Low-Level African Easterly Wave Activity and Its Relation to Atlantic Tropical Cyclogenesis in 2001

Robert S. Ross Department of Meteorology, The Florida State University, Tallahassee, Florida

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T. N. Krishnamurti Department of Meteorology, The Florida State University, Tallahassee, Florida

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Abstract

This paper provides new information on the low-level (850 hPa) structure and behavior of African easterly waves (AEWs) and relates this information to previous studies. Individual AEWs that occurred during June–September of 2001 are studied by a synoptic approach that employs Hovmöller diagrams, wave track maps, and case studies. The focus is on two AEW regimes in the lower troposphere over North Africa: a dry regime to the north of the African easterly jet (AEJ) coincident with the surface position of the monsoon trough near 20°N, and a wet regime to the south of the jet coincident with the near-equatorial rainbelt near 10°N. The following issues are addressed: the origin of the waves seen in the two wave regimes, relation of the wave activity to the mean positions of the surface monsoon trough and the 600–700-hPa AEJ, collocation of the tracks of the two wave regimes off the African coast, and diversity in low-level wave behavior that includes merging, splitting, and dissipation of the cyclonic vorticity centers associated with the wave troughs. The relationship between the waves following the two tracks is examined as well as the relationship between the low-level wave activity and Atlantic tropical cyclogenesis in 2001. It is shown that the two wave regimes can interact, and that both regimes were instrumental in Atlantic tropical cyclogenesis in 2001.

Corresponding author address: Robert S. Ross, Department of Meteorology, The Florida State University, Tallahassee, FL 32306-4520. Email: bross@coven.met.fsu.edu

Abstract

This paper provides new information on the low-level (850 hPa) structure and behavior of African easterly waves (AEWs) and relates this information to previous studies. Individual AEWs that occurred during June–September of 2001 are studied by a synoptic approach that employs Hovmöller diagrams, wave track maps, and case studies. The focus is on two AEW regimes in the lower troposphere over North Africa: a dry regime to the north of the African easterly jet (AEJ) coincident with the surface position of the monsoon trough near 20°N, and a wet regime to the south of the jet coincident with the near-equatorial rainbelt near 10°N. The following issues are addressed: the origin of the waves seen in the two wave regimes, relation of the wave activity to the mean positions of the surface monsoon trough and the 600–700-hPa AEJ, collocation of the tracks of the two wave regimes off the African coast, and diversity in low-level wave behavior that includes merging, splitting, and dissipation of the cyclonic vorticity centers associated with the wave troughs. The relationship between the waves following the two tracks is examined as well as the relationship between the low-level wave activity and Atlantic tropical cyclogenesis in 2001. It is shown that the two wave regimes can interact, and that both regimes were instrumental in Atlantic tropical cyclogenesis in 2001.

Corresponding author address: Robert S. Ross, Department of Meteorology, The Florida State University, Tallahassee, FL 32306-4520. Email: bross@coven.met.fsu.edu

1. Introduction

For over three decades the westward-propagating wave disturbances of the lower and middle troposphere over North Africa, known as African easterly waves (AEWs), have been studied by a variety of techniques, including synoptic case studies, composite and spectral techniques, and through output from numerical weather prediction (NWP) models. Currently there is great interest in the weather systems of North Africa, including AEWs, as evidenced by the JET2000 project (Thorncroft et al. 2003) and the major international research and field project known as the African Monsoon Multidisciplinary Analysis (AMMA) planned for 2006–08.

Burpee (1972), using spectral techniques to study the waves, concluded that the waves obtain their energy from combined barotropic–baroclinic instabilities of the African easterly jet (AEJ), a prominent maximum in the zonal wind of 12–15 m s−1 at 600 hPa and 15°N over North Africa in summer. He found maximum amplitude in the meridional wind spectra of 1–2 m s−1 near 700 hPa. Most studies of AEWs, including this study by Burpee, have emphasized the wave structure to the south of the AEJ at the jet level. However, there has been considerable interest in how AEW amplitude and structure vary in the vertical, and a number of studies have referred to AEW structure in the lower troposphere below the jet level. The focus of the present paper is on this low-level structure and the behavior of AEWs as seen at the 850-hPa level.

Carlson (1969) described AEWs, based on synoptic analysis, and showed that the wave trough at 10 000 ft has two vortices ahead of it at 2000 ft—one at 12°N, which he described as propagating from land to ocean, and one at 20°N, which he described as dissipating at the West African coast. Burpee (1974), using upper-air data and composite surface streamline maps, defined two tracks at the surface for cyclone–anticyclone pairs accompanying each AEW at 700 hPa—one along 20°N associated with the monsoon trough, and one along 5°–10°N related to the east–west-oriented rainfall maximum near 10°N in summer. Based on a composite study of AEWs occurring during the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE), Reed et al. (1977) found that disturbance centers propagated on the south side of the 700-hPa AEJ at the jet level drawing energy from barotropic energy conversion. To the north of the jet, baroclinic energy conversions were found in the baroclinic zone below the jet level. These investigators also found two surface centers associated with the upper wave: a stronger center located near 17°N in the monsoon trough and a weaker center located to the south in the zone of active convection near 11°N. The dual centers over land appeared to merge into one over the ocean.

Nitta and Takayabu (1985) found two different tracks for AEWs over North Africa. The southern track waves extended farther into the Atlantic than the northern track waves, but no merger of the two tracks was found. Coherence analysis indicated that the waves following the two tracks were coupled.

Reed et al. (1988a) used the European Centre for Medium-Range Weather Forecasts (ECMWF) data from 1985 at the 700- and 850-hPa levels to show that AEWs propagate across North Africa in two tracks, one to the north and one to the south of the AEJ, with the two tracks merging over the ocean near 15°N. Using this same dataset Reed et al. (1988b) calculated meridional wind variance for waves of 2.9–4.3-day periods. A meridional vertical section along 9°W of this wind variance is shown in Fig. 1. The circled plus sign in the figure has been added to indicate the mean position of the AEJ in their study. This figure shows that to the south of the jet the maximum wind variance exists at jet level, while to the north of the jet, maximum variance values are found below the jet level near 850 hPa. Note that if the waves are tracked at the 850-hPa level, as is done in the present study, two tracks for the waves will be seen.

Duvel (1990) studied AEWs for the period 1983–85 using ECMWF analyses and Meteosat data. Spectral analysis of the 850-hPa meridional wind component in the 2.8–5.1-day band revealed maximum wave amplitude at 20°N near the West African coast. A secondary maximum in amplitude developed in August and September at 7.5°N along the West African coast. Two tracks for the waves were inferred for late summer, with the tracks merging over the ocean at about 15°N.

