in terms of the dispersion of the various wave modes excited. The low-latitude response is, as expected, dominated by Kelvin and the gravest Rossby wave modes.
Coupled with an equatorial heat source, the sudden cooling of the lower troposphere over a localized area in the subtropics gives rise to a northeasterly wind surge and large-scale Walker and Hadley circulations reminiscent of periods of strong cold surges over East Asia. Finally, the effect of the presence of a mean wind is shown to modify the spatial extent of the equatorial circulation with a mean easterly wind favoring the formation of equatorially trapped Walker cells.
Abstract
Linear analyses of wave-CISK models often lead to the conclusion that the mechanism does not explain the scale selection of planetary-scale tropical motions. In particular, Kelvin or gravity wave-CISK tends to prefer cumulus-scale motions. On the other hand, numerical models with “positive-only” heating did not experience much difficulty in generating large scale disturbances of realistic structure. A previous study explained these numerical results in terms of linear Kelvin wave-CISK modes, whose wave packages exhibit remarkable resemblence to the simulated disturbances. However, the linear theory can not explain how the simulated disturbances can maintain a “wavenumber-l” structure against the explosive growth of short wavelength components.
A reexamination of the wave-CISK theories is carried out in this paper. It is argued that the CISK mechanism possess inherently a very severe form of nonlinearity that takes full effect for even infinitesimal perturbations. The linearized CISK theories, therefore, do not give a correct description of a CISK-driven system at any stage of its development. A simple 5-level 64-wave spectral model demonstrates that the nonlinear effects of wave-CISK alone is able to produce exponentially growing “wavenumber-1” flow patterns that propagate without change of shape. Since these flow patterns have well-defined structure, growth rate, and propagation speed, it is proposed to call them nonlinear wave-CISK modes.
The nonlinear wave-CISK modes exhibit all the characteristics of the linear Kelvin wave-CISK packages of Chang and Lim (1988). The scale selection problem of their linear Kelvin wave-CISK theory has, however, been resolved. Introduction of a diffusion damping has no significant effect on the propagation speed and structure of these modes. The overall structure of the modes depends mainly on the growth rate. Varying other parameters may affect the growth rate; but modes of the same growth rate look rather like each other, irrespective of the other parameters.
Abstract
Linear analyses of wave-CISK models often lead to the conclusion that the mechanism does not explain the scale selection of planetary-scale tropical motions. In particular, Kelvin or gravity wave-CISK tends to prefer cumulus-scale motions. On the other hand, numerical models with “positive-only” heating did not experience much difficulty in generating large scale disturbances of realistic structure. A previous study explained these numerical results in terms of linear Kelvin wave-CISK modes, whose wave packages exhibit remarkable resemblence to the simulated disturbances. However, the linear theory can not explain how the simulated disturbances can maintain a “wavenumber-l” structure against the explosive growth of short wavelength components.
A reexamination of the wave-CISK theories is carried out in this paper. It is argued that the CISK mechanism possess inherently a very severe form of nonlinearity that takes full effect for even infinitesimal perturbations. The linearized CISK theories, therefore, do not give a correct description of a CISK-driven system at any stage of its development. A simple 5-level 64-wave spectral model demonstrates that the nonlinear effects of wave-CISK alone is able to produce exponentially growing “wavenumber-1” flow patterns that propagate without change of shape. Since these flow patterns have well-defined structure, growth rate, and propagation speed, it is proposed to call them nonlinear wave-CISK modes.
The nonlinear wave-CISK modes exhibit all the characteristics of the linear Kelvin wave-CISK packages of Chang and Lim (1988). The scale selection problem of their linear Kelvin wave-CISK theory has, however, been resolved. Introduction of a diffusion damping has no significant effect on the propagation speed and structure of these modes. The overall structure of the modes depends mainly on the growth rate. Varying other parameters may affect the growth rate; but modes of the same growth rate look rather like each other, irrespective of the other parameters.
Abstract
The Asian monsoon is a planetary-scale circulation system powered by the release of latent heat, but important features of deep convection and rainfall distribution cannot be adequately represented by the large-scale patterns. This is mainly due to the strong influences of terrain that are important across a wide range of horizontal scales, especially over the Maritime Continent where the complex terrain has a dominant effect on the behavior of convective rainfall during the boreal winter monsoon. This chapter is a review and summary of published results on the effects on monsoon convection due to interactions between the Maritime Continent terrain and large-scale transient systems.
