scholarly journals Interpreting the Opposition between Two Block-Onset Forcing Mechanisms

2007 ◽  
Vol 64 (6) ◽  
pp. 2091-2104 ◽  
Author(s):  
Li Dong ◽  
Stephen J. Colucci
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Potential Vorticity ◽  
Long Waves ◽  
Synoptic Scale ◽  
Short Waves ◽  
Idealized Model ◽  
Dominant Wave ◽  
Critical Wavelength ◽  
Forcing Mechanisms

The opposition between two block-onset forcing mechanisms, previously identified in midtropospheric analyses over the Southern Hemisphere midlatitudes, is analytically interpreted with an idealized model. These mechanisms are the interaction (Finter) between deformation and potential vorticity and the advection (Fadv) of meridionally varying potential vorticity. Weather systems of concern, primarily consisting of planetary- and synoptic-scale waves, mostly fall into two regimes of zonal and meridional wavenumber space in which the opposition between the two block-onset forcing mechanisms is analytically derived. A synoptic interpretation of this opposition is schematically presented within the framework of barotropic dynamics. It is found that whether blocking occurs in diffluent or confluent flow depends upon the critical wavelength associated with the geostrophic flow. Blocking tends to take place in the diffluent flow of long waves in which Finter dominates over Fadv. In addition, blocking also tends to occur in the confluent flow of relative short waves in which Fadv prevails over Finter. An investigation of Rossby wave phase speeds in one diagnosed case reveals a lengthening with time of the dominant wave until it reaches the stationary wavelength on the block-onset day. In this context blocking may be understood as a stationarity and thus persistence of one of the two block-onset forcing mechanisms.

A Climatology of Rossby Wave Breaking on the Southern Hemisphere Tropopause

2011 ◽  
Vol 68 (4) ◽  
pp. 798-811 ◽  
Author(s):  
Thando Ndarana ◽  
Darryn W. Waugh
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Potential Vorticity ◽  
Seasonal Variations ◽  
Wave Breaking ◽  
Polar Front ◽  
Rossby Wave Breaking ◽  
South Pacific Ocean ◽  
Ocean Region ◽  
Summer Minimum

Abstract A 30-yr climatology of Rossby wave breaking (RWB) on the Southern Hemisphere (SH) tropopause is formed using 30 yr of reanalyses. Composite analysis of potential vorticity and meridional fluxes of wave activity show that RWB in the SH can be divided into two broad categories: anticyclonic and cyclonic events. While there is only weak asymmetry in the meridional direction and most events cannot be classified as equatorward or poleward in terms of the potential vorticity structure, the position and structure of the fluxes associated with equatorward breaking differs from those of poleward breaking. Anticyclonic breaking is more common than cyclonic breaking, except on the lower isentrope examined (320 K). There are marked differences in the seasonal variations of RWB on the two surfaces, with a winter minimum for RWB around 350 K but a summer minimum for RWB around 330 K. These seasonal variations are due to changes in the location of the tropospheric jets and dynamical tropopause. During winter the subtropical jet and tropopause at 350 K are collocated in the Australian–South Pacific Ocean region, resulting in a seasonal minimum in the 350-K RWB. During summer the polar front jet and 330-K tropopause are collocated over the Southern Atlantic and Indian Oceans, inhibiting RWB in this region.

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>

Are Recurrent Rossby wave packets linked to persistent extreme weather events in the Southern Hemisphere?

2020 ◽  
Author(s):  
Syed Mubashshir Ali ◽  
Olivia Martius ◽  
Matthias Röthlisberger
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Wave Packets ◽  
Extreme Weather Events ◽  
Synoptic Scale ◽  
Spatial And Seasonal Variation ◽  
Weather Events ◽  
Surface Weather ◽  
Winter Cold ◽  
Hot Spells

<p>Synoptic-scale Rossby wave-packets have a recurrent pattern during several episodes of persistent surface weather which is termed as 'recurrent Rossby wave-packets' (RRWP). They result in a statistically significant increase in winter cold and summer hot spells over large areas of the Northern Hemisphere mid-latitudes.</p><p>We present a global climatology of the RRWPs to study its spatial and seasonal variation. We also investigate the link of RRWPs to persistent surface extremes in the Southern Hemisphere (SH).  We find that RRWPs result in a statistically significant increase in winter cold and summer hot spells over broad areas in Australia and South America. Furthermore, we discuss the effects of climatological oscillations (Madden Julian Oscillation, ENSO, etc) on influencing the RRWPs.</p>

