Comparison of Controlling Parameters for Near-Equatorial Tropical Cyclone Formation between Western North Pacific and North Atlantic

西北太平洋和北大西洋近赤道热带气旋生成的控制因子比较

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  • In this study, the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones (NETCs) between the western North Pacific (WNP) and the North Atlantic (NA) are investigated. NETCs exhibit distinctive spatial variabilities in different basins. Over the past few decades, the majority of NETCs took place in WNP while none was observed in NA. The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods. It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear (VWS) are likely the primary controlling parameters. Compared with NA, larger low-level vorticity and smaller VWS appear over WNP. The increase of vorticity attributes a lot to the turning of northeast trade wind. NETCs in WNP tend to occur in the areas with VWS less than 9 m s−1, while the VWS in NA generally exceeds 10 m s−1. On the other hand, the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃ and the difference of mid-level moisture is not significant; thus, thermal factors have little contribution to the distinction of NETC activities between WNP and NA. Intraseasonal oscillation (ISO) and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis. More NETCs were generated in ISO active phase. Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process. The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.
    近赤道热带气旋(NETC)在不同的海域有明显的不同。据统计,大多数近NETC生成于西北太平洋,而北大西洋则一个都没有。本文研究了西北太平洋和北大西洋NETC的空间分布和控制因子的不同。利用统计方法研究了NETC空间分布差异的原因。结果表明,动力因子(如低层的相对涡度和垂直风切变)是主要的影响因子。相比北大西洋,西北太平洋在近赤道地区有更大的相对涡度和更小的垂直风切变。涡度的增加主要是东北信风的转向。西北太平洋的近赤道热带气旋往往出现在垂直风切变小于9 m s−1的区域,而北大西洋中的垂直风切变通常大于10 m s−1。然而两大洋的海表温度在近赤道地区都超过了26.5℃,对流层中层相对湿度差异不明显,热力因子对西北太平洋和北大西洋NETC活动的差异贡献不大。此外,西北太平洋的季节内振荡和天气尺度扰动也更有利于NETC的生成。季节内振荡活跃时期有更多的NETC产生。西北太平洋的天气尺度扰动通过正压能量转换过程,从平均流中获得更多的能量。总体不利的热力和动力条件导致了北大西洋NETC很少发生。
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  • Fig. 1.  Global distribution of near-equatorial (5°S–5°N) tropical cyclogenesis during 1970–2018 based on the IBTrACS dataset. The black dots represent the genesis positions of NETCs.

    Fig. 2.  The frequency of NETCs in different basins.

    Fig. 3.  The monthly frequency of NETCs in the WNP counted from 1970 to 2018.

    Fig. 4.  Monthly distributions of low-level relative vorticity (shaded; 10−5 s−1) and vertical wind shear of (contour; interval 3 m s−1 and the contour of 9 m s−1 is bold) in the WNP. The green dots represent the genesis positions of NETCs.

    Fig. 5.  Comparison of large-scale environmental factors between the WNP (left panels) and NA (right panels). (a–b) 850-hPa relative vorticity (shaded; 10−5 s−1) and wind (vector; m s−1); (c–d) 200-hPa divergence (shaded; 10−6 s−1) and wind (vector; m s−1); (e–f) vertical wind shear (VWS, contour; m s−1) and 500-hPa vertical motion (shaded; −102 Pa s−1); and (g–h) SST (shaded; ℃) and 600-hPa relative humidity (contour; %), respectively.

    Fig. 6.  Distributions of GPI in the (a) WNP and (b) NA near-equatorial regions during the NETC active season (DJFMAM). The purple dots represent the genesis positions of NETCs.

    Fig. 7.  Climatic mean of precipitation (shaded; mm day−1) and 200-hPa wind field (vector; m s−1). The purple dots are NETC genesis positions from 1970 to 2018.

    Fig. 8.  Frequency of the WNP NETCs in corresponding phase of ISO over 1974–2018.

    Fig. 9.  Compose distribution of OLR at the genesis time of NETCs after 20–90-day filtering. The purple dots represent the genesis locations of WNP NETCs during 1970–2018.

    Fig. 10.  Distributions of standard deviation of OLR in boreal winter in the WNP and NA during the NETC active season (DJFMAM). (a) WNP intraseasonal-scale filtering (20–90 days), (b) NA intraseasonal-filtering (20–90 days), (c) WNP synoptic-scale filtering (3–8 days), and (d) NA synoptic scale filtering (3–8 days).