Pytharoulis and Thorncroft (1999) used radiosonde data and the Met Office global analysis from 1995 to study the low-level AEW structures to the north of the AEJ. They pointed out that more attention should be given to the disturbances following the northern wave track, and to the relationship between the disturbances following the northern and southern tracks. Furthermore, they stated that disturbances in the northern track may very well play a role in tropical cyclone development over the Atlantic. These investigators raised a very important question regarding the nature of AEWs when they considered whether the AEWs are systems with two circulation centers at low levels but with one strong vorticity maximum at 700 hPa south of the AEJ, or if there are two different kinds of AEWs growing on either side of the jet. Based on cross-correlation analysis they found a high degree of coherence between the 700-hPa waves south of the AEJ and the low-level waves north of the jet, confirming the complicated structure of AEWs. They concluded that the 700-hPa and the low-level waves are not independent structures but are part of the same AEW.

Thorncroft and Hodges (2001) found a strong positive correlation between the frequency of 850-hPa wave activity at the West African coast between 10° and 15°N and the frequency of Atlantic tropical cyclones, based on 20 yr of data. Thus, tropical cyclone frequency may not vary with the total number of AEWs passing into the Atlantic, but with the number of waves that have significant low-level amplitude. It seems likely that the study of the low-level structure and behavior of AEWs may prove to be of considerable importance for our understanding of Atlantic tropical cyclone genesis.

A reasonable consensus of the above literature states that AEWs possess a complicated structure with two circulation centers at low levels, a stronger one to the north of the 600–700-hPa AEJ and a weaker one to the south of the jet, and with one strong vorticity maximum at 700 hPa, located to the south of the jet. Although not completely settled, it appears that the 700-hPa wave and the low-level waves are not independent structures but are part of the same AEW. As a matter of convenience, this paper will refer to the northern and southern wave structures at low-levels as “wave regimes,” but this should not be taken to imply that these wave structures are necessarily independent of each other or independent of the wave structure at 700 hPa. The low-level wave regime to the north of the AEJ is located around 20°N and is coincident with the surface position of the monsoon trough. These waves are relatively dry and draw their energy primarily from baroclinic dynamics. The low-level wave regime to the south of the AEJ is located near 10°N and is coincident with the near-equatorial rainbelt. These waves tend to produce abundant rainfall and are driven primarily by barotropic dynamics.

The literature is not conclusive as to the relationship between the two low-level wave regimes, including an understanding of whether the two wave regimes merge over the ocean or not. Previous studies have used the term “merging” without making a potentially important distinction that this paper will make (i.e., the difference between the “merging of wave tracks” and the “merging of waves” for the two low-level wave regimes). This distinction will be explained in section 3 and the potential significance of this distinction will be discussed in section 5. In addition to being inconclusive about the relationship between the two low-level wave regimes, the literature is also unclear as to the role of the low-level wave regimes with respect to Atlantic tropical cyclogenesis.

The goal of this paper is to provide new information about the low-level AEW structures. Extending the work of Pytharoulis and Thorncroft (1999), this paper focuses on the low-level waves following the northern track and on the relationship between the waves following the northern and southern tracks during the summer of 2001. Building on the work of Thorncroft and Hodges (2001), the paper treats the relationship between the 850-hPa wave activity and Atlantic tropical cyclogenesis in 2001. In sections 2, 3, and 4 the low-level structure and behavior of AEWs will be examined from a synoptic perspective, which focuses on the individual waves that occurred during June–September of 2001. Hovmöller diagrams (section 2), wave track maps (section 3), and individual synoptic case studies (section 4) will be utilized to study the two AEW regimes at 850 hPa. Issues such as the following will be addressed: the origin of the waves seen in the two wave regimes; relation of the wave activity to the mean positions of the surface monsoon trough and the 600–700-hPa AEJ; collocation of the tracks of the two wave regimes off the African coast; diversity in low-level wave behavior, which includes merging, splitting, and dissipation of the cyclonic vorticity centers associated with the wave troughs; and relation of the 850-hPa wave activity to tropical cyclogenesis in 2001. In section 5 the salient findings in this research will be summarized and suggestions for future research will be outlined.

2. Hovmöller diagrams

Hovmöller diagrams were constructed for the observed 850-hPa meridional wind component. The latitudes and the time periods for the Hovmöller diagrams were carefully chosen in order to maximize the amount of knowledge gained about AEWs. The rationale for constructing the diagrams at 20°N for June and July, and at 10°N for August and September will now be explained.

The benchmark analysis for this research is the ECMWF analysis with the Florida State University (FSU) physical initialization for the wind field, and the Tropical Rainfall Measuring Mission (TRMM) and the Special Sensor Microwave Imager (SSM/I) datasets for precipitation. These data were used to construct daily maps (1200 UTC) of 850-hPa vector wind and 24-h precipitation for the period 1 June–31 October 2001 in the region of AEW activity (5°S–30°N, 35°W–15°E). Isolines of 850-hPa relative vorticity were added to the maps to facilitate the identification of the waves. It was determined that relative vorticity values greater than 4.0 × 10−5 were associated with AEW troughs that were well defined and that could be tracked for several days. Maps of the daily locations of such vorticity centers were constructed for each month, June–September. These maps provided a definition of the regions of AEW activity at 850 hPa for each month, and it was found that there was a latitudinal shift in the activity from June to September. Figure 2 shows the distribution of 850-hPa vorticity centers in two latitude bands, 5°–14°N and 15°–24°N, for each month, expressed as a percentage of the total number of vorticity centers. In June and July, 66% and 76%, respectively, of the vorticity centers associated with AEW troughs occurred in the northern latitude band. By August and September, 57% and 78%, respectively, of the vorticity centers were found in the southern latitude band. Thus, there was a pronounced southward shift in the 850-hPa AEW activity as the summer progressed. The 850-hPa wave regime to the north of the AEJ, coincident with the surface position of the monsoon trough, was dominant in early summer, and by late summer, the 850-hPa wave regime located in the near-equatorial rainbelt to the south of the AEJ was dominant. The patterns seen in Fig. 2 provided the primary justification for constructing the Hovmöller diagrams using 20°N for June and July and 10°N for August and September. Duvel (1990) found a similar southward shift in 850-hPa AEW activity between the periods June–July and August–September. He found that the primary maximum in wave activity was located near 20°N during both periods but that a secondary maximum in activity appeared near 7.5°N at the West African coast in August–September.

