Proc. Fla. State Hort. Soc. 2001 Paper No. 63 Draft 17
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RELATIONSHIP BETWEEN FLORIDA FREEE INCIDENCE AND SEA SURFACE
TEMPERATURE IN THE TROPICAL PACIFIC
J. David Martsolf
Horticultural Sciences Dept.
University of Florida, Gainesville, F~lorid~a 32611
Key WVor~ds. freeze hazard, climate prediction, climate change, frost control, cold
protection, FAWN, DISC.
Abstract. A climate prediction section has been added to weather information provided by
the Florida Automated Weather Network (FAWN), which can be accessed via the Internet at
http://fawn. ifas.ufl.edul. A cold protection module is under development for the Decision
Information System for Citrus (DISC) which may utilize these new features. it is likely to
contain a tool that relates sea surface temperatures in the El NiflolSouthern Oscillation
(ENSO) region of the Tropical Pacific (120E to 70W9, 25N to 258) to the possibility of
damaging freezes in Florida. FAWN is expected to provide predictions of sea surface
temperatures as much as one year into the future. Since 1980 there have been six major
freezes in Florida. A rule suggested by this work is that if the winter is expected to be near
SFlorida Agricultural Experiment Station Journal Series No. R-08179. Gratefully acknowledged are the
Florida Citrus Production Research Advisory Council for funding DISC, the FAWN Consortium through
their implementation of the Climate Prediction component and in particular Dr. James W. Jones, Dr. Fedro
Zazueta, Dr. James J. O'Brien, and Mr. David Zierden, Dr. Howard Beck, Mr. John L. Jackson, members
of the Weather Advisory Committee and especially Mr. Peter D. Spyke, Mr. Norman Todd, Jr. Dr. Paul M.
Lyrene and Dr. Lar K. Jackrson, and the Japan Meteorological Agency, the original source of the monthly
mean sea surface temperature data.
neutral (ENSO sea surface temperatures between -0.7 and +0.5 OC of their mean), the
likelihood of a major freeze in Florida is 50%. In the past two decades 60%/ of the winters
have been near neutral ENSO winters. When the winter is predicted to be a strong El Niflo
or a strong La Niiia, the odds of a major freeze in Florida are low. Availability of, and
confidence in, predictions of the future phase of the ENSO is increasing. Utility of rules
relating freeze incidence to ENSO phases depends on effective delivery of ENSO phase
predictions to management decision makers. Both FAWN and DISC are expected to play
collaborative roles in this communication.
The University of Florida's Institute of Food and Agricultural Sciences (IFAS)
developed and maintains the Florida Automated Weather Network (FAWN, 2001).
During the fall of 2000, a test of a recently installed climate prediction component of
FAWN (Climate Prediction Section, 2001) led to the finding documented in this paper.
This test led to an examination of Florida freeze dates relative to phases of the El Nifio /
Southern Oscillation (ENSO) as one of the examples of how the information might be
used in decision making processes. One of these decision making aids is under
development for the Decision Information System for Citrus (DISC). See DeSousa
(2001), The DISC Team (2001) and Martsolf (1997, 1998). The Climate Prediction
Center (CPC, 2001) can be expected to provide forecasts of sea surface temperatures in
the ENSO region at least a year in the future. The purpose of the paper is to defend the
formation of rules that might be used conveniently in deciding how serious the threat of a
major freeze may be in the next winter given these predictions.
Materials and Methods
FA WN. The Florida Automated Weather NetwJork (FAWN, 2001) recently acquired a
climate prediction section (Climate Prediction section, 2001) a collaborative effort of
three Universities (The Florida Consortium, 1999). Questions about the possibility of a
relationship between freeze incidence and particular ENSO phases arose during a study
of this section of FAWN.
Florida Freezes. A list of the recent severe freezes in Florida has been published by
Attaway (1997). U~se of the published list, which is broadly accepted, avoids a rather
detailed task of defining a freeze in terms of temperature, wind speed, and impact on
vulnerable horticultural crops. The lists of freezes in Tables 1 and 2 are precisely as
Attaway published them with one exception. H;-e chose to define the 1983-85
combination of two freezes as one freeze. In this analysis the time scale has monthly
resolution so this combination in the list was split into the two parts. These two freezes
are as described separately by Dr. Attaway making the combination in the list but one of
two ways to handle the freezes. Attaway documents two categories of freezes which he
terms "impact" and "near impact." Attaway defines the impact freezes "as those which
made the greatest contribution to the geographical relocation of the Florida citrus industry
from north to south." The near impact freezes were "those which did severe damage, in
some cases reinforcing decisions made as a result of an impact freeze." In this work the
two categories are treated as one which is taken to be a list of the severe freezes.
