F

60 w eo

d) 10 July 1994

20 s

60°w

20 s

FIGURE 7 Trajectory of the cold air mass over South America during the intense cold episodes of July 1994. (A) 7 July, (B) 8 July, (C) 9 July, and (D) 10 July. Arrows indicate different pressure levels in the atmosphere. (Adapted from Sanchez and Silva Dias, 1996.)

ed with freezes caused by overnight radiative cooling in Florida. They move into the Gulf of México or Atlantic Ocean, becoming part of the warm Atlantic subtropical high. The third set of anticyclone paths (Fig. 8C) is associated with high-pressure areas that move toward Texas, but do not create severe freezes in Florida. Figure 8D shows the paths of a small subset of the many strong anticyclones that are linked to meridional winter flow and unusually cold weather across North America, but do not produce notable impacts across the southeastern United States.

3.3.5. Atmospheric Teleconnection Patterns Associated with Polar Outbreaks in the Americas and Their Interdecadal Variability

North and South American polar outbreaks that produce damaging freezes do not appear to be related to El Niño or La Niña. For southern Brazil, there is no clear signal of El Niño or La Niña impacts at interannual timescales on either the frequency or intensity of polar outbreaks in South America. The coldest events registered in July 1975 and June and July 1994 occurred during transition years between El Niño and La Niña.

What is observed is a tendency for warmer than normal winters during El Niño years with a lower probability of strong polar outbreaks in those winters. However, some of the events listed in Table 1 occurred during some El Niño or La Niña episodes.

Large-scale circulation factors controlling austral winter freeze events over subtropical and tropical latitudes are generally accompanied by major synoptic- to planetary-scale changes in the pressure systems over an extensive part of the Southern Hemisphere. These changes are reflected in a large-amplitude trough of the middle-latitude westerlies. The tropical extension (or penetration) of such large-amplitude troughs appears to be associated with synoptic- to planetary-scale events. The mechanism of downstream amplification across the Pacific into South America generally precedes heavy frost events. This is a well-known wave train of the midlatitudes that is seen to traverse across the Pacific Ocean toward South America, exhibiting the successive intensification of downstream troughs and ridges (Krishnamurti et al., in press).

The occurrence of severe North American freezes in the far southeastern United States has been linked to the positive phase of the Pacific-North American Oscillation (PNA) atmospheric teleconnection pattern

FIGURE 8 Partial track of the strongest wintertime anticyclones, beginning with points in their paths in southern Canada and then across the United States. Positions are based on 12 or 13 October Universal Time Zone (UTC) synoptic charts. The years labeled in (A)-(C) refer to years of Florida citrus freezes, while the tracks in (D) are from the remaining nonfreeze events. (From Rogers and Rolhi, 1991. With permission.)

FIGURE 8 Partial track of the strongest wintertime anticyclones, beginning with points in their paths in southern Canada and then across the United States. Positions are based on 12 or 13 October Universal Time Zone (UTC) synoptic charts. The years labeled in (A)-(C) refer to years of Florida citrus freezes, while the tracks in (D) are from the remaining nonfreeze events. (From Rogers and Rolhi, 1991. With permission.)

(Rogers and Rohli, 1991; Rohli and Rogers, 1993; Downton and Miller, 1993). This prominent mode of low-frequency variability in the Northern Hemisphere extratropics is observed in all months of the year except June and July. Its positive phase is characterized by amplification of the upper tropospheric ridge off the west coast of North America (Fig. 1A), which establishes a strong meridional flow of polar air southward toward the United States and Central America. The PNA positive phase can be forced by the tropical heating associated with El Niño in the tropical Pacific. It has been shown, however, that El Niño is not the only forcing mechanism of the PNA, and while the record of freezes in Florida is dominated by the occurrence of the PNA positive mode, the occurrence of El Niño during polar outbreaks and freezes is only occasional. Cold waves are, of course, synoptic events occurring over 2-6 days, and while the presence of a ridge over the western peripheries of the continents is almost mandatory, the long-duration El Niño is not or is merely coincidental. Furthermore, the nature of winters in the eastern United States associated with El Niño has changed since

1977, with relatively mild winters consistently prevailing in events since that year. El Niño is statistically linked to colder, but wetter than normal, winters along the Gulf of México coast and in Florida. The wetter and colder weather is typically linked to the presence of skies that are cloudier than normal, conditions not generally linked to polar outbreaks.

The PNA is linked to variability in the mean surface pressure of the Aleutian Low. Around 1977, the Pacific Ocean sea surface temperatures (SSTs) underwent a remarkable regime shift that has attracted considerable attention (Latif and Barnett, 1994; Mantua et al., 1997). The climate shift produced a deepening of the wintertime Aleutian Low and moved the PNA index into an almost continuously positive phase during subsequent winters. Freezes affecting North and Central America and the Florida citrus-growing regions began occurring with great rapidity, such as in 1977, 1981, 1982, 1983, 1985, 1989, and 1997, whereas they had previously occurred only about once per decade (Rogers and Rohli, 1991). In contrast, 1948-57 was a freeze-free period characterized by anomalously high pressure in the

Aleutian Low center and anomalously low index values of the PNA. A strong, persistent PNA pattern, within and among winters, substantially increases the probability that Florida will experience a widespread, severe tree-damaging freeze. Further comments on the long-term interdecadal variability of citrus freezes appeared in the earlier discussion.

Strong cold surges in México and Central America usually possess a positive PNA and a confluent subtropical jet over the Gulf of México and southeast United States that is also found during El Niño conditions (Schultz et al., 1998). In fact, Klaus (1973) and Schultz et al. (1998) presented evidence that cold surges in this region tended to be more numerous during the cold season after the El Niño year.

In South America, research on interdecadal variability in cold surges focuses on variability in cold frontal passages. Calbete (personal communication) identified cold fronts at three different latitudinal bands over 1975 -98, and noticed that the number of cold fronts reaching these latitudinal bands was larger during the period 1975 -84 as compared to the period 1987-98, regardless of the intensity of the cold air masses related to the fronts. Calbete (1996) also showed that cold air masses reach southern Brazil from April to October, and in some regions at high altitudes freezes can occur all year long. Table 3 summarizes the number of cold air masses and snow events during the whole period 1988-96. However, the occurrence of very strong freezes in southern Brazil may be more of a random short-time variability event, occurring anytime even though the number of cold fronts may have been lower in recent decades.

Lemos and Calbete (1996) and Marengo (submitted) identified the number of cold fronts affecting the coastal section of Brazil from 35°-20°S, encompassing the coffee-growing areas of southeastern Brazil. The number reaching this latitudinal band was larger during 1975-84 compared to the period 1987-95. For this

TABLE 3 Number of Cold Air Masses and Number of Snow Events in Southern Brazil during 1988-96 during the Cold Season

Month

Number of cold air masses

Number of snow events

April

0 0

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