The Hovmöller diagrams of observed 850-hPa wind at 20°N for June and July and at 10°N for August and September of 2001 are shown in Figs. 3a,b, respectively. Prior to construction of the Hovmöller diagrams all of the v-component wind fields were smoothed using a 21-point bivariate normal weighting scheme that reduces the amplitude of waves with wavelengths equal to 7 grid lengths (777 km) to one-half, filters out waves with wavelengths shorter than this, and leaves waves with longer wavelengths unaffected. This procedure filtered out small-scale oscillations and highlighted the AEW activity. In the Hovmöller diagrams the propagation of AEWs is shown by lines sloping from the upper right to the lower left that are numbered consecutively for each month. These lines separate the northerly wind component (above the line) from the southerly wind component (below the line) and thus depict the propagation of the trough of each AEW. These lines were entered based on inspection of the patterns seen in the Hovmöller diagrams and by examining the benchmark analysis described at the beginning of this section. Examination of Fig. 3a shows that in June six waves were tracked, and in July eight waves were tracked, with the sixth wave (wave 6) for that month subsequently developing into Tropical Storm Barry. Inspection of Fig. 3b reveals that in August six waves were tracked, with wave 3 subsequently developing into Tropical Storm Dean and wave 6 developing into Hurricane Erin. In September eight waves were tracked, with wave 3 developing into Tropical Depression 9, wave 7 developing into Hurricane Iris, and wave 8 developing into Tropical Storm Jerry.

The Hovmöller diagram for June and July along 20°N (Fig. 3a) shows that regular wave motion tends to develop around 0°–5°E in June and slightly farther to the east in the zone 0°–10°E in July. The waves reach their maximum amplitude in the region from the prime meridian to 25°W, in good agreement with other observational studies. Wave amplitude diminishes westward over the Atlantic Ocean. These northern track waves in June and July are dominated by strong baroclinic dynamics and are much drier than those found in the wave regime to the south in August and September.

The Hovmöller diagram for August and September along 10°N (Fig. 3b) shows that wave motion has become regular by 10°E, with maximum amplitude occurring between the prime meridian and 30°W, in good agreement with other observational studies.

3. Wave track maps

The ECMWF analysis with FSU physical initialization was used to construct daily maps (1200 UTC) of 850-hPa vector wind with superimposed isolines of relative vorticity in the region 5°S–30°N, 35°W–15°E for the period 1 June–31 October 2001. The maps also contain the observed 24-h precipitation as defined by TRMM and SSM/I data. These maps were carefully inspected to establish the track of each of the AEWs seen in the Hovmöller diagrams (Figs. 3a,b). The most useful tracking device was the westward migration of the cyclonic vorticity centers. An inventory of these waves is presented in Table 1. The longitude of origin of each wave is specified along with an indication of whether the origination occurs to the north (N) or south (S) of the mean position of the AEJ in each month. Several waves dissipate within the domain of observation (e.g., wave 2 in July has the designation, D23W, which means the wave dissipated at 23°W). A number of oscillations seen in the Hovmöller diagrams were produced by waves bearing distinct vorticity centers to the north and south of the AEJ. Such waves carry one number, but with two-letter designations: N and S. For example, wave 3 in July is designated 3N and 3S. (Furthermore, wave 3N dissipates at 17°W.) Wave 3 in August is comprised of two cyclonic vorticity centers, both located to the north of the mean position of the AEJ. This wave is indicated as 3NN and 3NS, meaning that both vorticity centers are to the north of the AEJ (first letter after the number) but one is to the north and one to the south in relation to each other (second letter after the number). In several instances waves that have two distinct cyclonic vorticity centers show a merging of vorticity centers. This can occur with a vorticity center on either side of the AEJ (e.g., wave 2N and 2S in August, where 2S merges with 2N at 26°W) or with two vorticity centers on the same side of the AEJ (e.g., wave 3NN and 3NS in August, where 3NS merges with 3NN at 5°W). Splitting of one vorticity center into two centers can also occur, as was the case with wave 3N and 3S in July. Prior to the period when 3N (3S) was propagating to the north (south) of the AEJ the wave was represented by one vorticity center located to the north of the AEJ. (See the map of July tracks in Fig. 4b.) Table 1 indicates that in 2001 three tropical storms, two hurricanes, and one tropical depression developed from AEWs. In the comments column of Table 1 it is indicated that Tropical Storms Barry and Dean developed from AEWs that had their origin to the north of the mean monthly position of the AEJ and underwent the process of development after they had propagated southwestward and had crossed to the south of the mean monthly position of that jet. Tropical Depression 9 developed from an AEW that resulted from the merger of a cyclonic vorticity center that originated to the north of the mean monthly position of the AEJ with a cyclonic vorticity center that developed to the south of the mean monthly position of that jet along the west coast of Africa. The merger occurred over the ocean to the south of the mean monthly position of the jet. Table 1 indicates that the two AEWs that developed into hurricanes in 2001 (Hurricanes Erin and Iris) originated to the south of the mean monthly position of the AEJ and that another wave from this wave regime developed into Tropical Storm Jerry.

The tracks for the waves listed in Table 1 are shown for each month, June–September, in Figs. 4a–d. In addition to the wave tracks, each map shows the mean monthly positions for the equatorial trough (monsoon trough over land) and the 700-hPa AEJ, as determined from monthly means in the 2001 National Centers for Environmental Prediction (NCEP) operational analysis of sea level pressure and 700-hPa wind, respectively. Brief highlights of the tracks for each month will now be given.

In June (Fig. 4a), all six of the AEWs have their origin to the north of the mean position of the AEJ, and out of eight waves in July (Fig. 4b), only waves 3S and 5 (Table 1) have their origin to the south of the mean position of the AEJ. In June the wave origin is near 0°–5°E, and in July the region of origin shifts slightly farther to the east to around 0°–10°E. The waves in these two months originate in the vicinity of the mean position of the monsoon trough and show a clear pattern of moving westward, and then southwestward, along this trough. Following a southwesterly track along the equatorial (monsoon) trough, these 850-hPa waves eventually move to the south of the mean position of the AEJ (Figs. 4a,b). Wave 6 in July (Fig. 4b) originated along the monsoon trough at 10°E to the north of the mean position of the AEJ, followed the trough westward and then southwestward, crossed to the south of the mean position of the AEJ near 15°W, and eventually developed into Tropical Storm Barry in the Atlantic. As mentioned above, Tropical Storm Dean in August (Fig. 4c) followed a similar scenario. This indicates that waves in the low-level wave regime located to the north of the mean position of the AEJ over the African continent can develop into tropical cyclones over the Atlantic once they cross to the south of the 700-hPa mean position of the AEJ. However, a majority of the low-level waves that crossed from the north to the south of the mean location of the AEJ in 2001 did not develop beyond the wave stage over the Atlantic. The processes that lead to development of some waves, and to nondevelopment of other waves, require further study.

