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Sukyoung Lee

Abstract

A two-layer quasigeostrophic model is used to study the equilibration of baroclinic waves. In this model, if the background flow is relaxed toward a jetlike profile, a finite-amplitude baroclinic wave solution can be realized in both supercritical and subcritical regions of the model’s parameter space. Analyses of the model equations and numerical model calculations indicate that the finite-amplitude wave equilibration hinges on the breaking of Rossby waves before they reach their critical latitude. This “jetward” wave breaking results in an increase in the upper-layer wave generation and a reduction in the vertical phase tilt. This change in the phase tilt has a substantial impact on the Ekman pumping, as it weakens the damping on the lower-layer wave for some parameter settings and enables the Ekman pumping to serve as a source of wave growth at other settings. Together, these processes can account for the O(1)-amplitude wave equilibration.

From a potential vorticity (PV) perspective, the wave breaking reduces the meridional scale of the upper-layer eddy PV flux, which destabilizes the mean flow. This is followed by a strengthening of the lower-layer eddy PV flux, which weakens the lower-layer PV gradient and constrains the growth of the lower-layer eddy PV. The same jetward wave breaking focuses the upper-layer PV flux toward the jet center where the upper-layer PV gradient is greatest. This results in an intensification of the upper-layer eddy PV relative to lower-layer eddy PV. Because of this large ratio, the upper-layer eddy PV plays the primary role in inducing the upper- and lower-layer eddy streamfunction fields, decreasing the vertical phase tilt. As a result, the Ekman pumping on the eddies is weakened, and for some parameter settings the Ekman pumping can even act as a wave source, contributing toward O(1)-amplitude wave equilibration. By reducing the horizontal shear of the zonal wind, the same wave breaking process weakens the barotropic decay, which also contributes to the wave amplification.

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Sukyoung Lee

Abstract

A two-layer quasigeostrophic model is used to study the effect of lower boundary Ekman pumping on the energetics of baroclinic waves. Although the direct impact of the Ekman pumping is to damp the total eddy energy, either the eddy available potential energy (EAPE) or the eddy kinetic energy (EKE), individually, can grow because of the Ekman pumping. Growth of EAPE is favored if the phase difference between the upper and lower wave fields is less than a quarter wavelength, and EKE is favored if the phase difference is greater than a quarter wavelength. A numerical model calculation shows that the EAPE growth occurs directly through the Ekman pumping and that the increased EAPE can in turn lead to further growth by strengthening the baroclinic energy conversion from zonal available potential energy to the EAPE. Through this indirect effect, the Ekman pumping can increase the net production of total eddy energy.

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Sukyoung Lee

Abstract

The effect of barotropic dynamics on atmospheric midlatitude storm tracks is investigated for the Northern Hemisphere winter season, using both observed data analyses and linear barotropic model experiments. It is shown that when the model flow is initialized with a realistic wave packet, barotropic processes alone can produce key features of the observed storm track structure. The author attributes this result both to the barotropic waveguide effect and to the fact that the geographical locations of barotropic growth coincide reasonably well with the baroclinic growth of the atmospheric storm track eddies.

This study also shows that barotropic dynamics are more relevant for storm tracks in lower latitudes than for those in higher latitudes, consistent with the fact that the lower-latitude storm tracks are more closely associated with the subtropical jet, rather than with the polar front jet that is driven by baroclinic eddies.

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Sukyoung Lee

Abstract

Linear stability analyses of three-dimensional time-mean flows in a two-level primitive equation model are presented. The model's stationary wave is generated by implementing idealized mountains at the lower boundary. Two sets of experiments are performed: the first with one mountain and the second with three mountains at equal distance from each other. Structures of streamfunctions and heat fluxes from the linearly unstable normal mode are compared with their bandpass transient counterparts in the nonlinear model simulation. The three-dimensional time-mean flow, about which the equations are linearized, is convectively unstable for both the one and three mountain cases. For the three mountain case, there is reasonable agreement between the linear mode and the bandpass transient eddies in terms of both the longitudinal location of the largest eddy amplitudes and the potential enstrophy budget, suggesting that the global mode can capture the correct structure of the climatological storm tracks for different reasons. For the one mountain case, however, the largest eddy amplitude of the linear mode extends farther downstream than that of the bandpass transient eddies.

The reasonable correspondence between the linear modes and the bandpass eddies for the three mountain case appears to be due to the relative proximity of successive unstable regions. Between these successive unstable regions are diffluent flows, resulting in increased deformation and enhanced horizontal diffusion, which plays an important role in dissipating transient eddy enstrophy. It is suspected that this locally enhanced dissipation represents strained eddies by the deformation field.

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Sukyoung Lee

Abstract

Using 16 years of NCEP–NCAR reanalysis data on the 200-mb surface, it is shown that in the deep Tropics, the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind. This acceleration is mainly due to transient eddies of intraannual and interannual timescales, and to those associated with the Madden–Julian oscillation (MJO). The interannual timescale eddy fluxes are dominated by eastward-propagating disturbances with zonal wavenumber 2 and a period of 2–4 yr, suggesting that these eddy fluxes may be tied to the El Niño–Southern Oscillation.

In the deep Tropics, the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the Hadley cells. The deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies. This result indicates that a nonzero obliquity of the earth is crucial for maintaining the present climate’s equatorial easterlies.

