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  • View in gallery

    Volume-integrated eddy kinetic energy (1018 J) over the North Pacific between 15 Jul and 30 Sep 2005. The tropical cyclone symbols mark the dates that tropical cyclones moved poleward of 40°N. The arrows mark the last date associated with a tropical cyclone that stayed south of 40°N. The areas are bounded in latitude by 35°–65°N. The western North Pacific extends between 130° and 160°E. The central North Pacific extends between 160°E and 160°W. The eastern North Pacific extends between 160° and 125°W.

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    Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 5–10 Sep 2005. The tropical cyclone symbols mark the location of TY Nabi at each time.

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    Contributions to the distribution of Ke at 0000 UTC 7 Sep 2005: (a) vertically averaged Ke (shaded, 105 J m−2), energy flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights (contours, 60-m intervals); (b) vertically averaged Ke [shaded as in (a)], 500-hPa heights (light gray contours), and vertically averaged Ke generation resulting from baroclinic conversion (contours; W m−1); (c) as in (b), but for the vertically averaged Ke tendency resulting from ageostrophic geopotential flux convergence; (d) vertically averaged Ke generation (shaded, W m−2), ageostrophic geopotential flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]; and (e) vertically averaged Ke flux convergence (shaded, W m−2), Ke flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]. The TC symbols mark the location of TY Nabi.

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    As in Fig. 3, but for 1000 UTC 8 Sep 2005.

  • View in gallery

    As in Fig. 3, but for 0000 UTC 9 Sep 2005.

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    Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 26–31 Jul 2005. The tropical cyclone symbols mark the location of TS Banyan at each time.

  • View in gallery

    As in Fig. 3, but for 0000 UTC 29 Jul 2005. The TC symbols mark the location of the ex-TS Banyan.

  • View in gallery

    As in Fig. 3, but for 0000 UTC 30 Jul 2005. The TC symbols mark the location of the ex-TS Banyan.

  • View in gallery

    Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 24–29 Aug 2005. The TC symbol marks the location of TY Guchol.

  • View in gallery

    Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel. The TC symbol marks the location of TY Guchol.

  • View in gallery

    Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel. The TC symbol marks the location of TY Saola.

  • View in gallery

    (a) Vertically averaged Ke (shaded, 105 J m−2), vertically averaged Ke generation resulting from baroclinic conversion (dark contours with negative contours dashed, W m−1), and 500-hPa heights (contours, 60-m intervals); (b) vertically averaged Ke generation (shaded, W m−2), ageostrophic geopotential flux vectors (reference vector in the lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]; and (c) vertically averaged Ke flux convergence (shaded, W m−2), Ke flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights. The TC symbol marks the location of TY Saola.

  • View in gallery

    Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel.

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Downstream Development Associated with the Extratropical Transition of Tropical Cyclones over the Western North Pacific

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  • 1 Department of Meteorology, Naval Postgraduate School, Monterey, California
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Abstract

The movement of a tropical cyclone into the midlatitudes involves interactions among many complex physical processes over a variety of space and time scales. Furthermore, the extratropical transition (ET) of a tropical cyclone may also result in a high-amplitude Rossby wave response that can extend to near-hemispheric scales. After an ET event occurs over the western portion of a Northern Hemisphere ocean basin, the high-amplitude downstream response often forces anomalous midlatitude circulations for periods of days to a week. These circulations may then be related to high-impact weather events far downstream of the forcing by the ET event. In this study, downstream development following ET events over the western North Pacific Ocean is examined. Local eddy kinetic energy analyses are conducted on four cases of North Pacific tropical cyclones of varying characteristics during ET into varying midlatitude flow characteristics during 15 July–30 September 2005. The goal is to examine the impact of each case on downstream development across the North Pacific during a period in which these events might increase the midlatitude cyclogenesis across the North Pacific during a season in which cyclogenesis is typically weak. Four typhoon (TY) cases from the summer of 2005 are chosen to represent the wide spectrum of variability in ET. This includes a case (TY Nabi 14W) that directly resulted in an intense midlatitude cyclone, a case in which a weak midlatitude cyclone resulted (TY Banyan 07W), a case in which the decaying tropical cyclone was absorbed into the midlatitude flow (TY Guchol 12W), and a case (TY Saola 17W) in which the tropical cyclone decayed under the influence of strong vertical wind shear. The variability in downstream response to each ET case is related to specific physical characteristics associated with the evolution of the ET process and the phasing between the poleward-moving tropical cyclone and the midlatitude circulation into which it is moving. A case of downstream development that occurred during September 2005 without an ET event is compared with the four ET cases.

