scholarly journals The Role of Stationary Waves in the Maintenance of the Northern Annular Mode as Deduced from Model Experiments

2006 ◽  
Vol 63 (11) ◽  
pp. 2931-2947 ◽  
Author(s):  
Heiner Körnich ◽  
Gerhard Schmitz ◽  
Erich Becker
Keyword(s):  
Lower Troposphere ◽  
Circulation Model ◽  
Stationary Wave ◽  
Stationary Waves ◽  
Mean Flow ◽  
Thermal Forcing ◽  
Annular Mode ◽  
Model Experiments ◽  
Wave Forcing ◽  
Orographic Forcing

Abstract The influence of stationary waves on the maintenance of the tropospheric annular mode (AM) is examined in a simple global circulation model with perpetual January conditions. The presented model experiments vary in the configurations of stationary wave forcing by orography and land–sea heating contrasts. All simulations display an AM-like pattern in the lower troposphere. The zonal momentum budget shows that the feedback between eddies with periods less than 10 days and the zonal-mean zonal wind is generally the dominating process that maintains the AM. The kinetic energy of the high-frequency eddies depends on the stationary wave forcing, where orographic forcing reduces and thermal forcing enhances it. The AMs in the model experiments differ in the superposed anomalous stationary waves and in the strength of the zonally symmetric component. If only orographic stationary wave forcing is taken into account, the mountain torque decelerates the barotropic wind anomaly, and thus acts to weaken the AM. However, the combined forcing of orography and land–sea heating contrasts produces a feedback between the anomalous stationary waves and the AM that compensates for the mountain torque. The different behavior of the model experiments results from the fact that only the thermal forcing changes the character of the anomalous stationary waves from external Rossby waves for orographic forcing alone to vertically propagating waves that enable the feedback process through wave–mean flow interaction. Only with this feedback, which is shown to be due to linear zonal–eddy coupling, does the model display a strong AM with centers of action over the oceans. The main conclusions are that this process is necessary to simulate a realistic northern AM, and that it distinguishes the northern from the southern AM.

scholarly journals Diabatic and Orographic Forcing of Northern Winter Stationary Waves and Storm Tracks

2009 ◽  
Vol 22 (3) ◽  
pp. 670-688 ◽  
Author(s):  
Edmund K. M. Chang
Keyword(s):  
Storm Track ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Storm Tracks ◽  
Minor Role ◽  
Stationary Waves ◽  
Global Circulation Model ◽  
Mean Flow ◽  
The Pacific ◽  
Eddy Fluxes

Abstract In this study, a dry global circulation model is used to examine the contributions made by orographic and diabatic forcings in shaping the zonal asymmetries in the earth’s Northern Hemisphere (NH) winter climate. By design, the model mean flow is forced to bear a close resemblance to the observed zonal mean and stationary waves. The model also provides a decent simulation of the storm tracks. In particular, the maxima over the Pacific and Atlantic, and minima over Asia and North America, are fairly well simulated. The model also successfully simulates the observation that the Atlantic storm track is stronger than the Pacific storm track, despite stronger baroclinicity over the Pacific. Sensitivity experiments are performed by imposing and removing various parts of the total forcings. In terms of the NH winter stationary waves in the upper troposphere, results of this study are largely consistent with previous studies. Diabatic forcings explain most of the modeled stationary waves, with orographic forcings playing only a secondary role, and feedbacks due to eddy fluxes probably play only minor roles in most cases. Nevertheless, results of this study suggest that eddy fluxes may be important in modifying the response to orographic forcings in the absence of zonal asymmetries in diabatic heating. On the other hand, unlike the conclusion reached by previous studies, it is argued that the convergence of eddy momentum fluxes is important in forcing the oceanic lows in the lower troposphere, in agreement with one’s synoptic intuition. Regarding the NH winter storm-track distribution, results of this study suggest that NH extratropical heating is the most important forcing. Zonal asymmetries in NH extratropical heating act to force the Pacific storm track to shift equatorward and the Atlantic storm track to shift poleward, attain a southwest–northeast tilt, and intensify. It appears to be the main forcing responsible for explaining why the Atlantic storm track is stronger than the Pacific storm track. Tibet and the Rockies are also important, mainly in suppressing the storm tracks over the continents, forcing a clearer separation between the two storm tracks. In contrast, asymmetries in tropical heating appear to play only a minor role in forcing the model storm-track distribution.

