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Modified from Rogers and Rohli, 1991.

Modified from Rogers and Rohli, 1991.

Northers in Texas; Nortes, Chocolatero, and Tehuano in México; Papagayo in Nicaragua and Guatemala; Atem-poralado in Honduras; and Invierno de las Chicharras in Venezuela.

Dalavalle and Bosart (1975) indicate that comparatively weak anticyclones (<1045 hPa) can also produce severe freezes over North America. The cold waves of the 1990s have been among the 10 worst events this century in the midwestern United States in terms of low temperature values (Rogers, 1997), but were associated with anticyclones of only moderate strength. The mid-January 1994 cold wave was associated with two fast-moving highs having intensities under 1040 hPa, the second of which was advected over the Plains behind an intense cyclone that moved across the Great Lakes. The early February 1996 cold wave was ushered in by the cold air advection behind a strong Alberta storm moving across the Midwest, producing an anticyclone that just exceeded 1040 hPa in intensity. Even the Florida freeze of 19-20 January 1997 was associated with an anticyclone of about 1042 hPa in intensity. The trajectory of this anticyclone was characteristic of that for Florida advective freezes (which are the most severe, as described in Section 3.4.2), but its intensity was vastly below normal and the bulk of the freeze damage was probably due to radiative cooling of the air.

As the cold front of a polar outbreak reaches the Gulf of México or the United States East Coast, a strong baro-clinic zone is typically established in conjunction with the warm adjacent ocean that is the site of subsequent cyclone development. Thus, as the anticyclone at the core of the polar air mass enters the United States, traveling southward, cyclogenesis begins occurring along the coastal areas. The developing cyclone will frequently evolve into a ferocious nor'easter along the eastern coast of the United States (Kocin and Uccellini, 1990), producing tremendous snowfalls along the Atlantic coast and thereby creating many societal impacts of its own. One example is the East Coast storm accompanying what is still regarded as the worst cold wave on record in many parts of the country—that of 8-11 February 1899 (Kocin et al., 1988). The East Coast cyclone developed while the cold wave was advancing across the midsection of the continent, producing a strong baroclinic zone across the southeastern United States. Intensification of the pressure gradient occurred across the anticyclone/cyclone couplet, producing strong cold air advection and a further decline in air temperature and wind chill from the Plains to the East Coast. Schultz et al. (1997) studied the United States su-perstorm of 12-14 March 1993, which developed after polar air, originating over Alaska and western Canada, brought northerlies exceeding 20 m sec-1 and temperature decreases up to 15°C over 24 hr into México and Central America. During this cold surge, topographically channeled northerlies along the Rocky and Sierra Madre Mountains advected cold air equatorward, reaching as far south as 7°N, and the dynamical forcing associated with the low-latitude upper level trough and confluent jet-entrance region over México and Central America, in addition to the topographic channeling, favored the extraordinary equatorward incursion of cold air.

Schultz et al. (1998), Steenburgh et al. (1998), and Klaus (1973) documented midlatitude cold surges affecting México and Central America, based on observations and regional modeling. A Central American cold surge is defined as the leading edge of a cold anticyclone originating poleward of México that has penetrated equatorward to at least 20°N. Although the topographies of the Rocky Mountains and the Sierra Madre undoubtedly play an important role in channeling the cold surge equatorward, different planetary-and synoptic-scale flow patterns can also be conducive to longevity of Central American cold surges. From 177 cases studied, they concluded that 75% of the cold surges had durations of 2-6 days, the same timescale as mobile disturbances in the westerlies. There does not appear to be any relationship between the temperature drop and the duration of the event, although cold surges penetrating to low latitudes (7°-10°N) have a weak tendency to persist longer than those that do not penetrate to low latitudes (15°-20°N). In addition, cold surges tend to reach their most equatorward extent where topographic features impede the progress of cold air; the temperature decreases in the postsurge air do not appear to be related to the latitude of the cold outbreak.

3.3.3. Conceptual Model for North-Central American Cold Surges

As with the South American illustration in Fig. 3, the North American conceptual model (Figs. 4A-4C) shows the evolution of the surface circulation components of a polar outbreak, based on the work of Dallavalle and Bosart (1975), Konrad and Colucci (1989), Kocin and Uccellini (1990), Konrad (1996), and Schultz et al. (1998), for severe North American cold waves. Many different synoptic circumstances can lead to polar outbreaks around North America; we illustrate just one here, which is associated with severe cold waves over the eastern United States.

The polar outbreak begins (Fig. 4A) with a strong polar anticyclone moving away from its source region across south-central Canada into the United States. The polar high likely originates over snow-covered plains and forests of northwestern Canada, acquiring its characteristics from radiative cooling during the long polar nights. The dense, cold air is linked to a large positive

FIGURE 4 Conceptual model of wintertime polar outbreaks in North-Central America. (Adapted from Dallavalle and Bosart, 1975; Kocin and Uccellini, 1990; Schultz et al., 1998; Konrad, 1996; Konrad and Colucci, 1989.)

pressure anomaly at the core of the air mass. The baro-clinic zone that is continually present south of the high encourages its south-southeastward motion by the process of cold air advection. The leading edge of the polar air approaches the Gulf of México in Fig. 4A, where strong baroclinicity is also developing. Within 12-24 hr (Fig. 4B), the cold front has entered México and a substantial cold outbreak begins in that country. Low-temperature records are also being set in the northern United States. A frontal wave develops in the Gulf baroclinic zone. The western boundary of the cold air mass is illustrated here as lying along the Rocky Mountains, a common barrier to the shallow cold air mass.

