Abstract

The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP–NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Niño–Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden–Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and hence explain some of the variance of the next month's 300-hPa streamfunction by solving the inverse problem.

These results are further evidence that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of circulation anomalies in higher latitudes.

Abstract

Previous observational work has demonstrated that the phase speed of oceanic equatorial Kelvin waves forced by the Madden–Julian oscillation (MJO) appears to vary substantially. Processes that are responsible for systematic changes in the phase speed of these waves are examined using an ocean general circulation model. The model was integrated for 26 yr with daily wind stress derived from the NCEP–NCAR reanalysis. The model is able to reproduce observed systematic changes of Kelvin wave phase speed reasonably well, providing a tool for the analysis of their dynamics.

The relative importance of the upper ocean background state and atmospheric forcing for phase speed changes is determined based on a series of model experiments with various surface forcings. Systematic changes in phase speed are evident in all model experiments that have different slowly varying basic states, showing that variations of the upper ocean background state are not the primary cause of the changes. The model experiments that include and exclude intraseasonal components of wind stress in the eastern Pacific demonstrate that wind stress changes to the east of the date line can significantly alter the speed of Kelvin waves initially generated over the western Pacific, which often results in a phase propagation faster than the free wave speed. These faster waves contribute to the systematic changes of phase speed evident in observations. Similar results are also obtained using a linear stratified model, eliminating nonlinearity as a possible cause of the phase speed changes.

Abstract

This paper promotes the view that African easterly waves (AEWs) are triggered by localized forcing, most likely associated with latent heating upstream of the region of observed AEW growth. A primitive equation model is used to show that AEWs can be triggered by finite-amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable. Heating close to the entrance region of the African easterly jet (AEJ) is shown to initiate AEWs downstream. The heating leads to an initial trough that reaches the West African coast about 5–7 days later, depending on the nature of the heating profile. After this, a structure that projects strongly onto the leading linear normal mode of the basic state becomes established, characterized by a number of westward-propagating disturbances that strongly resemble AEWs. The sensitivity of the forced AEWs to the nature and location of the heating profile is examined. AEWs are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the AEJ. In the present study, this was best achieved by lower tropospheric heating from shallow convection or upper-level heating and lower-level cooling from a stratiform precipitation profile. Both profiles have significant heating gradients in the vertical in the mid-to-lower troposphere. This triggering paradigm for the genesis of AEWs has consequences for the variability and predictability of AEWs at weather and climate time scales. In addition to the nature of the AEJ, often emphasized, it is crucial to consider the nature and variability of upstream heating triggers.

Abstract

The dynamics of convectively coupled equatorial waves (CCEWs) is analyzed in an idealized model of the large-scale atmospheric circulation. The model is composed of a linear rotating shallow-water system with a variable equivalent height, or equivalent gravity wave speed, which varies in space. This model is based on the hypothesis that moist convection acts to remove convective instability, therefore modulating the equivalent height of a shallow-water system. Asymptotic solutions are derived in the case of a small perturbation around a constant coefficient, which is assumed to be a mean moist equivalent height derived from satellite observations. The first-order solutions correspond to the free normal modes of the linear shallow-water system and the second-order flow is derived solving a perturbation eigenvalue problem. The asymptotic solutions are documented in the case of a zonally varying equivalent height and for wavenumbers and frequencies that are consistent with observations of CCEWs. This analysis shows that the dynamics of the secondary divergence and its impact on the full divergence varies mode by mode. For instance, for a negative equivalent height anomaly, which is interpreted as a moister background, the secondary divergence is nearly in phase with the primary divergence in the case of Kelvin waves—in contrast to mixed Rossby–gravity waves where the secondary divergence acts to attenuate the primary divergence. While highly idealized, the modeled waves share some features with observations, providing a mechanism for the relationship between CCEWs phase speed, amplitude, and horizontal structure.

Abstract

Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature, and humidity fields from open-ocean buoys, radiosondes, and gridded reanalysis products. The analysis shows that while some Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 4200–5900 km, westward phase speeds of 11.3–13.6 m s⁻¹, and maximum meridional wind anomalies at about 600 hPa. A warm, moist boundary layer is observed ahead of the waves, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux fluctuations in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally oriented inverted troughs, while in the east Pacific the waves are oriented southwest–northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that Pacific easterly waves are driven primarily by convective heating. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.

Abstract

The mean structure of African easterly waves (AEWs) over West Africa and the adjacent Atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space–time-filtered satellite-derived outgoing longwave radiation. These results are compared with previous studies and an idealized modeling study in a companion paper, which provides evidence that the waves bear a close structural resemblance to the fastest-growing linear normal mode of the summertime basic-state flow over Africa. There is a significant evolution in the three-dimensional structure of AEWs as they propagate along 10°N across West Africa. At this latitude, convection occurs in northerly flow to the east of the Greenwich meridian, then shifts into the wave trough, and finally into southerly flow as the waves propagate offshore into the Atlantic ITCZ. In contrast, to the north of the African easterly jet along 15°N convection remains in southerly flow throughout the waves’ trajectory. Along 10°N over West Africa, the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction, as in the companion paper. Offshore, and along 15°N, the relationship between the convection and dynamics is more complex, and not as easily explained in terms of dynamical forcing alone.

Abstract

A primitive equation model is used to study the linear normal modes of the African easterly jet (AEJ). Reanalysis data from the summertime mean (June–September; JJAS) flow is used to provide zonally uniform and wavy basic states. The structure and growth rates of modes that grow over West Africa on these basic states are analyzed. For zonally uniform basic states, the modes resemble African easterly waves (AEWs) as in many previous studies, but they are quite baroclinic and surface intensified.

