1. Introduction
Prior research has established a global climatology of the frequency and location of tropical cyclogenesis and empirically determined the environmental parameters supportive of tropical cyclone (TC) formation (e.g., Gray 1968, 1979). Specifically, six environmental parameters have been linked to TC genesis including sea surface temperatures (SSTs) greater than 26°C, sufficiently high midtropospheric relative humidity, conditional instability, enhanced lower-tropospheric relative vorticity, weak vertical wind shear, and sufficiently large planetary vorticity (e.g., Gray 1968; McBride and Zehr 1981; Emanuel and Nolan 2004). With the exception of planetary vorticity, these environmental genesis parameters influencing TC genesis are strongly influenced by the annual cycle (e.g., Gray 1968), while also exhibiting significant sensitivity to phenomena that include, but are not limited to, El Niño–Southern Oscillation (ENSO; e.g., Ramage and Hori 1981; Gray 1984, 1985), the Madden–Julian oscillation (MJO; e.g., Liebmann et al. 1994; Maloney and Hartmann 2000a,b), and convectively coupled equatorial waves (e.g., easterly waves; Burpee 1972; Frank and Roundy 2006; Schreck et al. 2012). Further complicating the impact of these phenomena on these environmental genesis parameters are the nonlinear, multiscale interactions among these phenomena (e.g., ENSO and MJO; Nakazawa 1988; Anyamba and Weare 1995; Li et al. 2012) and the differences in the annual cycles of these phenomena (e.g., Wheeler and Kiladis 1999; Roundy and Frank 2004; Frank and Roundy 2006).
However, subsequent research has noted that TC activity within a given basin tends to cluster in 2–3-week active periods (Gray 1979) with ~(10%–35%) of TCs forming in the presence of at least one preexisting TC in the same basin (e.g., Briegel and Frank 1997; Krouse and Sobel 2010; Ventrice et al. 2011). These multiple TC events (MTCEs) have been shown to occur more frequently during the peak of TC season in the western North Pacific (WPAC) compared to single TC events (STCEs; Krouse and Sobel 2010), potentially suggesting differences between MTCE and STCE environments. Indeed, prior work has suggested that environmental phenomena such as Rossby waves radiated from a preexisting TC may be more strongly supportive of MTCE development as each trough in the Rossby wave packet serves as a potential TC genesis location (e.g., Davidson and Hendon 1989; Carr and Elsberry 1995; Holland 1995; Ritchie and Holland 1999). MTCEs may also be favored within packets of mixed Rossby–gravity (MRG) waves or tropical depression (TD)-type disturbances with each trough in the wave packet serving as a potential location for TC genesis (e.g., Dickinson and Molinari 2002; Aiyyer and Molinari 2008; Ventrice et al. 2011). A series of convectively active Kelvin waves have also been associated with western North Pacific (WPAC) MTCE development through large-scale enhanced convection, enhanced lower-tropospheric cyclonic vorticity, and intertropical convergence zone (ITCZ) breakdown into multiple vortices (Ferreira and Schubert 1997; Schreck and Molinari 2011).
Moreover, MRG waves, TD-type waves, and Kelvin waves occur more frequently during the convectively active MJO due to large-scale enhanced convection (Dickinson and Molinari 2002; Aiyyer and Molinari 2008; Schreck and Molinari 2011; Schreck et al. 2012) and zonal wave accumulation (e.g., Webster and Chang 1988; Holland 1995; Maloney and Hartmann 2001). North Atlantic (NATL) MTCEs have also been shown to occur more frequently during high-amplitude convectively active MJO events potentially due to an extended African easterly jet, which yields a longer period of amplification for easterly waves along the jet (Ventrice et al. 2011). Moreover, MJOs that yield eastern North Pacific (EPAC) MTCEs may be associated with significantly stronger vertical wind shear compared to MJO events that yield STCEs (e.g., Maloney and Hartmann 2000a, 2001; Aiyyer and Molinari 2008). Both the MJO and ENSO and their associated basin-scale environmental changes (e.g., Gray 1984; Liebmann et al. 1994; Camargo and Sobel 2005) could substantially influence MTCE occurrence given that MTCEs may require a larger region of the basin to be favorable for TC genesis compared to STCEs.
Despite these aforementioned differences between MTCEs and STCEs identified in prior work, there has yet to be a multibasin climatology of MTCE number and location and the environmental factors responsible for their occurrence. In fact, MTCE climatologies only exist within the WPAC and, to lesser extent, the NATL, with many of these studies utilizing different TC subsets, investigating different aspects of MTCE climatologies, and examining the impact of different environmental factors on MTCE occurrence (e.g., Ritchie and Holland 1999; Krouse and Sobel 2010; Ventrice et al. 2011). The broader applicability of these prior MTCE studies to all basins is especially uncertain given the differences among basins in their large-scale environment (e.g., Gray 1968, 1979; Camargo et al. 2007), convectively coupled equatorial wave and MJO activity (e.g., Roundy and Frank 2004; Frank and Roundy 2006; Schreck et al. 2012), TC genesis pathways (McTaggart-Cowan et al. 2008, 2013), and TC size (Merrill 1984; Chavas and Emanuel 2010; Knaff et al. 2014).
In light of the uncertainties regarding MTCEs, the present study provides an MTCE climatology in the NATL, EPAC, and WPAC by building upon prior research. Specifically, the climatological number of MTCEs, location of MTCEs, spatiotemporal separation between TCs during MTCEs, and environmental factors responsible for MTCEs are examined. Based upon prior work, the present study seeks to investigate the following hypotheses: 1) MTCEs comprise a substantial portion of TC activity across all basins, 2) strong similarities exist among basins in the spatiotemporal separation between TCs during MTCEs, 3) MTCEs require an environment supportive of TC genesis on a larger-scale relative to STCEs, and 4) TC-induced Rossby wave radiation is important in triggering MTCE occurrence. The implications of the present study potentially include additional insight into the sensitivity of TC genesis to environmental conditions.
