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Statistical Characteristics and Mechanistic Analysis of Suddenly Reversed Tropical Cyclones over the Western North Pacific Ocean

doi: 10.1007/s00376-014-4064-9

  • Based on best track data of tropical cyclones (TCs) from the Japan Meteorological Agency, the characteristics of suddenly reversed TCs (SRTCs), which have turning angles usually approaching 180°, are statistically analyzed from 1949 to 2011 over the western North Pacific Ocean. The typical large-scale circulation patterns of SRTCs are investigated using reanalysis data and dynamical composite analysis. Results show that turnings mainly occur in low latitudes between 10°N and 20°N, and mainly west of 135°E. The majority of SRTCs reach their peak intensity at, or slightly before, the turning time and subsequently decrease at some variable rate. Specifically, SRTCs are divided into four types, each containing two groups (i.e. eight groups in total) in terms of the moving-direction changes. The moving speed of all SRTC types except the south-north type decreases to its lowest during the 24 h, corresponding to a significant reduction in the primary steering components. According to the analysis of the 13 typical flow patterns found in this study, we suggest that sudden track changes are caused by the reversal steering flow. The original balance of the background flow patterns are broken up by new systems, e.g. binary TCs or dispersion-induced anticyclones. Additionally, sudden track changes are often due to double ridge variations of the subtropical high or weakened/strengthened high pressure in the east and west, respectively.
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Manuscript received: 02 April 2014
Manuscript revised: 30 July 2014
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Statistical Characteristics and Mechanistic Analysis of Suddenly Reversed Tropical Cyclones over the Western North Pacific Ocean

  • 1. College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing 211101
  • 2. Beijing Aerospace Control Center, Beijing 100000

Abstract: Based on best track data of tropical cyclones (TCs) from the Japan Meteorological Agency, the characteristics of suddenly reversed TCs (SRTCs), which have turning angles usually approaching 180°, are statistically analyzed from 1949 to 2011 over the western North Pacific Ocean. The typical large-scale circulation patterns of SRTCs are investigated using reanalysis data and dynamical composite analysis. Results show that turnings mainly occur in low latitudes between 10°N and 20°N, and mainly west of 135°E. The majority of SRTCs reach their peak intensity at, or slightly before, the turning time and subsequently decrease at some variable rate. Specifically, SRTCs are divided into four types, each containing two groups (i.e. eight groups in total) in terms of the moving-direction changes. The moving speed of all SRTC types except the south-north type decreases to its lowest during the 24 h, corresponding to a significant reduction in the primary steering components. According to the analysis of the 13 typical flow patterns found in this study, we suggest that sudden track changes are caused by the reversal steering flow. The original balance of the background flow patterns are broken up by new systems, e.g. binary TCs or dispersion-induced anticyclones. Additionally, sudden track changes are often due to double ridge variations of the subtropical high or weakened/strengthened high pressure in the east and west, respectively.

1. Introduction
  • Climatological tropical cyclone (TC) tracks exhibit prominent geographic, latitudinal, seasonal and interannual variation. The variability in TC tracks on various time scales is sufficiently large to have tremendous social impacts. Therefore, improvement in TC track prediction has been a high priority since the birth of tropical weather forecasting. Since the 1990s, much significant improvement has been made in TC research (Aberson, 2001; Franklin et al., 2003). Satellite (Soden et al., 2001) and dropsonde data (Aberson and Franklin, 1999) are of great help. Moreover, a better understanding of dynamical models (Kurihara et al., 1998) and the mechanisms associated with TC motion (Wang et al., 1998; Emanuel, 1999) has provided much useful information.

    During nearly every western North Pacific summer season, one or more TCs are observed to undergo a peculiar type of sudden track change. Studies have shown that this sudden change in track may lead to the largest errors in TC track prediction (Lam, 1992; Chen and Luo, 1995; George and Gray,1997). As commonly seen, there is a class of special cases that may suddenly reverse at an angle of 180°, sometimes then moving stably in the reversed direction for a relatively long period. This implies that once the mechanism or process responsible is established, it continues to affect these suddenly reversed tropical cyclones (SRTCs).

