scholarly journals Convectively Coupled Equatorial Waves. Part III: Synthesis Structures and Their Forcing and Evolution

2007 ◽  
Vol 64 (10) ◽  
pp. 3438-3451 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Lower Troposphere ◽  
Wave Activity ◽  
Equatorial Region ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Upper Troposphere ◽  
Convectively Coupled Equatorial Waves ◽  
Eastern Hemisphere ◽  
Equatorial Wave

Abstract Building on Parts I and II of this study, the structures of eastward- and westward-moving convectively coupled equatorial waves are examined through synthesis of projections onto standard equatorial wave horizontal structures. The interaction between these equatorial wave components and their evolution are investigated. It is shown that the total eastward-moving fields and their coupling with equatorial convection closely resemble the standard Kelvin wave in the lower troposphere, with intensified convection in phase with anomalous westerlies in the Eastern Hemisphere (EH) and with anomalous convergence in the Western Hemisphere (WH). However, in the upper troposphere, the total fields show a mixture of the Kelvin wave and higher (n = 0 and 1) wave structures, with strong meridional wind and its divergence. The equatorial total fields show what may be described as a modified first internal Kelvin wave vertical structure in the EH, with a tilt in the vertical and a third peak in the midtroposphere. There is evidence that the EH midtropospheric Kelvin wave is closely associated with SH extratropical eastward-moving wave activity, the vertical velocity associated with the wave activity stretching into the equatorial region in the mid–upper troposphere. The midtropospheric zonal wind and geopotential height show a pattern that may be associated with a forced wave. The westward-moving fields associated with off-equatorial convection show very different behaviors between the EH midsummer and the WH transition seasons. In the EH midsummer, the total fields have a baroclinic structure, with the off-equatorial convection in phase with relatively warm air, suggesting convective forcing of the dynamical fields. The total structures exhibit a mixture of the n = 0, 1 components, with the former dominating to the east of convection and the latter to the west of convection. The n = 0 component is found to be closely connected to the lower-level n = 1 Rossby (R1) wave that appears earlier and seems to provide organization for the convection, which in turn forces the n = 0 wave. In the WH transition season the total fields have a barotropic structure and are dominated by the R1 wave. There is evidence that this barotropic R1 wave, as well as the associated tropical convection, is forced by the NH upper-tropospheric extratropical Rossby wave activity. In the EH, westward-moving lower-level wind structures associated with equatorial convection resemble the R1 wave, with equatorial westerlies in phase with the intensified convection. However, westward-moving n = −1 and n = 0 structures are also involved.

scholarly journals Convectively Coupled Equatorial Waves. Part I: Horizontal and Vertical Structures

2007 ◽  
Vol 64 (10) ◽  
pp. 3406-3423 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Composite Structures ◽  
Wave Structure ◽  
Wave Theory ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Ambient Flow ◽  
Convectively Coupled Equatorial Waves ◽  
Eastern Hemisphere ◽  
Equatorial Wave

Abstract Multilevel 15-yr ECMWF Re-Analysis (ERA-15) and satellite-observed brightness temperature (Tb) data for the period May–October 1992 are used to examine the horizontal and vertical structures of convectively coupled equatorial waves. Dynamical waves are isolated using a methodology developed previously. Composite structures of convectively coupled equatorial waves are obtained using linear regression/correlation between convection (Tb) and dynamical structures. It is found that the relationship depends on the ambient flow and the nature of the convective coupling, and varies between off-equatorial- and equatorial-centered convection, different hemispheres, and seasons. The Kelvin wave structure in the Western Hemisphere is generally consistent with classic equatorial wave theory and has its convection located in the region of low-level convergence. In the Eastern Hemisphere the Kelvin wave tends to have convection in the region of enhanced lower-tropospheric westerlies and a tilted vertical structure. The Kelvin wave also tends to have a third peak in zonal wind amplitude at 500 hPa and exhibits upward propagation into the lower stratosphere. Lower-tropospheric westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave structures and their relationship with convection are consistent with classic equatorial wave theory and the implied lower-tropospheric convergences. In the Eastern Hemisphere the WMRG and R1 waves have first baroclinic mode structures in the vertical. However, in the Western Hemisphere, the R1 wave has a barotropic structure. In the Eastern Hemisphere the R1 wave, like the Kelvin wave, tends to have equatorial convection in the region of enhanced lower-level westerlies, suggesting that enhanced surface energy fluxes associated with these waves may play an important organizing role for equatorial convection in this warm-water hemisphere. In the upper troposphere, eastward-moving Rossby–gravity (EMRG) and n = 1 gravity waves are found in the Eastern Hemisphere, and eastward-moving WMRG and R1 waves are found in the Western Hemisphere, suggestive of Doppler shifting of waves by the ambient flow.