The remainder of the manuscript is divided into three parts. Section 2 describes the data and methods used. Section 3a examines climatological MTCE number, location, and spatiotemporal separation between TCs during MTCEs. Section 3b compares MTCE and STCE composites in the EPAC and attempts to estimate the frequency of TC-induced Rossby wave radiation during MTCEs to provide insight into the potential environmental factors responsible for MTCEs. Section 4 summarizes the results and discusses future work.
2. Data and methods
a. Data
The MTCE climatology in the present study is constructed using TC data from each World Meteorological Organization (WMO) Regional Specialized Meteorological Center (RSMC) included in version 3 (revision 6) of the International Best Track Archive for Climate Stewardship (IBTrACS; Knapp et al. 2010). The National Hurricane Center is the RSMC in the EPAC and NATL, while the Japanese Meteorological Agency (JMA) is the RSMC in the WPAC. All 6-h IBTrACS TCs of tropical origin forming at or equatorward of 20°N in the NATL, EPAC, and WPAC from 1979–2010 are chosen for analysis. A latitude threshold of 20°N is conservatively chosen to exclude all TCs with potentially baroclinic origins given differences in the environment, genesis mechanisms, and structure of TCs with baroclinic origins to TCs with tropical origins (e.g., Hart 2003; McTaggart-Cowan et al. 2008; Guishard et al. 2009). TC genesis is defined as the first 6-h data point in which an RSMC defines a TC as either a tropical depression or a tropical storm. A small number of TCs are excluded in each basin due to the 1-min averaged maximum 10-m wind speed exceeding 34 kt (1 kt ≈ 0.51 m s−1) at TC genesis since the timing of TC genesis has likely been missed (Elsner et al. 1996; McTaggart-Cowan et al. 2008, 2013). To obtain a 1-min averaged maximum 10-m wind speed, the JMA 10-min averaged maximum 10-m wind speed is multiplied by 1.14. A wind speed threshold is not used to define TC genesis in the current study for two reasons: heterogeneous wind reporting criteria among forecast centers (e.g., 10- vs 1-min averaged wind speed) and occasionally substantial differences in wind speed estimates for TCs among forecast centers (Knapp et al. 2010).
To identify the environmental factors favoring MTCE occurrence, the daily-averaged 2.5° × 2.5° National Oceanographic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL) interpolated outgoing longwave radiation (OLR) data (Liebmann and Smith 1996) are used as a proxy for tropical moist deep convection. The large-scale wind field during MTCE occurrence is represented using the 6-h 0.5° × 0.5° National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR; Saha et al. 2010). The CFSR is chosen due to its better representation of best-track TC position, best-track TC intensity, the life cycle of best-track TC intensity, and composite TC structure relative to other reanalyses (e.g., Schenkel and Hart 2012; Wood and Ritchie 2014; Brammer and Thorncroft 2015).
b. Methods
1) MTCE, STCE, and MDR definitions
Similar to Krouse and Sobel (2010), an MTCE is defined as two or more TCs simultaneously occurring within the same basin. The present study places particular emphasis on examining the first 6-h time of MTCE occurrence with the TC genesis events occurring during this first 6-h time referred to as “new TC genesis events.” A TC is defined as an STCE if no other TCs occur within the same basin during the TC life cycle. MTCEs are compared against STCEs to determine if their climatological number, location, and potential environmental triggering factors are unique. Table 1 provides the mean and 95% confidence intervals for TC latitude, longitude, Julian day, and sample size for STCEs and new TC genesis events during MTCEs. The number of preexisting TCs and new TC genesis events during MTCEs that occur within the main development region (MDR) are examined in the NATL (10°–20°N, 90°–17°W; Goldenberg et al. 2001), EPAC (8°–20°N, 130°W–western Central American coast; Wang and Lee 2009), and WPAC (5°–21°N, 110°–160°E; Pun et al. 2013).
Lower limit of 95% confidence interval, mean, and upper limit of 95% confidence interval for best-track TC genesis latitude (°N), genesis longitude (°E), Julian day of TC genesis, and sample size for NATL, EPAC, and WPAC STCEs and new TCs during MTCEs. The confidence intervals are determined using a bootstrap approach.
2) Compositing methodology
To further isolate the environmental factors that potentially yield a favorable environment for MTCE occurrence, composites are constructed for both MTCEs and STCEs relative to their first 6-h occurrence time (e.g., Frank and Roundy 2006; Schenkel and Hart 2015). First, a 32-yr (1979–2010) daily and 6-h climatological mean are constructed for OLR and CFSR data, respectively, by retaining the annual cycle and the first four harmonics of the seasonal cycle (e.g., Schreck et al. 2011; Ventrice et al. 2011, 2012). Next, TC genesis-centered grids are fixed upon the TC genesis location at daily intervals from 20 days prior to 20 days after TC genesis (41 total grids). Each TC-centered composite grid has a uniform grid spacing of 250 km for the OLR data (2.5° × 2.5° native output grid) and 50 km for the CFSR data (0.5° × 0.5° native output grid). The composite grid consists of 20 000 km in the zonal direction and 6000 km in the meridional direction.
To separate the influence of different time scales in the composites, OLR and reanalysis data are filtered and composited according to three time–frequency bands (Sakaeda and Roundy 2014) including 1) the background state (time scales greater than 100 days), 2) intraseasonal time scales (time scales between 20 and 100 days), and 3) transient time scales (time scales less than 20 days). The background state includes contributions from the seasonal cycle, ENSO, and multidecadal variability. While the MJO strongly contributes to intraseasonal time scales (e.g., Madden and Julian 1971, 1994; Zhang 2005), more recent work has suggested that intraseasonal variability in some basins (e.g., EPAC) may also be driven by local factors (Rydbeck et al. 2013). Transient time scales are primarily dominated by TCs and their respective precursor disturbances in the composites. The filter is computed by applying a Fourier transform in time, zeroing out the coefficients outside of the selected bands, and performing an inverse Fourier transform on the zeroed-out data. Such an approach can yield Gibbs ringing especially due to the presence of TCs (Schreck et al. 2011; Aiyyer et al. 2012; Schreck et al. 2012), which should be kept in mind when interpreting these results since the TC is not removed prior to filtering. Composite analysis results are only presented for EPAC MTCEs and STCEs given that the NATL and WPAC exhibit complex significant differences with the EPAC.