    The dynamics determining an SRTC's motion is complex. Thus, such sudden changes are poorly predicted because short-term perturbations can be more unpredictable than longer-term straight tracks. For instance, prediction errors of TC Sarah (1989) and TC Hagibis (2007) at their points of turn may have been three to four times the annual mean prediction error (Holland and Lander, 1993; Potty et al., 2012). In offshore areas, failure to predict such suddenly changing TCs may have disastrous effects (Doyle et al., 2010). Despite the existence and potential regular occurrence of SRTCs, these special cases have received surprisingly little attention in the TC research community.

    In general, there is no commonly used definition of sudden TC track changes available in the literature (Wu et al., 2013). (Ni, 2012) suggested there are two approaches to identifying sudden TC track changes. One is to rely on subjective judgment to determine track types and mutation moments. For instance, (Wu et al., 2011) reported a change in track direction exceeding 60° over a 24-h period, with northwestward movement substituted by northeastward movement. Another approach is to account for the changes in the direction of movement by using a quantitative strategy. For example, it was stipulated by 85-906 project (Dai, 2013) that a sudden left-turning TC could be defined by a change in the direction of movement of >30° over a 12-h period. In (Wu et al., 2013), it is postulated that a 1.5 standard deviation of TC directional changes during 12 h (6 h) is approximately 47° (37°). Thus, a sudden northward-turning track change is defined if a track direction change exceeds 40° (37°) during a 12-(6-)h period. Moreover, (Chan et al., 1980), while discussing sudden TC changes in the Atlantic, defined a moving-direction change exceeding 20° in 12 h as a track turning from west to east. Besides, the moving-direction changes of a TC track in the initial 24 h and 24 h before dying out could also be taken into consideration when comparing straight and turned tracks (Chen et al., 2009).

    It has been suggested that the formation mechanism of a suddenly changed TC track is mainly related to the evolution of the large-scale circulation (Wu and Emanuel, 1993). The temporal and spatial distribution of a sudden track change is also closely connected to the seasonal evolution of the large-scale environment (Miller et al., 1988; Yuan et al., 2007). After analyzing suddenly reversed TCs from west to east, (Harr and Elsberry, 1991) concluded that the interactions between the subtropical high, monsoon system and midlatitude westerlies are major factors leading to changes in the large-scale steering flow. Besides, other potential mechanisms are episodic interactions with surrounding systems, such as other TCs (Carr III et al., 1997), tropical upper tropospheric trough (TUTT) cells, and midlatitude troughs (DeMaria, 1987). For example, TC Sarah (1989), which turned almost 180° from south to north, was mainly impacted by two mesoscale convective systems (Holland and Lander, 1993); the clockwise turning of TC Hagibis (2007) from west to east mainly resulted from the binary effect of TC Fujiwara (Potty et al., 2012); and the close to 150° turning of TC Lupit (2009) from west to east may have been related to the formation of an extratropical cyclone near Japan (Pokhil, 2012).

    Although there are a few studies in the literature that make a passing mention of events that resemble what we call here SRTCs, to the best of our knowledge no studies exist that have specifically addressed these cases of TC track change as a distinct phenomenon. In this paper, a composite analysis is conducted to reveal some important characteristics related to SRTCs. Section 2 describes the data and analysis method used. Standard features of SRTCs are defined in section 3, followed by the reporting in section 4 of statistical results on aspects of the variation in the temporal and spatial distribution, intensity and move translation of SRTCs. In addition, typical tracks are clustered and listed using regression models (Gaffney and Smyth, 1999; Gaffney, 2004); and on the basis of a dynamic composite method (Cheng et al., 2009), typical large-scale flow patterns are investigated and preliminary research conducted on the formation mechanisms of SRTCs. Finally, a summary of the key findings of this study are presented in section 5.

2. Data and method
  • TC track data adopted in this work are derived from the TC best track dataset of the Japan Meteorological Agency (JMA). The data include the TC center position (latitude and longitude) and intensity (the central pressure and maximum sustained wind speed) in the northwest Pacific Ocean (including the South China Sea) at 6 h intervals. The study also employs the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data products (Kalnay et al., 1996) on a horizontal resolution of 2.5°× 2.5° with 17 vertical levels, available four times daily.

