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Severe/very severe

Adapted from Marshall, 1983 and updated by J. Zullo (Centro de Pesquisas Agricolas [CEPAGRI], Universidade de Campinas, Sao Paulo, Brazil). No cold events that affected coffee were registered after the ones in winter 1994.

Adapted from Marshall, 1983 and updated by J. Zullo (Centro de Pesquisas Agricolas [CEPAGRI], Universidade de Campinas, Sao Paulo, Brazil). No cold events that affected coffee were registered after the ones in winter 1994.

air damming. As the cold air moves into lower latitudes (Fig. 3C), the blocking effect of the Andes diminishes (due to a more zonal orientation of the Andes to the north of 18°S). Cold advection produced by the southerly wind produces most of the local cooling at the leading edge of the surge, and the cooling is felt in southeastern Brazil and on the west side of Brazil adjacent to the Andes. Thus, the advance of the polar air outbreak along the subtropical Andes is set up by the topographic blocking of the flow.

Accompanying the cold surge is an area of surface high pressure. The highest pressure detected in central South America with a cold spell was 1044 hPa during the event of 15 July 1975 (Tarifa et al., 1977; Marengo, submitted), which was considered the strongest in the twentieth century. Cold wave central pressure intensities in southern Brazil rarely exceed 1038 hPa: e.g., 1035 hPa (June 1994, Marengo et al., 1997a) and 1033 hPa (July 1981, Haddock et al., 1981).

3.3.2. North and Central American Polar Outbreaks

Cold waves have long attracted the attention of North American weather forecasters and climatologists (Garriott, 1906; Cox, 1916). Cold waves have been defined by meteorologists in terms of (1) severity or rapidity of the drop in air temperature (Wendland, 1987), (2) the numerical values of the low temperatures or temperature departures that ultimately occur, and (3) the duration of the cold wave (varying time spans can be used) per the Cox (1916) quotation in the Introduction. To a lesser extent the cold wave can be characterized by the intensity of the polar anticyclone accompanying it (Rogers and Rohli, 1991). In North America, polar outbreaks are frequently observed east of the Rocky Mountains-Mexican Sierras. The most visible feature of the winter polar outbreaks is the surface anticyclone moving from its Alaskan/Yukon source region and associated with the southward transport of cold air across its core and eastern flank (Rogers and Rohli, 1991). SLPs associated with the core of the air mass can exceed 1070 hPa in northwestern Canada, but the central pressure decreases steadily with the southward movement of the polar outbreak, seldom exceeding 1055 hPa once the central high-pressure area enters the United States and 1040 hPa by the time it reaches the United States East Coast or the Gulf of México. The December 1983 and 1989 cold waves and citrus freezes were associated with polar highs with central pressures in excess of 1055 hPa when the anticyclones entered the northern Great Plains of the United States (Rogers and Rohli, 1991). Some examples of anticyclone central pressures in the northern Plains for severe cold events are given in Table 2.

The polar outbreaks are typically associated with record low temperatures (Konrad and Colucci, 1989) and produce numerous adverse socioeconomic impacts across many parts of the United States, including loss of vegetable crops and citrus, damage to homes and plumbing, human mortality, and disruption of transportation systems, among other problems. Air temperatures in the northern Great Plains reached —35°C during the 1989 polar outbreak, and it broke over 250 minimum temperature records across the United States, ultimately producing a citrus freeze in Florida (Rogers and Rohli, 1991). Colle and Mass (1995) have documented northerly surges of cold air that often move equatorward along the eastern side of the Rockies into México and have described the strongest surges that developed in the midwinter of 1994, with temperature decreases of 20°-30°C and pressure rises of 15-30 hPa within 24 hr. These cold surges have several regional names in North and Central America: Blue

TABLE 2 Strongest Anticyclones Detected over the Northern Great Plains on 1200 UTC (Universal Time Zone) Synoptic Charts for the Period 1899-1989 and Associated Agricultural Damage in Florida

Date

Intensity (pressure, hPa)

Damage

3 January 1766

Unknown

Severe

7-8 February 1835

Unknown

Very severe

30 December 1880

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