Sea Surface Temperature Data Set. In order to compare the date of the freezes with
the sea surface temperature in the ENSO region of the Tropical Pacific during the period
that included the date, a data set was acquired from the COAPS (2001). The data are a
list of temperature departures for each month. The temperature departures are both time
and space averages. This data set was developed and maintained by the Japan
Meteorological Agency (JMA). The JMA data set was the source of the departures of the
sea surface temperatures from their time and space mean values shown in Table 1 and to
plot to the temperature trace in Figure 1. The JMA data set was used to determine the
phase of ENSO during each of twenty winters between 1980 and 2000 as indicated in
Table 2. TIhe same JMA data set was used in all the analyses documented in this paper.
Near neutral ENSO Phase Definition. Near neutral is defined for this study to be
from 0.7 oC to + 0.5 oC. This zone is shown graphically as the shaded area in F~igure 1.
Most (e.g., Hansen et al., 1997) declare the neutral phase to be that period when the
spatial average sea surface temperature is within a half degree C of the time and space
average temperature, e.g. the zero departure line in Figure 1. Early in the deliberations
that led to this paper, the rather common definition of the neutral phase was used. But
the '85 Freeze that falls less than 0.2 C in the La Nifia territory (see Figure 1) made the
rule that would summarize the relationship between the freeze and the neutral zone rather
complex. So a near neutral zone was defined that would include all the freezes. The
analysis was made using the near neutral phase definition. The counts are of occurrences
of the near neutral (NN) phase (see Table 2).
Climate Prediction Center. At CPC (2001) one can find the description: "In 1970,
various federal weather and climate functions were consolidated into the National
Weather Service and placed in a new agency called the National Oceanic and
Atmospheric Administration. In the 1980's the National Weather Service established the
Climate Prediction Center (CPC), known at the time as the Climate Analysis Center,
located in Camp Springs, Maryland. CPC is best known for its US climate forecasts
based on El Nifio and La Nifia conditions in the tropical Pacifc." The purpose of the
CPC (2001) is declared to be: "...serve the public by assessing and forecasting the
impacts of short-term climate variability, emphasizing enhanced risks ofweather-related
extreme events, for use in mitigating losses and maximizing economic gains."
Methods. Two data sets, one of freeze dates (Attaway, 1997) and one of sea surface
temperature (SST, 2001), were compared on the same time axis (see Figure 1) to
visualize the ENSO phase during the incidence of the six major freezes. The ENSO
phases for each of the 20 winters were examined by the use of an ]Excel spreadsheet in
which the JMA, Index for the midpoint of each of the 240 months in the data set was
coded indicating which of the three phases, El Nifio (EL), La Nifia (LA), and near neutral
(NN). The results are documented in Table 2.
Tables 1 provides results of an analysis of the relationship between freeze dates and
the monthly values of the departure of the sea surface temperature from its mean value.
The average departure, -0.25C, seems to indicate a slight bias toward the La Nina, or the
cool side and the standard deviation, 0.243C, suggests that while there is some dispersion
to the data points within the near neutral zone, there is likely a relationship between the
freeze incidence and when the ENSO is in a near neutral state.
Table 2 summarizes observations made as to the phase of the ENSO for each of 20
winters in the two decades used in the study. There were 12 near neutral (NN) winters.
So 60% of the winters in the 20 year series were near neutral and those winters included
all six of the freezes. The percentage of the NN winters in which there were severe
freezes is 50%. There were 14 of 20 winters, or 70%, in which no major freeze occurred.
Conversely there were six major freezes in the 20 years; 30% of the winters contained
freezes. So instead of concluding that if the future is similar to the past, that the odds of
having a major freeze in any future year is 30%; one can look at the forecast of the sea
surface temperature for the winter and if it is a near neutral winter the odds of a major
freeze increase to 50%. But there is another observation that may be even more valuable.
If the forecast is for a strong El Nif o or La Nif la, the odds of a severe freeze are low.
Figure 1 illustrates the rule: major Florida freezes have occurred only during periods
when the sea surface temperatures in the Tropical Pacific are near neutral, during the
most recent two decades of the 20f Century. No major freezes occurred during a period
when the sea surface temperatures were outside the near neutral zone.
The most recent two decades were used in this study for several reasons. One of
these is rapid climate change is taking place. Many suggest the effect has been showing
up in such relationships since the mid to late 1970s (Zwiers and Weaver). A shorter time
span stretches out the graph so that the relationship between the short term freezes and
the longer term ENSO fluctuations becomes apparent.
