2634 MONTHLY WEATHER REVIEW VOLUME 128 Extratropical Transition of Tropical Cyclones over the Western North Paci®c. Part II: The Impact of Midlatitude Circulation Characteristics PATRICK A. HARR AND RUSSELL L. ELSBERRY Department of Meteorology, Naval Postgraduate School, Monterey, California TIMOTHY F. H OGAN Naval Research Laboratory±Monterey, Monterey, California (Manuscript received 18 January 1999, in ®nal form 7 December 1999) ABSTRACT Two characteristic midlatitude circulation patterns (labeled northwest and northeast) are found to be associated with extratropical transition (ET) of tropical cyclones over the western North Paci®c Ocean. Although in both cases the tropical cyclone moves poleward ahead of a midlatitude trough, the primary midlatitude circulation is either that trough or is a large quasi-stationary cyclone to the northeast of the poleward-moving tropical cyclone. Transition into a northwest pattern typically results in the development within 36 h of an intense extratropical cyclone that moves north±northeast. A tropical cyclone that moves into a northeast pattern enters into strong zonal ¯ow between the primary midlatitude circulation and the subtropical ridge to the southeast. These systems move rapidly eastward and do not intensify signi®cantly during the 36 h following transition. In Part I of this study, the ET of Typhoon (TY) David (1997) and the ET of TY Opal (1997) were investigated in terms of the formation of extratropical cyclone features. In this study, the same cases of ET are examined to de®ne interactions between the decaying tropical cyclone and these two general synoptic environments in terms of the distributions of heat and momentum ¯uxes and generation of kinetic energy between the cyclone and the environment. During transition into either circulation pattern, the tropical cyclone is initially impacted by upper-tropospheric eddy angular momentum ¯uxes associated with the juxtaposition of the midlatitude cir- culation. During transition into a northwest pattern, the tropical cyclone couples with the midlatitude baroclinic zone such that low-level eddy heat ¯uxes contribute to the extratropical cyclone development. During transition into a northeast pattern, the strong zonal ¯ow seems to prevent a direct interaction between the decaying tropical cyclone and the primary midlatitude circulation. Eddy heat ¯uxes do not increase and minimal baroclinic development occurs. The two types of extratropical transition also have signi®cant differences in the generation of kinetic energy during the reintensi®cation as an extratropical cyclone. Extratropical transition into a northwest pattern results in barotropic and baroclinic production of kinetic energy through direct solenoidal circulations that result from the coupling of the tropical cyclone and midlatitude trough. The movement of a tropical cyclone toward the large, quasi-stationary extratropical cyclone in the northeast pattern results in barotropic destruction of kinetic energy that inhibits signi®cant reintensi®cation. 1. Introduction mation of the TC structural characteristics and then a reintensi®cation as an extratropical cyclone. Through As a tropical cyclone (TC) begins to recurve into the examination of 30 ET cases over the western North midlatitude westerlies, the characteristic TC structure Paci®c, Klein et al. (2000) showed that the transfor- changes in response to gradients of atmospheric wind mation stage of the ET process proceeded through three (including vertical shear), temperature, and moisture. steps in a similar manner for all cases studied. The basic Often, the structural changes lead to the transition of physical characteristics of the transformation stage, the TC into an extratropical cyclone. Klein et al. (2000) which are identi®able in satellite imagery, are the asym- de®ne a conceptual model of extratropical transition metric patterns of cloud and precipitation due to envi- (ET) as a two-stage process that consists of a transfor- ronmental in¯ow of colder, drier (warm, moist) air in the western (eastern) portion of the outer TC circulation. The reintensi®cation stage is dominated by the evolution of extratropical cyclone features such as fronts and Corresponding author address: Patrick A. Harr, Code MR/Hp, De- partment of Meteorology, Root Hall, 589 Dyer Rd., Monterey, CA asymmetric wind, cloud, and precipitation patterns. 