The Antecedent Large-Scale Conditions of the ''Perfect Storms'' Of

2546 MONTHLY WEATHER REVIEW VOLUME 138

The Antecedent Large-Scale Conditions of the ‘‘Perfect Storms’’ of Late October and Early November 1991

JASON M. CORDEIRA AND LANCE F. BOSART Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

(Manuscript received 10 November 2009, in ﬁnal form 26 January 2010)

ABSTRACT

The ‘‘Perfect Storms’’ (PSs) were a series of three high-impact extratropical cyclones (ECs) that impacted North America and the North Atlantic in late October and early November 1991. The PSs included the Perfect Storm in the northwest Atlantic, a second EC over the North Atlantic that developed from the interaction of the PS with Hurricane Grace, and a third EC over North America commonly known as the ‘‘1991 Halloween Blizzard.’’ The PSs greatly impacted the North Atlantic and North America with large waves, coastal flooding, heavy snow, and accumulating ice, and they also provided an opportunity to investigate the physical processes that contributed to a downstream baroclinic development (DBD) episode across North America that culminated in the ECs. Downstream baroclinic development resulted from an amplification of the large-scale flow over the North Pacific that was influenced by anomalous tropical convection, the recurvature and extratropical transition of western North Pacific Tropical Cyclones Orchid, Pat, and Ruth, and the subsequent evolution of the extratropical flow. The progression of DBD occurred following the development of a negative PNA regime and the generation of baroclinic instability over North America associated with equatorward-displaced potential vorticity anomalies and poleward-displaced corridors of high moisture content. An analysis of the eddy kinetic energy tendency equation demonstrated that the resulting baroclinic conversion of eddy available potential energy into eddy kinetic energy during the cyclogenesis process facilitated the progression of DBD across North America and the subsequent development of the ECs.

1. Introduction damage (Table 1). A majority of the meteorological impact from the PS occurred in association with high a. Background and motivation winds, large waves, and coastal flooding (Table 1). EC1, The ‘‘Perfect Storms’’ (PSs) were a series of three however, mainly impacted North Atlantic shipping routes extratropical cyclones (ECs) that occurred between with numerous high wind and wave reports. EC2 was as- 25 October and 4 November 1991 that included the sociated with an early-season cold air outbreak and record Perfect Storm (Cardone et al. 1996; Junger 1997) and snow and ice accumulations over the Midwest. The pur- two additional ECs over the North Atlantic and central pose of this paper is to investigate the antecedent large- United States (EC1 and EC2, respectively). EC1 was an scale flow evolution over the North Pacific (NP) and the intense cyclone that resulted from the interaction be- physical processes that contributed to downstream de- tween the PS and Hurricane Grace (HG; 1991), whereas velopment over North America and culminated in the EC2 is colloquially known as the ‘‘1991 Halloween three ECs. Blizzard’’ in the Midwest. The PS, EC1, and EC2 were Six-hourly locations of the PS, EC1, EC2, and HG, notorious cyclones that resulted in more than 34 deaths along with the mean sea level pressure (MSLP) and and over $300 million (1991 U.S. dollars) in property time-mean 500-hPa geopotential heights from 25 October through 4 November, show the positions of the storms relative to the time-mean amplified flow over North Corresponding author address: Jason M. Cordeira Department America and the North Atlantic (Fig. 1a). The de- of Atmospheric and Environmental Sciences, University at Al- bany, State University of New York, ES-351, 1400 Washington velopment of HG followed a baroclinic pathway before Ave., Albany, NY 12222. the system transformed into a tropical cyclone (TC) via the E-mail: [email protected] tropical transition (TT) process on 25 October (not shown;

DOI: 10.1175/2010MWR3280.1

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TABLE 1. Summary of the meteorological and societal impacts of the PS, EC1, and EC2. Sources: The Environment Canada data are available online at http://www.meds-sdmm.dfo-mpo.gc.ca/ and the PS damage summary is available online at http://www.ncdc.noaa.gov/ oa/satellite/satelliteseye/cyclones/pfctstorm91/pfctstdam.html.

Impact Magnitude Location and time Source PS High winds 25.0 m s21 Environment Canada buoys 44141 Environment Canada; (sustained) and 44137 over NW Atlantic Eid et al. (1992) on 29–30 Oct High winds (gust) 28.0 (35.0) m s21 Milton, MA (Chatham, MA) Maglaras et al. (1995) on 31 Oct Maximum wave 28.5 (30.7) m Environment Canada buoy Environment Canada; heights 44141 (44137) on 29 Oct Eid et al. (1992) Heavy rain .100.0 mm Eastern Massachusetts on 30–31 Oct NCDC Property damage .$200 (.$100) East Coast (Massachusetts) Bigio (1992); NOAA (1992); million PS damage summary Coastal ﬂood N/A New England Maglaras et al. (1995) forecasting EC1 Rapid deepening 50.0 hPa (24 h)21 North Atlantic between 0000 UTC ECMWF ERA-40 30 Oct and 0000 UTC 31 Oct High winds (gust) .30.0 m s21 North Atlantic on 31 Oct DeAngelis (1992) Large ocean swells ;10.0 m North Atlantic on 31 Oct EC2 Heavy snow .50.0 (93.7) cm East-central through northeast MN This day in weather history (Duluth, MN) on 31 Oct–2 Nov for southeast Minnesota, Ice accumulation 5.0 (7.5) cm Southwest through north-central northeast Iowa, and western IA (south-central and southeast Wisconsin (Rieck and IA) on 31 Oct–2 Nov Boyne 2006) Property damage $68.0 ($11.7) million IA (MN) Daily record min 219.08/223.38/211.68C Duluth, MN on 4 Nov/Bismarck, ND on NCDC summary of the day temperatures 31 Oct/Chicago, IL on 4 Nov

Davis and Bosart 2004). Hurricane Grace subsequently further described this amplified flow pattern on the dy- interacted with the PS, underwent extratropical transition namic tropopause (DT) as the result of possible down- (ET), and influenced the development of EC1 on stream development, diabatic heating, and the modulation 29 October. The PS later retrograded toward the northeast of the DT by TCs and TCs undergoing ET. Bosart (2003) United States, completed a counterclockwise loop over briefly reemphasized the amplified flow pattern from the Gulf Stream, underwent TT, and became a weak a potential vorticity (PV) perspective (Hoskins et al. 1985) unnamed hurricane on 1 November (Pasch 1991; Pasch and suggested EC2 was a ‘‘twin perfect storm.’’ Limited and Avila 1992). Concurrently, EC1 rapidly crossed the research, however, has focused on the evolution of the North Atlantic with characteristics of a diabatic Rossby antecedent large-scale flow. vortex (e.g., Moore and Montgomery 2004, 2005) and Studies have shown that noteworthy cyclogenesis explosively deepened (SLP decrease of 50 hPa in 24 h; events over North America such as the ‘‘Cleveland Su- Fig. 1b) approximately 2 bergerons (Sanders and Gyakum perbomb’’ of 25–26 January 1978 (e.g., Hakim et al. 1980) between 30 and 31 October. Upstream, EC2 formed 1995, hereafter HBK95), the ‘‘Superstorm’’ of 12– in the western Gulf of Mexico downstream of a deep time- 14 March 1993 (e.g., Bosart et al. 1996, hereafter B96), mean trough over the western United States and pro- and the ‘‘1998 Canadian Ice Storm’’ of 5–9 January 1998 gressed toward Hudson Bay between 31 October and (e.g., Gyakum and Roebber 2001) are often preceded by 4 November 1991. large-scale flow amplifications over the eastern NP and western North America. These studies illustrated how b. Revisiting the ‘‘Perfect Storms’’ upstream amplified flow influenced the juxtaposition of Previous research on the PS has primarily focused on equatorward-displaced regions of high PV air (e.g., upper- the extreme sea state in the North Atlantic (Table 1; level positive PV anomalies) and poleward-displaced Cameron and Parkes 1992; Eid et al. 1992; Maglaras regions of warm, moist air (e.g., lower-level positive PV et al. 1995; Cardone et al. 1996). Cameron and Parkes anomalies represented by positive potential temperature (1992), however, first depicted the development of the anomalies) during the cyclogenesis process. In regions PS during an amplification of the 500-hPa flow (e.g., of strong midlatitude baroclinity, conditions were favor- ‘‘strongly zonal to strongly meridional’’) as suggested by able for a mutual intensification of the upper- and lower- Fig. 1 (cf. their Figs. 1–4). Nielsen-Gammon (2001) level PV anomalies (Hoskins et al. 1985) and ‘‘type B’’