The track maps for August and September (Figs. 4c,d) clearly show that a new 850-hPa wave regime has developed to the south of the mean location of the AEJ, where the waves propagate west-northwestward parallel to the jet axis. The wave regime to the north of the mean position of the AEJ is still in place, where the waves continue to follow the mean position of the monsoon trough as they propagate to the west and then to the southwest, producing a merger of wave tracks for the two regimes near 25°W off the West African coast in general agreement with the studies of Reed et al. (1988a) and Duvel (1990). Waves to the north of the mean location of the AEJ originate around 0°–10°E in August (Fig. 4c) and around 10°–15°E in September (Fig. 4d). Waves to the south of the mean location of the AEJ generally originate farther to the west in the region 0°–15°W in both August and September.

In August there are two cases where waves to the north and south of the mean position of the jet merge, wave 2N/2S and wave 4N/4S (Table 1 and Fig. 4c). Otherwise, the merging of the northern and southern wave regimes seen in Figs. 4c,d represents a merging of wave tracks rather than a merging of the waves, themselves. It should be noted that a merging of wave tracks is taken in this paper to mean that the tracks of the cyclonic vorticity centers associated with the wave troughs from the two wave regimes converge without the vorticity centers merging into one vorticity center (i.e., the vorticity centers remain out of phase one with the other). Merging of the waves is taken to mean that the two cyclonic vorticity centers do, in fact, merge into one vorticity center. Thus, this paper, while “stuck” with the terminology of merging from previous studies, draws a distinction between “convergence” of wave tracks and “merger” of vorticity centers. The potential significance of this distinction is discussed in section 5.

In section 1 of this paper it was pointed out that Pytharoulis and Thorncroft (1999) had stressed the need for more studies of the 850-hPa wave regime following the northern track (north of the AEJ), and of the relationship between the disturbances that follow the two tracks (north and south of the jet). One reason that they cited for giving more attention to the low-level waves to the north of the jet was their claim that some could move over the ocean and become associated with tropical cyclone formation. The present study has clearly responded to this call for additional research by providing new information on the role of the northern wave regime in Atlantic tropical cyclogenesis, and on the complex relationships between the waves in the two wave regimes.

4. Synoptic case studies

In this section, a detailed synoptic history will be presented for each of the six tropical cyclones that developed from AEWs in 2001 (Hurricanes Erin and Iris; Tropical Storms Barry, Dean, and Jerry; and Tropical Depression 9). The National Hurricane Center (NHC) report, “Atlantic Hurricane Season of 2001,” (Beven et al. 2003) will be utilized to document the synoptic history of each system as it developed from the wave stage over Africa into a tropical cyclone over the Atlantic. Particular attention will be given to Tropical Storm Barry that developed from a northern track wave, Hurricane Erin that developed from a southern track wave, and Tropical Depression 9 that developed from the merger of the northern and southern track waves. The particular focus on these three cases will serve to illustrate the diverse ways in which AEW activity at 850 hPa can evolve into tropical cyclones over the Atlantic.

a. Tropical Storm Barry

According to Beven et al. (2003), “Barry formed from a tropical wave that moved westward from the coast of Africa on 24 July.” Using the daily ECMWF analyses of 850-hPa wind and vorticity as described in section 3, this wave is tracked across West Africa in Fig. 4b as wave 6, whose cyclonic vorticity center is first located at 17°N, 12°E on 20 July. On successive days this vorticity center is located at 16°N, 4°E (21 July); 20°N, 5°W (22 July); 19°N, 14°W (23 July); 15°N, 19°W (24 July); 15°N, 24°W (25 July); and 13°N, 30°W (26 July). The position on 24 July represents a vorticity center (wave trough), which is poised to move westward from the coast of Africa, and this position and date clearly identify the wave as the one NHC has signified as the precursor to Barry. It is evident from the track in Fig. 4b that this wave originated to the north of the mean monthly position of the AEJ, and that it moved generally along the mean monthly position of the monsoon trough, eventually crossing to the south of the mean monthly position of the jet prior to exiting into the eastern Atlantic Ocean. The NHC report indicates that convection flared in the system 4 days after its exit from the coast of Africa, on 28 July, to the east of the Lesser Antilles. The system moved through the Caribbean Sea during 29–31 July as a wave with increasing convection. It emerged into the southeastern Gulf of Mexico on 1 August, and on 2 August the system rapidly intensified into a tropical depression and then into a tropical storm. The tropical storm made landfall at Santa Rosa Beach, Florida, on 6 August with 60-kt (31 m s−1) winds and a central pressure of 990 hPa.

The 850-hPa vector wind and relative vorticity, along with the observed 24-h precipitation, for Tropical Storm Barry’s precursor wave, during the period 22–26 July 2001, are shown in Figs. 5a–e. The wind field is from the ECMWF analysis with FSU physical initialization, and the precipitation is from TRMM and SSM/I datasets. The wave trough, with an associated distinct vorticity center, is seen to move southwestward from near 20°N, 5°W on 22 July (Fig. 5a) to near Dakar, Senegal (15°N, 19°W) by 24 July (Fig. 5c), from whence it moves west-southwestward, with several associated vorticity centers, into the Atlantic by 26 July (Fig. 5e). The wave is seen throughout the sequence to be well defined in the wind and vorticity fields, and is also seen to draw in moisture by pulling the near-equatorial rainbelt northward in the southerly flow to the east of the wave trough.

