2094 MONTHLY WEATHER REVIEW VOLUME 125

Observational Evidence for Alternate Modes of Track-Altering Binary Tropical Scenarios

LESTER E. CARR III, MARK A. BOOTHE, AND RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California (Manuscript received 11 July 1996, in ®nal form 13 January 1997)

ABSTRACT An observational study of western North Paci®c tropical (TC) revealed many cases of two TCs whose tracks were altered by processes that were quite different from the mutual advection (Fujiwhara-type) processes. Thus, four conceptual models are proposed to describe these track alterations. A conceptual model called direct interaction is proposed that is a modi®cation of one by Lander and Holland and has three modes: 1) a one-way in¯uence in which the track of a smaller TC that is embedded in the circulation of a larger TC has a cyclonic orbiting motion, but no signi®cant track alteration of the larger TC is apparent; 2) a similar case in which a mutual advection occurs with the tracks of both the smaller and larger TCs being altered; and 3) a subset of 2) in which the mutual advection includes an attraction component such that the two similarly sized TC circulations eventually merge into a larger circulation with a single center. During the 7-yr period (1989± 95), the one-way in¯uence, mutual interaction, and merger modes were detected seven, three, and two times. A semidirect interaction conceptual model is proposed in which the two TCs have a relative cyclonic rotation as in the Lander and Holland model, but the TCs are separated by 10Њ±20Њ longitude so that a direct (advective- type) interaction is excluded. Rather, the track alteration is attributed to an environmental ¯ow established by the juxtaposition of a TC on one side and a subtropical anticyclone cell on the opposite side. In an east±west orientation of the two TCs and a subtropical anticyclone cell to the east (west), the height gradient between the western (eastern) TC and the eastern (western) subtropical anticyclone establishes a poleward (equatorward) environmental steering ¯ow across the eastern (western) TC. In the 1989±95 sample, a semidirect interaction that altered the tracks of the eastern or the western TC occurred 18 and 14 times, respectively. An indirect interaction conceptual model is proposed in which the distinguishing feature is the Rossby wave dispersion-induced anticyclone to the east and equatorward of the western TC. This anticyclone imposes an equatorward (poleward) steering ¯ow across the eastern (western) TC. Several variations of the indirect interaction are possible depending on the separation distance, sizes of each TC, and their relative orientation. During the 7-yr period, an indirect interaction affecting the western TC or the eastern TC occurred 36 and 22 times, respectively. A fourth conceptual model of track alterations involving two TCs is proposed in conjunction with a reverse- oriented monsoon trough formation. The distinguishing feature of this conceptual model is the combination of the peripheral anticyclones of both TCs as the eastern TC moves into an east±west orientation and has a separation of 10Њ±20Њ longitude. In the 1989±95 sample, a reverse-oriented trough formation involving two TCs occurred seven times. The frequency of track alterations whenever two TCs are present emphasizes that forecasters must give special attention to such situations. The four conceptual models proposed here emphasize that the physical mechanisms are complex and in the vast majority of cases cannot be attributed to the mutual advection (Fujiwhara-type) process implied in the Lander and Holland model.

1. Introduction cyclone interaction, for which one of the de®ning char- Anomalous (TC) tracks such as non- acteristics is that the two TCs appear to rotate cyclon- climatological equatorward de¯ections, rapid poleward ically relative to an intermediate midpoint. Based on movement, and apparent rotation are often observed studies dating back to Fujiwhara (1923), the key factors when two tropical cyclones are present in a region. Such in the direct binary interaction are the separation dis- track anomalies have often been attributed by forecast- tance and the outer structure of each cyclone, ers to what will be referred to here as direct binary which determine the cyclonic rotation rate and the ten- dency for attraction or repulsion (Elsberry 1995). More speci®cally, two cyclones with a suitably small sepa- ration distance are presumed to be mutually advected Corresponding author address: Dr. Lester E. Carr, Department of by their cyclonic circulations. Meteorology, Naval Postgraduate School, Code MR/Cr, 589 Dyer Road, Room 254, Monterey, CA 93943-5114. Observed tracks in the western North Paci®c region, E-mail: [email protected] where multiple TCs are common (Neumann 1993), are

Unauthenticated | Downloaded 09/27/21 04:45 AM UTC SEPTEMBER 1997 CARR ET AL. 2095 frequently not compatible with a direct binary interaction explanation. Many of the anomalous track de¯ections oc- cur at separation distances that are too large to be a mutual advection, especially when one or both of the TC circu- lations are small. Dong and Neumann (1983) attributed some of the anomalous rotation rates or separation distance tendencies to environmental horizontal wind shear in the monsoon trough, although similar shear effects might be expected near the subtropical ridge. Based on a 7-yr compilation of western North Paci®c TCs, it is proposed that many of these anomalous track de¯ections may be explained by two other modes of binary TC interaction that are fundamentally different from the direct type. Two conceptual models that will be termed semidirect and indirect binary cyclone inter- action will be introduced and described with case stud- ies. Another mode of sometimes almost simultaneous FIG. 1. Conceptual model of binary cyclone interaction by Lander track alterations of two suitably separated TCs is a re- and Holland (1993) indicating the approach and capture, mutual orbit, verse-oriented monsoon trough formation (Carr and Els- the release and escape, and the merger stages. berry 1994; Lander 1996). This mode is not necessarily an actual TC interaction, because it just may be that an east±west juxtaposition of the two TCs happens to be tween the two vortices is indeed the centroid). They favorable for setting up the circulation that leads to near- emphasize that the track changes associated with the ly simultaneous poleward track changes. ``capture'' and ``release'' events typically occur sud- For the semidirect and indirect interactions and the denly prior to and subsequent to the period of mutual reverse trough formation (RTF), the track-altering in- cyclonic rotation (orbit). Thus, the successful forecast ¯uence is in conjunction with an adjacent anticyclone. of the tracks during and following the binary cyclone The key factor in the semidirect type is an advection interaction period will critically depend on the predic- established by a pressure gradient between one of the tion of the times of capture and release. Examples of TCs and a favorably aligned cell in the subtropical an- actual merger of the two cyclones, as would be expected ticyclone. A preliminary interpretation of the indirect from the attraction component of the , type is offered based on the Carr and Elsberry (1995) is found by Lander and Holland (1993) to be rare. Land- numerical simulations of monsoon gyres and large TCs er (1995) has documented such a merger case in which that generate a trailing anticyclone via Rossby wave separate cloud patterns can be identi®ed from the ge- dispersion. The presence and strength of such an anti- ostationary satellite imagery as the two centers complete cyclone between the two TCs is hypothesized to be the more than one cyclonic orbit. critical factor in the indirect interaction. In the RTF, the A more typical scenario is that a smaller cyclone is two TCs are part of a trough oriented southwest to north- swept into the horizontally and vertically sheared ¯ow east, and their trailing anticyclones combine to form a of the larger cyclone. This mode of direct interaction is nearly parallel anticyclonic circulation on the equator- more appropriately considered as a one-way in¯uence ward side that together establish a more poleward en- (Fig. 2a) because the motion of the larger TC is little vironmental steering ¯ow across one or both TCs. affected by the presence of the smaller TC. In terms of The extensive literature on the direct type will be the barotropic vortex stabilization mechanism of Carr brie¯y reviewed in section 2 to provide a contrast with and Williams (1989), the smaller vortex circulation is the semidirect and indirect types to be introduced in equivalent to a high azimuthal wavenumber (short sections 3 and 4, respectively. Following the description wavelength) perturbation on the larger vortex circula- of the RTF in section 5, the frequency and some char- tion. The horizontal shear of the larger vortex distorts acteristics of each of these track-altering situations based the smaller circulation, and elongation along the stream- on the 7-yr compilation will be given in section 6. lines must be accompanied by contraction in the cross-stream direction to conserve absolute vorticity in a barotropic sense (Smith et al. 1990). In nature, the 2. Direct binary interaction additional effect of vertical wind shear of the larger Lander and Holland (1993) have documented anom- (stronger) vortex may also contribute to the dissipation alous tracks associated with binary tropical cyclones. of the smaller vortex if it remains under the in¯uence They propose a conceptual model of direct interaction of the larger vortex for very long. (Fig. 1) with cyclonic rotation of the two vortices rel- A related mode of direct interaction called mutual in- ative to the centroid (generally assuming the two cir- teraction (Fig. 2b) occurs if the two TCs are close enough culations are of the same size so that the midpoint be- that a mutual cyclonic rotation (orbit) occurs, as in the

