Florida’s mild winters allow the state to play a vital role in supplying fresh vegetables for U.S. consumers. Producers also benefit from premium prices when low temperatures prevent production in most of the country. This study characterizes the influence of the El Niño–Southern Oscillation (ENSO) on the Florida vegetable industry using statistical analysis of the response of historical crop (yield, prices, production, and value) and weather variables (freeze hazard, temperatures, rainfall, and solar radiation) to ENSO phase and its interaction with location and time of year. Annual mean yields showed little evidence of response to ENSO phase and its interaction with location. ENSO phase and season interacted to influence quarterly yields, prices, production, and value. Yields (tomato, bell pepper, sweet corn, and snap bean) were lower and prices (bell pepper and snap bean) were higher in El Niño than in neutral or La Niña winters. Production and value of tomatoes were higher in La Niña winters. The yield response can be explained by increased rainfall, reduced daily maximum temperatures, and reduced solar radiation in El Niño winters. Yield and production of winter vegetables appeared to be less responsive to ENSO phase after 1980; for tomato and bell pepper, this may be due to improvements in production technology that mitigate problems associated with excess rainfall. Winter yield and price responses to El Niño events have important implications for both producers and consumers of winter vegetables, and suggest opportunities for further research.
The variability and unpredictability of weather are an ongoing challenge for Florida growers who are the dominant and, at times, the only U.S. source of fresh winter vegetables. In 1995, Florida accounted for 63% of the area harvested in the United States for 13 important winter vegetables, and essentially all of the U.S. winter production of tomato, bell pepper, snap bean, sweet corn, eggplant, escarole, and endive (FASS 1996). Vegetables provided annual revenues of $1.48 billion (farm value) to growers in the 1995–96 crop year (FASS 1997). Unexpected adverse weather conditions can cause financial hardship for many growers. Furthermore, although limited supplies of fresh vegetables may be available in spite of periods of adverse weather, the price can climb sharply, creating hardship for consumers with limited incomes. Greater understanding of unfolding climatic anomalies and their impact on Florida vegetable production could benefit both producers and consumers.
Although the El Niño–Southern Oscillation (ENSO) occurs within the tropical Pacific, its effects can be felt around much of the globe, where it can account for a substantial portion of the observed interannual variability of temperatures and precipitation. ENSO influences the climate of the southeastern U.S. coastal plain, including Florida: El Niño years tend to be cooler and wetter, and La Niña years tend to be warmer and drier than normal in the fall through the spring, with the strongest effect in the winter (Ropelewski and Halpert 1986; Kiladis and Diaz 1989; Hanson and Maul 1991;Sittel 1994a). Although mean winter temperatures are associated with ENSO, the temperature extremes that damage winter crops in southern Florida are controlled primarily by frontal systems that bring cold air from Canada. Analysis of agriculturally important freezes showed influence from the Pacific–North America circulation pattern, but not from the Southern Oscillation (Downton and Miller 1993).
The influence of ENSO-related weather variability on agriculture has been shown for cereal production in south Asia, Australia, and the North American prairies (Garnett and Khandekar 1992); corn yields in Zimbabwe (Cane et al. 1994) and the U.S. Midwest (Handler 1990;Carlson et al. 1996); the yields and combined value of several Australian crops (Nicholls 1985); wheat rust in the United States and China (Scherm and Yang 1995);and U.S. soybean futures prices (Keppenne 1995). In the United States, Handler (1990) showed that Florida field corn yields are correlated (|R| > 0.36) with ENSO-related Pacific sea surface temperatures (SST) for the winter, summer, and fall quarters. For four coastal plain states (Florida, Georgia, South Carolina, and Alabama) in the southeast United States, Hansen et al (1998) showed that ENSO phase influences yields of maize and tobacco, and the total values of maize, tobacco, soybean, and peanut.
