scholarly journals The Role of Wave Packets in Wave–Mean Flow Interactions during Southern Hemisphere Summer

2005 ◽  
Vol 62 (7) ◽  
pp. 2467-2483 ◽  
Author(s):  
Edmund K. M. Chang
Keyword(s):  
Life Cycle ◽  
Wave Packet ◽  
Southern Hemisphere ◽  
Wave Packets ◽  
Reanalysis Data ◽  
Heat Fluxes ◽  
Mean Flow ◽  
Single Wave ◽  
Dominant Wave ◽  
Flow Interactions

Abstract In this study, reanalysis data produced by the European Centre for Medium-Range Weather Forecasts for 14 Southern Hemisphere (SH) summer seasons have been analyzed. All cases of hemispheric transient eddy kinetic energy (TEKE) maxima have been identified, and the evolution of the local energetics and planetary-scale flow anomalies accompanying these TEKE growth/decay episodes are composited. The longitude–time evolution of the composite energetics shows the clear signature of a wave packet propagating eastward at a group velocity of about 27° longitude per day and undergoing a life cycle of growth and decay, with the energetics within a volume close to the wave packet center dominating the hemispheric mean energetics. When individual cases are examined, 52% are found to resemble the composite and have the energetics life cycle dominated by the evolution of a single wave packet, and an additional 21% are found to be dominated by the evolution of two wave packets having similar amplitudes. Only the remaining 27% can be regarded as having experienced TEKE growth and decay throughout much of the hemisphere. The zonal mean flow and eddy feedback anomalies (i.e., reduction in the meridional temperature gradient due to the effects of the eddy heat fluxes, as well as increase in the barotropic shear due to a narrowing of the midlatitude jet through the effects of the eddy momentum fluxes) associated with the cases dominated by the evolution of a single wave packet are also found to be dominated by anomalies close to the wave packet center. The fact that hemispheric wave growth/decay is often dominated by the evolution of a single wave packet has interesting dynamical consequences when the climatological basic flow is not zonally symmetric. When a wave packet propagates over regions of enhanced baroclinicity, it can extract more energy from the mean flow via baroclinic conversion, leading to its preferential growth. On the other hand, when a wave packet propagates over regions of weak baroclinicity, baroclinic conversion is suppressed; hence any packet growth must be due to other processes. By examining the location of wave packet peaks when hemispheric TEKE is at a maximum, it is observed that hemispheric mean TEKE peaks much more frequently when the dominant wave packet is located downstream of the region with strongest baroclinicity. In addition, the growth in TEKE for these cases is usually dominated by an increase in baroclinic conversion. In contrast, for the small number of cases in which the hemispheric mean TEKE maximum occurs when the dominant wave packet is located downstream of the region with weakest baroclinicity, the growth of the hemispheric TEKE is instead dominated by a reduction in barotropic dissipation.

Recurrent Rossby wave packets and persistent extreme weather

2021 ◽  
Author(s):  
S. Mubashshir Ali ◽  
Olivia Martius ◽  
Matthias Röthlisberger
Keyword(s):  
Southern Hemisphere ◽  
Spatial Patterns ◽  
Rossby Wave ◽  
Wave Packets ◽  
Reanalysis Data ◽  
Synoptic Scale ◽  
Background Flow ◽  
Dry Spell ◽  
Upper Level ◽  
Wet Spells

<p>Upper-level synoptic-scale Rossby wave packets are well-known to affect surface weather. When these Rossby wave packets occur repeatedly in the same phase at a specific location, they can result in persistent hot, cold, dry, and wet conditions. The repeated and in-phase occurrence of Rossby wave packets is termed as recurrent synoptic-scale Rossby wave packets (RRWPs). RRWPs result from multiple transient synoptic-scale wave packets amplifying in the same geographical region over several weeks.</p><p>Our climatological analyses using reanalysis data have shown that RRWPs can significantly modulate the persistence of hot, cold, dry, and wet spells in several regions in the Northern and the Southern Hemisphere.  RRWPs can both shorten or extend hot, cold, and dry spell durations. The spatial patterns of statistically significant links between RRWPs and spell durations are distinct for the type of the spell (hot, cold, dry, or wet) and the season (MJJASO or NDJFMA). In the Northern Hemisphere, the spatial patterns where RRWPs either extend or shorten the spell durations are wave-like. In the Southern Hemisphere, the spatial patterns are either wave-like (hot and cold spells) or latitudinally banded (dry and wet spells).</p><p>Furthermore, we explore the atmospheric drivers behind RRWP events. This includes both the background flow and potential wave-triggers such as the Madden Julian Oscillation or blocking. For 100 events of intense Rossby wave recurrence in the Atlantic, the background flow, the intensity of tropical convection, and the occurrence of blocking are studied using flow composites.</p>