b. Tropical Storm Dean

According to Beven et al. (2003), “Dean formed from a large tropical wave that crossed Dakar with minimal shower activity between 14 and 15 August.” Using daily ECMWF analyses, this wave is tracked across West Africa in Fig. 4c as wave 3, which first appears as two distinct cyclonic vorticity centers that are both located to the north of the mean August position of the AEJ. The daily analyses revealed that the two vorticity centers merged on 12 August at 17°N, 5°W. Successive positions for the merged vorticity center are 17°N, 8°W (13 August); 16°N, 14°W (14 August); 11°N, 20°W (15 August); 13°N, 27°W (16 August); and 9°N, 34°W (17 August). The positions for 14 and 15 August define a wave trough (cyclonic vorticity center) that crossed Dakar between those two dates and, therefore, identify the wave as the one NHC signified as the precursor to Dean. This wave clearly originated to the north of the mean August position of the AEJ and crossed to the south of that mean jet position between 14 and 15 August, as the wave moved along the mean monthly position of the monsoon trough. NHC’s report that the wave had “minimal shower activity” as it crossed Dakar is consistent with the wave’s origin in the relatively dry region to the north of the AEJ. The NHC report indicates that the wave moved westward and gradually began to develop thunderstorms. It finally developed into a tropical storm on 22 August and moved west-northwestward through the U.S. Virgin Islands. Dean encountered shear on 23 August and weakened to a tropical wave. The system reintensified to a tropical storm on 27 August when it attained its peak intensity of 60 kt and a central pressure of 990 hPa. Dean became extratropical by 28 August as it moved northward, remaining well to the east of the East Coast.

c. Hurricane Erin

According to Beven et al. (2003), “Erin can be traced back to a tropical wave that emerged from western Africa on 30 August.” This wave is tracked across West Africa in Fig. 4c as wave 6, whose cyclonic vorticity center is first located at 9°N, 10°E on 26 August. On successive days this vorticity center is located at 10°N, 5°E (27 August); 9°N, 5°W (28 August); 10°N, 8°W (29 August); 9°N, 17°W (30 August); 9°N, 25°W (31 August); and 12°N, 33°W (1 September). The position on 30 August represents a vorticity center (wave trough) that is emerging from western Africa into the eastern Atlantic and is obviously the wave that NHC has identified as the precursor to Erin. Unlike the waves that were precursors to Tropical Storms Barry and Dean, this wave originated and moved to the south of the mean August position of the AEJ. This was one of two AEWs at 850 hPa that developed into hurricanes in 2001, and both of these waves originated and moved westward remaining to the south of the mean position of the AEJ. [Wave 7S in September (see Table 1 and Fig. 4d) was the other southern track wave that developed into a hurricane in 2001, Hurricane Iris, which is discussed below in section 4e.]

The 850-hPa vector wind and relative vorticity, along with the observed 24-h precipitation for this precursor wave to Hurricane Erin are shown in Figs. 6a–e for the period 26–30 August. This sequence shows that a cyclonic vortex near 9°N, 10°E with a relatively weak vorticity center on 26 August (Fig. 6a) moved westward and dramatically intensified, exiting the west African coast by 30 August with a very well-defined vorticity center at 9°N, 17°W (Fig. 6e). By the time the system moved off the coast, it had a well-organized region of precipitation associated with it.

The NHC report indicates that the system almost immediately showed signs of tropical cyclone formation after it exited the west coast of Africa, with curvature in the bands of associated deep convection. It is estimated that a tropical depression formed by 1 September (the last plotted location for this system in Fig. 4c), located about 600 nautical miles west-southwest of the Cape Verde Islands, and this system strengthened to a tropical storm by 2 September. By 5 September, southwesterly shear caused the tropical cyclone to degenerate into an area of disturbed weather. The system regained tropical storm strength about 550 nautical miles north-northeast of the northern Leeward Islands by 7 September. Erin strengthened into a hurricane late on 8 September. While passing east of Bermuda on 9 September, Erin continued to strengthen, and it reached its peak intensity of 105 kt with a central pressure of 968 hPa on that day. The eye of the hurricane passed within about 90 nautical miles east-northeast of Bermuda, which was Erin’s point of closest approach to the island. The center passed just east of Cape Race, Newfoundland, on 12 September, as the system was weakening to just below hurricane strength. Shortly thereafter, Erin lost its tropical characteristics.

d. Tropical Depression 9

“Tropical Depression 9 formed from a tropical wave that moved westward from the coast of Africa on 11 September,” according to Beven et al. (2003). This wave is tracked in Fig. 4d as wave 3, which was composed of two cyclonic vorticity centers, one which originated on 8 September at 21°N, 10°E well to the north of the mean monthly position of the AEJ, and a second center that formed to the south of the mean monthly position of the AEJ along the west coast of Africa at 8°N, 13°W on 10 September. By 9 September the northern center had moved to 19°N, 2°W. On 10 September, when the southern center formed at 8°N, 13°W, the northern center was located at 21°N, 5°W. After 10 September the locations of the two vorticity centers are 20°N, 12°W and 9°N, 17°W (11 September); 18°N, 20°W and 9°N, 23°W (12 September); and finally 14°N, 32°W on 13 September when the two centers are seen to merge just to the south of the mean monthly position of the AEJ. Note that the northern center moved west-southwestward parallel to the mean monthly position of the monsoon trough and the southern center moved west-northwestward parallel to the mean monthly position of the AEJ prior to their merger on 13 September. When NHC refers to the “tropical wave that moved westward from the coast of Africa on 11 September,” they are referring to the southern vorticity center located at 9°N, 17°W on that date. From the positions mentioned above, the northern vorticity center was at 20°N, 13°W on 11 September, a location that was still inland over West Africa. As stated, these two vorticity centers merged on 13 September at 14°N, 32°W. It was this wave with the two merged vorticity centers that moved across the Atlantic, eventually developing into Tropical Depression 9.

The 850-hPa vector wind and relative vorticity, along with the observed 24-h precipitation for this scenario, are shown in Figs. 7a–e for the period 9–13 September 2001. Precipitation data were not available for 10–11 September. On 9 September (Fig. 7a) the northernmost vorticity center can be seen near 19°N, 2°W without any associated precipitation. On 10 September (Fig. 7b) the northern and southern vorticity centers are seen at 21°N, 5°W and 8°N, 13°W, respectively. On 11 September (Fig. 7c) two distinct vorticity centers are visible at 20°N, 12°W and 9°N, 17°W. These two vorticity centers are associated with the wave troughs of the northern track and southern track waves, respectively. The two wave troughs are still separate. By 12 September (Fig. 7d) the two vorticity centers are seen just off the West African coast, one lying to the north (18°N, 20°W) and one lying to the south (9°N, 23°W), in a unified trough that has developed from a merging of the northern track and southern track wave troughs. By 13 September (Fig. 7e) the unified wave, now with one consolidated vorticity center (14°N, 32°W), has moved farther off the West African coast.

