sign in sign up
<<
Home , Tropical cyclogenesis, Tropical cyclone basins, Tropical wave

SEPTEMBER 2006 FRANK AND ROUNDY 2397

The Role of Tropical Waves in Tropical

WILLIAM M. FRANK Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

PAUL E. ROUNDY CIRES, University of Colorado, Boulder, Colorado

(Manuscript received 7 October 2005, in final form 23 December 2005)

ABSTRACT

This paper analyzes relationships between activity and in all of the earth’s major tropical basins. Twenty-nine years of outgoing longwave radiation data and global reanalysis are filtered and analyzed to determine statistical relationships between wave activity in each basin and the corresponding cyclogenesis. Composite analyses relative to the storm genesis locations show the structures of the waves and their preferred phase relationships with genesis. Five wave types are examined in this study, including mixed Rossby–gravity waves, tropical-depression-type or easterly waves, equatorial Rossby waves, Kelvin waves, and the Madden–Julian oscillation. The latter is not one of the classical tropical wave types, but is a wavelike phenomenon known to have a strong impact on tropical cyclogenesis. formation is strongly related to enhanced activity in all of the wave filter bands except for the Kelvin band. In each basin the structure of each composite wave and the phase relationship between the wave and cyclogenesis are similar, suggesting consistent forcing mechanisms. The waves appear to enhance the local circulations by increasing the forced upward vertical motion, increasing the low-level at the genesis location, and by modulating the vertical shear. Convective anomalies of waves associated with genesis are detectable in the analyses as long as 1 month prior to genesis. This opens up the possibility of developing statistically based genesis forecasts.

1. Introduction why and how some disturbances undergo genesis re- mains elusive. Do the form quasi randomly a. Background from fortuitous interactions between mesoscale convec- Every year approximately 80 tropical cyclones form tive systems, or do predictable large-scale circulations determine the time and place of genesis? over the tropical (Gray 1985). Virtually all of About 80%–90% of all tropical cyclones form within them form over warm, tropical waters in locations that 20° of the . This raises the possibility that tropi- are all but devoid of in situ observations. As a result, cal cyclogenesis events may be modulated by the family there are a great many questions as to how and why of equatorial and near-equatorial waves that propagate they form. zonally in this band. The waves could cause cyclones to Tropical cyclones form within regions of preexisting form by organizing deep and/or by altering deep, precipitating convection. Prior to cyclogenesis the the flow in preferred areas. It is our hypothesis that a convection is only loosely organized into mesoscale ar- large fraction of tropical cyclones form when tropical eas of enhanced cloudiness that are often called tropical waves produce anomalously favorable conditions for disturbances. Only a small fraction of these tropical genesis. This paper examines relationships between disturbances form tropical cyclones, and understanding tropical cyclogenesis events and tropical wave activity. It focuses on the phase relationships between the waves and tropical cyclone formation. The primary goals are Corresponding author address: W. M. Frank, Department of Meteorology, The Pennsylvania State University, 503 Walker to improve the understanding of these relationships and Building, University Park, PA 16803. to explore the potential for long-range forecasts of E-mail: [email protected] tropical cyclogenesis using statistical models.