Testing the Limits and Breakdown of the Nonacceleration Theorem for Orographic Stationary Waves

2020 ◽  
Vol 77 (5) ◽  
pp. 1513-1529
Author(s):  
Nicholas J. Lutsko
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Surface Wind ◽  
Circulation Model ◽  
Wave Model ◽  
Stationary Wave ◽  
Surface Wind Speed ◽  
Maximum Height ◽  
Stationary Waves ◽  
Mean Flow

Abstract The nonacceleration theorem states that the torque exerted on the atmosphere by orography is exactly balanced by the convergence of momentum by the stationary waves that the orography excites. This balance is tested in simulations with a stationary wave model and with a dry, idealized general circulation model (GCM), in which large-scale orography is placed at the latitude of maximum surface wind speed. For the smallest mountain considered (maximum height H = 0.5 m), the nonacceleration balance is nearly met, but the damping in the stationary wave model induces an offset between the stationary eddy momentum flux (EMF) convergence and the mountain torque, leading to residual mean flow changes. A stationary nonlinearity appears for larger mountains (H ≥ 10 m), driven by preferential deflection of the flow around the poleward flank of the orography, and causes further breakdown of the nonacceleration balance. The nonlinearity grows as H is increased, and is stronger in the GCM than in the stationary wave model, likely due to interactions with transient eddies. The midlatitude jet shifts poleward for H ≤ 2 km and equatorward for larger mountains, reflecting changes in the transient EMFs, which push the jet poleward for smaller mountains and equatorward for larger mountains. The stationary EMFs consistently force the jet poleward. These results add to our understanding of how orography affects the atmosphere’s momentum budget, providing insight into how the nonacceleration theorem breaks down; the roles of stationary nonlinearities and transients; and how orography affects the strength and latitude of eddy-driven jets.

The Response of an Idealized Atmosphere to Orographic Forcing: Zonal versus Meridional Propagation

2016 ◽  
Vol 73 (9) ◽  
pp. 3701-3718 ◽  
Author(s):  
Nicholas J. Lutsko ◽  
Isaac M. Held
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Stationary Waves ◽  
Linear Regime ◽  
Mean Flow ◽  
Trapped Waves ◽  
Barotropic Model ◽  
Flow Modification ◽  
Control Climate ◽  
Orographic Forcing

Abstract A dry atmospheric general circulation model is forced with large-scale, Gaussian orography in an attempt to isolate a regime in which the model responds linearly to orographic forcing and then to study the departures from linearity as the orography is increased in amplitude. In contrast to previous results, which emphasized the meridional propagation of orographically forced stationary waves, using the standard Held–Suarez (H–S) control climate, it is found that the linear regime is characterized by a meridionally trapped, zonally propagating wave. Meridionally trapped waves of this kind have been seen in other contexts, where they have been termed “circumglobal waves.” As the height of the orography is increased, the circumglobal wave coexists with a meridionally propagating wave and for large-enough heights the meridionally propagating wave dominates the response. A barotropic model on a sphere reproduces this trapped wave in the linear regime and also reproduces the transition to meridional propagation with increasing amplitude. However, mean-flow modification by the stationary waves is very different in the two models, making it difficult to argue that the transitions have the same causes. When adding asymmetry across the equator to the H–S control climate and placing the orography in the cooler hemisphere, it becomes harder to generate trapped waves in the GCM and the trapping becomes sensitive to the shape of the orography. The barotropic model overestimates the trapping in this case. These results suggest that an improved understanding of the role of circumglobal waves will be needed to understand the stationary wave field and its sensitivity to the changes in the zonal-mean climate.

