Abstract

The present study investigates relative contributions of different time-scale variations of environmental factors to the tropical cyclone (TC) genesis over the western North Pacific (WNP) during July–August–September–October (JASO). Distinct from previous studies that are concerned with large-scale spatial patterns during a certain period, the present study focuses on local and instantaneous conditions of the TC genesis. Analysis shows that the contribution of convection and lower-level vorticity to the TC genesis is mainly due to intraseasonal and synoptic variations. The contribution of vertical wind shear is largely related to synoptic variations. The contribution of midlevel specific humidity is almost 2 times more from intraseasonal variations than from synoptic variations. The contribution of sea surface temperature (SST) to the TC genesis is mainly due to interannual and intraseasonal variations. The barotropic energy for synoptic-scale disturbances during the TC genesis comes mainly from climatological mean flows over the southwest quadrant and from intraseasonal wind variations over the northeast quadrant of the WNP, respectively. The contribution of interannual variations to the TC genesis is enhanced over the southeast quadrant of the WNP. More TCs form under weak easterly and westerly vertical shears, respectively, during El Niño developing and decaying JASO. The contribution of interannual variations of SST tends to be larger during El Niño decaying than during developing JASO.

1. Introduction

The classic work by Gray (1968) identified some basic environmental factors for tropical cyclone (TC) genesis. Those factors include sea surface temperature (SST), conditional instability, relative humidity in the middle level, cyclonic absolute vorticity in the lower level, anticyclonic relative vorticity in the upper level, and vertical wind shear between the upper and lower levels. Particularly, the western North Pacific (WNP) is the basin of the most active TC genesis around the globe. In this region, the large deep pool of warm water with mean SST above 29°C supplies abundant moisture needed for the development and intensification of TCs. Meantime, the lower-level winds in boreal summer–fall over the WNP feature a convergence between the monsoon westerlies and the trade wind easterlies and a meridional shear line, which are favorable for the TC genesis (Ritchie and Holland 1999; Molinari and Vollaro 2013).

Previous studies have shown that the TC genesis over the WNP is subjected to variations on various time scales (Li 2012). For example, the TC genesis has a pronounced interannual variation, which is closely related to the El Niño–Southern Oscillation (ENSO) (e.g., Chen et al. 1998; Wang and Chan 2002; Camargo et al. 2007; Wu et al. 2012; Cao et al. 2014a). During El Niño summers, the frequency of TC genesis is above normal in the southeast quadrant and below normal in the northwest quadrant of the WNP (Wang and Chan 2002). In addition to ENSO, the TC genesis frequency over the WNP is found to be susceptible to influences of ENSO Modoki (Chen and Tam 2010), the SST gradient between the southwestern Pacific and the western Pacific warm pool (Zhan et al. 2013), the Pacific meridional mode (Zhang et al. 2016), and the SST anomalies in the tropical Indian Ocean, to the east of Australia, and in the northern tropical Atlantic (Zhan et al. 2011; Zhou and Cui 2011; Huo et al. 2015; Cao et al. 2016). Cao et al. (2014b) showed that the interannual variation of the monsoon trough exerts a strong control on the TC genesis over the WNP through modifying lower-level convergence, cyclonic vorticity, and moisture based on idealized numerical model simulations.

Previous studies indicated that the Madden–Julian oscillation (MJO; Madden and Julian 1994), a major intraseasonal oscillation (ISO) in the tropics, plays an important role in the TC genesis and development over the WNP (e.g., Liebmann et al. 1994; Maloney and Hartmann 2001; Kim et al. 2008; Camargo et al. 2009; Huang et al. 2011; Cao et al. 2012). MJO may regulate the TC genesis through the change of background lower-level convergence and vorticity, vertical wind shear, and midlevel humidity (Maloney and Hartmann 2001; Camargo et al. 2009). The dynamic impact of circulations in association with MJO on the TC genesis is through barotropic eddy kinetic energy conversion by which synoptic-scale disturbances obtain energy from the large-scale flows (Maloney and Hartmann 2001; Hsu et al. 2011).

Previous studies are mostly concerned with the environmental conditions of the TC genesis on a large spatial scale averaged in a certain period. For example, studies about interannual variations of TC genesis compared seasonal mean fields between El Niño and La Niña years (e.g., Wang and Chan 2002; Camargo et al. 2007; Wu et al. 2012; Cao et al. 2014a). It is shown that the location of TC genesis over the WNP shifts eastward in El Niño years as a result of the relative humidity and vorticity changes (e.g., Camargo et al. 2007). Nevertheless, TC genesis is still observed over the WNP in La Niña years. This indicates that anomalies in seasonal mean fields cannot fully explain the interannual variations in the TC genesis. Studies about intraseasonal variations of TC genesis compared the environmental fields between the intraseasonal wet and dry phases (e.g., Camargo et al. 2009; Huang et al. 2011; Cao et al. 2012). The TC genesis over the WNP displays an increase in the intraseasonal wet phase and a decrease in the intraseasonal dry phase (Kim et al. 2008; Huang et al. 2011; Zhao et al. 2015). Nevertheless, TC genesis is not exclusive in the intraseasonal dry phase. These studies provide evidence that favorable large-scale environmental conditions averaged in a certain period increase the number of TC genesis. However, they cannot explain the TC genesis when the mean large-scale environmental condition is unfavorable. This is because of a mismatch of the time and spatial scales between the background field change and the TC genesis. The mean field refers to an average state in a certain period, whereas the TC genesis is a local instantaneous phenomenon whose condition is a combination of different time-scale variations. It is possible that the local instantaneous condition is satisfied, but the average field in a certain period is unfavorable for the TC genesis. It is also possible that the average field in a certain period is satisfied, but the local instantaneous condition is unfavorable. Thus, it is necessary to examine the state at a locality to find out the relative contributions of different time scales to the TC genesis.

