1. Introduction
The mid-latitude flow can interact with the tropics in many ways. One such way involves
the progression of a tropical cyclone (TC) into the mid-latitudes where it can come under the
influence of transient upper-level disturbances. This tropical-extratropical interaction known as
extratropical transition (ET) has been the focus of a number of studies in the recent literature (see
Jones et al. 2003 for a thorough review). A second way in which the extratropics can exert an
influence of the tropics is by providing a non-classical mechanism for tropical cyclogenesis to
occur. Typically, the initial TC development occurs farther poleward (25˚- 30˚N) than TCs that
form from African waves or along monsoon troughs in the Atlantic and Pacific ocean basins,
respectively. [Recent work by Davis and Bosart (2003, 2004) has identified several examples of
tropical cyclogenesis originating from mid-latitude precursor disturbances.]
Numerous studies in the literature have shown that ET is a common occurrence in the
Atlantic (Hart and Evans, 2003), western Pacific (Klein et al. 2000), and Australian (Foley and
Hanstrum 1992; Sinclair 2002) basins. The reader is referred to Jones et al. (2002) for a complete
review of the topic. Hart and Evans (2003) found that relative to the number of TCs, ET is much
more frequent in the Atlantic than in any other basin. Hart and Evans estimate that nearly 50% of
all Atlantic tropical cyclones undergo ET. In the western Pacific, approximately 25% of all
western Pacific tropical cyclones undergo ET (Klein et al., 2000). The only basin to experience
frequent tropical cyclogenesis and not experience ET is the eastern North Pacific basin. The
potential for ET events in the eastern Pacific has come under increasing scrutiny because of the
impact these systems have on the precipitation totals in the southwestern United States
(Corbosiero et al, 2009).
[Broadly speaking, ET is the conversion of a symmetric, vertically stacked, warm-core
tropical cyclone with a maximum intensity in the lower troposphere into an asymmetric, coldcore and tilted extratropical cyclone with a maximum intensity in the upper troposphere. The
evolution of the ET is sensitive to the interaction of the decaying tropical cyclone and the midlatitude circulation.] Over the last several years, several different studies have attempted to
classify the evolution of an ET event. Early case-studies of ET in the northwest Pacific identified
three separate types of ET evolutions: 1) complex when the tropical cyclone interacted with a
surface baroclinic zone; 2) compound when the tropical cyclone interacted with a surface lowpressure system (Sekioka 1956, 1970, 1972 a, b; Matano and Sekioka 1971a, b; Mohr 1971;
Brand and Guard 1979); and 3) straying when the tropical cyclone remnants dissipate when
moving into the mid-latitude environment (Sekioka and Matano 1990). Foley and Hanstrum
(1994)
defined
two
types
of
ET
over
the
southeast
Indian
Ocean
as
being
either cradled or captured based on the interactions between the decaying tropical cyclone and
the midlatitude circulation. Recently Hart (2002) developed a cyclone phase diagram from two
parameters calculated directly from the three-dimensional height field. One parameter
determines whether the cyclone exhibits a warm or cold core. The second parameter provides a
measure of the asymmetries in the thermal structure. The Hart phase space diagrams can be used
to examine the lifecycle of all cyclones and can determine the approximate time that
extratropical transition occurred.
As a TC translates poleward, it often increases its forward motion, encounters cooler seasurface temperatures (SSTs), and increased vertical shear as it begins to interact with a higher
latitude upper-level trough/jet system during the ET process. In satellite imagery, the inner core
of the tropical cyclone loses its symmetric appearance and gradually takes on the appearance of
an extratropical cyclone (EC). The increase in translation speed further contributes to the
asymmetric structure. Klein et al. (2000) showed that the inner-core of the tropical cyclone
becomes asymmetric as strong frontogenesis develops on the poleward side of the tropical
cyclone center. The system gradually takes on the appearance of a traditional mid-latitude
cyclone. Further indications include an increase in the radius of gale force winds, asymmetries in
the wind and precipitation fields, and a decrease in sea surface temperature beneath the tropical
cyclone.
ET is clearly reflected in the cloud and precipitation patterns of the transitioning TC. The
nearly symmetric wind and precipitation distributions that are concentrated about the circulation
center of the tropical cyclone evolve to produce strong and expansive asymmetric wind and
precipitation distributions. During ET, the cloud and precipitation shield expands ahead and to
the left of the tropical cyclone center, in response to the synoptic-scale forcing associated with
the approaching trough. ET events can be tremendous rain makers as the large-area of
synoptically-driven ascent can act on abundant tropical moisture. Hurricanes Agnes (1972,
Bosart and Dean 1991; Bosart and Carr 1978; DiMego and Bosart 1982a,b) and Floyd (1999,
Atallah and Bosart 2003; Colle 2003) are two such historic examples. Due to the expansion of
the area covered by clouds and precipitation when the tropical cyclone moves poleward, heavy
precipitation can occur over land without the tropical cyclone center making landfall.
In addition to interacting with a pre-existing tropical cyclone, mid-latitude troughs can
also trigger disturbances that could evolve into tropical cyclones. Higher latitude upper-level
cold-core disturbances on occasion can initiate ordinary baroclinic cyclogenesis in the
subtropics. A subset of these baroclinic developments may in turn become TCs via the tropical
transition (TT) process where TT is used to describe the transition of a cold-core baroclinic EC
into a warm-core TC.
Vertical wind shear associated with the higher latitude upper-level
disturbance acts to organize the developing convection and generate low-level vorticity (Bosart
and Bracken 2002). Forcing for large-scale ascent associated with the higher-latitude trough can
be effective in triggering deep convection because deep instability can be created equally well by
cooling aloft ahead of the upper-level disturbance as opposed to hating from below. In the
environment favorable for ascent a positive feedback process between deep convection and
cyclonic vorticity production by stretching in convectively driven updrafts acts to weaken the
vertical wind shear and allows the growing disturbance to develop a warm core. Shear reduction
and attendant upper-level potential vorticity (PV) destruction through diabatically driven
processes can weaken and eliminate the original triggering baroclinic disturbance, resulting in
the development of a warm-core disturbance and anticyclonic outflow aloft via the TT process
provided sea surface temperatures (SSTs) are sufficiently warm (at last 23-24 C) to sustain the
deep convection . Notable examples of TT include TC Diana (1984; Bosart and Bartlo 1991),
TC Michael (2000; Davis and Bosart 2003, 2004), TC Humberto (2001; Dvis and Bosart 2006),
and TC Catarina (2004), the first documented western South Atlantic TC (McTaggart-Cowan et
al. 2006) .
The purpose of this paper is to examine in detail the remarkable events of late August
1992. During this period, the ET of TC Lester over the southwestern United States, the first-ever
documented ET in this region occurred. At the same time, powerful TC Andrew devastated
South Florida and made a second landfall along the central Louisiana coast. Andrew was then
steered poleward ahead of the same baroclinic system responsible for the ET of Hurricane Lester
just days prior. Finally, the upscale impact of Lester and Andrew on the synoptic-scale flow led
to the downstream development and the generation of a “vorticity seed” in the western Atlantic.
Despite favorable sea-surface temperatures and low vertical wind shear, no tropical system
developed. All of these events are directly related to each other.
The paper is organized as follows: Section 2 describes the data source and methodology.
Section 3 provides a brief description of Hurricanes Lester and Andrew. The climatology and
results are presented in Section 4.