Vegetables are grown for the fresh market throughout Florida across a considerable range of latitudes, with possible climatic influence from two oceans. Mild winters in the southern half of the peninsula permit year-round vegetable production. In contrast to field crops, vegetables are generally irrigated and therefore unaffected by periods of drought. They are, however, quite vulnerable to freezing conditions in the fall and winter, as demonstrated by the unanticipated 19 January 1997 freeze that cost Florida’s winter vegetable industry more than $200 million and displaced thousands of migrant workers (Sharp 1997). Little is known about the vulnerability of Florida’s fresh vegetable industry to ENSO-related weather variability, or how ENSO impacts vary spatially or with time of year. The purpose of this study is to characterize the influence of ENSO phase on cool season fresh vegetables grown in Florida, and on weather variables (rainfall, temperatures, solar radiation, and freezes) that may influence vegetable production. Hypotheses about the relationships among ENSO, weather, and vegetable production were formulated and tested by statistical analyses of historical data. Effects of ENSO interactions with location and with time of year are also examined.
The approach used in this study was to analyze statistical dependency of annual values of crop and weather variables on ENSO phases from historical records. For consistency with vegetable crop reporting conventions in Florida and operational definitions of an ENSO year, a year is denoted from the previous October to the current September for all crop, meteorological, and climate variables.
This study used the Center for Ocean–Atmospheric Prediction Studies (COAPS) classification of ENSO phases (El Niño, neutral, La Niña) based on 5-month running means of Japan Meteorological Agency (JMA) SST anomalies, spatially averaged over the tropical Pacific from 90° to 150°W and 4°N to 4°S. A year is classified as El Niño (La Niña) if SST anomalies are at least +0.5°C (⩽−0.5°C for La Niña) for at least six consecutive months, and if this 6-month period starts before October and includes October through December (Sittel 1994b). The classification was extended to include the period prior to the start of the JMA SST records in 1949 (D. Legler 1997, personal communication) from spatially averaged monthly mean Pacific SST fields reconstructed from available data (1868–1949) using an orthogonal projection technique (Meyers et al. 1999). The period from 1929 to 1996 includes 14 El Niño (1930, 1931, 1941, 1952, 1958, 1964, 1966, 1970, 1973, 1977, 1983, 1987, 1988, and 1992) and 14 La Niña events (1939, 1943, 1945, 1950, 1955, 1956, 1957, 1965, 1968, 1971, 1972, 1974, 1976, and 1989). To test sensitivity of results to the criteria used to classify years by ENSO phase, analyses of yield response were repeated using another common classification of years based on the Southern Oscillation index (SOI) (Ropelewski and Halpert 1996).
Daily minimum and maximum temperature and rainfall data were obtained from the National Weather Service Summary of the Day database for seven locations (Fig. 1) in central and southern Florida (1931–95) (EarthInfo 1996). These data were used to calculate mean daily minimum and maximum temperature and mean monthly rainfall totals for the fall (preceding October–December) and winter (January–March) quarters, which had valid daily observations for all 3 months of the quarter. The stations were chosen to represent the important cool-season vegetable-producing regions in Florida, and are at least 95% complete for all three variables. The extreme minimum temperature, number of days with freezing temperatures, and degree-days below 0°C (DD0),
were calculated for each year that had valid data for at least 90% of the days from the preceding October to the current March, where Td is the minimum temperature on day d of the period of n days considered. These simple indices of cold stress hazard include measures of the severity (extreme minimum temperature) and frequency (number of days with freezing temperatures) of freezes (Waylen 1988), and the combination of frequency and severity (DD0). Distributions of the number of freezes and DD0 were positively skewed; to approximate a normal distribution for statistical analyses, each observation y was transformed to y′ = log(y + 1). Quarterly mean daily solar radiation for four locations (Fig. 1) in central and southern Florida (1961–90) was derived from monthly values from the National Solar Radiation Data Base (NREL 1992). Figure 2 shows winter weather statistics averaged across the selected locations.
Official records of marketable crop yields, areas planted and harvested, production, farmgate prices, and total value for Florida crop reporting districts for vegetable crops (tomato, bell pepper, potato, strawberry, sweet corn, snap bean, and watermelon, 1946–1996) are from the Florida Agricultural Statistics Service (FASS 1976–81). District yields were not reported in 1971. Yields for crop reporting districts were combined into averages for northern, central, and southern Florida (Fig. 1). Marketable yields, prices, and production of vegetables (tomato, bell pepper, and snap bean, 1929–80; sweet corn, 1950–80) for particular seasons are from Rose (1977) through 1974, and the Florida Agricultural Statistics Service (FASS 1976–81) for 1975–80, with seasons defined by the month of harvest (e.g., a winter crop is harvested in January–March). Although crop yields were no longer reported on a quarterly basis after 1980, reported quarterly areas harvested and monthly production sold available for tomato and sweet corn were used to calculate quarterly marketable yields for these crops after 1980. Sampling procedures and the method of calculating yields were consistent before and after 1980 (S. Zonner 1997, personal communication). Quarterly mean prices for 1981–96 were calculated from monthly prices weighted by monthly production sold. All price data were deflated to a December 1995 basis using quarterly values of the U.S. urban consumer price index (Bureau of Labor Statistics).