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FIG. 1. (a) The 6-hourly locations of HG, PS, EC1, and EC2 and the time-mean 500-hPa geopotential heights from 25 Oct through 4 Nov 1991 (dam, solid line). Tropical symbols denote categorical intensity and characteristics of HG and the PS. (b) MSLP of HG, PS, EC1, and EC2 is given by a combination of the National Hurricane Center best-track data, ECMWF ERA-40 analyses, and surface data. The 1 and 2 bergeron (at 458N) deepening rates are given as reference.

cyclogenesis (Petterssen and Smebye 1971). Cyclogenesis of 41 strong cold-season cyclones between 1975 and 1985 of this nature is consistent with the Eady model for baro- in the eastern NP could be traced to the ascent of warm clinic growth (Eady 1949; Hoskins and Valdes 1990) air within the baroclinic environment of upstream cy- whereby baroclinic instability is favored in the presence clones in the western NP. Similarly, Chang et al. (2002) of vertical shear (given an upshear tilt between regions of suggested that the subsequent downstream energy prop- upper- and lower-level PV) and reduced static stability agation could maintain synoptic-scale eddies over less [associated with the vertical structure of an upper-level baroclinic regions through an extension of the ‘‘storm PV anomaly (Thorpe 1985)]. track.’’ Because large-scale ﬂow ampliﬁcations and DBD Baroclinic instability/growth can also impact the down- can alter the energetic characteristics of the atmosphere stream propagation and intensity of eddy kinetic energy in response to a conversion between the zonal and eddy

Ke in association with downstream baroclinic development components of the available potential energy (APE) (DBD; Chang and Orlanski 1993; Orlanski and Sheldon and kinetic energy (KE), the Ke tendency can be used to 1993, 1995; Chang et al. 2002; Danielson et al. 2004). For identify the physical processes that contribute to DBD example, Danielson et al. (2004) found that the domi- (e.g., Orlanski and Sheldon 1993; Danielson et al. 2006; nant source for downstream energy propagation in 20 Harr and Dea 2009).

Unauthenticated | Downloaded 10/11/21 03:25 AM UTC JULY 2010 C O R D E I R A A N D B O S A R T 2549 c. Paper goals and organization TABLE 2. Summary of kinematic and thermodynamic variables partitioned for the Ke budget. Terms subscripted with an ‘‘m’’ The goals of this paper are to 1) elucidate the key represent the 30-day average, while lowercase and primed terms players in the NP that contributed to the large-scale flow represent the eddy component calculated by subtracting the total amplification and the initiation of DBD and 2) inves- from the 30-day average. tigate the physical processes that contributed to DBD Partition Parameter across North America and the development of the PS, V 5 Vm 1 v Horizontal (vector) wind EC1, and EC2 using an analysis of the Ke tendency. This v 5 vm 1 v9 Vertical (pressure) velocity paper will be organized as follows. Section 2 will outline F5Fm 1 f Geopotential height the data and methods used for the study. Sections 3 and 4 a 5 am 1 a9 Specific volume will present an overview of the Northern Hemisphere (NH) and NP flow evolutions, whereas section 5 will investigate the Ke tendency during the period of DBD. as gridded brightness temperature (Tb)datafromthe Section 6 will discuss the results in the context of the Cloud Archive User Service (CLAUS; Hodges et al. 2000) development of the PS, EC1, and EC2, and section 7 will at 0.5830.58 horizontal and 3-h temporal resolutions. offer conclusions. The Ke tendency was derived by partitioning the kinematic and thermodynamic parameters into mean (30-day average) and eddy (derived by subtracting 6-h 2. Data and methods parameter values from the mean) components and fol- The structure of the NH, NP, and North American lowed the methodologies of Orlanski and Sheldon (1993), large-scale flow patterns was examined by using the Decker and Martin (2005), Danielson et al. (2006), and Pacific–North American (PNA) teleconnection index Harr and Dea (2009). The Ke tendency equation in (Wallace and Gutzler 1981; Barnston and Livezey 1987; pressure coordinates is given by Feldstein 2002) and the Arctic Oscillation (AO) as ›K › v9f a measure of the ‘‘zonal index cycle’’ (Rossby et al. 1939; e ( ) 5 $ (vf)a v9a9 [$ (VKe)] Rossby and Willett 1948; Thompson and Wallace 2000) ›t ›p between October and November 1991. Daily values of ›(v9K ) e v (v $V ) 1 residual, (1) the PNA index were calculated from the 2.5832.58 ›p m gridded 0000 and 1200 UTC National Centers for Envi- 2 ronmental Prediction–National Center for Atmospheric where Ke 5 ½jvj . The variables in (1) and partitioning Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996; procedure is further presented in Table 2. The first three Kistler et al. 2001) of 500-hPa geopotential height fields terms on the right-hand side (rhs) of (1) represent the over fixed domains following Archambault et al. (2008). advection of eddy geopotential heights by the eddy ve- Daily values of the AO were obtained from the Climate locity [i.e., 2(v $f)] and were expanded in flux form Prediction Center (CPC) and were constructed by pro- following Orlanski and Katzfey (1991). The first term in jecting the daily 1000-hPa height anomalies onto the (1) represents the ageostrophic geopotential flux (AGF) leading empirical orthogonal function of the monthly divergence. The nondivergent component of the AGF mean 1000-hPa height anomaly poleward of 208N(more was removed using Eq. (4.5) from Orlanski and Sheldon information is available online at http://www.cpc.noaa. (1993). The AGF divergence is dynamically consistent gov/products/precip/CWlink/daily_ao_index/history/ with the advection of absolute vorticity by the irrota- history.shtml). tional component of the horizontal wind. The second Gridded datasets of the 40-yr European Centre for term represents (in an average sense) the baroclinic