3) Analysis of TC-induced Rossby wave radiation
4) Statistical significance methodology
The present study utilizes three types of statistical significance tests. Linear correlation coefficients are tested to determine if they are statistically significantly different from 0 at the 95% confidence interval using a two-tailed t test (e.g., Camargo and Sobel 2005; Sakaeda and Roundy 2014). A Kolmogorov–Smirnov goodness-of-fit statistical significance test is used to determine whether two samples are drawn from the same distribution using a 95% confidence interval (e.g., Camargo and Sobel 2005, 2010; Chavas and Emanuel 2010).
For the MTCE and STCE composites, a 1000-sample bootstrap approach is used to determine whether the composited anomalies are statistically significantly different from zero at the 95% confidence interval (e.g., Roundy and Frank 2004; Frank and Roundy 2006; Ventrice et al. 2011). Specifically, a new distribution of anomalies is constructed for a given variable at each grid point by randomly selecting anomalies (with replacement) for times in which a best-track TC is present. The sample size of each new distribution is defined as the number of STCEs and new TC genesis events during MTCEs. A mean is then calculated for the new distribution of anomalies with the process being repeated until there are 1000 means. The distribution of 1000 means is then used to compute the 95% confidence interval of the mean to determine whether the anomalies are statistically significantly different from zero. A similar approach is also used to test the statistical significance of medians.
3. Results
a. Climatology of MTCE and STCE number, location, and separation between TCs
1) Number of MTCEs and STCEs
The total number of STCEs and MTCEs with two, three, and four TCs in the NATL, EPAC, and WPAC are presented in Fig. 1a. For comparison, the total number of uniquely named TCs during STCEs and MTCEs in the NATL, EPAC, and WPAC are also presented in Fig. 1b. Figure 1a reveals that STCEs occur more frequently than MTCEs with the ratio of MTCEs to STCEs being greatest in the EPAC (~0.8) followed by the WPAC (~0.5) and the NATL (~0.3). Consistent with Ventrice et al. (2011), MTCEs in all basins most commonly occur with two TCs. Despite the number of STCEs being greater than MTCEs, Fig. 1b shows that a substantial number of TCs within each basin occur during MTCEs with numbers ranging from 57% in the EPAC to 47% in the WPAC and 34% in the NATL. The WPAC TC fraction (~47%) during MTCEs is slightly larger than previous higher-end estimates (~31%; Krouse and Sobel 2010).
Number of (a) MTCEs with two, three, and four TCs and STCEs within the NATL, EPAC, and WPAC and (b) uniquely named TCs for MTCEs and STCEs within the same basins.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Additional insight is provided by examining the total monthly number of MTCEs and STCEs in the NATL, EPAC, and WPAC (Fig. 2). Figure 2 reveals strong similarities between the monthly number of MTCEs and STCEs as suggested by significant correlations computed using a two-sided t test (0.79 ≤ R ≤ 0.90; p ≪ 0.01). However, the MTCE number peaks one month later than STCEs in both the EPAC and WPAC, similar to Krouse and Sobel (2010), with MTCEs constituting the majority of TC activity in the EPAC and WPAC during peak MTCE activity.
Monthly number of STCEs and MTCEs in the NATL, EPAC, and WPAC. Linear correlation coefficients between the monthly number of STCEs and MTCEs in each basin are provided along with their p values.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Next, the annual number of STCEs (Fig. 3a) and MTCEs (Fig. 3b) in the NATL, EPAC, and WPAC is also shown. Figures 3a and 3b reveal that the annual number of MTCEs and STCEs in the WPAC are significantly negatively correlated according to a two-sided t test (R = −0.70; p ≪ 0.01) whereas MTCEs and STCEs in the NATL and EPAC exhibit nonsignificant correlations. Nonsignificant correlations of MTCE number with ENSO indices (not shown) in each basin suggest that ENSO is not significant in forcing MTCE activity. Larger interannual and interdecadal variability in MTCE number compared to STCEs is noted with a substantial decrease in EPAC and WPAC MTCE numbers following the early-to-mid 1990s (Fig. 3b).
Time series of annual number of (a) STCEs and (b) MTCEs in the NATL, EPAC, and WPAC. The linear correlation coefficients between STCEs and MTCEs in each basin are also provided along with their p values.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
These results demonstrate that TCs frequently occur as part of MTCEs in the EPAC, the WPAC, and, to a lesser extent, the NATL, especially during the peak of TC season of certain years. While similarities between the monthly number of MTCE and STCEs in each basin suggest that similar factors in the annual cycle trigger MTCEs and STCEs, differences in the annual number of MTCEs and STCEs may suggest that each responds differently to both interannual and intraseasonal time scales.
2) MTCE and STCE location
Figure 4 depicts STCE location (black TC symbol) and STCE number (shading) on a 5° latitude × 5° longitude grid for the NATL (Fig. 4a), EPAC (Fig. 4b), and WPAC (Fig. 4c). Similar plots are also constructed for new TC genesis locations during MTCEs in the NATL (Fig. 5a), EPAC (Fig. 5b), and WPAC (Fig. 5c). Comparison of Figs. 4 and 5 reveals that STCE genesis locations are spread more uniformly throughout the basin compared to new TC genesis locations especially in the EPAC and NATL. Figure 5 also reveals that a large majority of EPAC (~85%) and NATL (~75%) new TC genesis locations during MTCEs form in the eastern MDR (green rectangle in Figs. 5a,b), while most STCE genesis locations are located to the west (Figs. 4a,b). In contrast to the EPAC and NATL, both new TC genesis locations during MTCEs and STCEs are spread throughout the WPAC.