According to the NHC report this wave reached the Caribbean Sea on 16 September and formed into a depression on 19 September in the southwestern Caribbean Sea. The depression made landfall in Nicaragua on 20 September. The NHC report further states that the tropical wave that spawned Tropical Depression 9 crossed Central America and was responsible for the formation of the eastern Pacific Hurricane Juliette on 21 September.

e. Hurricane Iris

There are two references to the wave that developed into Hurricane Iris that are found in Beven et al. (2003). These two references were compared to the track of the wave shown in Fig. 4d as wave 7 (derived from daily ECMWF analyses of 850-hPa wind and vorticity as described in section 3), to confirm that wave 7 was, indeed, the precursor to Hurricane Iris. The two references are 1) “The precursor of Iris was a poorly defined tropical wave that moved westward across the tropical Atlantic during the last days of September” and 2) “. . . the tropical wave reached 50°W on the 3rd of October . . .” Figure 4d shows that wave 7 originated and moved to the south of the mean September position of the AEJ. The wave moved from its origin at 6°N, 0° on 25 September to a position along the west coast of Africa near 8°N, 12°W on 27 September, and to 7°N, 30°W by 30 September. This last position fits with the NHC location of 50°W on 3 October, giving a reasonable motion of approximately 7° day−1 between 30 September and 3 October. This information was deemed sufficient to identify wave 7 as the precursor to Hurricane Iris. A tropical depression formed southeast of Barbados on 4 October, and tropical storm status was attained by 5 October to the south-southeast of Puerto Rico. Tropical storm Iris became a hurricane near the Barahona Peninsula of the Dominican Republic by 6 October. Iris became a powerful category 4 hurricane on the Saffir–Simpson hurricane scale by 8 October and made landfall in southern Belize on 9 October. Iris intensified just before landfall and the maximum winds peaked at 125 kt with a minimum pressure of 948 hPa.

f. Tropical Storm Jerry

Beven et al. (2003) first identifies the precursor to Tropical Storm Jerry as “a westward-moving tropical wave that crossed the west coast of Africa and entered the tropical Atlantic on 1 October.” This southern track wave is tracked in Fig. 4d as wave 8, whose cyclonic vorticity center is first located at 5°N, 1°E on 28 September. On successive days this vorticity center is located at 6°N, 10°W (29 September); 7°N, 15°W (30 September); 9°N, 19°W (1 October); 11°N, 27°W (2 October); and 11°N, 32°W (3 October). The position on 1 October represents a vorticity center (wave trough), which had crossed the west coast of Africa and entered the tropical Atlantic, and this position and date clearly identifies the wave as the one NHC has signified as the precursor to Jerry. The NHC report indicates that on 6 October this wave developed into a tropical depression to the east-southeast of Barbados. The depression strengthened into Tropical Storm Jerry on 7 October, and the sustained winds increased to their maximum speed of 45 kt as the storm approached the Windward Islands. On the 8 October, Jerry moved into the eastern Caribbean Sea and dissipated to the south of Puerto Rico.

5. Concluding remarks

In this paper the low-level (850 hPa) structure and behavior of AEWs observed during June–October 2001 has been examined in a synoptic approach that included the use of Hovmöller diagrams, wave track maps, and individual case studies. The results of this study are consistent with the prevailing view of AEWs found in the literature, that is, that the waves possess a complicated structure with two circulation centers at low levels, a stronger one to the north of the 600–700-hPa AEJ and a weaker one to the south of the jet, and with one strong vorticity maximum at 700 hPa, located to the south of the jet. Although not completely settled in the literature, it appears that the 700-hPa wave and the low-level waves are not independent structures but are part of the same AEW. As a matter of convenience, this paper has referred to the northern and southern wave structures at low-levels as “wave regimes,” but this was not taken to imply that these wave structures are necessarily independent of each other or independent of the wave structure at 700 hPa.

A pronounced southward shift in the 850-hPa AEW activity was found as the summer progressed, in agreement with the work of Duvel (1990) and others. The 850-hPa wave regime to the north of the AEJ, coincident with the surface position of the monsoon trough, was dominant in June and July, and by August and September the 850-hPa wave regime located along the near-equatorial rainbelt to the south of the AEJ was dominant. Accordingly, the Hovmöller diagrams of 850 hPa observed meridional wind component that were used to track the waves were constructed along 20°N for June and July, and along 10°N for August and September. In the Hovmöller diagrams (Figs. 3a,b), six waves were tracked in June, eight in July, six in August, and eight in September.

The track of each of the AEWs seen in the Hovmöller diagrams was determined using daily maps of ECMWF analysis with FSU physical initialization. Each track was based on successive locations of the cyclonic vorticity center located in each wave trough. These tracks (Figs. 4a–d) revealed that in June all six of the AEWs had their origin to the north of the mean position of the AEJ, and in July six of the eight waves tracked had their origin to the north of the mean position of this jet. The waves in these two months originated in the vicinity of the mean position of the monsoon trough and moved westward, and then southwestward, along this trough and eventually moved to the south of the mean position of the AEJ. The track maps for August and September confirmed that a new 850-hPa wave regime had developed to the south of the mean location of the AEJ, where the waves propagated west-northwestward parallel to the jet axis. The wave regime to the north of the mean position of the AEJ was still in place, and these waves continued to follow the mean position of the monsoon trough as they propagated to the west and then to the southwest, producing a merger of wave tracks for the two regimes near 25°W off the West African coast.

The wave track maps revealed that the low-level wave activity in 2001 was rich in diversity. Some waves had two cyclonic vorticity centers, one to the north and one to the south of the mean position of the AEJ, or two cyclonic vorticity centers on the same side of the jet, typically to the north of the jet. Most of these vorticity centers remained separate as they propagated westward, but some of them merged, producing a more vigorous wave trough. On the other hand, splitting of one vorticity center into two centers was observed, leading to the formation of two wave troughs.

In 2001 Tropical Storms Barry and Dean developed over the Atlantic basin from AEWs that had their origin to the north of the AEJ. In both cases the waves moved southwestward to a position to the south of the AEJ as they exited from the African continent into the Atlantic. Tropical Depression 9 developed over the Atlantic from an AEW that resulted from the merger of cyclonic vorticity centers located to the north and south of the mean monthly position of the AEJ. The merger of these vorticity centers occurred over the Atlantic to the south of the AEJ position. Tropical Storm Jerry and Hurricanes Erin and Iris developed over the Atlantic from AEWs that had their origin to the south of the mean monthly position of the AEJ. Detailed synoptic histories of all of these systems were presented in section 4, and a clear and unambiguous linkage was established between the AEW over Africa and the named system over the Atlantic.