Higher-frequency anomalies attributed primarily to weather variability were separated from lower-frequency trends attributed to changes in technology and management using a harmonic smoothing technique (Press et al. 1989) that applies a low-pass filter covering a specified period to detrended, Fourier-transformed data. Each crop data series was smoothed with a filtering period of 7.0 yr (i.e., frequency of 0.143 yr−1) (Hansen et al. 1998). Figure 3 shows observed and smoothed winter vegetable yields. The variability about the smoothed trends of yield and production series generally increased with increasing trends. Therefore analyses were performed on the ratios of observed to smoothed values rather than on anomalies to avoid giving excessive weight to periods of higher variability.
Hypothesized influences of ENSO phase on crop (marketable yield, price, production, and value) and weather variables (mean daily extreme temperatures, rainfall, solar radiation, and indices of freeze hazard) to ENSO phase were tested by analysis of variance (ANOVA, Steel and Torrie 1980). To test the hypothesis that the influence of ENSO on crops differs in different regions within Florida, the yield response to ENSO phase was examined for five crops (bell pepper, strawberry, sweet corn, snap bean, and watermelon) grown throughout Florida. Four crops (tomato, sweet corn, bell pepper, and snap bean) for which records were available for the fall, winter, and spring harvest seasons were examined to test the hypothesis that the influence of ENSO on crop variables changes with time of year. A significant response of individual crops and locations or seasons was inferred only if standardized values for the combined set of crops responded significantly to ENSO phase or its interaction with location or season. Duncan’s multiple range test identified which ENSO phases differed significantly (P < 0.05) in their effects on a crop or weather variable. The same procedure was used to analyze the influence of ENSO phase on weather variables.
Preliminary analyses suggested that winter tomato and sweet corn yields were less sensitive to ENSO phase after quarterly reporting was discontinued in 1980. To test the hypothesis that yield response to ENSO changed, the winter yield ratio series of tomato and sweet corn were divided into early (⩽1980) and late (1981–96) periods. La Niña and neutral phases were combined for this analysis because only one La Niña occurred from 1981 to 1996, and winter yields did not differ significantly between La Niña and neutral years through 1980.
Information needed to calculate recent quarterly yields was available for tomato and sweet corn. Linear correlation and multiple regression were used to test hypothesized influences of weather variables on the yields of these two crops. For each weather variable, a single time series was derived as the simple average of standardized values from the seven (four for solar radiation) weather locations. Correlation and preliminary multiple regression analyses indicated that mean precipitation, daily maximum temperatures, and DD0 were the most influential weather variables on tomato and sweet corn yields. A model of yield ratios as a linear function of the three weather variables and an intercept was solved by least squares regression separately for tomato and sweet corn for all available years through 1980 and after 1980.
Fall and winter weather
For the winter months, mean daily maximum temperatures and monthly rainfall showed strong responses to ENSO phase (P < 0.001) at all seven locations analyzed. Rainfall was an average of 54.5% higher, and higher at all locations (P < 0.001) in El Niño than in neutral or La Niña winters (Fig. 4a). Fall rainfall was higher for the set of locations (P < 0.001) and for Fort Myers (P < 0.01) in El Niño than in neutral or La Niña years. Winter mean daily maximum temperature was lower in El Niño than in neutral or La Niña years at every location; the average decrease in El Niño years across the locations was 1.6°C (Fig. 4b). ENSO phase influenced mean daily minimum temperatures for the set of locations in the fall (P < 0.01) and winter (P < 0.001), and for Fort Pierce in the winter (P < 0.05). Fall and winter solar radiation responded significantly to ENSO phase for the set of four locations (P < 0.001). For each location, solar radiation was lower (P < 0.01) in El Niño than in neutral or La Niña winters; the decrease in El Niño years averaged 1.0 MJ m−2 day−1 or 7.5% across the four locations (Fig. 4c). Fall solar radiation was lower (P < 0.05) in El Niño than in neutral or La Niña years at the two northern locations. For winter rainfall, mean maximum temperatures, and mean solar radiation, the variance among ENSO phases was greater than the variance among locations. Three indices of cold stress hazard (minimum annual temperatures, the number of days with freezing temperatures, and degree-days below 0°C) (Table 1) were not significantly related to ENSO phase for the seven locations analyzed. These results confirm previously reported relationships between ENSO and rainfall and temperatures in the fall and winter in Florida, and support the hypothesized influence of ENSO on solar radiation, but not on the frequency or severity of freezes.