© 2006 American Meteorological Society

MWR3204 2398 MONTHLY WEATHER REVIEW VOLUME 134 b. Conditions at the genesis location portions of the MT. It is probable that most storms form in conjunction with some sort of external pertur- The climatological conditions associated with tropi- bation of the and that pure ITCZ breakdown cal cyclogenesis are relatively well established. Briegel occurs only during times when such interactions are and Frank (1997) summarized seasonal studies of cy- absent. clogenesis (e.g., Gray 1968, 1979, 1985). Collectively, The ITCZ is perturbed by a variety of external cir- they show that in the current most tropical cy- culations. For example, Briegel and Frank (1997) clones form in regions that exhibit four characteristics: showed that in the northwest Pacific 1) sea surface temperatures above 26.5°–27.0°C have a tendency to form in the ITCZ when a midlati- coupled with a relatively deep oceanic mixed layer; 2) tude trough is north-northwest of the eastern end of the cyclonic low-level relative vorticity and planetary vor- MT. However, the most common modulations of the ticity; 3) weak to moderate (preferably easterly) verti- large-scale flow in the low latitudes are due to zonally cal shear; and 4) organized deep convection in an propagating equatorial and near-equatorial waves. This area with large-scale ascending motion and high study examines some aspects of the role of those tropi- midlevel . However, these characteristics are cal waves in the formation of tropical cyclones. not fully independent, and they tend to be present over widespread areas of the tropical oceans much of the c. Tropical waves time. Because tropical cyclogenesis is infrequent at any one location, the conditions that cause individual A recent global of tropical wave activity storms to form must differ from the climatological con- (Roundy and Frank 2004a,b, hereafter RF04) analyzed ditions in nature and/or degree. the contributions to the total variance in outgoing long- Composite analyses (e.g., McBride and Zehr 1981) wave radiation (OLR) by each of the five prominent and numerous observational case studies (e.g., Zehr wave types with periods of 2 days or longer. One of 1992) have shown that most storms form in regions that these is the Madden–Julian oscillation (MJO; Madden exhibit both anticyclonic upper-level relative vorticity and Julian 1994). Three others are classical equatorial and cyclonic low-level vorticity in the immediate vicin- waves that, in idealized form, are either symmetric or ity of the prestorm disturbance. Furthermore, genesis is antisymmetric about the equator (though in practice favored in regions of weak easterly shear (supported by they are usually stronger in one hemisphere than the modeling studies beginning with Kurihara and Tuleya other). These are equatorial Rossby waves (ER), mixed 1981, 1982), though there is often strong vertical shear Rossby–gravity waves (MRG), and Kelvin waves. The just a few hundred kilometers away from the incipient fifth wave type is confined to a single hemisphere, is storm location. Other case studies support the concept westward propagating, and is known as the tropical- that tropical cyclogenesis may require the depression-type wave (TD type, also referred to as an to be preconditioned to high values of humidity in the easterly waves or African waves). There have been sev- low to midtroposphere to inhibit negative effects of eral studies of the theoretical structures of these waves downdrafts (Bister and Emanuel 1997). (e.g., Matsuno 1966; Zhang and Webster 1989), and While we have improved our knowledge of the con- their observed structures (e.g., Reed et al. 1977; Hen- ditions present at the time and place of genesis, we have don and Liebmann 1991; Madden and Julian 1994; less understanding of the circulations that produce Takayabu 1994; Wheeler and Kiladis 1999; Wheeler et them. The most common location for cyclogenesis is al. 2000; Straub and Kiladis 2002; Kiladis et al. 2005). within or close to the intertropical convergence zone RF04 found that the low-frequency MJO and ER (ITCZ), particularly in the portions of the ITCZ that waves combined to explain at least 15% of the total exhibit a trough (MT) configuration with OLR variance between 20°N and 20°S latitude, and in westerly flow equatorward of the trough axis (e.g., some regions they explained at least 50% of the sub- Briegel and Frank 1997; Ritchie and Holland 1999). A seasonal local variance. The higher-frequency waves MT provides an environment that satisfies all of the produced smaller percentages of the total variance in criteria for genesis mentioned above. Both observa- the , but in many areas, particularly in the tional studies (e.g., Ferreira and Schubert 1997) and , they were also major contribu- model simulations (e.g., Wang and Frank 1999) suggest tors to the variance. Because OLR is at least a crude that a MT may break down spontaneously to form indicator of rainfall in the Tropics, the results of RF04 tropical cyclones if it remains unperturbed for a long showed that the tropical waves play a major role in enough period of time. However, external influences on modulating the tropical rainfall. The circulations of the MT can accelerate the genesis process in preferred these waves also modulate the large-scale wind fields. SEPTEMBER 2006 FRANK AND ROUNDY 2399 d. Waves and cyclogenesis The OLR data are daily means on a 2.5° grid and were The idea that waves can trigger tropical cyclogenesis obtained from the Climate Diagnostics Center. Daily is not new. It has been known for decades that about wind estimates for 850 and 200 hPa were obtained from half of the hurricanes in the North Atlantic form from the National Centers for Environmental Prediction– African waves (e.g., Frank and Clark 1980), and there National Center for Atmospheric Prediction reanalysis. have been many studies of this type of genesis (e.g., The tropical storm genesis dates and locations are Carlson, 1969; Landsea, 1993; Thorncroft and Hodges, based on the archived best tracks of the National Oce- 2001; and others). Molinari and Vollaro (2000), Mal- anic and Atmospheric Administration (NOAA) Tropi- oney and Hartmann (2001), Liebmann et al. (1994), and cal Prediction Center and the U. S. Navy’s Joint Ty- others have showed effects of the MJO on North Pacific phoon Warning Center. They were obtained from Dr. tropical cyclogenesis. Dickinson and Molinari (2002) ’s Web site. The time at which the dis- analyzed events of cyclogenesis in the northwest Pacific turbance was first classified as a tropical depression was associated with MRG waves. Bessafi and Wheeler used as the time of genesis. (2006) and Hall et al. (2001) analyzed the relationships b. Spectral analysis and filtering between the MJO and ER, MRG, and Kelvin waves and genesis over the southern and in the Figure 1 (from RF04), is a spectral analysis of the Australian region, respectively. OLR data with frequency as the ordinate and wave- We propose that the zonally propagating waves in number as the abscissa. Values are normalized by di- the tropical atmosphere play major roles in causing viding by an estimated background spectrum (RF04; tropical cyclogenesis by enhancing local conditions in Wheeler and Kiladis 1999). Negative wavenumbers de- environmentally favorable areas. They can enhance the note westward-propagating waves. Also shown in this potential for genesis in several ways: by increasing up- figure are the regions of the spectral space that were ward vertical motion and convection (which also tends assumed to correspond to the indicated wave types to lead to increased deep-layer moisture), by increasing (outlined by heavy gray lines). These areas represent the low-level vorticity, and/or by altering the local ver- the bands over which the OLR and wind data were tical shear pattern. Some types of waves may also trig- spectrally filtered to isolate the individual wave types. ger convection in regions of converging zonal wind, Filters applied by Wheeler and Kiladis (1999) are out- where wave energy tends to accumulate (Ritchie and lined in heavy-dashed lines for comparison. Though the Holland 1999). The larger-scale waves, such as the MJO spectra of the same waves may differ between OLR and and ER, can alter the mean zonal wind over large winds, our filters are broad and, with the exception of enough time and space scales to affect the mean flow in the filter for the Kelvin wave, they extend to wavenum- which the smaller and higher-frequency waves propa- ber 1 or 0 over their entire frequency ranges, allowing gate. Finally, some of the lower-frequency waves might them to capture both the strongest OLR signals and the also affect sea surface temperatures sufficiently to have corresponding larger spatial scales that tend to have some effect on genesis, but this is not explored in the more power in the winds. Thin black lines in Fig. 1 current study. indicate theoretical solutions of the shallow-water wave This paper explores the relationships between the equations for the indicated equivalent depths. The major tropical wave types and tropical cyclone forma- large box in the upper left of the figure is a combined tion in each of the six major . TD-type and MRG wave band. In practice it is hard to Satellite imagery and wind data from global analysis entirely separate these two higher-frequency wave fields are used to determine relationships between types using this kind of spectral filtering because they wave activity and cyclogenesis in each region and to occupy overlapping wavenumber–frequency space, but analyze the anomaly fields at the genesis location pro- our composite analysis below makes both wave types duced by each wave type. The data and methodology apparent where they occur. are described in section 2. The results are presented in The spectral analysis and filtering techniques are de- section 3, and discussion and conclusions are presented scribed in detail in RF04. The OLR and wind data were in section 4. spectrally filtered for each of the outlined regions shown in Fig. 1. It is important to note that just because 2. Methodology the filter bands include the domains of the solutions of the shallow-water wave equations, not everything that a. Data passes through the bandpass filter is necessarily repre- The data used to isolate and analyze the wave activity sentative of that theoretical wave type. and the tropical cyclones cover the years 1974–2002. The filters applied here did not include cross- 2400 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 1. Normalized wavenumber–frequency spectrum of OLR. Spectral bands of the four filters used in this project are indicated by the heavy solid lines. Bands applied by Wheeler and Kiladis (1999) are included for comparison (heavy dashed lines). Thin solid lines show the shallow-water-model solutions in a dry, motionless atmosphere for the equivalent depths indicated by the labels. equatorial symmetry constraints. RF04 found that esis cases where tropical waves would have their strong- while all wave types are present in both hemispheres, est effects. These boxes contain the majority of the gen- there are major differences in the amplitude of the esis locations in each basin except the North Atlantic. wave activity (Fig. 2). The MJO is by far the most active The amount of variance in each wave band was esti- wave type in the , while the mated for each genesis event at the location and date of higher-frequency waves are all much more prominent genesis. Genesis events were classified as to whether or in the Northern Hemisphere. Relaxing the symmetry not they occurred when waves were active within that constraints allows the composites (discussed below) to basin. Time series of variance were estimated for each show the natural asymmetry of the waves. grid point by smoothing the squared filtered anomaly with a running mean in time. The windows for the run- c. Composites and other analyses ning means were 51 days for the MJO, 21 days for the The locations of the genesis events that occurred dur- ER band, and 11 days for both the MRG–TD-type and ing the 29-yr period are shown by the small circles in Kelvin bands (somewhat longer than the dominant pe- Fig. 3. Also shown are six rectangular areas labeled 1–6. riod in each of these bands). A wave band was consid- These regions were chosen as the analysis areas for the ered active when its OLR variance was locally above a six tropical cyclone basins: 1) north Indian Ocean, 2) threshold value (see the appendix). Note that though south Indian Ocean, 3) northwest , 4) wave dynamics controls much of the variability of indi- South Pacific Ocean, 5) northeast Pacific Ocean, and 6) vidual wave types, variability in one filter band is not North . The boxes were restricted to necessarily independent from variability in the other latitudes equatorward of 20° in order to focus on gen- bands and in tropical cyclones, as activity of each of

Fig 1 live 4/C SEPTEMBER 2006 FRANK AND ROUNDY 2401

FIG. 2. Same as in Fig. 1, but for the Northern and Southern Hemispheres separately.

