A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

2007 ◽  
Vol 64 (3) ◽  
pp. 849-868 ◽  
Author(s):  
Sukyoung Lee ◽  
Seok-Woo Son ◽  
Kevin Grise ◽  
Steven B. Feldstein
Keyword(s):  
Wave Propagation ◽  
Zonal Wind ◽  
Wave Breaking ◽  
Observational Studies ◽  
Mean Flow ◽  
Radiative Relaxation ◽  
Meridional Overturning ◽  
Rossby Wave Propagation ◽  
The Tropics ◽  
Pv Gradient

Abstract Observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi-periodic poleward propagation. A set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation. This study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the Tropics. These negative anomalies arise from meridional overturning/breaking of waves that originate in midlatitudes. This wave breaking homogenizes the potential vorticity (PV) within the region of negative zonal wind anomalies, and also leads to the formation of positive zonal wind anomalies in the subtropics. Subsequent equatorward radiation of midlatitude waves is halted, which results in wave breaking at the poleward end of the homogenized PV region. This in turn generates new positive and negative zonal wind anomalies, which enables a continuation of the poleward propagation. The shielding of the homogenized PV region from equatorward wave propagation allows the model’s radiative relaxation to reestablish undisturbed westerlies in the Tropics, while extratropical westerly anomalies arise from eddy vorticity fluxes. The above process indicates that the poleward zonal mean anomaly propagation is caused by an orchestrated combination of linear Rossby wave propagation, nonlinear wave breaking, and radiative relaxation. The importance of the meridional wave propagation and breaking is consistent with the fact that the poleward propagation occurs only in the parameter space of the model where the PV gradient is of moderate strength. Implications for predictability are briefly discussed.

Teleconnections from Tropics to Northern Extratropics through a Southerly Conveyor

2005 ◽  
Vol 62 (11) ◽  
pp. 4057-4070 ◽  
Author(s):  
Zhuo Wang ◽  
C-P. Chang ◽  
Bin Wang ◽  
Fei-Fei Jin
Keyword(s):  
Wave Propagation ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Propagation Theory ◽  
Wave Source ◽  
Tropical Forcing ◽  
Rossby Wave Source ◽  
Rossby Wave Response ◽  
Rossby Wave Propagation ◽  
The Tropics

Abstract Rossby wave propagation theory predicts that Rossby waves in a tropical easterly flow cannot escape from the Tropics to the extratropics. Here the authors show that a southerly flow component in the basic state (a southerly conveyor) may transfer a Rossby wave source northward; thus, a forcing embedded in the deep tropical easterlies may excite a Rossby wave response in the extratropical westerlies. It is shown that the southerly conveyor determines the location of the effective Rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing, provided that the tropical response can reach the southerly conveyor. A stronger southerly flow favors a stronger extratropical response, and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows.

Extratropical–Tropical Interaction during Onset of the Australian Monsoon: Reanalysis Diagnostics and Idealized Dry Simulations

2007 ◽  
Vol 64 (10) ◽  
pp. 3475-3498 ◽  
Author(s):  
Noel E. Davidson ◽  
Kevin J. Tory ◽  
Michael J. Reeder ◽  
Wasyl L. Drosdowsky
Keyword(s):  
Wave Propagation ◽  
High Latitude ◽  
Monsoon Trough ◽  
Background Flow ◽  
Australian Monsoon ◽  
Subtropical Jet ◽  
Westerly Winds ◽  
Rossby Wave Propagation ◽  
Upper Level ◽  
The Tropics

Abstract The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics. For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical–tropical interaction is northeastward Rossby wave propagation from the cyclogenesis region toward the Tropics at upper levels. At these levels, westerly winds extend to nearly 10°S and provide a favorable background flow for such propagation. The process eventually results in the amplification of an equatorward-extending midlatitude upper trough and tropical ridge, which appears to trigger the development of the underlying monsoon trough. To test the hypothesis, the influence of high-latitude cyclogenesis on the tropical circulation is investigated with the aid of an idealized, dry, three-dimensional, baroclinic wave channel model. The initial state consists of (i) a zonally constant baroclinic region centered on 40°S, from which the high-latitude cyclogenesis develops, (ii) a weak monsoon trough at 15°S, and (iii) a subtropical jet at 25°S. The major findings from the simulations are as follows: 1) There is evidence of northeastward Rossby wave propagation from the cyclogenesis region toward low latitudes. 2) Consistent with theoretical studies, the subtropical jet plays a key role by providing a favorable westerly background flow for group propagation into the Tropics. 3) High-latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity, and divergence in the monsoon trough. 4) Vorticity and divergence changes are consistent with enhancement of the monsoon trough (increases in low-level cyclonic vorticity) and the potential for triggering a large-scale convective outbreak (changes in upper-level divergence).