Intraseasonal Periodicity in the Southern Hemisphere Circulation on Regional Spatial Scales

2017 ◽  
Vol 74 (3) ◽  
pp. 865-877 ◽  
Author(s):  
David W. J. Thompson ◽  
Brian R. Crow ◽  
Elizabeth A. Barnes
Keyword(s):  
Southern Hemisphere ◽  
Time Scales ◽  
Spatial Scales ◽  
Wave Packets ◽  
Lower Troposphere ◽  
Wave Activity ◽  
Heat Fluxes ◽  
Time Structure ◽  
Upper Troposphere ◽  
Activity Form

Abstract Wave activity in the Southern Hemisphere extratropical atmosphere exhibits robust periodicity on time scales of ~20–25 days. Previous studies have demonstrated the robustness of the periodicity in hemispheric averages of various eddy quantities. Here the authors explore the signature of the periodicity on regional spatial scales. Intraseasonal periodicity in the Southern Hemisphere circulation derives from out-of-phase anomalies in wave activity that form in association with extratropical wave packets as they propagate to the east. In the upper troposphere, the out-of-phase anomalies in wave activity form not along the path of extratropical wave packets, but in their wake. The out-of-phase anomalies in wave activity give rise to periodicity not only on hemispheric scales, but also on synoptic scales when the circulation is sampled along an eastward path between ~5 and 15 m s−1. It is argued that 1) periodicity in extratropical wave activity derives from two-way interactions between the heat fluxes and baroclinicity in the lower troposphere and 2) the unique longitude–time structure of the periodicity in upper-tropospheric wave activity derives from the contrasting eastward speeds of the source of the periodicity in the lower troposphere (~10 m s−1) and wave packets in the upper troposphere (~25 m s−1).

scholarly journals Implications of the Semigeostrophic Nature of Rossby Waves for Rossby Wave Packet Detection

2015 ◽  
Vol 143 (1) ◽  
pp. 26-38 ◽  
Author(s):  
Gabriel Wolf ◽  
Volkmar Wirth
Keyword(s):  
Wave Packet ◽  
Rossby Wave ◽  
Hilbert Transform ◽  
Wave Packets ◽  
Meridional Wind ◽  
Coordinate Space ◽  
Single Wave ◽  
Planetary Scale ◽  
Complex Demodulation ◽  
The Hilbert Transform

Abstract Upper-tropospheric Rossby wave packets have received increased attention recently. In most previous studies wave packets have been detected by computing the envelope of the meridional wind field using either complex demodulation or a Hilbert transform. The latter requires fewer choices to be made and appears, therefore, preferable. However, the Hilbert transform is fraught with a significant problem, namely, a tendency that fragments a single wave packet into several parts. The problem arises because Rossby wave packets show substantial deviations from the almost-plane wave paradigm, a feature that is well represented by semigeostrophic dynamics. As a consequence, higher harmonics interfere with the reconstruction of the wave envelope leading to undesirable wiggles. A possible cure lies in additional smoothing (e.g., by means of a filter) or resorting to complex demodulation (which implies smoothing, too). Another possibility, which does not imply any smoothing, lies in applying the Hilbert transform in semigeostrophic coordinate space. It turns out beneficial to exclude planetary-scale wavenumbers from this transformation in order to avoid problems in cases when the wave packet travels on a low wavenumber quasi-stationary background flow.

ENSO and SAM influence on the generation of long episodes of Rossby Wave Packets during Southern Hemisphere Summer

2021 ◽  
Author(s):  
Iago Perez ◽  
Marcelo Barreiro ◽  
Cristina Masoller
Keyword(s):  
Interannual Variability ◽  
Southern Hemisphere ◽  
Rossby Wave ◽  
Southern Oscillation ◽  
Wave Packets ◽  
Southern Annular Mode ◽  
Frequency Of Occurrence ◽  
Atmospheric Conditions ◽  
Mean Flow ◽  
Time Duration