Medium-Range Weather Forecasts (ECMWF) Re-Analysis conversion (BC) of eddy APE into Ke and is observed (ERA-40; Uppala et al. 2005), with 1.125831.1258 hor- during vertical displacements of warm and cold air izontal grid spacing, 13 vertical levels (1000–50 hPa), and (Lorenz 1955). For example, the BC term is positive 6-h temporal resolution constituted the primary data associated with processes involving the ascent of warm source for the large-scale overview and Ke tendency air (e.g., frontogenesis, warm air advection, and deep analysis. Additionally, the North Pacific overview is moist convection). The third term represents the vertical aided by infrared satellite (IR) imagery from the National flux divergence and is typically very small when in- Climatic Data Center (NCDC) through their Global In- tegrated over the depth of the atmosphere (Orlanski and ternational Satellite Cloud Climatology Project (ISCCP) Sheldon 1993), while the fourth and fifth terms represent

B1 Browse System (GIBBS) archive (more information horizontal and vertical Ke ﬂux divergence, respectively. available online at http://www.ncdc.noaa.gov/gibbs/) and The sixth term represents barotropic Ke generation, or

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6 2 21 FIG. 2. Time-mean 300-hPa streamfunction (dark contours every 10 3 10 m s ) and streamfunction anomaly (shaded according to scale every 10 3 106 m2 s21 with negative values dashed) from an 11-day weighted climatology for (a) 10–20 Oct, (b) 15–25 Oct, (c) 20– 30 Oct, and (d) 25 Oct–4 Nov 1991. Figures were generated using the 2.58 NCEP–NCAR reanalysis.

the Ke tendency due to a conversion of KE between the across North America. The Ke analysis will be com- zonal mean and eddy flow. All other processes, which plemented by quasigeostrophic (QG) and PV diag- include frictional dissipation, are contained in the sev- nostics in order to identify important features that enth term (residual). contribute to the Ke tendency that are commonly rep- The vertically integrated Ke tendency was used to resented by traditional methods. investigate the downstream propagation of Ke, interactions between Ke maxima for the case of DBD, and 3. Time-mean hemispheric overview sources of Ke generation within the PSs. Upon vertical integration between 1000 and 50 hPa, (1) can be sim- Eleven-day overlapping periods of the time-mean 1 plified to 300-hPa streamfunction and streamfunction anomaly over the NP illustrate a flow pattern characterized by flat ›K e ridges over the western NP (R1) and western North ’$ (vf)a v9a9 [$ (VKe)] ›t America (R2) and by troughs over the central NP (T1)

v (v $Vm) 1 residual. (2) and eastern North America (T2) during the 10–20 Oc- tober period (Fig. 2a). The large-scale flow amplified The terms on the rhs of (2) relate to Ke generation via considerably by the 25 October–4 November period and AGF convergence, BC (thermally direct processes), and was accompanied by the deepening and westward mi- barotropic generation, and Ke translation via Ke flux gration of trough T2 into western North America via (KEF) divergence (Chang 1993; Orlanski and Chang discontinuous retrogression (e.g., Palmeń and Newton 1993; Orlanski and Sheldon 1993, 1995; Danielson et al. 1969, 152–153; Figs. 2a–d). The large-scale flow further 2006). Equation (2) will be used in section 5 for the case amplified over the NP as ridge R2 retrogressed, in- of DBD between 24 and 31 October. While DBD can be creased in amplitude, and became collocated with pro- observed through traditional synoptic-scale analyses, gressive ridge R1 near 1508W (Figs. 2c,d). Finally, the the rhs of (2) will help identify the relative importance large-scale flow amplification occurred as alternating of individual physical processes toward Ke propagation streamfunction anomalies developed (represented by R3, T1, R2, and T2, respectively) between the tropical 1 Terms three and five from (1) are approximately zero when Pacific and western North America between 25 October vertically integrated, given the top and bottom boundary condi- and 4 November (Fig. 2d). This streamfunction anomaly tions (v ; 0) on the vertical velocity. pattern suggests possible modulation of the extratropical

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6 2 21 FIG. 3. 300-hPa streamfunction anomaly (shaded according to scale every 8 3 10 m s ) from 1 Oct to 15 Nov 1991 averaged from (a) 308 to 508N latitude and (b) 408 to 608N latitude and displayed from 908Eto08. Labels R1, T1, R2, and T2 in (a) are as in Fig. 2 with symbols denoting the genesis longitude of the PS (star), EC1 (circle), and EC2 (square). The dashed lines in (b) denote the phase speed c and group velocity cg of the streamfunction anomaly pattern between 23 October and 15 November.

flow pattern in association with anomalous tropical the NP flow. The influence of synoptic-scale transient convection, similar to responses analyzed by Jin and disturbances on the time-mean structure of the NP flow Hoskins (1995), Newman and Sardeshmukh (1998), and is investigated in section 4b. Trenberth et al. (1998), among others, and is investigated The large-scale flow amplification coincided with a further in section 4a. transition of the PNA and AO indices from positive to The retrogression of ridge R2 into the eastern NP and strongly negative from 11 to 23 and from 22 to 29 October, trough T2 into western North America is illustrated by respectively (Fig. 4). The PNA index value transitioned westward-moving negative and positive streamfunction from 11.92 on 11 October to 22.78 on 23 October and anomalies, respectively, between 8 and 15 October from represented the largest positive-to-negative transition (for a time–longitude perspective (Fig. 3a). The progression a comparable time scale) for any October between 1948 of ridge R1 into the central NP, shown by eastward-moving and 2009. The 22.78 PNA index value represents the positive streamfunction anomalies from 1308E to 1808 lowest daily PNA index value for any October in the 62-yr between 8 and 15 October, resulted in the collocation of climatology and highlights the significance of the ampli- ridges R1 and R2 along 1508W by 22 October (Fig. 2c). fied flow over the eastern NP and western North America. Note that ridge R1 is outside of the 308–508N and 408– The value of the AO subsequently transitioned from 608N latitude bands after ;22 October. Downstream 11.00 on 22 October to 23.04 on 29 October, as the PNA development across North America is illustrated by index trended positively, and likely occurred in associa- eastward-progressing regions of alternating positive and tion flow amplification during the period of downstream negative streamfunction anomalies with a group velocity development between the NP and North Atlantic. 21 cg of ;18 m s between 24 and 31 October (Fig. 3b). A slowly progressive ridge–trough–ridge pattern with a phase speed c of ;2ms21, initiated between 22 and 24 4. North Pacific overview October, persisted across the eastern NP and North a. Tropical influences America until ;7 November. This slowly progressive pattern suggests that the time-mean streamfunction Variability of tropical convection has been shown to anomaly pattern between 25 October and 4 November influence the structure of the extratropical flow over the (Fig. 2d) is representative of the day-to-day variability of NP (e.g., Simmons 1982; Branstator 1985; Jin and Hoskins

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FIG. 4. Time series of the PNA teleconnection (solid) and AO (dashed) index values from 1 Oct to 15 Nov 1991. Maximum and minimum values are noted in October.