    Fig. 11.  Distributions of 850-hPa total kinetic energy tendency $ \dfrac{{\partial K'}}{{\partial {\rm{t}}}}$ (10−6 m2 s−3) in (a) WNP and (b) NA during the NETC active season (DJFMAM); (c) the term of $ - \overline {{{\left({u'} \right)}^2}} \dfrac{\partial }{{\partial x}}\overline u $ in WNP; (d) the term of $ - \overline {u'v'} \dfrac{\partial }{{\partial {\rm{y}}}}\overline u $ in NA; (e) the term of $ - \overline {u'v'} \dfrac{\partial }{{\partial x}}\overline v $ in WNP; and (f) the term of $ - \overline {{{\left({v'} \right)}^2}} \dfrac{\partial }{{\partial {\mathop{\rm y}\nolimits} }}\overline v $ in NA.

    Table 1.  BDI values associated with the key genesis parameters in the WNP and NA

    VariableBDI (WNP – NA)
    850-hPa relative vorticity 1.92
    SST 1.91
    200–850-hPa wind shear−0.99
    500-hPa vertical motion (isobaric)−0.63
    600-hPa relative humidity 0.32
    Download: Download as CSV

    Table 2.  Contributions of each term on the right-hand side of Eq. (2) to $ \delta $GPI

    TermValueProportion (%)
    $ \delta $GPI3.10/
    $ \delta $GPI-VOR(Term1)1.2641
    $ \delta $GPI-VWS(Term2)0.9731
    $ \delta $GPI-MPI(Term4)0.5317
    $ \delta $GPI-W(Term5)0.21 7
    $ \delta $GPI-RH(Term3)0.13 4
    Download: Download as CSV
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Comparison of Controlling Parameters for Near-Equatorial Tropical Cyclone Formation between Western North Pacific and North Atlantic

    Corresponding author: Xuyang GE, xuyang@nuist.edu.cn
  • 1. Key Laboratory of Meteorological Disaster of Ministry of Education/Joint International Research Laboratory of Climate and Environment Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China
  • 2. University of Colorado, Colorado Spring, Colorado 80918, USA
Funds: Supported by the National Natural Science Foundation of China (42088101)

Abstract: In this study, the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones (NETCs) between the western North Pacific (WNP) and the North Atlantic (NA) are investigated. NETCs exhibit distinctive spatial variabilities in different basins. Over the past few decades, the majority of NETCs took place in WNP while none was observed in NA. The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods. It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear (VWS) are likely the primary controlling parameters. Compared with NA, larger low-level vorticity and smaller VWS appear over WNP. The increase of vorticity attributes a lot to the turning of northeast trade wind. NETCs in WNP tend to occur in the areas with VWS less than 9 m s−1, while the VWS in NA generally exceeds 10 m s−1. On the other hand, the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃ and the difference of mid-level moisture is not significant; thus, thermal factors have little contribution to the distinction of NETC activities between WNP and NA. Intraseasonal oscillation (ISO) and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis. More NETCs were generated in ISO active phase. Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process. The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.

西北太平洋和北大西洋近赤道热带气旋生成的控制因子比较

近赤道热带气旋(NETC)在不同的海域有明显的不同。据统计,大多数近NETC生成于西北太平洋,而北大西洋则一个都没有。本文研究了西北太平洋和北大西洋NETC的空间分布和控制因子的不同。利用统计方法研究了NETC空间分布差异的原因。结果表明,动力因子(如低层的相对涡度和垂直风切变)是主要的影响因子。相比北大西洋,西北太平洋在近赤道地区有更大的相对涡度和更小的垂直风切变。涡度的增加主要是东北信风的转向。西北太平洋的近赤道热带气旋往往出现在垂直风切变小于9 m s−1的区域,而北大西洋中的垂直风切变通常大于10 m s−1。然而两大洋的海表温度在近赤道地区都超过了26.5℃,对流层中层相对湿度差异不明显,热力因子对西北太平洋和北大西洋NETC活动的差异贡献不大。此外,西北太平洋的季节内振荡和天气尺度扰动也更有利于NETC的生成。季节内振荡活跃时期有更多的NETC产生。西北太平洋的天气尺度扰动通过正压能量转换过程,从平均流中获得更多的能量。总体不利的热力和动力条件导致了北大西洋NETC很少发生。
1.   Introduction
  • It has been well realized that a significant Coriolis parameter is necessary for the formation of tropical cyclones (TCs). Coriolis force acts to spin up the vorticity of the initial tropical disturbance (Gray, 1968). In general, TCs usually occur at a certain distance away from the equator (i.e., 5 degrees). However, more and more near-equatorial tropical cyclones (NETCs) were found from the observation. For example, Typhoon Sarah (March 1956) formed at 1.7°N with its intensity reaching 70 knots at 4.1°N as observed by an aircraft (Fortner, 1958). Typhoon Kate (October 1970) formed, developed, and reached typhoon intensity while remaining within equatorward of 5°N (Holliday and Thompson, 1986). Typhoon Vamei (December 2001) formed near Singapore with its circulation center at 1.5°N and the maximum sustained surface wind was 39 m s−1 according to naval ship reports (Chang et al., 2003; Chambers and Li, 2007). When the Coriolis parameter is set to zero in idealized numerical simulations, a weak vortex can still develop into a TC, and the initial time of intensification is about the same as vortices located at 10°N (Steenkamp et al., 2019; Kilroy et al., 2020). These studies suggest that the Coriolis force may not be as important as indicated in earlier literature in the genesis of TCs.

