1. Introduction
After African easterly waves (AEWs) were identified by Carlson (1969a, b), Burpee (1974) found that these waves propagate along two distinct tracks—one following the Saharan thermal low and the other in the rainy zone of West Africa. AEWs along the northern and southern tracks are denoted, respectively, as AEWns and AEWSs. These two tracks of AEWs were later confirmed by post–First Global Atmospheric Research Programme (GARP) Global Experiment (FGGE)] data (e.g., Reed et al. 1988; Lau and Lau 1990) and modern reanalyses (e.g., Pytharoulis and Thorncroft 1999; Fink et al. 2004). The northern track appears to be underneath the midtropospheric Saharan high, north of the African easterly jet, while the southern track lies along the south side of the African easterly jet. Reed et al. (1988) observed that the population of AEWns is likely larger than AEWSs, while Chen (2006) estimated that the ratio between populations of AEWn and AEWs is 2.5: 1. In addition, it has been shown that the genesis mechanisms and propagation properties of AEWns and AEWSs differ distinctly from each other (Chen 2006).
It was estimated by Landsea (1993) that approximately 60% of the tropical storms and moderate hurricanes, and over 80% of the intense hurricanes, in the North Atlantic are formed from AEWs. Using an automated tracking method to identify AEWs, Thorncroft and Hodges (2001) found that AEWSs make the largest contribution to the formation of North Atlantic tropical cyclones. However, based on the 2.5: 1 population ratio between AEWns and AEWSs, and some preliminary observations, Chen (2006) noted that the contribution of AEWns to tropical cyclogenesis might not be negligible. Furthermore, adopting Thorncroft and Hodges’ (2001) approach, Hopsch et al. (2007) showed that the conversion rate of AEWns to tropical cyclones is much lower than that of AEWSs. Therefore, the purpose of this study is to identify the contributions of tropical cyclogeneses from AEWns and AEWSs to the total number of tropical cyclones that develop from AEWs. Because the National Hurricane Center (NHC) operationally tracks AEWs (e.g., Pasch et al. 1998), this information could be useful to forecasters if the distinction between AEWns and AEWSs is made.
To accomplish this scientific goal, the manual backtracking approach used by Chen (2006) to trace AEWns and AEWss is adopted to track those transformed into tropical cyclones for the period of 1979–2006. This manual tracking method should help clarify the differences in AEW tracks that previous studies using automatic tracking had difficulties revealing (Chen 2006). Details of this approach are described, and an example of an application of this approach to an AEWn and AEWS is provided in section 2. Statistics and locations of tropical cyclones transformed from AEWns and AEWSs are presented in section 3, while changes that occur in AEWns to facilitate their transformation into tropical cyclones are discussed in section 4. Finally, concluding remarks are provided in section 5.
2. Data and case identification
Tropical cyclones recorded by the Atlantic Hurricane Dataset Reanalysis Project (Landsea et al. 2004) during July–October from 1979 to 2006 were analyzed. Using streamline charts and vorticity prepared from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) for 1979–2002 (Uppala et al. 2005) and the National Centers for Environmental Prediction (NCEP)/Department of Energy Global Reanalysis 2 for 2003–06 (Kanamitsu et al. 2002), the backtracking procedure of Chen (2006) is employed to track AEWs. Starting from a tropical cyclone’s formation location, the AEW from which the tropical cyclone formed is backtracked to its genesis location. Examples of a tropical cyclone developed from an AEWn and an AEWS are shown in Figs. 1 and 2, respectively. These two cases were chosen because they were found to be representative of typical AEWn and AEWS tropical cyclones.