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merger process would commence, which is consistent with the rareness of mergers in nature. Falkovich et al. (1995) attribute the merger to a redistribution of the outer vorticity ®eld of each TC by the adjacent TC circulation such that the asymmetric ¯ows across the centers reduce the separation distance. In summary, the direct binary TC interaction implies that two tropical cyclones somehow become close enough that the outer circulation of the larger TC be- comes the advecting current (steering) for the smaller TC (Fig. 2a), or they mutually advect (Fig. 2b). Merger (Fig. 2c) is rare because the separation distance has to become so small. No signi®cant contribution from the FIG. 2. Three modes of direct binary tropical cyclone interaction: (a) One-way in¯uence in which the track of a smaller TC (solid TC environment or from a TC±environment interaction is symbol, dashed streamlines) that is embedded in the circulation of a required. However, the spatial variations of the envi- larger TC (open TC symbol, solid streamlines) has an orbiting motion ronment may contribute to the apparent rotation, or one prior to dissipation but causes no signi®cant track alteration of the of the vortices may come close enough to an adjacent larger TC because its circulation is too small; (b) a similar physical mechanism as in (a), except a mutual advection occurs with the tracks cyclone or anticyclone to overcome the direct interac- of both the smaller and the larger TCs being altered; (c) a subset of tion effect (Holland and Dietachmayer 1993). As the (b) in which the mutual advection includes an attraction component smaller TC is advected by the outer circulation of the such that the two similarly sized circulations eventually merge into larger TC, the horizontal or vertical shear tends to dis- a larger circulation with a single center. sipate the smaller TC circulation. A type of self-induced end to a binary cyclone interaction may occur as the Lander and Holland (1993) model in Fig. 1. However, it beta-induced gyres form, especially if the anticyclone is not assumed that the two TCs are of equal size, so the gyre to the east of the eastern TC becomes larger than centroid is closer to the larger TC center, which has a the corresponding cyclonic gyre to the west (Holland proportionately smaller de¯ection. In the sense that the and Dietachmayer 1993). Escape from the cyclonic ro- major track alteration is for the smaller TC, which will tation of the direct binary TC interaction occurs when again tend to be dissipated via horizontal and vertical the eastern TC vortex becomes embedded in, and is shear, the mutual interaction will be considered to involve de¯ected around, this anticyclone to the east. the same physical mechanisms as in the one-way in¯u- ence. However, the merger (Fig. 2c) of two nearly equal- sized TCs that somehow become close enough that their 3. Semidirect interaction cyclonic circulations overlap will be considered a subset a. Conceptual model of the mutual interaction in Fig. 2b. Regardless of how rare such a merger is, it has at- Carr and Elsberry (1994) introduced a multiple TC tracted great interest as a scienti®c problem. Although synoptic pattern in the western North Paci®c that will be Chang (1983) suggested the lower-tropospheric in¯ows de®ned here as a semidirect binary TC interaction (Fig. associated with the secondary circulation of each cy- 3a). By de®nitions, the two TCs must be suf®ciently clone could cause a mutual attraction, this effect would separated that a direct interaction is not occurring, which not be large away from the central core. Chan (1993) for typical sizes of TCs usually only occurs if the sep- describes the tendency for attraction in terms of the aration is less than 10Њ latitude (Brand 1970; Dong and asymmetric and symmetric vorticity advections. For dif- Neumann 1983). The second requirement is that the two ferent outer wind structures and initial separation dis- TCs must be oriented approximately east±west and suf- tances, the region between the vortices may have pos- ®ciently close (north or south) to the subtropical ridge itive or negative vorticity gradients. Chan shows the axis, as shown in Fig. 3a. The key feature of the semi- separation distance will decrease (vortices will tend to direct interaction is that the height gradient between the merge) if this intermediate region has a positive vorticity western TC and the eastern anticyclonic circulation sub- gradient. The mechanisms involved during vortex merg- jects the eastern TC to moderately strong (10±15 kt) and er have been explored with a discrete vortex patch model predominantly poleward steering ¯ow (Fig. 3b). Simi- by Ritchie and Holland (1993), an adaptive grid model larly, the height gradient between the eastern TC and the by Holland and Dietachmayer (1993), and a dual-nested western anticyclonic circulation subjects the western TC model by Falkovich et al. (1995). The ®rst two models to moderately strong, and predominantly equatorward, isolate a straining effect of the horizontal shear of one steering ¯ow (Fig. 3c). vortex circulation on that of the other vortex as the Notice the TCs in Fig. 3 are suf®ciently separated critical factor leading to merger. Ritchie and Holland that their circulations do not overlap, as in a direct in- indicate that two equal-sized tropical cyclones would teraction. Thus, any relative cyclonic rotation about a have to approach within 150±300 km before such a midpoint in a semidirect interaction must be attributed