    According to statistics, there were 213 NETCs in the world from 1900 to 2017 (Steenkamp et al., 2019). The formation mechanism of NETCs has been investigated recently. In the case of Vamei (2001), the channeling and strengthening of the cross-equatorial surge wind improved the background shear vorticity near the equator. The Borneo vortex migrated to the open water region and stayed long enough to strengthen into a typhoon (Chang et al., 2003). Different peak seasons between NETCs (December–May) and off-equatorial TCs (July–October) are also revealed (Holliday and Thompson, 1986; Yi and Zhang, 2010; Li et al., 2019). Large-scale circulations are changed during the alternation of summer and winter (Li et al., 2019). With this regard, background vorticity makes significant contributions to NETC formation, including the promotion of mid-level outflow, the equivalent Coriolis effect via enhanced absolute vorticity, and the merge of hot towers (Deng and Li, 2020).

    However, few studies have compared the distribution of NETCs in different basins. The studies by Fu et al. (2012) and Peng et al. (2012) suggest that TC genesis in the western North Pacific (WNP) differs from that in North Atlantic (NA) in the large-scale background environmental conditions, disturbance types, and sources. The monsoon flow provides favorable large-scale conditions for the formation of TCs in the WNP and various sources of synoptic-scale disturbances can evolve into the pre-existing vortex (Fu et al., 2007), while beneficial conditions for TC genesis in the NA are mainly provided by the easterly trade wind.

    Interestingly, no NETC has been found in NA. The nearest TC to the equator in NA is Isidore (1990), and its initial latitude is 7.2°N. To our knowledge, there is no any literature study on the large-scale environmental factors precluding TCs from occurring near the equator in NA. It prompts us to investigate why there is no NETC in NA from the perspective of large-scale environmental factors. Different from the investigation on the controlling parameters of NETC peak seasons in the WNP (Li et al., 2019), our study focuses on the comparison between the WNP and NA. The objectives of our study are to (1) analyze the spatial distribution of global NETCs and (2) understand the different roles of environmental conditions in the WNP and the NA. The effects of intraseasonal oscillation (ISO) and synoptic-scale dicturbances on the genesis of NETCs in the WNP and NA are further discussed. The cause of the absence of NETC in NA will then be drawn.

    The remaining parts of this paper are organized as follows. Data and methods are described in Section 2. In Section 3, the distributions of global NETCs during 1970–2018 are presented. In Section 4, the relative roles of large-scale environmental parameters controlling the formation of NETCs in the WNP and NA are analyzed. In Section 5, the influences of ISO and synoptic-scale perturbations on NETC genesis in the WNP and NA are presented. Finally, the conclusions are given in Section 6.

2.   Data and methods
  • The best-track data for TCs are from the International Best Track Archive for Climate Stewardship (IBTrACS) version v04r00 (Knapp et al., 2010), which contains the most complete global set of historical TCs. NETCs are defined as in Li et al. (2019), namely, tropical depressions formed initially within 5 degrees of the equator and then strengthened to tropical storm category of 17.2 m s−1 or higher. The large-scale environmental fields are obtained from the six-hourly ECMWF Interim reanalysis (ERAI; Dee et al., 2011), with horizontal resolution of 1.5° × 1.5°. Daily mean outgoing longwave radiation (OLR) is obtained from the NOAA polar-orbiting satellites on a 2.5° latitude–longitude grid (Liebmann and Smith, 1996). ISO information is from the Australian Bureau of Meteorology, where the all-season Real-time Multivariate Madden–Julian oscillation index (RMM) is constructed (Wheeler and Hendon, 2004). The ISO phase is the two-dimensional phase space defined by RMM series 1 (RMM1) and RMM series 2 (RMM2). The ISO active phase is defined as the phase in which the magnitude of vector RMM1 and RMM2 exceeds 1.