Plan view of locations (black TC symbol) and numbers of new TC genesis events during STCEs (color shaded) within a 5° longitude × 5° latitude grid within the (a) NATL, (b) EPAC, and (c) WPAC. The dark green rectangle denotes the MDR in each basin.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
As in Fig. 4, but for the number of MTCEs.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
The clustering of new TC genesis locations during MTCEs in the eastern NATL and EPAC MDR may suggest that there are unique environmental factors in these regions that favor MTCEs (e.g., African easterly jet; Ventrice et al. 2011). In contrast, the uniform distribution of both STCEs and new TC genesis location throughout the WPAC may be due to much of the WPAC being supportive of TC genesis throughout a large part of the year (e.g., Gray 1968; Camargo et al. 2007; Tippett et al. 2011).
3) Horizontal distance between TCs during MTCEs
The horizontal distance between new TC genesis and preexisting TCs during MTCEs is shown with the preexisting TC location (black TC symbol) and preexisting TC number (shading) on a 500 km × 500 km grid centered on new NATL (Fig. 6a), EPAC (Fig. 6b), and WPAC (Fig. 6c) TC genesis location. Violin plots of the zonal location of preexisting TCs relative to new TCs during MTCEs are also presented for a more quantitative assessment (Fig. 7). Figure 6 reveals that the majority of preexisting TCs are located west of new TC genesis locations in the NATL (67%), EPAC (80%), and WPAC (72%). Figures 6 and 7 both reveal remarkably similar kernel density estimates and median preexisting TC zonal location relative to new TC location among basins. Specifically, median values are −1640, −1830, and −2010 km in the NATL, EPAC, and WPAC (e.g., Frank 1982; Briegel and Frank 1997; Ritchie and Holland 1999), respectively, with these values being nonsignificantly different from one another according to a bootstrap approach. Kolmogorov–Smirnov significance testing of the zonal distance between TCs fails to reject the null hypothesis that the two samples are taken from the same distribution (0.11 ≤ p ≤ 0.32).
Plan view of locations (black TC symbol) and numbers of preexisting TC locations relative to the new TC genesis event location during MTCEs (color shaded) within a 500 km × 500 km grid in the (a) NATL, (b) EPAC, and (c) WPAC. The location of new TC genesis is denoted by the large red TC symbol.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Violin plot of zonal distance (km) between new TC genesis and preexisting TCs during MTCEs in the NATL, EPAC, and WPAC. The violin plot displays the median (white circle), interquartile range (thick dark gray bar), minimum and maximum values (thin dark gray bars), and kernel density estimate (color shading).
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
The similarities in zonal distance between preexisting TC location and new TC genesis among basins is remarkable given the differences in the environment (e.g., Gray 1968; Roundy and Frank 2004; Frank and Roundy 2006) and in TC genesis and size among basins (e.g., Merrill 1984; Chavas and Emanuel 2010; McTaggart-Cowan et al. 2013). The range of zonal distances between TCs in each basin is also similar to the zonal wavelength of easterly waves (e.g., Burpee 1974; Reed et al. 1977; Nitta et al. 1985), TC-induced Rossby wave radiation (e.g., Ferreira and Schubert 1997; Ritchie and Holland 1999; Krouse et al. 2008), and mixed Rossby–gravity waves (e.g., Dickinson and Molinari 2002; Roundy and Frank 2004; Frank and Roundy 2006), suggesting that some combination of these phenomena may be responsible for MTCE formation. Moreover, the preference for the new TC genesis to occur eastward of preexisting TCs during MTCEs in all basins may suggest that MTCE occurrence is consistent with TC genesis due to the MJO (Liebmann et al. 1994; Frank and Roundy 2006; Ventrice et al. 2011), convectively coupled equatorial waves (e.g., Dickinson and Molinari 2002; Roundy and Frank 2004; Frank and Roundy 2006), and TC-induced Rossby wave radiation (e.g., Ferreira and Schubert 1997; Ritchie and Holland 1999; Krouse et al. 2008).
4) Temporal separation between TC genesis events during MTCEs
The total number of preexisting TC genesis events is shown relative to new TC genesis in one-day bins (Fig. 8) for the NATL, EPAC, and WPAC. Figure 8 reveals that a large majority of preexisting TCs form within 4 days prior to the day of new TC genesis in the NATL (81%), EPAC (68%), and WPAC (71%). In fact, median values of temporal separation between TCs are consistent among the NATL (3.00 days), EPAC (3.25 days), and WPAC (3.25 days; e.g., Ritchie and Holland 1999; Krouse and Sobel 2010), with these values being nonsignificantly different from each other according to a bootstrap approach. The peak number in preexisting TC genesis events also occurs one day prior to new TC genesis followed by a sharp decrease on the day of new TC genesis. Kolmogorov–Smirnov significance testing fails to reject a null hypothesis that assumes that the two samples in Fig. 8 are taken from the same distribution (0.26 ≤ p ≤ 0.80).
Number of preexisting TC genesis occurrences relative to number of days to new TC genesis during NATL, EPAC, and WPAC MTCEs utilizing one-day bins.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
The consistency between the formation time scales of preexisting TCs before new TC genesis (3.00–3.25 days) provides additional evidence that the processes responsible for MTCE occurrence are likely similar among basins. The 3.00–3.25-day time scale is also consistent with the threshold time scale used by Ritchie and Holland (1999) (i.e., 72 h before new TC genesis) to define TC-induced Rossby wave radiation induced tropical cyclogenesis in the WPAC further suggesting the importance of TC-induced Rossby wave radiation to MTCE occurrence.
b. Environmental factors contributing to MTCE occurrence
1) Composite analysis of factors responsible for EPAC MTCEs and STCEs
The composite analysis primarily focuses on the intraseasonal time scales since these anomalies are most strongly significantly different between the MTCE and STCE composites. In contrast, the data for the background state and transient time scales are not shown because there are generally no significant differences between MTCEs and STCEs beyond the transient anomalies associated with the preexisting TC. Such a result further suggests that ENSO is not crucial to EPAC MTCE occurrence. The composite analysis examines EPAC MTCEs and STCEs, which are not characteristic of either NATL or WPAC MTCEs and STCEs.