Atmospheric general circulation and waves simulated by a Venus AORI GCM with topographical and radiative forcings

2021 ◽  
Author(s):  
Masaru Yamamoto ◽  
Takumi Hirose ◽  
Kohei Ikeda ◽  
Masaaki Takahashi
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Circulation Model ◽  
Stationary Wave ◽  
Static Stability ◽  
Small Scale ◽  
Mean Flow ◽  
Short Period ◽  
Low Latitudes ◽  
Thermal Tides

<p>General circulation and waves are investigated using a T63 Venus general circulation model (GCM) with solar and thermal radiative transfer in the presence of high-resolution surface topography. This model has been developed by Ikeda (2011) at the Atmosphere and Ocean Research Institute (AORI), the University of Tokyo, and was used in Yamamoto et al. (2019, 2021). In the wind and static stability structures similar to the observed ones, the waves are investigated. Around the cloud-heating maximum (~65 km), the simulated thermal tides accelerate an equatorial superrotational flow with a speed of ~90 m/s<sup></sup>with rates of 0.2–0.5 m/s/(Earth day) via both horizontal and vertical momentum fluxes at low latitudes. Over the high mountains at low latitudes, the vertical wind variance at the cloud top is produced by topographically-fixed, short-period eddies, indicating penetrative plumes and gravity waves. In the solar-fixed coordinate system, the variances (i.e., the activity of waves other than thermal tides) of flow are relatively higher on the night-side than on the dayside at the cloud top. The local-time variation of the vertical eddy momentum flux is produced by both thermal tides and solar-related, small-scale gravity waves. Around the cloud bottom, the 9-day super-rotation of the zonal mean flow has a weak equatorial maximum and the 7.5-day Kelvin-like wave has an equatorial jet-like wind of 60-70 m/s. Because we discussed the thermal tide and topographically stationary wave in Yamamoto et al. (2021), we focus on the short-period eddies in the presentation.</p>

The Dynamical Response to Snow Cover Perturbations in a Large Ensemble of Atmospheric GCM Integrations

2009 ◽  
Vol 22 (5) ◽  
pp. 1208-1222 ◽  
Author(s):  
Christopher G. Fletcher ◽  
Steven C. Hardiman ◽  
Paul J. Kushner ◽  
Judah Cohen
Keyword(s):  
Snow Cover ◽  
General Circulation ◽  
Circulation Model ◽  
Wave Activity ◽  
Dynamical Response ◽  
Mean Flow ◽  
Surface Cooling ◽  
Fall Season ◽  
Annular Mode ◽  
Northern Annular Mode

Abstract Variability in the extent of fall season snow cover over the Eurasian sector has been linked in observations to a teleconnection with the winter northern annular mode pattern. Here, the dynamics of this teleconnection are investigated using a 100-member ensemble of transient integrations of the GFDL atmospheric general circulation model (AM2). The model is perturbed with a simple persisted snow anomaly over Siberia and is integrated from October through December. Strong surface cooling occurs above the anomalous Siberian snow cover, which produces a tropospheric form stress anomaly associated with the vertical propagation of wave activity. This wave activity response drives wave–mean flow interaction in the lower stratosphere and subsequent downward propagation of a negative-phase northern annular mode response back into the troposphere. A wintertime coupled stratosphere–troposphere response to fall season snow forcing is also found to occur even when the snow forcing itself does not persist into winter. Finally, the response to snow forcing is compared in versions of the same model with and without a well-resolved stratosphere. The version with the well-resolved stratosphere exhibits a faster and weaker response to snow forcing, and this difference is tied to the unrealistic representation of the unforced lower-stratospheric circulation in that model.

scholarly journals The Building Blocks of Northern Hemisphere Wintertime Stationary Waves

2020 ◽  
Vol 33 (13) ◽  
pp. 5611-5633 ◽  
Author(s):  
Chaim I. Garfinkel ◽  
Ian White ◽  
Edwin P. Gerber ◽  
Martin Jucker ◽  
Moran Erez
Keyword(s):  
Temperature Field ◽  
Northern Hemisphere ◽  
General Circulation ◽  
Circulation Model ◽  
Building Blocks ◽  
Stationary Wave ◽  
Heat Fluxes ◽  
Boreal Winter ◽  
Stationary Waves ◽  
Western Asia