Standardized annual yield ratios (i.e., observed/smoothed) of a set of six vegetable crops (tomato, bell pepper, strawberry, snap bean, and watermelon, 1929–96; sweet corn, 1950–96) showed a significant (P < 0.05) response to the current year’s ENSO phase, with mean yield ratios lower in El Niño than in La Niña years for the set of crops. Watermelon was the only crop within the set that showed the opposite tendency. None of the individual crops showed a significant response to ENSO phase.
Yield statistics for the northern, central, and southern regions of Florida were available for five of the crops (bell pepper, strawberry, snap bean, and watermelon, 1946–70, 1972–96; sweet corn, 1951–70, 1972–96). Region did not interact significantly with either the current or previous ENSO phase for this set of crops or for any of the individual crops. These results do not support the hypothesis that ENSO affects crops differently at different latitudes in Florida.
Effect of season
ENSO phase significantly influenced quarterly yield ratios of the four crops used to test the influence of time of year on the relationship between ENSO and crop production (Table 2). The highly significant ENSO phase by season interaction supports the hypothesis that the season in which vegetable crops were harvested modified the impact of ENSO on crop yields. ENSO phase did not influence any of the fall- or spring-harvested crops, but affected yield ratios of all of the winter-harvested crops. Winter yield ratios of all four crops were lower in El Niño years than in neutral or La Niña years (Fig. 5). Mean yields of tomato, bell pepper, and sweet corn up to 1980 decreased more than 25% in El Niño years (Table 2). ENSO phases based on the SOI (Ropelewski and Halpert 1996) tended to be a poorer predictor of quarterly vegetable yields; overall yield responses of bell pepper and snap bean, and winter yield responses of sweet corn and snap bean were not significant (P = 0.05) using the SOI-based classification.
Although FASS stopped reporting quarterly vegetable yields after 1980, yields for tomato and sweet corn were extended through 1996 from published records. ANOVA of the extended series for the two crops combined show a significant ENSO phase (El Niño vs non–El Niño) effect and phase by period interaction (Table 3), supporting the hypothesis that yield response to ENSO phase changed after 1980. ENSO phase response was not significant for either crop after 1980. As expected from the previous analysis of seasonal effects on ENSO response, yield ratios through 1980 were lower in El Niño than in non–El Niño years. However, tomato and sweet corn yields did not respond significantly to El Niño events between 1981 and 1996. Analyses using ENSO phases based on SOI showed the same phase by period interaction.
Influence of weather
Simple correlation results (Table 4) for all available years suggest that several weather variables influenced yields. For tomato, maximum temperature and rainfall showed the strongest association with yields, whereas maximum temperature and freezes showed significant association with sweet corn. For both crops, increasing yields were associated with increasing maximum temperatures, and with decreasing rainfall and degree-days below freezing. Multiple regression gives a better indication of the independent contribution of each of the weather variables considered (precipitation, maximum temperatures, and DD0) to yields when some of the weather variables are strongly correlated (e.g., DD0 vs Min-T, Rain vs Rad, and Rain vs Tmax) (Table 4). For all years, multiple regression results (Table 5) were generally consistent with correlation results. The overall regression was not significant for sweet corn.
The most surprising difference in correlation results between the early (⩽1980) and late (>1980) periods was the apparent change in the direction of the association between yields and rainfall (Table 4). Although the positive correlations in the later period were not significantly different from zero, the changes in correlation coefficients from the early to the late period were significant for both tomato (P < 0.05) and sweet corn (P < 0.01). Multiple regression results showed the same change in the sign of partial correlations with rainfall, and a change in the sign of partial correlations for maximum temperature, which were positive for all years and after 1980, but negative through 1980 (Table 5). ANOVA of winter mean temperatures, rainfall, and solar radiation did not show any interaction between ENSO phase and period (i.e., before vs after 1980) effects, suggesting that the apparent change in crop response to ENSO was not due to a change in the regional influence of ENSO on climate.