<p>Rossby Wave Packets (RWPs) are key to the improvement of  long-range forecasting and for the prediction of sub-seasonal extremes. Several studies have focused on their properties, such as time duration, trajectory, areas of detection and dissipation as well as interannual variability in the northern hemisphere, but only a few of them have focused in the southern hemisphere. Here we study the influence of low-frequency climate modes on RWPs during southern hemisphere summer using NCEP DOE 2 Reanalysis data. Focusing on long-lived RWPs, which we define as RWPs with a lifespan above 8 days,  we determine how El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) modify their frequency of occurrence and their main areas of detection and dissipation. We found that during El Niño and negative SAM years, the number of long-lived RWPs is maximum. In addition, years with the highest amount of long-lived RWPs show a zonally symmetric and narrow upper level jet that is shifted northward from its climatological position. On the other hand, when the jet is shifted southward, particularly in the southeastern Pacific, during positive SAM phases, only a small number of long-lived RWPs is detected. Therefore, negative SAM conditions provide a background mean flow that favours the occurrence of long-lived RWPs while positive SAM conditions have the opposite effect. The dependence on ENSO phase is not as symmetric: while El Niño sets atmospheric conditions that favour the formation of long-lived RWPs, La Niña years present high interannual variability in the frequency of occurrence. Furthermore, in El Niño events the main formation area is between 61-120ºE and the main dissipation area between 300-359ºE. During La Niña events, the main formation area is located by 241-300ºE and no main dissipation area is identified. In the case of positive SAM two main formation areas appear at 61-120ºE and 241-300ºE and two main dissipation areas within 121-180 and 301-359ºE. Lastly in the case of negative SAM one main formation area at 241-300ºE is detected and no main dissipation area is detected. The robustness of the results was tested repeating the analysis using data from the ERA5 Reanalysis and supports the finding that the maximum number of long-lived RWPs occur during negative SAM and El Niño years</p>

scholarly journals Zonal Flow Regime Changes in a GCM and in a Simple Quasigeostrophic Model: The Role of Stratospheric Dynamics

2009 ◽  
Vol 66 (5) ◽  
pp. 1366-1383 ◽  
Author(s):  
Isabella Bordi ◽  
Klaus Fraedrich ◽  
Michael Ghil ◽  
Alfonso Sutera
Keyword(s):  
General Circulation ◽  
Baroclinic Instability ◽  
Three Dimensional ◽  
Vertical Wind Shear ◽  
Circulation Model ◽  
Zonal Flow ◽  
Heat Fluxes ◽  
Mean Flow ◽  
Single Wave

Abstract The atmospheric general circulation is characterized by both single- and double-jet patterns. The double-jet structure of the zonal mean zonal wind is analyzed in Southern Hemisphere observations for the two calendar months of November and April. The observed features are studied further in an idealized quasigeostrophic and a simplified general circulation model (GCM). Results suggest that capturing the bimodality of the zonal mean flow requires the parameterization of momentum and heat fluxes associated with baroclinic instability of the three-dimensional fields. The role of eddy heat fluxes in generating the observed double-jet pattern is ascertained by using an analytical Eady model with stratospheric easterlies, in which a single wave disturbance interacts with the mean flow. In this model, the dual jets are generated by the zonal mean flow correction. Sensitivity of the results to the tropospheric vertical wind shear (or, equivalently, the meridional temperature gradient in the basic state’s troposphere) is also studied in the Eady model and compared to related experiments using the simplified GCM.

scholarly journals Evaluation of the tropospheric flows to a major Southern Hemisphere stratospheric warming event using NCEP/NCAR Reanalysis data with a PSU/NCAR nudging MM5V3 model

2008 ◽  
Vol 8 (2) ◽  
pp. 7953-7976
Author(s):  
K. Wang
Keyword(s):  
Southern Hemisphere ◽  
Polar Region ◽  
Lower Stratosphere ◽  
Polar Vortex ◽  
Reanalysis Data ◽  
Heat Fluxes ◽  
Stratospheric Warming ◽  
Flow Conditions ◽  
Excessive Heat ◽  
Novel Approach

Abstract. Previous studies of the exceptional 2002 Southern Hemisphere (SH) stratospheric warming event lead to some uncertainty, namely the question of whether excessive heat fluxes in the upper troposphere and lower stratosphere are a symptom or cause of the 2002 SH warming event. In this work, we use a hemispheric version of the MM5 model with nudging capability and we devised a novel approach to separately test the significance of the stratosphere and troposphere for this year. We paired the flow conditions from 2002 in the stratosphere and troposphere, respectively, against the conditions in 1998 (a year with displaced polar vortex) and in 1948 (a year with strong polar vortex that coincided with the geographical South Pole). Our experiments show that the flow conditions from below determine the stratospheric flow features over the polar region. Regardless of the initial stratospheric conditions in 1998 or 1948, when we simulated these past stratospheres with the troposphere/lower stratosphere conditions constrained to 2002 levels, the simulated middle stratospheres resemble those observed in 2002 stratosphere over the polar region. On the other hand, when the 2002 stratosphere was integrated with the troposphere/lower stratosphere conductions constrained to 1948 and 1998, respectively, the simulated middle stratospheric conditions over the polar region shift toward those of 1948 and 1998. Thus, our experiments further support the wave-forcing theory as the cause of the 2002 SH warming event.