Hurricane Bonnie (19–30 August 1998) originated from an AEWn, and the track of this hurricane and its parent AEWn are shown in Fig. 1a. On 19 August, this tropical cyclone develops ahead of the trough of an easterly wave at 600 mb south of the easterly jet in the subtropical Atlantic. This trough is coupled with a closed cyclonic vortex at 925 mb (Fig. 1c). The vertical structure of this easterly wave is illustrated by the longitude–height cross section of vorticity (ζ) and vorticity tendency (ζt) at 15°N (Fig. 1b). During the formation of Hurricane Bonnie, both ζ(15°N) and ζt(15°N) extend vertically beyond 400 mb, and the tropical cyclogenesis occurs where ζt is positive. It will be shown later that the vertical extent of this vortex is not different from that of an AEWS transforming into a hurricane. The AEWn that formed Hurricane Bonnie is backtracked to its genesis location in the vicinity of the Saharan thermal low, north of the 600-mb African easterly jet, as indicated by a closed cyclonic vortex at 925 mb (Fig. 1f). The low-level vortex is not coupled with any noticeable disturbance at 600 mb (Fig. 1d). Consistent with previous studies (e.g., Pytharoulis and Thorncroft 1999; Chen 2006), large ζ and ζt of this AEWn at 17.5°N only exist in the lower troposphere (Fig. 1e).
On 21 September 2000, Hurricane Isaac (21 September–1 October 2000) formed ahead of the trough of a 600-mb easterly wave (Fig. 2a), where ζt is positive (Fig. 2b). A 925-mb vortex just off the West African coast is coupled with this wave (Fig. 2c). Both the ζ and ζt centers of this vortex at 12.5°N extend vertically up to 400 mb. After backtracking to its genesis location on 13 September 2000, the wave appears to originate near 30°E south of the African easterly jet (Fig. 2d). Note that centers of ζ(12.5°N) and ζt(12.5°N) (Fig. 2e) only exist in the midtroposphere. During this genesis stage, there is no significant signal of this disturbance (Fig. 2f), as the description for AEWSs provided by Chen (2006).
3. Tropical cyclone formation from AEWs: Statistics and spatial distributions
Formation locations of tropical cyclones during the hurricane seasons (July–October) of 1979–2006 are shown in Fig. 3a. Two groups of tropical cyclones emerge: one is distributed along intertropical convergence zone (ITCZ), and the other covers the Caribbean Sea, the Gulf of Mexico, and the western North Atlantic region. Tropical cyclones in the ITCZ group are mostly transformed from AEWs, while a majority of those in the latter group are formed from other non-AEW processes (e.g., Lawrence et al. 1998). After classifying the AEW-transformed tropical cyclones into those originating from AEWSs (Fig. 3b) and AEWns (Fig. 3c), it is found that most of the AEWSs (AEWns) are clustered along the ITCZ east of 50°W (west of 30°W). In addition, it appears that AEWns contribute much more to the tropical cyclone formation in the western North Atlantic/Caribbean Sea than AEWSs.
Using a classification scheme similar to Landsea (1993) and Chen et al. (2006), North Atlantic tropical cyclones are categorized, based on the Saffir–Simpson scale (Simpson and Riehl 1981), into the following three groups (Fig. 4a): group 1, tropical depression, tropical storm, and category 1 tropical cyclones; group 2, categories 2–3 tropical cyclones; and group 3, categories 4–5 tropical cyclones. Note that this grouping of tropical cyclones is slightly different from that of Landsea (1993). Statistically, 58% of the total North Atlantic tropical cyclones are formed from AEWs, and of these 32% are contributed by AEWns and 26% by AEWSs (Fig. 4b). The population ratio of AEWSs and AEWns is 1: 2.5 (Chen 2006), and results in this study indicate that this ratio is 1: 1.2 for AEWs transformed into tropical cyclones. Thus, AEWSs form tropical cyclones twice as effectively as AEWns, although AEWns contribute more to tropical cyclone formation than AEWSs because of their larger population. Thorncroft and Hodges (2001) developed an automatic tracking approach with a threshold value of vorticity (≥0.5 × 10−5 s−1) to trace AEWS. This approach was expanded of late by Hopsch et al. (2007) to analyze ERA-40. Regardless of the exact conversion rate, these two studies found that AEWSs are more effective than AEWns in the conversion of AEWS to tropical cyclones. The effectiveness of AEWS in tropical cyclogenesis will be addressed further later in this section.