ENSO phase significantly (P < 0.05) influenced annual prices of the set of crops for which quarterly data were available (tomato, bell pepper, sweet corn, and snap bean). ENSO phase interacted significantly (P < 0.01) with season to influence winter prices (P < 0.001). ANOVA for individual crops supported the hypothesized influence of ENSO phase on prices only for bell pepper and snap bean in the winter harvest season (P < 0.01), when prices of both crops were higher ($0.39 kg−1 or 25.3% for bell pepper, and $0.24 kg−1 or 15.2% for snap bean) in El Niño than in neutral or La Niña years (Fig. 6).
Production and value
Annual production ratios of vegetable crops responded significantly to ENSO phase (P < 0.05) and its interaction with season (P < 0.001), with significant responses in both the winter (P < 0.001) and the spring (P < 0.05). The only significant ENSO phase response of individual crops in particular seasons was for winter tomato (P < 0.01); production anomalies averaged 20.5 Gg, or 22.8% higher in La Niña than in neutral or El Niño winters (Fig. 7). Analysis of the subset of production ratios through 1980 showed a stronger (P < 0.01) overall ENSO phase effect, and significant reduction of winter production in El Niño years, for each of the four crops. Annual value ratios of these crops also responded significantly to ENSO phase (P < 0.05) and its interaction with season (P < 0.05), with significant responses in the winter (P < 0.01) and the spring (P < 0.05). Tomato value anomalies averaged $26.3 million, or 22.1% higher in La Niña than in neutral or El Niño winters (P < 0.05, Fig. 8). For the spring harvest season, snap bean value anomalies averaged $3.01 million, or 14.1% higher in El Niño than in neutral or La Niña years (P < 0.05).
The observed influence of ENSO on fall and winter precipitation and temperatures generally agrees with what is already known (Ropelewski and Halpert 1986;Kiladis and Diaz 1989; Sittel 1994a). Although mean daily maximum and minimum temperatures were correlated (Table 4), response to ENSO phase was more consistent among locations for daily maximum than for daily minimum temperatures. This is likely due to the effect of ENSO-related cloud cover on radiational heating in the daytime. The lack of significant relationship between ENSO and measures of freeze hazard agrees with Downton and Miller’s (1993) analysis of agriculturally important freezes in central Florida.
Yield responses of Florida winter vegetables to ENSO can be explained as a response to ENSO-related weather variability, although the evidence does not suggest that a single weather variable has a dominant influence on yield response to ENSO. Low daytime temperatures and solar radiation in El Niño winters translate into reduced photosynthesis and growth rates and delayed crop development. Increased rainfall may directly reduce yields. For example, excess rainfall can leach applied fertilizers, and can raise water tables, creating anaerobic conditions that can damage root systems. Long periods of wet foliage favor the development of foliar diseases such as bacterial spot [Xanthomonas campestris pv. vesicatoria (Doidge) Dye.], the most damaging disease of tomato and bell pepper in Florida (Pohronezny and Volin 1983; Pohronezny et al. 1986; Simons 1987). High rainfall amounts also have an indirect influence because of their association with reduced daytime temperatures and solar radiation. Freezing temperatures influenced winter vegetable yields, but in a manner that does not appear to be related to ENSO. Even though ENSO explained a significant portion of the interannual variability of winter vegetable yields, the yield variability due to freeze damage does not seem to be predictable from ENSO phases.