Corresponding author address: Patrick A. Harr, Department of Meteorology, Naval Postgraduate School, Monterey, CA 93943-5114. Email: paharr@nps.edu

Abstract

The movement of a tropical cyclone into the midlatitudes involves interactions among many complex physical processes over a variety of space and time scales. Furthermore, the extratropical transition (ET) of a tropical cyclone may also result in a high-amplitude Rossby wave response that can extend to near-hemispheric scales. After an ET event occurs over the western portion of a Northern Hemisphere ocean basin, the high-amplitude downstream response often forces anomalous midlatitude circulations for periods of days to a week. These circulations may then be related to high-impact weather events far downstream of the forcing by the ET event. In this study, downstream development following ET events over the western North Pacific Ocean is examined. Local eddy kinetic energy analyses are conducted on four cases of North Pacific tropical cyclones of varying characteristics during ET into varying midlatitude flow characteristics during 15 July–30 September 2005. The goal is to examine the impact of each case on downstream development across the North Pacific during a period in which these events might increase the midlatitude cyclogenesis across the North Pacific during a season in which cyclogenesis is typically weak. Four typhoon (TY) cases from the summer of 2005 are chosen to represent the wide spectrum of variability in ET. This includes a case (TY Nabi 14W) that directly resulted in an intense midlatitude cyclone, a case in which a weak midlatitude cyclone resulted (TY Banyan 07W), a case in which the decaying tropical cyclone was absorbed into the midlatitude flow (TY Guchol 12W), and a case (TY Saola 17W) in which the tropical cyclone decayed under the influence of strong vertical wind shear. The variability in downstream response to each ET case is related to specific physical characteristics associated with the evolution of the ET process and the phasing between the poleward-moving tropical cyclone and the midlatitude circulation into which it is moving. A case of downstream development that occurred during September 2005 without an ET event is compared with the four ET cases.

Corresponding author address: Patrick A. Harr, Department of Meteorology, Naval Postgraduate School, Monterey, CA 93943-5114. Email: paharr@nps.edu

1. Introduction

Extratropical transition (ET) has been described as an evolutionary process by which a tropical cyclone (TC) loses tropical characteristics and becomes more extratropical in nature (Jones et al. 2003). Based on an examination of a set of ET cases that occurred over the western North Pacific, Klein et al. (2000) developed a conceptual model of ET that involved two stages: a transformation stage and a reintensification stage. The transformation stage is defined by an interaction between a recurving TC and a preexisting, lower-tropospheric baroclinic zone and the associated vertical wind shear. During the transformation stage, lower-tropospheric temperature advection, frontogenesis, and a vertical motion dipole become established, the TC warm core disperses, and interactions between the midlatitude baroclinic zone and the TC are characterized by slantwise ascent/descent on isentropic surfaces (Klein et al. 2000). On satellite imagery, the transformation stage has an asymmetric pattern in cloud and precipitation structure due to the inflow of relatively cool, dry environmental air in the western section and warm, most air in the eastern section of the outer TC circulation (Harr et al. 2000).

The deepening of the ex-TC as a baroclinic cyclone is defined as the reintensification stage of ET. This stage is marked by the evolution of features characteristic of extratropical cyclones such as fronts and asymmetric wind, cloud, and precipitation patterns (Harr et al. 2000). In several prior studies of ET (DiMego and Bosart 1982; Foley and Hanstrum 1994; Harr and Elsberry 2000; Harr et al. 2000; Sinclair 2002), reintensification of the TC remnants has been described as being similar to Petterssen–Smebye type B extratropical cyclogenesis (Petterssen and Smebye 1971). In this condition, low-level cyclone development occurs when an area of upper-level positive vorticity advection becomes superposed upon a low-level frontal zone (Petterssen 1955; Sutcliffe and Forsdyke 1950). Klein et al. (2002) and Ritchie and Elsberry (2007) found that the phasing between the poleward-moving TC and a midlatitude trough was critical for Petterssen–Smebye type B extratropical cyclogenesis to occur such that the TC remnants deepen in the reintensification stage of ET.

There is no accepted definition of ET (Jones et al. 2003). Extratropical transition is considered an evolution from a tropical cyclone to the transformation stage and to the reintensification stage as an extratropical cyclone. However, not all ET events proceed to the reintensification stage (Klein et al. 2000). In this study, the ET process is considered complete when reintensification as an extratropical cyclone occurs or when the decaying tropical cyclone is no longer identifiable as an independent circulation.

Recent research has examined the impact of the ET process on the evolution of the midlatitude flow downstream of the location of the movement of the decaying TC into the midlatitudes. In particular, Harr et al. (2008) demonstrated that predictability was typically reduced across the North Pacific following an ET event over the western North Pacific. In their analysis, reduced predictability was defined by increased variance among members of the ensemble prediction system (EPS) from the Global Forecast System (GFS) model run by the National Centers for Environmental Prediction (NCEP). Anwender et al. (2008) expanded on the analysis of Harr et al. (2008) to show that predictability was also reduced based on variability among members of the European Centre for Medium-Range Weather Forecasts (ECMWF) EPS. The analysis by Anwender et al. (2008) also examined periods of reduced predictability following ET events over the North Atlantic. Therefore, there is a need to increase understanding of the impact of the ET process on the evolution of the downstream flow patterns. The studies of Harr et al. (2008) and Anwender et al. (2008) have shown that the downstream impact of ET events may reach near hemispheric scales.

