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Gareth J. Berry
and
Chris D. Thorncroft

Abstract

To examine the dynamical role of convection in African easterly wave (AEW) life cycles the Weather Research and Forecasting (WRF) model is used to simulate the evolution of a single AEW from September 2004. The model simulations are validated against corresponding numerical weather prediction analyses and the mean fields closely resemble composite structures from previous studies. A potential vorticity (PV) thinking approach is used to highlight the interactions between dynamics and convection.

Organized deep convection embedded within the AEW has a large contribution to the synoptic-scale mean PV and energetics of the AEW. The PV tendency is maximized in the lower troposphere, consistent with the vertical gradient in diabatic heating rates in the areas of convection. By examining terms in the Lorenz energy cycle, it is shown that diabatic heating associated with convection is as important as adiabatic energy conversion in producing eddy available potential energy of the synoptic AEW, implying that AEWs are best described as hybrid adiabatic and diabatic structures. The net effect of convection is succinctly described using a simulation whereby the parameterizations associated with convection are switched off at the midpoint of the model run. This perturbation experiment shows that, although the AEW continues to propagate westward with a similar phase speed, the net PV value continually weakens with time. This result proves that convection is vital for the maintenance of the AEW as it propagates across West Africa and suggests that without active convection the synoptic AEW cannot persist for an extended length of time.

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Victor M. Torres
and
Chris D. Thorncroft

Abstract

A review of the mean state over the tropical eastern Pacific (EPAC) and the Intra-Americas Sea (IAS) region is provided to assess the characteristics that impact the development and genesis of easterly waves (EWs). The EPAC–IAS region is characterized by complex topography, the Western Hemisphere warm pool, the ITCZ at 10°N, and predominant deep convection over the Panama Bight around 9°N, 78°W. A prominent easterly jet at 600 hPa of about 5.5 m s−1, is oriented approximately parallel to the Mexican coast. The jet is characterized by a strip of high potential vorticity (PV) on the cyclonic shear side and low PV on the anticyclonic side. This distribution of PV satisfies the necessary conditions for barotropic instability: the Charney–Stern condition, as well as the Fjørtoft condition. Together these conditions suggest the potential for barotropic growth of EWs over the EPAC region. The mean high PV region over the EPAC is created in association with two different populations of cloud/convection systems: stratiform and shallow, with the former being key for the creation of positive PV anomalies at midlevels. Evidence is also provided that suggests that the low PV region arises in association with sources of negative PV anomalies over the Sierra Madre region likely resulting from frequent dry convection. This is a key and novel result that is basic for the setting up of a negative meridional PV gradient and fundamental for the Charney–Stern condition associated with barotropic instability and growth of EWs.

Significance Statement

The tropical eastern Pacific is influenced by synoptic easterly waves that impact daily weather in the region and can trigger tropical cyclones. This research explores the nature of a midlevel jet that supports the development of easterly waves in this basin. The jet is established in association with moist convection over the ocean that leads to a midlevel potential vorticity maximum equatorward of the jet and, frequent dry convection over the Mexican Sierra Madre region that leads to a low-level potential vorticity minimum poleward of the jet. This finding highlights the need to better understand, and ultimately predict, these potential vorticity sources in order to better understand and predict the nature of the easterly wave developments in this region.

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Matthew A. Janiga
and
Chris D. Thorncroft

Abstract

The geographic and diurnal variability of moist convection over tropical Africa and the east Atlantic is examined using the Tropical Rainfall Measuring Mission (TRMM) satellite and related to the variability of the convective environment. The stratiform rain fraction is highest within oceanic and continental regions just north of the equator. Both regions have high column relative humidity (CRH). In both monsoon and semiarid continental regions, stratiform rain fractions are significantly higher on days when the CRH is high, which suggests a relationship between these quantities. Large convective systems with high echo tops dominate the rainfall over the Sahel. The importance of CAPE and shear to the development of these types of systems is suggested by the fact these systems are especially common on days when the CAPE and shear are unusually high.

Both deep convective and stratiform conditional rain rates increase with the size and echo-top height of convective systems. According to the TRMM Precipitation Radar (PR) near-surface rain rate, the highest deep convective and stratiform conditional rain rates occur off the coast of West Africa. However, comparisons between the PR near-surface rain rate and rain rates computed from Z–R relationships from the literature suggest that deep convective conditional rain rates over the Sahel are underestimated by the TRMM precipitation algorithm. Over the Sahel, small (large) convective systems produce most of the rainfall in the afternoon (early morning). This is associated with enhanced convective rainfall in the afternoon and stratiform in the early morning. The transition from small to large convective systems as convection propagates away from topographic features is also observed.