(i) OLR anomalies
The analysis begins by examining time–longitude plots of OLR anomalies (shaded) and intraseasonal-filtered OLR anomalies (black contours) meridionally averaged between 500 km south and north of new TC genesis during MTCEs (Fig. 9a), STCE genesis (Fig. 9b), and the difference between STCEs and MTCEs (Fig. 9c). While TCs can significantly project onto intraseasonal time scales, comparison of the anomalies and intraseasonal-filtered anomalies can serve as a litmus test for whether the filtered anomalies are filtering artifacts. Plan view plots of OLR anomalies (shaded), total OLR (black contours), and 850-hPa anomalous wind vectors at day 8 prior (Fig. 10), day 4 prior (Fig. 11), and the day of MTCE and STCE occurrence (Fig. 12) are presented, respectively. In Figs. 10–12, panels (a)–(c) depict MTCEs, STCEs, and the difference between the two, respectively.
Days relative to event occurrence vs zonal distance from genesis location of OLR anomalies (W m−2; shaded) and intraseasonal-filtered OLR anomalies (W m−2; black contours) for EPAC (a) MTCEs and (b) STCEs, and (c) their differences meridionally averaged between 500 km south and north of TC formation. The black dashed lines labeled TC and P denote the location of the TC and preexisting TC, respectively; while ISO+ (ISO−) denotes the envelope of positive (negative) intraseasonally filtered OLR anomalies. Anomalies are only shown if they are significantly different from zero at the 95% confidence interval.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Plan distance-from-genesis view of OLR anomalies (W m−2; shaded), total OLR (W m−2; black contours), and 850-hPa anomalous wind vectors (m s−1; black vectors) 8 days prior to EPAC (a) MTCEs and (b) STCEs, and (c) their differences. OLR anomalies and anomalous wind vectors are only shown if they are significantly different from zero at the 95% confidence interval.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
As in Fig. 10, but 4 days prior to MTCE and STCE occurrence.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
As in Fig. 10, but for the day of MTCE and STCE occurrence. The location of the new TC and median location of the preexisting TC are denoted by the orange TC symbol without a label and an orange TC symbol labeled P, respectively.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Figure 9a reveals significant basin-scale negative intraseasonal OLR anomalies (denoted ISO− in Fig. 9a) that initially propagate rapidly eastward in the two weeks prior to MTCE occurrence. These composited intraseasonal anomalies presented here and throughout the rest of this section for the MTCE composites strongly resemble composites of all EPAC TC genesis events associated with intraseasonal time scales (e.g., MJO; Maloney and Hartmann 2000a,b, 2001). The rapid eastward propagation slows as these basin-scale negative intraseasonal OLR anomalies reach the domain center coinciding with the excitation of a packet of westward propagating waves (e.g., Nakazawa 1986, 1988; Aiyyer and Molinari 2008). The westward propagating waves have a period of ~4 days, a zonal wavelength of ~1800 km, and an eastward group velocity (Fig. 9a) consistent with characteristics of easterly waves or mixed Rossby–gravity waves (e.g., Burpee 1974; Reed et al. 1977; Dickinson and Molinari 2002; Diaz and Aiyyer 2013). The westward propagating wave packet serves as a source of multiple high-amplitude precursor disturbances for the preexisting TC and new TC (labeled P and TC in Fig. 9a), exhibiting strong similarities to an EPAC MTCE case study (Aiyyer and Molinari 2008). Figures 10a, 11a, and 12a demonstrate the slow eastward propagation of basin-scale negative OLR anomalies as the anomalies gradually increase in magnitude and shrink in horizontal scale prior to the MTCE emerging from the convective envelope. In contrast, Figs. 9b, 10b, 11b, and 12b do not exhibit any significant OLR anomalies prior to STCE genesis other than synoptic-scale OLR anomalies associated with the STCE. The differences between the MTCE and STCE composited OLR anomalies may suggest that basin-scale, enhanced intraseasonal convection is of primary importance in triggering MTCEs due to 1) the associated large-scale conditions that are supportive of both enhanced convection and TC genesis over a sufficiently large region (i.e., ~1800-km zonal distance between TCs in Fig. 6) and 2) the excitation of a series of TC precursor disturbances (e.g., easterly waves) that appear to be initiated by the arrival of intraseasonal convection (Aiyyer and Molinari 2008).
(ii) Lower-tropospheric zonal wind anomalies
Time–longitude plots of 850-hPa zonal wind anomalies (shaded) and intraseasonal-filtered 850-hPa zonal wind anomalies (black contours) meridionally averaged between 1000 km south and the latitude of new TC genesis during MTCEs (Fig. 13a), STCE genesis (Fig. 13b), and the difference between STCEs and MTCEs (Fig. 13c) are shown in Fig. 13. Figure 13a reveals significant eastward propagating positive and negative intraseasonal zonal wind anomalies in the western and eastern halves of the domain, respectively, in the weeks prior to MTCEs that appear to be coupled to the intraseasonal convective anomalies (Fig. 9a; Maloney and Hartmann 2000a, 2001; Aiyyer and Molinari 2008). Figures 10a, 11a, 12a, and 13a depict a strong negative zonal gradient in intraseasonal zonal wind anomalies maximized near the domain center suggestive of enhanced zonal wave accumulation (e.g., Webster and Chang 1988; Holland 1995; Maloney and Hartmann 2001). Composited 850-hPa relative vorticity anomalies (not shown) suggest that the intraseasonal zonal wind anomalies are not associated with significant relative vorticity anomalies, which contradicts prior work (e.g., Maloney and Hartmann 2000a, 2001; Aiyyer and Molinari 2008). However, these prior studies did not test the relative vorticity anomalies for significance, which is an important caveat given that nonsignificant intraseasonal relative vorticity anomalies are present within the MTCE composites. Additionally, 850-hPa relative vorticity composites do not show any evidence of ITCZ breakdown although this may be an artifact of the compositing process (e.g., Ferreira and Schubert 1997; Wang and Magnusdottir 2005, 2006). While the lower-tropospheric intraseasonal zonal wind anomalies may yield favorable conditions for TC genesis via enhanced zonal wave accumulation, their impacts upon MTCE occurrence appear to be secondary given that MTCEs are triggered 20 days after intraseasonal zonal wind anomaly onset.