Downstream from an ET event, the midlatitude circulation often becomes perturbed because of interaction between the decaying TC and the midlatitude circulation into which it is moving. These perturbations may impact downstream development of the extratropical circulation. Orlanski and Sheldon (1993) defined the process of dispersion and spreading of energy in a growing unstable system that is characteristic of high-frequency baroclinic waves as downstream baroclinic development. In this framework, downstream baroclinic development may be involved in the generation of observed disturbances that may not be anticipated based solely on an examination of local available baroclinity. The process of downstream development has often been examined using diagnostics of the local eddy kinetic energy (Orlanski and Katzfey 1991; Orlanski and Chang 1993; Orlanski and Sheldon 1993; Danielson et al. 2006a,b). Chang (1993) and Danielson et al. (2004) found that downstream development associated with collections of cold-season cyclones over the eastern North Pacific was more dependent on the flux of eddy kinetic energy (Ke) from upstream cyclones than from local baroclinic conversion.

Orlanski and Katzfey (1991) concluded that ageostrophic geopotential fluxes from an upstream circulation comprised the major source of an increase in Ke during the developmental stage of a downstream cyclone. Furthermore, the radiation of ageostrophic geopotential fluxes downstream was the major sink of Ke in the upstream cyclone during its decay stage. Orlanski and Sheldon (1995) defined a synoptic model of downstream development by considering the transfer of Ke between individual synoptic-scale systems due to an ageostrophic geopotential flux. In their three-stage model (Fig. 3 in Orlanski and Sheldon 1995), a Ke center downstream of a midlatitude trough weakens as it radiates energy via ageostrophic geopotential fluxes through a downstream ridge. The convergence of these fluxes contributes to a new downstream Ke center that also grows through baroclinic conversion of eddy available potential energy (Ae) as cold air sinks into the base of the trough. This Ke center radiates energy through the trough, which contributes to a new Ke center downstream of the trough. This Ke center then grows by baroclinic conversion from Ae to Ke as warm air rises. The process may then continue as the Ke center on the downstream side of the trough may radiate energy through the downstream ridge.

During 15 July–30 September 2005 (Fig. 1), there were several periods when volume-integrated Ke successively increased from the western North Pacific to the eastern North Pacific in association with ET over the western North Pacific. The four cases of ET identified in Fig. 1 are examined to define the sensitivity of the downstream development process to varying TC and midlatitude circulation characteristics. The period is chosen to not include October as there is a general increase in Ke due to the increase in the intensity of the westerly flow and equatorward extension of the midlatitude westerlies. Also, the impact of ET on downstream development may be important for contributing to strong midlatitude cyclogenesis events during a season in which they do not normally occur. The start date of 15 July is chosen because there is a 1 month gap in TC activity between mid-June and mid-July. Finally, 18–23 September is examined as it is the only period in Fig. 1 in which significant downstream development occurred without an ET event.

Results indicate that there are direct and indirect impacts of the ET process on downstream development. Furthermore, the impacts are dependent upon the phasing between the decaying TC and the midlatitude circulation into which it is moving. Section 2 defines the methodology and the individual cases are examined in section 3. The results are discussed in section 4.

2. Data and method

In this study, the local Ke analysis is calculated using the NCEP Final (FNL) Global Data Assimilation System analyses. The FNL analysis incorporates observations with a cutoff period of 5 h past the synoptic time. The output fields from the FNL analyses have a horizontal resolution of 1° latitude–longitude, a temporal resolution of 6 h, and 26 vertical levels from 1000 to 10 hPa. Also, surface variables, sea level variables, and many other miscellaneous atmospheric and surface observations are contained in the FNL dataset. This study utilizes the 0000 and 1200 UTC analyses of geopotential height, relative humidity, vertical velocity, temperature, zonal and meridional wind components, and mean sea level pressure at vertical levels from 1000 to 100 hPa in 50-hPa increments.

The Ke budget is computed by partitioning a vector quantity as a sum of a mean and perturbation as V = Vm + v, and the three vector quantities refer to horizontal wind componrnts. Scalar quantities are defined similarly as A = Am + a. The mean quantities are defined as a 30-day time average. The time that each TC either completed ET or was no longer identifiable as a unique circulation was defined to be the center time on which 30-day means were calculated (Table 1). Based on the above definitions, Ke is defined as ½|v|2. For each case, the time period of study is chosen to be representative of conditions surrounding ET.

From Orlanski and Sheldon (1995), the time tendency of Ke is defined in pressure coordinates as
i1520-0493-137-4-1295-e1
The horizontal gradient operator is defined as p. The first term on the right-hand side represents the generation of Ke and is discussed further below. The second and third terms on the right-hand side of (1) represent the horizontal and vertical divergence of the Ke fluxes. The fourth term is the barotropic generation term, which represents the Ke tendency due to the conversion of kinetic energy between the eddy and mean flow. All other factors, which include dissipation, are contained in the residual. The barotropic generation term and terms contained in the residual are typically less important for cyclone growth and decay (Decker and Martin 2005) and are not addressed in this study. Orlanski and Sheldon (1995) related the generation of Ke(−v · pϕ) to the conversion from Ae to Ke by baroclinic processes as
i1520-0493-137-4-1295-e2
The term −ωα represents the baroclinic conversion from Ae to Ke and the convention is that a negative ωα represents a conversion from Ae to Ke. The last two terms on the right-hand side of (2) represent horizontal and vertical energy fluxes. The term (vϕ)a is the ageostrophic geopotential flux, which has been defined by Orlanski and Sheldon (1995) as
i1520-0493-137-4-1295-e3
where k is the unit vertical vector. The expression in (3) defines the ageostrophic geopotential flux to be the eddy geopotential flux with the nondivergent part removed (Orlanski and Sheldon 1993; Chang and Orlanski 1993). Because emphasis is on the horizontal propagation of energy, the terms defined above are integrated in the vertical from 1000 to 100 hPa. This leads to a simplification as the vertical flux terms become very small. The integrals are normalized by the gravitational constant to define units of joules per square meter for Ke and watts per square meter for the budget terms in (1)(3).