Fig 2 live 4/C 2402 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 3. The rectangular boxes show the six cyclone basins defined for this study, and the individual dots show genesis locations for the individual storms for the period 1974–2002. these may vary with the activity of total OLR. Each determine whether the cyclone season coincides with genesis event was classified as to whether it occurred in the mean seasonal wave activity within the region. Each the positive or negative OLR anomaly region of each panel of Fig. 4 shows the mean annual cycles of wave- wave type. filtered OLR variance and tropical cyclogenesis within Composites of total unfiltered OLR, OLR anoma- the rectangular analysis area of the indicated basin. In lies, and the 850- and 200-hPa wind anomalies were each panel the heavy black line is an index of the num- each constructed relative to the genesis point and time ber of tropical cyclones per month that form within the lags from the genesis date for each spectral band in box. The OLR variance shown in the other lines is just each basin. These composites included only those gen- the square of the filtered anomalies averaged over the esis events that occurred during periods when the filter basin and the day of the year. band of interest was locally active at the genesis point An important caveat is that in some cases waves may (by using the threshold discussed in the appendix). The impact tropical cyclone formation without producing composite structures were compared with theoretical significant effects upon the OLR pattern, such as by wave structures and were used to determine the phase altering the vertical shear and/or increasing the low- relationships between genesis and wave activity. The level vorticity. Examples of this include the roles of the Kelvin wave analysis was limited to storms forming MJO in genesis in the northeast Pacific and the North equatorward of 10°N and 10°S, because Kelvin wave Atlantic (discussed below). activity tends to be more concentrated near the equator In the Southern Hemisphere the results are straight- than the other wave types. forward. The annual cycles of OLR variance in the Time–longitude composite diagrams were con- MJO and ER bands tend to vary in phase with the structed for each filter band and region relative to the cyclone season. The MRG–TD wave activity is in phase genesis point. These were used to determine whether with the cyclone season, but the cyclones themselves the wave anomaly could be detected propagating into might be contributing a significant part of the signal. and out of the genesis location. Kelvin wave activity in the South Indian Ocean clearly A 1000-sample bootstrap significance test was ap- varies in phase with the cyclone season, and this pattern plied to the composite OLR and wind anomalies, as- is similar but less distinct in the South Pacific. suming the null hypothesis that the anomalies were not The patterns are much more variable in the Northern significantly different from zero. This was done by ran- Hemisphere basins. The cyclone season of the north domly selecting (with replacement) from the set of gen- Indian Ocean basin differs from the other basins in that esis dates included in the composite, applying the av- the spends most of the late summer eraging to each new set, and locating the zero anomaly over land and relatively far north of the equator. within the resulting distribution. Hence, it has two distinct genesis peaks in the spring and autumn. In this basin the first peak coincides with 3. Results peaks in MJO, ER, and Kelvin band activity. None of these wave types is particularly active during the second a. Seasonal variations of wave activity and genesis peak of the cyclone season. The MRG–TD-type and A useful climatological question is the following: ER bands have sharp peaks in activity during the How do the annual cycles of variance in each wave middle and late summer, but these do not coincide with band compare with the annual cycles of genesis in each either cyclone peak. basin? Each of the six tropical cyclone basins has its The northeast Pacific shows only weak seasonal re- own unique climatology. Therefore, a first step is to lationships between wave activity and cyclone forma- SEPTEMBER 2006 FRANK AND ROUNDY 2403 tion. The MRG–TD-type waves are somewhat more season in most basins except the North Indian. The active during cyclone season than in the other months warm season is defined here as June–October in the and Kelvin waves remain active during the earliest part Northern Hemisphere and December–April in the of the cyclone season. The northwest Pacific shows Southern Hemisphere. When the open bar is taller than strong annual cycles in MRG–TD-type and ER wave the shaded bar, it means that more storms formed when activity that coincides well with the cyclone season. The that wave type was active than would be expected if the other wave types do not vary in phase with the cyclone storms formed at random times during the year (Figs. season. In the North Atlantic the MRG–TD-type waves 5a–f) or during the warm season only (Figs. 5g–l). vary roughly in phase with the cyclone season, while the When the open bar is shorter than the shaded bar, it other three wave types do not. shows up as a line partway up the shaded bar. In this Comparing Figs. 2 and 4 it is clear that the low- latter case, fewer storms formed when the OLR vari- frequency MJO band and ER band variances that ance was above the threshold than would have been dominate the Southern Hemisphere spectrum are expected by chance. strongly seasonal, and they vary in phase with the cy- Figures 5a–f show that in general storms tend to form clone season in the two Southern Ocean basins and for more often when the MJO, ER, and MRG–TD-type the first peak of the North Indian season. Activity in waves are active than would be explained if the storms the Kelvin band tends to follow the same pattern, occurred randomly throughout the year. The exception though the cycles are somewhat less distinct than for is for the MJO band in the Atlantic. Kelvin waves do the MJO and ER bands. not show this tendency except in the South Indian All of the wave types (except the MJO) are more Ocean (for a more detailed analysis, see Bessafi and active in the Northern than in the Southern Hemi- Wheeler 2006). However, the apparent relationships sphere. This is particularly true for the MRG–TD-type between active waves and genesis could simply be be- band, which varies strongly and in phase with the cy- cause the waves and cyclones have similar seasonal clone season in the North Atlantic and the northwest cycles in some basins, as noted above. Hence, Figs. 5g–l, Pacific. which are comparing events only in the warm seasons, are generally better indicators of possible relationships b. Individual genesis events and local wave activity between genesis and anomalous wave activity in the Figure 4 suggests that there are seasonal relation- region. ships between basin mean wave activity and cyclone The warm-season results in Fig. 5 indicate that cyclo- formation in each basin, but these relationships could genesis is clearly related to above-normal wave activity be coincidental. The waves might be active away from in the MJO, ER, and MRG–TD-type bands in the the genesis points, and they do not necessarily show all northeast Pacific basin and to a lesser extent the north periods of wave activity, because these are not always Indian basin. The Atlantic basin shows such relation- locked to the seasonal cycle. Therefore, it is important ships for the ER and MRG–TD-type bands but not the to examine the level of local wave activity at the time of MJO. In contrast, the northwest Pacific shows no such each individual genesis event. As noted above, tropical relationship for any wave band, and the two Southern cyclogenesis is affected by circulations that occur over a Hemisphere basins show only a weak relationship for broad range of scales, but it is the conditions within a the single MRG–TD-type band. Genesis does not ap- few hundred kilometers of the storm center that are pear to be associated with above-normal warm-season thought to be crucial in determining which disturbances Kelvin wave activity in any basin, and Fig. 4 suggests will form cyclones. This section examines the structures that these waves would most impact genesis during of the waves that are present when tropical cyclones northern spring. Although these results suggest that in form and the locations of the forming storms within the many cases (e.g., the MJO in the northwest Pacific) wave circulation. The goal is to infer what types of flow waves may not tend to be more active than normal perturbations promote genesis. when genesis occurs, our further analysis (discussed be- In Figs. 5a–l, the open (or white) vertical bars show low) suggests that they still influence genesis. the percentage of the total number of cyclones in that Figures 6a–f show, for each of the four major wave basin that formed when the OLR variance exceeded bands, the fraction of the cyclones that form within the the threshold for each wave type. The shaded bar shows negative OLR anomaly of that band. For each filtered the percentage of the days when the threshold was ex- dataset the positive and negative areas are nearly equal ceeded for that wave type. The data in Figs. 5a–f are for in size (each covering 50% of the area in space and the entire season, while the data in Figs. 5g–l are for the time, typically Ϯ1%–2%). Therefore, a shaded bar with warm season only, which approximates the hurricane a value greater than 50% means that storms formed 2404 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 4. Mean annual cycles of storm genesis (storms per month) and of OLR variance in the indicated filter bands for each of the six cyclone basins. The line colors are as follows: cycle of cyclogenesis (black); MJO (heavy blue); ER (thin blue); MRG–TD type (red); and Kelvin (green). Variance values are the squared filtered OLR anomalies averaged over the day of the year and smoothed for plotting with a 5-day running mean.

