The Effects of Gap-Wind-Induced Vorticity, the Monsoon Trough, and the ITCZ on East Paciﬁc Tropical Cyclogenesis

1312 MONTHLY WEATHER REVIEW VOLUME 142

HEATHER M. HOLBACH AND MARK A. BOURASSA Department of Earth, Ocean, and Atmospheric Science and Center for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

(Manuscript received 8 July 2013, in ﬁnal form 26 November 2013)

ABSTRACT

Tropical cyclogenesis in the eastern North Paciﬁc (EPAC) basin is related to gap-wind-induced surface relative vorticity, the monsoon trough, and the intertropical convergence zone (ITCZ). There are several gaps in the Central American mountains, on the eastern edge of the EPAC basin, through which wind can be funneled to generate surface wind jets (gap winds). This study focuses on gap winds that occur over the Gulf of Papagayo and Gulf of Tehuantepec. Quick Scatterometer (QuikSCAT) 10-m equivalent neutral winds are used to identify gap wind events that occur during May through November of 2002–08. Dvorak ﬁx locations, Gridded Satellite (GridSat) infrared (IR) data, and National Hurricane Center (NHC) tropical cyclone (TC) reports are used to track the disturbances during the study period. Surface vorticity is tracked using the QuikSCAT winds and the contribution of surface vorticity from the gap winds to the development of each disturbance is categorized as small, medium, or large. Cross-calibrated multiplatform surface wind data are used to verify the tracking of QuikSCAT-computed surface vorticity and to identify when the monsoon trough and the ITCZ are present. It is found that gap winds are present over the Gulf of Papagayo and Gulf of Tehuantepec for about 50% of the QuikSCAT coverage days and that these gap winds appear to contribute to the development of disturbances in the EPAC. Considerably more TCs form when the monsoon trough is present versus the ITCZ and the majority of the contributions from the gap winds also occur when the monsoon trough is present.

1. Introduction Montgomery 2001). ‘‘Top down’’ theories (Ritchie and Holland 1997; Simpson et al. 1997) also exist. According The circumstances under which a tropical cyclone to these theories, mesoscale convective vortices produce (TC) forms are still a highly debated and not well- a midlevel cyclonic vorticity maximum that enhances understood topic within the ﬁeld of meteorology. Many the low-level cyclonic vorticity. Although none of these theories exist for tropical cyclone formation, such as theories can completely explain tropical cyclogenesis, conditional instability of the second kind (CISK; Charney they all propose that there must be an initial region of and Eliassen 1964; Ooyama 1964) and wind-induced low-level, or surface, cyclonic vorticity present. An in- surface heat exchange (WISHE; Emanuel 1986; Emanuel creased understanding of tropical cyclogenesis would et al. 1994). In CISK and WISHE, which are both ‘‘bot- allow forecasters to more accurately predict which dis- tom up’’ theories, the feedback loop begins at the surface. turbances will develop into TCs and improve the general Other bottom-up theories involve low-level cyclonic rel- knowledge about TCs. ative vorticity (hereafter simply referred to as vorticity) This study focuses on tropical cyclogenesis in the maxima, induced by convection, that merge over time eastern North Paciﬁc (EPAC) basin. This basin is unique to create a larger low-level cyclonic vorticity maxi- in that it has the highest density of TC formation of all of mum (Montgomery and Enagonio 1998; Enagonio and the tropical cyclone basins (Renard and Bowman 1976). It has been suggested that most of the TCs that form in the EPAC originate from tropical disturbances formed in Corresponding author address: Heather M. Holbach, Center for the Atlantic basin or from easterly waves that traverse Ocean–Atmospheric Prediction Studies, The Florida State Uni- versity, 2000 Levy Ave., Building A Suite 292, Tallahassee, FL the Atlantic Ocean (Avila 1991; Avila and Pasch 1992; 32306. Shapiro 1986). In this model, some of the disturbances or E-mail: [email protected] easterly waves that do not generate TCs in the Atlantic