Finite-amplitude gravity waves in the atmosphere: travelling wave solutions

2017 ◽  
Vol 826 ◽  
pp. 1034-1065 ◽  
Author(s):  
Mark Schlutow ◽  
R. Klein ◽  
U. Achatz
Keyword(s):  
Wave Packet ◽  
Wave Packets ◽  
Vertical Component ◽  
Travelling Wave ◽  
Moving Frame ◽  
Travelling Wave Solutions ◽  
Mean Flow ◽  
Large Wave ◽  
Modulation Equations ◽  
Wave Solutions

Wentzel–Kramers–Brillouin theory was employed by Grimshaw (Geophys. Fluid Dyn., vol. 6, 1974, pp. 131–148) and Achatz et al. (J. Fluid Mech., vol. 210, 2010, pp. 120–147) to derive modulation equations for non-hydrostatic internal gravity wave packets in the atmosphere. This theory allows for wave packet envelopes with vertical extent comparable to the pressure scale height and for large wave amplitudes with wave-induced mean-flow speeds comparable to the local fluctuation velocities. Two classes of exact travelling wave solutions to these nonlinear modulation equations are derived here. The first class involves horizontally propagating wave packets superimposed over rather general background states. In a co-moving frame of reference, examples from this class have a structure akin to stationary mountain lee waves. Numerical simulations corroborate the existence of nearby travelling wave solutions under the pseudo-incompressible model and reveal better than expected convergence with respect to the asymptotic expansion parameter. Travelling wave solutions of the second class also feature a vertical component of their group velocity but exist under isothermal background stratification only. These waves include an interesting nonlinear wave–mean-flow interaction process: a horizontally periodic wave packet propagates vertically while draining energy from the mean wind aloft. In the process it decelerates the lower-level wind. It is shown that the modulation equations apply equally to hydrostatic waves in the limit of large horizontal wavelengths. Aside from these results of direct physical interest, the new nonlinear travelling wave solutions provide a firm basis for subsequent studies of nonlinear internal wave instability and for the design of subtle test cases for numerical flow solvers.

scholarly journals Wave–Mean-Flow Interactions in Moist Baroclinic Life Cycles

2017 ◽  
Vol 74 (7) ◽  
pp. 2143-2162 ◽  
Author(s):  
Ray Yamada ◽  
Olivier Pauluis
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Moisture Flux ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Latent Heating ◽  
Mean Flow ◽  
Momentum Exchange ◽  
Flow Interactions

Abstract Previous studies show that the moist Eliassen–Palm (EP) flux captures a greater eddy momentum exchange through form drag than the dry EP flux in the midlatitude climate. This suggests that the eddy moisture flux acts to decrease the baroclinicity of the zonal jet. This study investigates such a mechanism in moist baroclinic life cycles, which are simulated in an idealized general circulation model with large-scale condensation as the only moist process. The runs are analyzed using a linear diagnostic based on the Kuo–Eliassen equation to decompose the jet change into parts driven by individual forcing terms. It is shown that the wave-induced latent heating drives an indirect Eulerian-mean cell on the equatorward flank of the jet, which acts to reduce the baroclinicity in that region. The eddy sensible heat fluxes act to reduce the baroclinicity near the center of the jet. The moist baroclinic forcing strengthens as the amount of initially available moisture increases. The effect of the eddy moisture flux on the transformed Eulerian-mean (TEM) and isentropic dynamics is also considered. It is shown that the circulation and EP flux on moist isentropes is around 4 times as strong and extends farther equatorward than on dry isentropes. The equatorward extension of the moist EP flux coincides with the region where the baroclinic forcing is driven by latent heating. The moist EP flux successfully captures the moisture-driven component of the baroclinic forcing that is not seen in the dry EP flux.

scholarly journals Comments on “A New Theory for the Generation of the Equatorial Subsurface Countercurrents”

2005 ◽  
Vol 35 (8) ◽  
pp. 1494-1496 ◽  
Author(s):  
Frédéric Marin ◽  
Bach Lien Hua ◽  
Richard Schopp
Keyword(s):  
Momentum Equation ◽  
Heat Fluxes ◽  
Driving Mechanism ◽  
Mean Flow ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Flow Interactions ◽  
Transformed Eulerian Mean ◽  
Zonal Momentum

Abstract From a numerical simulation of the Atlantic Ocean, Jochum and Malanotte-Rizzoli provide evidence that the equatorial subsurface countercurrents can be triggered by tropical instability waves through eddy–mean flow interactions in a low-Rossby-number regime. Adapting the transformed Eulerian mean formalism to a shoaling jet, they propose eddy heat fluxes to be the driving mechanism for the subsurface countercurrents. Here it is shown that such a formalism relying on the existence of a residual meridional streamfunction cannot be applied to a shoaling jet, so that the eddy heat fluxes term in the zonal momentum equation cannot be rigorously justified. Moreover, the role of the zonal pressure gradient that was dropped in their study needs to be reassessed. Despite this mathematical questioning of Jochum and Malanotte-Rizzoli’s framework, the authors agree with them that eddy heat fluxes may contribute to the dynamics of the subsurface countercurrents.

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