  • To composite the large-scale circulations of similar cases, we take advantage of the moving coordinate system for tracking TCs. For this purpose, relative positions of surrounding systems also basically remain stable. Meanwhile, the TC is always located in the center of the area, which can be expressed by (Li et al., 2005)

    $$ \overline{S}(x,y)=\dfrac{1}{N}\sum_{n=1}^N{S_n (x,y)} , $$

    where N is the sample number, S(x,y) is the physical quantity field at a certain time, \(\overlineS\)(x,y) is the mean value, and (x,y) are the coordinates of the selected area. In this study, we employ geopotential height, and zonal (u) and meridional (v) winds, to obtain composite images of the environment field where TCs move 12 h before and after their turning points. There are 40 grid points in the x and y directions, and the distance between each grid point is 2.5°.

3. Statistical characteristics of SRTCs
  • In the model illustrated in Fig. 1, Θ represents the turning angle during the 6-h period and (B, E), (C, F) and (D, G) represent the positions 6, 12 and 24 h (before, after) the point of turn. The calculation of turning angles, such as Θ, is consistent with (Dai, 2013), in which the moving-direction changes during both the 12 and 24 h period are counted. Likewise, we utilize the same algorithm to calculate the turning angles during the 12- and 24-h period. Considering the identification methods for sudden track change described in section 1, we take the threshold of direction change for the SRTCs as greater than any of the above-mentioned standard derivations. An SRTC track change is defined as having occurred if: (1) The TC moves for longer than 24 h before and after turning without its before and after paths crossing. This excludes any abnormal turning as a result of stagnation or rapid rotation. In other words, AD and AG in Fig. 1 must remain almost parallel. (2) The TC turning angle needs to possess one or more of the following characteristics: (i) the track-direction change exceeds 90° during the 6 h period; (ii) the turning angle is ≤90° but >45° (90°) during the 6 (12) h period; (iii) the turning angle is ≤90° but >45° (90°) during the 6 and 12 h (24 h) period. In some cases, TCs move very slowly near their turning points, which leads to adjacent points being denser and affecting the turning angle calculation. However, under the above circumstances, most TCs also show suddenly reversed track changes. Hence, point (iii) is supplementary to point (ii), which has already been commented upon in (Wu et al., 2013). With regard to the above rules, a total of 100 TCs during a 63-yr period (1949-2011) are employed as the sample in this study.

    For the purposes of discussion, the turning direction is measured from due east, west, south and north. The SRTCs are then divided into four types (EW, WE, NS and SN), respectively referring to movement from east to west, west to east, north to south, and south to north. Furthermore, each type is separated into two further groups that detail the difference between their rotational movement (i.e. clockwise or anticlockwise), as this feature is closely connected to our analysis of the possible mechanism involved. Thus, our sample of SRTCs includes four types and eight groups. The four types (EW, WE, NS and SN) account for 23% (6 clockwise, 17 anticlockwise), 47% (36 clockwise, 11 anticlockwise), 17% (4 clockwise, 13 anticlockwise), and 13% (5 clockwise, 8 anticlockwise) of the total, respectively. Clearly, type WE represents the largest proportion of the total 100 SRTC events, which is an aspect that deserves particular attention.

  • From Fig. 2, which shows the turning points calculated within 5°×5° grids, we can see that most of the turnings occur in the South China Sea (0°-20°N, 100°-120°E), in waters near the Philippines (5°-20°N, 120°-150°E), and in waters near Taiwan Island (20°-30°N, 110°-130°E), accounting for 39%, 26% and 10% of the total, respectively. Furthermore, we can conclude that turnings mainly occur at lower latitudes between 10° and 20°N, and mainly west of 135°E.

    As illustrated in Fig. 3a, the annual mean frequency of SRTCs is 1.5, and the maximum in a year is 5 (in 1968, 1984, 1994, and 2000). On average, SRTC activity is quite irregular throughout the year, but with a 71% occurrence in July to November, which is consistent with the period of highest frequency of TC genesis.

    Figure 1.  Model of SRTC track segments during the 24 h before and after turning: "T" marks the turning point; Θ represents the turning angle during the 6 h period.

    Figure 2.  Distribution of turning points within 5°× 5° grids.

    Figure 3.  (a) Annual variation, (b) inter-monthly variation, (c) monthly variation, and (d) numbers of each type in different stages.

    Figure 4.  Occurrence period of (a) the maximum intensity and (b) the maximum attainable intensity of SRTCs. "T" marks the turning point, and a minus or plus sign means before or after 12 h around the turning time, respectively.