DOI: 10.1175/MWR-D-13-00218.1

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1997; Zehnder and Powell 1999). These studies found that cyclonic surface vorticity occurring in the presence of an easterly wave, the ITCZ, or the monsoon trough could lead to the development of a TC. Reanalysis data also used to complete another case study showed enhanced low-level flow through the Isthmus of Tehuantepec that occurred ahead of a 700-mb African easterly wave (Molinari et al. 2000). However, without any observa- tional data available for verification, Molinari et al. could only hypothesize that when the 700-mb wave moved westward into the EPAC, it coupled with the cyclonic surface vorticity generated by the wind jet through the Isthmus of Tehuantepec to initiate cyclogenesis. These previous studies focused on the gap winds generated through the Isthmus of Tehuantepec and did not consider how gap winds generated over the Gulf of Papagayo might contribute to tropical cyclogenesis in the EPAC. The data to confirm the theories from the previous studies were also limited, mainly to ship and buoy observations, prior to the launch of the Quick Scatterometer (QuikSCAT) in 1999. The QuikSCAT remotely sensed wind data tremendously increased the spatial and temporal coverage of surface winds over the FIG. 1. Topographical map depicting the mountain gaps in the Central American mountains (Chelton et al. 2000). Red circles oceans. There are two objectives for this study: 1) to use indicate the mountain gaps and the black arrows indicate the di- QuikSCAT wind data to determine how gap-wind- rection of the flow through the gaps of interest to this study. induced surface vorticity over the Gulf of Papagayo and Gulf of Tehuantepec contribute to tropical cyclogenesis in the EPAC and 2) to determine how the ITCZ and the propagate into the EPAC. However, there is another monsoon trough interact with the gap winds to influence unique feature to this basin, which is the presence of the tropical cyclogenesis in the EPAC. Central American mountains on the western edge of the Two case studies, Hurricane Celia (2004) and Hurri- basin. This mountain range has peak elevations of 2000– cane John (2006), are used to illustrate and discuss the 3000m and several mountain gaps (Fig. 1) where the methods and the results of the study. Section 2 discusses elevation reaches only about 250 m (Steenburgh et al. the data and the process for identifying gap winds and 1998). This study aims to determine how surface vor- summarizes the gap wind events that occurred from May ticity generated by winds funneled through the gaps in to November of 2002–08. Next, the process for tracking the Central American mountains, creating a surface wind the disturbances is presented in section 3, followed by jet (gap winds) over the Gulf of Papagayo and the Gulf of a description of the method for calculating and tracking Tehuantepec, influences the development of TCs in the the propagation of surface vorticity (section 4). In sec- EPAC. tion 5, the propagation of surface vorticity seen in Many of the previous studies related to this topic have QuikSCAT is verified using cross-calibrated multiplat- investigated how the Central American mountains in- form (CCMP) wind data. A description of gap wind con- teract with African easterly waves, the intertropical contribution categories and a summary of the contributions vergence zone (ITCZ), and the monsoon trough. Studies follow in section 6. Section 7 contains a comparison using computer models found that flow incident on ter- between the occurrence of gap wind contributions, the rain similar to the Central American mountains is monsoon trough, and the ITCZ. The conclusions are deflected around the mountains and through the gaps, given in section 8. generating cyclonic surface vorticity in the lee of the mountains (Mozer and Zehnder 1996; Zehnder 1991; 2. Identifying gap wind events Zehnder and Gall 1991). Other studies have used a combination of computer models and reanalysis data to The Jet Propulsion Laboratory’s (JPL’s) 12.5-km look at specific TCs to determine how the mountains version 3 QuikSCAT wind product (Fore et al. 2014) is influenced their development (Farfan and Zehnder used to analyze surface wind and vorticity fields over the

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FIG. 2. Example of daily coverage by QuikSCAT.

EPAC. This dataset includes an improved algorithm of Papagayo that create a surface wind jet with a fanlike for retrieving wind measurements in rain-contaminated pattern. A method for automating the detection of the gap regions as well as in rain-free areas, which greatly reduces wind events is developed in which thresholds for spatial the noise in the surface vorticity fields. SeaWinds on coverage, wind speed, and wind direction must be met. The QuikSCAT was a Ku-band scatterometer that pro- thresholds are maximized for each gulf to minimize the vided surface vector wind data from 19 July 1999 to 23 false detection of nongap wind events such as the presence November 2009. It had an 1800-km-wide, single swath of rain-contaminated vectors. For the Gulf of Tehuantepec, and a repeat period of approximately 4 days (Huddleston the median wind direction must fall between 3108 and 608 and Spencer 2001), providing approximately twice-daily (meteorological convention). The median wind direction coverage at a given location (Fig. 2). The actual sampling for the Gulf of Papagayo must fall between 188 and 1108. pattern is less regular (Schlax et al. 2001), particularly at The peak wind speed for each of these gap wind events is the latitudes of the Central American gaps. Data in the then determined by identifying the 90th percentile wind wind product include time, latitude, longitude, surface speed out of all of the values in the specified region for the wind speed (10-m equivalent neutral; Kara et al. 2008), given overpass. The median wind speed threshold for the 2 wind direction, and a rain flag. Gulf of Tehuantepec is 3.9 m s 1 and for the Gulf of 2 To investigate the impact of the gap winds on tropical Papagayo the threshold is 4.0 m s 1. After the automated cyclogenesis, it is first necessary to identify when the gap detection process is completed, a visual inspection is per- wind events occurred. For gap winds to be present there formed to eliminate the remaining nongap wind events. must be enhanced northerly winds over the Gulf of The distribution of the peak wind speed of the gap Tehuantepec or enhanced easterly winds over the Gulf winds for each gulf (Fig. 3) is used to categorize the gap