To illustrate the mismatch between the large-scale average field and the TC genesis locality, we show two examples of TC genesis. The first one is a case on 26 July 1988 that is a La Niña year. This example is used to demonstrate the TC genesis when the interannual component (representing the seasonal mean anomaly field) is unfavorable. The TC formed around 21°N and 141°E along with obvious cyclonic wind and positive humidity anomalies (Fig. 1a). The three components (interannual, intraseasonal, and synoptic variations) of 850-hPa wind, 700-hPa specific humidity, and outgoing longwave radiation (OLR) (the decomposition of the three components will be explained later) show different distributions. Note that the intraseasonal and interannual components at a specific day can represent their respective values within a certain time period as a result of their slow variations. At that day, the interannual component displays anticyclonic vorticity and small positive humidity anomalies around the location of the TC genesis (Fig. 1b). The intraseasonal component provides a very favorable condition for the TC genesis with large cyclonic vorticity and positive specific humidity anomalies (Fig. 1c). The synoptic component of winds is favorable, whereas that of specific humidity is small (Fig. 1d). The temporal evolution of variables at the TC genesis location shows that the TC genesis occurs when the intraseasonal component is favorable (positive vorticity, low OLR, high specific humidity) and the synoptic vorticity field shifts to a favorable state under unfavorable interannual component (negative vorticity) (Figs. 1e–g). In this case, the TC genesis is mainly attributed to intraseasonal and synoptic components when the seasonal mean state (represented by an interannual component) is unfavorable.

Fig. 1.

The (a) original 850-hPa wind (vector) and 700-hPa specific humidity anomalies (shaded, g kg−1) and those on the (b) interannual, (c) intraseasonal, and (d) synoptic time scales on 26 Jul 1988. The time evolution of anomalies of (e) 850-hPa relative vorticity (10−5 s−1), (f) OLR (W m−2), and (g) 700-hPa specific humidity (g kg−1) averaged in a 7.5° × 7.5° box over the TC genesis location on interannual (black), intraseasonal (red), and synoptic (blue) time scales from 20 to 28 Jul 1988. The orange lines denote the original anomalies. The black symbols in (a)–(d) represent the center of TC genesis. The inverted triangles (e)–(g) denote the formation time of TC.

Fig. 1.

The (a) original 850-hPa wind (vector) and 700-hPa specific humidity anomalies (shaded, g kg−1) and those on the (b) interannual, (c) intraseasonal, and (d) synoptic time scales on 26 Jul 1988. The time evolution of anomalies of (e) 850-hPa relative vorticity (10−5 s−1), (f) OLR (W m−2), and (g) 700-hPa specific humidity (g kg−1) averaged in a 7.5° × 7.5° box over the TC genesis location on interannual (black), intraseasonal (red), and synoptic (blue) time scales from 20 to 28 Jul 1988. The orange lines denote the original anomalies. The black symbols in (a)–(d) represent the center of TC genesis. The inverted triangles (e)–(g) denote the formation time of TC.

The second case is a TC genesis on 8 August 2011 when the ISO is in the dry phase. This example is used to demonstrate the TC genesis when the intraseasonal component is unfavorable. This TC formed around 22°N and 137°E (Figs. 2a–d). Apparently, the intraseasonal components of vorticity and specific humidity are unfavorable (Fig. 2c), but the interannual components of vorticity and specific humidity are favorable for the TC genesis (Fig. 2b), and large cyclonic vorticity of the synoptic component provides a favorable condition (Fig. 2d). The temporal evolution of variables at the location of the TC genesis indicates that the TC genesis is mainly due to a shift of synoptic component to a favorable condition under the favorable state of interannual component (Figs. 2e–g). In this case, the TC genesis is mainly attributed to interannual and synoptic components when the intraseasonal state is unfavorable.

Fig. 2.

As in Fig. 1, but for the TC genesis on 8 Aug 2011. The time evolution is from 2 to 10 Aug 2011.

Fig. 2.

As in Fig. 1, but for the TC genesis on 8 Aug 2011. The time evolution is from 2 to 10 Aug 2011.

The above two cases show that the TC genesis may occur when the seasonal mean or intraseasonal phase is unfavorable. Thus, the relationship between TC genesis and mean state change does not reflect properly the contributions of different components. In this study, the statistical analysis is conducted on local and instantaneous conditions of the TC genesis on different time scales. In particular, this study documents quantitatively the relative contributions of different time-scale variations of various environmental factors to the TC genesis. Three questions are to be addressed in this study. The first one is which time-scale variation of one particular environmental factor contributes most to the TC genesis. The second one is whether contributions of various time-scale variations to the TC genesis differ over the different regions of the WNP. The third one is how the contributions of different time-scale variations differ between the El Niño developing and decaying years.

In the remainder of this paper, section 2 describes the data and methods used in the present study. Section 3 presents the percent contributions of different time-scale variations of environmental factors to the TC genesis over the WNP. In section 4, we compare regional differences of percent contributions of different time-scale variations of environmental factors to the TC genesis. In section 5, we compare the contributions of different time-scale variations of factors between El Niño developing and decaying years. Section 6 examines the influence of the TC signal to the contributions of three time-scale variations of relative vorticity. A summary and discussion are given in section 7.

2. Data and methods

The time and position of the TC genesis over the WNP are obtained from the National Climate Data Center’s International Best Track Archive for Climate Stewardship (IBTrACS), version 3 (Knapp et al. 2010). The TC best-track data include 6-hourly longitude and latitude of the TC center and the maximum sustained wind speed. TC genesis is defined as the first record of TC best-track data when the maximum wind speeds reach 25 kt (~12.9 m s−1) over the WNP. The analysis is focused on the region extending from 120°E to the date line and from the equator to 25°N. A total of 530 TC geneses were identified over the above domain during July–August–September–October (JASO) for the period 1979–2013. The TC genesis over the South China Sea [SCS, (0°–25°N, 100°–120°E)] is discussed in section 4 to make a comparison with the WNP. A total of 101 TC geneses were detected over the SCS during the same period. For the period 1982–2013 when SST data are available, there are 489 TC geneses over the WNP during JASO. We have performed a parallel analysis using the maximum wind speed of 33 kt (~17 m s−1) as the criterion for the TC genesis. The obtained results are almost the same though there are some differences in the details.