scholarly journals The Influence of the QBO on the Propagation of Equatorial Waves into the Stratosphere

2012 ◽  
Vol 69 (10) ◽  
pp. 2959-2982 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Lesley Gray
Keyword(s):  
Wave Amplitude ◽  
Lower Stratosphere ◽  
Longwave Radiation ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Upper Troposphere ◽  
Zonal Winds ◽  
Equatorial Wave ◽  
The Impact

Abstract The variation of stratospheric equatorial wave characteristics with the phase of the quasi-biennial oscillation (QBO) is investigated using ECMWF Re-Analysis and NOAA outgoing longwave radiation (OLR) data. The impact of the QBO phases on the upward propagation of equatorial waves is found to be consistent and significant. In the easterly phase, there is larger Kelvin wave amplitude but smaller westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave amplitude due to reduced propagation from the upper troposphere into the lower stratosphere, compared with the westerly phase. Differences in the wave amplitude exist in a deeper layer in summer than in winter, consistent with the seasonality of ambient zonal winds. There is a strong evidence of Kelvin wave amplitude peaking just below the descending westerly phase, suggesting that Kelvin waves act to bring the westerly phase downward. However, the corresponding evidence for WMRG and R1 waves is less clear. In the lower stratosphere there is zonal variation in equatorial waves. This reflects the zonal asymmetry of wave amplitudes in the upper troposphere, the source for the lower-stratospheric waves. In easterly winters the upper-tropospheric WMRG and R1 waves over the eastern Pacific region appear to be somewhat stronger compared to climatology, perhaps because of the accumulation of waves that are unable to propagate upward into the lower stratosphere. Vertical propagation features of these waves are generally consistent with theory and suggest a mixture of Doppler shifting by ambient flows and filtering. Some lower-stratosphere equatorial waves have a connection with preceding tropical convection, especially for Kelvin and R1 waves in winter.

scholarly journals Linking African Easterly Wave Activity with Equatorial Waves and the Influence of Rossby Waves from the Southern Hemisphere

2018 ◽  
Vol 75 (6) ◽  
pp. 1783-1809 ◽  
Author(s):  
Gui-Ying Yang ◽  
John Methven ◽  
Steve Woolnough ◽  
Kevin Hodges ◽  
Brian Hoskins
Keyword(s):  
Southern Hemisphere ◽  
Rossby Waves ◽  
Lower Troposphere ◽  
Strong Relationship ◽  
Wave Theory ◽  
Wave Activity ◽  
Equatorial Waves ◽  
Horizontal Structure ◽  
Positive Vorticity ◽  
Equatorial Wave

Abstract A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby–gravity (WMRG) waves, and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broadband-filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave that tilts eastward below the African easterly jet and westward above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centers within AEWs and equatorial WMRG waves. Although the waves do not phase lock, positive vorticity centers amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southward flow) and east of this in the lower troposphere (northward flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northward flow. In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: (i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3–4 days) and (ii) they enable westward-moving RWs to propagate into the tropical waveguide from the SH. The RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastward group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism.

scholarly journals A simple nonlinear model of low frequency (interseasonal) oscillations in the tropical atmosphere