Climatology of Anticyclonic and Cyclonic Rossby Wave Breaking on the Dynamical Tropopause in the Southern Hemisphere

2011 ◽  
Vol 24 (4) ◽  
pp. 1239-1251 ◽  
Author(s):  
Jie Song ◽  
Chongyin Li ◽  
Jing Pan ◽  
Wen Zhou
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Zonal Wind ◽  
Potential Vorticity ◽  
Wave Breaking ◽  
Austral Summer ◽  
Austral Winter ◽  
Rossby Wave Breaking ◽  
Weather Forecasts ◽  
Medium Range

Abstract The characteristics of the climatological distribution of the anticyclonic (LC1) and cyclonic (LC2) Rossby wave breaking (RWB) in the Southern Hemisphere (SH) are investigated by calculating the occurrence frequency of the LC1- and LC2-like stratospheric potential vorticity (PV) streamers in the SH during the austral summer [December–February (DJF)] and wintertime [June–August (JJA)] on several isentropic surfaces by using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) daily dataset. The results show that 1) on the equatorward flank of the climatological midlatitude jet (MLJ), the LC1-like PV streamers are frequently found over the central oceanic regions, whereas the LC2-like PV streamers are almost absent. On the poleward flank of the climatological MLJ, both types of PV streamers are frequently observed and the LC2-like PV streamers predominate; 2) the regions where the occurrences of the PV streamers are frequent overlap the weak zonal wind regions; and 3) in austral winter, a “double-jet” setting is evident in two regions of the SH [the double-jet upstream (DU) and the spilt jet region]. In the double-jet setting regions, the LC1-like PV streamers are frequently found both in the DU and the split-jet regions, while the occurrence of the LC2-like PV streamers is frequent in the split-jet region but is rather infrequent in the DU region.

scholarly journals The Planetary- and Synoptic-Scale Interactions in a Southeast Pacific Blocking Episode Using PV Diagnostics

2005 ◽  
Vol 62 (6) ◽  
pp. 1901-1916 ◽  
Author(s):  
John P. Burkhardt ◽  
Anthony R. Lupo
Keyword(s):  
Potential Vorticity ◽  
Synoptic Scale ◽  
Atlantic Region ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Scale Interactions ◽  
Planetary Scale ◽  
Southeast Pacific ◽  
Forcing Mechanisms ◽  
Environmental Prediction

Abstract The synoptic- and planetary-scale forcing in two blocking anticyclones occurring over the southeast Pacific Ocean was examined using potential vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short-lived. Some studies of NH blocking dynamics have also shown that the extent to which the planetary- and synoptic-scale and planetary–synoptic-scale interaction forcing that contribute to the genesis and maintenance of Pacific and Atlantic region events can be different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH was carried out in order to determine whether or not these events are associated with similar dynamic mechanisms to those in the NH. Using the National Center for Atmospheric Research and National Centers for Environmental Prediction (NCAR–NCEP) reanalyses dataset and applying a low-pass filter to the relevant variables, the authors examined the scale interactions associated with two blocking events that occurred during July and August 1986 and applied potential vorticity diagnostics. Results demonstrate that blocking in the southeast Pacific was associated with similar synoptic features, and the forcing mechanisms on the planetary, synoptic scales, and interactions were more similar to North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrated that blocking events in the NH were associated with synergistically interacting synoptic- and planetary-scale waves, while in the SH, blocking events resulted from the superposition of synoptic and planetary waves. This result may explain the paucity of blocking occurrences and their tendency to be weaker and less persistent over much of the SH.

The Role of Deformation and Potential Vorticity in Southern Hemisphere Blocking Onsets

2005 ◽  
Vol 62 (11) ◽  
pp. 4043-4056 ◽  
Author(s):  
Li Dong ◽  
Stephen J. Colucci
Keyword(s):  
Southern Hemisphere ◽  
Potential Vorticity ◽  
Geostrophic Wind ◽  
Positive Contribution ◽  
Synoptic Scale ◽  
Planetary Scale ◽  
Sheared Flow ◽  
Westerly Flow ◽  
Quasigeostrophic Model