3. Analysis

The case of Typhoon (TY) Nabi, which is defined to have completed reintensification as an extratropical cyclone at 0000 UTC 8 September 2005, is presented first as a fundamental case of downstream development during an ET event. The ET of Tropical Storm (TS) Banyan in July 2005 represents a midsummer case in which an ET of a weak TC significantly modified the circulation over the North Pacific. Although amplitudes of Ke centers were small, a significant downstream development across the North Pacific was associated with the flux of energy from Banyan during the transformation and reintensification stages of ET. By contrast, no direct impact on the downstream circulation occurred as the very small TY Guchol began ET but became absorbed into a preexisting midlatitude circulation on 25 August 2005. However, the merger of TY Guchol with a developing midlatitude trough increased the generation of Ke in the trough and also increased the downstream dispersion of Ke. Finally, TY Saola moved poleward at the end of September into an environment of very strong westerly wind shear and dissipated without completing ET. Although the volume-integrated Ke over the eastern North Pacific increased (Fig. 1) following the dissipation of TY Saola, the increase is not attributed to the decaying TC.

For each case, the synoptic evolution associated with the TC and downstream circulation prior to, and during the ET period is defined. For key times associated with the TY Nabi and TS Banyan cases, the distribution of the total vertically integrated Ke is defined followed by the vertically integrated contributions to Ke tendency from the generation of Ke via (−v · pϕ) and the convergence of the advective fluxes (− · VKe). Finally, the contributions to Ke generation from the vertically integrated baroclinic conversion of Ae(−αω) and the convergence of the ageostrophic geopotential fluxes [− · (vϕ)a] are presented. Following the establishment of the basic framework of ET influence on Ke transport during TY Nabi and TS Banyan, the cases of TY Guchol and TY Saola are summarized with plan views of the vertically averaged Ke and total energy flux vectors. Additionally, plan views of several energy budget terms are included for TY Saola.

a. Typhoon Nabi (September 2005)

As TY Nabi began to recurve on 0000 UTC 5 September 2005 (Fig. 2a), the midlatitude flow had a zonal orientation that extended toward a mature low east of the Kamchatka Peninsula and a weak trough south of Alaska. Over the next 24 h, Nabi moved to the point of recurvature at 31.4°N, 130.0°E (Fig. 2b). The 500-hPa ridge poleward of Nabi intensified as the trough previously over the Sea of Japan moved east. The mature cyclone previously east of the Kamchatka Peninsula (Fig. 2a) moved north of the Aleutian Islands and the trough over the eastern North Pacific began to dig southward. Nabi moved northeastward to 37.5°N, 133.0°E by 0000 UTC 7 September 2005 (Fig. 2c). The 500-hPa ridge, which had been poleward of Nabi, continued to amplify (Fig. 2c) and moved northeast of Nabi. Downstream of this ridge, a trough amplified along 170°E. The trough over the eastern North Pacific became cut off near 43°N, 128°W. By 0000 UTC 8 September 2005, Nabi had moved northeast of Japan (Fig. 2d) and the ridge immediately downstream of Nabi continued to amplify. The central North Pacific trough also amplified. Over the eastern North Pacific, the cutoff low off the west coast of North America became more separated from the midlatitude flow and a shortwave trough moved into the extreme northern portion of the Gulf of Alaska. By 0000 UTC 9 September (Fig. 2e), the 500-hPa trough associated with the ex-Nabi extratropical cyclone became closed and was nearly vertically stacked with the surface circulation south of the Kamchatka Peninsula. The downstream ridge and trough couplet continued to move eastward. However, the ridge over the eastern North Pacific amplified and the shortwave trough that was over the very northern Gulf of Alaska at 0000 UTC 8 September (Fig. 2d) moved southeastward and amplified along the west coast of North America. By 0000 UTC 10 September, the ex-TY Nabi had become a primary low pressure center over the central North Pacific (Fig. 2f). The eastern North Pacific ridge–trough couplet amplified as the ridge built over Alaska, and the digging trough over the west coast of North America continued to move southward and became a closed upper-level low pressure system. Between 5 and 10 September, the movement of Nabi into the midlatitudes changed a mostly zonal circulation to a high-amplitude pattern of a series of ridges and troughs that extended to the west coast of North America.