1995; Newman and Sardeshmukh 1998; Trenberth et al. duration of the midlatitude streamfunction anomaly 1998; Barsugli and Sardeshmukh 2002); concomitantly pattern over the extratropical eastern NP and North associated with variability in the PNA index (e.g., America (Fig. 3b). To first order, the negative stream- Hoskins et al. 1977; Horel and Wallace 1981; Simmons function anomaly that crossed the date line between et al. 1983; Plumb 1985; Black and Dole 1993; Cash and 26 October and 15 November represents enhanced cy- Lee 2001; Franzke and Feldstein 2005; Mori and Watanabe clonic flow in the midlatitudes influenced by an increase 2008). In the present case, IR imagery, cold brightness in the zonal flow on the northern edge a subtropical temperatures (Tb , 250 K), and the 300-hPa divergence anticyclone modulated by the tropical convection (not pattern depicted a coherent region of eastward-moving shown). The positive and negative streamfunction anom- (c ’ 6ms21) tropical convection from 13 to 23 October aly couplet ;608 downstream represents subsequently and between 1208 and 1658E (Fig. 5). A period of slower enhanced anticyclonic and cyclonic flow in the eastern (c ’ 2ms21) eastward-moving convection persisted Pacific and North America, respectively, likely as a result from 26 October to 15 November and between 1508E of the upstream flow modification (Fig. 3b). and 1808. The influence of tropical convection on the extra- The phase speeds, location, and duration of the tropical flow during the 1991 event is further investigated eastward-moving convection between 13 October and from a composite of 19 MJO and coupled OKW events in 15 November suggest the convection was associated with the tropical Pacific between 1974 and 2006 (Fig. 6). These the Madden–Julian oscillation (MJO; Madden and Julian 19 events were identified following Roundy and Kravitz 1971, 1972, 1994), where c ’ 6ms21 and convection (2009) for the positive phase of ENSO (El Ninõ) with coupled to an oceanic Kelvin wave (OKW), where c ’ Ninõ-3.4 region sea surface dynamic heights greater than 2ms21, respectively (Hendon et al. 1998; Roundy and one standard deviation above the long-term mean, during Kiladis 2006). High-resolution MJO indices available from the months of October, November, and December, and CPC (see online at http://www.cpc.noaa.gov/products/ centered on the equator and 1508E (Table 3). The 19 precip/CWlink/MJO/mjo.shtml) confirm this hypothesis events were composited 6 days before (day 26), concur- (not shown). The MJO and OKW events occurred in rent with (day 0), and 6 days after (day 16) the minimum association with increasing El Ninõ conditions (warming outgoing longwave radiation (OLR) value associated with sea surface temperatures, weakening trade winds, and in- the convection coupled to the MJO crossed 1508E. creasing sea surface dynamic heights during the fall of Composite 300-hPa geopotential height (hereafter 1991) in the tropical Pacific (Janowiak 1993). The phase height) and wind anomalies, with low-latitude OLR, il- speed and duration of the convection coupled to the lustrate a negative height anomaly over the central NP OKW in the tropical Pacific between 26 October and and a positive height anomaly over the Gulf of Alaska on 15 November was coincident with the phase speed and day 26 (Fig. 6a) that increased in magnitude with minor

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FIG. 5. CLAUS brightness temperature (K, shaded according to scale) and 300-hPa divergence (contoured every 0.5 3 1025 s21 from 0.5–2.0 3 1025 s21) averaged between 58Nto 58S. Approximate phase speed c of observed brightness temperature maxima are given. In- frared satellite imagery every 3 days beginning on 7 October is shown for comparison (day of month given within triangle). Satellite domain is cropped between 58N and 58S and between 908E and 1758W. phase changes by day 0 (Fig. 6b). The evolution of the California. The composite analysis for the 19 MJO and composite flow pattern qualitatively agrees with the evo- coupled OKW events over the tropical Pacific illustrates lution of the time-mean and time–longitude 300-hPa that anomalous tropical convection likely influenced the streamfunction and streamfunction anomaly patterns midlatitude flow during the 1991 event, and promoted the in late October and early November (Figs. 2 and 3). The development of positive and negative 300-hPa height anomalous cyclonic flow associated with the negative anomalies over the Gulf of Alaska and the western height anomaly over the central NP likely influenced the United States, respectively. growth of the positive height anomaly over the Gulf of b. Tropical–extratropical influences Alaska in conjunction with a poleward transport of low PV air from the subtropics, which is investigated in sec- The variability of the NP flow between September and tion 4b (Fig. 6b). The day 16 composite depicts anoma- November is strongly influenced by the life cycles of lous westerly flow across the NP at 358N, likely associated TCs in the western NP (e.g., Harr and Elsberry 2000; with the strengthening of a subtropical anticyclone over Klein et al. 2000; Jones et al. 2003; Riemer et al. 2008; the west Pacific (noted by an anomalous anticyclonic Reynolds et al. 2009). The 6-h locations of three western 300-hPa circulation in Fig. 6c). Statistically significant NP TCs and time-mean 300-hPa streamfunction and positive and negative height anomalies persisted over nondivergent wind magnitudes demonstrate that TCs the Gulf of Alaska and the western United States, re- Orchid, Pat, and Ruth recurved and underwent ET on spectively, as a positive height anomaly developed over the western periphery of time-mean subtropical anticy- the Gulf of Mexico. The latter anomaly suggests an clones over the west Pacific (Fig. 7). Orchid reintensified enhanced subtropical jet over Mexico in conjunction as an EC (980–972 hPa) on 14 October, given its fa- with a negative height anomaly to the southwest of vorable proximity to the upstream trough over Japan

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FIG. 6. Composite 300-hPa geopotential height anomalies (m, shaded according to color scale), 300-hPa wind vector anomalies (m s21, magnitude given by reference vector), and OLR (W m22, shaded according to grayscale) for the 19 MJO and coupled Kelvin wave events given in Table 3 at (a) ‘‘Day 26’’, (b) ‘‘Day 0’’, and (c) ‘‘Day 16.’’ The dark black contours indicate the statistical signiﬁcance of the 300-hPa geopotential height anomalies (solid and dashed for positive and negative anomalies, respectively) at the 90% and 95% conﬁdence levels as determined by a two-sided Student’s t test. The star marks the location of 08 and 1508E.