This study has contributed new information with regard to two issues that have not been fully resolved in previous studies of AEWs, issues that may be related. These issues are 1) the relationship between waves in the two low-level wave regimes, particularly after the waves move from the African continent into the Atlantic, which includes the issue of merging; and 2) the relationship of the low-level wave activity to tropical cyclogenesis over the Atlantic.

In section 3 a distinction was made between the convergence of the tracks of the cyclonic vorticity centers for the waves in the two wave regimes (convergence of wave tracks) and the actual merging of the cyclonic vorticity centers, themselves. A convergence of tracks could leave two rather weak waves propagating westward over the Atlantic, while a merger of cyclonic vorticity centers could lead to a more vigorous wave, possibly one that would be more likely to develop into a tropical cyclone. Stated another way, the tracks of the two wave regimes (tracks of the cyclonic vorticity centers) could converge, with the cyclonic vorticity centers remaining out of phase, or the cyclonic vorticity centers could come into phase and merge (combine). Previous studies have not been in agreement on whether or not the tracks merge (converge, in this paper) and have essentially not dealt at all with the issue of actual mergers of the vorticity centers themselves. The present study found that mergers of cyclonic vorticity centers from the two wave regimes was the exception rather than the rule, and that the convergence of the two low-level wave tracks (tracks of the cyclonic vorticity centers) was more common. This issue should be examined in future studies, particularly since the exact nature of the interaction of the two wave regimes over the Atlantic may be related to tropical cyclogenesis.

This study has clearly shown that waves in the low-level wave regime located to the north of the mean position of the AEJ over the African continent can develop into tropical cyclones over the Atlantic once they cross to the south of the mean position of that jet. Future studies should determine if the low-level positive vorticity anomalies move “through” or “underneath” the actual daily position of the 700-hPa AEJ (not just the mean position) and, if so, what dynamical processes are involved. In 2001 a majority of the low-level waves that crossed from the north to the south of the mean location of the AEJ did not develop beyond the wave stage over the Atlantic. The processes that lead to development of some waves, and to nondevelopment of other waves, also requires further study. It seems likely that the study of the low-level structure and behavior of AEWs will continue to be of considerable importance for the understanding of Atlantic tropical cyclogenesis.

Acknowledgments

This research has been supported by NSF Grant ATM-0419618 and NASA Grant NAG5-13563.

REFERENCES

  • Beven, J. L., S. R. Stewart, M. B. Lawrence, L. A. Avila, J. L. Franklin, and R. J. Pasch, 2003: Annual summary: Atlantic hurricane season of 2001. Mon. Wea. Rev., 131 , 14541484.

    • Search Google Scholar
    • Export Citation
  • Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci., 29 , 7790.

  • Burpee, R. W., 1974: Characteristics of North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci., 31 , 15561570.

    • Search Google Scholar
    • Export Citation
  • Carlson, T. N., 1969: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97 , 716726.

    • Search Google Scholar
    • Export Citation
  • Duvel, J. P., 1990: Convection over tropical Africa and the Atlantic Ocean during northern summer. Part II: Modulation by easterly waves. Mon. Wea. Rev., 118 , 18551868.

    • Search Google Scholar
    • Export Citation
  • Nitta, T., and Y. Takayabu, 1985: Global analysis of the lower tropospheric disturbances in the tropics during the northern summer of the FGGE year. Part II: Regional characteristics of the disturbances. Pure Appl. Geophys., 123 , 272292.

    • Search Google Scholar
    • Export Citation
  • Pytharoulis, I., and C. Thorncroft, 1999: The low-level structure of African easterly waves in 1995. Mon. Wea. Rev., 127 , 22662280.

  • Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE. Mon. Wea. Rev., 105 , 317333.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., A. Hollingsworth, W. A. Heckley, and F. Delsol, 1988a: An evaluation of the performance of the ECMWF operational forecasting system in analyzing and forecasting tropical easterly wave disturbances over Africa and the tropical Atlantic. Mon. Wea. Rev., 116 , 824865.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., E. Klinker, and A. Hollingsworth, 1988b: The structure and characteristics of African easterly wave disturbances as determined from the ECMWF operational analysis/forecast system. Meteor. Atmos. Phys., 38 , 2233.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., and K. Hodges, 2001: African easterly wave variability and its relationship to Atlantic tropical cyclone activity. J. Climate, 14 , 11661179.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., and Coauthors, 2003: The JET2000 project: Aircraft observations of the African easterly jet and African easterly waves. Bull. Amer. Meteor. Soc., 84 , 337351.

    • Search Google Scholar
    • Export Citation

Fig. 1.
Fig. 1.

Meridional section along 9°W of the meridional wind variance (m2 s−2) for waves of the 2.9–4.3-day period for August–September 1985. Circled plus sign indicates the mean position of the African easterly jet (after Reed et al. 1988b).

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 2.
Fig. 2.

Percent of observed 850-hPa relative vorticity centers of magnitude greater than 4.0 × 10−5 in two latitude bands, 5°–14°N and 15°–24°N, distributed by month, June–September 2001, in the AEW region (5°S–30°N, 35°W–15°E).

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 3.
Fig. 3.

Hovmöller diagrams of analyzed (observed) 850-hPa meridional wind component (m s−1) in the longitude band 15°E–35°W for (a) June–July 2001 along 20°N and (b) August–September 2001 along 10°N. Positive values (southerly wind component) are shown with solid lines and shading, and negative values (northerly wind component) are shown with broken lines. The propagation of AEWs is depicted by lines sloping from the upper right to the lower left and numbered consecutively for each month.

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 4.
Fig. 4.

Tracks of the AEWs depicted in the Hovmöller diagrams (Figs. 3a,b) and listed in Table 1: (a) June, (b) July, (c) August, and (d) September. The position is based on the location of the cyclonic vorticity center(s) associated with the wave. The mean monthly position of the equatorial trough (monsoon trough over land) is indicated by a broken line, and the mean monthly position of the 700-hPa AEJ is indicated by a solid arrow, both based on NCEP operational analyses.

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 5.
Fig. 5.