At 0000 UTC 7 September, two Ke centers existed over the western North Pacific (Fig. 3a). Total energy flux [(vϕ)a + (VKe)] from the Ke center over the Sea of Japan that was associated with the decaying TY Nabi was directed into the midlatitude circulation and downstream into the second Ke center over the western North Pacific that was on the upstream side of the central North Pacific trough. The baroclinic contribution to Ke production over the region of the ET of Nabi (Fig. 3b) was due to the upward motion of warm air on the eastern side of the decaying TC (Harr and Elsberry 2000) such that there is conversion of Ae to Ke. However, Ke was dispersed via divergence of (vϕ)a (Fig. 3c) such that negative −v · pϕ exists in the region of the ET (Fig. 3d). The Ke was dispersed to the midlatitudes where convergence of (vϕ)a contributes to the accumulation of energy in the region poleward of the decaying Nabi centered near 50°N, 141°E (Fig. 3c). Immediately downstream was a region of Ke flux divergence (Fig. 3e) as the flux VKe contributed to the transport of energy through the ridge that has amplified immediately downstream of the decaying Nabi.

As Ke was transported through the ridge over the western North Pacific, there was an increase in the generation of Ke on the upstream side of the central North Pacific trough near 47°N, 163°E (Fig. 3d). Energy accumulation downstream of the generation area was then due to the convergence of ageostrophic geopotential fluxes (Fig. 3c) and baroclinic conversion of Ae to Ke as cold air sinks on the western portion of the trough (Fig. 3b). Furthermore, the VKe flux vectors (Fig. 3e) indicate that Ke was transported toward the base of the trough over the central North Pacific. The accumulation of energy near the base of the central North Pacific trough centered at 43°N, 166°E (Fig. 3e) coincided with the increase in the 500-hPa height gradient as the trough extended southeastward over the central North Pacific. However, a total Ke center had not yet appeared on the downstream side of the central North Pacific trough (Fig. 3a).

Over the next 24 h (Fig. 4a), the primary Ke centers remain associated with the ET of Nabi and the central North Pacific trough, which had expanded to contain a Ke center on the downstream side of the trough between 180° and 170°W. Over the Sea of Japan baroclinic processes continued to produce Ke during the ET of Nabi (Fig. 4b) and the Ke was dispersed to the midlatitudes via ageostrophic geopotential flux divergence (Figs. 4c,d). Energy generated north of the ex-Nabi was transported through the ridge over the western North Pacific and into the upstream side of the central North Pacific trough near 44°N, 172°E (Figs. 4d,e).

On the eastern side of the central North Pacific trough, baroclinic conversion (Fig. 4b) of Ae to Ke occurred via warm air rising and contributed to the Ke center. Energy that was transported through the base of the trough near 40°N, 180° (Fig. 4a) was transported (Fig. 4e) downstream to the cyclonic shear side of the flow over a region of 45°–55°N, 180°–170°W. From this region, there is a split in the energy transport with part being recirculated to the upstream side of the trough and part being transported downstream over the northern portion of the Gulf of Alaska. Therefore, the generation, dispersion, and transport of Ke increased during 0000 UTC 7–8 September as the ET of Nabi continued to supply energy to the midlatitudes, and the Ke centers on both sides of the central North Pacific trough amplified.

Over the next 24 h (Fig. 5a), the amplitudes of the Ke centers increased over the central and eastern North Pacific. The Ke in the ex-TC, which came mainly from baroclinic production located southeast of the Kamchatka Peninsula (Fig. 5b), continued to be dispersed via divergence of ageostrophic geopotential fluxes east of the ex-TC center (Fig. 5c), which are directed between 165° and 177°E (Fig. 5d) toward the upstream side of the central North Pacific trough. In the idealized model of downstream development in Orlanski and Sheldon (1995), the Ke center on the upstream side of the trough weakens as energy is dispersed to the downstream Ke center. However, in this case, the upstream Ke center continued to receive Ke from the ex-TC as defined by the convergence of the ageostrophic geopotential flux (Figs. 5c,d).

Baroclinic conversion contributed to the Ke center along the eastern side of the central North Pacific trough (Fig. 5b). This Ke center near 55°N, 165°E in Fig. 5a was dispersed by the ageostrophic geopotential fluxes (Fig. 5d) and transported (Fig. 5e) such that a portion contributed to the increase in the Ke center associated with the digging trough that moved southward along the western coast of North America (Figs. 2e,f).

The TY Nabi is a case in which a significant impact on downstream development was made by the ET of the TC as it reintensified into an extratropical cyclone. Prior to the ET of Nabi, the midlatitude circulation contained weakening baroclinic midlatitude features that coincided with a low-energy, highly zonal circulation pattern across the western North Pacific (i.e., 5–7 September in Fig. 1). Ageostrophic geopotential flux convergence deposited Ke poleward of Nabi as it began to move out of the tropics. Eddy kinetic energy was also transported by ageostrophic geopotential fluxes into a developing energy center immediately downstream over the central North Pacific. The ET of TY Nabi continued to provide a source of Ke to the central North Pacific trough as the trough amplified and Ke began to be dispersed downstream to the eastern North Pacific. The extension of increased Ke over the central North Pacific is evident in Fig. 1 as a broad maximum in volume-averaged Ke over the central North Pacific during the period of 0000 UTC 8 September–1200 UTC 9 September.

b. Tropical Storm Banyan (July 2005)

Tropical Storm Banyan moved into a weak midsummer midlatitude circulation over the western North Pacific (Fig. 6a). Banyan began to interact with the midlatitude circulation on 0000 UTC 26 July 2005 as it began to be encompassed by a weak midlatitude trough (Fig. 6a). The subtropical 500-hPa ridge was located directly east of Banyan and amplified northward as Banyan moved poleward across Japan (Figs. 6b,c). By 0000 UTC 29 July TS Banyan completed ET and became an extratropical cyclone with a closed circulation at 500 hPa (Fig. 6d). The upper-level ridge immediately downstream of the ex-Banyan circulation continued to amplify northward (Fig. 6d) as a shortwave trough began to dig southward on the eastern side of the ridge. As the ex-Banyan circulation weakened over the central North Pacific, the shortwave trough continued to amplify as it moved around the primary Gulf of Alaska low pressure system (Figs. 6e,f).