    The seasonal evolutions of the four SRTC types (EW, WE, NS and SN) are distinct, and closely related to the turning direction. Related to the seasonal evolution of the large-scale circulation, type EW SRTCs are mainly found in August, type WE in July, type NS in November, and type SN in September. For our purpose of studying other parameters, a useful definition of an SRTC is needed. Accordingly, the 72 h after TC genesis is defined as the "initial stage", 72 h before dying out as the "later stage", and the remainder as the "intermediate stage". Their occurrence frequencies are illustrated in Fig. 3b, and the total number of SRTCs in each stage accounts for 17%, 66% and 17%, respectively. More precisely, after changing direction, the majority of TCs last for more than 72 h, and sudden changes tend to occur in the middle of a TC's lifespan.

  • Figure 4a shows the maximum intensity of SRTCs within, before and after 12 h around the turning time Overall, SRTCs tend to show comparable contribution in each period, accounting for 31%, 39% and 30%, respectively. Specifically, the occurrence of peak intensity in type EW increases to 52%, while the remaining three types correspond to the mean state. This means that virtually all of the SRTCs reach their peak intensity at, or slightly before, the turning time and subsequently decrease at some variable rate.

    In the JMA datasets, TC intensities are classified into four categories: tropical depression (maximum wind <33 kn); tropical storm (maximum wind between 34 and 47 kn); severe tropical storm (maximum wind between 48 and 63 kn); and typhoon (maximum wind >64 kn). According to these criteria, the proportion of SRTCs reaching typhoon intensity only account for 38%. Thus, we can conclude that TCs with weaker intensity tend to have larger turning angles.

  • (Gaffney and Smyth, 1999) indicated that scattered TC tracks can be described well by small groups of tracks, or TC "regimes" (Gaffney and Smyth, 1999). In this section, a cluster analysis technique is used to obtain the mean regression trajectories of all SRTCs in the eight groups. (Gaffney, 2004) provides us with a detailed description of the clustering methodology, which was then later applied to extratropical cyclones over the North Atlantic (Gaffney et al., 2007). Here, to compare the difference between SRTCs with clockwise and anticlockwise rotation, the mass center positions of the eight groups are retained prior to clustering. After obtaining the mass center positions of each group, we regard them as the turning point. We then move the tracks in the corresponding area in order to obtain the mean tracks.

    The mean regression trajectories for the eight groups are illustrated in Fig. 5. The variation in the density of the dots indicates the SRTC moving speed. The trajectories' initial positions are marked by black solid circles, so as to measure the starting location and moving direction. Note that along the mean regression trajectories, many points remain close to one another within the turning area, suggesting a reduction of moving speed before changing direction.

    Clearly, the mass centers (triangles) are mainly distributed within (20°-30°N, 125°-140°E), while type NS deviates to the south of 20°N. As we can see, the observed tracks for types EW and WE can move for a longer period after turning, whereas most of the tracks for types NS and SN often deviate to the east or west after turning, indicating more perturbations in their synoptic surroundings. In particular, type WE occupies a much larger area, mainly over the Pacific Ocean. The typical tracks of types EW and WE are somewhat similar, tending to the north 36 h after turning and measured clockwise and anticlockwise, respectively. Interestingly, the anticlockwise mean track of type NS presents an obvious inverted-V shape, and is often limited within a long narrow strip. Additionally, the clockwise mean track of type SN is V-shaped, which is consistent with relatively small changes in direction. Since the mean trajectory is mainly used to represent the whole moving trend, it is unavoidable that there is much greater deviation between some special cases and the average track. Even though only a few cases possess multiple turning points, the specific stages that meet the definition outlined in section 3.1 are chosen in this study.after turning. The purpose of this section is to familiarize the forecaster with the speed features of SRTCs. At the point of turn, nearly all of SRTCs are moving their slowest, with the average speed of movement being slightly below 10 km h-1. Meanwhile, the rate of acceleration of SRTCs after turning is different among each type, demonstrating that the speed is closely related to the direction of movement (data not shown).

  • The largest errors in mean forecast position are associated with sudden track changes and mainly due to acceleration

    Figure 5.  Actual track (gray solid circles) and mean regression track (black solid circles) of each sample. The triangles are clockwise and anticlockwise mass centers, and the large black solid circles are the starting points.