Within another 24 -48 hr, the polar high reaches Texas (Fig. 4C) and is no longer supported by cold air advection. It is at its greatest peril of dissipating, especially over the relatively warm environment around the Gulf of México. It has weakened substantially, but is being maintained to some extent by the relatively cold continental surface. The cyclone developing in the Gulf in Fig. 4B has slowly migrated northeastward, undergoing a process of secondary redevelopment (Kocin and Uccellini, 1990) that leads to cyclogenesis off the East Coast. The East Coast cyclone will rapidly evolve as an upper air trough of cold polar air (not illustrated) begins to lend support via positive vorticity advection. Severe cold weather and windy conditions now prevail over the southeastern United States. In the final stages (Fig. 4C), both features of the anticyclone-cyclone couplet have received upper level support from the waves in the upper troposphere. The Gulf Coastal anticyclone begins redeveloping eastward, assisted by negative (anticyclonic) vorticity advection occurring ahead of another upper level ridge approaching from the west. It does not greatly reintensify due to its destabilization and proximity to the warm ocean. The cyclone has evolved into a major East Coast winter snowstorm, aided by upper air positive (cyclonic) vorticity advection east of the trough that was, in a large part, created and amplified by the preceding polar outbreak in the Great Plains and the Midwest. The warm front that marks the western boundary of the polar outbreak now tends to move eastward as new western synoptic systems push eastward.

3.3.4. Tracks of Cold-Core Anticyclones and Trajectories of Polar Outbreaks in the Americas

As indicated in the previous sections, there is no direct relationship between the intensity of the cold-core anticyclone and the degree of cooling in regions affected by the polar outbreak associated with that high-

FIGURE 5 Partial track of wintertime cold-core anticyclones related to intense freeze events in southern Brazil. The track indicates the daily paths of the anticyclones from southern Chile, along the Andes, toward southern Brazil (Marengo, submitted).

pressure center. It is also important to consider the track, or path, of the anticyclone and the trajectory of the cold air moving from higher to lower latitudes.

The tracks of some South American anticyclones associated with cold surges (1975, 1979, 1981, 1990, 1993, and 1994), and occasionally freezing conditions in southern and southeastern Brazil, are presented in Fig. 5. Since it was not possible to plot the track of every anticyclone listed in Table 1 due to the lack of weather maps and surface charts for those cases, this figure should not be taken as comprehensive of all cases of major cold surge events. The most prominent characteristics of each path are the eastward trajectory coming from the Pacific, then a turn around the Andes to follow a northward trajectory until it reaches approximately 25°S; there, it turns toward the southeast and southern Brazil-northern Argentina, ultimately reaching the subtropical Atlantic where it is finally assimilated by the subtropical high. In general, the anticyclones take 24-48 hr to travel from the coast of Chile to southern Brazil. There is a tendency for the anticyclones to become somewhat stationary over southern Brazil during severe freeze events, as in 1975, 1981, and 1994.

a) 24 June 1994

b) 25 June 1994

d) 27 June 1994

FIGURE 6 Trajectory of the cold air mass over South America during the intense cold episodes of June 1994. (A) 24 June, (B) 25 June, (C) 26 June, and (D) 27 June. Arrows indicate different pressure levels in the atmosphere. (Adapted from Sanchez and Silva Dias, 1996.)

The strong polar outbreaks of June and July 1994 can be analyzed in conjunction with Fig. 5. The case for 2427 June 1994 is shown in Figs. 6A-6D. On 24 June (Fig. 6A), air coming from the South Pacific reached northern Argentina. On 25 June (Fig. 6B), the trajectory of cold air coming from upper levels above the Pacific turned toward the west and reached western Amazonia, affecting southeastern Peru on 26 June (Fig. 6C). It is interesting to notice that the parcel coming from the middle troposphere above the South Pacific reached central Brazil, while the air parcels from lower levels affected southeast Brazil and eventually reached the coastal region. The 7-10 July 1994 case (Figs. 7A-7D) is significantly different from the June 1994 case, considering the origin of the air parcels. The air parcels that reached western Amazonia and central and southeastern Brazil came from air masses over the South Atlantic. There are indicators that the polar air parcels did not reach such low latitudes as in the June 1994 event. Air came from upper levels of the troposphere and descended until it reached western Amazonia on July 10 (Fig. 7D). This indicates that the air mass reaching southeastern Brazil in July 1994 showed more oceanic characteristics than that of June 1994.

The tracks of surface anticyclones associated with polar outbreaks in North America (listed in Table 2) are presented in Figs. 8A to 8D. Figure 8A shows the path of the strong anticyclones associated with the severe freezes of 1894-95, 1899, 1962, 1983, 1985, and 1989, which produced extensive citrus damage in Florida. It comprises the synoptic setting associated with Fig. 4. The most prominent characteristics of each path are a southward movement across the Plains toward eastern Texas and a subsequent abrupt eastward or northeastward turn toward the Atlantic coast. In general, the anticyclones take 24-48 hr to traverse the distance from the Canadian border to Texas. Rogers and Rohli (1991) indicate that Florida freeze damage generally occurs in the mornings when the high is in Texas. Dalavalle and Bosart (1975) point out that movement of the composite polar anticyclone is linked to changes in forcing mechanisms; cold air ad-vection drives the southward motion, and anticyclonic vorticity advection aloft supports the eastward curvature and reintensification. Figure 8B shows the tracks of slow-moving anticyclones, which generally track east of those in Fig. 8A, crossing over Missouri or Arkansas. This second kind of anticyclone is associat-

a) 7 July 1994

b) 8 July 1994

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