As Banyan moved poleward from 26 July, it contributed to an increase in the volume integrated Ke over the western North Pacific (Fig. 1). Because of the weak midsummer midlatitude circulation, significant transport of Ke downstream did not occur until ET was complete on 0000 UTC 29 July (Fig. 7a) and the ex-Banyan circulation moved farther into the midlatitude circulation to be located at the southern tip of the Kamchatka Peninsula. Baroclinic processes contributed to the Ke center east of the Kamchatka Peninsula (Fig. 7b) and this energy was dispersed over the ridge downstream of the ex-Banyan circulation (Figs. 7c,d). The dispersion of Ke from ex-Banyan and a separate midlatitude cyclone north of Banyan led to a transport of Ke down the eastern side of the ridge along 177°E (Fig. 7e). This process contributed to the development and downstream movement of the Ke center at 55°N, 175°W (Fig. 7a) that was associated with the shortwave trough near 50°N, 170°W.

Over the next 24 h (Fig. 8a), the Ke center associated with ex-Banyan persisted to the east of the Kamchatka Peninsula as the baroclinic production of Ke increased (Fig. 8b) and the dispersion due to ageostrophic geopotential fluxes also increased (Fig. 8c). However, the downstream dispersion of Ke weakened and the recirculation of Ke, as defined by the magnitude of the ageostrophic geopotential fluxes directed toward the ex-Banyan circulation, increased (Fig. 8d) as the ex-Banyan circulation became broader and more vertically aligned. The Ke center on the western side of the eastern North Pacific trough (Fig. 8a) moved downstream in association with the flux divergence of VKe near 52°N, 148°W (Fig. 8e).

In the case of TY Nabi, the energy transport downstream into the midlatitude circulation of the North Pacific began prior to the initiation of ET. During the midsummer case of a weaker circulation, TS Banyan, the downstream dispersion of Ke from the TC was delayed until the ET process was essentially complete. However, the ET of the TC resulted in a source of Ke that initiated a downstream development cycle that resulted in successive increases in volume-averaged Ke from the western to eastern North Pacific between 26 and 30 July (Fig. 1).

c. Typhoon Guchol (August 2005)

The TY Guchol was a small TC that began interaction with a midlatitude trough that was approaching from the northwest on 0000 UTC 24 August (Fig. 9a). As the trough moved southeastward over the western North Pacific, the vertical wind shear eroded the structure of TY Guchol such that the decaying TC became absorbed into the midlatitude circulation during 0000 UTC 25–26 August (Figs. 9b,c). Following the absorption of the TC into the trough, a low pressure system formed at sea level and a ridge at 500 hPa built rapidly to the east (Fig. 9d). As the sea level low pressure system lifted to the northeast, the ridge continued to build over Alaska (Fig. 9e) and a trough began to dig southeastward toward the northwestern coast of North America (Figs. 9e,f).

The decaying TY Guchol and the midlatitude trough over the western North Pacific were both contributing to the total energy flux across the North Pacific as they were beginning to merge at 0000 UTC 25 August (Fig. 10a). The merger of the two circulations resulted in a sharp increase in total volume-integrated Ke over the central North Pacific that also began at 0000 UTC 25 August (Fig. 1). At the completion of the merger (Fig. 10b), a relative maximum in Ke increased on the eastern side of the trough southeast of the Kamchatka Peninsula and the total energy flux increased downstream of the trough. Over the next 24 h (Fig. 10c), the relative maximum in Ke along the eastern portion of the trough was maintained and the energy flux downstream increased. The downstream development process resulted in an increase in the Ke on the downstream side of the ridge that built along 170°W and the subsequent amplification of the trough over the eastern North Pacific. The maximum in volume-integrated Ke over the eastern North Pacific was reached 12 h later at 1200 UTC 27 August (Fig. 1).

The merger of the decaying TC and trough increased the overall Ke on the downstream side of the trough and increased the flow of Ke across the North Pacific. Therefore, Guchol had an indirect influence on the downstream development, which may have had less amplitude had Guchol not been present to merge with the trough over the western North Pacific.

d. Typhoon Saola (September 2005)

Typhoon Saola is also examined as a case in which strong vertical wind shear resulted in the dissipation of the TC before it could enter the reintensification stage of ET. However, the remnants did not merge with a midlatitude circulation but continued to dissipate as the remaining circulation and moisture were advected rapidly downstream (Fig. 11).