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tropical anticyclone and the relative strengths of the an- ticyclonic cells to the west and east, will determine the subsequent steering ¯ow for the western TC. Most aspects of the Lander and Holland (1993) con- ceptual model of the direct interaction apply in the semi- direct interaction. Given the two track sequences in Figs. 3b and 3c, a relative motion diagram such as in Fig. 1 would indeed indicate an ``approach,'' a cyclonic rotation (orbit), and an ``escape,'' especially if the eastern TC recurves. However, these two TCs would never get suf- ®ciently close for the motion of one TC to be due to an advection determined directly by the circulation of the other TC. Thus, the physical mechanisms involved in the semidirect interaction are distinctly different from the direct interaction implied by Lander and Holland (1993) based on an advective (Fujiwhara-type) effect.

b. Case study 1 The primary purpose of this case study is to illustrate a representative semidirect interaction involving Super- FIG. 3. (a) Arrangement of key synoptic circulations in the semidirect (ST) Seth and Tropical Storm (TS) Verne dur- binary tropical cyclone interaction. Notice that no anticyclone is present ing November 1991. The complete evolution is de- between the tropical cyclones, which must be within 10Њ±20Њ longitude scribed by Carr and Elsberry (1994). Only selected anal- but not be embedded in the other's circulation. (b) Impact of the poleward steering ¯ow (broad arrow) across the eastern TC in (a) and the ap- yses will be included here to illustrate the key events. proximate motion vector (narrow arrow). (c) Impact of the equatorward From 0000 UTC 2 November to 0000 UTC 5 Novem- steering ¯ow (broad arrow) across the western TC in (a) and the ap- ber, ST Seth is steadily moving out of the deep Tropics proximate motion vector (narrow arrow) that depends on the relative on a heading of 300Њ (Fig. 4a). The corresponding Navy strengths of the ␤-effect propagation and the steering effects. Operational Global Analysis and Prediction System (NO- GAPS) analyses (not shown) indicate Seth is within the to environmental ¯ows. This is called a semidirect bi- southeasterly ¯ow to the southwest of a strong subtropical nary TC interaction because two TCs must exist, and it anticyclone cell, which is consistent with Seth's steady is the juxtaposition of a TC on one side and a subtropical 11-kt (1 kt ഠ 0.5msϪ1) translation early in the period. anticyclone cell on the opposite side that establishes the As ST Seth approaches the subtropical anticyclone axis environmental steering for that TC. Whereas an envi- on 6 November (Fig. 5a), the translation speed decreases ronmental ¯ow is not necessary for occurrence of rel- to 2±4 kt (Fig. 4a). A slight asymmetry in the 30-kt ative rotation in a direct interaction, semidirect inter- isotach pattern with a larger area in the northwesterly action cannot occur without the presence of favorably ¯ow is consistent with a near stall in translation. On 0000 positioned environmental circulations. In a semidirect UTC 6 November, TS Verne is near 11ЊN, 160ЊE (Fig. interaction, the eastern TC is acting to inhibit the re- 4b), which is about 20Њ longitude and 10Њ latitude to the curvature of the western TC (Fig. 3c), and/or the western east-southeast of ST Seth (Fig. 5a). TC is concurrently acting to encourage the recurvature Because Seth is temporarily stalled, TS Verne closes of the eastern TC (Fig. 3b). The ``and/or'' terminology to a separation distance of 15Њ longitude and is almost above allows for variations in the environmental in¯u- directly east of ST Seth at 0000 UTC 8 November (Fig. ence as determined by the relative outer wind strengths 5b). This separation distance and orientation, plus the of the TCs and the strengths of the anticyclonic circu- presence of a strong subtropical anticyclone cell to the lations. For example, the track of the western TC might west of ST Seth, meet the criteria for a semidirect in- be altered by the eastern TC±western anticyclone with- teraction. Notice the 30-kt isotach on the west side of out alteration of the eastern TC track, and vice versa. ST Seth is considerably larger than that on the east side Notice that the semidirect interaction will tend to be (Fig. 5b), which is an indicator that Seth is under the transitory. The eastern TC typically moves rapidly around in¯uence of a net southward steering ¯ow. Indeed, the the eastern subtropical anticyclone cell (Fig. 3b) and re- track of Seth on 8 November is south of west. One curves into the westerlies, which terminates the semi- interpretation is that the expected beta-effect propaga- direct interaction. Similarly, the retardation of the ex- tion toward the northwest (see Elsberry 1995) is being pected poleward motion of the western TC will make the offset by this southward environmental steering estab- favorable juxtaposition in Fig. 3c a transient phenomena. lished between TS Verne and the subtropical anticyclone After the eastern TC is no longer present, the position cell to the west. Consequently, Seth drifts almost west- of the western TC, with respect to the reestablished sub- ward toward the subtropical anticyclone cell.