As shown in Figs. 4c–e, slightly less than one-half of tropical cyclones in group 1 and a greater majority of tropical cyclones in groups 2 and 3 are formed from AEWs. In group 1 (Fig. 4c), both AEWns and AEWSs appear to contribute comparably to the tropical cyclone formation. However, there is a more noticeable difference in contribution to tropical cyclone formation between AEWns and AEWSs in groups 2 and 3, with more group 2 (3) tropical cyclones developing from AEWns (AEWSs; Figs. 4d,e). In addition, the tendency for more tropical cyclones in the western North Atlantic/Caribbean Sea (west of 45°W) to develop from AEWns than AEWSs is clear (Fig. 5). Particularly in group 2 (Fig. 5b) and group 3 (Fig. 5c), the number of tropical cyclones that form west of 45°W originating from AEWns is over 3 times more than that from AEWSs.
To help understand why differences exist between the genesis locations of tropical cyclones developing from AEWns and AEWSs, a scatter diagram of daily mean precipitable water (W) versus outgoing longwave radiation (OLR), averaged over a 5° square centered at each AEW when it propagates through 17.5°W (near the coast of West Africa), is presented in Fig. 6a. Because OLR is a proxy of cumulus convection that is maintained by water vapor supply, this type of scatter diagram gives useful information regarding the differences in the amounts of moisture and convection associated with AEWns and AEWSs.
As indicated with statistical significance, scatters (W versus OLR) associated with AEWns and AEWSs shown in Fig. 6a form two clearly separated groups, indicating that AEWns are generally drier and weaker in convection than AEWSs as they move off the African continent. These results are generally consistent with the drier conditions in the vicinity of the Saharan thermal low where AEWns develop, relative to the wetter environment along the southern flank of the midtropospheric African easterly jet where AEWSs develop. Because tropical cyclogenesis is very dependent on the presence of cumulus convection (e.g., Kwon and Mak 1990), the drier and less convective AEWns should take longer than the more moist AEWSs to transform into tropical cyclones, explaining the tendency for tropical cyclogenesis associated with AEWns to occur further west than AEWSs. Further substantiating this inference, scatter diagrams of W versus a lifetime of AEW before tropical cyclogenesis (TAEW) and OLR versus TAEW (Figs. 6b,c, respectively) also have distinct groupings. The drier and less convective AEWn group has longer TAEW, by about 3 days, than the wetter and more convective AEWS group.
4. Budget analyses of AEWs: Water vapor and vorticity
To illustrate the differences in the evolution of AEWns and AEWSs as they cross the tropical Atlantic, daily mean values of various terms in the moisture and vorticity budgets are averaged over a 5° square area (indicated by []) centered on the AEWs at points along their trajectory across the ocean (Figs. 7b–d, f–h). Mean 925-mb streamlines superimposed with precipitable water (Fig. 7a), and mean 850-mb streamlines superimposed with vorticity (Fig. 7e) are also presented to show the average environmental conditions present along the AEW paths. Note the presence of the West African monsoon trough, which is associated with relatively large values of low-level moisture and vorticity, which are basic environmental factors conducive to tropical cyclogenesis (Gray 1968, 1979).
a. Water vapor budget
It is indicated in Fig. 7b that [W] of AEWns is relatively small in the vicinity of the Saharan thermal low, but, on average, doubles in magnitude by the time the waves have crossed 20°W. In contrast, [W] of AEWSs along the southern flank of the midtropospheric African easterly jet (indicated by blue dots in Fig. 7b) tends to be much larger than [W] of AEWns while in the African continent. AEWSs are moistened slightly during their westward propagation within the continent, and appear to moisten at a slightly faster rate just after crossing the western African coast. Evidently, the monsoon trough is an important source of water vapor to AEWs and, in particular, to AEWns, as they propagate into the North Atlantic.
According to the water vapor budget equation, the change in precipitable water is caused by the convergence/divergence of water vapor flux (∇ · Q, where Q is the vertically integrated water vapor flux from surface to 300 mb) and precipitation (P). Thus, the increase in [W] of AEWs should be followed by a corresponding increase in water vapor supply by [−∇ · Q] (Fig. 7c). While still within the African continent, [−∇ · Q] of AEWSs tends to be noticeably larger than that of AEWns, which is likely a result of the drier conditions in the regions AEWns propagate relative to those over which AEWSs propagate. However, because of the westward path of AEWSs and the southwestward path of AEWns on the African continent, these waves tend to be located in similar regions at approximately the time they reach 20°W (Figs. 3b,c). Furthermore, at 20°W AEWns and AEWSs are embedded within the West African monsoon trough, and [−∇ · Q] has a peak with similar magnitude in both waves, which confirms that this monsoon trough is an important water vapor source for AEWs.