Strong confluent flow existed over the western North Pacific during the dissipation of TY Saola (Fig. 11a). Contributions to the energy flux across the North Pacific were from the midlatitude trough approaching from the northwest and the combination of TY Saola and the trough east of Japan that was responsible for the recurvature of Saola. Although the Ke increased rapidly over the eastern North Pacific during 0000 UTC 26–28 September (Figs. 1 and 11b,c), the energy flux from Saola was weak. Weak influence on the downstream transport of Ke by the remnants of TY Saola was primarily evident by the total flux of energy (Fig. 11b) and the direction of the ageostrophic geopotential fluxes (Fig. 12b) from the region of the ex-TC located near 40°N, 162°E at 0000 UTC 27 September. Although TY Saola did not transition to an extratropical cyclone, it did contribute to a region of baroclinic production of Ke centered at 45°N, 169°E (Fig. 12a) at the entrance to the strong zonal flow that extended across the central North Pacific. This energy was dispersed to a region of energy gain near 49°N, 159°E (Fig. 12b), which was then being transported downstream toward the northern portion of the Gulf of Alaska (Fig. 12c). Although the development of the primary cyclone over the Gulf of Alaska (Fig. 11c) continued to increase the Ke over the eastern North Pacific over the next 24 h, the contribution from the remnants of TY Saola as defined by the ageostrophic geopotential fluxes at 0000 UTC 27 September (Fig. 12b) did not continue because of the dissipation of the remnant TC. The development over the Gulf of Alaska initiated a downstream development over western North America. However, the overall contribution of Saola in the evolution of Ke across the North Pacific was small and only had a brief influence on the overall increase in Ke, which is evident as a small increase over the western and central North Pacific at 0000 UTC 27 September (Fig. 1).

e. Non-ET event (September 2007)

One pronounced downstream development event not associated with any TC occurred during 18–23 September (Fig. 1). At 0000 UTC 20 September (Fig. 13a), which was prior to the initiation of the downstream development, the total Ke across the North Pacific was weak. The west to east propagation of Ke developed in a manner similar to that defined by Orlanski and Sheldon (1995) as a midlatitude trough moved over the western North Pacific (Fig. 13b). Energy was transported to the downstream side of the trough near 53°N, 177°E (Fig. 13c) as a ridge formed over the central North Pacific. Energy was then transported over the ridge into a developing trough over the eastern North Pacific (Fig. 13d). Therefore, the development of the trough over the western North Pacific led to a downstream development and subsequent intensification of a Ke center and digging trough over the eastern North Pacific 4 days later.

4. Conclusions

During 15 July–30 September 2005, 14 TCs occurred over the western North Pacific (Fig. 1). Four of these TCs moved poleward of 40°N and began the transformation stage of ET. Two of the ET cases (TY Nabi and TS Banyan) resulted in reintensification as extratropical cyclones. One ET case (TY Guchol) merged with a larger midlatitude trough and TY Saola dissipated as it moved into a strong upper-level westerly midlatitude flow. In all of the four ET cases, there was a progression of maxima in the volume-integrated Ke that began over the western North Pacific and proceeded to the eastern North Pacific (Fig. 1).

The period of study was chosen to examine the influence of ET in downstream development prior to October when the baroclinic energetics increase in association with the strengthening westerly flow. For each case, a period of days was chosen to analyze the distribution of Ke surrounding or just after the time that the TC completed ET or was no longer identifiable as an independent circulation. The four cases examined in this study represented a variety of TC characteristics and midlatitude flow patterns in which energetics analyses have been used to describe the characteristics of downstream development associated with the ET of TCs.

The ET of TY Nabi (7–9 September) represents what might be considered a typical case in which the decaying TC phases favorably with an upstream midlatitude trough and reintensifies as a strong extratropical cyclone. A pronounced downstream development pattern occurred following the ET of Nabi. The downstream development was enhanced as energy from ex-Nabi maintained the upstream energy center longer than might have occurred in a typical downstream development scenario. That is, instead of the transport of energy from the upstream to downstream side of the trough, the dispersion of energy from ex-Nabi provided a source to the upstream energy center. This is defined in Fig. 1 as a prolonged period of relatively high volume-integrated Ke values over the western and central North Pacific between 7 and 9 September.

In contrast to TY Nabi, TS Banyan (27–29 July) was a large but weak tropical storm that moved poleward during midsummer. However, the ET of Banyan did result in a strong extratropical cyclone and a pronounced downstream development that was due in part to the poleward transport of warm, rising air. The energy was dispersed via ageostrophic geopotential fluxes into the weak midlatitude flow and contributed to downstream development over the central and eastern North Pacific (Fig. 1). Because of the weak baroclinic environment across the western North Pacific at the start of the transformation stage of ET during TS Banyan, significant downstream progression of Ke did not begin until the remnants of Banyan had reintensified as an extratropical cyclone. For this region, relative maxima in vertically integrated Ke across the western and central North Pacific during Banyan are short lived (Fig. 1).

Typhoon Guchol (26–27 August) was a very small typhoon that did not complete ET and became absorbed into a midlatitude trough. Because of the strength of the midlatitude trough, the vertically integrated Ke over the western North Pacific had increased by 1200 UTC 23 August, which was prior to the ET of Guchol (Fig. 1). However, the sudden increase in Ke over the central then eastern North Pacific occurred following the absorption of Guchol into the midlatitude trough.