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TABLE 3. Dates of OLR minima at 1508E associated with MJO elevation of ;600 hPa of the 320-K surface within the and OKW events between October and December 1974–2006 for axis of the western NP ridge (Fig. 8a). The ET of Orchid ˜ Nino-3.4 dynamic height values greater than one standard de- and Pat (OP) occurred between 14 and 16 October viation above the long-term mean. Events were identified following Roundy and Kravitz (2009). in association with anticyclonic wave breaking period one (AWB1; e.g., Thorncroft et al. 1993) on the 320-K Dates Dates Dates surface. AWB1 resulted in a zonal contraction of the 3 Nov 1982 11 Nov 1990 26 Nov 1997 downstream trough over the central NP, and produced 11 Dec 1982 17 Dec 1990 25 Dec 1997 a PV distribution consistent with a Kona low (KL1) to the 7 Oct 1986 24 Oct 1991 2 Oct 2002 northwest of Hawaii on 16 October (Fig. 8b; Simpson 13 Nov 1986 30 Dec 1991 22 Nov 2002 10 Dec 1986 14 Oct 1994 9 Nov 2004 1952; Ramage 1962; Morrison and Businger 2001; Otkin 11 Dec 1989 8 Dec 1994 and Martin 2004; Moore et al. 2008). A quasi-zonal, 4 Oct 1990 21 Oct 1997 enhanced extratropical PV gradient (waveguide; e.g., Hoskins and Ambrizzi 1993) was subsequently estab- (e.g., Harr and Elsberry 2000), and absorbed Pat in the lished across the NP on 17 October (not shown). western NP (Fig. 7a). The time-mean NP jet stream A period of cyclonic wave breaking (CWB1) was (NPJ) subsequently strengthened to ;60 m s21 in re- observed over the Sea of Okhotsk as evidenced by the sponse to (i) potentially warm upper-level outflow from ‘‘PV hook’’ (e.g., HBK95; Bosart 1999) on 18 October Ruth (suggested by inset IR imagery at 0000 UTC (Fig. 8c). CWB1 preceded short-lived downstream ridge 26 October) in the equatorward-entrance region of the and trough amplification (R1 and T1 in Figs. 2a,b) over NPJ between 24 and 27 October (Fig. 7b) and (ii) in- the central NP by 20 October (Fig. 8d). A second period tensification of the subtropical anticyclone in association of AWB (AWB2) and quasi-zonal flow again developed with anomalous tropical convection located near 1658E over the central NP and produced a second Kona low (Fig. 5). Ruth recurved and underwent ET equatorward (KL2) feature on 22 October (Fig. 8e). The aforemen- of the time-mean NPJ between 26 and 31 October, and tioned outflow from TC Ruth (R) and the eastward subsequently weakened as an EC (Fig. 7b). progression of anomalous tropical convection sub- The locations of TCs Orchid (O) and Pat (P) are sequently strengthened the NPJ over the western NP marked by localized maxima of layer-averaged 925– between 22 and 24 October (Figs. 8f,g). 850-hPa cyclonic (;2.0 3 1024 s21) relative vorticity and The quasi-zonal NPJ extended eastward in proximity PV [.1.5 PV units (PVU; 1 PVU [ 1026 Km2 kg21 s21)] to KL2 between 24 and 26 October and influenced cy- on the 320-K potential temperature (hereafter 320 K) sur- clogenesis over the central NP and a second period of face to the south of the Kamchatka Peninsula at 0000 UTC CWB (CWB2) by 26 October (Fig. 8g). Cyclogenesis on 14 October (Fig. 8a). Potentially warm anticy- occurred as a region of high PV air, located over the Sea clonic outflow from these TCs is indicated by a pressure of Okhotsk on 24 October, merged with the KL2 PV

6 2 21 21 FIG. 7. Time-mean streamfunction (contoured every 10 3 10 m s ) and nondivergent wind speed (shaded in m s according to scale) from (a) 11–15 October with the 12-h locations of TCs Orchid and Pat. (b) As in (a), but from 26 to 30 October and for TC Ruth. Inset IR imagery is shown at 0000 UTC 7 and 26 October, respectively. Symbols denote categorical intensity and tropical characteristics.

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26 2 21 21 FIG. 8. The PV units [(1 PVU [ 10 Km kg s ), shaded according to scale], pressure (black contours every 50 hPa), wind (pennant, full barb, and half barb denote 25, 5, and 2.5 m s21, respectively) on the 320-K isentropic surface, and 925–850-hPa layer- averaged relative vorticity (solid red lines every 0.5 3 1024 s21 starting at 0.5 3 1024 s21) at 0000 UTC on even numbered days (a)–(h) between 14 and 28 Oct 1991. The O, P, R, AWB#, CWB#, and KL# labels indicate the positions or occurrence of TCs Orchid, Pat, and Ruth; anticyclonic and cyclonic wave breaking; and Kona low features.

maximum. This PV merger facilitated the poleward amplification occurred along 1208 and 908W. As a result, transport of low PV air, possibly influenced by deep regions of high and low PV air were transported into moist convection and latent heat release in the vicinity western and central North America, respectively (Fig. 8h). of KL2, into the Bering Sea and the formation of a Rex Although the ET of TCs Orchid, Pat, and Ruth were block over the central NP by 28 October (Fig. 8h). The relatively uneventful with respect to significant cyclone above processes involving KL2 and the poleward flow reintensification and downstream flow amplification, downstream is consistent with the composite 300-hPa these TCs, in conjunction with the progression of the geopotential height and wind over the central NP in MJO and OKW into the central Pacific, influenced the Figs. 6a,b. Downstream development was subsequently development of zonal waveguides formed by subtropical evident between 26 and 28 October as trough and ridge anticyclogenesis and AWB (Schwierz et al. 2004; Martius

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2 22 FIG. 9. Vertically averaged (1000–50 hPa) Ke (m s , contoured and shaded) from 10 Oct to 9 Nov 1991 averaged between 408 and 608N and displayed from 908Eto08. Labels a, b, and A–F located Ke maxima are discussed in section 5. The dashed line indicates the phase speed c for eastward-progressing Ke in the eastern Pacific. The group velocities cg between Ke maxima A and C and between C and E are noted. et al. 2008). The relative importance of the interactions for the initiation of downstream development (e.g., between the MJO, OKW, and TCs and the subsequent Chang 1993). evolution of the extratropical flow during this event cannot be explicitly determined from the current in- 5. Downstream development vestigation. The results, however, suggest a possible re- a. Eddy kinetic energy lationship between the MJO, OKW, and TCs, and the subsequent evolution of the extratropical flow. For ex- A time–longitude analysis of vertically averaged Ke be- ample, the interaction of outflow associated with TC tween 10 October and 9 November is presented in Fig. 9.

Ruth and the NPJ was likely modulated by the upper- High concentrations of Ke between 13 and 16 October level circulation induced by subtropical anticyclogenesis coincided with locally enhanced meridional flow in the as the MJO and OKW progressed across the western western and central NP between 1308W and 1308Eas Pacific (Figs. 5 and 7b). It is therefore possible to spec- a result of the recurvature and ET of TCs Orchid and Pat ulate that anomalous tropical convection may influence (Fig. 8a). Subsequently, high concentrations of Ke were the subtropical circulation in a way that not only in- found within an envelope of slowly eastward-progressing tensifies the NPJ, but also determines whether or not (c ’ 2ms21) maxima that extended from 1808 to a recurving TC and its outflow can interact with the 1308W between 18 October and 9 November. This broad extratropical waveguide. In the present case, the sub- Ke maximum corresponded to a region of anomalous sequent intensification of the zonal waveguides and southerly flow situated between negative and positive periods of CWB appeared to be sufficient conditions 300-hPa streamfunction anomalies in the central and

Unauthenticated | Downloaded 10/11/21 03:25 AM UTC 2558 MONTHLY WEATHER REVIEW VOLUME 138 eastern NP (Fig. 3b), located ;608 downstream of the Ke growth over the central NP near 1808 and was collo- eastward-progressing tropical convection coupled to the cated with the envelope of slowly eastward progressing

OKW (Fig. 5). Two episodes of downstream develop- features discussed in section 5a. The Ke decay via BC ment, a–b and A–E, were initiated from this broad Ke occurred along 1208W between 24 and 27 October and maximum between 18 and 23 October. likely inhibited the progression of the initial downstream