The daily satellite OLR as a proxy for deep convection is obtained from the National Oceanic and Atmospheric Administration (NOAA) starting from 1974 (Liebmann and Smith 1996). This study employs daily horizontal winds at 850 and 200 hPa and specific humidity at 700 hPa from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data starting from 1948 (Kalnay et al. 1996). The NOAA and NCEP–NCAR datasets have a horizontal resolution of 2.5° in latitude and longitude. The time period analyzed is from 1979 to 2013. The daily mean SST with a 0.25° horizontal resolution is extracted from the NOAA optimum interpolation 1/4° daily SST data starting from September 1981 (Reynolds et al. 2007). The original SST data are converted to 1° horizontal resolution. Note that because of the limitation of the length of SST data, only the period 1982–2013 is used to discuss the contribution of SST variations to the TC genesis.

Previous studies have shown distinct peaks of spectrum in atmospheric and oceanic quantities on intraseasonal (10–90 day) and synoptic (<10 day) bands (Hsu et al. 2011). Thus, we separate the whole original daily anomalous fields into interannual (>90 day), intraseasonal (10–90 day), and synoptic (3–8 day) components. Then, we analyze anomalous fields during JASO. Following Cao et al. (2017), the 10–90-day (3–8 day) variation is obtained using a 9-day (2 day) running mean minus a 91-day (9 day) running mean. We use a 91-day running mean to obtain the interannual variation (>90 days). Based on the average over the WNP domain, the percent variance accounted for by interannual variation is about 10% for OLR and vorticity, about 20% for vertical wind shear and humidity, respectively. The percent variance accounted for by intraseasonal variation is about 30% for OLR and vorticity, and about 40% for vertical wind shear and humidity, respectively; while the percent variance accounted for by synoptic variation is about 40% for OLR and vorticity, and about 30% for vertical wind shear and humidity, respectively.

Previous studies have shown that the dynamic effect of circulations associated with ISO and interannual variation on the TC genesis is through barotropic eddy kinetic energy conversion (Maloney and Hartmann 2001; Hsu et al. 2011; Wu et al. 2012). In the formula of barotropic energy conversion, the basic flows are separated into long-term daily mean (climatological) winds, interannual (>90 day), and intraseasonal (10–90 day) wind anomalies. Eddy winds within 3–8-day periods are obtained by applying the running mean method to daily winds.

We use the ENSO index obtained from the NOAA/Climate Prediction Center (CPC). Warm and cold periods are determined based on a threshold of +/−0.5°C for 3-month running mean of ERSST.v5 SST anomalies in the Niño-3.4 region (5°N–5°S, 120°–170°W). When the Niño-3.4 index is larger than 0.5°C and ENSO reaches a mature phase in the winter, a year is defined as the El Niño developing year. Then the next year is defined as the El Niño decaying year. The El Niño developing years selected during the analysis period are 1982, 1987, 1991, 1994, 1997, 2002, 2004, 2006, and 2009 (Wu et al. 2009; Li 2012). Corresponding, the next years are the El Niño decaying years. We classify 1987 as an El Niño developing year because it not only precedes an ENSO mature winter, but also its circulation is close to other El Niño developing years (Wu et al. 2009). For the 9 selected El Niño developing JASO, we identify a total of 148 TC geneses over the WNP. A total of 126 TC geneses are identified over the WNP in the El Niño decaying JASO.

3. Contributions of different time-scale variations to the TC genesis

In this section, we analyze contributions of environmental parameters to the TC genesis on different time scales over the WNP during JASO. According to previous studies (Gray 1968; Maloney and Hartmann 2001; Camargo et al. 2009; Hsu et al. 2011), six environmental factors are considered, including SST, midlevel moisture as measured by 700-hPa specific humidity, vertical instability as measured by OLR, vertical shear of zonal wind as calculated by the difference of zonal wind between 200 and 850 hPa, lower-level relative vorticity at 850 hPa, and barotropic energy conversion at the 850-hPa level. First, for each TC genesis, we calculated daily anomalous values of these quantities on different time scales averaged in a 7.5° × 7.5° box encompassing the TC genesis location except for SST for which the average is in a 5° × 5° box. Then, the anomalous values of these quantities are partitioned into different bins based on the total anomalies (the sum of anomalies on interannual, intraseasonal, and synoptic time scales). Corresponding to each bin, the number of TC genesis and the averaged total anomalies as well as the averaged anomalies on different time scales are calculated. The percent contributions of different time-scale variations are calculated by dividing the averaged anomaly on one time scale by the absolute value of the averaged total anomaly in the same bin. The percentage of the TC genesis in each bin is calculated by dividing the number of TC genesis within that bin by the total number of TC genesis. Note that the magnitude of the averaged total anomaly may be smaller than the averaged anomaly on one time scale as the averaged anomalies on the three time scales may be of opposite sign. Thus, the percent contribution on one time scale can exceed 100%. The total contribution of interannual, intraseasonal, and synoptic variations add up to be equal to 100% for each bin. Figure 3 displays the percent of OLR, 850-hPa relative vorticity, vertical shear of zonal wind, and 700-hPa specific humidity anomalies accounted for by variations on interannual, intraseasonal, and synoptic time scales along with the percentage of the TC genesis frequency (the probability density function or PDF in short).

Fig. 3.

Percent (%) distribution of anomalies of (a) OLR, (b) 850-hPa relative vorticity, (c) vertical shear of zonal wind between 200 and 850 hPa, and (d) 700-hPa specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales associated with the 530 TCs over the WNP during JASO of 1979–2013. The solid line (scale at right) indicates the PDF distribution of TC genesis number. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

Fig. 3.