Abstract The relative importance of interactions between deformation and potential vorticity (PV) as a block-onset mechanism is examined in 30 cases of atmospheric blocking over the Southern Hemisphere (SH). The blocking cases are diagnosed with a quasigeostrophic model for the u component of the geostrophic wind tendency. In this model, two mechanisms, the advection of the meridional gradient of PV and interactions between deformation and PV, can force the weakening of westerly flow or increasing easterly flow associated with blocking. The first forcing mechanism, which does not directly include deformation, indicates that the advection of equatorward increasing cyclonic PV (or equatorward decreasing anticyclonic PV) could force a local weakening of geostrophic westerlies or increasing easterlies. The second forcing mechanism, which represents the net effect of interactions between deformation and PV, indicates that eastward increasing PV embedded in a cyclonically sheared flow or equatorward increasing PV coincident with a stretching (diffluent) flow could each force a weakening in the westerlies. While deformation is a distinct signature of blocking, it may not always actively participate in the formation of blocking. Advection and interaction contributions generally opposed each other in both the diagnosed blocking and nonblocking cases. Weakening westerlies associated with block onset would occur when one effect (usually the advection effect) contributes more negatively to the wind tendency than the opposing, positive contribution from the other effect. When deformation is actively involved in the formation of blocking, self-interactions between synoptic-scale PV and deformation and self-interactions between planetary-scale PV and deformation contribute more importantly than synoptic-to-planetary-scale interactions between PV and deformation fields to the weakening of westerlies associated with block onsets.

Method for Identifying and Clustering Rossby Wave Breaking Events in the Northern Hemisphere

2021 ◽  
pp. 17-28
Author(s):  
A. V. Gochakov ◽  
◽  
O. Yu. Antokhina ◽  
V. N. Krupchatnikov ◽  
Yu. V. Martynova ◽  
...  
Keyword(s):  
Rossby Wave ◽  
Potential Vorticity ◽  
Wave Breaking ◽  
Large Scale ◽  
Method Development ◽  
Spatial Clustering ◽  
Potential Temperature ◽  
Reanalysis Data ◽  
Rossby Wave Breaking ◽  
Dynamic Phenomena

Many large-scale dynamic phenomena in the Earth’s atmosphere are associated with the processes of propagation and breaking of Rossby waves. A new method for identifying the Rossby wave breaking (RWB) is proposed. It is based on the detection of breakings centers by analyzing the shape of the contours of potential vorticity or temperature on quasimaterial surfaces: isentropic and iserthelic (surfaces of constant Ertel potential vorticity (PV)), with further RWB center clustering to larger regions. The method is applied to the set of constant PV levels (0.3 to 9.8 PVU with a step of 0.5 PVU) at the level of potential temperature of 350 K for 12:00 UTC. The ERA-Interim reanalysis data from 1979 to 2019 are used for the method development. The type of RWB (cyclonic/anticyclonic), its area and center are determined by analyzing the vortex geometry at each PV level for every day. The RWBs obtained at this stage are designated as elementary breakings. Density-Based Spatial Clustering of Applications with Noise algorithm (DBSCAN) was applied to all elementary breakings for each month. As a result, a graphic dataset describing locations and dynamics of RWBs for every month from 1979 to 2019 is formed. The RWB frequency is also evaluated for each longitude, taking into account the duration of each RWB and the number of levels involved, as well as the anomalies of these parameters.

A novel identification and tracking method of weather-relevant 3D Potential vorticity streamers

2021 ◽  
Author(s):  
Christoph Fischer ◽  
Elmar Schömer ◽  
Andreas H. Fink ◽  
Michael Riemer ◽  
Michael Maier-Gerber
Keyword(s):  
Rossby Wave ◽  
Potential Vorticity ◽  
Wave Breaking ◽  
Extreme Weather ◽  
Individual Characteristics ◽  
Distance Functions ◽  
Extreme Weather Events ◽  
Rossby Wave Breaking ◽  
Feature Vectors ◽  
Weather Events