Fig 4a–c live 4/C SEPTEMBER 2006 FRANK AND ROUNDY 2405

FIG.4.(Continued)

Fig 4d–f live 4/C 2406 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 6. For each basin and filter band, the percentage of the FIG. 5. For each basin and wave band the shaded bar is the storms that formed within the negative OLR anomaly region for percent of time that the wave variance is above the threshold that filter band. Filter bands are numbered as in Fig. 5. value for that basin, while the open (white) bar is percentage of the genesis events that occurred when the wave variance was above the threshold. The filter-band numbers are 1 (MJO), 2 such regions, because even though the strongest (ER), 3 (MRG–TD-type), and 4 (Kelvin). (a)–(f) Computed for MJO convection is elsewhere, MJO-band convection the entire cyclone season, and (g)–(l) for the warm season only is still locally enhanced at the time and place of gene- (see text for definitions of the warm seasons). sis during many of the events in which it plays a role. Although these results suggest that some wave preferentially in the negative OLR anomaly region of types may only exceed the variance threshold during that wave band. Because negative OLR anomalies are less than half of all the storm formations in some basins, associated with the more convectively active portions of our composites (below) suggest that many of these the waves, this represents a tendency for storms to form waves still show clear relationships to genesis when the where waves are enhancing the convection above back- waves are active. ground values. The MRG–TD-type combined wave band also shows In all six basins the storms form in the negative OLR a clear tendency for storms to form in the negative anomalies of the MJO band between about 60%–70% OLR anomaly regions, with percentages ranging from a of the time, and these results are significantly different low of about 65% in the South Pacific to a high of from the 50% line at the 99% level (97% for the At- around 85% in the Atlantic. Departures are significant lantic) using a bootstrap test. Storms also have a clear at better than the 99% level in all basins. The only two tendency to form in the negative OLR anomaly regions basins with percentages below 75% are in the Southern of ER waves in all basins. For ER waves the departures Hemisphere, where this wave band is least active. In- from the 50% line are all significant at the 99% level. terpretation of these results is complicated because a Maloney and Hartmann (2001) suggest that the MJO portion of the signals of the storms themselves fall into influences east Pacific cyclogenesis through far-field ef- this band. fects, meaning that the strongest active convection of Storms show a small preference for forming in the the MJO is not in the east Pacific when genesis occurs. negative anomalies of the Kelvin wave band in all ba- Our method successfully diagnoses the MJO impacts in sins. Although the percentages of storms that do so are SEPTEMBER 2006 FRANK AND ROUNDY 2407 only 55%–65%, the departures from 50% are signifi- the latitude of maximum anomalous westerly flow. The cant at the 97% level or above. storms form well within the negative OLR anomaly of the MJO, indicated by the heavy, solid contours. c. Composite wave structures at genesis Hence, the storms are forming in a region of MJO- In general, the composites for each wave type show induced low-level zonal confluence and convergence as similar structures in each basin. Recall that the com- well as MJO-enhanced convection. They are also form- posites are performed only for times when wave activity ing at the time when this favorable, anomalous westerly was above the threshold. For brevity, this discussion flow region reaches the eastern edge of the monsoon focuses on composites from the 850-hPa level and for trough, which is a region of strong, persistent convec- the northwest and northeast Pacific basins and from the tion and low-level cyclonic vertical vorticity. This loca- 200-hPa level for the northwest Pacific only. Results for tion in the background flow is sometimes referred to as the other basins are summarized and compared with the confluence point due to the change in direction the composites for these regions. We also performed from monsoonal (to the west) to trade east- null-case composites for each basin by compositing erlies (to the east) on the equatorward side of the ITCZ relative to the same genesis locations and days of the (e.g., Briegel and Frank 1997). This confluence point is year, but for years with no tropical cyclone present. thought to be a region favorable for zonal wave energy These null cases showed only a relatively smooth back- accumulation (Briegel and Frank 1997; Ritchie and ground reflecting the mean flow of the region with no Holland 1999). Because the genesis location lies near evidence of anomalous OLR, vorticity, or other flow the northern boundary of the westerly surge, the wave patterns. This result suggests that when our genesis- is also increasing the vorticity at that point. centered composites show clear wave structures, they The wind anomalies for the ER band in the north- provide evidence of preferred phase relationships to west Pacific show a classic ER wave pattern in the genesis. Northern Hemisphere. There is a strong cyclonic gyre Figure 7 shows the composites for the northwest Pa- centered just northwest of the genesis location. The cific at the time of genesis. In each of these figures the anomalous flow in the Southern Hemisphere is weak by grid has been shifted zonally for each case so that 0° comparison. The cyclonic gyre is located about one- longitude is the mean longitude of the storm genesis quarter wavelength to the west of the center of the locations. However, the grid is not shifted meridionally, negative OLR anomaly. The preferred region of gen- so 0° on the vertical axis is the latitude of the equator. esis with respect to ER waves is within the region of The reason for not shifting the grid meridionally is that large-scale cyclonic flow of the wave anomaly and is some of the wave types maintain a close alignment with also within the negative OLR anomaly region (Fig. 7b). the equator and are somewhat less distinct when the The anomalous rotational flow of the ER wave rein- grid is shifted north–south and data are then compos- forces the vorticity of the monsoon trough. Further- ited relative to the storm genesis point. A small open more, the storm forms during the time that the wave- circle is plotted at the mean location of the storm gen- enhanced convection reaches the confluence point of esis points, and for the northwest Pacific this is at 0° the ITCZ. relative longitude and about 11°N actual latitude. The For the MRG–TD-type band the pattern is similar shaded contours show the background OLR pattern, and is shown in Fig. 7c. The structure near the genesis which suggests that the mean genesis location is at the location