Evidence of downstream Ke propagation and the ﬁrst development event (a–b) into North America (Fig. 10c). downstream development episode (a–b) was observed After 27 October, Ke growth via BC was located over in conjunction with CWB1 on 18 October near 1408E North America and illustrates the modulation of DBD (Fig. 8c). However, this downstream development epi- (A–E) to over the western North Atlantic and the pre- sode was short lived and failed to progress beyond dominant contribution to Ke growth within Ke maxi- ;1208W (Fig. 9). The second downstream development mum F (Fig. 10c). Barotropic processes weakly contributed episode (A–E) was subsequently initiated in conjunc- toward Ke growth between 19 and 23 October in the tion with CWB2 on 23 October and progressed from central NP and toward Ke decay in the western North 1708Wto08 between 23 and 31 October (Fig. 9). This Atlantic between 28 and 31 October (Fig. 10d). Com- downstream development episode was initiated in con- pared to the contributions to the Ke tendency via AGF junction with the development of Ke maxima A and B in divergence and BC, the contributions via barotropic the central and eastern NP between 23 and 25 October processes were considerably weaker [consistent with

(Fig. 9). The generation of Ke maximum C continued the Orlanski and Katzfey (1991) and Chang (1993)]. progression of Ke across North America and occurred The AGF divergence and BC terms from (2) were following the large-scale ﬂow ampliﬁcation character- analyzed in proximity to Ke maxima A–E for the period ized by a negative PNA regime (Fig. 4). After 27 Oc- of DBD between 24 and 30 October (Figs. 11–14). The tober the group velocity of Ke maxima C–E increased analysis of Ke maxima A–E is complemented by ana- 21 (from ;18 to ;30 m s ), suggesting Ke modulation in lyses of the tropospheric thermal and PV structures to the presence of baroclinic processes and evidence of reconcile linkages between Ke centers and observed DBD (Chang 1993). synoptic-scale features. The Ke maximum A (hereafter Downstream development episode A–E, likely mod- A, B, etc.) was located to the east of KL2 and was dis- ulated by baroclinic processes and encompassing the sipating as inferred by the AGF vectors extending cyclogenesis of the PS and EC1, will be subsequently poleward toward B on 24 October (Figs. 11a,c). The referred to as DBD whereas all other references to down- Ke growth within B on 24 October was suggested via stream Ke progression will be referred to as ‘‘downstream AGF convergence along the eastern periphery of the 2 22 development.’’ The generation of Ke in conjunction with 300 m s Ke contour and in a region where the NPJ the cyclogenesis of EC2 over North America occurred was quasi zonal near 508N (Fig. 11c). Further growth of separate from the period of DBD (label F in Fig. 9), and B occurred as advection of low PV air by the 300-hPa will not be emphasized. DBD episode A–E between the irrotational wind increased and enhanced AGF con- central NP and the North Atlantic between 24 and 31 vergence over the Aleutian Islands between 24 and October is next investigated through an analysis of select 26 October (Figs. 11a,d and 12a,d). terms from the Ke tendency equation. The Ke maximum B peaked in magnitude (.600 m2 s22) in conjunction with persistent upstream b. Eddy kinetic energy budget AGF convergence and downstream BC on 26 October

Contributions to the Ke tendency from (2) for the Ke (Figs. 12a,b). The Ke growth via BC occurred in an en- distribution shown in Fig. 9 are presented from 19 Oc- vironment characterized by cold air advection (sug- tober to 3 November in Fig. 10. The dominant contri- gested by negative 1000–500-hPa thickness advection) bution to the Ke tendency is arguably KEF divergence, and was collocated with a coherent region of high PV air which inﬂuences to the horizontal translation of Ke. The (i.e., a cyclonic PV anomaly) to the west of a cyclone along average KEF divergence for an enclosed Ke center (e.g., the west coast of the United States (Figs. 12c,d). The Ke 2 22 the Ke enclosed within the 300 m s contour), how- maximum B subsequently weakened by 27 October in ever, is approximately zero; balanced by upstream KEF association with AGF divergence, while downstream, C divergence and downstream KEF convergence (Fig. 10b; developed via AGF convergence (Fig. 12a inset). e.g., Orlanski and Sheldon 1993). Positive and negative The West Coast cyclone redeveloped over the Rocky contributions to the Ke tendency (hereafter Ke growth Mountains and was associated with a large region of and decay, respectively) for individual enclosed Ke cen- warm air advection over the central and north Plains ters (labeled a, b, and A–F) were mostly determined by on 28 October (Fig. 13c). The thermally direct process

AGF divergence (Fig. 10a). The BC also contributed to rapidly strengthened the Ke within maximum C via BC

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FIG. 10. Contributions to the vertically averaged Ke tendency via (a) ageostrophic geopotential ﬂux divergence, (b) Ke ﬂux divergence, (c) baroclinic conversion, and (d) barotropic processes. Positive (negative) values of the Ke 2 22 2 22 tendency are plotted in red (blue) at 100 and 300 (2100 and 2300) m s with the vertically averaged Ke (m s , shaded according to scale) between 24 October and 3 November, from 1208E and 08, and averaged between 408 and 608N. Labels a, b, and A–F locate Ke maxima discussed in section 5.

and influenced the development of D via AGF conver- with a partition of Ke maximum D into D and E located gence on 28 October (Figs. 13a–c). Warm air advection along the PV streamer with enhanced contributions to persisted into 29 October and contributed to an ampli- the Ke tendency via BC over eastern Canada and the fication of the large-scale flow through QG height ten- western North Atlantic (Figs. 14b,c). The Ke growth via dency principles and continued Ke growth via BC within BC occurred in proximity to D in association with ther- C. As a result, AGF persisted between C and D. mal advections along the PS cold and warm fronts, and in The large-scale flow over eastern North America proximity to E in association with the frontogenetic underwent AWB on 30 October in conjunction with ageostrophic circulation within the entrance region of poleward-displaced low PV air over eastern Canada and the North Atlantic jet on 30 October (Figs. 14b,c). an equatorward-displaced filament of high PV air [i.e., Further evidence for the partition of D into D and E PV streamer (e.g., Appenzeller and Davies 1992)] over is observed in association with horizontal stretching de- the western North Atlantic (Fig. 14d). AWB coincided formation along the large-scale axis of dilation suggested

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2 22 FIG. 11. (a) Vertically averaged Ke (m s , shaded according to scale) and AGF vectors (black) and AGF divergence contoured every 200 m2 s22 day21 starting at 2900 m2 s22 day21 with negative (positive) values contoured blue (red). (b) As in (a), but for baroclinic conversion. (c) 300-hPa wind speed (m s21; shaded according to scale), 1000–500-hPa thickness [dashed contours every 6 dam with value less (greater) than 540 dam colored blue (red)], and SLP (contoured black every 4 hPa). (d) 300-hPa PV (PVU; shaded according to scale), velocity potential (contours every 2 3 106 m2 s21, negative values dashed), irrotational wind vectors (m s21; reference vector given at 10 m s21), and 700-hPa vertical velocity (white contours every 3 mbar s21 with negative values dashed) at 0000 UTC 24 Oct 1991. The heavy 2 22 solid 300 m s contour in (a) and (b) indicates the bounds of the speciﬁc area averaged in Fig. 15. Ke centers from Fig. 9 are labeled in (a) and (b). High (H) and low (L) pressure centers are marked in (c) with additional synoptic features mentioned in section 4. The ‘‘Xs’’ in (d) mark the locations of maxima in the PV distribution. by the southwest–northeast-oriented thickness gradient The warm seclusion process was likely associated with a over Atlantic Canada, the thinning PV streamer, and the reduction in baroclinic conversion, enhanced barotropic production of two PV anomalies to the southeast of New decay, and frictional dissipation (not shown).