FIG. 3. Distribution of gap wind event peak wind speeds (black line) for (a) the Gulf of Papagayo and (b) the Gulf of Tehuantepec. The green line indicates the 25th percentile wind speed, the red line indicates the median wind speed, and the blue line indicates the 75th percentile wind speed.

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FIG. 4. Examples of (a) a weak and (b) a moderate gap wind event over the Gulf of Papagayo and (c) a strong and (d) a very strong gap wind event over the Gulf of Tehuantepec. wind events. Figure 3 shows that there is a larger spread for the period of 2002–08. This table shows that the gap in the peak wind speeds of the Gulf of Tehuantepec gap winds are present over the Gulf of Papagayo for about winds than in the Gulf of Papagayo gap winds. Also, the 53% of the QuikSCAT coverage days and over the Gulf Gulf of Tehuantepec gap winds are typically stronger of Tehuantepec for about 49% of the QuikSCAT cov- than the Gulf of Papagayo gap winds. The median peak erage days. Previous studies have developed climatol- gap wind speed for the Gulf of Tehuantepec is approx- ogies using 25- and 12.5-km JPL v2 QuikSCAT winds 2 imately 11.7 m s 1 whereas the median peak gap wind (Brennan et al. 2010; Cobb et al. 2002) for gale-force 2 2 speed for the Gulf of Papagayo is approximately 8.9 m s 1. (34 kt or ;17.5 m s 1) or stronger gap wind events for There are four categories (weak, moderate, strong, and very strong; Fig. 4) of gap wind strengths determined for each gulf that contain 25% of the gap wind events. The TABLE 1. Wind speed threshold values of weak, moderate, strong, category thresholds are given in Table 1. and very strong gap wind events for the Gulf of Papagayo and Gulf of Tehuantepec. QuikSCAT coverage exists for the Gulf of Papagayo for about 86% or 1290 of the 1498 days in this study Gap wind Gulf of Gulf of whereas the QuikSCAT coverage of the Gulf of Te- strength Papagayo Tehuantepec 2 2 huantepec is signiﬁcantly better at about 97% (1446 days). Weak 4.5 # U , 7.9 m s 1 4.6 # U , 9.7 m s 1 2 2 Table 2 summarizes the number of gap wind events Moderate 7.9 # U , 8.9 m s 1 9.7 # U , 11.7 m s 1 21 21 that occurred over the Gulf of Papagayo and the Gulf Strong 8.9 # U , 10.1 m s 11.7 # U , 14.1 m s $ 21 $ 21 of Tehuantepec during the months of May–November Very strong U 10.1 m s U 14.1 m s