1996 ◽  
Vol 3 (1) ◽  
pp. 29-40 ◽  
Author(s):  
D. Luo
Keyword(s):  
Nonlinear Interaction ◽  
Wave Equations ◽  
Narrow Range ◽  
Lower Troposphere ◽  
Equatorial Region ◽  
Wave Number ◽  
Convective Heating ◽  
Kelvin Wave ◽  
Intensity Parameter ◽  
Zonal Wavenumber

Abstract. In this paper, for a prescribed normalized vertical convective heating profile, nonlinear Kelvin wave equations with wave-CISK heating over equatorial region is reduced to a sixth-order nonlinear ordinary differential equation by using the Galerkin spectral method in the case of considering nonlinear interaction between first and second baroclinic modes. Some numerical calculations are made with the fourth- order Rung-Kutta scheme. It is found that in a narrow range of the heating intensity parameter b, 30-60-day oscillation can occur through linear coupling between first and second baroclinic Kelvin wave-CISK modes for zonal wave-number one when the convective heating is confined to the lower and middle tropospheres. While for zonal wavenumber two, 30-60-day oscillation can be observed in a narrow range of b only when the convective heating is confined to the lower troposphere. However, in a wider range of this heating intensity parameter, 30-60-day oscillation can occur through nonlinear interaction between the first and second baroclinic Kelvin wave-CISK modes with zonal wavenumber one for three vertical convective heating profiles having a maximum in the upper, middle and lower tropospheres, and the total streamfield of the nonlinear first and second baroclinic Kelvin wave-CISK modes possesses a phase reversal between the upper- and lower-tropospheric wind fields. While for zonal wavenumber two, no 30-60-day oscillations can be found. Therefore, it appears that nonlinear interaction between vertical Kelvin wave-CISK modes favours the occurrence of 30-60-day oscillations, particularly, the importance of the vertical distribution of convective heating is re-emphasised.

Uncertainties of Estimates of Inertia–Gravity Energy in the Atmosphere. Part II: Large-Scale Equatorial Waves

2009 ◽  
Vol 137 (11) ◽  
pp. 3858-3873 ◽  
Author(s):  
N. Žagar ◽  
J. Tribbia ◽  
J. L. Anderson ◽  
K. Raeder
Keyword(s):  
Large Scale ◽  
Normal Modes ◽  
Lower Troposphere ◽  
Physical Space ◽  
Significant Degree ◽  
Hadley Cell ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Energy Propagation ◽  
Zonal Wavenumber

Abstract This paper analyzes the spectra and spatiotemporal features of the large-scale inertia-gravity (IG) circulations in four analysis systems in the tropics. Of special interest is the Kelvin wave (KW), which represents between 7% and 25% of the total IG wave (zonal wavenumber k ≠ 0) energy. The mixed Rossby–gravity (MRG) mode comprises between 4% and 15% of the IG wave energy. At the longest scales, the KW spectra are fitted by a law while the MRG energy spectrum appears flat. At shorter scales both modes follow a −3 law. Energy spectra of the total IG wave motion at long zonal scales (zonal wavenumber smaller than 7) have slopes close to −1. The average circulation associated with KW is characterized by reverse flows in the upper and lower troposphere consistent with the ideas behind simple tropical models. The inverse projection is used to quantify the role of Kelvin and MRG waves in current analysis systems in the upper troposphere over the Indian Ocean. At these levels, easterlies between 10°S and 30°N are represented by the KW to a significant degree while the cross-equatorial flow toward the descending branch of the Hadley cell at 10°S is associated with the MRG waves. The transient structure of equatorial waves is presented in the space of normal modes defined by the zonal wavenumbers, meridional Hough functions, and the vertical eigenfunctions. The difference in the depth of the model domain in DART–CAM and NCEP–NCAR on one hand and ECMWF and NCEP on the other appears to be one reason for different wave propagation properties. In the latter case the vertical energy propagation is diagnosed by filtering the propagating KW modes back to physical space. The results agree with the linear theory of vertically propagating equatorial waves.