In addition to [W], precipitation also changes in response to [−∇ · Q] variations. To depict rainfall associated with AEWs, ΔOLR (=235 W m−2 − OLR) is used as a proxy (Arkin and Ardanuy 1989). As shown in Fig. 7d, [ΔOLR] of AEWSs increases slightly after crossing the African coast, but [ΔOLR] of AEWns is almost quadrupled. Despite the differences while inside the African continent, [ΔOLR] of the two types of AEWs becomes similar over the North Atlantic west of 50°W.
b. Vorticity budget
It was demonstrated in Chen (2006), and shown in Figs. 1 and 2, that the maximum ζ values of AEWn and AEWS within the continent appear in the lower and middle troposphere, respectively. Despite this difference, after moving over the ocean, vorticity associated with these two types of AEWs eventually grows throughout a similar depth of the troposphere.
It is inferred from the increase in [−∇ · Q] as AEWs propagate across the coast of West Africa (indicated by a dashed line in every panel of Fig. 7) into the monsoon trough that vertical motion of these waves (Fig. 7f) and the corresponding convergence in the lower troposphere (not shown) also increases over this monsoon trough. It is thus expected that the dynamic process of vortex stretching also strengthens over the eastern tropical Atlantic (Fig. 7g), following the increases of vertical motion and convergence of water vapor flux of AEWs (Fig. 7c). A result of this dynamic process is a deepening of the AEW, as indicated by a significant increase in vorticity tendency (ζt) across the monsoon trough (Fig. 7h). Because of its abundant environmental moisture and low-level vorticity, this trough functions as a stimulus to facilitate the transformation of AEWs into tropical cyclones. These results echo findings by Gray and Landsea (1992) that the hurricane frequency fluctuates with western Sahel rainfall and the strength of the West African monsoon trough.
5. Concluding remarks
Using the manual backtracking approach of Chen (2006) to trace AEWs that originate along the southern flank of the midtropospheric African easterly jet (designated as AEWS) and over the Saharan thermal low north of this jet (designated as AEWn), North Atlantic tropical cyclones formed by these two types of waves were identified. This manual backtracking approach provides an alternative to objective approaches such as the automated tracking approach utilized by Thorncroft and Hodges (2001). The data used for this study include the Atlantic Hurricane Database Reanalysis Project (Landsea et al. 2004) for 1979–2006, ERA-40 for 1979–2002, and NCEP/DOE Global Reanalyses for 2003–06. Major findings of our analyses are as follows:
Approximately 58% of the tropical cyclones in North Atlantic were formed from AEWs, which is very similar to Landsea’s (1993) estimate. Of these tropical cyclones formed from AEWs, approximately 32% and 26% were formed from AEWns and AEWSs, respectively. Because the population ratio of these two types of AEWS is 2.5:1 (Chen 2006), the conversion rate from AEWSs to tropical cyclones is twice as effective as AEWns.
Because AEWns tend to be less moist than AEWSs, they tend to take longer and travel farther westward across the tropical North Atlantic than AEWSs, before developing into tropical cyclones. Thus, AEWns tend to contribute more to tropical cyclogeneses in the western North Atlantic/Caribbean Sea than AEWSs.
The West African monsoon trough supplies both types of AEWs with moisture and enhances low-level vorticity. This monsoon trough is particularly important in supplying AEWns with the moisture necessary to gain similar properties as AEWSs, and eventually become tropical cyclones.
Findings of this study substantiate the contribution of AEWns to the North Atlantic tropical cyclone formation, and, in addition, offer a different perspective to explore possible deficiencies in numerical forecasts of North Atlantic tropical cyclogenesis. In particular, numerical forecasts of the characteristics of AEWns and the role played by the West African monsoon trough on the development of AEWs could be examined.
Acknowledgments
This study was initiated during our previous research effort in search of the maintenance mechanism of the African monsoon circulation and the activity of African easterly waves. That effort was originally supported by the NSF Grant ATM 0136220. This study was finalized through the partial support from the Cheney Research Foundation. We would also like to thank two anonymous reviewers for their constructive comments to enrich our discussion in this paper.