As in Fig. 9, but for 850-hPa zonal wind anomalies (m s−1) meridionally averaged between 1000 km south and the latitude of new TC genesis.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
(iii) Upper-tropospheric zonal wind anomalies
It is also instructive to examine time–longitude plots of 200-hPa zonal wind anomalies (Fig. 14) that are constructed similarly to the previously presented plots of lower-tropospheric zonal wind anomalies (Fig. 13). Plan view plots of 200-hPa anomalous wind vectors and 200-hPa temperature anomalies (shaded; Figs. 15–17) are also presented, similar to Figs. 10–12. The MTCE composites (Fig. 14a) reveal significant eastward propagating basin-scale positive and negative intraseasonal zonal wind anomalies in the eastern and western halves of the domain, respectively, coupled to the convective anomalies in the weeks prior to MTCE occurrence. Figures 15–17 reveal a strong positive zonal gradient in upper-tropospheric zonal wind anomalies maximized at the domain center yielding enhanced large-scale upper-tropospheric divergence (e.g., Maloney and Hartmann 2000a, 2001; Aiyyer and Molinari 2008).
As in Fig. 13, but for 200-hPa zonal wind anomalies (m s−1).
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Plan distance-from-genesis view of 200-hPa temperature anomalies (K; shaded) and 200-hPa anomalous wind vectors (m s−1; black vectors) 8 days prior to EPAC (a) MTCEs, (b) STCEs, and (c) their differences. Anomalies are only shown if they are significantly different from zero at the 95% confidence interval.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
As in Fig. 15, but 4 days prior to MTCE and STCE occurrence.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
As in Fig. 15, but on the day of MTCE and STCE occurrence. The location of the new TC and median location of the preexisting TC are denoted by the orange TC symbol without a label and an orange TC symbol labeled P, respectively.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Given the structure of the lower-and upper-tropospheric zonal winds, 850–200-hPa zonal vertical wind shear anomalies are discussed next in a time–longitude plot in Fig. 18 structured similarly to Fig. 13. Large-scale intraseasonal positive zonal vertical wind shear anomalies reduce the climatological easterly vertical wind shear in the eastern half of the domain, while the western half of the domain is characterized by negative intraseasonal vertical wind shear anomalies that enhance the climatological easterly vertical wind shear in the weeks prior to MTCE occurrence (e.g., Maloney and Hartmann 2000a,b, 2001). While the intraseasonal positive vertical wind shear anomalies in the eastern half of the domain appear to yield more favorable conditions for TC genesis (e.g., Gray 1968, 1979; Emanuel and Nolan 2004), the easterly vertical wind shear anomalies in the western half of the domain are located too far south to impact the MTCEs (Figs. 10–12 and 15–17). Figure 17a is suggestive of a reduction in vertical wind shear in the region of TC genesis (e.g., Maloney and Hartmann 2000a, 2001), contradicting the results of Aiyyer and Molinari (2008). The delayed onset time between the basin-scale upper-tropospheric intraseasonal zonal wind anomalies and its associated upper-tropospheric divergence anomalies and reduced vertical wind shear anomalies during MTCEs may suggest that these anomalies are not of primary importance to MTCE occurrence.
As in Fig. 14, but for the 850–200-hPa zonal vertical wind shear anomalies (m s−1).
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
(iv) Upper-tropospheric temperature anomalies
Composited 200-hPa temperature anomalies are depicted in time–longitude plots in Fig. 19, which are structured similarly to the time–longitude plots in Fig. 9. The most significant differences in Fig. 19 are the occurrence of positive temperature anomalies in the eastern half of the STCE domain prior to STCEs (Fig. 19b) and negative temperature anomalies at the same region and time in the MTCE composite (Fig. 19a). Both sets of composited temperature anomalies appear to be associated with the background state (not shown) rather than intraseasonal time scales in the weeks prior to MTCE occurrence. Plan view plots (Figs. 15–17) suggest that the largest differences are concentrated at and to the north of the STCE and MTCE genesis latitude. The negative temperature anomalies in the MTCE composite are indicative of enhanced convective instability compared to STCEs creating more favorable conditions for TC genesis (Gray 1968; Camargo et al. 2007; Tippett et al. 2011). The relatively long temporal scales of the temperature anomalies compared to the MTCE may suggest that these temperature anomalies are not a primary forcing for MTCE occurrence.
As in Fig. 9, but for 200-hPa temperature anomalies (K).
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
2) Implications of composite analysis
The composites presented here suggest that intraseasonal time scales may precondition the large-scale MTCE environment compared to STCEs primarily due to the arrival of enhanced intraseasonal convection and the excitation of multiple TC precursor disturbances. More secondary impacts of intraseasonal time scales on MTCE occurrence include 1) enhanced lower-tropospheric zonal wave accumulation, 2) basin-scale upper-tropospheric divergence, 3) large-scale reductions in zonal vertical wind shear, and 4) enhanced convective instability (e.g., Maloney and Hartmann 2000a, 2001; Aiyyer and Molinari 2008). The intraseasonal time scales in the MTCE composites can likely be attributed to a combination of the MJO (e.g., Madden and Julian 1971, 1994; Zhang 2005) and local intrinsic intraseasonal variability (Rydbeck et al. 2013). However, with the exception of strongly enhanced intraseasonal convection, MTCE and STCE occurrence is comparable during enhanced intraseasonal convection (not shown), suggesting that intraseasonal time scales are not the only factor determining whether MTCEs or STCEs occur (Ventrice et al. 2011).