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TABLE 2. Summary of the total number of gap wind events for TC reports from the NHC (www.nhc.noaa.gov/pastall. May–November of 2002–08. The columns give the number of days shtml#tcr; Avila et al. 2006; Pasch et al. 2009). The TC that gap winds are present for each year for both gulfs and the total reports contain information on the development of the number of days (percentage of days) having QuikSCAT coverage for each Gulf. TCs along with the times at which any sort of disturbance, such as an easterly wave, may have crossed the Year Gulf of Papagayo Gulf of Tehuantepec Central American mountains and led to the develop- 2002 116 105 ment of the storm. 2003 108 98 With the information provided by the Dvorak Fix 2004 114 101 Archive and the NHC TC reports, it is possible to track 2005 77 106 2006 112 106 the disturbances between QuikSCAT overpasses using 2007 74 102 Gridded Satellite (GridSat) infrared (IR) data to track 2008 81 88 the cloud clusters. The GridSat IR data, a composite Total gap wind days 682 706 product of IR data from global geostationary satellites Total coverage days 1290 (86%) 1446 (97%) (Knapp et al. 2011), are provided by the National Cli- matic Data Center (NCDC). The dataset has 8-km spatial grid spacing and 3-h temporal spacing. Tracking the Gulf of Tehuantepec, events that primarily occur the cloud clusters allows for the timing and location of during the winter months. Therefore, the summary of the disturbance to be confirmed in the QuikSCAT sur- gap wind events developed for this study provides in- face vorticity plots. Using the tracks of the disturbances, sight into the summer gap winds in this region. the TC reports, and the GridSat IR data make it possible to locate and track the precursor disturbances with a good degree of confidence. 3. Track disturbances The GridSat IR imagery shows the propagation of Storm-track information for this study is obtained the disturbances that developed into Hurricane Celia from two sources. Dvorak fix locations, based on the (Fig. 6) and Hurricane John (Fig. 7). Hurricane Celia Dvorak (1984) technique, for all named tropical cy- was classified as a tropical depression at 0000 UTC clones, disturbances, and invests for each hurricane 19 July 2004. Using the Dvorak fix locations and the season in the EPAC basin from 2002 to 2008, are re- GridSat IR imagery, the disturbance that developed trieved from the Dvorak Fix Archive (Cossuth et al. into Celia is tracked back to Central America where it 2013). Unique system ID code, tropical cyclone name, emerged in the EPAC on 11 July 2004. Hurricane John fix latitude, fix longitude, fix time, hours prior to cyclo- was classified by the NHC as a tropical depression at genesis, and Dvorak current intensity number (Dvorak 0000 UTC 28 August 2006. Dvorak fix locations begin at 1984) are among the variables included in these data. 0000 UTC 25 August 2006. Examination of the GridSat Dvorak fix locations are often available prior to classi- IR imagery reveals that the disturbance that developed fication by the National Hurricane Center (NHC), into John entered the EPAC on 24 August 2006. which aids in tracking the disturbances. The Dvorak fix After determining the time that the disturbances are locations for Hurricane Celia and Hurricane John are near each of the gaps, it is possible to observe whether given in Fig. 5. The second source of track information is gap winds are present. For cases in which there is limited

FIG. 5. Dvorak ﬁx locations for (a) Hurricane Celia beginning at 0000 UTC 18 Jul 2004, and (b) Hurricane John beginning at 0000 UTC 25 Aug 2006. The L indicates the location where each storm became a depression. Locations where they became tropical storms and hurricanes are denoted by their respective symbols.

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FIG. 6. GridSat IR imagery showing the progression of the disturbance that develops into Hurricane Celia. Yellow circles indicate the location of the disturbances at each time shown. Red dots indicate the Dvorak fix location of the disturbances for times that fix locations exist. The spatial region for each panel is 58S–308N, 608–1308W. coverage by QuikSCAT, CCMP wind data (discussed in Also, there are no gap winds present over the Gulf of section 5) is used to determine if there was a gap wind Tehuantepec as John passed by on 27 August 2006. event present. An overpass by QuikSCAT at 1100 UTC 11 July 2004 (Fig. 8), as Hurricane Celia was entering the 4. Vorticity EPAC near the Gulf of Papagayo, reveals that strong gap winds are present over the Gulf of Papagayo. There For this study we ignore the diabatic and baroclinic is also an overpass by QuikSCAT at 1200 UTC 13 July vorticity generation mechanisms and focus on the adi- 2004 (Fig. 8) when Celia’s disturbance is to the south of abatic and barotropic mechanisms through which the the Gulf of Tehuantepec. This overpass shows the pres- gap winds generate surface vorticity. There are three ence of very strong gap winds over the Gulf of Tehuan- general mechanisms (Fig. 9) by which the structure of tepec. Rain contamination is present to the south of the the gap winds can generate surface vorticity. The first Tehuantepec gap wind jet, which is identified by the mechanism is through horizontal shear. The strength of presence of the very strong across-swath wind vectors. the gap winds and their fanlike pattern create a shear in This rain contamination is one of the drawbacks to using the surface wind field. For the Northern Hemisphere QuikSCAT winds and must be considered when ana- (NH), if the jet is oriented as shown in Fig. 9, positive or lyzing the surface vorticity computed from the wind cyclonic vorticity is generated on the eastern side of the vectors. For Hurricane John, an overpass by QuikSCAT jet and negative or anticyclonic vorticity is created on at 1200 UTC 24 August 2006 (Fig. 8) shows that mod- the western side. The second way that the gap winds can erate gap winds are present over the Gulf of Papagayo generate surface vorticity is through the addition of and very strong westerly winds are present to the south. curvature in the jet. In the NH, if the jet curves to the In section 7, evidence is provided that these westerly east as in Fig. 9, the positive vorticity on the eastern side winds are due to the presence of the monsoon trough. of the jet is enhanced while the negative vorticity on the