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a neutral point (or ``break'' in the ridge) to the north of Seth in the analysis would normally suggest that re- curvature is at least possible, if not probable. However, the position of the neutral point is an artifact of the presence of the western TC circulation and shifts sub- stantially to the east when the circulation of the western TC is removed. Despite appearances to the contrary, recurvature of the eastern TC is unhindered and quite probable, and recurvature of the western TC is either improbable or may be signi®cantly delayed. Although the translation speed of TS Verne has decreased to about 9 kt on 8 November (Fig. 4b), the track is still north- westward toward ST Seth. By 0000 UTC 10 November (Fig. 5c), ST Seth, TS Verne, and the western and eastern subtropical ridge cells are oriented east±west. The separation distance be- tween Seth and Verne is about 15Њ longitude. As indi- cated in the conceptual model (Fig. 3c), Seth is em- bedded in an environmental steering ¯ow toward the south that is established between Verne to the east and the western subtropical anticyclone cell. Seth continues to drift west-southwest at 7±8 kt during this period (Fig. 4a). According to the eastern TC conceptual model (Fig. 3b), TS Verne should be experiencing a poleward steer- ing ¯ow between ST Seth to the west and the eastern subtropical anticyclone cell. When a northwestward beta-effect propagation is added, the actual track be- comes oriented to the north-northwest. On 0000 UTC 12 November (Fig. 5d), the separation distance has greatly increased between ST Seth, which has drifted southward (Fig. 4a), and TS Verne, which has recurved on a poleward and eastward track (Fig. 4b). A characteristic of the eastern TC track in a semi- direct interaction is that deceleration does not occur as in a normal recurvature of a single TC around the sub- tropical anticyclone. Indeed, the translation speed of TS Verne increased from about 6 kt early on 9 November to 12 kt during recurvature around 0600 UTC 11 No- vember. As the criteria for a semidirect interaction for Verne are terminated, further acceleration occurs fol- lowing recurvature as TS Verne comes under the in¯u- ence of the strong midlatitude westerlies. The eastern subtropical anticyclonic circulation quick- ly extends westward (Fig. 5d) as Verne departs, perhaps FIG. 4. Joint Typhoon Warning Center best track for (a) Superty- with a contribution from peripheral ridging to the south- phoon Seth during 0000 UTC 4 November±12 November 1991 and (b) Tropical Storm Verne during 6±12 November 1991. Positions are east of Verne associated with Rossby wave dispersion. given each 12 h, with the 0000 UTC position indicated by the date Notice that the 30-kt isotach has shifted from the west along the track. side of ST Seth on 10 November (Fig. 5c) to the east side on 12 November, which indicates a reversal of the environmental steering from southward to northward. In the NOGAPS analyses of the Seth±Verne case (Fig. That is, without the presence of TS Verne to the east, 5b), there is an apparent extension of the eastern anti- the semidirect interaction period for ST Seth ends. Con- cyclonic circulation poleward of the eastern TC (Verne), sistent with this steering ¯ow change, ST Seth ended the which normally would be considered a hindrance to extended period of southward drift around 12 UTC 11 recurvature. However, the anticyclonic extension is in November and began to move on a west-northwest track part associated with the presence of the circulation of (Fig. 4a). Seth was rapidly decreasing in intensity on 12 the eastern TC and disappears when the circulation of November, perhaps because it was interacting with the the eastern TC is removed. Conversely, the presence of mountainous topography of , , and it

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FIG. 5. NOGAPS 500-mb streamline isotach analysis at 0000 UTC on (a) 6, (b) 8, (c) 10, and (d) 12 November 1991. The isotachs (thick lines) begin at 30 kt, and the contour interval is 20 kt, and the western (eastern) TC symbol is for Supertyphoon Seth (Tropical Storm Verne). was being sheared by stronger upper-level as it jiwhara-type mutual advection. In this case, the sepa- approached the east coast of Asia. As Seth dissipated, it ration distance was never less than 14Њ latitude (840 n came under the in¯uence of the lower-tropospheric north- mi), which is signi®cantly greater than the 750-n mi east monsoon and began to move southwestward again separation distance Brand (1970) found to be an upper (Fig. 4a). limit for binary TC interaction. In addition, the angular Motion relative to the midpoint between Seth and rotation of the two TCs from 0000 UTC 9 to 0000 UTC Verne (Fig. 6) during the sequence exhibits most of the 11 November is about 7Њ per 12 h (Fig. 6), which would key indicators that Lander and Holland (1993) associ- not be expected until the TCs are within 660 n mi (see ated with a direct (Fujiwhara-like) binary interaction Brand 1970; his Fig. 2). Furthermore, the sizes and sep- (Fig. 1). After an ``approach'' phase with very little aration of the cloud patterns of Seth and Verne in sat- relative rotation, a curved ``orbit'' phase occurs during ellite imagery (not shown) do not indicate that the TCs which a relatively constant separation distance is main- are undergoing a direct binary TC interaction. Thus, the tained, then a fairly sharp ``release'' turn follows, and Seth and Verne case illustrates that analysis of the rel- ®nally ``escape'' occurs with very little relative rotation. ative motion diagram alone is insuf®cient to determine As indicated in section 3a, the mechanism involved in the mechanism causing the relative motion. the semidirect interaction cannot be explained by a Fu- Carr and Elsberry (1994) present other case studies

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FIG. 6. Positions each 12 h of Supertyphoon Seth (left) and Tropical Storm Verne (right) relative to the midpoint between them from 0000 UTC 5 November to 0000 UTC 12 November 1991. involving semidirect interaction. During the Tropical Cyclone Motion Experiment in 1990, Typhoon (TY) Zola (Fig. 7b) moved rapidly to the north-northwest between TY Yancy (Fig. 7a) to the west and a subtrop- ical ridge cell to the east. Notice the translation speed of Zola increased from about 8 kt late on 19 November to around 19 kt early on 21 November when the semi- direct interaction was occurring. Around this time, the track of TY Yancy had turned westward. Although this turn is consistent with the expected behavior of the west- ern TC in a semidirect interaction, this case is compli- cated because the circulation of TY Yancy was also interacting with the topography of Taiwan. The angular rotation relative to the midpoint between Yancy and Zola (not shown) closely resembles that for Seth±Verne (Fig. 6). However, the minimum separation distance ex- ceeded 15Њ latitude, and the satellite imagery (not shown) also indicates well-separated cloud patterns. Thus, the direct interaction mechanism could not be the explanation for the apparent cyclonic rotation of Yancy and Zola. FIG. 7. Tracks as in Fig. 4, except for (a) Typhoon Yancy during 16± 4. Indirect interaction 22 August 1990 and (b) Typhoon Zola during 17±23 August 1990. a. Conceptual model will be presented to illustrate the variations that are A number of variations of indirect interactions (Fig. possible. The effect of the indirect interaction on the 8) are possible because of the variety of relative posi- western TC may at times be rather subtle; that is, a tions and intensities of the two TCs. However, the com- properly separated eastern TC may offset the interven- mon feature is an intervening anticyclone to the south- ing anticyclone effect and inhibit the expected poleward east of the western TC and how that anticyclone affects, motion of the western TC. Although the equatorward or is affected by, the motion of one or both of the TCs. de¯ection of the eastern TC may not be very large, a After a somewhat generic conceptual model is presented south-of-west track is not expected from climatology. to illustrate the general characteristics, some case studies In all cases, the recognition of the existence and char-