Percent (%) distribution of anomalies of (a) OLR, (b) 850-hPa relative vorticity, (c) vertical shear of zonal wind between 200 and 850 hPa, and (d) 700-hPa specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales associated with the 530 TCs over the WNP during JASO of 1979–2013. The solid line (scale at right) indicates the PDF distribution of TC genesis number. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

More TCs (384 cases) occur when the total OLR anomalies are between −60 and 0 W m−2 (Fig. 3a). The intraseasonal and synoptic variations have a comparable positive contribution to the TC genesis when the total OLR anomalies are below −20 W m−2. When the total OLR anomalies are between −20 and 0 W m−2, the intraseasonal variation has a dominant contribution. When the total OLR anomalies are positive, the synoptic variation has a negative effect on the TC genesis. There are 90 TCs (about 17% of total) that occur with positive OLR anomalies. Those results indicate that the major positive contributions of convection are due to intraseasonal and synoptic variations when the convection is favorable to the TC genesis, whereas the major negative effect on the TC genesis is from synoptic variation when the convection is unfavorable to the TC genesis.

About 15% of TCs over the WNP appear when the lower-level vorticity anomalies are between 0 and 0.5 × 10−5 s−1 (Fig. 3b). In this range, the positive contribution of intraseasonal variation is much larger than that of synoptic variation. When the total vorticity anomalies are more than 0.5 × 10−5 s−1, the positive contribution of the synoptic component is comparable to that of the intraseasonal component. The contribution of the interannual component to the TC genesis is small. In addition, there are 76 TCs (about 14% of total) that form when the total vorticity anomalies are less than zero, in which the largest negative effect is due to the synoptic component.

Both convection and lower-level relative vorticity have large contributions to the TC genesis over the WNP through synoptic and intraseasonal variations. In comparison, the synoptic component has a larger contribution when the anomalies are larger, while the intraseasonal component has a larger contribution when the anomalies are smaller. It indicates that the synoptic-scale waves and ISOs play an important role in the TC formation through modulating the convection and lower-level relative vorticity (e.g., Liebmann et al. 1994; Maloney and Hartmann 2001; Cao et al. 2012). The cyclonic lower-level disturbances serve as precursors to the TC genesis (Zehr 1992). Cao et al. (2014c) showed that intraseasonal fluctuation of the monsoon trough exerts a strong control on the TC genesis through dynamic (relative vorticity) and thermodynamic (moisture) effects by numerical experiments using an idealized model. Interactions between a lower-middle level cyclonic vorticity during the active ISO phase and a vortex lead to the generation of outflow at the middle level, which can promote the upward penetration of ascending motion. The convection is important to intensify the lower-level disturbances through convective heating (Gray 1968).

About 91% (484 cases) of TCs over the WNP occur when the vertical zonal wind shear anomalies are between −10 and 10 m s−1 (Fig. 3c). Thus, indeed, weak vertical wind shear is a favorable condition for the TC genesis because strong vertical wind shear disrupts the development of deep convection and the building up of an upper-level warm core (Gray 1979; Cheung 2004). Over the WNP, climatological vertical wind shear is weak easterly wind on average, less than −5 m s−1. More TCs appear when vertical wind shear anomalies are between 0 and 5 m s−1 (Fig. 3c). In this range, the contribution of synoptic variation is comparable to that of intraseasonal variation and the positive anomalies lead to weak total wind shear. When the total vertical wind shear anomalies are between −5 and 0 m s−1, the positive synoptic anomalies indicate a positive contribution as it reduces the mean easterly shear, but it is overwhelmed by negative interannual and intraseasonal anomalies. When the total vertical wind shear anomalies are less than −5 m s−1, the contribution of synoptic variation is comparable to that of interannual variation, whereas the intraseasonal variation appears to be the most unfavorable component. When the total vertical wind shear anomalies are more than 5 m s−1, the interannual variation is the least unfavorable component as the corresponding anomaly is the smallest.

High humidity in the middle level is a favorable condition for TC genesis (Gray 1979; Cheung 2004). If the middle level is too dry, the cooling induced by evaporation enhances downdrafts, thus inhibiting sustainable convective development. Meantime, a dry middle level also inhibits convection development because of entrainment of dry air by ascending parcels. About 64% of TCs over the WNP are observed when the specific humidity anomalies are between 0 and 2 g kg−1, in which the contribution by the intraseasonal variation is almost 2 times than that by the synoptic variation (Fig. 3d). It indicates that the ISO plays an important role in the TC genesis through modifying the specific humidity. This is consistent with Camargo et al. (2009) who suggested that the midlevel relative humidity has the largest contribution to the MJO modulation of TCs. When the total humidity anomalies are negative, synoptic, intraseasonal, and interannual variations may all be unfavorable albeit on different ranges. Quite a few TCs (152 in total, accounting for about 29% of total) occur when specific humidity is unfavorable. The percentage is higher compared to the other three dynamic factors. This suggests that the moisture field is not as critical as the dynamic fields for the TC genesis over the WNP.

A warm enough ocean surface provides the necessary energy and surface heat flux for development of convection (Gray 1968). When the total SST anomalies are positive, the interannual and intraseasonal components have a larger contribution than the synoptic component (Fig. 4). A pronounced feature is that there are half of the TCs (about 50% of total of 265) that form when total SST anomalies are negative, which is mainly due to interannual and intraseasonal components. Thus, instantaneous SST does not appear to be a good indicator for the TC genesis. This is consistent with Chan and Liu (2004) who indicated that the typhoon activity over the WNP has no significant relationship with the local SST. The fact that TCs occur when local SST anomalies are negative is likely because of the high mean SST background in the WNP warm pool region (Wang and Chan 2002). This again suggests that local thermodynamic field is not as critical as the dynamic fields for the TC genesis over the WNP. We have examined SST anomalies one and two days before the TC genesis and found little change in the above features.

Fig. 4.

As in Fig. 3, but for SST associated with the 489 TCs during JASO of 1982–2013.

Fig. 4.

As in Fig. 3, but for SST associated with the 489 TCs during JASO of 1982–2013.