Finally, TY Saola (26–28 September) began ET as it encountered the very large vertical wind shear and strong zonal flow over the western North Pacific. These factors led to the dissipation of the ex-Saola circulation. However, extension of the warm, moist tropical air into the strong zonal flow led to the production of Ke by baroclinic processes. This energy was dispersed into the midlatitude flow and may have had a minor impact on a large downstream development event that occurred over the eastern North Pacific.

Based on a progression of relative maxima in the vertically integrated Ke from the western to eastern North Pacific (Fig. 1), five occurrences of downstream development occurred over the North Pacific during the period from 15 July to 30 September. Four of the occurrences were associated with the ET of a TC over the western North Pacific. However, there was a large amount of variability associated with the role of a poleward-moving, decaying TC in influencing downstream development. In just this limited sample, a variety of TC and midlatitude flow characteristics were identified with downstream development. It is not necessary for the ET process to result in a reintensifying extratropical cyclone for there to be a significant influence on downstream development. Because of the many dynamic and thermodynamic processes involved during ET, the impact of ET on downstream development is typically not forecast well. One key to improving predictability is increased understanding of the role(s) of various processes associated with ET. However, an equally important key is to explain the variability caused by the many ways in which a poleward-moving, decaying TC may interact with the midlatitude flow into which it is moving.

Acknowledgments

This research was supported by the Office of Naval Research, Marine Meteorology Program and the National Science Foundation’s Climate and Large-Scale Dynamics.

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Fig. 1.
Fig. 1.

Volume-integrated eddy kinetic energy (1018 J) over the North Pacific between 15 Jul and 30 Sep 2005. The tropical cyclone symbols mark the dates that tropical cyclones moved poleward of 40°N. The arrows mark the last date associated with a tropical cyclone that stayed south of 40°N. The areas are bounded in latitude by 35°–65°N. The western North Pacific extends between 130° and 160°E. The central North Pacific extends between 160°E and 160°W. The eastern North Pacific extends between 160° and 125°W.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 2.
Fig. 2.

Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 5–10 Sep 2005. The tropical cyclone symbols mark the location of TY Nabi at each time.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 3.
Fig. 3.

Contributions to the distribution of Ke at 0000 UTC 7 Sep 2005: (a) vertically averaged Ke (shaded, 105 J m−2), energy flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights (contours, 60-m intervals); (b) vertically averaged Ke [shaded as in (a)], 500-hPa heights (light gray contours), and vertically averaged Ke generation resulting from baroclinic conversion (contours; W m−1); (c) as in (b), but for the vertically averaged Ke tendency resulting from ageostrophic geopotential flux convergence; (d) vertically averaged Ke generation (shaded, W m−2), ageostrophic geopotential flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]; and (e) vertically averaged Ke flux convergence (shaded, W m−2), Ke flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]. The TC symbols mark the location of TY Nabi.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 4.
Fig. 4.

As in Fig. 3, but for 1000 UTC 8 Sep 2005.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 5.
Fig. 5.

As in Fig. 3, but for 0000 UTC 9 Sep 2005.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 6.
Fig. 6.

Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 26–31 Jul 2005. The tropical cyclone symbols mark the location of TS Banyan at each time.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 7.
Fig. 7.

As in Fig. 3, but for 0000 UTC 29 Jul 2005. The TC symbols mark the location of the ex-TS Banyan.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 8.
Fig. 8.

As in Fig. 3, but for 0000 UTC 30 Jul 2005. The TC symbols mark the location of the ex-TS Banyan.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 9.
Fig. 9.

Mean sea level pressure (grayscale shading and contours below 1000 hPa in 4-hPa intervals) and 500-hPa heights (contours at 60-m intervals) for 0000 UTC 24–29 Aug 2005. The TC symbol marks the location of TY Guchol.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 10.
Fig. 10.

Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel. The TC symbol marks the location of TY Guchol.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 11.
Fig. 11.

Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel. The TC symbol marks the location of TY Saola.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 12.
Fig. 12.

(a) Vertically averaged Ke (shaded, 105 J m−2), vertically averaged Ke generation resulting from baroclinic conversion (dark contours with negative contours dashed, W m−1), and 500-hPa heights (contours, 60-m intervals); (b) vertically averaged Ke generation (shaded, W m−2), ageostrophic geopotential flux vectors (reference vector in the lower right, 105 W m−1), and 500-hPa heights [as defined in (a)]; and (c) vertically averaged Ke flux convergence (shaded, W m−2), Ke flux vectors (reference vector in lower right, 105 W m−1), and 500-hPa heights. The TC symbol marks the location of TY Saola.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Fig. 13.
Fig. 13.

Vertically averaged Ke (shaded, 105 J m−2) and energy flux vectors (reference vector in lower right, 105 W m−1) and 500-hPa heights (contours, 60-m intervals) for the times indicated at the top of each panel.

Citation: Monthly Weather Review 137, 4; 10.1175/2008MWR2558.1

Table 1.

TCs examined in this study with the time that ET was completed or the system was no longer identified as an independent circulation. This time also defined the center of the 30-day period used to construct the mean fields.

Table 1.
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