Rossby Wave Breaking and Transport between the Tropics and Extratropics above the Subtropical Jet

2013 ◽  
Vol 70 (2) ◽  
pp. 607-626 ◽  
Author(s):  
Cameron R. Homeyer ◽  
Kenneth P. Bowman
Keyword(s):  
Rossby Wave ◽  
Wave Breaking ◽  
Lower Stratosphere ◽  
Potential Temperature ◽  
Breaking Waves ◽  
Breaking Wave ◽  
Mean Flow ◽  
Rossby Wave Breaking ◽  
Subtropical Jet ◽  
The Tropics

Abstract Rossby wave breaking is an important mechanism for the two-way exchange of air between the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere. The authors present a 30-yr climatology (1981–2010) of anticyclonically and cyclonically sheared wave-breaking events along the boundary of the tropics in the 350–500-K potential temperature range from ECMWF Interim Re-Analysis (ERA-Interim). Lagrangian transport analyses show net equatorward transport from wave breaking near 380 K and poleward transport at altitudes below and above the 370–390-K layer. The finding of poleward transport at lower levels is in disagreement with previous studies and is shown to largely depend on the choice of tropical boundary. In addition, three distinct modes of transport for anticyclonic wave-breaking events are found near the tropical tropopause (380 K): poleward, equatorward, and symmetric. Transport associated with cyclonic wave-breaking events, however, is predominantly poleward. The three transport modes for anticyclonic wave breaking are associated with specific characteristics of the geometry of the mean flow. In particular, composite averages show that poleward transport is associated with a “split” subtropical jet where the jet on the upstream side of the breaking wave extends eastward and lies poleward and at lower altitudes of the subtropical jet on the downstream side, producing a substantial longitudinal overlap between the two jets. Equatorward transport is not associated with a split subtropical jet and is found immediately downstream of stationary anticyclones in the tropics, often associated with monsoon circulations. It is further shown that, in general, the transport direction of breaking waves is determined primarily by the relative positions of the jets.

scholarly journals How can we understand the global distribution of the solar cycle signal on the Earth's surface?

2016 ◽  
Vol 16 (20) ◽  
pp. 12925-12944 ◽  
Author(s):  
Kunihiko Kodera ◽  
Rémi Thiéblemont ◽  
Seiji Yukimoto ◽  
Katja Matthes
Keyword(s):  
Solar Cycle ◽  
Surface Temperature ◽  
Zonal Wind ◽  
High Solar Activity ◽  
Mean Flow ◽  
Central Pacific ◽  
Solar Signal ◽  
The Tropics ◽  
Solar Influence ◽  
The Impact

Abstract. To understand solar cycle signals on the Earth's surface and identify the physical mechanisms responsible, surface temperature variations from observations as well as climate model data are analysed to characterize their spatial structure. The solar signal in the annual mean surface temperature is characterized by (i) mid-latitude warming and (ii) no overall tropical warming. The mid-latitude warming during solar maxima in both hemispheres is associated with a downward penetration of zonal mean zonal wind anomalies from the upper stratosphere during late winter. During the Northern Hemisphere winter this is manifested by a modulation of the polar-night jet, whereas in the Southern Hemisphere, the upper stratospheric subtropical jet plays the major role. Warming signals are particularly apparent over the Eurasian continent and ocean frontal zones, including a previously reported lagged response over the North Atlantic. In the tropics, local warming occurs over the Indian and central Pacific oceans during high solar activity. However, this warming is counterbalanced by cooling over the cold tongue sectors in the southeastern Pacific and the South Atlantic, and results in a very weak zonally averaged tropical mean signal. The cooling in the ocean basins is associated with stronger cross-equatorial winds resulting from a northward shift of the ascending branch of the Hadley circulation during solar maxima. To understand the complex processes involved in the solar signal transfer, results of an idealized middle atmosphere–ocean coupled model experiment on the impact of stratospheric zonal wind changes are compared with solar signals in observations. Model integration of 100 years of strong or weak stratospheric westerly jet condition in winter may exaggerate long-term ocean feedback. However, the role of ocean in the solar influence on the Earth's surface can be better seen. Although the momentum forcing differs from that of solar radiative forcing, the model results suggest that stratospheric changes can influence the troposphere, not only in the extratropics but also in the tropics through (i) a downward migration of wave–zonal mean flow interactions and (ii) changes in the stratospheric mean meridional circulation. These experiments support earlier evidence of an indirect solar influence from the stratosphere.

scholarly journals Planetary Wave Breaking and Nonlinear Reflection: Seasonal Cycle and Interannual Variability

2006 ◽  
Vol 19 (23) ◽  
pp. 6139-6152 ◽  
Author(s):  
John T. Abatzoglou ◽  
Gudrun Magnusdottir
Keyword(s):  
Interannual Variability ◽  
Wave Breaking ◽  
Large Scale ◽  
Planetary Wave ◽  
Wave Train ◽  
Zonal Flow ◽  
Reanalysis Data ◽  
Nonlinear Reflection ◽  
Meridional Overturning ◽  
Pv Gradient