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Michael J. Ventrice
and
Chris D. Thorncroft

Abstract

The role of convectively coupled atmospheric Kelvin waves (CCKWs) on African easterly wave (AEW) activity is explored over tropical Africa during boreal summer. Examination of the pre-Alberto AEW in 2000 highlights the observation that the convective trigger for the initiation of the AEW was generated by a strong CCKW and that the subsequent intensification of the AEW at the West African coast was associated with a second CCKW. Composite analysis shows that, generally, AEW activity increases during and after the passage of the convectively active phase of strong CCKWs. The increase in AEW activity is consistent with convective triggering at the leading edge of the convective phase of the CCKW. This convective triggering occurs in a region where the background low-level easterly vertical wind shear is increased by the CCKW. As the AEW propagates westward through the convectively active phase of the CCKW, it can develop in an environment favorable for convection. It is also shown that this phase of the CCKW is characterized by enhanced meridional vorticity gradients in the core of the African easterly jet suggesting that enhanced mixed barotropic–baroclinic growth may also be responsible for enhanced AEW activity there.

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Alan Brammer
and
Chris D. Thorncroft

Abstract

African easterly waves (AEWs) are objectively tracked between West Africa and the tropical Atlantic based on the CFSRv2 data for 1979 to 2012. The characteristics of the troughs of the AEWs at the West African coast are explored and related to whether they favor tropical cyclogenesis over the eastern Atlantic. A logistic regression model was used to determine the optimum combination of predictors that relate AEW characteristics to tropical cyclogenesis. The most skillful model for genesis over the eastern Atlantic consisted of four variables of the AEWs dynamics over the coastal region and the absolute number of days from the peak in the AEW season. Using this diagnostic an equal number of favorable developing and nondeveloping waves were compared through a composite difference analysis. Favorable developing waves had significantly higher moisture content in the lower troposphere to the northwest of the trough as they exited the West African coast compared to favorable nondeveloping waves. Trajectory analysis for all the waves revealed that as the AEWs transition over the West African coast the troughs are typically open to the environment ahead and to the northwest of the trough. For developing waves this means that moist air is ingested into the lower levels of the system, while for nondeveloping waves dry air is ingested. At this point in the AEW life cycle this difference may be fundamental in determining whether a favorable wave can develop or not.

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Matthew A. Janiga
and
Chris D. Thorncroft

Abstract

Using data from the Tropical Rainfall Measuring Mission (TRMM), the modulation of convection by African easterly waves (AEWs) is investigated over regions of the east Atlantic and tropical Africa. To explain the modulation of convection, the large-scale environment (lift, moisture, conditional instability, and shear) is also examined as a function of AEW phase in each region.

Over semiarid portions of tropical Africa, unconditional rain rates are greatest in the northerly phase of AEWs due to the strong adiabatic forcing for ascent. Along the Guinea Coast, the western coast of Africa, and over the east Atlantic—where forcing for ascent is weaker—rainfall is shifted toward the trough where the air is moist. Significant contrasts in the characteristics of convection as a function of AEW phase—comparable in magnitude to regional contrasts—are also observed. In all regions, large and high echo-top convective systems are more sensitive to AEW phase than small and low echo-top systems. In semiarid regions, deep convection and large high echo-top convective systems account for a large fraction of the rainfall in the ridge and northerlies. Stratiform and small low echo-top convective systems dominate in the trough and southerlies. Convective system height and conditional rain rates increase with conditional instability and system sizes may increase with shear. Over the east Atlantic, stratiform fractions and convective system sizes and echo-top heights are greatest in the trough while the ridge is dominated by shallow convection. This is primarily related to the presence of moist air in the trough and dry air in the ridge.

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Alan Brammer
and
Chris D. Thorncroft

Abstract

This study presents a large-scale trajectory analysis of African easterly waves (AEWs) across West Africa and the eastern Atlantic. Back trajectories were initialized at multiple pressure levels from in and around the vortex center of the AEW troughs to reveal the source regions of environmental inflow. The trajectory analysis highlights a changing influence of environmental air on AEW troughs. Over West Africa, monsoonal flow dominates with source regions of air from the southwest and east to northeast influencing the trough. As the AEW troughs leave West Africa, flow from the northwest becomes increasingly important. Cluster analysis highlighted that the contribution of trajectories from the northwest increased as the AEW troughs move westward and that this cluster also had high variability in environmental characteristics.

Correlation analysis of outgoing longwave radiation around the troughs with environmental characteristics 72 h earlier was conducted on 443 AEWs. This analysis reveals that the impact of the various source regions on convective activity within the AEW troughs is consistent with the cluster trajectory analysis. While the AEW troughs were over West Africa, convection was sensitive to midlevel equivalent potential temperature around the trough and to the northeast of the trough axis. Over the West African coast and Cape Verde basin, the correlation analysis captures the changing flow regime with sensitivity to west of the trough axis at midlevels and northwest at low levels. These results highlight that the cool and dry low-level northerly trade winds over the Canary and Cape Verde basin can be a crucial influence on AEWs as they leave West Africa.

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Chris D. Thorncroft
,
Nicholas M. J. Hall
, and
George N. Kiladis

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.

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Nicholas M. J. Hall
,
George N. Kiladis
, and
Chris D. Thorncroft

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.

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George N. Kiladis
,
Chris D. Thorncroft
, and
Nicholas M. J. Hall

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.

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