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FIG. 7. As in Fig. 6, but for Hurricane John. western side is slightly reduced. Finally, the presence of as average vorticity) are created from the QuikSCAT westerly winds due to the monsoon trough south of the winds. The average vorticity calculation is adapted from gap wind jet can also enhance the positive vorticity on the work of Bourassa and McBeth-Ford (2010). This the southern side of the jet in the NH as displayed in calculation uses the circulation theorem to compute the Fig. 9. If the jet is also curved while the monsoon trough circulation around a ‘‘shape,’’ which is then divided by is present, this vorticity can be further enhanced. the area of the shape to give the average vorticity at the Once the gap wind events are identiﬁed, plots of area- center of the shape. The shape used for the area aver- averaged surface relative vorticity (hereafter referred to aging is dependent upon the diameter chosen, which is

FIG. 8. Plots of wind vectors displaying the gap wind events that inﬂuenced the development of (a),(b) Hurricane Celia at 12.5-km grid spacing and (c) Hurricane John at 25-km grid spacing. The black line in (c) indicates the track of Hurricane John.

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FIG. 9. Vorticity mechanisms for the gap wind jets. Black arrows indicate wind vectors. Red circular arrows with a plus sign in the middle indicate regions of positive vorticity and blue circular arrows with a negative sign in the middle indicate regions of negative vorticity for the NH. a multiple of the distance between adjacent wind vectors used a linear interpolation. Also, if there are too many (12.5 km). A shape with a diameter of 12.5 km will be points (.20%; Bourassa and McBeth-Ford 2010) miss- a square and a shape with a diameter of 25 km will be ing around the circumference of the shape, the average a diamond. When considering shapes with diameters vorticity is set to missing. equal to an odd number of adjacent wind vector cells Average vorticity calculated from the QuikSCAT (12.5, 37.5, 62.5 km, ...) or an even number (25, 50, overpass of Hurricane Celia’s disturbance at 1100 UTC 75 km, ...), the shape will become more circular as the 11 July 2004 shows that the gap winds over the Gulf of diameter increases (Fig. 10). As the shape becomes Papagayo generate a region of positive surface vorticity more circular the random error is reduced in the average that is subsequently tracked in the QuikSCAT over- vorticity calculation. A diameter of 50 km is used in this passes (Fig. 11). Even with the presence of rain con- study to decrease the amount of noise present in the tamination near the Tehuantepec gap wind jet at 1200 average vorticity plots while still allowing the vorticity to UTC 13 July 2004, positive surface vorticity generated be tracked. In calculating the circulation about the by the gap winds is observed (Fig. 11). This positive shape, a spline ﬁt is used to interpolate between adjacent surface vorticity created by the Tehuantepec gap winds good wind vectors. This is a slight improvement to the appears to then propagate toward the disturbance and calculation of Bourassa and McBeth-Ford (2010) who add to its positive surface vorticity. For Hurricane John,

FIG. 10. Shapes for diameters of (a) 12.5, (b) 25, (c) 37.5, and (d) 50 km (Bourassa and McBeth-Ford 2010). The bold lines show the perimeter of the shape.