    The modified genesis potential index (GPI; Murakami and Wang, 2010) is applied to represent environmental variables contributing to TC genesis, which additionally incorporates the vertical motion term and is defined as:

    $$ \hspace{-16pt} {\rm{GPI = Term1 \times Term2 \times Term3 \times Term4 \times Term5,}} $$ (1)

    where Term1 = |105η|3/2, η (s−1) is the absolute vorticity at 850 hPa; Term2 = (1 + 0.1${V_{\rm{s}}}$)−2, $ {V_{\rm{s}}}$ (m s−1) is the magnitude of the vertical wind shear (VWS) between 200 and 850 hPa; Term3 = (RH/50)3, RH (%) is the 600-hPa relative humidity; Term4 = ($ {V_{\rm{p}}} $/70)3, $ {V_{\rm{p}}}$ is the maximum potential intensity (MPI) following Emanuel (2000); and Term5 = (−$ \omega $ + 0.1)/0.1, $ \omega $ (Pa s−1) is the 500-hPa vertical wind velocity. According to Li et al. (2013), the deviation of the total GPI can be calculated by taking the logarithm as follows:

    $$\begin{aligned}[b] {\rm{\delta GPI}} & = {\rm{\alpha 1 \,\times \,\delta Term1 + \alpha 2 \,\times \,\delta Term2 + \alpha 3\, \times\,\delta Term3 }}\\ & +{\rm{\alpha 4 \,\times \,\delta Term4 + \alpha 5 \,\times\, \delta Term5,}} \end{aligned}$$ (2)

    where

    $$ \left\{ {\begin{array}{*{20}{l}} {\begin{array}{*{20}{l}} {{\rm{\alpha 1}} = \overline {{\rm{Term2}}} \;\times\; \overline {{\rm{Term3}}} \;\times\; \overline {{\rm{Term4}}} \;\times\; \overline {{\rm{Term5}}} }\\ {{\rm{\alpha 2}} = \overline {{\rm{Term1}}} \;\times\; \overline {{\rm{Term3}}} \;\times\; \overline {{\rm{Term4}}} \;\times\; \overline {{\rm{Term5}}} }\\ {{\rm{\alpha 3}} = \overline {{\rm{Term1}}} \;\times\; \overline {{\rm{Term2}}} \;\times\; \overline {{\rm{Term4}}} \;\times\; \overline {{\rm{Term5}}} } \;.\\ {{\rm{\alpha 4}} = \overline {{\rm{Term1}}} \;\times\; \overline {{\rm{Term2}}} \;\times\; \overline {{\rm{Term3}}} \;\times\; \overline {{\rm{Term5}}} }\\ {{\rm{\alpha 5}} = \overline {{\rm{Term1}}} \;\times\; \overline {{\rm{Term2}}} \;\times\; \overline {{\rm{Term3}}} \;\times\; \overline {{\rm{Term4}}} } \end{array}} \end{array}} \right. $$ (3)

    The bar denotes the average of WNP or NA climatic mean, and δ denotes the area-averaged difference between the two basins (WNP−NA). Selected area is 0°–4.5°N, 129°–179.5°E for WNP, and 0°–4.5°N, 0.5°–51°W for NA. The overland values are excluded in the computation. The box difference index (BDI; Peng et al., 2012) is applied to quantitatively measure the differences of large-scale environmental variables controlling the cyclogenesis near the equatorial regions between the WNP and NA. The definition of the index for a variable M is as below,

    $$ {\rm{BDI = }}\frac{{{M_{{\rm{WNP}}}} - {M_{{\rm{NA}}}}}}{{{\sigma _{{\rm{WNP}}}} + {\sigma _{{\rm{NA}}}}}}, $$ (3)

    where $M_{\text{WNP}}$ and $ \sigma _{\text{WNP}} $ ($ M_{\text{NA}} $ and $ \sigma _{\text{NA}} $) represent the mean and standard deviation of the variable in the WNP (NA) basin, respectively. The area selected for the computation of BDI is the same as ${\rm{\delta }}$GPI. The barotropic kinetic energy (BKE) equation is used to investigate the dynamical process of background environmental flow in modulating NETC genesis and defined as:

    $$\frac{{\partial {K^\prime }}}{{\partial t}} = - \overline {{{\left({{u^\prime }} \right)}^2}} \frac{\partial }{{\partial x}}\bar u - \overline {{u^\prime }{v^\prime }} \frac{\partial }{{\partial y}}\bar u - \overline {{u^\prime }{v^\prime }} \frac{\partial }{{\partial x}}\bar v - \overline {{{\left({{v^\prime }} \right)}^2}} \frac{\partial }{{\partial y}}\bar v, $$ (4)

    where $ K' $ is the kinetic energy of transient disturbances; $ u' $ and $ v' $ are the 850-hPa zonal and meridional wind components at synoptic timescale, after using the technique of 3–8-day Lanczos band pass filtering (Duchon, 1979); and $ \overline{u} $ and $ \overline{v} $ are the climate average of zonal and meridional wind from December to May.

3.   Global NETC distribution
  • Figure 1 shows that the global distribution of NETC genesis during 1970–2018. There are five basins with NETC activities, including the WNP, South Pacific (SP), central Pacific (CP), North Indian (NI), and South Indian (SI) oceans. Note that the occurrence of NETCs has large variations from basin to basin.