The apparent change in yield response to ENSO around 1980 is more difficult to explain. Although part of the change can be attributed to weak 1988 and 1992 El Niño events that had little influence on rainfall, the strong 1982–83 El Niño had a large, positive impact on winter rainfall (Fig. 2) but little reduction of yields (Fig. 3). We can speculate that changes in production technology may have reduced crop sensitivity to ENSO by reducing the negative impacts of excess rainfall. For example, expanding use of integrated pest management in the early 1980s (Pohronezny et al. 1986) and the introduction of resistant cultivars (Scott and Jones 1989) around 1990 have reduced the susceptibility of winter tomato and bell pepper to bacterial spot damage in years with high fall and winter rainfall. Furthermore, the widespread use of black plastic mulch since the mid 1970s and expanding use of microirrigation since the mid 1980s tend to reduce the negative impacts of excess rainfall on tomato and bell pepper. Plastic mulch reduces leaching losses of fertilizer nitrogen (Locascio et al. 1985; Sweeney et al. 1987) and provides some protection from soil- and residue-borne disease organisms. Microirrigation reduces the risk of flooding during periods of heavy rainfall relative to the traditional subsurface irrigation method. Although these changes in production technology might explain the apparent shift in response to ENSO for tomato and bell pepper, similar shifts in production technology are not apparent for sweet corn or snap bean.
Analysis of only annual vegetable data would have overlooked important ENSO influences on the Florida vegetable industry that occur only in the winter harvest season. The winter ENSO signal is important because of Florida’s unique role as the nation’s most important supplier of fresh winter vegetables. We consider it unfortunate that quarterly yield reporting was discontinued for most vegetables after 1980, especially given the possibility that ENSO influence on winter vegetable yields may have changed around that time.
Prices, and areas planted and harvested are human decision variables; explaining their relationship to ENSO is more difficult than for weather and yields. Although decreased yields in El Niño years should tend to increase prices by decreasing the supply, competition from other regions with different ENSO response, supply elasticity of prices, and possible adjustments in areas harvested in Florida and elsewhere could modify the effect. Florida’s high (72%, mean of 1981–91; Van Sickle 1994) share of the U.S. winter (December–April) market for snap bean can help explain the sensitivity of its prices to ENSO-related yield variability. Bell pepper showed a price response to ENSO while tomato did not, even though their winter market shares were similar (45% for bell pepper vs 56% for tomato, mean of 1981–91) and tomato showed more evidence of a production response to ENSO phase. The smaller size of the bell pepper market may make its prices more sensitive to changes in production. ENSO influence on the production and value of winter tomatoes is important because tomato is by far the most important vegetable crop to Florida’s economy (29.7% of the total value of vegetables in 1996; FASS 1997).
Because production is the product of yields (a biological response) and decisions about areas planted and harvested, ENSO effects on the supply of winter vegetables from Florida cannot be predicted from yields alone. Statistical tests could detect an effect on production for only one of the four crops (tomato) that showed significant winter yield responses to ENSO, apparently because of variations in harvested areas. Similarly, the value of production depends on production and price. The total value of winter tomato production responded significantly to ENSO phase, reflecting the influence of ENSO on production and lack of influence on prices.
Results from this study have important implications for consumers and producers. Evidence of reduced production and increased prices of winter vegetables raises the concern that lower-income consumers in particular could have difficulty meeting nutritional requirements in El Niño years. With information about what type of weather to expect in a particular growing season, Florida farmers could adjust crop mixtures, areas planted, or crop management to reduce the adverse effects of higher rainfall and lower temperatures and radiation in El Niño winters. Farmers in other regions with mild winters may have opportunities to take advantage of higher prices for bell pepper and snap beans in El Niño winters.
This study raises several questions that warrant further research. First, the possibility that reduced supplies and increased prices of winter vegetables may reduce their consumption in El Niño winters could be tested with historical data. Second, although imports from Mexico may reduce the impacts on consumers, the influence of ENSO on the major winter vegetable producing region in western Mexico is not yet known. Third, we do not yet have enough evidence to establish the reason for the apparent shift in yield response to ENSO around 1980, or to determine its implications for future prediction or mitigation. Finally, further research is needed to determine how to use information about ENSO impacts on Florida’s weather and fresh winter vegetable industry to improve planting and management decisions.
The authors express their appreciation to Ayse Irmak for entering data, Shirley Zonner at FASS for verifying the consistency of crop data, and George Hochmuth for providing insights about changes in vegetable production technology. This research was funded in part from a grant from NOAA Office of Global Programs entitled, “Regional Assessments and Applications for Effects of Seasonal-to-Interannual Climate Variability.”
Corresponding author address: Dr. James W. Hansen, Agricultural and Biological Engineering, University of Florida, P.O. Box 110570, Gainesville, FL 32611-0570.
* Florida Agricultural Experiment Station Journal Series Number R-05984.