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though the horizontal wind shear makes the vortex bar- otropically stable in the inner region, the outer region of a large TC is not stable. A wave train of a peripheral anticyclone and then a cyclonic circulation farther to the southeast tend to develop in the wake of a large TC as it propagates to the northwest under the in¯uence of beta-effect propagation. As Carr and Elsberry (1995, their Fig. 11) demonstrate, the distortion of an initially symmetric vortex on a beta plane creates anticyclonic (cyclonic) vorticity advection tendencies to the east and equatorward (west and poleward) of the propagating FIG. 8. Schematic as in Fig. 3 except for an indirect binary tropical vortex. In addition to creating a wave train behind the cyclone interaction with the Rossby wave dispersion peripheral an- vortex owing to the anticyclonic vorticity advection, the ticyclone of the western TC (solid TC symbol) creating a poleward steering (broad arrow) across the western TC and an equatorward cyclonic vorticity advection facilitates a northwestward steering across the eastern TC (open TC symbol). The storm motion propagation into the subtropical ridge. vectors are indicated by a narrow arrow. A typical scenario for an indirect interaction (Fig. 8) is that the western TC is large, the peripheral anticyclone acter of the indirect interaction may alert the forecaster to the southeast is large or is becoming larger, and the to the possibility of nonclimatological TC tracks. eastern TC is small. Then the western TC will be ex- The distinguishing feature of this conceptual model periencing a poleward de¯ection owing to the interac- of indirect interaction is the existence of a peripheral tion with the peripheral anticyclone (Carr and Elsberry (or trailing) anticyclone to the southeast of a relatively 1995). The combination of the peripheral anticyclone large western TC (Fig. 8). As indicated above, it is the and the subtropical anticyclone to the east will tend to amplitude and orientation of this peripheral anticyclone establish a equatorward steering ¯ow across the eastern that is the key to the motion tendencies of both the TC (Fig. 8). Notice that a relative rotation about a mid- western TC and the eastern TC. One mechanism for point of the two TCs will be anticyclonic in an indirect developing a peripheral anticyclone is the signi®cant interaction, whereas the rotation is cyclonic in a semi- Rossby wave train behind a large TC (Shapiro and Ooy- direct interaction. ama 1990, their Fig. 2) or a monsoon gyre as shown in Another variation of this indirect interaction model Fig. 9, which is from Carr and Elsberry (1995). Al- (Fig. 8) is that the western TC is not so large, and its

FIG. 9. Rossby wave dispersion-induced peripheral anticyclone development illustrated by (a)±(c) model streamfunction for the TC-only control experiment of Carr and Elsberry (1995) at times indicated. Contour interval is 1 ϫ 106 m2 sϪ1, negative (positive) contours are dashed (solid), and the zero contour has been suppressed for clarity. (d)±(f) As in (a)±(c) except for the monsoon gyre- only control experiment.

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FIG. 10. Streamline and isotach analyses as in Fig. 5 except at 1200 UTC on (a) 16, (b) 17, (c) 18, and (d) 19 September 1991. The TC symbols denote Typhoon Nat (west), Tropical Storm Luke (middle), and Supertyphoon Mireille (east).

peripheral anticyclone is not so strong or is not allowed dencies associated with each TC. Because the Rossby to develop owing to the superposition of the eastern TC wave dispersion increases with a stronger outer wind at too small of a separation distance from the western circulation in the TC (e.g., Fiorino and Elsberry 1989), TC. In this scenario, the expected poleward de¯ection of a larger TC will tend to have a stronger peripheral an- the western TC in conjunction with the trailing anticy- ticyclone to the southeast and a stronger cyclonic vor- clone is small, or is not as large as expected, because the ticity advection poleward and westward. Thus, the sep- cyclonic vorticity advection to the northwest of the east- aration distance, sizes of each TC, and their relative ori- ern TC prevents the peripheral anticyclone of the western entation are critical factors in an indirect interaction (Fig. TC from developing. Similarly, the south-of-west steer- 8). ing ¯ow across the eastern TC, as in the ®rst scenario, does not develop. Then the eastern TC may approach the b. Case study 2 western TC so that the separation distance and east±west orientation criteria of the semidirect interaction in Fig. 3 Tropical Storm (TS) Luke was embedded in a large may become satis®ed. cyclonic circulation. For example, the cyclonic circu- In both of these scenarios, the forecaster must be aware lation at 500 mb around TS Luke at 1200 UTC 16 of the anticyclonic and cyclonic vorticity advection ten- September 1991 (Fig. 10a) was nearly as large as the

Unauthenticated | Downloaded 09/27/21 04:45 AM UTC SEPTEMBER 1997 CARR ET AL. 2103 subtropical anticyclone cell to the northeast. Such a large combined cyclonic circulation with the tropical storm is expected to generate a strong peripheral anti- cyclone (Carr and Elsberry 1995). By 17 September (Fig. 10b), the peripheral anticyclone trailing the com- bined cyclone±TS Luke circulation is connected with the subtropical anticyclone to the northeast and an an- ticyclone over the southern Philippines associated with the southwest monsoon ¯ow. Thus, a poleward steering component is established across TS Luke much as in the numerical model (Fig. 9) and the conceptual model (Fig. 8). This poleward steering is indicated by the 30-kt (15 m sϪ1) isotach to the southeast of Luke. This pattern continues on 18 September (Fig. 10c) with an extended anticyclone oriented southwest to northeast and a large 30-kt isotach to the east of TS Luke. Between 17 and 18 September (Fig. 10d), TS Luke is also interacting with a midlatitude trough. The corresponding track changes for TS Luke are shown in Fig. 11a. At the time corresponding to Fig. 10a, TS Luke was moving north- westward at about 14 kt (7 m sϪ1). As the peripheral anticyclone develops on 17±18 September (Figs. 10b, c), TS Luke has a more poleward track (Fig. 11a). Fi- nally, TS Luke recurves with a sharp eastward turn in conjunction with the interaction with the midlatitude trough. The indirect interaction in this case is primarily on the small TY (later a supertyphoon) Mireille that was east-southeast of the peripheral anticyclone trailing the TS Luke±cyclonic circulation on 17 September (Fig. 11b). Notice the convection associated with the cyclonic circulation and the cloud-free zone to the east and south- east as expected from subsidence in the peripheral an- ticyclone (Fig. 12a). Notice also the small size of Mir- eille at this time. Thus, Mireille should move as a cork- in-a-stream with the steering ¯ow, which is modi®ed by the peripheral ridge (Fig. 10b) to be westward or even south of west. This is consistent with the nonclimato- logical track (Fig. 11b) of Mireille during 17±18 Sep- tember. As Mireille continued to be small (Fig. 12b), no signi®cant cyclonic vorticity advection would be ex- pected to the northwest that might modify the large peripheral ridge that was generated in conjunction with the TS Luke±cyclonic circulation (Figs. 10c,d). Notice that the combination of the trough connecting TS Luke and TY Nat in the with the extended FIG. 11. Tracks as in Fig. 4 except for (a) Tropical Storm Luke anticyclone oriented southwest-northeast (Fig. 10c) during 14±19 September 1991 and (b) Supertyphoon Mireille during forms a reverse-oriented monsoon trough (Carr and Els- 15±28 September 1991. berry 1994; Lander 1996).