    Observational analyses indicate that a deep-layer mean steering (e.g. 1000 to 150 or 100 hPa) is better correlated with storm motion than any single-level steering. (Franklin, 1990) concluded that the deep-mean flow averaged over the inner-core region within 3° of latitude from the center is a most useful indicator of the TC's motion, while it appears to be more appropriate to define the flow averaged over the 5°-7° latitude radial band as the environmental steering flow. Here, we calculate the zonal and meridional speeds of the steering flows [U(S) and V(S)] and the actual translation speeds correlated with the SRTCs. The mean wind averaged within the 5°× 5° grid with TC as the level between 850 and 300 hPa is calculated (Pike, 1985). Note that the steering flows defined include not only environmental steering flow, but also secondary steering flow.

    As depicted in Fig. 6, the primary steering flow of the four SRTC types (EW, WE, NS and SN) are separately decomposed into their u and v components. On the whole, the calculated steering is highly consistent with the SRTC translation speed. In particular, type EW is accompanied by the slowing of eastward movement and meridional acceleration, while type WE is characterized by rapid slowing of westward movement, as described by (Carr III and Elsberry, 1995) and Wu et al. (2011, 2013). Likewise, type NS starts to slow down northward about 24 h prior to the turning time, while type SN shows a reduction of southward movement.

4. Analysis of the large-scale steering flow patterns for each type
  • In order to demonstrate the different evolutions of the large-scale circulation, the steering flow is determined between 850 and 300 hPa by the vertical average of several levels, which depends upon the TC intensity (Pike, 1985). Following this method, charts of large-scale mean steering are attained through compositing similar cases' environment fields. We then present (Figs. 7-10) the large-scale mean steering at 12 h before (left-hand panels) and after (center panels) the turning time using the NCEP-NCAR reanalysis wind field. The right-hand panels, meanwhile, demonstrate the changes in 24 h mean geopotential height before and after the turning time. In other words, each row of three panels in Figs. 7-10 shows a typical circulation pattern corresponding to the 13 classes listed with prominent signs in Table 1. Taking Fig. 7 as an example, the three panels in the first row represent cases of binary TC systems with a TC located to the west of the SRTC, while the three panels in the second row represent cases where the co-existing TC is located to the east of SRTC. The panels in the third row are the non-binary TC cases. Under the conditions represented in the fourth row, the SRTCs are situated at the double ridge lines of the subtropical high in the northeast and southeast, respectively.

    Figure 6.  Meridional and zonal actual speed [U(T) and V(T)] and average steering flows [U(S) and V(S)] of the four types of TCs during the 24 h before and after turning. "T" on the x-axis represents the turning point; the y-axis indicates speed; and a plus or minus sign represents the direction.

    More precisely, the centers of these figures are accorded with the SRTCs' centers owing to the dynamic composite. A similar application of the dynamical composite method can be found in (Li et al., 2005). In total, there are 40 grid points in the x and y directions, and each grid point distance is 2.5°. Besides, the shading in the left and center panels in Figs. 7-10 indicate the areas with wind speed >8 m s-1. The dotted and solid lines of the right-hand panels indicate the low and high systems (low, TCs or midlatitude troughs; high, subtropical high or high pressure zone). "T" marks the turning time, and a plus or minus sign represents 6 or 12 h before or after the turning time, respectively. Each of these patterns is discussed in more detail below.

  • To investigate the formation mechanisms of type EW SRTCs, a conceptual model is first established. At the beginning, such SRTCs are steered by the westerly flow, with the high pressure located to the southeast or northeast of the SRTC. It is commonly seen that a high pressure ridge behind the upper trough merges into a weak high pressure, which is forced by the eastward movement of the trough, indicating that the height of the base controls the direction of synoptic systems. In this case, the SRTC is formed by the strengthened easterly flow resulting from the westward movement of the high pressure zone.