England and Greenland (Figs. 14c,d). The Ke maximum E subsequently became associated with the life cycle of c. Summary of DBD EC1 on 31 October (not shown), whereas D weakened as the PS underwent the warm seclusion process and A summary of the Ke and the contributions to the Ke collocated with the 1000–500-hPa thickness minimum to tendency from (2), including the residual term (calcu- the southeast of New England on 31 October (Fig. 14c). lated by approximating the time derivative of Ke as

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FIG. 12. As in Fig. 11, but for 0000 UTC 26 Oct 1991 with (a) inset at 1800 UTC 26 Oct 1991. a finite difference between consecutive 6-h time pe- a period of growth via BC and decay via AGF divergence riods), is presented in Fig. 15 for periods of Ke . on 27–30 October (Fig. 15c). Recall that C was briefly 2 22 300 m s for maxima B, C, and D. The Ke growth rate, influenced by AGF convergence while over the western defined by dividing the vertical average of each quantity United States on 26 October (Fig. 12a inset). AGF di- on the rhs of (2) by the vertically averaged Ke within the vergence within C promoted Ke growth within Ke maxi- area enclosed by the 300 m2 s22 contour (Fig. 15a), was mum D via AGF convergence on 28–30 October used to illustrate the quasi-Lagrangian, normalized (Fig. 15d). Over the northwest Atlantic, D was also de- contribution of each term to the Ke tendency. The fined by a period of growth via BC on 28–30 October. 2 22 300 m s Ke contour was chosen to isolate Ke centers Barotropic processes weakly contributed to the decay of from one another and to contain the Ke centers of interest Ke within B and D, and Ke growth within C (Figs. 15b–d). throughout a majority of their life cycles. Finally, the residual term influenced the decay of all three

The Ke growth rates demonstrate that Ke maximum B Ke maxima. The large contributions via the residual term was deﬁned by a period of growth via AGF convergence likely represent frictional dissipation, but could also have on 24–25 October and via BC on 25–26 October, and arisen from errors associated with the horizontal, vertical, by a period of decay via AG divergence on 27 October and temporal resolution of the ERA-40 and the ﬁnite-

(Fig. 15b). Downstream, Ke maximum C was deﬁned by difference approximation for the time derivative of Ke.

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FIG. 13. As in Fig. 11, but for 0000 UTC 28 Oct 1991.

6. Discussion America may have influenced the eastward progression of Ke into the North Atlantic. The NP overview and Ke analysis illustrated that con- The DBD episode appeared to be strongly influenced ditions were favorable for Ke generation and downstream by the large-scale flow evolution in the vicinity of 1208W development in the central NP following a complex in- that favored Ke growth via BC over North America terplay between the modulation of the extratropical flow (Fig. 10c). The results suggest that the development of via anomalous tropical convection; the recurvature and ridge R2 over the Gulf of Alaska and trough T2 over the ET of western NP TCs Orchid, Pat, and Ruth; and the western United States (i.e., a negative PNA regime) subsequent evolution of the extratropical. Two down- largely contributed toward DBD across North America. stream development episodes were initiated over the NP Further validation is observed from the flow patterns in conjunction with AWB and CWB. The first episode prior to three DBD episodes over North America ana- (a–b) failed to progress eastward across North America, lyzed by Orlanski and Sheldon (1993) and Decker and whereas the second episode (A–E) became the DBD Martin (2005) that were characterized by PNA index episode. The fact that similar Rossby wave breaking values as low as 22.35, 21.93, and 21.75, respectively. events in the central NP produced dissimilar downstream It is interesting to note that the failed downstream responses suggests the basic state over western North development episode (a–b) occurred during a period

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FIG. 14. As in Fig. 11, but for 0000 UTC 30 Oct 1991. characterized by a positive and trending-negative PNA by negative and trending-positive PNA regimes, mid- regime. latitude baroclinic instability (and therefore the poten-

A likely mechanism for enhancing Ke growth via BC tial for cyclogenesis and Ke growth via BC) could and promoting DBD across North America with a flow become maximized where equatorward-displaced high pattern characterized by a negative PNA regime comes PV air interacts with regions of enhanced midlatitude from HBK95 and B96. HBK95 and B96 suggested that baroclinity in the presence of poleward-displaced cor- a ridge located over the Gulf of Alaska and a trough ridors of high moisture. over the western United States can influence cyclogen- In the present case, cyclogenesis occurred several esis by establishing a conduit for the extraction of high times during the DBD episode, as high-latitude PV PV air of Arctic origin into the midlatitudes over west- anomalies became displaced equatorward and resulted ern North America, similar to that observed in this case in Ke growth via BC (Figs. 12–14b,d). A summary of the (Figs. 2c,d and 8f–h). The process can also be extended noteworthy cyclonic PV anomalies and their proximity to a flow pattern characterized by a positive PNA regime to poleward-displaced corridors of high moisture con- (or a trending-positive PNA regime) and the establish- tent [represented by precipitable water (PW) plumes] ment of a conduit for the extraction of high PV air into during the cyclogenesis of the PS, EC1, and EC2 is eastern North America. In flow patterns characterized presented Fig. 16. The coupling of the PV anomalies

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FIG. 15. (a) Vertically averaged Ke and (b)–(d) vertically averaged Ke tendency (solid line) and contributions to the Ke tendency (stacked columns colored according to key at bottom of ﬁgure) between (b) 0000 UTC 24 Oct and 1200 UTC 27 Oct, (c) 1200 UTC 27 Oct 2 22 and 1800 UTC 31 Oct, and (d) 0000 UTC 28 Oct and 1800 UTC 31 Oct 1991. Values are averaged within the 300 m s Ke contour identiﬁed as the heavy solid contours in Figs. 11–14. The Ke tendency and contributions terms are presented as a normalized growth rate 21 (day ) by dividing the vertical average of each quantity by the vertically averaged Ke.

with regions of enhanced low-level baroclinity is repre- and energetics, respectively, of baroclinic wave growth sented by the 700-hPa Eady baroclinic growth rate s for within the midlatitude storm track. baroclinic instability defined by Figure 16 illustrates the tracks of three long-lived PV anomalies: PV-PS, PV-EC1, and PV-EC2 as they cir- 1 cumnavigated the NH.2 The PV anomalies PV-PS and s 5 0.31fUzN . (3) PV-EC1 can be located in proximity to Ke maxima D and E on 28 and 30 October in Figs. 13d and 14d, re- Here, f is the Coriolis parameter, Uz is the vertical derivative of the horizontal wind between 850 and spectively. The PV anomaly PV-EC2 can be located to 600 hPa, and N is the Brunt–Vaïsa¨la¨ frequency over the the west of the Kamchatka Peninsula on 24 October and same layer. The PW plumes in Fig. 16 represent regions was discussed briefly in section 4b for its role in CWB2 where the effect of the static stability parameter can be (Figs. 8f and 11a). Mergers of these PV anomalies with reduced, in the presence of upward motion and latent other regions of high PV are noted between PV-PS heat release in a moist environment, and lead to larger and the PV anomaly associated with HG (PV-HG), baroclinic growth rates than those identified by the Eady and between PV-EC2 and a second PV anomaly that baroclinic growth rate parameter. The Eady baroclinic growth rate was used by Hoskins and Valdes (1990) and 2 These PV anomalies were found to merge and fracture with Chang and Orlanski (1993) to interpret a climatology time; however, only the primary maxima are displayed for clarity.