Barotropic energy conversion is an important way through which large-scale circulation changes on intraseasonal and interannual time scales have a significant effect on the TC genesis (Maloney and Hartmann 2001; Wu et al. 2012; Feng et al. 2014). As shown in Fig. 5, about 66% of TCs over the WNP appear when the barotropic energy conversion anomalies are between 0 and 20 × 10−5 m2 s−3. In general, the synoptic-scale disturbances mainly obtain the energy from climatological mean flow and intraseasonal variation of circulation when the barotropic energy conversion is favorable to the TC genesis. Particularly, when the barotropic energy conversion anomalies are between 0 and 10 × 10−5 m2 s−3, the contribution from climatological mean flows is almost 2 times more than that from the intraseasonal variation of circulation. This is mainly associated with monsoon trough. Previous studies have found that 70%–80% of TCs over the WNP form in the monsoon trough region, which features a wind convergence and a meridional wind shear (Ritchie and Holland 1999; Feng et al. 2014). The two important terms of the barotropic energy conversion are due to the meridional shear of zonal wind and the zonal convergence of zonal wind. When the barotropic energy conversion is less than zero, the intraseasonal variation has the largest negative effect. The interannual variation of circulation has a small effect on the development of synoptic-scale disturbances. There are 134 TCs (about 25% of total) that cannot be accounted for by the barotropic energy conversion. In such cases, other types of energy conversions may play a role in the development of synoptic-scale disturbances (Hsu et al. 2011).

Fig. 5.

Percent distribution of 850-hPa barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) associated with the 530 TCs over the WNP during JASO of 1979–2013. The solid line (scale at right) indicates the PDF distribution of TC genesis numbers. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

Fig. 5.

Percent distribution of 850-hPa barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) associated with the 530 TCs over the WNP during JASO of 1979–2013. The solid line (scale at right) indicates the PDF distribution of TC genesis numbers. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

From the above statistics, we observe cases when a single factor is unfavorable for the TC genesis. In such cases, the TC occurrence may be attributed to other factors. To illustrate this, we plot vorticity and OLR anomalies for the 152 TC cases when the total specific humidity anomalies are negative on the TC genesis days (Fig. 6). Among the 152 TC cases, there are 84 TC cases in which both vorticity and OLR anomalies are favorable. There are another 38 TC cases when either OLR or vorticity anomalies are favorable.

Fig. 6.

The scatter diagram of TC genesis according to the relative vorticity (10−5 s−1) and OLR (W m−2) anomalies when the specific humidity anomalies are less than zero. The four digit numbers denote the number of TC genesis falling in the respective sections.

Fig. 6.

The scatter diagram of TC genesis according to the relative vorticity (10−5 s−1) and OLR (W m−2) anomalies when the specific humidity anomalies are less than zero. The four digit numbers denote the number of TC genesis falling in the respective sections.

4. The comparison of relative contributions among the subregions

In this section, we examine whether there are regional differences in the contributions of different time-scale variations of environmental parameters to the TC genesis. The WNP is divided into four subregions based on the latitude of 15°N and longitude of 150°E. This division, though subjective, provides a nearly equal area for the four quadrants. Meantime, the SCS is also included in the comparison. The total averaged positive contributions of different time-scale variations of environmental parameters to the TC genesis are calculated by the sum of the product of individual percent contribution and the corresponding percentage of the TC genesis number in the bins when total specific humidity, vorticity, and barotropic energy conversion anomalies are positive or total OLR anomalies are negative.

One feature to note is that the relative contributions of intraseasonal and synoptic variations of OLR, vorticity, and specific humidity over the northwest and southwest quadrants are similar to those over the whole WNP domain (Figs. 7a–c). This is likely because of a larger number of TC geneses over the northwest and southwest quadrants (199 and 177) than over the northeast and southeast quadrants (75 and 84). A similar feature is observed over the SCS region except for vorticity (Fig. 7b). The contribution of relative vorticity to the TC genesis is much larger from the intraseasonal variation than that from the synoptic variation over the SCS, whereas the contribution of relative vorticity from the intraseasonal and synoptic variations is comparable over the WNP (Fig. 7b). It indicates that the effect of vorticity on the TC genesis is dominated by the intraseasonal variation over the SCS. This may be because synoptic-scale waves are mainly located over the WNP, not over the SCS. Meantime, Li and Zhou (2013) have suggested that the MJO and quasi-biweekly oscillations have obvious impacts on the frequency of TC genesis over the SCS.

Fig. 7.

The percent of TC genesis with positive contributions from anomalies of (a) OLR; (b) relative vorticity; (c) specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales; and (d) barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) over the whole WNP, northwest, northeast, southwest, and southeast quadrants of WNP and the SCS.

Fig. 7.

The percent of TC genesis with positive contributions from anomalies of (a) OLR; (b) relative vorticity; (c) specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales; and (d) barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) over the whole WNP, northwest, northeast, southwest, and southeast quadrants of WNP and the SCS.

A second feature is that there is a major difference in the lower-level barotropic energy conversion between the northeast and southwest quadrants of the WNP. Over the northeast quadrant, synoptic-scale disturbances mainly obtain energy from the intraseasonal variation of environmental flow, whereas over the southwest quadrant, the barotropic energy conversion from climatological mean circulation is much larger than that from the intraseasonal variation of circulation (Fig. 7d). This difference may be largely due to climatological location of the monsoon trough, which extends eastward to 150°E (figure not shown). This is a favorable region for barotropic energy conversion from climatological mean flow to synoptic-scale disturbances (Wu et al. 2012).

Another feature is that the relative contributions of four environmental factors in relation to interannual variations are obviously enhanced over the southeast quadrant. In particular, the specific humidity contribution from the interannual component becomes larger than that from the synoptic component (Fig. 7c) and the relative vorticity from the interannual component is close to that from synoptic component (Fig. 7b). This may be because the response of dynamic and thermodynamic fields to the SST anomalies in the equatorial central and eastern Pacific is the most obvious over the southeast quadrant of the WNP (Wang and Chan 2002). Because of the persistence of the SST anomalies, the anomaly fields have a large interannual component. As such, during El Niño years, more TCs occur over the southeastern quadrant of the WNP as a result of the favorable conditions (Wang and Chan 2002; Camargo et al. 2007; Li 2012; Wu et al. 2012).