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FIG. 11. Swaths of surface vorticity from QuikSCAT showing the progression of the surface vorticity that is tracked as the disturbance develops into Hurricane Celia. Reds indicate positive surface vorticity and blues indicate negative surface vorticity. The spatial region for each panel is 08–258N, 708–1208W. Black circles indicate the location of the disturbance based on the GridSat IR imagery whereas black dots indicate the Dvorak fix location of the disturbance for times that fix locations are available. the average vorticity calculated from the QuikSCAT spacing, which allows for tracking the propagation of overpass at 1200 UTC 24 August 2006 reveals that the surface vorticity in between overpasses of the dis- significant positive surface vorticity is present on the turbances by QuikSCAT. south side of the Gulf of Papagayo gap wind jet (Fig. 12). Latitude-averaged (78–178N) Hovmoller€ diagrams of The contrast between the easterly Papagayo gap wind CCMP surface vorticity are created to observe whether jet and the westerly winds to the south amplifies the the surface vorticity is propagating. These Hovmoller€ amount and strength of positive surface vorticity gen- diagrams have longitude on the x axis and time, in- erated by the gap winds. This initial positive surface creasing from bottom to top, on the y axis. Regions of vorticity is tracked as it develops into Hurricane John positive surface vorticity oriented from the bottom right (Fig. 12). to the top left indicate that the positive surface vorticity is propagating from east to west. The surface vorticity for the disturbance that developed into Hurricane Celia 5. Verification of QuikSCAT vorticity using CCMP can be tracked in the Hovmoller€ diagram of CCMP lati- Because of the limited temporal coverage of the dis- tude-averaged surface vorticity (Fig. 13). This Hovmoller€ turbances by QuikSCAT and rain contamination in- diagram further suggests that the surface vorticity gen- fluences on QuikSCAT, the CCMP ocean surface wind erated by the Papagayo gap winds on 11 July 2004 and by data record is used for verification to confirm the prop- the Tehuantepec gap winds on 13 July 2004 contributed to agation of the surface vorticity seen in the QuikSCAT Celia’s surface vorticity. A noteworthy feature on this overpasses. CCMP combines cross-calibrated wind speeds Hovmoller€ diagram is that the dipole of surface vorticity from three microwave radiometer sensors, cross-calibrated generated by the Tehuantepec gap winds on 13 July 2004 wind speeds and directions from two scatterometers, is seen as a disruption in the propagation of the positive and ship and buoy wind data with background winds surface vorticity. It is unclear exactly how the negative from the European Centre for Medium-Range Weather surface vorticity of this dipole influences the development Forecasts (ECMWF) to produce an ocean surface wind of the disturbance due to the lack of temporal resolution. (10 m) data record on a 0.258 latitude–longitude grid However, it does appear as though the positive surface (Atlas et al. 2011). This dataset has 6-h temporal vorticity of the disturbance continues to propagate

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FIG. 12. As in Fig. 11, but for Hurricane John. eastward after the interaction. For Hurricane John, surface vorticity is not as significant as that present on the Hovmoller€ diagram of CCMP latitude-averaged 24 August 2006, and there is no coverage of the region surface vorticity (Fig. 13) also confirms that the sur- by QuikSCAT on 23 August to verify either the source face vorticity created by the Papagayo gap wind jet on or the magnitude of this surface vorticity. Also worth 24 August 2006, propagates westward as Hurricane mentioning is that surface divergence fields calculated John develops. This diagram also shows that some from CCMP (not shown) identify corresponding re- positive surface vorticity was present near the longi- gions of convergence (divergence) with positive (nega- tude of the Gulf of Papagayo on 23 August 2006. This tive) surface vorticity.

FIG. 13. Hovmoller€ diagram of CCMP surface vorticity averaged over 78–178N (a) from 10 Jul to 19 Jul 2004 for Hurricane Celia and (b) from 23 Aug to 30 Aug 2006 for Hurricane John. Vertical black lines denote the longitudes of the Gulf of Tehuantepec and the Gulf of Papagayo. The black lines beginning at 18 Jul 2004 and 25 Aug 2006 show Dvorak ﬁx locations for Celia and John, respectively.

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FIG. 14. Sample QuikSCAT average vorticity plots indicating (a) a large contribution from Gulf of Papagayo gap winds, (b) medium contribution from Gulf of Papagayo gap winds, and (c) a small contribution from Gulf of Tehuantepec gap winds. Reds indicate positive average vorticity and blues indicate negative average vorticity.