    Figure 1.  Global distribution of near-equatorial (5°S–5°N) tropical cyclogenesis during 1970–2018 based on the IBTrACS dataset. The black dots represent the genesis positions of NETCs.

    More specifically, the number of NETCs in the WNP exceeds 100, so WNP is the most frequent NETC genesis area compared with other basins (Fig. 2). The second NETC active basin is the SI (29) and the others are NI (15), SP (11), and CP (6). In contrast, the eastern Pacific (EP), NA, and South Atlantic (SA) are void of TCs near the equatorial regions. Although 798 TCs formed in the entire NA during the period of interest, none of them formed within 5 degrees north from the equator. Hence, an interesting question arises here as why no NETC occurs in Atlantic. It is worthwhile to examine the impacts of large-scale environmental factors accounting for the discrepancies between the WNP and NA. For EP, it bears many similarities with NA. Hence, we mainly focus on the comparison between the WNP and NA.

    Figure 2.  The frequency of NETCs in different basins.

4.   Large-scale controlling parameters
  • Environmental conditions associated with tropical cyclogenesis are relatively well established (Briegel and Frank, 1997; Emanuel, 2000). TC forms in regions that exhibit five characteristics: warm sea surface temperature; cyclonic low-level relative vorticity and planetary vorticity; weak to moderate VWS; organized deep convection in an area with large-scale ascending motion; and high mid-level humidity (Frank and Roundy, 2006; Li et al., 2019). Figure 3 shows the monthly frequency of NETCs in the WNP. During 1970–2018, 102 NETCs originated in the WNP and 56 of them occurred in December–May (DJFMAM). The monthly ratio of NETCs to total TCs decreases from March to August and increases from August to January of the ensuing year. NETCs are inactive in boreal summer especially from July to October (Holliday and Thompson, 1986; Yi and Zhang, 2010; Li et al., 2019). The cross-equatorial flow north of the equator turns clockwise, resulting in the decrease of vorticity in summer (Li et al., 2019).

    Figure 3.  The monthly frequency of NETCs in the WNP counted from 1970 to 2018.

    To reveal the relationship between the seasonal variation of low-level vorticity and NETC activities, monthly distributions of NETCs and 850-hPa vorticity in the WNP are displayed in Fig. 4. The evolution of low-level vorticity has a good correspondence with the seasonal variation of NETCs. The vorticity in the near-equatorial region is generally positive from November to May and negative from June to October, consistent with NETC active and inactive seasons. Meanwhile, moderate VWS (broadly defined as 850–200-hPa shear magnitude ranging between 5 and 10 m s−1) is conducive to tropical cyclogenesis and intensification (Rogers et al., 2020). NETCs tend to form where VWS is less than 9 m s−1 as shown in Fig. 4. Statistical analysis by McGauley and Nolan (2011) showed that most TCs formed with moderate wind shear values (in the 5–10-m s−1 range). It likely depends on the relationship among the magnitude of VWS, the TC motion, and the lower-level mean flows. It is also sensitive to the orientation of VWS. In this study, NETCs are formed over the moderate VWS region, indicating that moderate VWS cannot inhibit the NETCs therein.

    Figure 4.  Monthly distributions of low-level relative vorticity (shaded; 10−5 s−1) and vertical wind shear of (contour; interval 3 m s−1 and the contour of 9 m s−1 is bold) in the WNP. The green dots represent the genesis positions of NETCs.

    Large-scale environmental conditions are averaged over the NETC active season (DJFMAM) to investigate the background differences between the WNP and NA near-equatorial regions. Figures 5a and 5b compare the low-level wind and vorticity fields and between the WNP and NA. Generally, easterly wind prevails in the equatorial regions of both basins. However, the 850-hPa northeasterly trade wind in the WNP basin turns anticlockwise when approaching the equator, leading the formation of maximum cyclonic vorticity near 5°N. The easterly wind flow in the NA near the equator is relatively straight, with little cyclonic vorticity therein. Quantitatively, the low-level vorticity near the equator is less than 0.6 × 10−5 s−1 in the NA, whereas it is much higher (≥ 1.0 × 10−5 s−1) near most of the equatorial region in the WNP. Numerous studies (Gray, 1968; Hendricks et al., 2004; Montgomery et al., 2006) suggested that the environmental low-level vorticity plays an important role in cyclogenesis. Li et al. (2019) also found that the low-level vorticity is one of the major factors for distinctive seasonal variability of NETCs in the WNP. By this reasoning, the rare environmental low-level vorticity in the NA near-equatorial region likely explains the lack of NETCs in this region.