toward the northwest on 2±3 August while intensifying c. Case study 3 from 25 to 50 kt (Fig. 14a). By 3 August (Fig. 13b), The synoptic sequence (Fig. 13) during the indirect some building of an anticyclone is evident behind Cait- interaction of TS Caitlin and TY (later a supertyphoon) lin, and this anticyclone is beginning to connect to a Doug is similar to that of Luke and Mireille in Fig. 10. quasi-stationary anticyclone over the central Philip- Caitlin is the western TC and Doug is about 15Њ lon- pines. The appearance of a 30-kt isotach between Caitlin gitude east and 5Њ latitude south of Caitlin on 0000 UTC and the anticyclone just south of is indicative of 2 August 1994 (Fig. 13a). Caitlin is moving rapidly a strong environmental steering across Caitlin, which

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FIG. 12. Geostationary Meteorological Satellite infrared imagery for Typhoon Nat (west), Tropical Storm Luke (middle), and Super- (east) at 0300 UTC on (a) 17 and (b) 19 September 1991. moves rapidly across Taiwan and the Taiwan Strait with is being dominated by the strong steering in conjunction little change in intensity and makes around 0000 with the anticyclone south of Japan. By contrast, the UTC 4 August (Fig. 14a). At this time (Fig. 13c), the peripheral anticyclone is a weaker circulation, and Cai- anticyclone south of Japan, the peripheral anticyclone tlin is moving so rapidly that its effect on the motion trailing Caitlin, and the anticyclone over the Philippines of Caitlin is small. create a northeast±southwest axis between Caitlin and Between 0000 UTC 2 August and 3 August, Doug Doug. Caitlin dissipates over mainland China by 0000 moved slightly north of west and intensi®ed from 20 to UTC 5 August (Figs. 13d and 14a). 40 kt (Fig. 14b). Doug was in¯uenced by the peripheral Although the required peripheral anticyclone devel- anticyclone that developed in the wake of Caitlin on 3± oped as in Fig. 8a, no conclusive evidence of a north- 4 August (Figs. 13b,c). During this period, Doug turned ward displacement of Caitlin that is expected for the south of west (Fig. 14b), as expected for the eastern TC western TC in an indirect interaction is found, perhaps in the conceptual model (Fig. 8). By 5 August (Fig. because Caitlin crossed Taiwan during the period. Yeh 13d), Doug was still east of the peripheral ridge trailing and Elsberry (1993) have shown that TCs approaching Caitlin. However, Caitlin had dissipated, and Doug had the southern portion of the Central Mountain Range are rapidly intensi®ed to 115 kt and likely had strong cy- de¯ected southward by a blocking action, which would clonic vorticity advection to the north and west (Carr tend to offset the anticipated poleward de¯ection as- and Elsberry 1995). Thus, it is unlikely that the pe- sociated with an indirect interaction scenario. However, ripheral anticyclone in Fig. 8a remained a dominant a more likely explanation is that the motion of Caitlin effect on Doug. During 5 August, Doug resumed a

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FIG. 13. Streamline and isotach analyses as in Fig. 5 except on (a) 2, (b) 3, (c) 4, and (d) 5 August 1994. The TC symbols denote Tropical Storm Caitlin (west) and Supertyphoon Doug (east). north-of-west track (Fig. 14b) and then later recurved clones of both TCs as an eastern TC moves into an east± north of Taiwan. west orientation with a separation of 10Њ±20Њ longitude. Although the period of indirect interaction persisted This combination will be favored if both TCs are large only about 2 days and had little effect on the western or have strong outer winds so that the peripheral anti- TC (Caitlin), the effect on the track of the eastern TC cyclones are also large, as described in section 4a. If (Doug) was considerable. The period with a noncli- the two TCs are closer to 10Њ longitude apart, their matological south-of-west track apparently delayed the combined cyclonic circulation area may have the same recurvature and displaced westward the overall track effect as a very large TC, or a TC embedded in a mon- relative to what might have been anticipated from the soon gyre, and have considerable anticyclonic vorticity northwestward track prior to, say, 2 August (Fig. 14b). advection to the east and equatorward (Carr and Els- A proper evaluation of the onset of the indirect inter- berry 1995), so that peripheral anticyclogenesis is vig- action with Caitlin could have then potentially improved orous and widespread. Further displacement of the east- the track forecast and would have considerable skill ern TC into a position north of east, as in Fig. 15 with measured relative to the climatology and persistence a merger of the peripheral anticyclones into a similarly forecast. oriented anticyclone (ridge) axis, creates a reverse-ori- ented trough situation (Carr and Elsberry 1994; Lander 5. Binary cyclone contribution to reverse-oriented 1996) in which the two TCs are embedded. Conse- trough formation quently, a change to a poleward-oriented track should be expected for both TCs. These track changes can occur a. Conceptual model quite dramatically and are not predicted well (Lander The distinguishing feature of this conceptual model 1996). (Fig. 15) is the combination of the peripheral anticy- This reverse trough formation often follows a semi-

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FIG. 15. Schematic as in Fig. 3 except for two TCs that are separated by 10Њ±20Њ longitude and lead to a reverse-oriented monsoon trough formation and the merger of peripheral anticyclones to form an ex- tended anticyclone axis oriented southwest to northeast in the North- ern Hemisphere.

the effect of the subtropical anticyclone, the peripheral anticyclone becomes the dominant effect (Fig. 15). The eastern TC will still move poleward, and the establish- ment of the combined and extended anticyclone axis will then favor recurvature of the western TC as well.