6. Gap wind contributions Tehuantepec for each year in this study. Overall, the Gulf of Papagayo (Gulf of Tehuantepec) gap winds con- To quantify the contributions of the Papagayo and tributed to the surface vorticity of 84 (43) of the 191 storm Tehuantepec gap winds to the development of each of systems investigated. The majority of the systems with the systems investigated in this study, three contribution contributions from the gap winds over the Gulf of categories are developed. The gap winds are said to Papagayo fall within the medium and large contribu- make a large contribution to the development of the tion categories whereas the majority of contributions disturbance 1) if they produce cyclonic surface vorticity from the Gulf of Tehuantepec gap winds fall within the that visually appears to provide the largest source of small contribution category. Over the seven years in- surface vorticity for the initial disturbance or 2) if the cluded in this study, 107 (148) systems had no contri- gap winds are present with westerly winds to their south bution from the Papagayo (Tehuantepec) gap winds. creating the largest source of surface vorticity (Fig. 14). A medium contribution occurs if the gap winds visually appear to produce surface vorticity that contributes to 7. Monsoon trough and ITCZ interactions the initial disturbance along with another significant As discussed in section 6, the Gulf of Papagayo and source not associated with the gap winds (Fig. 14). Fi- Gulf of Tehuantepec gap winds contribute to surface nally, the gap winds are categorized as making a small vorticity as disturbances develop into invests or TCs. contribution to the development if they visually appear However, the presence of gap winds alone is not a suf- to contribute surface vorticity to an existing region of pos- ficient indicator of whether a disturbance will develop. itive surface vorticity after development begins (Fig. 14). Gap winds are present over both gulfs for about 50% of For Hurricane Celia, there is no other initial source of the QuikSCAT coverage days investigated, but invests positive surface vorticity, other than that produced by and TCs do not occur every time these gap winds are the Papagayo gap winds, when the disturbance first entered the EPAC. Therefore, it is determined that the gap winds over the Gulf of Papagayo make a large contri- TABLE 3. Summary of the number of systems (TCs and invests) bution to the development of Hurricane Celia. The gap to which gap winds over the Gulf of Papagayo (Gulf of Te- winds over the Gulf of Tehuantepec create surface huantepec) make small, medium, large, or no contribution to their vorticity that contributes to the development of Celia development, for each year in this study. The total number of storms for each year along with the total number of storms that had after the initial surface vorticity is generated and thus small, medium, large, or no contribution by the gap winds is also make a small contribution. In the case of Hurricane shown. John, the Hovmoller€ diagram of CCMP surface vorticity averaged over 78–178N (Fig. 13) confirms that the initial Total Year Small Medium Large None storms surface vorticity generated by the Papagayo gap winds and the westerly winds develops into John. Therefore, 2002 2 (8) 9 (2) 3 (0) 12 (16) 26 2003 3 (4) 5 (1) 7 (2) 19 (27) 34 the Gulf of Papagayo gap wind event is determined 2004 2 (5) 10 (1) 3 (1) 14 (22) 29 to make a large contribution to the development of 2005 1 (6) 7 (0) 3 (0) 14 (19) 25 Hurricane John. 2006 0 (3) 10 (2) 4 (0) 17 (26) 31 Table 3 gives the number of systems (TCs and invests) 2007 1 (1) 3 (0) 3 (0) 13 (19) 20 that have small, medium, large, or no contribution from 2008 0 (4) 7 (3) 1 (0) 18 (19) 26 Total 9 (31) 51 (9) 24 (3) 107 (148) 191 the gap winds over the Gulf of Papagayo and Gulf of

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FIG. 15. Generalized illustration of the wind patterns that create (a) the ITCZ and (b) the monsoon trough.

observed. To gain a better understanding of the gap A summary of the number of TCs that have contri- winds’ role in the development of the systems, their butions from the Gulf of Papagayo and the Gulf of contributions are evaluated in terms of coincidence with Tehuantepec gap winds while 1) strong westerly winds the monsoon trough and the ITCZ. Figure 15 shows a (monsoon trough) are present or 2) weak westerly or general schematic of the ITCZ and the monsoon trough. easterly winds (ITCZ) are present is given in Table 4. A The ITCZ occurs when the northeast trade winds from total of 118 TCs occurred over the 7 yr of this study, and the NH converge with the southeast trade winds from the 98 of these TCs occurred while the monsoon trough was Southern Hemisphere (SH) whereas the monsoon trough present. The Gulf of Papagayo gap winds contribute occurs because of a convergence of the NH northeast to 44 of these 98 TCs and the Gulf of Tehuantepec trades and the monsoon southwesterlies. gap winds contribute to 20. It is hypothesized that the To observe when the monsoon trough or ITCZ is proximity and alignment of the Gulf of Papagayo with present, longitude-averaged Hovmoller€ diagrams of the monsoon trough is the main reason that the Gulf the CCMP zonal wind component are examined. These of Papagayo gap winds contribute to signiﬁcantly more plots average the zonal wind component from 808 to TCs when the monsoon trough is present compared to 1408W; latitude is on the x axis and time, increasing from the Gulf of Tehuantepec gap winds. When the westerlies the bottom to the top, is on the y axis. Positive values are weak or the ITCZ becomes more prominent, only indicate that westerly winds are the dominant zonal 20 TCs are observed. Gulf of Papagayo gap winds con- wind direction and negative values indicate that easterly tribute to the development of 14 of these storms and winds are dominant. Strong westerly winds (average Gulf of Tehuantepec gap winds contribute to 9. 2 zonal wind speed $2ms 1) indicate that the monsoon 2 trough is present whereas weak westerlies (0 m s 1 # 2 8. Conclusions average zonal wind speed , 2ms 1) or easterlies indicate the ITCZ is more prominent. Latitude positions This study provides an insight into the summer of the storm tracks are then plotted over the Hovmoller€ gap winds over the Gulf of Papagayo and the Gulf of diagrams to compare the presence of the monsoon trough Tehuantepec. It is found that gap winds occur for about or the ITCZ with storm occurrences and the contribution 50% of the QuikSCAT coverage days for both gaps. that the gap winds make to the development of each These gap winds are also found to generate positive storm. The dates when gap winds are present are also surface vorticity that appears to contribute to the for- plotted on these diagrams for comparison. The Hovmoller€ mation of TCs in the EPAC. Gap winds over the Gulf diagram of longitude-averaged CCMP zonal wind of Papagayo contribute to approximately 44% (49%) of component for August 2006 (Fig. 16) suggests that the the storm systems (TCs) investigated and the Gulf of westerly winds observed south of the Papagayo gap Tehuantepec gap winds contribute to approximately wind event on 24 August 2006, are associated with the 23% (25%) of the storm systems (TCs) investigated. presence of the monsoon trough. The CCMP longitude- The majority of the storms with contributions from the averaged zonal wind Hovmoller€ diagram for September Papagayo gap winds fall within the medium or large 2006 is also shown in Fig. 16. These diagrams show that contribution categories whereas the majority of the the majority of the storms that occurred during August contributions by the Tehuantepec gap winds fall in the and September 2006 developed while the monsoon small contribution category. Additionally, the location trough was present. at which a system is classiﬁed as a TC is not an indication