shows a wave propagating toward the west- eastern edge of the monsoon trough portion of the northwest (confirmed by analysis of lag composites, not ITCZ. This has been shown to be a favored location for shown). In this band the wave anomaly circulation in- genesis by Briegel and Frank (1997), Briegel (2002), dicates that the genesis location occurs within the west- Ritchie and Holland (1999), and others. The heavy ern edge of the low-level cyclonic circulation associated black solid and dashed lines represent the band-filtered with the wave and in northeasterly anomalous flow. It OLR negative and positive anomalies, respectively, for also forms within the region of negative OLR anomaly. the appropriate wave type. The vectors show the Dickinson and Molinari (2002) analyzed cases of MRG anomalous winds for that filter band. waves in the northwest Pacific moving toward the The northwest Pacific MJO composite shows a wind northwest and transforming into TD-type waves after structure resembling a large Kelvin wave with westerly they reach the monsoon trough. The composites sup- winds along the equator (Fig. 7a). Typhoons in this port their conclusions and suggest that this movement region tend to form near the easternmost edge of the off the equator generally occurs at or about the time of low-level westerly wind surge and within the eastern genesis when the MRG–TD-type disturbances reach and equatorward side of a cyclonic gyre, just north of the confluence point of the ITCZ. Their case study in- 2408 MONTHLY WEATHER REVIEW VOLUME 134 SEPTEMBER 2006 FRANK AND ROUNDY 2409 cluded three different genesis events that each occurred wave band shows slightly enhanced convection (nega- in somewhat different flow patterns. Our composite re- tive OLR anomaly) at the time and place of cyclogen- sult suggests that the preferred genesis point in an esis, but there is no clear sign of a Kelvin wave in the MRG wave train is in anomalous northeasterly 850-hPa composite wind structure. flow within counterclockwise gyres as the waves move The composites for the remaining four basins are off the equator. Case 3 in Fig. 6m of Dickinson and qualitatively similar to those for the northwest and Molinari (2002) is most similar to the composite behav- northeast Pacific with the following exceptions. ior. In the south Indian and South Pacific basins the cy- The Kelvin wave–band composite (Fig. 7d) suggests clones tend to form within, rather than at the eastern a relationship between cyclogenesis and Kelvin-band edge, of the westerly flow of the MJO. (The MJO OLR anomalies, but the relationship is less clear in the anomalous flow in the south Indian basin is west- wind data because the classic Kelvin wave structure is northwesterly.) In the Atlantic genesis occurs within an hard to discern. (There is some evidence of a Kelvin MJO westerly flow anomaly that is concentrated in the wavelike westerly surge at 850 hPa near and east of the vicinity of the ITCZ within a small negative OLR genesis point, but there is also an easterly surge north anomaly. and west of the genesis point.) Kelvin wave structures MRG–TD-type anomalies are weaker and less dis- are not clearly discernable in the winds in any basin, but tinct in the two Southern Hemisphere basins than they this does not necessarily mean that Kelvin waves never are in the four Northern Hemisphere ones. This is par- affect genesis—some storms that develop close to the ticularly true in the South Pacific. However, they do equator, especially in the Pacific ITCZ during the show similar structures and phase relationships to those northern spring or in the Indian Ocean during the in the Northern Hemisphere. The lagged composite southern summer, might be impacted by them. We have structures suggest that MRG waves may occasionally seen relatively rare cases in which Kelvin waves might influence cyclogenesis in the Indian basin, in a manner have influenced genesis in the northeast Pacific during similar to that discussed above in the northwest Pacific May, including during May 2005. (e.g., Dickinson and Molinari 2002). This needs further investigation, but it is clear that such The upper-level composites for the MJO, ER, and events are not frequent enough to produce a strong MRG–TD-type bands in the northwest Pacific are signal in the statistics. shown in Fig. 9. Results for the northeast Pacific are Figures 8a–d shows the same four composites for generally similar (not shown). Many of the upper-level genesis in the northeast Pacific. The results are quali- composites show wave structures more clearly than tatively similar. The shaded background OLR fields in- those at 850 hPa. For example, Fig. 9b shows anticy- dicate that genesis in this region tends to occur within clonic and cross-equatorial from the develop- the western part of the monsoon trough. Storms tend to ing storm being integrated into a wind pattern that is form near the eastern edge of the westerly wind symmetric across the equator suggestive of an n ϭ 1ER anomaly and within the negative OLR anomaly of the wave, where n is the meridional wavenumber. Compar- MJO. As in the northwest Pacific, the genesis location ing the 200- and 850-hPa composite anomalies for each is at the poleward edge of the MJO westerly anomaly, wave type shows that the upper-level winds are roughly so the wind surge is increasing the low-level rotation as opposite in direction to the low-level winds suggesting well as the convergence at this location. For the ER that each of these wave anomalies is dominated by a band the storms form within the cyclonic rotation of the baroclinic, first internal mode vertical structure for the ER wave (southeast of the center of rotation) and portions of the wave near the genesis location. This within the negative OLR anomaly. They also form tendency for the upper- and lower-level winds to be in within the low-level cyclonic rotation of the MRG–TD- opposite directions is generally true in the other four type waves and the region of the negative (enhanced basins as well, with the lone exception being the MRG– convection) OLR anomaly. Once again, the Kelvin TD-type waves in the South Pacific basin, where the

←

FIG. 7. Composite of the OLR anomalies (contours), 850-mb wind anomalies, and total OLR (shading) relative to the genesis point and date in the NW Pacific for (a) MJO, (b) ER, (c) MRG–TD-type, and (d) Kelvin filter bands. The contour interval for OLR anomalies is2WmϪ2, with minimum contours at Ϯ1WmϪ2. For all wave types except the Kelvin wave, the first contour is significantly different from zero at Ͼ95% level, and the second contour exceeds the 99% level. For the Kelvin wave, the second contour generally exceeds the 95% level. Wind anomaly vectors are plotted only when they exceed the 90% level for a difference from zero. 2410 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 8. Same as in Fig. 7, but for the northeast Pacific. SEPTEMBER 2006 FRANK AND ROUNDY 2411

