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- Author or Editor: Sukyoung Lee x
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Abstract
This study investigates the mechanism behind the recent boreal summer circulation trend pattern and associated high surface temperature anomalies over the Russian Far East. This circulation pattern includes a prominent anticyclone over the Kamchatka Peninsula where heat extremes have been trending upward. Observational analysis and numerical model simulations indicate that latent heating anomalies centered over Yakutia, west of Kamchatka Peninsula, can excite this anticyclone and the downstream circulation trend pattern. However, this anticyclone alone is insufficient for generating the anomalously high temperature over the region. Instead, the high temperature emerges when there is an upstream precursor that resembles the Eurasian circulation trend pattern. Warm advection by this upstream circulation initiates a positive temperature anomaly over the Russian Far East, one week prior to the onset of the anticyclone in this region. As this anticyclone develops, the temperature anomalies further intensify by adiabatic warming and shortwave radiative heating. If upstream circulation anomalies are opposite to those of the Eurasian trend pattern, the initial temperature over the Russian Far East is anomalously negative. As a result, the adiabatic warming and shortwave radiative heating within this anticyclonic region are unable to bring the temperature to an extreme condition. These findings indicate that the temperature extremes over the Russian Far East are contributed by a combination of remote and local circulation forcings and provide insights into subseasonal forecasts of heat waves over this region.
Abstract
This study investigates the mechanism behind the recent boreal summer circulation trend pattern and associated high surface temperature anomalies over the Russian Far East. This circulation pattern includes a prominent anticyclone over the Kamchatka Peninsula where heat extremes have been trending upward. Observational analysis and numerical model simulations indicate that latent heating anomalies centered over Yakutia, west of Kamchatka Peninsula, can excite this anticyclone and the downstream circulation trend pattern. However, this anticyclone alone is insufficient for generating the anomalously high temperature over the region. Instead, the high temperature emerges when there is an upstream precursor that resembles the Eurasian circulation trend pattern. Warm advection by this upstream circulation initiates a positive temperature anomaly over the Russian Far East, one week prior to the onset of the anticyclone in this region. As this anticyclone develops, the temperature anomalies further intensify by adiabatic warming and shortwave radiative heating. If upstream circulation anomalies are opposite to those of the Eurasian trend pattern, the initial temperature over the Russian Far East is anomalously negative. As a result, the adiabatic warming and shortwave radiative heating within this anticyclonic region are unable to bring the temperature to an extreme condition. These findings indicate that the temperature extremes over the Russian Far East are contributed by a combination of remote and local circulation forcings and provide insights into subseasonal forecasts of heat waves over this region.
Abstract
A forced, nonlinear barotropic model on the sphere is shown to simulate some of the structure of the observed Northern Hemisphere midlatitude storm tracks with reasonable accuracy. For the parameter range chosen, the model has no unstable modes with significant amplitude in the storm track regions; however, several decaying modes with structures similar to the storm track are discovered. The model's midlatitude storm tracks also coincide with the location of a waveguide that is obtained by assuming that the horizontal variation of the time-mean flow is small compared with the scale of the transient eddies. Since the model is able to mimic the behavior of the observed storm tracks without any baroclinic dynamics, it is argued that the barotropic waveguide effects of the time-mean background flow acting on individual eddies are partially responsible for the observed storm track structure.
Abstract
A forced, nonlinear barotropic model on the sphere is shown to simulate some of the structure of the observed Northern Hemisphere midlatitude storm tracks with reasonable accuracy. For the parameter range chosen, the model has no unstable modes with significant amplitude in the storm track regions; however, several decaying modes with structures similar to the storm track are discovered. The model's midlatitude storm tracks also coincide with the location of a waveguide that is obtained by assuming that the horizontal variation of the time-mean flow is small compared with the scale of the transient eddies. Since the model is able to mimic the behavior of the observed storm tracks without any baroclinic dynamics, it is argued that the barotropic waveguide effects of the time-mean background flow acting on individual eddies are partially responsible for the observed storm track structure.