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Streamlines at (a) 600 and (c) 925 mb superimposed with zonal wind speed (shadings), and (b) longitude–height cross section of vorticity (ζ; contours) and vorticity tendency (ζt; shadings) at 12.5°N on 1200 UTC 19 Sep 1998, the approximate time at which Hurricane Bonnie formed. (e) The same as (b), but the cross section is at 17.5°N at 1200 UTC 10 Aug 1998. The trajectory of Hurricane Bonnie is denoted by thick lines in (a) and (c) marked with hurricane symbols at its 0000 UTC positions. (d), (f) Same as (a) and (c), but valid at 1200 UTC 10 Aug 1998, the approximate time at which the AEW that became Hurricane Bonnie formed. The AEW track is depicted by a thick line marking the 0000 UTC position of AEW’s trough, and the thick cross denotes its location at the time (d) and (e) are valid.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Streamlines at (a) 600 and (c) 925 mb superimposed with zonal wind speed (shadings), and (b) longitude–height cross section of vorticity (ζ; contours) and vorticity tendency (ζt; shadings) at 12.5°N on 1200 UTC 19 Sep 1998, the approximate time at which Hurricane Bonnie formed. (e) The same as (b), but the cross section is at 17.5°N at 1200 UTC 10 Aug 1998. The trajectory of Hurricane Bonnie is denoted by thick lines in (a) and (c) marked with hurricane symbols at its 0000 UTC positions. (d), (f) Same as (a) and (c), but valid at 1200 UTC 10 Aug 1998, the approximate time at which the AEW that became Hurricane Bonnie formed. The AEW track is depicted by a thick line marking the 0000 UTC position of AEW’s trough, and the thick cross denotes its location at the time (d) and (e) are valid.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Streamlines at (a) 600 and (c) 925 mb superimposed with zonal wind speed (shadings), and (b) longitude–height cross section of vorticity (ζ; contours) and vorticity tendency (ζt; shadings) at 12.5°N on 1200 UTC 19 Sep 1998, the approximate time at which Hurricane Bonnie formed. (e) The same as (b), but the cross section is at 17.5°N at 1200 UTC 10 Aug 1998. The trajectory of Hurricane Bonnie is denoted by thick lines in (a) and (c) marked with hurricane symbols at its 0000 UTC positions. (d), (f) Same as (a) and (c), but valid at 1200 UTC 10 Aug 1998, the approximate time at which the AEW that became Hurricane Bonnie formed. The AEW track is depicted by a thick line marking the 0000 UTC position of AEW’s trough, and the thick cross denotes its location at the time (d) and (e) are valid.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Same as Fig. 1, but for the formations of Hurricane Isaac at 1200 UTC 21 Sep 2000 and its parent AEW on 13 Sep 2000.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Same as Fig. 1, but for the formations of Hurricane Isaac at 1200 UTC 21 Sep 2000 and its parent AEW on 13 Sep 2000.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Same as Fig. 1, but for the formations of Hurricane Isaac at 1200 UTC 21 Sep 2000 and its parent AEW on 13 Sep 2000.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean 925-mb streamlines of July–September superimposed with formation locations of tropical cyclones originated from (a) AEW- (red dots) and non-AEW-related processes (light blue dots), (b) AEWns (blue triangles), and (c) AEWSs (red triangles) during the hurricane seasons of 1979–2006. Genesis locations of AEWSs and AEWns are marked by blue and red open triangles in (b) and (c), respectively, superimposed with their trajectories (dashed lines) before transforming into tropical cyclones. The 600-mb easterly jet core (with isotach ≤−7 m s−1) is depicted with a thick solid line in all panels.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean 925-mb streamlines of July–September superimposed with formation locations of tropical cyclones originated from (a) AEW- (red dots) and non-AEW-related processes (light blue dots), (b) AEWns (blue triangles), and (c) AEWSs (red triangles) during the hurricane seasons of 1979–2006. Genesis locations of AEWSs and AEWns are marked by blue and red open triangles in (b) and (c), respectively, superimposed with their trajectories (dashed lines) before transforming into tropical cyclones. The 600-mb easterly jet core (with isotach ≤−7 m s−1) is depicted with a thick solid line in all panels.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean 925-mb streamlines of July–September superimposed with formation locations of tropical cyclones originated from (a) AEW- (red dots) and non-AEW-related processes (light blue dots), (b) AEWns (blue triangles), and (c) AEWSs (red triangles) during the hurricane seasons of 1979–2006. Genesis locations of AEWSs and AEWns are marked by blue and red open triangles in (b) and (c), respectively, superimposed with their trajectories (dashed lines) before transforming into tropical cyclones. The 600-mb easterly jet core (with isotach ≤−7 m s−1) is depicted with a thick solid line in all panels.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