The MTCE composite results may also be influenced by other TCs occurring in the weeks prior to MTCEs, which is investigated in time–longitude plots of EPAC, NATL, and WPAC TC number during EPAC MTCEs (Fig. 20a), STCEs (Fig. 20b), and the difference between MTCEs and STCEs (Fig. 20c) meridionally summed from 1500 km south to 1500 km north of TC formation. Figure 20c suggests that the largest differences of nearly 10–40 [~(4%–17%)] additional TCs are localized to the west of new TC genesis primarily associated with the preexisting TC ~10 days prior to and the formation of new TCs in the ~10 days after new TC genesis. Thus, Fig. 20 conservatively suggests that the composited anomalies are not an artifact of enhanced TC numbers prior to day 10 before new TC genesis with these preexisting TCs likely serving to enhance the composited anomalies thereafter.
Days relative to genesis vs zonal distance from genesis location of NATL, EPAC, and WPAC TC number in the EPAC (a) MTCEs and (b) STCEs, and (c) their differences summed from 1500 km south and north of TC formation. The grid has been smoothed once with a 9-pt smoother.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
It is also important to emphasize that significant complex differences exist between EPAC MTCE and STCE composites and similar composites in the NATL and WPAC (not shown). Specifically, intraseasonal time scales may be necessary to EPAC MTCE occurrence due to the relatively small region of the EPAC that is favorable for TC genesis (e.g., strong meridional SST, humidity and vertical wind shear gradients; Gray 1968, 1979; Camargo et al. 2007; Wood and Ritchie 2015). Thus, the intraseasonal anomalies may serve to temporarily expand the portion of the EPAC that is favorable for TC genesis yielding more supportive conditions for EPAC MTCEs. EPAC MTCEs may also be different from those in the NATL and WPAC due to the combination of predominantly nonbaroclinic genesis events in the EPAC (McTaggart-Cowan et al. 2013) and the reduced influence of convectively coupled equatorial waves and the MJO on EPAC TC genesis compared to the NATL and WPAC (Frank and Roundy 2006; Schreck et al. 2012).
3) Examination of TC-induced Rossby wave radiation during MTCEs
To examine the role of TC-induced Rossby wave radiation during MTCEs, TC-induced Rossby wave zonal group velocities are calculated from Eq. (1) for meridional wavenumbers n = 0–2 for MTCEs in the NATL, EPAC, and WPAC (Fig. 21). Figure 21 shows that less than half of MTCEs in the NATL (~44%), EPAC (~45%), and WPAC (~38%) meet the criteria [section 2b(3)] for determining whether MTCEs potentially occur because of TC-induced Rossby wave radiation. Of those MTCEs meeting this criterion, nearly all (>97%) of their Rossby wave zonal group velocities are positive, suggesting eastward TC-relative Rossby wave energy radiation from the preexisting TC to the new TC (e.g., Davidson and Hendon 1989; Carr and Elsberry 1995; Holland 1995; Ritchie and Holland 1999). It is important to note that the shallow-water median zonal group velocity of the n = 1, 2 TC-induced Rossby waves in the EPAC is comparable to the zonal group velocity of the westward moving wave packet in the EPAC MTCE composite (~3 m s−1; Fig. 9) and the EPAC MTCE case study of Aiyyer and Molinari (2008) (~4 m s−1). In the remainder of the present study, the n = 1 TC-induced Rossby wave is used because of 1) the strong agreement between the zonal group velocity for the n = 1 TC-induced Rossby wave and the MTCE composites and 2) prior modeling work showing that TCs radiate Rossby wave energy most strongly for n = 1 TC-induced Rossby waves (Krouse et al. 2008).
Violin plot of TC-induced Rossby wave zonal group velocity (m s−1) during MTCEs computed using Eq. (1). The violin plot displays the median (white circle), interquartile range (thick dark gray bar), minimum and maximum values (thin dark gray bar), and kernel density estimate (shading). The sample sizes of each distribution are located above each violin.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
Next, the strength of agreement between theory and observations is demonstrated by comparing scatterplots of the observed zonal distance between TCs versus the shallow-water zonal wavelength of the n = 1 TC-induced stationary Rossby wave [Eq. (2)] in the NATL (Fig. 22a), EPAC (Fig. 22b), and WPAC (Fig. 22c). The shallow-water zonal wavelength is expressed as an integer multiple (shaded dots) since TC-induced Rossby waves can radiate Rossby waves for multiple wavelengths beyond the TC (e.g., Shapiro and Ooyama 1990; Carr and Elsberry 1995; Ferreira and Schubert 1997). The large majority of observed zonal distances between TCs within each basin fall within 1–2 integer multiples of the shallow-water zonal wavelength of TC-induced Rossby waves in the NATL (96%), EPAC (79%), and WPAC (83%). Figure 22 shows significant correlations between the observed zonal distance between TCs versus the shallow-water zonal wavelength of TC-induced Rossby waves in the NATL (R = 0.58; p ≪ 0.01), EPAC (R = 0.90; p ≪ 0.01), and WPAC (R = 0.26; p = 0.01), respectively. The particularly strong correlations in the EPAC are expected given 1) the similarity between the median EPAC zonal environmental steering flow (~2 m s−1; not shown) and the steering flow identified in idealized modeling as likely being most favorable for TC-induced Rossby wave radiation [e.g., ~(2–4) m s−1; Krouse et al. 2008] and 2) the previously mentioned strong similarity among the eastward group velocities calculated here (Fig. 21) with the group velocity estimated in Aiyyer and Molinari (2008) and from the composites presented here (Fig. 9). Weaker yet still significant correlations in the NATL and WPAC compared to the EPAC occur primarily due to an overprediction of the zonal wavelength of Rossby waves from Eq. (2) for cases with large TC-relative zonal environmental steering flows.
