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Mingyu Park
and
Sukyoung Lee

Abstract

According to baroclinic adjustment theory, the isentropic slope maintains its marginal state for baroclinic instability. However, the recent trend of Arctic warming raises the possibility that there could have been a systematic change in the extratropical isentropic slope. In this study, global reanalysis data are used to investigate this possibility. The result shows that tropospheric isentropes north of 50°N have been flattening significantly during winter for the recent 25 years. This trend pattern fluctuates at intraseasonal time scales. An examination of the temporal evolution indicates that it is the planetary-scale (zonal wavenumbers-1–3) eddy heat fluxes, not the synoptic-scale eddy heat fluxes, that flatten the isentropes; synoptic-scale eddy heat fluxes instead respond to the subsequent changes in isentropic slope. This extratropical planetary-scale wave growth is preceded by an enhanced zonal asymmetry of tropical heating and poleward wave activity vectors. A numerical model is used to test if the observed latent heating can generate the observed isentropic slope anomalies. The result shows that the tropical heating indeed contributes to the isentropic slope trend. The agreement between the model solution and the observation improves substantially if extratropical latent heating is also included in the forcing. The model temperature response shows a pattern resembling the warm-Arctic–cold-continent pattern. From these results, it is concluded that the recent flattening trend of isentropic slope north of 50°N is mostly caused by planetary-scale eddy activities generated from latent heating, and that this change is accompanied by a warm-Arctic–cold-continent pattern that permeates the entire troposphere.

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Sukyoung Lee
and
Yohai Kaspi

Abstract

The structure and stability of Jupiter’s atmosphere is analyzed using transformed Eulerian mean (TEM) theory. Utilizing the ammonia distribution derived from microwave radiometer measurements of the Juno orbiter, the latitudinal and vertical distribution of the vertical velocity in the interior of Jupiter’s atmosphere is inferred. The resulting overturning circulation is then interpreted in the TEM framework to offer speculation of the vertical and meridional temperature distribution. At midlatitudes, the analyzed vertical velocity field shows Ferrel-cell-like patterns associated with each of the jets. A scaling analysis of the TEM overturning circulation equation suggests that in order for the Ferrel-cell-like patterns to be visible in the ammonia distribution, the static stability of Jupiter’s weather layer should be on the order of 1 × 10−2 s−1. At low latitudes, the ammonia distribution suggests strong upward motion, which is reminiscent of the rising branch of the Hadley cell where the static stability is weaker. Taken together, the analysis suggests that the temperature lapse rate in the midlatitudes is markedly smaller than that in the low latitudes. Because the cloud-top temperature is nearly uniform across all latitudes, the analysis suggests that in the interior of the weather layer, there could exist a temperature gradient between the low- and midlatitude regions.

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Cory Baggett
and
Sukyoung Lee

Abstract

In the framework of the Lorenz energy cycle, the climatological and eddy life cycle characteristics of the generation of eddy available potential energy through Ekman pumping (EEPE) are evaluated using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data (1979–2011). EEPE exhibits an annual cycle that is maximized during a given hemisphere’s winter, with maximum values in the midtroposphere of the midlatitudes.

Spectral analysis of the Southern Hemisphere storm track reveals that positive EEPE is associated with an anomalously small vertical phase tilt. A composite analysis of the Southern Hemisphere eddy life cycle reveals a maximum in EEPE that occurs after the peak eddy amplitude. Eddy life cycles during winter with large values of EEPE have higher values of eddy available potential energy and eddy kinetic energy than life cycles with small EEPE. However, baroclinic energy conversion remains unenhanced in life cycles with large values of EEPE. The lack of enhancement of baroclinic conversion is related to the small vertical phase tilt associated with positive EEPE. Instead, barotropic energy conversion is muted, and it is this muted barotropic decay that results in an amplification of eddy kinetic energy. There is no evidence of reflecting critical latitudes playing a role in this reduction of barotropic decay, as found in previous modeling studies. Rather, during Southern Hemisphere winter, this reduction coincides with the presence of a turning latitude on the equatorward side of the storm track.

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Lei Wang
and
Sukyoung Lee

Abstract

The authors use a quasigeostrophic (QG) two-layer model to examine how eddies modify the meridional asymmetry of a zonal jet. The initial asymmetry is introduced in the model’s “radiative equilibrium state” and is intended to mimic a radiatively forced poleward jet shift simulated by climate models. The calculations show that the initial “poleward” jet shift in the two-layer model is amplified by eddy potential vorticity fluxes. This eddy-accentuation effect is greater as the baroclinicity of the equilibrium state is reduced, suggesting that seasonal variations in baroclinicity may help explain observed and modeled jet-shift sensitivity to season. The eddy-accentuated jet shift from the corresponding radiative equilibrium state is more clearly visible in the slowly varying, eddy-free reference state of Nakamura and Zhu. This reference state formally responds only to nonadvective, nonconservative processes, but ultimately arises from the advective eddy fluxes. The implication is that fast eddies are capable of driving a slowly varying jet shift, which may be balanced by nonconservative processes such as radiative heating/cooling.

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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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Steven Feldstein
and
Sukyoung Lee

Abstract

Data from a 2025-day Geophysical Fluid Dynamics Laboratory aquaplanet GCM integration are used to examine the temporal evolution of the zonal index, defined as the principal component of the first EOF of the zonally and vertically integrated absolute angular momentum. This EOF represents meridional displacements of the subtropical jet. Positive (negative) values of the zonal index correspond to poleward (equatorward) displacements of the subtropical jet and are referred to as the high (low) zonal index.