(a) Occurrence frequency of total (NTC; light gray) and AEW-transformed (NAEW→TC; dark gray) North Atlantic tropical cyclones (per season) in terms of maximum tangential wind speed. The Saffir–Simpson scale of tropical cyclones is marked at the top of (a), where C indicates category. The three groups of tropical cyclones are numbered in each panel. (b) Histogram of average number (NTC; per season) of total North Atlantic tropical cyclones (Total; gray) and those originated from the AEW (dark gray), AEWn (dark), and AEWS (black). The percentage of the total population is provided on the top of each bar. (c)–(e) Same as (b), but for groups 1–3 tropical cyclones, respectively.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
(a) Occurrence frequency of total (NTC; light gray) and AEW-transformed (NAEW→TC; dark gray) North Atlantic tropical cyclones (per season) in terms of maximum tangential wind speed. The Saffir–Simpson scale of tropical cyclones is marked at the top of (a), where C indicates category. The three groups of tropical cyclones are numbered in each panel. (b) Histogram of average number (NTC; per season) of total North Atlantic tropical cyclones (Total; gray) and those originated from the AEW (dark gray), AEWn (dark), and AEWS (black). The percentage of the total population is provided on the top of each bar. (c)–(e) Same as (b), but for groups 1–3 tropical cyclones, respectively.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
(a) Occurrence frequency of total (NTC; light gray) and AEW-transformed (NAEW→TC; dark gray) North Atlantic tropical cyclones (per season) in terms of maximum tangential wind speed. The Saffir–Simpson scale of tropical cyclones is marked at the top of (a), where C indicates category. The three groups of tropical cyclones are numbered in each panel. (b) Histogram of average number (NTC; per season) of total North Atlantic tropical cyclones (Total; gray) and those originated from the AEW (dark gray), AEWn (dark), and AEWS (black). The percentage of the total population is provided on the top of each bar. (c)–(e) Same as (b), but for groups 1–3 tropical cyclones, respectively.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Formation locations of group (a) 1, (b) 2, and (c) 3 tropical cyclones formed from AEWns (red dots) and AEWSs (blue triangles). Genesis locations of AEWns and AEWSs are marked by red open dots and blue open triangles, respectively, while their trajectories are depicted by dashed lines in corresponding color. Numbers of tropical cyclones formed west of 45°W (dashed line) originated from AEWns (N) and AEWSs (S) are given at the right of each panel.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Formation locations of group (a) 1, (b) 2, and (c) 3 tropical cyclones formed from AEWns (red dots) and AEWSs (blue triangles). Genesis locations of AEWns and AEWSs are marked by red open dots and blue open triangles, respectively, while their trajectories are depicted by dashed lines in corresponding color. Numbers of tropical cyclones formed west of 45°W (dashed line) originated from AEWns (N) and AEWSs (S) are given at the right of each panel.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Formation locations of group (a) 1, (b) 2, and (c) 3 tropical cyclones formed from AEWns (red dots) and AEWSs (blue triangles). Genesis locations of AEWns and AEWSs are marked by red open dots and blue open triangles, respectively, while their trajectories are depicted by dashed lines in corresponding color. Numbers of tropical cyclones formed west of 45°W (dashed line) originated from AEWns (N) and AEWSs (S) are given at the right of each panel.