The SST contributions to the TC genesis have no obvious regional differences (figure not shown) except over the southeast quadrant of the WNP. When the total SST anomalies are positive in the southeast quadrant, the intraseasonal component has a larger contribution than the synoptic component, but the interannual component has a small contribution. Meantime, there are almost 62% of the TCs (49/79) that occur when the total SST anomalies are negative. Thus, local SST is not a favorable factor for the TC genesis over the southeast quadrant of the WNP.

5. A comparison of El Niño developing and decaying years

Previous studies have shown different characteristics of TC activity over the WNP during El Niño developing and decaying years. TC genesis frequency tends to increase during El Niño developing years, but decrease during El Niño decaying years (Li 2012). Here, we make a comparison of the contributions of different time-scale variations of factors between the two types of years. In the following, we first compare the contributions of different factors to the TC genesis on different ranges. Then, we examine the overall contributions of different factors. Because of the limited number of TCs, we only consider the whole WNP region in this section.

The relation of TC genesis to OLR anomalies displays similar features in El Niño developing and decaying years. Overall, there is a positive contribution from intraseasonal and synoptic variations when the total OLR anomalies are favorable, and there is a negative effect due to synoptic variation when the total OLR anomalies are unfavorable (Figs. 8a,e). Similar features are observed in the relation to vorticity anomalies (Figs. 8b,f) and specific humidity anomalies (Figs. 8d,h). A detailed difference for vorticity is that about 61% (91/148) of TCs form when the lower-level vorticity anomalies are between 0.5 and 2 × 10−5 s−1 in El Niño developing JASO, whereas about 58% of TCs (77/126) appear when the anomalies are between 0 and 1.5 × 10−5 s−1 in El Niño decaying JASO (Figs. 8b,f). A major feature to note for vertical wind shear is that there are more TC geneses under anomalous easterly shear in El Niño developing years (Fig. 8c), but under anomalous westerly shear in El Niño decaying years (Fig. 8g). Those differences in effects of vorticity and vertical wind shear may be explained by a different wind response over the WNP to ENSO-related SST anomalies in the tropical Indo-Pacific region. During El Niño developing years, warm SST anomalies in the equatorial central-eastern Pacific induce an anomalous lower-level cyclone over the WNP as a Rossby wave–type response with anomalous westerly winds located at lower latitudes (Wang et al. 2003; Wu et al. 2003). Opposite wind anomalies develop at the upper level. Thus, there are anomalous easterly vertical shears over the WNP. In contrast, during El Niño decaying years, ENSO-induced north Indian Ocean warming and central North Pacific cooling together induce an anomalous lower-level anticyclone over the WNP (Wang et al. 2003; Wu et al. 2014). This contributes to lower-level anomalous easterly winds at lower latitudes, leading to anomalous westerly vertical shears.

Fig. 8.

As in Fig. 3, but in (left) El Niño developing JASO and (right) El Niño decaying JASO.

Fig. 8.

As in Fig. 3, but in (left) El Niño developing JASO and (right) El Niño decaying JASO.

There are several notable differences in the relation of TC genesis to SST anomalies between El Niño developing and decaying years. Apparently, there are more TC geneses with positive SST anomalies in El Niño decaying JASO (85/126) than in El Niño developing JASO (45/148). This change appears to be mainly due to the increase of contributions from interannual components (Figs. 9a–b). In the developing years, the positive SST contributions are mainly due to intraseasonal component and the negative SST effects are due to interannual and intraseasonal components (Fig. 9a). In the decaying years, the positive SST contributions are mainly due to interannual component (Fig. 9b).

Fig. 9.

As in Fig. 4, but in (top) El Niño developing JASO and (bottom) El Niño decaying JASO.

Fig. 9.

As in Fig. 4, but in (top) El Niño developing JASO and (bottom) El Niño decaying JASO.

The relation of the TC genesis to the barotropic energy conversion does not show notable differences between the El Niño developing and decaying years. A difference to note is that the positive contribution in relation to interannual variation is enhanced in the developing years. This is associated with a wind response over the WNP to ENSO-related SST anomalies. In both types, the positive contribution is mainly due to climatological mean flow and intraseasonal wind variations and the negative effect is mainly due to the intraseasonal wind variations (Figs. 10a,b). This feature is the same in all years (Fig. 5).

Fig. 10.

As in Fig. 5, but in (top) El Niño developing JASO and (bottom) El Niño decaying JASO.

Fig. 10.

As in Fig. 5, but in (top) El Niño developing JASO and (bottom) El Niño decaying JASO.

Comparison of the total positive contribution shows that the intraseasonal and synoptic variations of convection, vorticity, and specific humidity have a much larger contribution to the TC genesis than the interannual variation in both the El Niño developing and decaying JASO (Figs. 11a–c). The barotropic energy conversion also displays similar features in the two types of years with climatological mean flow and intraseasonal wind variations providing the dominant source of barotropic energy for the synoptic disturbances (Fig. 11d). A main difference between the two types of years is in the interannual component. The interannual component has a positive contribution to the TC genesis in the El Niño developing JASO, but a negative or negligible effect in the El Niño decaying JASO. This feature agrees with previous studies (Li 2012) that showed that more TCs tend to form in the El Niño developing years than in the El Niño decaying years over the WNP. Note, however, that the contribution of the interannual component is small even in the El Niño developing JASO. This result indicates that the ENSO-related seasonal mean anomalies have a small direct contribution to the TC genesis over the WNP. This, however, cannot exclude the possibility that ENSO may indirectly affect the TC genesis via modulating the intraseasonal and synoptic variations. For example, the intensity of ISOs over the WNP is enhanced in El Niño developing years (Wu and Cao 2017). Another feature is that the interannual variation plays a main role in the contribution difference of relative vorticity, specific humidity, and barotropic energy conversion between the El Niño developing and decaying JASO. For OLR, the intraseasonal variation has the largest contribution to the difference between the two types of years.