    Figure 5.  Comparison of large-scale environmental factors between the WNP (left panels) and NA (right panels). (a–b) 850-hPa relative vorticity (shaded; 10−5 s−1) and wind (vector; m s−1); (c–d) 200-hPa divergence (shaded; 10−6 s−1) and wind (vector; m s−1); (e–f) vertical wind shear (VWS, contour; m s−1) and 500-hPa vertical motion (shaded; −102 Pa s−1); and (g–h) SST (shaded; ℃) and 600-hPa relative humidity (contour; %), respectively.

    Strong upper-level divergence exists in both the WNP and NA near-equatorial regions (Figs. 5cd). The difference is that the upper troposphere wind is east in WNP, while it is west in NA. Hence, the magnitude of VWS in NA is obviously larger than that in WNP (Figs. 5ef). Specifically, the averaged VWS in WNP averaged over 0°–4.5°N, 129°–179.5°E is 7.3 m s−1, while that in NA over 0°–4.5°N, 0.5°–51°W is 11.0 m s−1. Finocchio and Majumdar (2017) also showed that the magnitude of deep-layer shear in the NA in the mean zonal wind profile (from the equator to 30°N) is larger than that in the WNP from a statistical perspective. The effect of VWS on the formation and intensity change of TCs has been widely examined (McBride and Zehr, 1981; Ge et al., 2013). In the presence of VWS, TC vortex tends to tilt vertically with height (Zhang and Kieu, 2006), which is not conducive to the organizational development of vortex. Larger VWS is a possible factor in suppressing NETCs genesis in the NA.

    Vertical motion is closely related to convective instability of atmospheric environment, which moistens the air column and thus favors deep convection (Kilroy and Smith, 2016). The vertical motion distribution of the two oceans is also compared (Figs. 5ef). There is a maximum vertical movement belt along 145°–165°E in the WNP, which is less than −6 × 102 Pa s−1. The maximum vertical motion in the NA is greater than −6 × 102 Pa s−1 in most regions. The large low-level vorticity in the WNP near-equatorial region, together with strong vertical motion and appropriate VWS provide favorable dynamic conditions for the formation of NETCs.

    The differences of large-scale thermodynamic parameters between the two basins are further compared. Figures 5gh display the distribution of 600-hPa relative humidity (RH) and sea surface temperature (SST). It is apparent that SST in WNP is significantly higher than that in NA. Nevertheless, SST in the NA near equator region is generally higher than 27.5℃, exceeding the threshold value of 26.5℃. Although a high SST implies a greater MPI, it does not warrant a genesis by itself. RH near the equator of both basins is about 60%, indicating that the mean SST and mid-level RH are not the critical factors for the distinctive distribution between WNP and NA.

    BDI values for key environmental variables are computed (Table 1). As expected, the low-level relative vorticity is the most distinctive parameter among selected variables. SST is the second. The middle-level RH of the two basins is similar, so the BDI value is the smallest. Magnitudes of VWS and mid-level vertical motion lie in between SST and RH. In short, WNP has greater low-level vorticity, higher SST, weaker VWS, and larger mid-level vertical motion and RH in the near-equatorial region compared with NA. This result generally accounts for the distinctive features of NETCs between WNP and NA.

    VariableBDI (WNP – NA)
    850-hPa relative vorticity 1.92
    SST 1.91
    200–850-hPa wind shear−0.99
    500-hPa vertical motion (isobaric)−0.63
    600-hPa relative humidity 0.32

    Table 1.  BDI values associated with the key genesis parameters in the WNP and NA

    GPI [Eq. (1)] is computed to further investigate the relative influence of large-scale environmental parameters on NETC genesis. Figure 6 shows the mean GPI distributions from December to May in WNP and NA near-equatorial regions. The GPI value in WNP is larger and covers broader near-equatorial region than that in NA. GPI distribution coincides well with the occurrence of NETCs as shown in Fig. 1. In contrast, the value of GPI in NA is much smaller around the focused area, which agrees with the fact that no NETC formed in the NA. Since the distribution of GPI represents reasonably well the geographical contrast of NETCs, δGPI [Eq. (2)] is applied to reveal the relative contribution of the key environmental parameters.

    Figure 6.  Distributions of GPI in the (a) WNP and (b) NA near-equatorial regions during the NETC active season (DJFMAM). The purple dots represent the genesis positions of NETCs.

    δGPI in WNP is averaged over a rectangular area (0°–4.5°N, 129°–179.5°E), covering the main region of NETC formation, and the area in the NA is 0°–4.5°N, 0.5°–51°W correspondingly.

    As shown in Table 2, the total value of δGPI is 3.10 and δGPI is primarily contributed by low-level absolute vorticity (δGPI-VOR) and VWS (δGPI-VWS). These two factors contribute 72% of δGPI. The proportion of the term related to vertical motion (δGPI-W) is about 7%. The remaining two terms are the GPI changed by relative humidity (δGPI-RH) and MPI (δGPI-MPI). The results further suggest the high sensitivity of NETCs to dynamic factors. The 800-hPa maximum relative vorticity and vertically averaged horizontal shear are considered to be more important than 925–400-hPa water vapor content and SST for TC development in the WNP (Fu et al., 2012), while the thermodynamic variables are identified as more important in the NA (Peng et al., 2012).