FIG. 9. Same as in Fig. 7, but with the 200-mb winds in the northwest Pacific. The Kelvin wave band is not shown. anomalous wind patterns are indistinct in this band. The southwesterly vertical shear of composite MRG– The commonly observed first internal mode vertical TD-type winds is significantly different from zero in the structure would allow the waves to modulate vertical northwest and northeast Pacific and Atlantic basins. shear in a manner that might influence genesis. For Within the limits of the current analysis, there do not example, the climatological mean 850–200-hPa vertical appear to be significant anomalies in the vertical shear shear of the zonal wind averaged over 5°–12.5°Nis associated with the Kelvin wave. The MJO band shows westerly over the Atlantic between 40° and 70°W for genesis occurring in a region of significantly enhanced July–September. Shear of the composite MJO flow easterly shear in all basins except the north Indian, over the central tropical Atlantic near the genesis point while in the ER band there are significant easterly (not shown), is easterly and significantly different from shear anomalies in the north Indian, south Indian, zero. This shows that the MJO often reduces the total northwest Pacific, and perhaps the northeast Pacific ba- westerly shear over the Atlantic when genesis occurs. sins but none in the South Pacific or Atlantic. Wave- 2412 MONTHLY WEATHER REVIEW VOLUME 134 induced shear anomalies may enhance genesis by re- 20°E of the genesis point in the north and south Indian ducing the total shear in regions where the background Ocean, 5 days prior and 30°E in the South Pacific and shear is inhibitive. Atlantic, 8 days prior and 35°–40°E in the northwest Altogether, the composites portray clear and rela- Pacific, and 15 days prior and 100°E in the northeast tively consistent phase relationships between tropical Pacific. cyclogenesis and tropical wave activity. For example, In the higher-frequency MRG–TD-type band the northwest Pacific typhoons tend to form within the cy- negative OLR anomaly associated with genesis tends to clonic anomalies associated with the MJO, ER, and become significant to the east of the genesis point from MRG–TD-type waves. This suggests that the waves en- 3–6 days prior to the formation of the storm, with the hance the tendency for genesis in part by increasing the longest lead time being in the northeast Pacific. Genesis local low-level vorticity. Because storms are observed occurs very soon after the negative OLR anomaly be- to form within the convectively active portions of these comes significant at the genesis point (0–1 days). The waves, the results suggest that large-scale enhancement signal is stronger in the Northern Hemisphere, which of deep convection enhances storm genesis. The MJO was expected because these waves are much stronger in and ER waves also produce an easterly shear anomaly general there. at the genesis location. Kelvin-band Hovmoellers suggest significant nega- tive OLR anomalies 3–7 days prior to genesis in most d. Timing of wave-induced genesis basins. The anomalies are less clear than in the other Time–longitude diagrams were computed for the bands, but they tend to arrive at the longitude of the OLR anomalies in each filter band and for each basin. genesis location on the day of or the day after genesis. The OLR anomalies were plotted at the mean latitude of genesis events that occurred when the wave band of 4. Summary and discussion interest was active. These diagrams were used to esti- mate how long before genesis the wave anomalies could The results show that three classes of equatorial be detected. If a wave is detectable several days before waves play significant roles in the formation of many of genesis occurs, it implies that forecasts of tropical cy- the 80 or so tropical cyclones that form each year within clogenesis based on wave projections should be fea- the six major cyclone basins. These wave types include sible. By detectible, we mean that the composite OLR the MJO, equatorial Rossby waves, and high-frequency anomalies are significantly different from zero at the westward-moving waves including mixed Rossby– 99% level. Figure 10 shows these diagrams for the gravity waves and tropical-depression-type distur- MJO, ER, MRG–TD-type, and Kelvin bands in the bances. Equatorial Kelvin waves do not appear to play northwest Pacific. Figure 11 shows the diagrams for the a major role in tropical cyclogenesis, except in isolated first three of these bands in the northeast Pacific. events, especially during the Northern Hemisphere The significant eastward-moving negative OLR spring or during the southern summer in the southern anomalies of the MJO reached the genesis location Indian Ocean (Bessafi and Wheeler 2006). The remain- from 5 days (south Indian) to 15 days (northwest Pa- der of this discussion will consider only the first three cific) prior to the time of genesis in all basins, except in wave bands. the Atlantic. In the Atlantic, the negative anomaly at Analyses of the seasonal variations of wave activity the genesis point exceeded the 95% level about 5 days and cyclogenesis in the six global cyclone basins shows prior to genesis. In each of the basins the negative OLR interesting variations from region to region, but they anomalies become significant to the west of the genesis provide only suggestions of relationships between the location much earlier—17 days prior to genesis in the waves and genesis. Some regions have cyclone seasons northwest Pacific and 18 to more than 30 days in the that vary in phase with different types of waves, while other basins. These precursor anomalies first become others show less seasonal variation in wave activity. significant 45°W of the genesis point in the north Indian When basin-scale wave activity is computed at the and northwest Pacific, 50°W in the south Indian and times of genesis events, clearer patterns emerge. In two northeast Pacific, 100°W in the northeast Pacific, and Northern Hemisphere basins (e.g., the north Indian and 40°W in the North Atlantic. northeast Pacific) there is a distinct tendency for gen- The negative OLR anomalies in the ER band arrive esis to occur more often when wave variance in any of at the genesis location one to two days prior to the time the three filter bands is above the threshold (see the of genesis in all six basins. These westward-moving appendix). In the North Atlantic the same pattern is anomalies first become significant in the composites at seen for the ER and MRG–TD-type bands. The rela- the 99% confidence level about 4 days prior to and tionship of the MJO to genesis in the North Atlantic is SEPTEMBER 2006 FRANK AND ROUNDY 2413

FIG. 10. Composite Hovmoeller diagrams of OLR anomalies in the northwest Pacific for (a) MJO, (b) ER, (c) MRG–TD-type, and (d) Kelvin filter bands. The contour interval is2WmϪ2, with minimum contours at Ϯ1WmϪ2. The confidence levels for the difference from zero are approximately the same as those given in Fig. 7. more complicated because only about one-quarter of by far-field effects of the MJO than by its small contri- the storms that form there do so when the MJO band is bution to local convection, as suggested by Maloney considered locally active in OLR. The composites sug- and Hartmann (2001) in relation to genesis in the east gest that these storms may be more strongly influenced Pacific. In contrast to the other three Northern Hemi- 2414 MONTHLY WEATHER REVIEW VOLUME 134

FIG. 11. Same as in Fig. 10, but for the northeast Pacific.