AbstractAn intermediate-complexity moist general circulation model is used to investigate the forcing of stationary waves in the Northern Hemisphere boreal winter by land–sea contrast, horizontal heat fluxes in the ocean, and topography. The additivity of the response to these building blocks is investigated. In the Pacific sector, the stationary wave pattern is not simply the linear additive sum of the response to each forcing. In fact, over the northeast Pacific and western North America, the sum of the responses to each forcing is actually opposite to that when all three are imposed simultaneously due to nonlinear interactions among the forcings. The source of the nonlinearity is diagnosed using the zonally anomalous steady-state thermodynamic balance, and it is shown that the background-state temperature field set up by each forcing dictates the stationary wave response to the other forcings. As all three forcings considered here strongly impact the temperature field and its zonal gradients, the nonlinearity and nonadditivity in our experiments can be explained, but only in a diagnostic sense. This nonadditivity extends up to the stratosphere, and also to surface temperature, where the sum of the responses to each forcing differs from the response if all forcings are included simultaneously. Only over western Eurasia is additivity a reasonable (though not perfect) assumption; in this sector land–sea contrast is most important over Europe, while topography is most important over western Asia. In other regions, where nonadditivity is pronounced, the question of which forcing is most important is ill-posed.

scholarly journals Intraseasonal Dynamical Evolution of the Northern Annular Mode

2005 ◽  
Vol 18 (18) ◽  
pp. 3820-3839 ◽  
Author(s):  
Brent A. McDaniel ◽  
Robert X. Black
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Dynamical Evolution ◽  
Stationary Wave ◽  
Index Of Refraction ◽  
Annular Mode ◽  
Wave Forcing ◽  
Planetary Scale ◽  
The North ◽  
The North Atlantic

Abstract The Northern Hemisphere annular mode (NAM) accounts for a significant fraction of the extratropical wintertime atmospheric variability. The dynamics of NAM events have been studied on monthly time scales, but little is known about the physical mechanisms that give rise to NAM variability on shorter time scales. Composite diagnostic analyses based on daily NAM indices are performed with a goal of identifying the dominant processes responsible for the growth and decay of large-amplitude positive and negative NAM events on short intraseasonal time scales. Transformed Eulerian mean, piecewise potential vorticity inversions, and regional Plumb flux diagnoses are employed to deduce the proximate forcings of the zonal-mean wind tendency during maturing and declining NAM stages. A remarkable degree of reverse symmetry is observed between the zonal-mean dynamical evolution of positive and negative NAM events. Anomalous equatorward and downward (poleward and upward) Eliassen–Palm fluxes are observed during the maturation of positive (negative) NAM events, consistent with index of refraction considerations and an indirect downward stratospheric influence. The associated patterns of anomalous wave driving provide the primary forcing of the zonal wind tendency field. Spectral analyses reveal that both the stratospheric and tropospheric patterns of wave driving are primarily due to low-frequency planetary-scale eddies. Regional wave activity flux diagnoses further illustrate that this wave-driving pattern represents the zonal-mean manifestation of planetary-scale anomalies over the North Atlantic that are linked to local anomalies in stationary wave forcing. The decay of NAM events coincides with the collapse in the pattern of anomalous stationary wave forcing over the North Atlantic region. Our diagnostic results indicate that both (i) synoptic eddies and (ii) direct downward stratospheric forcing provide second-order reinforcing contributions to the intraseasonal dynamical evolution of NAM events.

Interpreting Stationary Wave Nonlinearity in Barotropic Dynamics

2010 ◽  
Vol 67 (7) ◽  
pp. 2240-2250 ◽  
Author(s):  
Lei Wang ◽  
Paul J. Kushner
Keyword(s):  
General Circulation ◽  
Wave Model ◽  
Stationary Wave ◽  
Stationary Waves ◽  
Mean Flow ◽  
Model Studies ◽  
The Difference ◽  
Wave Models ◽  
Wave Nonlinearity ◽  
Basic Features

Abstract Stationary wave nonlinearity describes the self-interaction of stationary waves and is important in maintaining the zonally asymmetric atmospheric general circulation. However, the dynamics of stationary wave nonlinearity, which is often calculated explicitly in stationary wave models, is not well understood. Stationary wave nonlinearity is examined here in the simplified setting of the response to localized topographic forcing in quasigeostrophic barotropic dynamics in the presence and absence of transient eddies. It is shown that stationary wave nonlinearity accounts for most of the difference between the linear and full nonlinear response, particularly if the adjustment of the zonal-mean flow to the stationary waves is taken into account. The separate impact of transient eddy forcing is also quantified. Wave activity analysis shows that stationary wave nonlinearity in this setting is associated with Rossby wave critical layer reflection. A nonlinear stationary wave model, similar to those used in baroclinic stationary wave model studies, is also tested and is shown to capture the basic features of the full nonlinear stationary wave solution.