    TermValueProportion (%)
    $ \delta $GPI3.10/
    $ \delta $GPI-VOR(Term1)1.2641
    $ \delta $GPI-VWS(Term2)0.9731
    $ \delta $GPI-MPI(Term4)0.5317
    $ \delta $GPI-W(Term5)0.21 7
    $ \delta $GPI-RH(Term3)0.13 4

    Table 2.  Contributions of each term on the right-hand side of Eq. (2) to $ \delta $GPI

    The root causes of environmental differences between the two basins are then discussed. The inter tropical convergence zone (ITCZ) in WNP is stronger than that in NA, and it is closer to the equator than that in EP (Fig. 7). The ITCZ in WNP could provide larger barotropic instability and convergence of water vapor (Figs. 5gh), which is benefitial to TCs genesis near the equator. The high-level westerlies exist in the subtropical region east of 180°, including the east–central Pacific and NA; while strong easterlies stand in the WNP near-equatorial region. Due to the easterly trade wind at the lower level, strong westerly VWS appears in NA and weak easterly VWS in WNP (Figs. 5ef). The significant difference of subtropical jet stream and ITCZ are possibly responsible for the contrast of TC genesis near the equator.

    Figure 7.  Climatic mean of precipitation (shaded; mm day−1) and 200-hPa wind field (vector; m s−1). The purple dots are NETC genesis positions from 1970 to 2018.

5.   ISO and synoptic-scale perturbation
  • Tropical cyclogenesis on the equator is possible as long as there is a sufficient reservoir of relative vorticity initially to be organized (Kilroy et al., 2020). Low-level relative vorticity can be enhanced through the interaction between ISO and synoptic-scale disturbances. The magnitude and phase of ISO affect tropical cyclogenesis by modulating convection, low-level vorticity, and VWS (Liebmann et al., 1994; Klotzbach, 2010). The convective phase of ISO favors tropical cyclogenesis by amplifying high-frequency waves that in turn act as TC precursors (Sobel and Maloney, 2000). To illustrate the relationship between the WNP NETCs and ISO activities, the frequency of ISO phases corresponding to NETCs genesis over 1974–2018 (because of the lack observation of OLR before 1974) is presented (Fig. 8). During this period, 100 NETCs were generated and 62 of them were associated with ISO active phases. Statistics analysis (Pan et al., 2010) showed that the ratio of the number of TCs associate with strong ISO events is about twice as much as that of TCs associated with weak/no ISO events during June to October in WNP, while the ratio is about 1.6 for NETCs from December to May.

    Figure 8.  Frequency of the WNP NETCs in corresponding phase of ISO over 1974–2018.

    To describe the spatial relationship between ISO and NETCs, OLR is composited at the genesis time of NETCs after filtering 20–90-day disturbances (Fig. 9). In the composite field, OLR is negative all over the WNP near-equatorial region. The negative center is between 150° and 165°E, corresponding well with the centralized genesis location of NETCs. The environment with active convection is more conducive to NETC genesis. The essence of ISO is the eastward transport of cumulus convection, which is an important source of TC energy. The composite OLR of ISO phases 5 and 6 is negative over the WNP near-equatorial region, especially between 130° and 160°E (Pan et al., 2010). The condensational latent heat released by ISO activities provides a good environmental background for the occurrence and enhancement of NETCs.

    Figure 9.  Compose distribution of OLR at the genesis time of NETCs after 20–90-day filtering. The purple dots represent the genesis locations of WNP NETCs during 1970–2018.

    The existence of persistent, organized, and deep moist convection is the primary requirement for TC genesis (Bracken and Bosart, 2000). Synoptic-scale waves are important precursors for tropical cyclogenesis and can be modulated by ISO (Crosbie and Serra, 2014). To reveal the difference of ISO and synoptic-scale perturbations between WNP and NA, the spatial standard deviation of OLR in boreal winter is investigated (Fig. 10). It can be clearly seen that the standard deviation of OLR after 20–90-day filtering in WNP is significantly greater than that in NA (Figs. 10a, b). The standard deviation in the WNP near-equatorial region exceeds 18 W m–2 in most areas, whereas it is much smaller in the NA region. Meanwhile, the standard deviation of OLR after 3–8-day filtering in the WNP near-equatorial region is also generally larger than that in NA region (Figs. 10c, d), implying that WNP has more synoptic-scale perturbations as well.