<p>Potential vorticity streamers (PVSs) are elongated quasi-horizontal filaments of stratospheric air in the upper troposphere related to, for example, Rossby wave breaking events. They are known to be related to partly extreme weather events in the midlatitudes and subtropics and can also be involved in (sub-)tropical cyclogenesis. While several algorithms have been developed to identify and track PVSs on planar isentropic surfaces, less is known about the evolution of these streamers in 3D, both climatologically but also for a better understanding of individual weather events. Furthermore, characteristics of their 3D shape have barely been considered as a predictor for high impact weather events like (sub-)tropical cyclones.</p><p>We introduce a novel algorithm for detection and identification of PVSs based on image processing techniques which can be applied to 2D and 3D gridded datasets. The potential vorticity was taken from high resolution isentropic analyses based on the ERA5 dataset. The algorithm uses the 2 PVU (Potential Vorticity Unit) threshold to identify and extract anomalies in the PV field using signed distance functions. This is accomplished by using a stereographic projection to eliminate singularities and keeping track of the reduced distortions by storing precomputed distance maps. This approach is computationally efficient and detects more interesting structures that exhibit the general behavior of PVSs compared to existing 2D techniques.</p><p>For each identified object a feature vector is computed, containing the individual characteristics of the streamers. In the 3D case, the algorithm looks at the structure en bloc instead of operating individually on multiple 2D levels. This also makes the identification stable regarding the seasonal cycle. Feature vectors contain parameters about quality, intensity and shape. In the case of 2D datasets, best-fitting ellipses computed from the statistical moments are regarded as a description of their shape. For 3D datasets, recent visualizations show that the boundary of these structures could be approximated by quadric surfaces . The feature vectors are also amended by tracking information, for example splitting and merging events. This low-dimensional representation serves as base for ERA5 climatologies. The data will be correlated with (sub-)tropical cyclone occurrence to spot useful and novel predictors for cyclone activity and preceding Rossby Wave Breaking events.</p><p>Overall, this new type of PVS identification algorithm, applicable in 2D or 3D, allows to diagnose the role of PVS in extreme weather events, including their predictability in ensemble forecasts.</p>

Teleconnections between Tropical Pacific SST Anomalies and Extratropical Southern Hemisphere Climate

2014 ◽  
Vol 28 (1) ◽  
pp. 56-65 ◽  
Author(s):  
Laura M. Ciasto ◽  
Graham R. Simpkins ◽  
Matthew H. England
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Rossby Wave ◽  
Wave Train ◽  
Cold Season ◽  
Tropical Pacific ◽  
Central Pacific ◽  
Sst Variability ◽  
Sst Anomalies

Abstract Teleconnections from tropical Pacific sea surface temperature (SST) anomalies to the high-latitude Southern Hemisphere (SH) are examined using observations and reanalysis. Analysis of tropical Pacific SST anomalies is conducted separately for the central Pacific (CP) and eastern Pacific (EP) regions. During the austral cold season, extratropical SH atmospheric Rossby wave train patterns are observed in association with both EP and CP SST variability. The primary difference between the patterns is the westward displacement of the CP-related atmospheric anomalies, consistent with the westward elongation of CP-related convective SST required for upper-level divergence and Rossby wave generation. Consequently, CP-related patterns of SH SST, Antarctic sea ice, and temperature anomalies also exhibit a westward displacement, but otherwise, the cold season extratropical SH teleconnections are largely similar. During the warm season, however, extratropical SH teleconnections associated with tropical CP and EP SST anomalies differ substantially. EP SST variability is linked to largely zonally symmetric structures in the extratropical atmospheric circulation, which projects onto the southern annular mode (SAM), and is strongly related to the SH temperature and sea ice fields. In contrast, CP SST variability is only weakly related to the SH atmospheric circulation, temperature, or sea ice fields and no longer exhibits any clear association with the SAM. One hypothesized mechanism suggests that the relatively weak CP-related SST anomalies are not able to substantially impact the background flow of the subtropical jet and its subsequent interaction with equatorward-propagating waves associated with variability in the SAM. However, there is currently no widely established mechanism that links tropical Pacific SST anomalies to the SAM.

scholarly journals EROSION AROUND A PILE DUE TO CURRENT AND BREAKING WAVES

1988 ◽  
Vol 1 (21) ◽  
pp. 102 ◽  
Author(s):  
E.W. Bijker ◽  
C.A. De Bruyn
Keyword(s):  
Shear Stress ◽  
Breaking Waves ◽  
Velocity Profiles ◽  
Long Waves ◽  
Orbital Velocity ◽  
Bottom Shear Stress ◽  
Normal Waves ◽  
Short Waves ◽  
A Current

Tests have been performed on a vertical pile subject to current only and to a combination of current with normal waves and current with breaking waves. The scour around the pile produced by current only is decreased by normal short waves superimposed upon that current and increased when breaking waves are superimposed upon the current. After analysis of the velocity profiles in the undisturbed area upstream of the pile and next to the pile, the following explanation is found for this phenomenon. When normal short waves are superimposed upon a current, the bottom shear stress of the combination of current with waves is increased more in the undisturbed area than next to the pile in the scour area. This results in a decrease of the scour around the pile. Due to the large values of the orbital velocity under breaking waves this effect is reversed for the combination of a current with breaking and relatively long waves. This results in an increase of the scour around the pile.

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