    It is important to note that this type of SRTC is usually part of a binary TC system, meaning it is accompanied by a co-existing TC for a certain period of time. We further divide these co-existing TCs into two categories, called western TCs (Fig. 7a) and eastern TCs (Fig. 7d). In Figs. 7-10, the SRTC centers labeled with TC marks are located in (0, 0). If there are binary TCs, only the SRTC is labeled. In the former category, with the northwestward movement of the western TC, the high pressure ridge not only moves westward but also strengthens (Fig. 7b). Simultaneously, the subtropical high develops into a strong and stable high pressure zone (Fig. 7c) and shifts slightly to the north, leading to the sudden track change. Note that the sudden track change occurs at the bifurcation point of northeasterly and southeasterly winds. In the latter group, the high pressure zone is separated by the low pressure zone by the co-existing TC (Fig. 7e), with northwest movement of the eastern TC. Figure 7f clearly shows that the high pressure zone in the north strengthens steadily because of the northward movement of the weakened upper-level trough. Even though the circulation of the anticyclone cell to the south becomes much stronger, the 15 m s-1 isotach has shifted from the south to the north side of the SRTC (Fig. 7e), suggesting a reduction in the U component to the south. Additionally, the majority of cases in this group are rotating anticlockwise from due east.

    Figure 7.  Mean steering flow of type EW SRTCs at 12 h before (left panels) and after (center panels) the turning time, and the 24 h mean geopotential height before and after the turning time (right panels). Each grid point distance is 2.5°. The centers of the figures correspond to the centers of SRTCs. The shading indicates wind speed >8 m s-1.

    For non-binary TC categories, there are also two kinds of flow pattern. For SRTCs in the former group (i.e. western TCs), the high pressure in the south is embedded into another higher center in the northwest due to the eastward movement of the upper-level trough, providing slightly stronger southward winds (Fig. 7g). As seen in Fig. 7i, the sudden track change occurs when the TC is met by the western extension of the subtropical high and dominated by the enhanced easterly winds. It seems that the background field is a typical Ω shape, suggesting TCs enlarge in scale after turning. During the beginning phase of SRTC evolution in the latter category (i.e. eastern TCs), SRTCs are located at the double ridge lines of the subtropical high in the northeast and southeast, respectively (Fig. 7j). It is notable that the double ridges differ in gradient, indicating a change of steering flow. It is also evident from Fig. 7k that the southern ridge extends all the way to the southwest of the TC, resulting in the development of a small high center in the west. SRTCs are steered by enhanced easterly winds and the subtropical ridge retreats eastward with the orientation changing to northwest-southeast (Fig. 7l). More than half of SRTCs in this category are rotating anticlockwise from due east.

  • Again, a conceptual model is first put forward. Initially, SRTCs of this type are dominated by easterly winds provided by the high pressure zone or the subtropical high to the north. With continuous advection of negative vorticity, the zonal high-pressure zone ruptures into two high-pressure systems, enclosed and massive. Usually, sudden track changes are caused by the development of a high pressure center in the southeast, with enhanced southwesterly flows.

    As discussed in section 3, statistical results show that this type of SRTC makes up over one third of the total 100 SRTC events. Likewise, the large-scale flow patterns of this type are more complicated than for the other three. Accordingly, we divide them into five categories. Typical flow patterns (Fig. 8c) associated with the first category are: (1) large-amplitude troughs, extending southward from the westerlies and located within a few hundred miles to the west of the TC center (Fig.8a), and (2) well-marked low-latitude troughs building northward into the westerlies, and weak troughs between two separate high cells (Fig. 8b). It can be concluded from the above analysis that sudden track changes result from a neutral point (or "break") in the high-pressure zone to the north of the TC, as is the main reason for the eastward turning.

    It should be pointed out that this type of SRTC is also often accompanied by a co-existing TC in a binary TC system, and again we divide this phenomenon into eastern and western TCs. For SRTCs in the eastern TCs category, they are dominated by the extended subtropical high moving to the west (Fig. 8d). The northern anticyclone is separated into two cells, in the northeast and in the west, respectively (Fig. 8e). The key feature of this second flow pattern category is that the western TC and the eastern anticyclone make the SRTC vulnerable to the strong southwesterly steering flow (Fig. 8e). By the same token, the western TC is subject to strong equatorward steering flow. For SRTCs in the western TCs category, the subtropical high that provides easterly flow at first quickly retreats eastward as the binary TCs depart or the upper-level trough moves eastward. Consequently, eastern TCs tend to facilitate a sudden track change of western TCs (Fig. 8g), and eastern TCs tend to undergo poleward movement (Fig. 8h). Finally, eastern TCs are captured into the westerlies (Fig. 8i). Note that the 18 m s-1 isotach shifts from the northeast to the south side of the SRTC, which indicates a reversal of the environmental steering from eastward to westward. The common feature of the two category with binary TCs is that a trailing anticyclone is generated behind the eastern TC via Rossby wave dispersion. Moreover, (Luo and Ma, 2001) demonstrated that the dispersion-induced anticyclone could increase the density of the isolines between an eastern TC and the subtropical high, and then accelerate the eastern TC.