Observed 850-hPa vector wind and relative vorticity (10−5 s−1), along with observed 24-h precipitation (mm day−1), for wave 6 in July (precursor to Tropical Storm Barry) during the 5-day period 22–26 Jul 2001: (a) 1200 UTC 22 Jul 2001, (b) 1200 UTC 23 Jul 2001, (c) 1200 UTC 24 Jul 2001, (d) 1200 UTC 25 Jul 2001, and (e) 1200 UTC 26 Jul 2001.

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 6.
Fig. 6.

Same as in Fig. 5, but for wave 6 in August (precursor to Hurricane Erin) during the 5-day period 26–30 Aug 2001: (a) 1200 UTC 26 Aug 2001, (b) 1200 UTC 27 Aug 2001, (c) 1200 UTC 28 Aug 2001, (d) 1200 UTC 29 Aug 2001, and (e) 1200 UTC 30 Aug 2001.

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Fig. 7.
Fig. 7.

Same as in Fig. 5, but for wave 3 in September (precursor to Tropical Depression 9) during the 5-day period 9–13 Sep 2001: (a) 1200 UTC 9 Sep 2001, (b) 1200 UTC 10 Sep 2001, (c) 1200 UTC 11 Sep 2001, (d) 1200 UTC 12 Sep 2001, and (e) 1200 UTC 13 Sep 2001. Precipitation data were not available for 10–11 Sep 2001.

Citation: Monthly Weather Review 135, 12; 10.1175/2007MWR1996.1

Table 1.

AEWs tracked June–September 2001 (15°E–35°W).

Table 1.
Save
  • Beven, J. L., S. R. Stewart, M. B. Lawrence, L. A. Avila, J. L. Franklin, and R. J. Pasch, 2003: Annual summary: Atlantic hurricane season of 2001. Mon. Wea. Rev., 131 , 14541484.

    • Search Google Scholar
    • Export Citation
  • Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci., 29 , 7790.

  • Burpee, R. W., 1974: Characteristics of North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci., 31 , 15561570.

    • Search Google Scholar
    • Export Citation
  • Carlson, T. N., 1969: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97 , 716726.

    • Search Google Scholar
    • Export Citation
  • Duvel, J. P., 1990: Convection over tropical Africa and the Atlantic Ocean during northern summer. Part II: Modulation by easterly waves. Mon. Wea. Rev., 118 , 18551868.

    • Search Google Scholar
    • Export Citation
  • Nitta, T., and Y. Takayabu, 1985: Global analysis of the lower tropospheric disturbances in the tropics during the northern summer of the FGGE year. Part II: Regional characteristics of the disturbances. Pure Appl. Geophys., 123 , 272292.

    • Search Google Scholar
    • Export Citation
  • Pytharoulis, I., and C. Thorncroft, 1999: The low-level structure of African easterly waves in 1995. Mon. Wea. Rev., 127 , 22662280.

  • Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE. Mon. Wea. Rev., 105 , 317333.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., A. Hollingsworth, W. A. Heckley, and F. Delsol, 1988a: An evaluation of the performance of the ECMWF operational forecasting system in analyzing and forecasting tropical easterly wave disturbances over Africa and the tropical Atlantic. Mon. Wea. Rev., 116 , 824865.

    • Search Google Scholar
    • Export Citation
  • Reed, R. J., E. Klinker, and A. Hollingsworth, 1988b: The structure and characteristics of African easterly wave disturbances as determined from the ECMWF operational analysis/forecast system. Meteor. Atmos. Phys., 38 , 2233.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., and K. Hodges, 2001: African easterly wave variability and its relationship to Atlantic tropical cyclone activity. J. Climate, 14 , 11661179.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., and Coauthors, 2003: The JET2000 project: Aircraft observations of the African easterly jet and African easterly waves. Bull. Amer. Meteor. Soc., 84 , 337351.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    Meridional section along 9°W of the meridional wind variance (m2 s−2) for waves of the 2.9–4.3-day period for August–September 1985. Circled plus sign indicates the mean position of the African easterly jet (after Reed et al. 1988b).

  • Fig. 2.

    Percent of observed 850-hPa relative vorticity centers of magnitude greater than 4.0 × 10−5 in two latitude bands, 5°–14°N and 15°–24°N, distributed by month, June–September 2001, in the AEW region (5°S–30°N, 35°W–15°E).

  • Fig. 3.

    Hovmöller diagrams of analyzed (observed) 850-hPa meridional wind component (m s−1) in the longitude band 15°E–35°W for (a) June–July 2001 along 20°N and (b) August–September 2001 along 10°N. Positive values (southerly wind component) are shown with solid lines and shading, and negative values (northerly wind component) are shown with broken lines. The propagation of AEWs is depicted by lines sloping from the upper right to the lower left and numbered consecutively for each month.

  • Fig. 4.

    Tracks of the AEWs depicted in the Hovmöller diagrams (Figs. 3a,b) and listed in Table 1: (a) June, (b) July, (c) August, and (d) September. The position is based on the location of the cyclonic vorticity center(s) associated with the wave. The mean monthly position of the equatorial trough (monsoon trough over land) is indicated by a broken line, and the mean monthly position of the 700-hPa AEJ is indicated by a solid arrow, both based on NCEP operational analyses.

  • Fig. 5.

    Observed 850-hPa vector wind and relative vorticity (10−5 s−1), along with observed 24-h precipitation (mm day−1), for wave 6 in July (precursor to Tropical Storm Barry) during the 5-day period 22–26 Jul 2001: (a) 1200 UTC 22 Jul 2001, (b) 1200 UTC 23 Jul 2001, (c) 1200 UTC 24 Jul 2001, (d) 1200 UTC 25 Jul 2001, and (e) 1200 UTC 26 Jul 2001.

  • Fig. 6.

    Same as in Fig. 5, but for wave 6 in August (precursor to Hurricane Erin) during the 5-day period 26–30 Aug 2001: (a) 1200 UTC 26 Aug 2001, (b) 1200 UTC 27 Aug 2001, (c) 1200 UTC 28 Aug 2001, (d) 1200 UTC 29 Aug 2001, and (e) 1200 UTC 30 Aug 2001.

  • Fig. 7.

    Same as in Fig. 5, but for wave 3 in September (precursor to Tropical Depression 9) during the 5-day period 9–13 Sep 2001: (a) 1200 UTC 9 Sep 2001, (b) 1200 UTC 10 Sep 2001, (c) 1200 UTC 11 Sep 2001, (d) 1200 UTC 12 Sep 2001, and (e) 1200 UTC 13 Sep 2001. Precipitation data were not available for 10–11 Sep 2001.

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