Abstract Forty-six years of daily averaged NCEP–NCAR reanalysis data are used to identify the occurrence of planetary wave breaking (PWB) in the subtropical upper troposphere. As large-amplitude waves propagate into the subtropics where the zonal flow is weak, they may break. PWB is diagnosed by observing the large-scale meridional overturning of potential vorticity (PV) contours on isentropic surfaces near the subtropical tropopause. PWB occurs most often during summer, and almost exclusively over the subtropical ocean basins in the Northern Hemisphere. The seasonal evolution of the zonal flow (and the associated latitudinal PV gradient) regulates the location and frequency of PWB. Significant interannual variability in PWB is associated with well-known modes of climate variability. One of the most interesting dynamical consequences of PWB is the possibility of nonlinear reflection poleward out of the wave-breaking region. Modeling studies have found nonlinear reflection following PWB. Observations show that about 36% of all PWB events are followed by nonlinear reflection back into midlatitudes. In these cases, a poleward-arching wave train can be seen propagating away from the wave-breaking region following breaking. It is suggested that a sufficiently strong latitudinal PV gradient must be present downstream of the wave-breaking region for reflection to take place. The proportion of PWB events that is reflective stays rather constant through the year, with slightly higher numbers in spring and fall compared to those in winter and summer.

Does Topographic Form Stress Impede Prograde Ocean Currents?

2021 ◽  
Author(s):  
YUE BAI ◽  
YAN WANG ◽  
ANDREW L. STEWART
Keyword(s):  
Wave Propagation ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Bottom Friction ◽  
Stress Generation ◽  
Momentum Balance ◽  
Mean Flow ◽  
Rossby Wave Propagation ◽  
Circumpolar Current ◽  
Current Systems

AbstractTopographic form stress (TFS) plays a central role in constraining the transport of the Antarctic Circumpolar Current (ACC), and thus the rate of exchange between the major ocean basins. Topographic form stress generation in the ACC has been linked to the formation of standing Rossby waves, which occur because the current is retrograde (opposing the direction of Rossby wave propagation). However, it is unclear whether TFS similarly retards current systems that are prograde (in the direction of Rossby wave propagation), which cannot arrest Rossby waves. An isopycnal model is used to investigate the momentum balance of wind-driven prograde and retrograde flows in a zonal channel, with bathymetry consisting of either a single ridge or a continental shelf and slope with a meridional excursion. Consistent with previous studies, retrograde flows are almost entirely impeded by TFS, except in the limit of flat bathymetry, whereas prograde flows are typically impeded by a combination of TFS and bottom friction. A barotropic theory for standing waves shows that bottom friction serves to shift the phase of the standing wave’s pressure field from that of the bathymetry, which is necessary to produce TFS. The mechanism is the same in prograde and retrograde flows, but is most efficient when the mean flow arrests a Rossby wave with a wavelength comparable to that of the bathymetry. The asymmetry between prograde and retrograde momentum balances implies that prograde current systems may be more sensitive to changes in wind forcing, for example associated with climate shifts.

scholarly journals How can we understand the solar cycle signal on the Earth's surface?

2016 ◽  
Author(s):  
Kunihiko Kodera ◽  
Rémi Thiéblemont ◽  
Seiji Yukimoto ◽  
Katja Matthes
Keyword(s):  
Solar Cycle ◽  
Surface Temperature ◽  
Zonal Wind ◽  
High Solar Activity ◽  
Late Winter ◽  
Mean Flow ◽  
Central Pacific ◽  
Solar Signal ◽  
The Tropics ◽  
The Impact

Abstract. To understand solar cycle signals on the Earth's surface and identify the physical mechanisms responsible, surface temperature variations from observations as well as climate model data are analyzed to characterize their spatial structure. The solar signal in the annual mean surface temperature is characterized by (i) mid-latitude warming and (ii) no warming in the tropics. The mid-latitude warming during solar maxima in both hemispheres is associated with a downward penetration of zonal mean zonal wind anomalies from the upper stratosphere during late winter. During Northern Hemisphere winter this is manifested in a modulation of the polar-night jet whereas in the Southern Hemisphere the subtropical jet plays the major role. Warming signals are particularly apparent over the Eurasian continent and ocean frontal zones, including a previously reported lagged response over the North Atlantic. In the tropics, local warming occurs over the Indian and central Pacific oceans during high solar activity. However, this warming is counter balanced by cooling over the cold tongue sectors in the southeastern Pacific and the South Atlantic, and results in a very weak zonally averaged tropical mean signal. The cooling in the ocean basins is associated with stronger cross-equatorial winds resulting from a northward shift of the ascending branch of the Hadley circulation during solar maxima. To understand the complex processes involved in the solar signal transfer, results of an idealized middle atmosphere–ocean coupled model experiment on the impact of stratospheric zonal wind changes are compared with solar signals in observations. The model results suggest that both tropical and extra-tropical solar surface signals can result from circulation changes in the upper stratosphere through (i) a downward migration of wave–zonal mean flow interactions and (ii) changes in the stratospheric mean meridional circulation. These experiments support earlier evidence of an indirect solar influence from the stratosphere.

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