Abstract
This study puts forward a mechanism for the observed upwelling in the tropical upper troposphere and lower stratosphere. In this hypothesis, the tropical upwelling is driven by momentum transport by Rossby waves that are generated by tropical convection. To test this hypothesis, model runs are conducted using an axisymmetric, global, primitive equation model. In these runs, the effect of Rossby waves is included by driving the model with observed fields of large-scale eddy momentum flux convergence. The resulting overturning circulation includes both meridional flow from the intertropical convergence zone (ITCZ) to the equator and rising motion in the tropical tropopause transition layer (TTL). This circulation therefore helps to explain the transport of moisture from the lower portion of the TTL in the ITCZ to the equatorial cold-point tropopause, where tropopause cirrus layers frequently occur.
Abstract
This study puts forward a mechanism for the observed upwelling in the tropical upper troposphere and lower stratosphere. In this hypothesis, the tropical upwelling is driven by momentum transport by Rossby waves that are generated by tropical convection. To test this hypothesis, model runs are conducted using an axisymmetric, global, primitive equation model. In these runs, the effect of Rossby waves is included by driving the model with observed fields of large-scale eddy momentum flux convergence. The resulting overturning circulation includes both meridional flow from the intertropical convergence zone (ITCZ) to the equator and rising motion in the tropical tropopause transition layer (TTL). This circulation therefore helps to explain the transport of moisture from the lower portion of the TTL in the ITCZ to the equatorial cold-point tropopause, where tropopause cirrus layers frequently occur.
Abstract
A strategy is adopted that applies the mean meridional circulation (MMC) equation to two different steady states of a primitive equation model. This allows for the investigation of the mechanisms behind the sensitivity of the Hadley cell structure to individual source terms in an axisymmetric circulation. Specifically, the strategy allows the MMC response to the individual source terms to be partitioned into direct and indirect components.
The model's Hadley cell strengthens and broadens as the viscosity of the model is increased. It is found that a substantial portion of this sensitivity is attributable to diabatic heating and surface friction changes, which are ultimately induced by changes in viscosity. Similar behavior is found as the meridional gradient of the radiative–convective equilibrium temperature is increased, except that in this case the indirect response arises through the viscosity and surface friction change. In both cases, the changes in the static stability change are found to be of secondary importance.
It is found that the latitudinal extent of the Hadley cell is more sensitive to the meridional temperature gradient than to the static stability. However, when the static stability is decreased (increased) by a sufficient amount, the Hadley cell becomes narrower (broader). Additional analyses indicate that the change in Hadley cell width is a response to the change in Hadley cell strength.
Abstract
A strategy is adopted that applies the mean meridional circulation (MMC) equation to two different steady states of a primitive equation model. This allows for the investigation of the mechanisms behind the sensitivity of the Hadley cell structure to individual source terms in an axisymmetric circulation. Specifically, the strategy allows the MMC response to the individual source terms to be partitioned into direct and indirect components.
The model's Hadley cell strengthens and broadens as the viscosity of the model is increased. It is found that a substantial portion of this sensitivity is attributable to diabatic heating and surface friction changes, which are ultimately induced by changes in viscosity. Similar behavior is found as the meridional gradient of the radiative–convective equilibrium temperature is increased, except that in this case the indirect response arises through the viscosity and surface friction change. In both cases, the changes in the static stability change are found to be of secondary importance.
It is found that the latitudinal extent of the Hadley cell is more sensitive to the meridional temperature gradient than to the static stability. However, when the static stability is decreased (increased) by a sufficient amount, the Hadley cell becomes narrower (broader). Additional analyses indicate that the change in Hadley cell width is a response to the change in Hadley cell strength.
Abstract
A linear-stochastic model is applied to the 10-day low-pass streamfunction field at 300, 500, and 850 mb for 40 winter seasons of Northern Hemisphere NCEP–NCAR reanalysis data. The linear operator is derived from the observed multilevel covariances, allowing for statistical representation of nonlinear processes. While all empirical normal modes of the system are decaying, increase in the streamfunction variance is possible through nonmodal growth. When the evolution of the streamfunction field following the optimal perturbation is predicted, the Pacific–North American teleconnection pattern (PNA) is found to be the most probable state of the atmosphere. Sixty-eight percent (70%) of positive (negative) PNA events are found to follow high projections onto the leading optimal, suggesting the PNA arises through constructive interference between the decaying modes and may be treated as a linear response to Gaussian white noise stochastic forcing. Implications for PNA timescale and onset mechanisms are also discussed.