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).

Tropical Precipitation Variability and Convectively Coupled Equatorial Waves on Submonthly Time Scales in Reanalyses and TRMM

2013 ◽  
Vol 26 (10) ◽  
pp. 3013-3030 ◽  
Author(s):  
Ji-Eun Kim ◽  
M. Joan Alexander
Keyword(s):  
Diurnal Cycle ◽  
Low Frequency ◽  
Tropical Rainfall Measuring Mission ◽  
Precipitation Variability ◽  
Department Of Energy ◽  
Equatorial Waves ◽  
Tropical Precipitation ◽  
Convectively Coupled Equatorial Waves ◽  
Precipitation Characteristics ◽  
Equatorial Wave

Abstract Tropical precipitation characteristics are investigated using the Tropical Rainfall Measuring Mission (TRMM) 3-hourly estimates, and the result is compared with five reanalyses including the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim), Modern Era Retrospective Analysis for Research and Applications (MERRA), National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis (NCEP1), NCEP–U.S. Department of Energy (DOE) reanalysis (NCEP2), and NCEP–Climate Forecast System Reanalysis (CFSR). Precipitation characteristics are evaluated in terms of the mean, convectively coupled equatorial wave activity, frequency characteristics, diurnal cycle, and seasonality of regional precipitation variability associated with submonthly scale waves. Generally the latest reanalyses such as ERA-Interim, MERRA, and CFSR show better performances than NCEP1 and NCEP2. However, all the reanalyses are still different from observations. Besides the positive mean bias in the reanalyses, a spectral analysis revealed that the reanalyses have overreddened spectra with persistent rainfall. MERRA has the most persistent rainfall, and CFSR appears to have the most realistic variability. The diurnal cycle in NCEP1 is extremely exaggerated relative to TRMM. The low-frequency waves with the period longer than 3 days are relatively well represented in ERA-Interim, MERRA, and CFSR, but all the reanalyses have significant deficiencies in representing convectively coupled equatorial waves and variability in the high-frequency range.

scholarly journals Intraseasonal Variability in a Dry Atmospheric Model

2007 ◽  
Vol 64 (7) ◽  
pp. 2422-2441 ◽  
Author(s):  
Hai Lin ◽  
Gilbert Brunet ◽  
Jacques Derome
Keyword(s):  
North Atlantic ◽  
Intraseasonal Variability ◽  
Atmospheric Model ◽  
Wave Activity ◽  
Western Hemisphere ◽  
Mean Flow ◽  
The North ◽  
Eastern Hemisphere ◽  
The North Atlantic ◽  
Wave Activity Flux

Abstract A long integration of a primitive equation dry atmospheric model with time-independent forcing under boreal winter conditions is analyzed. A variety of techniques such as time filtering, space–time spectral analysis, and lag regressions are used to identify tropical waves. It is evident that oscillations with intraseasonal time scales and a Kelvin wave structure exist in the model tropical atmosphere. Coherent eastward propagations in the 250-hPa velocity potential and zonal wind are found, with a speed of about 15 m s−1. The oscillation is stronger in the Eastern Hemisphere than in the Western Hemisphere. Interactions between the tropical and extratropical flows are found to be responsible for the simulated intraseasonal variability. Wave activity flux analysis reveals that a tropical influence occurs in the North Pacific region where a northeastward wave activity flux is found associated with the tropical divergent flow in the western and central Pacific. In the North Atlantic sector, on the other hand, a strong extratropical influence is observed with a southward wave activity flux into the Tropics. The extratropical low-frequency variability develops by extracting kinetic energy from the basic mean flow and through interactions with synoptic-scale transient eddies. Linear experiments show that the tropical atmospheric response to the extratropical forcing in the North Atlantic leads to an eastward-propagating wave in the tropical easterly mean flow of the Eastern Hemisphere.

scholarly journals Convectively Coupled Equatorial Waves in High-Resolution Hadley Centre Climate Models