Fig. 11.

The percent of TC genesis with positive contributions from anomalies of (a) OLR; (b) relative vorticity; (c) specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales; and (d) barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) over the whole WNP during the El Niño developing and decaying JASO. The percent has been rescaled by multiplying the ratio of the TC numbers in the El Niño developing and decaying JASO over the total TC numbers in all the JASO. The difference of percent of four factors between the El Niño developing and decaying JASO is also shown at the right side.

Fig. 11.

The percent of TC genesis with positive contributions from anomalies of (a) OLR; (b) relative vorticity; (c) specific humidity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales; and (d) barotropic energy conversion from interannual variation (red bar), intraseasonal variation (green bar), and climatological mean (yellow bar) over the whole WNP during the El Niño developing and decaying JASO. The percent has been rescaled by multiplying the ratio of the TC numbers in the El Niño developing and decaying JASO over the total TC numbers in all the JASO. The difference of percent of four factors between the El Niño developing and decaying JASO is also shown at the right side.

6. The influence of TC signal to relative vorticity

Hsu et al. (2008) indicated that the TCs contribute significantly to the seasonal mean and the intraseasonal and interannual variance of lower-level vorticity over the WNP along the TC tracks. Bi et al. (2015) showed that the TC impact depends upon the variable and the spatial resolution. One issue of concern is whether the results obtained based on total fields include significant TC effects. To address this issue, we make a parallel analysis about the contribution of lower-level relative vorticity on the three time scales after removing the TC vortex from the total wind fields.

Following Hsu et al. (2008) and Bi et al. (2015), we used Kurihara et al. (1995)’s TC-removing technique. The procedure to remove TC is the same as that in Bi et al. (2015), which can correctly separate the TC and environmental components. Based on the wind fields with the TC signal removed, we analyze anomalies of 850-hPa relative vorticity on interannual, intraseasonal, and synoptic time scales without TC effects.

Figure 12 shows the percent distribution of anomalies of 850-hPa relative vorticity on three time scales in relation to 530 TCs associated with TC-removed winds. About 19% of TCs over the WNP appear when the lower-level vorticity anomalies are between 0 and 0.5 × 10−5 s−1. The positive contribution of intraseasonal variation is much larger than that of synoptic variation in this range. When the total vorticity anomalies are more than 0.5 × 10−5 s−1, the positive contribution of the synoptic component is comparable to that of the intraseasonal component. In addition, 71 TCs (about 13% of total) form when the total vorticity anomalies are less than zero, in which the largest negative effect is due to synoptic component. Those results are nearly the same as Fig. 3b, which is based on total winds. A difference to note is that more TCs occur under moderate vorticity anomalies (between −0.5 and 1.5 × 10−5 s−1) and the number of TCs is reduced under vorticity anomalies larger than 2 × 10−5 s−1 when the TC signals are removed.

Fig. 12.

Percent (%) distribution of anomalies of 850-hPa relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales associated with the 530 TCs over the WNP during JASO of 1979–2013 with TC-removed winds. The solid line (scale at right) indicates the PDF distribution of TC genesis number. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

Fig. 12.

Percent (%) distribution of anomalies of 850-hPa relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales associated with the 530 TCs over the WNP during JASO of 1979–2013 with TC-removed winds. The solid line (scale at right) indicates the PDF distribution of TC genesis number. The digit number above the x axis indicates the corresponding number of TCs at the respective range.

Figure 13 shows the regional differences in the contributions of different time-scale variations of vorticity to the TC genesis with TC-removed winds. Figure 13 suggests that the contribution of relative vorticity from intraseasonal and synoptic variations is larger than that from interannual variations over the WNP and subregions, and the contribution of interannual component is larger in the southeast quadrant than in the other three quadrants. These results are similar to those based on total winds (Fig. 7b).

Fig. 13.

The percent of TC genesis with positive contributions from anomalies of relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales with TC-removed winds over the whole WNP, northwest, northeast, southwest, and southeast quadrants of WNP and the SCS.

Fig. 13.

The percent of TC genesis with positive contributions from anomalies of relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales with TC-removed winds over the whole WNP, northwest, northeast, southwest, and southeast quadrants of WNP and the SCS.

Figure 14 shows the percent of TC genesis with positive contributions from anomalies of relative vorticity on three time scales during the El Niño developing and decaying JASO with TC-removed winds. From Fig. 14, all the three time scales contribute positively to the TC genesis in the El Niño developing years, whereas the intraseasonal and synoptic variations contribute positively to the TC genesis in the El Niño decaying years. The difference between the two types of years is mainly due to the contribution of interannual variation. The results are nearly the same as those based on total winds (Fig. 11b).

Fig. 14.

The percent of TC genesis with positive contributions from anomalies of relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales over the whole WNP during the El Niño developing and decaying JASO with TC-removed winds. The percent has been rescaled by multiplying the ratio of the TC numbers in the El Niño developing and decaying JASO over the total TC numbers in all JASO. The difference of percent of four factors between the El Niño developing and decaying summers is also shown at the right side.

Fig. 14.

The percent of TC genesis with positive contributions from anomalies of relative vorticity on interannual (red bar), intraseasonal (green bar), and synoptic (orange bar) time scales over the whole WNP during the El Niño developing and decaying JASO with TC-removed winds. The percent has been rescaled by multiplying the ratio of the TC numbers in the El Niño developing and decaying JASO over the total TC numbers in all JASO. The difference of percent of four factors between the El Niño developing and decaying summers is also shown at the right side.

As noted by Bi et al. (2015), the vorticity field is likely a variable that is impacted largely by the TC. The TC effect in other variables is expected to be smaller than the vorticity because of the different spatial scale. Thus, we conclude that the TC signal does not alter the major results of the contributions of variations on interannual, intraseasonal, and synoptic time scales to the TC genesis over the WNP obtained based on total winds.