Abstract
A linear-stochastic model is applied to the 10-day low-pass streamfunction field at 300, 500, and 850 mb for 40 winter seasons of Northern Hemisphere NCEP–NCAR reanalysis data. The linear operator is derived from the observed multilevel covariances, allowing for statistical representation of nonlinear processes. While all empirical normal modes of the system are decaying, increase in the streamfunction variance is possible through nonmodal growth. When the evolution of the streamfunction field following the optimal perturbation is predicted, the Pacific–North American teleconnection pattern (PNA) is found to be the most probable state of the atmosphere. Sixty-eight percent (70%) of positive (negative) PNA events are found to follow high projections onto the leading optimal, suggesting the PNA arises through constructive interference between the decaying modes and may be treated as a linear response to Gaussian white noise stochastic forcing. Implications for PNA timescale and onset mechanisms are also discussed.
Abstract
The dynamical processes associated with block evolution are investigated by analyzing a GCM run, forced with perpetual January conditions. The core of the analysis lies on the temporal evolution of the blocks and on vorticity budget terms obtained from appropriate compositing procedures on a 350-mb model output. The results from the budget analysis are examined with barotropic model experiments, which allow the investigation of the influence of an individual dynamical process on block evolution.
Results are presented for two composite blocks, one close to the Atlantic storm track and the other farther downstream. Although these two blocks are found to develop differently, they share the following characteristics. During the decay linear processes dominate, and the high- and low-frequency eddy fluxes contribute equally toward prolonging the lifetime of the blocks by 2 to 3 days. While the time average of the budget yields results that are consistent with previous diagnostic studies, it is shown that such an approach exaggerates the role played by high-frequency eddies.
The barotropic model experiments show that the nonlinear self-interaction of the composite block anomaly plays a minimal role in the block evolution. It is the remaining part of the composite low-frequency eddy flux that contributes significantly toward the block evolution, indicating that case-to-case variability of the individual blocking events can be substantial, and that the nonlinearity of a slowly moving, nonsteady component of the flow plays an important role for the individual blocking events. The model experiments also demonstrate that the effect of divergence is crucial for correctly reproducing the structure of the blocking high. The implications of these results, as they apply to some of the prominent blocking theories, are also discussed.
Abstract
The dynamical processes associated with block evolution are investigated by analyzing a GCM run, forced with perpetual January conditions. The core of the analysis lies on the temporal evolution of the blocks and on vorticity budget terms obtained from appropriate compositing procedures on a 350-mb model output. The results from the budget analysis are examined with barotropic model experiments, which allow the investigation of the influence of an individual dynamical process on block evolution.
Results are presented for two composite blocks, one close to the Atlantic storm track and the other farther downstream. Although these two blocks are found to develop differently, they share the following characteristics. During the decay linear processes dominate, and the high- and low-frequency eddy fluxes contribute equally toward prolonging the lifetime of the blocks by 2 to 3 days. While the time average of the budget yields results that are consistent with previous diagnostic studies, it is shown that such an approach exaggerates the role played by high-frequency eddies.
The barotropic model experiments show that the nonlinear self-interaction of the composite block anomaly plays a minimal role in the block evolution. It is the remaining part of the composite low-frequency eddy flux that contributes significantly toward the block evolution, indicating that case-to-case variability of the individual blocking events can be substantial, and that the nonlinearity of a slowly moving, nonsteady component of the flow plays an important role for the individual blocking events. The model experiments also demonstrate that the effect of divergence is crucial for correctly reproducing the structure of the blocking high. The implications of these results, as they apply to some of the prominent blocking theories, are also discussed.
Abstract
Intraseasonal variability of the zonal-mean tropical tropopause height is shown to be modulated by localized tropical convection. Most of this convective activity is identified as being part of the Madden–Julian oscillation. While the convection is highly localized over the Pacific warm pool, a large-scale circulation response to the convective heating rapidly warms most of the tropical troposphere and cools most of the lowest few kilometers of the tropical stratosphere. These changes in temperature fields raise the tropical tropopause at most longitudes within 10 days of the convective heating maximum.
Abstract
Intraseasonal variability of the zonal-mean tropical tropopause height is shown to be modulated by localized tropical convection. Most of this convective activity is identified as being part of the Madden–Julian oscillation. While the convection is highly localized over the Pacific warm pool, a large-scale circulation response to the convective heating rapidly warms most of the tropical troposphere and cools most of the lowest few kilometers of the tropical stratosphere. These changes in temperature fields raise the tropical tropopause at most longitudes within 10 days of the convective heating maximum.