sphere basins, in the northwest Pacific warm-season ac- gardless of whether they are more active than average. tivity does not tend to be above average in any wave The two Southern Hemisphere basins behave much like band when genesis occurs. This does not mean that the the northwest Pacific in this respect. Both the south waves are unrelated to genesis in the northwest Pacific, Indian and South Pacific basins show no tendency to as there is a clear tendency for storms to occur in cer- form cyclones when the basin has enhanced MJO or tain portions of the waves when they are present, re- ER activity. However, they do show a weak tendency SEPTEMBER 2006 FRANK AND ROUNDY 2415 for cyclogenesis when MRG–TD-type activity is above larly, the smaller and higher-frequency MRG–TD-type normal. These higher-frequency waves are not as fre- wave band shows a consistent pattern of storms forming quent or as strong in the Southern Hemisphere as they within the region of enhanced low-level rotation and are in the Northern Hemisphere. within the negative OLR (convectively enhanced) re- The basins are large, and incipient cyclones are not. gion of the wave. Therefore, a more compelling case for wave–genesis Most of the world’s tropical cyclones form within or relationships comes from analysis of wave conditions at very close to the circulations of MT ITCZs. This is both the time and place of genesis events. The scarcity particularly true for the two basins examined most of data over the tropical oceans makes it difficult to closely in this paper (e.g., the northwest and northeast reliably analyze individual waves in many cases, but Pacific). Within these regions we hypothesize that the compositing techniques are capable of identifying mean waves accelerate the cyclogenesis process within a cli- conditions at the time of genesis. matologically favorable region. From the results of the The composites of the filtered OLR and wind current study it would appear that the MJO, ER, and anomalies relative to the genesis points yield several MRG–TD-type waves all do this by enhancing the important, general results: amount of large-scale convection in the region as well as the low-level rotation. The MJO and ER waves also • In each of the six basins the storms form in conjunc- produce a favorable easterly shear anomaly at the gen- tion with a wave structure that strongly resembles the esis location. These waves often modulate the time and expected wave structure associated with that filter place of cyclone formation within a monsoon trough. band. An unperturbed trough can eventually become un- • In each basin the storms tend to form at nearly the stable and break down, forming one or more cyclones same location within the composite wave structure. (e.g., Ferreira and Schubert 1997; Wang and Frank • All of the composites except those for the Kelvin 1999). The latter study found that simulations of an wave show approximate flow reversal between the idealized monsoon trough designed to resemble condi- 850- and 200-hPa levels, suggesting that the relevant tions in the northwest Pacific during August tended to wave structure is the baroclinic, first internal vertical produce ITCZ breakdown after about 10–14 days. mode, allowing the waves to both modulate vertical However, because monsoon troughs are almost con- shear and to produce opposing vorticity anomalies stantly perturbed by equatorial waves, we suggest that between the lower and upper . most cyclogenesis events occur when one or more of • All of the wave types seen in the composites are these waves accelerate the process. strongly asymmetric across the equator, though most At locations that are far removed from an MT, wave- of the ER composites do show opposite hemisphere induced genesis can still occur, but the process is likely circulations, consistent with theoretical wave struc- to take longer. Tropical cyclone formation from TD- tures. type waves in the easterly trade flow is particularly The MJO consistently produces storms near the east- common in the North Atlantic. ern edge and to the poleward side of the low-level west- Last, the composite analyses show that on average, erly wind anomaly and within the eastern and equator- the wave anomalies associated with tropical cyclogen- ward portion of a cyclonic gyre. Thus, the MJO en- esis are detectable prior to genesis for all wave types hances both the convergence and the low-level rotation and basins. The lead time is up to a month for the MJO at the genesis location. (In the Southern Hemisphere and up to 3 weeks for the ER wave in the northeast the primary effect seems to be the enhancement of the Pacific. This holds promise for the possible use of wave rotation.) The composites also show a tendency for the indices as predictors in statistical models for forecasting MJO to cause easterly vertical shear anomalies at the the probability of tropical cyclogenesis. Preliminary genesis location in most basins, and this would tend to tests of such procedures are ongoing and are producing be favorable for storm formation. The storms also form encouraging results. Other wave forecasts are already well within the region of locally enhanced convection. being used at the Research The ER waves also appear to enhance genesis by Centre in using procedures described in increasing both the large-scale rotation and convection, Wheeler and Weickmann (2001). as in this band the storms form within the low-level While the above analyses provide evidence that cyclonic gyre and about two days after the wave’s con- tropical cyclogenesis is frequently associated with equa- vective anomaly first reaches the genesis point. The ER torial wave perturbations, and is probably enhanced by waves also cause significant easterly shear anomalies at them, much remains to be learned about the mecha- the genesis locations in about half of the basins. Simi- nisms involved. We are in the process of performing 2416 MONTHLY WEATHER REVIEW VOLUME 134 more detailed analyses that will examine how the TD-type, and Kelvin bands, respectively. Though indi- anomalous wave circulations affect the vorticity, verti- vidual storms might contribute more or less than these cal shear, and other conditions at the time and place of values because of differences between the sizes and genesis. The results of this ongoing work will be pre- translation speeds, our estimates are appropriate for sented in a future paper. our statistical applications. We tried an alternative method of generating an artificial storm OLR signature Acknowledgments. This research was supported by that would not have included any wave signal, and re- the National Science Foundation Grants ATM-0003351 sults were consistent with those given above. and ATM-0233881. Professor John Molinari and two anomymous reviewers provided useful comments that REFERENCES dramatically improved the presentation of our results. George Kiladis graciously provided an additional re- Bessafi, M., and M. Wheeler, 2006: Modulation of south Indian Ocean tropical cyclones by the Madden–Julian Oscillation view and comments. and convectively coupled equatorial waves. Mon. Wea. Rev., 134, 638–656. APPENDIX Bister, M., and K. A. Emanuel, 1997: The genesis of Hurricane Guillermo: TEXMEX analysis and a modeling study. Mon. Wea. Rev., 125, 2662–2682. Determining the Threshold for Active Wave Briegel, L. M., 2002: The structure and variability of the ITCZ of Periods the western North Pacific. Ph.D. dissertation, Department of Meteorology, The Pennsylvania State University, 212 pp. Though much of the activity in the wave-filtered ——, and W. M. Frank, 1997: Large-scale influences on tropical OLR is contributed by large-scale waves, it is important cyclogenesis in the western North Pacific. Mon. Wea. Rev., to acknowledge that the storms themselves contribute 125, 1397–1413. Carlson, T. N., 1969: Synoptic histories of three African distur- to the filtered data as well. A correction for this con- bances that developed into Atlantic hurricanes. Mon. Wea. tribution must be applied to our statistics that depend Rev., 97, 256–276. on wave activity. The contribution of the storms to vari- Dickinson, M., and J. Molinari, 2002: Mixed Rossby-gravity waves ance in our filtered datasets tends to be small but can and western Pacific tropical cyclogenesis. Part I: Synoptic make the waves appear to be active on the genesis date evolution. J. Atmos. Sci., 59, 2183–2196. when perhaps they are not. Wheeler et al. (2000) ap- Ferreira, R. N., and W. H. Schubert, 1997: Barotropic aspects of ITCZ breakdown. J. Atmos. Sci., 54, 261–285. plied a wave activity threshold equal to the mean vari- Frank, N. L., and G. B. Clark, 1980: Atlantic tropical systems of ance of their filtered wave index time series before 1979. Mon. Wea. Rev., 108, 966–972. making their regression composites, and included activ- Gray, W. M., 1968: Global view of the origin of tropical distur- ity in their composites only during periods when a time bances and storms. Mon. Wea. Rev., 96, 669–700. series of windowed variance exceeded that threshold. ——, 1979: Hurricanes: Their formation, structure and likely role in the tropical circulation. Meteorology over the Tropical We selected the same activity threshold, plus a correc- Oceans, D. B. Shaw, Ed., Royal Meteorological Society, 155– tion estimated to be the contribution of the mean storm 218. to variance in the band. We estimated this contribution ——, 1985: Tropical cyclone climatology. WMO Tech. Doc. as follows. We constructed a storm-centered mean lon- WMO/TD-72, Vol. 1, WMO, Geneva, Switzerland, 145 pp. gitude–time array of unfiltered OLR anomalies associ- Hall, J. D., A. J. Matthews, and D. J. Karoly, 2001: The modula- ated with all Northern Hemisphere storms in our tion of tropical cyclone activity in the Australian region by the Madden–Julian Oscillation. Mon. Wea. Rev., 129, 2970– dataset that lasted longer than 10 days. We found the 2982. mean storm-centered OLR patterns for each day of the Hendon, H. H., and B. Liebmann, 1991: The structure and annual life of the storms, giving the relevant mean temporal a variation of antisymmetric fluctuations of tropical convection well as spatial structures. and their association with Rossby-gravity waves. J. Atmos. We plotted the resulting mean and did not discern Sci., 48, 2127–2140. Kiladis, G. N., K. H. Straub, and P. T. Haertel, 2005: Zonal and any distinct wave structures other than the signature of vertical structure of the Madden–Julian oscillation. J. Atmos. the mean storm itself. We then applied the filters to this Sci., 62, 2790–2809. mean storm, squared the resulting anomalies, and Kurihara, Y., and R. E. Tuleya, 1981: A numerical simulation passed the same moving averages over the result that study on the genesis of a tropical storm. Mon. Wea. Rev., 109, we applied to calculate wave activity as discussed in the 1629–1653. text. The estimated contribution of the storms to the ——, and ——, 1982: Influence of environmental conditions on the genesis of a tropical storm. Intense Atmospheric Vortices, variance in each wave band is just this variance estimate L. Bengtsson and J. Lighthill, Eds., Springer-Verlag, 71–79. at the time and place of genesis. The values are 10.3, Landsea, C. W., 1993: A climatology of intense (or major) Atlan- Ϫ 30.2, 83, and 1.7 (W m 2)2 for the MJO, ER, MRG– tic hurricanes. Mon. Wea. Rev., 121, 1703–1713. SEPTEMBER 2006 FRANK AND ROUNDY 2417