    In the fourth flow pattern category, similar to Fig. 8e, the dominant anticyclone is split into two cells in the east and northwest (Fig. 8j), respectively. The remarkable feature of this case is that there is an apparent anticyclone circulation in the southeast. As pointed out by (Carr III and Elsberry, 1995), the extended anticyclone circulation is partly due to the presence of the enlarging circulation of TCs and the northeast movement of the subtropical high, which is presented in Fig. 8k. The southwesterly flow established in the southeast of TCs is mainly owing to the joint effect of the anticyclone and subtropical high.

    As can be seen from Fig. 8m, the flow pattern of the fifth case involves the TC being embedded in the double ridges of the subtropical high. In the early stage, SRTCs are mainly dominated by the subtropical high, which is oriented from northwest to southeast. Upper-level troughs, which cannot directly affect TCs, nevertheless force the subtropical ridge to retreat all the way, resulting in the southerly deviation of its climate position (Fig. 8o). Furthermore, the orientation of the subtropical ridge shifts to west-east or northeast-southwest, suggesting a reversal of the steering flow (Fig. 8n). It is proposed that the sudden track change can be explained by the gradual disappearance of the northern line of the subtropical high.

  • As illustrated in Fig. 9, type NS SRTCs are always located between two high pressure cells in the northwest and northeast, respectively. In general, the sudden track changes are directly due to a weakened high cell in the east or a strengthened high cell in the west. The remarkable feature of this type is that the meridional winds are significantly stronger than the zonal winds. Two observed models are introduced and described in following paragraphs.

    As shown in Fig. 9a, SRTCs of this type are often accompanied by multiple other TCs, located in the southwest and southeast. The interaction with these multiple other TCs comprises a much more complex situation, and thus makes forecasting that much harder. During the 12-h period, the high cell in the east and the eastern TC are temporarily stalled. In particular, the eastern TC is a distance of close to 15 grids away from the SRTC, whereas the high cell in the west is under development and slightly shifted to the north due to the westward movement of the western TC (Fig. 9c). The meridional wind isotach (Fig. 9b) around the TC can be regarded as an indicator that the TC is influenced by moderately southward steering flow.

    Figure 8.  As in Fig. 7, but for type WE SRTCs.

    Figure 9.  As in Fig. 7, but for type NS SRTCs.

    Figure 10.  As in Fig. 7, but for type SN SRTCs.

    Under the non-binary TC category, in the early stage, SRTCs are steered by the northward winds, with a powerful subtropical high extending to the north and south (Fig. 9d). During the process of longwave adjustment, the high cell in the west is embedded into the high ridge, which is behind the upper-level trough (Fig. 9f). In other words, the longwave trough not only hinders the westward movement of the high cell in the east, but also facilitates the eastward movement of the high cell in the west. In particular, SRTCs in this case remain stable for over 48 h after turning.

  • Similar to type NS SRTCs, type SN SRTCs are also located between high pressure cells in the east and west. The prominent difference between the two types is that, in type SN, the high pressure cell in the west is not enclosed and massive. Two observed models are discussed in the following paragraphs.

    In the early stage, binary TCs are observed in the southeast. It is likely that the southward movement of SRTCs is mainly due to the Fujiwara effect for the much weaker high ridge in the west (Fig. 10a). As clearly seen in Fig. 10b, southwesterly winds are suddenly enhanced, not only restricting the mutual rotation of the binary TCs, but also weakening the western portion of the subtropical ridge, resulting in the track suddenly changing. (Guard, 1983) attributed the sudden strengthening of the southwesterly flow in the south or west of the TC to propagations of monsoon surge. (Liu and Chan, 2003) also reported that the presence of a large TC can be attributed to the southwesterly surge synoptic pattern, which needs to be further investigated.

    As discussed in section 3, SRTCs always tend to deviate to the east or west after a short period of southward movement. In the latter case without binary TCs, we find that this can be ascribed to the much weaker ridge in the west, suggesting weaker meridional wind isotachs in the west (Fig. 10d). Meanwhile, the eastern high pressure cell is suddenly split into two cells in the north and south, respectively, for negative vorticity advection (Fig. 10e). It is proposed that the effect on the TC of the peripheral anticyclone in the southeast remains strong, with an enhanced southwesterly flow.