Composites of various quantities are used to examine the temporal evolution of both the zonal-mean zonal winds and the eddy fields associated with either index. The high index is initiated by an enhancement in the equatorward wave activity propagation, which causes the subtropical jet to move in a poleward direction. The low index is led by a weakening in the equatorward propagation, which results in an equatorward displacement of the subtropical jet. More importantly, for the high index, the corresponding increase in eddy forcing is confined to a brief period near onset, resulting in rapid growth and slow decay of the zonal wind anomaly. This suggests that the high index state is “impulsively” forced by the eddies rather than maintained by eddy-zonal mean feedback. In contrast, for the low index, a reduction in the eddy forcing extends throughout the entire persistent episode, although the weakest eddy forcing occurs during onset. The authors believe that such forcing causes the low index anomaly to persist for a longer period of time than the high index anomaly and also results in a similar growth and decay rate for the low index. Furthermore, case studies show that the onset for both indices is associated with wave breaking. For the high (low) index, this wave breaking takes the form of filamentation (blocking).

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Sukyoung Lee
and
Changhyun Yoo

Abstract

An increase in the poleward heat or energy transport is often ascribed to a strengthening of the equator-to-pole gradient in temperature or in the top-of-the-atmosphere (TOA) net radiation. While this attribution conforms to the well-established flux–gradient relationship, a counterexample is shown here, demonstrating that a forced atmospheric circulation, triggered by enhanced convection over the western tropical Pacific warm pool and suppressed convection over the eastern tropical Pacific and Indian Oceans, can cause the equator-to-pole gradient in the TOA net radiation to increase.

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Qian Li
and
Sukyoung Lee

Abstract

The formation of a narrow band of the deep winter mixed layer (hereinafter “mixed layer wedge”) in the Indo–western Pacific Southern Ocean is examined using an eddy-resolving Parallel Ocean Program (POP) model simulation. The mixed layer wedge starts to deepen in June, centered at 47.5°S, with a meridional scale of only ~2° latitude. Its center is located ~1° north of the model’s Subantarctic Front (SAF). The Argo-based observed mixed layer is similarly narrow and occurs adjacent to the observed SAF. In the small sector of 130°–142°E, where the SAF is persistent and the mixed layer is deepest, the formation of the narrow mixed layer wedge coincides with destratification underneath the mixed layer. This destratification can be attributed primarily to the downwelling branch of a jet-scale overturning circulation (JSOC). The JSOC, which was reported in an earlier study by the authors, is driven by eddy momentum flux convergence and is therefore thermally indirect: its descending branch occurs on the warmer equatorward flank of the SAF, promoting destratification during the warm season. The model-generated net air–sea heat flux reveals a similar wedge-like feature, indicating that the flux contributes to the mixed layer depth wedge, but again this feature is preconditioned by the JSOC. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. These findings suggest that the eddy-driven JSOC associated with the SAF plays an important role in initiating the narrow, deep mixed layer wedge that forms north of the SAF.

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Cory Baggett
and
Sukyoung Lee

Abstract

One of the challenging tasks in climate science is to understand the equator-to-pole temperature gradient. The poleward heat flux generated by baroclinic waves is known to be central in reducing the equator-to-pole temperature gradient from a state of radiative–convective equilibrium. However, invoking this relationship to explain the wide range of equator-to-pole temperature gradients observed in past climates is challenging because baroclinic waves tend to follow the flux–gradient relationship such that their poleward heat flux is proportional to the equator-to-pole temperature gradient and zonal available potential energy (ZAPE). With reanalysis data, the authors show the existence of poleward heat transport by planetary-scale waves that are independent of the flux–gradient relationship and baroclinic instability. This process arises from a forced tapping of atmospheric ZAPE by planetary-scale waves that are triggered by enhanced tropical convection over the Pacific warm pool region. The Rossby waves excited by this tropical convection propagate northeastward over the Pacific Ocean and constructively interfere with the climatological stationary waves at higher latitudes. During polar night, when the current warming is most rapid, the forced tapping of ZAPE by planetary-scale waves produces a substantially greater warming than that by the synoptic-scale eddy fluxes that presumably arise from baroclinic instability.

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Steven Feldstein
and
Sukyoung Lee

Abstract

The authors address the question of whether or not eddy feedback plays an important role in driving the anomalous relative angular momentum associated with the zonal index (ZI) in the atmosphere. For this purpose, composites of anomalous relative angular momentum and anomalous eddy angular momentum flux convergence (eddy forcing) are examined with National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis data.

By using an empirical orthogonal function analysis, it is found that ZI behavior dominates the summer season of both hemispheres and also the winter season of the Southern Hemisphere. For the summer season, the ZI is characterized by meridional displacements of the midlatitude eddy-driven jet, and for the Southern Hemisphere winter it is characterized by a simultaneous movement of the subtropical and eddy-driven jets in the opposite direction.

For the ZI of each of the above seasons, unfiltered eddy forcing did not exhibit a prominent eddy feedback. However, suggestive evidence for a feedback by high-frequency eddies (period less than 10 days) was found. These eddies act to prolong the lifetime of the ZI anomalies against the dissipative influences of both low-frequency (period greater than 10 days) and cross-frequency (eddy fluxes that involves the product of high- and low-frequency disturbances) eddy forcing and the friction torque.

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