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Scatter diagrams of (a) OLR vs precipitable water (W), (b) OLR vs the lifetime of AEWs (TAEW), and (c) W vs TAEW. Values of OLR and W are averaged over a 5° square area centered at each AEW when it propagates across 17.5°W. TAEW is the period of time (in 6-h interval) between the genesis of an AEW and the formation of its related tropical cyclone. Scatters are grouped into AEWn (solid dots) and AEWS (open circles) with their mean position marked by a star. Oblongs cover the 95% confidence level of each group. A separation line of bivariate normal random variables (von Storch and Zwiers 1999) is added between the two groups. Probability of misclassification is given next to the separation line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Scatter diagrams of (a) OLR vs precipitable water (W), (b) OLR vs the lifetime of AEWs (TAEW), and (c) W vs TAEW. Values of OLR and W are averaged over a 5° square area centered at each AEW when it propagates across 17.5°W. TAEW is the period of time (in 6-h interval) between the genesis of an AEW and the formation of its related tropical cyclone. Scatters are grouped into AEWn (solid dots) and AEWS (open circles) with their mean position marked by a star. Oblongs cover the 95% confidence level of each group. A separation line of bivariate normal random variables (von Storch and Zwiers 1999) is added between the two groups. Probability of misclassification is given next to the separation line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Scatter diagrams of (a) OLR vs precipitable water (W), (b) OLR vs the lifetime of AEWs (TAEW), and (c) W vs TAEW. Values of OLR and W are averaged over a 5° square area centered at each AEW when it propagates across 17.5°W. TAEW is the period of time (in 6-h interval) between the genesis of an AEW and the formation of its related tropical cyclone. Scatters are grouped into AEWn (solid dots) and AEWS (open circles) with their mean position marked by a star. Oblongs cover the 95% confidence level of each group. A separation line of bivariate normal random variables (von Storch and Zwiers 1999) is added between the two groups. Probability of misclassification is given next to the separation line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean (a) 925-mb streamlines superimposed with precipitable water (W) and (e) 850-mb streamlines with vorticity (ζ), and (b) daily values of W averaged over a 5° square area centered at the daily mean locations of each AEWn (red dots) and AEWS (blue dots) that eventually transformed into tropical cyclones. (c), (d), (f), (g), and (h) Same as (b), but for convergence of water vapor flux (−∇ · Q), ΔOLR [=235 (W m−2) − OLR], 850-mb vertical velocity (ω), 850-mb vortex stretching (−f ∇ · V), and 850-mb vorticity tendency (ζt), respectively. The mean distributions of these values with respect to the longitudinal location for AEWns and AEWSs are indicated by solid red (blue) lines. The longitudinal location of the coast of West Africa is indicated by a dashed line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean (a) 925-mb streamlines superimposed with precipitable water (W) and (e) 850-mb streamlines with vorticity (ζ), and (b) daily values of W averaged over a 5° square area centered at the daily mean locations of each AEWn (red dots) and AEWS (blue dots) that eventually transformed into tropical cyclones. (c), (d), (f), (g), and (h) Same as (b), but for convergence of water vapor flux (−∇ · Q), ΔOLR [=235 (W m−2) − OLR], 850-mb vertical velocity (ω), 850-mb vortex stretching (−f ∇ · V), and 850-mb vorticity tendency (ζt), respectively. The mean distributions of these values with respect to the longitudinal location for AEWns and AEWSs are indicated by solid red (blue) lines. The longitudinal location of the coast of West Africa is indicated by a dashed line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
Mean (a) 925-mb streamlines superimposed with precipitable water (W) and (e) 850-mb streamlines with vorticity (ζ), and (b) daily values of W averaged over a 5° square area centered at the daily mean locations of each AEWn (red dots) and AEWS (blue dots) that eventually transformed into tropical cyclones. (c), (d), (f), (g), and (h) Same as (b), but for convergence of water vapor flux (−∇ · Q), ΔOLR [=235 (W m−2) − OLR], 850-mb vertical velocity (ω), 850-mb vortex stretching (−f ∇ · V), and 850-mb vorticity tendency (ζt), respectively. The mean distributions of these values with respect to the longitudinal location for AEWns and AEWSs are indicated by solid red (blue) lines. The longitudinal location of the coast of West Africa is indicated by a dashed line.
Citation: Journal of Climate 21, 24; 10.1175/2008JCLI2523.1