5. Discussion and conclusions
  • In this study, we first make a detailed statistical analysis of 100 cases of SRTCs identified over a period of 63 years in the western North Pacific. Four types comprising two groups each (eight groups in total) are then proposed in order to describe the sample according to the directional features of the SRTCs. Moreover, the mean regression tracks of the SRTCs in the eight groups are obtained using a cluster analysis technique. Results suggest, based on dynamic composite analysis that several mechanisms could contribute to the formation of the SRTCs. Many of these are derived from forecasts and from general pattern recognition, which are poorly understood. We list 13 flow patterns here to provide an indication of the complexity and variety of the potential mechanisms and a basis for further research.

    Remarkable differences are found in the 100 SRTC events in terms of their spatial distributions. SRTCs mainly occur at lower latitudes between 10°N and 20°N, and mainly west of 135°E, correlated with the variable influence of the synoptic situation on specific TCs. SRTC activity is quite irregular throughout the whole year, and the seasonal evolutions of the four SRTC types are distinctly different from each other. Almost all SRTCs reach their peak intensity at, or slightly before, the point of turning, and subsequently decrease at some variable rate. Besides, we conclude TCs with weaker intensity tend to have larger turning angles. Analysis also shows striking differences in speed variation as SRTCs approach the point of turning. Nearly all SRTCs are moving at their slowest around the point of turning, but the rate of acceleration after turning is different for each type, indicating that the speed of movement is closely related to the moving-direction change. On the whole, the motion of a TC during the turning period is consistent with the steering flow. Specifically, type WE SRTCs are characterized by a rapid slowing of westward movement, as described by (Carr III and Elsberry, 1995) and Wu et al. (2011, 2013).

    Atmospheric variations can also be decomposed into three prominent categories: namely, intraseasonal oscillation (ISO), Madden-Julian Oscillation (MJO) and quasi-biweekly oscillation (QBO) (Wu et al., 2011, 2013; Li and Zhou, 2013). Even though the main points of Wu et al. (2011, 2013) and (Li and Zhou, 2013) are different, it is clear that the fundamental findings are close to some degree. Regardless, it can be concluded that the goal of the above studies is to explore the interaction between the high-/low-frequency steering flows and sudden TC track changes or other aspects. However, we suggest that the purpose of this paper is to refine the mechanisms leading to TC reversed turning. Additionally, it has long been recognized that the steering flow is often related to the large-scale system, e.g. the subtropical high, monsoon trough, and other synoptic features (e.g. the upper-level trough, ridge). In other words, understanding TC movement primarily relies on our ability to predict the evolution of these large-scale systems surrounding the TC vortex, which can also shed light on sudden TC track changes.

    Thus, based on fundamental theories about TCs, we can state some important rules captured by the dynamic composite analysis. Overall, sudden track changes in the 100 events are primarily the result of the domination of reversal steering flow. Specifically, the original balance of the background flow patterns is broken up by new systems, such as binary TCs or energy dispersion anticyclones. In particular, binary TCs often lead to the rupture of the zonal high pressure zone, or propagate a high pressure ridge to prevent the SRTC from moving westward. The majority of the former rotate anticlockwise from due east, while the latter rotate clockwise from due west. The typical inverted Ω shape and the anticyclone dispersion induced by the subtropical high are also critical signs, which are associated with the scale variations of SRTCs. Additionally, sudden track changes are often accompanied by orientation variation in double ridges of the subtropical high, or weakened/strengthened high pressure in the east and west. The former only belongs to type WE SRTCs, whose turnings are mainly caused by the southern high pressure, whereas the latter belongs to both type NS and type SN SRTCs.

    It should be acknowledged that our study relies upon the concept of steering flow. Some studies suggest that the size of the TC may also play an important role in its movement (Carr III and Elsberry, 1995). It is also proposed that the position and intensity of the TC may contribute to its turning (Zhang et al., 2013), which could be further detailed and evaluated through diagnosis and numerical simulation studies. Finally, thermodynamic processes may bias the influence of steering flow in terms of the direction of a TC track (Chan et al., 2002), which should be taken into consideration in future work.




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