93943-5114. Both stages of the transition of a TC into an extratropical E-mail: [email protected] cyclone involve many complex physical processes (Pal- Unauthenticated | Downloaded 09/30/21 04:15 PM UTC AUGUST 2000 HARR ET AL. 2635 men 1958; DiMego and Bosart 1982a,b; Anthes 1990; 1992; Thorncroft et al. 1993; Evans et al. 1994; Schultz Sinclair 1993; Browning et al. 1998). et al. 1998). This study and a companion study (Harr and Elsberry In this study, the northwest and northeast circulation 2000) address how the reintensi®cation of the decaying patterns are generalized in section 3 based on composite TC as an extratropical cyclone is affected by the char- circulation patterns from 30 ET cases over the western acter of the midlatitude circulation into which the de- North Paci®c. Using the cases of Typhoon (TY) David caying TC moves. In the Harr and Elsberry (2000) study, (September 1997) and TY Opal (June 1997) as exam- frontogenesis was used as a proxy for the change from ples, the role of each midlatitude circulation is then a TC structure to an asymmetric extratropical cyclone examined more fully in terms of the heat and momentum structure. In this part, it will be shown that the midlat- ¯uxes (section 4) and the budget of kinetic energy (sec- itude circulation impacts the exchange of heat and mo- tion 5) during reintensi®cation. The diagnostic frame- mentum between the midlatitudes and TC, and the gen- work used in section 4 to compare the effect of envi- eration of kinetic energy during the reintensi®cation ronmental sources of PV on a decaying TC is similar stage of ET. to that used by Molinari et al. (1995) to diagnose the Klein et al. (2000) show the change in structural char- interactions leading to intensi®cation of Hurricane Elena acteristics during the transformation stage of ET is due in the Gulf of Mexico during August 1985. Although to the movement of the TC into the midlatitude envi- both the David and Opal cases have large upper-level ronment. Similarly, the formation of frontal regions dur- eddy angular momentum ¯uxes toward the TC as it ing the initial steps of reintensi®cation is dominated by moves into the midlatitudes, a signi®cant difference is the interaction between the TC and the midlatitude bar- found due to eddy heat ¯uxes at lower levels that affect oclinic zone and is relatively independent of the mid- the baroclinic energy conversions. latitude circulation pattern. However, Harr and Elsberry Previous studies of the kinetic energy budget asso- (1999) found that the ®nal structural characteristics of ciated with the ET of tropical cyclones over the North the reintensifying extratropical cyclone differ greatly Atlantic (Palmen 1958; Vincent and Chang 1975; Kor- depending on the character of midlatitude circulation negay and Vincent 1976; Chien and Smith 1977; into which the TC moves. In a northwest pattern, the DiMego and Bosart 1982b) have varying results that coupling of the TC remnants with the eastward-moving may be due to different environments around the trans- forming TC. The kinetic energy budgets associated with midlatitude trough located northwest of the TC is so the reintensi®cation stage of TY Opal and TY David vigorous that an intense extratropical cyclone with well- over the western North Paci®c are examined in section de®ned frontal characteristics forms to become the pri- 5 and interpreted with respect to the in¯uence of each mary circulation over the region. A weaker trough exists midlatitude circulation pattern. upstream in the northeast pattern, which is dominated by a preexisting, quasi-stationary extratropical cyclone northeast of the TC. In this case, the coupling between 2. Data the midlatitude trough and the TC is reduced due to the During June±October 1994±98, 30 cases of extra- in¯uence of the preexisting midlatitude cyclone, and tropical transition over the western North Paci®c oc- only weak warm frontogenesis occurs to the northeast curred (Table 1 in Klein et al. 2000). Environmental of the reintensifying TC. ®elds analyzed by the Naval Operational Global At- Various research groups have examined the in¯uence mospheric Prediction System (NOGAPS) at the Fleet of a midlatitude trough on the intensity change of the Numerical Meteorology and Oceanography Center (Ho- TC. An upstream upper-level trough, which may be gan and Rosmond 1991) are used to de®ne the midlat- characterized as a potential vorticity (PV) maximum, itude circulation and calculate the various diagnostic may intensify the TC through eddy ¯uxes of angular quantities that will be used to examine the interactions momentum (Molinari and Vollaro 1989, 1990) and po- between the TC and midlatitude environment. One ben- tential vorticity (Molinari et al. 1995, 1998). However, e®t of the NOGAPS ®elds is the operational use of these (and other) studies suggest other internal mech- satellite water vapor winds that provide an additional anisms (i.e., enhanced air±sea ¯uxes, initiation of an data source, particularly in cloud-free regions of the eyewall replacement cycle) may contribute during the middle and upper troposphere where this investigation intensi®cation of a TC. Similarly, other aspects of the into the interactions between the TC and midlatitudes midlatitude environment such as decreasing sea surface is based. temperature, increasing vertical wind shear, and the im- Although no formal de®nition exists for specifying port of eddy ¯uxes to the TC, may contribute to the ET when ET has occurred, Harr and Elsberry (2000) pro- of a TC. A framework for the environmental interaction pose use of a frontogenesis parameter to de®ne the time during ET is provided by generalized relationships be- of ET.