Scatterplot of observed zonal distance (km) between TCs during MTCEs and predicted zonal wavelength of meridional wavenumber n = 1 TC-induced stationary Rossby wave (km) during (a) EPAC, (b) NATL, and (c) WPAC MTCEs. The predicted zonal wavelength of the TC-induced stationary Rossby wave is computed from Eq. (2). The linear correlation coefficients between the two quantities are also provided along with their p values. Note that one large outlier in the WPAC data is included in the correlations, but not shown in the plot in order to increase the readability of plots.
Citation: Journal of Climate 29, 13; 10.1175/JCLI-D-15-0048.1
The eastward zonal group velocities and the strong agreement of the zonal distance between TCs and Rossby waves from observations and shallow water theory may suggest that TC-induced Rossby wave radiation is a trigger mechanism for a substantial fraction of MTCEs. Certain basins (e.g., EPAC) may be more favorable for TC-induced Rossby wave radiation by providing the thresholds of environmental easterly vertical wind shear and cyclonic horizontal wind shear theorized to be supportive of Rossby wave radiation (Krouse et al. 2008; Krouse and Sobel 2010). Moreover, the intraseasonal variability detailed in the prior section may also temporarily enhance the environmental favorability of TC-induced Rossby wave radiation via alterations in both environmental vertical and horizontal wind shear. These results together with prior work suggest that MTCEs may be forced by a combination of environmental phenomena interacting across a multitude of scales ranging from synoptic scales (e.g., TCs and associated Rossby wave radiation) to global scales (e.g., intraseasonal time scales including the MJO).
4. Summary and discussion
The present study represents one attempt at providing a multibasin climatology of MTCE number, location, the spatiotemporal separation between TCs during MTCEs, and the potential environmental factors responsible for triggering MTCEs. The primary conclusions of the present study are 1) a large fraction of all TCs in the NATL, EPAC, and WPAC occur during MTCEs; 2) the spatiotemporal separation between TCs is remarkably similar among basins despite differences in the environment and TC structure; 3) intraseasonal time scales may precondition the atmospheric environment to favor MTCE occurrence in the EPAC; and 4) a substantial fraction of MTCEs may be triggered by TC-induced Rossby wave radiation from a preexisting TC. Specifically, 34%–57% of TCs within the tropics occur during MTCEs with EPAC and NATL MTCEs constituting the largest and smallest fraction of TC activity, respectively. Analysis of monthly MTCE number reveals that the seasonal cycle of MTCEs and STCEs are strongly related in all three basins. However, examination of annual TC number reveals larger variability in MTCE numbers compared to STCEs with nonsignificant correlations between MTCEs and STCEs in the EPAC and NATL, suggesting differing responses to both interannual and intraseasonal variability.
With regards to climatological location, new TCs during MTCEs generally form within the eastern and central MDR in the EPAC and NATL, which is east of the climatological formation location of STCEs. In contrast, new TCs during MTCEs and STCEs in the WPAC are both uniformly spread throughout the basin. The majority of preexisting TCs [~(67%–80%)] in each basin occur west of new TCs during MTCEs, exhibiting a remarkably consistent zonal distance between TCs among basins as demonstrated by median values ranging from 1640 to 2010 km. The temporal separation between TCs during MTCEs is also consistent among basins with median values ranging between 3.00 and 3.25 days.
Composites of EPAC MTCEs and STCEs may suggest that the MTCE environment is preconditioned for MTCE occurrence primarily due to the arrival of enhanced intraseasonal convection and the subsequent excitation of a packet of TC precursor disturbances. Intraseasonal lower- and upper-tropospheric zonal wind anomalies and upper-tropospheric temperature anomalies and their associated environmental impacts (e.g., vertical wind shear and convective instability) may play a secondary role in MTCE occurrence given the 2–3-week delay between the onset of these anomalies and MTCE occurrence. The composites also suggest that intraseasonal time scales are likely one of several factors determining whether MTCEs or STCEs occur. It is important to emphasize that these results in the EPAC cannot be extended to the NATL and WPAC given the complex environmental differences among these basins. Last, TC-induced Rossby wave radiation may be an important trigger for MTCEs in all basins, as suggested by 1) eastward TC-induced Rossby wave zonal group velocities from the preexisting TC toward the new TC in 38%–45% of cases with values being comparable to composites and case studies and 2) significant correlations (0.26 ≤ R ≤ 0.90; p ≤ 0.01) among the observed zonal distance between TCs during MTCEs and the predicted zonal wavelength of stationary n = 1 TC-induced Rossby waves radiated from preexisting TCs.
Together with prior work, the present study suggests that a substantial fraction of TCs occur as part of MTCEs with MTCEs exhibiting remarkably similar characteristics among the NATL, EPAC, and WPAC. While the present study has addressed many questions regarding MTCEs, there remain additional questions that have been raised due to this study including the following:
Is the location and timing of TC genesis more predictable if a preexisting TC is present in the NATL, EPAC, or WPAC?
How do the composite results for MTCE and STCEs in the EPAC differ from the NATL and WPAC?
Are composites of MTCEs with three or more TCs different from composites with two TCs?
How would a Southern Hemisphere MTCE climatology compare to the Northern Hemisphere climatology presented here?
These questions remain the focus of ongoing research.
Acknowledgments
This research was supported by an NSF AGS Postdoctoral Research Fellowship. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the NSF. This research has benefited from critical and constructive input from three anonymous reviewers, Naoko Sakaeda of SUNY/Albany, Alan Brammer of SUNY/Albany, Robert Hart of FSU, Paul Roundy of SUNY/Albany, Kristen Corbosiero of SUNY/Albany, Corey Barton of FSU/GFDI, George Kiladis of NOAA/ESRL, and John Molinari of SUNY/Albany. The author would also like to thank Lance Bosart and Daniel Keyser of SUNY/Albany for their support and insights. This work would not have been possible without the availability of the CFSR dataset from NCEP. Interpolated OLR data was provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, from their web site at http://www.esrl.noaa.gov/psd/. All computations were completed using NCL provided by NCAR and figures were completed in GrADS provided by COLA/IGES and Matplotlib written by John D. Hunter.
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