    Figure 10.  Distributions of standard deviation of OLR in boreal winter in the WNP and NA during the NETC active season (DJFMAM). (a) WNP intraseasonal-scale filtering (20–90 days), (b) NA intraseasonal-filtering (20–90 days), (c) WNP synoptic-scale filtering (3–8 days), and (d) NA synoptic scale filtering (3–8 days).

    The dynamical processes involving the contribution of environmental factors to formation of NETCs are investigated by use of the BKE equation, which can represent the barotropic energy conversion from the background mean flows to TCs (Ha et al., 2013; Guo and Ge, 2018). BKE is contributed by four dynamical processes, namely, the zonal convergence shear of zonal wind ($-\overline{\left(u^{\prime}\right)^{2}} \dfrac{\partial}{\partial x} \bar{u} $), meridional shear of zonal wind ($-\overline{u^{\prime} v^{\prime}} \dfrac{\partial}{\partial \mathrm{y}} \bar{u}$), zonal shear of meridional wind ($-\overline{u^{\prime} v^{\prime}} \dfrac{\partial}{\partial x} \bar{v}$), and meridional convergence of meridional wind ($ -\overline{\left(v^{\prime}\right)^{2}} \dfrac{\partial}{\partial \mathrm{y}} \bar{v}$). Synoptic-scale systems such as tropical disturbances obtain more energy from mean flows in the WNP near-equator region compared with that in the NA region (Figs. 11a, b). Obviously, the convergence and meridional shear of zonal wind play more important roles (Figs. 11c, d) in the development of disturbances. Wave number is proportional to the negative longitudinal stretch of the basic flow along the equator $ (\dfrac{\partial \bar{u}}{\partial x}<0 ) $. Wave energy produced by tropical convection will accumulate in regions of convergence of the zonal wind (Webster and Chang, 1988). Meanwhile, the meridional shear of zonal wind will increase the barotropic instability, which helps the waves to break down into multiple tropical disturbances (Ferreira and Schubert, 1997; Maloney and Hartmann, 2001).

    Figure 11.  Distributions of 850-hPa total kinetic energy tendency $ \dfrac{{\partial K'}}{{\partial {\rm{t}}}}$ (10−6 m2 s−3) in (a) WNP and (b) NA during the NETC active season (DJFMAM); (c) the term of $ - \overline {{{\left({u'} \right)}^2}} \dfrac{\partial }{{\partial x}}\overline u $ in WNP; (d) the term of $ - \overline {u'v'} \dfrac{\partial }{{\partial {\rm{y}}}}\overline u $ in NA; (e) the term of $ - \overline {u'v'} \dfrac{\partial }{{\partial x}}\overline v $ in WNP; and (f) the term of $ - \overline {{{\left({v'} \right)}^2}} \dfrac{\partial }{{\partial {\mathop{\rm y}\nolimits} }}\overline v $ in NA.

6.   Conclusions and discussion
  • In this study, differences in the spatial distribution and controlling parameters for formation of NETCs in the WNP and NA are compared. The majority of NETCs formed in the WNP during winter season, whereas none was generated in NA. To reveal the possible mechanisms for the absence of NETCs in NA, large-scale environmental factors are firstly investigated. In contrast with NA, WNP has more favorable dynamic and thermodynamic conditions in the near-equatorial region during the NETC active season. Specifically, WNP has larger low-level vorticity, higher sea surface temperature, greater relative humidity, stronger vertical motion, and smaller VWS. The change of GPI is primarily contributed by low-level absolute vorticity and VWS. The low-level vorticity is increased by the shear effect, which is induced by the anticlockwise turning of the northeasterly trade wind near the equator in boreal winter. Dynamically, TC embedded in an environment with larger inertial stability has greater potential to resist the vertical tilting. NETCs tend to have weaker inertial stability and are more sensitive to VWS compared with off-equatorial TCs. The large low-level vorticity in the WNP near-equatorial region, together with strong vertical motion and appropriate VWS provide favorable dynamic conditions for the formation of NETCs in this region.

    The formation of NETCs is modulated by ISO. NETCs in the WNP are concentrated in ISO active phases. The composite OLR after 20–90-days filtering at the NETCs genesis time are totally negative values. The standard deviation of ISO and synoptic-scale disturbances in the WNP near-equatorial region are much larger than in the NA. Synoptic-scale disturbances tend to gain more energy in the WNP from mean flows through barotropic energy conversion process. The overall unfavorable dynamic and thermodynamic conditions in the NA result in the absence of NETCs there. The warm water on the equatorial sea surface is continuously transported westward, forming the WNP warm pool. The SSTs of both oceans exceed the threshold of TC genesis. However, SST has other indirect effects on the formation of TCs. For example, it will accelerate sea surface evaporation and increase the water vapor in the low atmospheric layer. It will also increase the static stability because of the faster warming in the upper troposphere, which is disadvantageous for TC genesis. The role of SST is complex and needs further study.

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