The Role of Linear Interference in the Annular Mode Response to Tropical SST Forcing

2011 ◽  
Vol 24 (3) ◽  
pp. 778-794 ◽  
Author(s):  
Christopher G. Fletcher ◽  
Paul J. Kushner
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Opposite Sign ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Stationary Wave ◽  
Annular Mode ◽  
Wave Response ◽  
Mode Response

Abstract Recent observational and modeling studies have demonstrated a link between eastern tropical Pacific Ocean (TPO) warming associated with the El Niño–Southern Oscillation (ENSO) and the negative phase of the wintertime northern annular mode (NAM). The TPO–NAM link involves a Rossby wave teleconnection from the tropics to the extratropics, and an increase in polar stratospheric wave driving that in turn induces a negative NAM anomaly in the stratosphere and troposphere. Previous work further suggests that tropical Indian Ocean (TIO) warming is associated with a positive NAM anomaly, which is of opposite sign to the TPO case. The TIO case is, however, difficult to interpret because the TPO and TIO warmings are not independent. To better understand the dynamics of tropical influences on the NAM, the current study investigates the NAM response to imposed TPO and TIO warmings in a general circulation model. The NAM responses to the two warmings have opposite sign and can be of surprisingly similar amplitude even though the TIO forcing is relatively weak. It is shown that the sign and strength of the NAM response is often simply related to the phasing, and hence the linear interference, between the Rossby wave response and the climatological stationary wave. The TPO (TIO) wave response reinforces (attenuates) the climatological wave and therefore weakens (strengthens) the stratospheric jet and leads to a negative (positive) NAM response. In additional simulations, it is shown that decreasing the strength of the climatological stationary wave reduces the importance of linear interference and increases the importance of nonlinearity. This work demonstrates that the simulated extratropical annular mode response to climate forcings can depend sensitively on the amplitude and phase of the climatological stationary wave and the wave response.

scholarly journals Atmospheric blocking in an aquaplanet and the impact of orography

2020 ◽  
Vol 1 (2) ◽  
pp. 293-311
Author(s):  
Veeshan Narinesingh ◽  
James F. Booth ◽  
Spencer K. Clark ◽  
Yi Ming
Keyword(s):  
General Circulation ◽  
Geographical Location ◽  
Dynamical Evolution ◽  
Circulation Model ◽  
Stationary Wave ◽  
Life Cycles ◽  
Stationary Waves ◽  
Atmospheric Blocking ◽  
The Impact ◽  
Block Frequency

Abstract. Many fundamental questions remain about the roles and effects of stationary forcing on atmospheric blocking. As such, this work utilizes an idealized moist general circulation model (GCM) to investigate atmospheric blocking in terms of dynamics, geographical location, and duration. The model is first configured as an aquaplanet, then orography is added in separate integrations. Block-centered composites of wave activity fluxes and height show that blocks in the aquaplanet undergo a realistic dynamical evolution when compared to reanalysis. Blocks in the aquaplanet are also found to have similar life cycles to blocks in model integrations with orography. These results affirm the usefulness of both zonally symmetric and asymmetric idealized model configurations for studying blocking. Adding orography to the model leads to an increase in blocking. This mirrors what is observed when comparing the Northern Hemisphere (NH) and Southern Hemisphere (SH), where the NH contains more orography and thus more blocking. As the prescribed mountain height increases, so do the magnitude and size of climatological stationary waves, resulting in more blocking overall. Increases in blocking, however, are not spatially uniform. Orography is found to induce regions of enhanced block frequency just upstream of mountains, near high pressure anomalies in the stationary waves, which is poleward of climatological minima in upper-level zonal wind, while block frequency minima and jet maxima occur eastward of the wave trough. This result matches what is observed near the Rocky Mountains. Finally, an analysis of block duration suggests blocks generated near stationary wave maxima last slightly longer than blocks that form far from or without orography. Overall, the results of this work help to explain some of the observed similarities and differences in blocking between the NH and SH and emphasize the importance of general circulation features in setting where blocks most frequently occur.

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