Abstract
Coherent baroclinic wave packets are present in the Southern Hemisphere, most clearly in the summer season. These coherent packets are also found in a hierarchy of models of nonlinear baroclinic instability-a two-layer quasigeostrophic (QG) model on a β-plane, a two-level primitive equation (PE) model, and a general circulation model. The flows are chaotic, but the packet itself can remain remarkably coherent, despite the complex evolution of the flow within the packet. In both QG and PE models, the packets become more robust as the supercriticality of the flow is reduced. In both models and the observations, the packets move with a group velocity that is greater than the phase speed of the individual disturbances, so that these disturbances exhibit downstream development. The structure of the baroclinic waves in the packet as a function of longitude resembles the life cycles of sinusoidal baroclinic waves as a function of time. More than one packet can exist in the domain at the same time. In the QG model, the number of packets increases in a systematic way as the length of the channel increases.
Abstract
Coherent baroclinic wave packets are present in the Southern Hemisphere, most clearly in the summer season. These coherent packets are also found in a hierarchy of models of nonlinear baroclinic instability-a two-layer quasigeostrophic (QG) model on a β-plane, a two-level primitive equation (PE) model, and a general circulation model. The flows are chaotic, but the packet itself can remain remarkably coherent, despite the complex evolution of the flow within the packet. In both QG and PE models, the packets become more robust as the supercriticality of the flow is reduced. In both models and the observations, the packets move with a group velocity that is greater than the phase speed of the individual disturbances, so that these disturbances exhibit downstream development. The structure of the baroclinic waves in the packet as a function of longitude resembles the life cycles of sinusoidal baroclinic waves as a function of time. More than one packet can exist in the domain at the same time. In the QG model, the number of packets increases in a systematic way as the length of the channel increases.
Abstract
This paper investigates the effect of baroclinic eddies on the structure of the Hadley cell. Self-consistent calculations of both axisymmetric and nonaxisymmetric circulations allow an unambiguous estimate of baroclinic eddy effects on the structure of the Hadley cell. Furthermore, a diagnostic analysis allows us to partition the influence of baroclinic eddies into “direct” and “indirect” responses. The former refers to the meridional circulation attributable to the explicit eddy fluxes while the latter refers to the meridional circulation attributable to part of other processes, such as surface friction and diabatic heating changes, which are in fact induced by the baroclinic eddies. For a realistic parameter range, it is found that these indirect responses are comparable to the direct response.
While the direct response of the eddies is always found to be a strengthening of the Hadley cell, the indirect response can either strengthen or dampen the Hadley cell. When the thermal driving of the atmosphere is moderate, baroclinic eddies always amplify and broaden the Hadley cells. On the other hand, if the thermal driving over the Tropics and subtropics becomes sufficiently strong, the net effect of baroclinic eddies is to dampen (strengthen) the Hadley cell above (below) the height level of maximum diabatic heating. An explanation for this behavior is given in terms of competition between the Hadley cell driving by the eddy fluxes (both direct and indirect) and damping of the Hadley cell by potential temperature mixing.
Abstract
This paper investigates the effect of baroclinic eddies on the structure of the Hadley cell. Self-consistent calculations of both axisymmetric and nonaxisymmetric circulations allow an unambiguous estimate of baroclinic eddy effects on the structure of the Hadley cell. Furthermore, a diagnostic analysis allows us to partition the influence of baroclinic eddies into “direct” and “indirect” responses. The former refers to the meridional circulation attributable to the explicit eddy fluxes while the latter refers to the meridional circulation attributable to part of other processes, such as surface friction and diabatic heating changes, which are in fact induced by the baroclinic eddies. For a realistic parameter range, it is found that these indirect responses are comparable to the direct response.
While the direct response of the eddies is always found to be a strengthening of the Hadley cell, the indirect response can either strengthen or dampen the Hadley cell. When the thermal driving of the atmosphere is moderate, baroclinic eddies always amplify and broaden the Hadley cells. On the other hand, if the thermal driving over the Tropics and subtropics becomes sufficiently strong, the net effect of baroclinic eddies is to dampen (strengthen) the Hadley cell above (below) the height level of maximum diabatic heating. An explanation for this behavior is given in terms of competition between the Hadley cell driving by the eddy fluxes (both direct and indirect) and damping of the Hadley cell by potential temperature mixing.