Previous attempts (e.g. H-ansen, et al., 1997) have revealed a rather weak relationship
between citrus yield and any of the three phases of ETNSO in spite of the fact that citrus
yield is clearly influenced by freezes. So it would seem that this finding of a rather
convincing relationship between a particular phase of ENSO and freeze incidence calls
these studies into question. But Hansen, et al. (1997) documents significance when a
year's delay was inserted into the analysis. The freeze date, as opposed to the impact of
the freeze on yield, seems more likely to be associated with a current neutral phase, since
the ENSO phases and freezes are both weather phenomena. One might imagine that
during these neutral periods there is a deficiency of forcing in either direction, leaving the
weather to fluctuate more widely, and within such broad variations are rare events that
permit a Florida freeze to occur. One of these is a deep trough in the jet stream
permitting large volumes of artic air to be pressed down through the mid-West and into
Florida. Another is the unusually dry condition upwind that permits the air mass to arrive
with a minimum of moderation from energy that would otherwise by transferred from
water and water vapor (Martsolf, 1999). Even though freezes are rare events and remain
difficult to forecast with precision, horticulturists have been innovative in development of
methods to decrease the vulnerability of their crops to such events (Martsolf, 2000).
Increases in the precision of estimates of the likelihood of freeze incidence seem likely to
be of value in such decisions.
There is a 60% chance of having a near neutral winter using the occurrences during
the period from 1980 to 2000 as a guide. When a near neutral ENSO winter is
anticipated the odds of having a major freeze in Florida is 50%. Strong El Nif io phases
have occurred in only 20% of the winters. Strong La Nifia phases have occurred in only
20% of the 20 winters. No freezes have occurred in either the strong EN or strong LN
phases so the odds of one occurring in a future strong EN or LN seem small. A new rule
replaces the old rule which suggested there is about a 30% chance of a freeze in Florida
in any given year because climate predictions indicate whether the winter will be near
neutral or not up to a year in advance. The new rule suggests that when a near neutral
ENSO phase winter is predicted, the odds of a major freeze in Florida increase to 50%.
Attaway, J. A. 1997. A History of Florida Citrus Freezes. Fla. Science Source., Lake
Alfred, FL., 368 pp.
Beck H. and J. Jackson. 1998. Florida Automated Weather Network. Proc. 7"
International Conference on Computers in ~Agriculture, Orlando, FL, ASAE, St.
Joseph, MI. pp. 595-601.
Climate Prediction Center. 2001. CPC, http://www.cpc. ncep.noaal~gov/
Climate Prediction Section. 2001. http://fawn.ifas.uff.edulpreview/cpred/, FAWN, IFAS,
Center for Ocean-Atmospheric Prediction Studies. 2001. (http://www.coaps .fsu.edul)
DeSousa, Jackie. 2001. Weather patterns demand an attentive eye. FFVA Harvester
Florida Automated Weather Network. 2001. http://fawn.ifas. ufl.edul, FAWN IF;AS, UF.
Hansen, J. W., A. Irmak, and J. W. Jones. 1997. El Niiio Southern Oscillation
Influences on Florida Crop 'Yields. Soil and Crop Sci. Soc. of Fla. Proc 57:12-16.
Martsolf, J. David. 1997. Acquiring weather information via a decision making system
(DISC). Proc. Fla. State Hort. Soc. 110:92-96.
Martsolf, J. David. 1999. What fr-eezes of the past century taught us. Proc. Fla. State
Hort. Soc. 112: 95-98.
Martsolf, J. David. 2000. Diversity of frost protection methodology. Proc. Fla. State H-ort.
Martsolf, J. D>., R. M. Peart, H. W. Beck, P. D. Spyke, Norman TTodd, Chet Townsend,
and J. K. Schueller. 1998. DISC makes progress with information integration. Proc.
Fla. State Hort. Soc. 111:144-147.
Sea Surface Temperatures. 2000. S ST (ftp://www. coaps.fsu.edulpublJ MASST_1 ndex/)
The DISC Team. 2001. DISC Project progress report. Citrus Industry 812(2):20,21,24.
The Florida Consortium. 1999. El Nino, La Nina and Florida's Climate: effects on
agriculture and forestry. COAPS/FSU, IFAS/UF, RSMAS/UM.
Zwiers, F. W. and A. J. Weaver. 2000. The causes of 20t Century warming. Science 290
Table 1. Relationship between recent Impact and Near Impact Freezes (as categorized by
Attaway, 1997) and the departure of sea surface temperatures from their mean in the
Table 2. Indication of the phase of ENSO associated with the incidence of major freezes in the
last two decades of the 20th Century.
EN LN NN
Where: NN is Near Neutral
EN is El Nino
LN is La Nina
ENSO is El Nino I Southern Oscillation
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