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FIG. 16. Tracks of vertically averaged (1000–50 hPa) PV anomalies (from the 30-day time mean) associated with the development of the PS, EC1, and EC2 at 1200 UTC 29 Oct, 30 Oct, and 1 Nov 1991, respectively. Day numbers and open circles (closed circles) mark the locations of the PV anomalies at 0000 (1200) UTC. The outlined sectors bound the aforementioned dates for vertically averaged PV (PVU; shaded according to blue–gray scale), precipitable water (mm, shaded according to red scale), and the Eady baroclinic growth rate parameter at 700 hPa (contoured every 0.25 day21 starting at 1.5 day21). The outlined sectors are chosen for the time period closest to the maximum in Eady baroclinic growth. Inset: PV anomaly latitude as a function of time.

influenced the cyclogenesis of EC2 (PV-EC29; Fig. 16). An alternative perspective for enhancing Ke growth On the other hand, a PV fracture is noted near 1808 at via BC and promoting DBD across North America high latitudes on 25 October and highlights the Arctic comes from the distribution of cool-season precipita- source region for PV-EC1. tion and cyclone tracks over the United States during A time–latitude summary of the PV anomaly locations flow patterns characterized by negative PNA regimes. (Fig. 16 inset) demonstrates that the PV anomalies be- Archambault et al. (2008) showed that synoptic-scale came displaced into the midlatitudes (south of ;508N) cool-season precipitation over the northeastern United between 28 October and 3 November (Fig. 16). The Eady States was enhanced during negative PNA regimes (cf. baroclinic growth rate subsequently reached a maximum their Figs. 3 and 4). This finding is consistent with to the south and east of the PV anomalies as they collo- previous results that the PNA regime and precipitation cated with regions of midlatitude baroclinity, and influ- in parts of the Northeast and Ohio Valley is negatively enced cyclogenesis and Ke growth via BC. The large correlated (Leathers et al. 1991; Notaro et al. 2006). (.2.5 day21) Eady baroclinic growth rate located be- Similarly, Rodionov (1994) and Isard et al. (2000) dem- tween PV-PS and the HG PW plume highlights the ex- onstrate that cyclones responsible for a bulk of the cool- plosive baroclinic growth observed during the genesis of season precipitation over the Great Lakes region tended the PS and EC1 at 1200 UTC on 29 October. to exhibit a more southwest–northeast track (e.g., the

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Colorado storm track) associated with the more-west- upstream AGF convergence and matured in association ward location of the upstream trough during a negative with BC. The analysis, however, also suggested that the PNA regime. The more meridional storm track sub- DBD episode was primarily inﬂuenced by the growth of sequently inﬂuenced a poleward transport of Gulf of a Ke maximum over western North America almost en- Mexico moisture into the Great Lakes region (Rodionov tirely in association with BC.

1994). The PNA regime and Ke growth via BC over North Positive contributions to the Ke tendency over North America would then also be negatively correlated if a America via BC occurred with a ridge located over the bulk of the cool-season precipitation in the northeastern Gulf of Alaska and a trough over the western United U.S. and Great Lakes region resulted from thermally States that was characterized by a strongly negative and direct processes such as warm air advection, frontogen- trending-positive PNA regime. The amplification of the esis, and/or deep moist convection. flow was likely influenced by the presence of anomalous The evidence suggests that the evolution of a ridge tropical convection coupled to the MJO and an OKW in over the Gulf of Alaska and a trough over the western the tropical Pacific. Results from a composite analysis of United States, indicative of a negative and subsequently 19 MJO and OKW events suggested that anomalous trending-positive PNA regime, influences baroclinic in- tropical convection influenced the development of pos- stability over North America, a more active storm track, itive upper-level geopotential heights over Alaska and and enhanced Ke growth via BC. As a result, DBD may negative geopotential heights over the western United be more likely to progress across North America during States (i.e., a negative PNA regime). a flow pattern initially characterized by a negative PNA The evolution of the large-scale flow prior to the PSs regime than during a flow pattern characterized by a of 1991 demonstrates that the initiation and progression positive PNA regime in the eastern NP and western of DBD across North America may be influenced by the United States. eastward progression of anomalous tropical convection across the tropical Pacific and the subsequent configu- ration of the large-scale flow over the eastern NP and 7. Conclusions western North America. The results show that flow The ‘‘Perfect Storms’’ of 1991 were a series of three patterns characterized by a negative PNA regime, or ECs that adversely impacted the North Atlantic and trending-positive PNA regime, is most likely to be as-

North America. While previous studies highlighted an sociated with enhanced Ke growth via BC over North amplification of the large-scale flow prior to the de- America. In the present case, Ke growth via BC over velopment of the PS, EC1, and EC2, this study in- western North America facilitated the progression of vestigated the tropical and extratropical flow over the DBD across the continent into the North Atlantic and NP to ascertain the physical processes that contrib- produced conditions favorable for the development of uted to the large-scale flow amplification and a sub- the PSs. sequent episode of DBD across North America. Downstream development in the NP occurred twice before progressing across North America as an epi- Acknowledgments. This research was supported by sode of DBD. The first downstream development National Science Foundation Grants ATM-0304254 episode, initiated on 19 October by CWB along and ATM-0553017. Discussions with Prof. Paul Roundy a zonal waveguide established by the recurvature and (University at Albany, SUNY) contributed to the man- ET of TCs Orchid and Pat, failed to progress into uscript. The authors thank Olivia Martius (Institute for North America. The second period of downstream Atmospheric and Climate Science, ETH Zu¨rich) and two development, initiated on 24 October by CWB along anonymous reviewers for comments that helped to clarify a second zonal waveguide established by a zonal and improve the manuscript. The authors also thank Carl elongation of the NPJ in association with TC Ruth and Schreck (University at Albany, SUNY) for providing the the progression of anomalous tropical convection into the Hovmo¨ller plotting routine. central Pacific, progressed across North America as an episode of DBD and featured the development of the PS and EC1. REFERENCES An analysis of the contributions to the Ke tendency for Appenzeller, C., and H. C. Davies, 1992: Structure of strato- individual Ke maxima generally agreed with previous spheric intrusions into the troposphere. Nature, 358, studies by Chang (1993), Orlanski and Sheldon (1993, 570–572. 1995), and Danielson et al. (2004, 2006) of DBD where Archambault, H. M., L. F. Bosart, D. Keyser, and A. R. Aiyyer, downstream Ke maxima initially grew in association with 2008: Influence of large-scale flow regimes on cool-season

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