2009 ◽  
Vol 22 (8) ◽  
pp. 1897-1919 ◽  
Author(s):  
Gui-Ying Yang ◽  
Julia Slingo ◽  
Brian Hoskins
Keyword(s):  
High Resolution ◽  
Energy Conversion ◽  
Phase Speed ◽  
Atmospheric Model ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Near Surface ◽  
Model Version ◽  
Convectively Coupled Equatorial Waves ◽  
Hadley Centre

Abstract A methodology for diagnosing convectively coupled equatorial waves is applied to output from two high-resolution versions of atmospheric models, the Hadley Centre Atmospheric Model, version 3 (HadAM3), and the new Hadley Centre Global Atmospheric Model, version 1 (HadGAM1), which have fundamental differences in dynamical formulation. Variability, horizontal and vertical structures, and propagation characteristics of tropical convection and equatorial waves, along with their coupled behavior in the models, are examined and evaluated against a previous comprehensive study of observed convectively coupled equatorial waves using the 15-yr ECMWF Re-Analysis (ERA-15) and satellite observed data. The extent to which the models are able to represent the coupled waves found in real atmospheric observations is investigated. It is shown that, in general, the models perform well for equatorial waves coupled with off-equatorial convection. However, they perform poorly for waves coupled with equatorial convection. Convection in both models contains much-reduced variance in equatorial regions, but reasonable off-equatorial variance. The models fail to simulate coupling of the waves with equatorial convection and the tendency for equatorial convection to appear in the region of wave-enhanced near-surface westerlies. In addition, the simulated Kelvin wave and its associated convection generally tend to have lower frequency and slower phase speed than that observed. The models are also not able to capture the observed vertical tilt structure and signatures of energy conversion in the Kelvin wave, particularly in HadAM3. On the other hand, models perform better in simulating westward-moving waves coupled with off-equatorial convection, in terms of horizontal and vertical structures, zonal propagation, and energy conversion signals. In most cases both models fail to simulate well a key picture emerging from the observations, that some wave modes in the lower troposphere can act as a forcing agent for equatorial convection, and that the upper-tropospheric waves generally appear to be forced by the convection both on and off the equator.

scholarly journals Effect of Overturning Circulation on Long Equatorial Waves: A Low-Frequency Cutoff

2018 ◽  
Vol 75 (5) ◽  
pp. 1721-1739 ◽  
Author(s):  
Amanda Back ◽  
Joseph A. Biello
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
Meridional Circulation ◽  
Low Frequency ◽  
Hadley Cell ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Background Flow ◽  
Overturning Circulation ◽  
Equatorial Wave

Zonally long tropical waves in the presence of a large-scale meridional and vertical overturning circulation are studied in an idealized model based on the intraseasonal multiscale moist dynamics (IMMD) theory. The model consists of a system of shallow-water equations describing barotropic and first baroclinic vertical modes coupled to one another by the zonally symmetric, time-independent background circulation. To isolate the effects of the meridional circulation alone, an idealized background flow is chosen to mimic the meridional and vertical components of the flow of the Hadley cell; the background flow meridionally converges and rises at the equator. The resulting linear eigenvalue problem is a generalization of the long-wave-scaled version of Matsuno’s equatorial wave problem with the addition of meridional and vertical advection. The results demonstrate that the meridional circulation couples equatorially trapped baroclinic Rossby waves to planetary, barotropic free Rossby waves. The meridional circulation also causes the Kelvin wave to develop an equatorially trapped barotropic component, imparting a westward-tilted vertical structure to the wave. The total energy of the linear system is positive definite, so all waves are shown to be neutrally stable. A critical layer exists at latitudes where the meridional background flow vanishes, resulting in a minimum frequency cutoff for physically feasible waves. Therefore, linear Matsuno waves with periods longer than the vertical transport time of the meridional circulation do not exist in the equatorial waveguide. This implies a low-frequency cutoff for long equatorial waves.

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