For wavy basic states the modes have a longitudinal structure determined by the AEJ. They have a surface-intensified baroclinic structure upstream and a deep barotropic structure downstream, as confirmed by energy conversion diagnostics. These modes look remarkably similar to the composite easterly wave structures found by the authors in a companion paper. The similarity extends to the phase relationship of vertical velocity with streamfunction, which resembles OLR composites, suggesting a dynamical influence on convection.

Without damping, the mode for the wavy basic state has a growth rate of 0.253 day⁻¹. With a reasonable amount of low-level damping this mode is neutralized. It has a period of 5.5 days and a wavelength of about 3500 km. Further results with monthly mean basic states show slight variations, as the wave packet essentially follows displacements of the jet core. Experiments focused on specific active and passive years for easterly waves (1988 and 1990) do not yield significantly different results for the modes. These results, and in particular, the stability of the system, lead to the conclusion that barotropic–baroclinic instability alone cannot explain the initiation and intermittence of AEWs, and a finite-amplitude initial perturbation is required.

Abstract

The intrusion of lower-stratospheric extratropical potential vorticity into the tropical upper troposphere in the weeks surrounding the occurrence of sudden stratospheric warmings (SSWs) is examined. The analysis reveals that SSW-related PV intrusions are significantly stronger, penetrate more deeply into the tropics, and exhibit distinct geographic distributions compared to their climatological counterparts. While climatological upper-tropospheric and lower-stratospheric (UTLS) PV intrusions are generally attributed to synoptic-scale Rossby wave breaking, it is found that SSW-related PV intrusions are governed by planetary-scale wave disturbances that deform the extratropical meridional PV gradient maximum equatorward. As these deformations unfold, planetary-scale wave breaking along the edge of the polar vortex extends deeply into the subtropical and tropical UTLS. In addition, the material PV deformations also reorganize the geographic structure of the UTLS waveguide, which alters where synoptic-scale waves break. In combination, these two intrusion mechanisms provide a robust explanation describing why displacement and split SSWs—or, more generally, anomalous stratospheric planetary wave events—produce intrusions with unique geographic distributions: displacement SSWs have a single PV intrusion maximum over the Pacific Ocean, while split SSWs have intrusion maxima over the Pacific and Indian Oceans. It is also shown that the two intrusion mechanisms involve distinct time scales of variability, and it is highlighted that they represent an instantaneous and direct link between the stratosphere and troposphere. This is in contrast to higher-latitude stratosphere–troposphere coupling that occurs indirectly via wave–mean flow feedbacks.

Abstract

Convectively coupled equatorial waves, as previously detected in studies of wavenumber-frequency spectra of tropical clouds, are studied in more detail. Composite dynamical structures of the waves are obtained using linear regression between selectively filtered satellite-observed outgoing longwave radiation (OLR) data, and various fields from a global reanalysis dataset. The selective filtering of the OLR was designed to isolate the convective variations contributing to spectral peaks that lie along the equatorial wave dispersion curves for equivalent depths in the range of 12–50 m. The waves studied are the Kelvin, n = 1 equatorial Rossby (ER), mixed Rossby–gravity, n = 0 eastward inertio–gravity, n = 1 westward inertio–gravity (WIG), and n = 2 WIG waves.

The horizontal structures of the dynamical fields associated with the waves are all generally consistent with those calculated from inviscid equatorial β-plane shallow water theory. In the vertical, there are statistically significant structures spanning the depth of the troposphere, and for all but the ER wave there are associated vertically propagating signals extending into the equatorial stratosphere as well. In zonal cross sections, the vertical structure of the temperature anomaly field appears, for all but the ER wave, as a “boomerang”-like shape, with the “elbow” of the boomerang occurring consistently at the 250-hPa level. The tilts of the boomerang imply upward phase propagation throughout most of the troposphere, and downward phase propagation above. The deep convection of the waves occurs in regions of anomalously cold temperatures in the lower troposphere, warm temperatures in the upper troposphere, and cold temperatures at the level of the tropopause. Such a vertical structure appears to indicate that waves of relatively short vertical wavelengths (L _z ∼ 10 km) are indeed important for the coupling of large-scale dynamics and convection. The deeper structure of the convectively coupled ER wave, on the other hand, is thought to be an indication of the effects of basic-state vertical shear. Finally, the scales of the waves in the equatorial lower stratosphere that are forced by the convectively coupled equatorial waves are quite consistent with those found in many previous studies.

Abstract

A statistical study of the three-dimensional structure of the Madden–Julian oscillation (MJO) is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space–time filtered outgoing longwave radiation (OLR) data. MJO convection is generally preceded by low-level convergence and upward motion in the lower troposphere, while subsidence, cooling, and drying prevail aloft. This leads to moistening of the boundary layer and the development of shallow convection, followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection. After the passage of the heaviest rainfall, a westerly wind burst region is accompanied by stratiform precipitation, where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion. The evolution of the heating field leads to a temperature structure that favors the growth of the MJO. The analysis also reveals distinct differences in the vertical structure of the MJO as it evolves, presumably reflecting changes in its vertical heating profile, phase speed, or the basic-state circulation that the MJO propagates through.

The dynamical structure and the evolution of cloud morphology within the MJO compares favorably in many respects with other propagating convectively coupled equatorial waves. One implication is that the larger convective envelopes within the Tropics tend to be composed of more shallow convection along their leading edges, a combination of deep convection and stratiform rainfall in their centers, and then a preponderance of stratiform rainfall along their trailing edges, regardless of scale or propagation direction. While this may ultimately be the factor that governs the dynamical similarities across the various wave types, it raises questions about how the smaller-scale, higher-frequency disturbances making up the MJO conspire to produce its heating and dynamical structures. This suggests that the observed cloud morphology is dictated by fundamental interactions with the large-scale circulation.