Liebmann, B., H. H. Hendon, and J. D. Glick, 1994: The relation- Straub, K. H., and G. N. Kiladis, 2002: Observations of a convec- ship between tropical cyclones of the western Pacific and tively coupled Kelvin wave in the eastern Pacific ITCZ. J. Indian Oceans and the Madden–Julian Oscillation. J. Meteor. Atmos. Sci., 59, 30–53. Soc. , 72, 401–411. Takayabu, Y. N., 1994: Large-scale cloud disturbances associated Madden, R. A., and P. R. Julian, 1994: Observations of the 40– with equatorial waves. Part I: Spectral features of the cloud 50-day tropical oscillation—A review. Mon. Wea. Rev., 122, disturbances. J. Meteor. Soc. Japan, 72, 433–448. 814–837. Thorncroft, C., and K. Hodges, 2001: African easterly wave vari- Maloney, E. D., and D. L. Hartmann, 2001: The Madden–Julian ability and its relationship to Atlantic tropical cyclone activ- Oscillation, barotropic dynamics, and North Pacific tropical ity. J. Climate, 14, 1166–1179. cyclone formation. J. Atmos. Sci., 58, 2545–2558. Wang, H., and W. M. Frank, 1999: Two modes of tropical cyclo- Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial genesis: An idealized simulation. Preprints, 23d Conf. on area. J. Meteor. Soc. Japan, 44, 25–43. Hurricanes and Tropical Meteorology, Dallas, TX, Amer. McBride, J. L., and R. M. Zehr, 1981: Observational analysis of Meteor. Soc., 923–924. tropical cyclone formation. Part II: Comparison of non- Wheeler, M., and G. N. Kiladis, 1999: Convectively coupled equa- developing vs. developing systems. J. Atmos. Sci., 38, 1132– torial waves: Analysis of clouds and temperature in the wave- 1151. number-frequency domain. J. Atmos. Sci., 56, 374–399. Molinari, J., and D. Vollaro, 2000: Planetary and synoptic-scale influence on eastern Pacific tropical cyclogenesis. Mon. Wea. ——, and K. Weickmann, 2001: Real time momitoring of modes Rev., 128, 3296–3307. of coherent synoptic to intraseasonal convective variability. Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure Mon. Wea. Rev., 129, 2677–2694. and properties of African waves disturbances as observed ——, G. N. Kiladis, and P. J. Webster, 2000: Large-scale dynami- during phase III of GATE. Mon. Wea. Rev., 105, 317–333. cal fields associated with convectively coupled equatorial Ritchie, E. A., and G. J. Holland, 1999: Large-scale patterns as- waves. J. Atmos. Sci., 57, 613–640. sociated with tropical cyclogenesis in the western Pacific. Zehr, R. M., 1992: Tropical cyclogenesis in the western North Mon. Wea. Rev., 127, 2027–2043. Pacific. NOAA Tech. Rep. NESDIS 61, 181 pp. [Available Roundy, P. E., and W. M. Frank, 2004a: A climatology of waves in from the U. S. Department of Commerce, NOAA/NESDIS, the equatorial region. J. Atmos. Sci., 61, 2105–2132. 5200 Auth. Rd., Washington, DC 20233.] ——, and ——, 2004b: Effects of low-frequency wave interactions Zhang, C., and P. J. Webster, 1989: Effects of zonal flows on on intraseasonal oscillations. J. Atmos. Sci., 61, 3025–3040. equatorially trapped waves. J. Atmos. Sci., 46, 3632–3652.