The Maritime Continent topographic features strongly affect both the demarcation of the boreal summer and winter monsoon regimes and the asymmetric seasonal marches during the transition seasons. In the western part of the region, the complex interactions that lead to variability in deep convection are primarily controlled by the cold surges and the synoptic-scale Borneo vortex. The Madden–Julian oscillation (MJO) reduces the frequency of weaker surges through an interference with their structure. It also influences convection, particularly on the diurnal cycle and when synoptic activities are weak. When both surges and the Borneo vortex are present, interactions between these circulations with the terrain can cause the strongest convection, which has included Typhoon Vamei (2001), which is the only observed tropical cyclone that developed within 1.5° of the equator.
The cold surges are driven by midlatitude pressure rises associated with the movement of the Siberian high. Rapid strengthening of surge northeasterly winds can be explained as the tropical response via a geostrophic adjustment process to the pressure forcing in the form of an equatorial Rossby wave group. Dispersion of meridional modes leads to a northeast–southwest orientation that allows the surge to stream downstream through the similarly oriented South China Sea. This evolution leads to a cross-equatorial return flow and a cyclonic circulation at the equator, and thus a mechanism for equatorial cyclogenesis. Although the narrow width of the southern South China Sea facilitates strengthening of the cold surge, it also severely restricts the likelihood of cyclone development so that Vamei remains to be the only typhoon observed in the equatorial South China Sea.
Climate variations from El Niño–Southern Oscillation to climate change may impact the interactions between the large-scale motion and Maritime Continent terrain because they lead to changes in the mean flow. The thermodynamic effects on the interaction between MJO and the monsoon surges and Borneo vortex over the complex terrain also need to be addressed. These and other questions such as any possible changes in the likelihood of equatorial tropical cyclogenesis as a result of climate change are all important areas for future research.
Abstract
The Asian monsoon is a planetary-scale circulation system powered by the release of latent heat, but important features of deep convection and rainfall distribution cannot be adequately represented by the large-scale patterns. This is mainly due to the strong influences of terrain that are important across a wide range of horizontal scales, especially over the Maritime Continent where the complex terrain has a dominant effect on the behavior of convective rainfall during the boreal winter monsoon. This chapter is a review and summary of published results on the effects on monsoon convection due to interactions between the Maritime Continent terrain and large-scale transient systems.
The Maritime Continent topographic features strongly affect both the demarcation of the boreal summer and winter monsoon regimes and the asymmetric seasonal marches during the transition seasons. In the western part of the region, the complex interactions that lead to variability in deep convection are primarily controlled by the cold surges and the synoptic-scale Borneo vortex. The Madden–Julian oscillation (MJO) reduces the frequency of weaker surges through an interference with their structure. It also influences convection, particularly on the diurnal cycle and when synoptic activities are weak. When both surges and the Borneo vortex are present, interactions between these circulations with the terrain can cause the strongest convection, which has included Typhoon Vamei (2001), which is the only observed tropical cyclone that developed within 1.5° of the equator.
The cold surges are driven by midlatitude pressure rises associated with the movement of the Siberian high. Rapid strengthening of surge northeasterly winds can be explained as the tropical response via a geostrophic adjustment process to the pressure forcing in the form of an equatorial Rossby wave group. Dispersion of meridional modes leads to a northeast–southwest orientation that allows the surge to stream downstream through the similarly oriented South China Sea. This evolution leads to a cross-equatorial return flow and a cyclonic circulation at the equator, and thus a mechanism for equatorial cyclogenesis. Although the narrow width of the southern South China Sea facilitates strengthening of the cold surge, it also severely restricts the likelihood of cyclone development so that Vamei remains to be the only typhoon observed in the equatorial South China Sea.
Climate variations from El Niño–Southern Oscillation to climate change may impact the interactions between the large-scale motion and Maritime Continent terrain because they lead to changes in the mean flow. The thermodynamic effects on the interaction between MJO and the monsoon surges and Borneo vortex over the complex terrain also need to be addressed. These and other questions such as any possible changes in the likelihood of equatorial tropical cyclogenesis as a result of climate change are all important areas for future research.