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FIG. 16. Hovmoller€ diagram of CCMP zonal wind averaged over 808–1408W for (a) August 2006 and (b) September 2006. Vertical black lines denote the latitudes of the Gulf of Papagayo and Gulf of Tehuantepec and thick black line segments indicate the presence of gap winds over each gulf. The green line beginning at 25 Aug 2006 shows Dvorak fix latitudes for Hurricane John while the gray lines indicate the Dvorak fix latitudes of other TCs that developed during August and September 2006. Blues indicate westerly winds and reds indicate easterly winds. of whether gap winds may have contributed to the de- more favorable for development. However, it should be velopment of the disturbance. noted that the presence of the gap winds themselves is The larger influence of the gap winds over the Gulf of not sufficient for cyclogenesis to occur because gap winds Papagayo is a new result that has not been observed in are present far more often than cyclogenesis occurs. the EPAC prior to this study. Previous studies have fo- Although the gap winds appear to contribute to cy- cused on the influence that Tehuantepec gap winds have clogenesis in the EPAC, it is difficult to say how large on cyclogenesis in this region and not on the influence of this contribution is. Because of the number of storms the Papagayo gap winds. The proximity and alignment that fall within the small and medium contribution cat- of the Papagayo gap winds to the monsoon trough egories, for both gap wind regions, it appears that cyclo- westerlies is the most likely reason that they appear to genesis is influenced by the gap-wind-induced, low-level have a larger influence on cyclogenesis in the EPAC. surface vorticity combining with a large-scale feature Approximately 76% (44/58) of the TCs with contribu- such as the monsoon trough or the ITCZ. As noted by tions from the Papagayo gap winds occurred while the many of the previous studies and NHC TC reports monsoon trough was present and approximately 69% (www.nhc.noaa.gov/pastall.shtml#tcr), African easterly (20/29) of the TCs with contributions from the gap winds waves might also play a large role in EPAC tropical over the Gulf of Tehuantepec occurred while the mon- cyclogenesis and thus would be another large-scale soon trough was present. Only 20 out of 118 TCs formed feature worth investigating in the future. African east- when the westerly winds were weak or when the ITCZ erly waves provide one of many possible future research was present. This suggests that the presence of the mon- topics that could be conducted to confirm the actual role soon trough creates a much more favorable environment that the gap winds play in cyclogenesis in the EPAC. for cyclogenesis and that surface vorticity generated by A coordinated dropsonde study (i.e., Helms and Hart the gap winds appears to make the environment even 2013) could be used to examine changes in the vertical

TABLE 4. Summary of the number of TCs that occurred while strong westerlies (monsoon trough) were present or weak westerlies or easterlies (ITCZ) were present along with the contribution from the Gulf of Papagayo (Gulf of Tehuantepec) gap winds.

Gap wind and Small Medium Large No gap wind no contribution contribution contribution contribution Total Strong westerlies 47 (47) 7 (31) 4 (13) 25 (6) 15 (1) 98 Weak westerlies or easterlies 4 (7) 2 (4) 2 (8) 9 (1) 3 (0) 20 Total 51 (54) 9 (35) 6 (21) 34 (7) 18 (1) 118

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