Consistent with the above findings, in an idealized GCM with a fixed equinox insolation and a sea surface temperature field symmetric across the equator, the tropical upper-tropospheric zonal wind is westerly. This is in part because such a GCM does not retain a transient meridional circulation arising from the seasonal cycle and because the horizontal eddy momentum flux convergence due to the MJO is stronger than that in the observations.

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Sukyoung Lee

Abstract

An idealized, linear barotropic model on an f plane is used to demonstrate the existence of a zonally localized storm track in the absence of any kind of local instability; with a background flow consisting of two jets separated by a local minimum, two distinctly localized eddy streamfunction variance (and eddy kinetic energy) peaks emerge on either side of the local jet minimum. In contrast, the decrease of the eddy vorticity variance at the jet minimum is essentially negligible. As the stretching deformation field of the background flow strengthens, the transient eddy is deformed irreversibly in the deformation region. The resulting enstrophy cascade toward smaller scale plays an important role in terminating the model's storm tracks, and in this case the second storm track downstream of the local jet minimum is substantially weakened.

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Sukyoung Lee

Abstract

Multiple zonal jets are investigated with a two-level primitive equation model on the sphere in which both baroclinicity and planetary radius are varied. As in the case for a two-layer quasigeostrophic model on a β-plane channel, it is found both that the Rhines scale successfully predicts the meridional scale of the multiple zonal jets, and that these jets are maintained in part by an eddy momentum flux divergence associated with slow baroclinic waves at the interjet minimum.

A scaling analysis suggests that n jets∝ (a/θm)1/2, with the constraints ζe ≡ 8 sin2 f (θ m/▵θ ) > 1 and n jets ≥ 1, where n jets is the number of the jets, a the planetary radius, θm one-half of the pole-to-equator potential temperature difference, ξe the supercriticality of the two-layer Phillips model, Δθ the potential temperature difference between the two levels, and ϕ the latitude. The number of jets simulated by the model agrees with this scaling, provided that L jeta, where L jet is the jet scale.

In model runs with a large planet where multiple zonal jets exist, the time–mean eddy heat flux is found to be consistent with the diffusive picture of Held and Larichev. In contrast, for the model runs with the planetary size equal to that of Earth, baroclinic adjustment is found to be more relevant. These results are consistent with the finding that in the large-planet (Earth-like) model runs, the jet/eddy scale is smaller than (comparable to) the corresponding planetary radius.

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Sukyoung Lee

Abstract

This article describes the generation and maintenance of persistent zonal jets in a two-layer quasigeostrophic, β-plane channel model, focusing on the transition from a one to a two jet state. For weak and moderate values of surface friction and supercriticality, the transition occurs abruptly as the width of the baroclinically unstable region of the initial flow is gradually increased. Across the transition point, the persistent two jet state is characterized by a smaller value of eddy energy than that for the one jet state. This reduction in eddy energy is due to increased barotropic energy conversion from the eddies to the zonal mean flow. Consistent with this result, the abrupt emergence of two persistent jets is accompanied by sharply defined eddy momentum flux divergence maxima at the critical latitudes between the two jet maxima. Essentially the same behavior is found in the transition from two to three, and three to four jets.

When two or more jets are present, baroclinically growing waves are found to exist along inter-jet minima, which are referred to as “inter-jet disturbances.” More importantly, the momentum fluxes of the interjet disturbances diverge at the interjet minimum, further decelerating the jets. Unstable normal modes similar to the inter-jet disturbances are also found. It is argued that the systematic wave absorption at the critical latitudes and the momentum flux divergence by the interjet disturbances may play a central role in the persistence of the multiple jets.

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Sukyoung Lee

Abstract

By analyzing El Niño and La Niña composites with 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data, evidence is presented here that the surface air temperature of the Arctic winter (December–February) is anomalously warm during La Niña and cold during El Niño. Surface and top-of-the-atmosphere energy fluxes were used to calculate the composite zonal-mean poleward moist static energy transport. The result shows that the La Niña warming in the Arctic is associated with an increased poleward energy transport in the extratropics. The opposite characteristics are found for El Niño. Because the total tropical convective heating is more localized during La Niña than El Niño, these findings suggest that the Arctic surface air temperature anomalies associated with ENSO may be attributed to the tropically excited Arctic warming mechanism (TEAM). In the tropics, consistent with previous studies, the anomalous poleward energy transport is positive during El Niño and negative during La Niña. Given the debate over whether a warmer world would take on more El Niño–like or La Niña–like characteristics, the findings of this study underscore the need for further investigation of tropical influence on polar climate.

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Sukyoung Lee and Steven Feldstein

Abstract

The characteristics of two distinct types of wave breaking in an aquaplanet general circulation model (GCM) are described. A systematic analysis of wave breaking is possible because when a baroclinic wave packet is present, the wave breaking tends to occur in the vicinity of the packet crew.

Although the refractive index is strictly valid only for linear, quasigeostrophic flows, in this GCM the refractive index is shown to be useful for categorizing two types of wave breaking. Empirical orthogonal function (EOF) analysis is performed using the refractive index obtained (by averaging over one carrier wavelength) at the center of the wave packet. Composite Eliassen–Palm fluxes and upper-level potential vorticity, corresponding to either phase of the first EOF of the refractive index, are consistent with the two types of wave breaking.

It is found that the strength of the meridional shear in the upper troposphere is related to the type of wave breaking. Implications of these results for storm track variability in the atmosphere are also discussed.

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