b. Case study 4 Three separate cyclonic circulations are found in an west±east-oriented trough between 15Њ and 20ЊN at 0000 UTC 2 October 1993 (Fig. 16a). The middle and eastern circulations are associated with TS (later TY) Flo and TY (later ST) Ed, respectively. No anticyclonic ¯ow is found between Flo and Ed, although a peripheral an- ticyclonic circulation appears to develop to the southeast of Ed. Notice that Ed has a 20-kt isotach to the northeast, which is indicative of an environmental steering toward the northwest. Whereas Ed (Fig. 17b) did move north- westward at about 10 kt during 2 October, TS Flo (Fig. 17a) was drifting westward at about 3 kt within the trough. On 3 October (Fig. 16b) Ed is slightly north of east of Flo, and the peripheral anticyclone appears to have connected the subtropical and monsoonal anticyclones to the northeast and south, respectively. This connection is more obvious on 4 October (Fig. 16c) with an ex- tended anticyclonic circulation oriented west-southwest to east-northeast. Notice also the monsoon trough with FIG. 14. Tracks as in Fig. 4 except for (a) Tropical Storm Caitlin during 30 July±5 August 1994 and (b) Supertyphoon Doug during the three embedded cyclonic circulations has a similar 31 July±9 August 1994. (reverse) orientation as well. Supertyphoon Ed now has a 30-kt isotach to the northeast, which would seem to indicate a continued northwestward motion. Although Ed is moving in the expected direction on 4 October, direct interaction as in Fig. 3. These patterns are similar the translation speed has decreased to about 6 kt, which in that the two TCs in both cases do not have an an- appears to be inconsistent with a stronger steering ¯ow ticyclone between them and they are in an approximate implied by the 30-kt isotach. Meanwhile, TY Flo is east±west orientation near the subtropical anticyclone continuing to move westward at about 8 kt. axis. In the case of the semidirect interaction, the effect A critical period in the motion of both Ed and Flo of the subtropical anticyclone is dominant, especially in occurs during 4 October, since both storms turn to the leading to recurvature of the eastern TC as in Fig. 3. If northeast (Fig. 17). This direction is consistent with the the presence of the two TCs or external effects weakens steering ¯ow along the southern edge of the reverse-

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FIG. 16. Streamlines and isotach analyses as in Fig. 5 except on (a) 2, (b) 3, (c) 4, and (d) 5 October 1993. The TC symbols denote Typhoon Flo (west) and Supertyphoon Ed (east). oriented monsoon trough (Fig. 16d). A 20-kt isotach peripheral anticyclone trailing Luke forms a connection now extends toward the northeast along this steering between the subtropical and monsoon anticyclones to ¯ow, which is also consistent with the northeastward the northeast and south, respectively, which is analogous motion of both storms. Already by 6 October, ST Ed is to the evolution in Fig. 16. Thus, reverse-oriented mon- translating at 18 kt (Fig. 17b). soon trough and a similar southwest±northeast orien- The tracks of Ed and Flo are consistent with the con- tation of the extended anticyclone axis are evident in ceptual model in Fig. 15. Both storms recurve at rela- Fig. 10c. The large 30-kt isotach to southeast of Luke tively low latitudes well south of the subtropical anti- is not connected to a similar isotach associated with TS cyclone axis at any time in the map series (Fig. 16). Nat just north of Luzon, Philippines, as was the case Clearly, the establishment of the reverse-oriented mon- for the 20-kt isotach in Fig. 16c. Whereas TS Nat had soon trough and the combined peripheral anticyclone been drifting westward on 16±17 September corre- on the equatorward side was a primary factor in the sponding to the analyses in Figs. 10a,b, on 18 September synoptic sequence and the track changes. Nat began to move east-northeastward (Fig. 18a). This track change is consistent with the establishment of the extended anticyclone to the southeast of Nat in Fig. 10c. c. Case study 5 This motion continued on 19 September, corresponding Another case of two TCs contributing to reverse-ori- to the analysis in Fig. 10d, and might have been ex- ented trough formation was included in case study 3 in pected to lead to an northeastward track out of the Trop- section 4b. The peripheral anticyclone associated with ics if the reverse-oriented trough and extended periph- TS Luke (Fig. 10) was discussed in conjunction with eral anticyclone persisted. In contrast to the northwest- its effect on TS Mireille to the southeast. However, the ward de¯ections of the storm motion vectors relative to

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FIG. 18. (a) Track of TY Nat during 15 September±2 October 1991. (b) Continuation of streamlines and isotach analyses from Fig. 10 FIG. 17. Tracks as in Fig. 4 except for (a) Typhoon Flo during 28 except on 21 September 1991. Location of TY Nat (Mireille) is in- September±8 October 1993 and (b) Supertyphoon Ed during 1±7 dicated by the western (eastern) TC symbol. October. the steering ¯ows for the TCs in the conceptual models anticyclone of the western TC (Nat) as implied by the in Figs. 8 and 15, the small circulation of TY Nat would conceptual model in Fig. 8. Nevertheless, the synoptic not have had signi®cant ␤-effect propagation, and it pattern on 19 September (Fig. 10d) resembles the con- would have been carried along in the southwest to north- ceptual model. On 20 September (not shown), the west- east steering. ward motion and the growth in size of TY Mireille began An indirect interaction (Fig. 8) involving Nat and to weaken the anticyclone between the two TCs in Fig. Mireille is proposed as the explanation for the termi- 10d. By 21 September (Fig. 18b), the extended anti- nation of the east-northeast track of Nat on 21 Septem- cyclone has been split into a subtropical anticyclone to ber (Fig. 18a). As described above, the intervening an- the northeast and another anticyclone over the Philip- ticyclone between Nat and Mireille evolved in response pines. Thus, Nat is no longer being steered by the east- to the Luke circulation, rather than as the peripheral northeast steering in association with the earlier reverse

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ber. This is another example of how the relative rotation diagram may be misleading as to the physical mecha- nisms leading to track alterations, as it is unreasonable that the small Nat would cause such an orbiting motion of the large Mireille.