7. Summary and discussion

TC activity is potentially controlled by the large-scale circulation patterns and thermodynamical conditions (Gray 1968; Camargo et al. 2007). This study examines the relative contributions of six environmental factors (convection, lower-level relative vorticity, vertical zonal wind shear, midlevel moisture, SST, and barotropic energy conversion) to the TC genesis over the WNP from a local and instantaneous point of view and compares the contributions of interannual, intraseasonal, and synoptic variations of these environmental factors. The regional differences in relative contributions of environmental factors are also examined among subdomains of the WNP. In the end, we compare the contributions of different time-scale variations of factors between El Niño developing and decaying JASO.

The results indicate that the major contributions of convection and lower-level vorticity to the TC genesis over the WNP are due to intraseasonal and synoptic variations, with little contribution from interannual variation. Synoptic variation accounts for the major negative effects of convection and lower-level vorticity when they are unfavorable for the TC genesis. Most TCs are generated when vertical zonal wind shear anomalies are between 0 and 5 m s−1 with comparable contributions from synoptic and intraseasonal variations. When vertical zonal wind shear anomalies are between −5 and 0 m s−1, the contribution of synoptic variation is overwhelmed by interannual and intraseasonal variations. The contribution of specific humidity to the TC genesis is 2 times larger from intraseasonal variation than from synoptic variation. The positive contribution of SST anomalies to the TC genesis is mainly due to interannual and intraseasonal variations. The synoptic-scale disturbances mainly obtain the energy from the climatological mean flow and the intraseasonal variation of circulation.

The relative contributions of environmental factors to the TC genesis show several regional differences. Over the northeast and southwest quadrants of the WNP, synoptic-scale disturbances mainly obtain energy from intraseasonal variation of winds and climatological mean flow, respectively. The contributions of convection, vorticity, specific humidity, and barotropic energy conversion in relation to interannual variation are obviously enhanced over the southeast quadrant of the WNP. The intraseasonal variation of relative vorticity has a much larger contribution than the synoptic variation to the TC genesis over the SCS.

The relation of TC genesis over the WNP to convection, lower-level vorticity, specific humidity, and barotropic energy conversion is similar in El Niño developing and decaying JASO. Comparison shows that more TCs over the WNP form under weak easterly and westerly vertical shears, respectively, during El Niño developing and decaying JASO. The positive contribution of interannual SST anomalies tends to be larger during the El Niño decaying than developing JASO. The contributions of interannual variations of different factors, though small, are positive during El Niño developing JASO, but negative or negligible during El Niño decaying JASO. With respect to the contribution difference between the El Niño developing and decaying JASO, the interannual variation plays a main role in specific humidity, relative vorticity, and barotropic energy conversion and the largest contribution difference in OLR is from the intraseasonal variation.

The present analysis compared contributions of different time-scale variations to the TC genesis between the El Niño developing and decaying years. The results indicate some differences in the contribution of interannual component to the TC genesis over the WNP between these two types of years. These differences may be associated with the impacts of ENSO as well as other climate modes. In addition to ENSO, the environmental condition over the WNP is subjected to influences of other climate modes. For example, Zhang et al. (2016) showed that the positive Pacific meridional mode (PMM) favors the occurrence of TCs over the WNP while the negative PMM phase inhibits the occurrence of TCs there. We have examined the contributions of the three time-scale variations in 7 positive PMM years (1986, 1988, 1990, 1992, 1994, 1995, and 2004) and 7 negative PMM years (1983, 1997, 1998, 1999, 2008, 2011, and 2012) selected based on one standard deviation of the PMM index of Zhang et al. (2016, see their Fig. 4). The composite anomalies for 115 TC geneses in the 7 positive PMM years and 88 TC geneses in the 7 negative PMM years show relative contributions of the three time-scale variations similar to those based on all years (1979–2013). Comparison indicates that the differences between the positive and negative PMM years depend upon the specific variable (figures not shown). In a similar manner, we analyzed the effect of spring Atlantic meridional mode (AMM) on the contributions of different time-scale variations to the TC genesis over the WNP. Comparison shows that there are no notable differences in contributions of seasonal mean anomalies between the positive and negative AMM years (figures not shown). Further studies are needed in the future to unravel the relative roles of ENSO and other climate modes in modulating the interannual variations of the TC genesis over the WNP.

In general, the seasonal predictability of the TC genesis mainly comes from the slowly evolving external forcing (such as local and remote SST anomalies) and the large-scale atmospheric circulation patterns (Zhan et al. 2012). The present study shows that half of the TCs occur when the total SST anomalies are negative and the interannual variation of the large-scale variables only accounts for a small part of the TC genesis over the WNP. This indicates that the ability of seasonal prediction of TC genesis using seasonal mean SST and the associated atmospheric anomalies is quite limited. However, the interannual component plays a main role in the contribution difference of relative vorticity, specific humidity, and barotropic energy conversion between the El Niño developing and decaying years. This suggests that the year-to-year change of the TC genesis has some predictability in relation to the interannual SST and atmospheric anomalies. For intraseasonal prediction of TC genesis, the ability increases as the contribution of intraseasonal variation accounts for a large part of the TC genesis.

The present analysis unravels the relative contributions of interannual, intraseasonal, and synoptic variations of environmental factors to the TC genesis over the WNP. Our method is distinct from previous studies in that we aim at local and instantaneous conditions of the TC genesis. Further studies are needed to investigate the relative contributions of different time-scale variations of the environmental factors to the TC genesis over other regions, such as the Atlantic Ocean, and compare the differences between the WNP and the Atlantic Ocean basins.

Acknowledgments

The comments of three anonymous reviewers led to the improvement of this paper. This study is supported by the National Key Research and Development Program of China Grant 2016YFA0600603, and the National Natural Science Foundation of China Grants 41505048, 41461164005, 41275001, and 41475074. The NCEP–NCAR reanalysis data and NOAA OLR data were obtained via ftp://ftp.cdc.noaa.gov/. The SST data were obtained from ftp://ftp.cdc.noaa.gov/Datasets/noaa.oisst.v2.highres. The IBTrACS data were obtained from http://www.ncdc.noaa.gov/ibtracs/index.php.

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