6. Characteristics of track-altering cases with two TCs As part of a 7-yr evaluation (1989±95) of the synoptic environment and associated track changes of western North Paci®c TCs, the frequency of occurrence, dura- tion, and the separation distances involved in each of the three TC interactions and RTF have been summa- rized (Table 1). A few cases with TC intensities less than 25 kt have been included in Table 1, where this FIG. 19. Relative rotation diagram of 6-h positions relative to the midpoints between TY Nat (left) and TY Mireille (right) based on a was necessary to estimate the beginning/ending time or direct binary interaction interpretation (JTWC 1991). the separation distance at onset time. Although the em- phasis here is on the track alterations of the two TCs by the mechanisms described above, other environmen- trough formation. Rather, the subtropical ridge is rees- tal effects might have been present: a monsoon gyre, or tablished to the north of Nat (Fig. 18b), which then a third TC, etc. Since the synoptic environment can only reverses direction and drifts west-northwestward (Fig. be de®ned from the 0000 and 1200 UTC NOGAPS anal- 18a). yses, an event must begin or end on these times. Because This indirect interaction interpretation of the motion the time resolution is only 12 h, not too much weight of Nat is in contrast to the Lander and Holland (1993) should be given to the durations in Table 1, especially type of interpretation that a direct interaction of Nat and in view of the generally small number of events in each Mireille has occurred based on the relative rotation di- category. Although the separation distances were cal- agram (Fig. 19) from JTWC (1991). First, this relative culated from positions recorded to 0.1Њ latitude/longi- motion diagram is with respect to the midpoint between tude, these positions have some uncertainty and the 12-h the two TCs. If the large size of Mireille and small size time resolution contributes to a lack of precision in the of Nat had been taken into account, the relative rotation values in Table 1. of Nat with respect to Mireille would have been much As described in section 2, and the conceptual model larger. It is clear from Fig. 18b that the two cyclonic in Fig. 2, the direct interaction has three modes, except circulations are too far separated to be interacting via a that the merger cases are a subset of the mutual inter- mutual interaction. Even though this 10Њ longitude plus action cases. Since seven one-way in¯uence (Fig. 2a) 8Њ latitude separation is evident in Fig. 19, an apparent cases were detected in the 7 years (Table 1), on average cyclonic rotation is inferred beginning late 21 Septem- only one such event occurs each season. The mutual

TABLE 1. Summary of the three modes (see indicated ®gures for conceptual models) track-altering binary tropical cyclone interactions observed in the western North Paci®c during 1989±95. Means and standard deviations are denoted by an overbar and SD, respectively. Because the merger type of direct interaction mode is a subset of the mutual type, values are enclosed in brackets. The time estimates are rather subjective because other effects than binary interaction may be occurring. Thus, the length of time is considered to be the entire period in which the effect is possibly occurring, rather than only the time in which the effect is judged to be having a clear effect on the track. Time of the minimum separation distance (Min, Time) in the rightmost columns is given as a percentage of the event duration (starting at 0% and terminating at 100%) to account for the different duration times of each event.

Separation distances (Њlat) Duration (hr) Onset Minimum Min, Time (%) Mode Type No. TÅ SD SÅ SD SÅ SD TÅ SD Direct One-way 7 40 13 12 1 8 3 80 23 (Fig. 2) mutual 3 52 9 4 2 1 1 77 41 [merger] [2] [57] [4] [5] [2] [0] [0] [100] [0] Semidirect Eastern 18 42 23 15 3 13 3 69 35 (Fig. 3) Western 14 57 29 15 3 13 3 58 27 Indirect Eastern 22 58 40 20 4 19 4 52 39 (Fig. 8) Western 36 55 36 19 4 17 4 65 36 All interactions 99 Ð Ð Ð Ð Ð Ð Ð Ð RTF (Fig. 15) 7 45 15 14 6 13 5 60 42

Unauthenticated | Downloaded 09/27/21 04:45 AM UTC 2110 MONTHLY WEATHER REVIEW VOLUME 125 interaction (Fig. 2b) cases are even more rare, with tends to advect the western (eastern) TC poleward roughly one event each 2 years. In this sample, two of (equatorward), the interaction event tends to be self- the three mutual attraction cases proceeded to merger limiting. (Fig. 2c). Recalling the lack of precise times, the average While the total number of interactions is not that duration of the one-way in¯uence was 40 h. The average large, considering this is for a 7-yr period, the track separation distance at the onset time was 12Њ latitude, modi®cations during these periods can cause signi®cant and the minimum separation was 8Њ latitude, which oc- forecast errors. Clearly, the indirect interaction that may curred on average at 80% of the duration time (Table be initiated at such large separation distances (ϳ 20Њ 1). For this small sample of mutual interactions, the latitude) is a more likely event. The semidirect inter- average onset separation distance was only 4Њ latitude. action may also be initiated at a fairly large separation In these cases, the minimum separation distance was 1Њ (ϳ 15Њ latitude) and thus occurs 2±3 times per year. latitude, and this occurred about three-quarters (76%) This interaction can cause signi®cant nonclimatological of the time in the event. However, this is misleading TC motions. Finally, the direct interaction requires the because two of the three cases merged, in which the smallest separation distances, especially the mutual in- minimum separation distance was zero and the time was teraction, and thus is rather rare. A general trend is that 100% (at the end of the event). In summary, a rather the magnitudes of these track alterations are inversely striking distinction between the one-way in¯uence and proportional to separation distances and also then to mutual in¯uence cases in these direct interactions is the frequency of occurrence. larger separation distances. Mutual interaction persists The special event of two TCs becoming aligned for longer and merger follows rather rapidly, although this a reverse-oriented trough formation (RTF) was only is a very small sample and the time resolution is only found seven times during 1989±95 (Table 1). In general, 12 h here. the tracks of both TCs are being affected, and the po- As emphasized in section 3a and the conceptual mod- leward turns may be quite dramatic. The mean duration el in Fig. 3, the semidirect interaction occurs at larger of the event is among the smallest. Average onset sep- separation distances so that the advection of one TC aration distance was 14Њ latitude and the average min- solely by the circulation of the other TC is not a possible imum distance was only slightly smaller. Thus, it is not physical explanation for this interaction. Whether the clear that the two TC circulations are actually interact- interaction involved the eastern TC (18 cases) or the ing, although the smaller separation distances seem to western TC (14 cases), the average separation distance be more favorable for a combination of the peripheral at onset was 15Њ latitude with a standard deviation of anticyclones that is an essential feature in the track mod- 3Њ (Table 1). The average minimum separation distance i®cations (Fig. 15). was 13Њ latitude for both the eastern TC and western TC with standard deviations of only 3Њ latitude. As de- Acknowledgments. This research has been supported scribed in section 3a the semidirect interaction tends to by the Of®ce of Naval Research Marine Meteorology be transient. This is especially true for the eastern TC Program. The assistance of Chris Kent, Sean White, and (only 42-h duration) as it moves rapidly to the northwest other colleagues is gratefully acknowledged. Mrs. Pen- between the western TC and the subtropical anticyclone ny Jones skillfully prepared the manuscript in many cell to the east. By contrast, the average time the western forms. TC is under the in¯uence of the semidirect interaction is about 57 h. The longer time the western TC is under the in¯uence of an equatorward steering is because the REFERENCES eastern TC is generally moving slowly as it approaches Brand, S., 1970: Interaction of binary tropical cyclones of the western the western TC. Thus, the conditions in Fig. 3c tend to North Paci®c. J. Appl. Meteor., 9, 433±441. persist longer than the conditions in Fig. 3b. 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