The ENSO-associated probabilities give the frequency
with which precipitation (as represented by the gridded product
of UEA-CRU) was observed to be in the lower-, middle-, and
upper-third of the 1950-1996 precipitation distribution during 
El Nino and La Nina events. For this analysis, El Nino and 
La Nina events are defined to be the 10 years during the 
1950-1996 period for which the NINO3.4 SST anomaly index 
(averaged SST anomaly over region 5S-5N; 170W-120W). 
Both the precipitation amounts and the NINO3.4 index are defined 
for 3-month averaged seasons, and for 12 overlapping seasons in the year.
The years that were demarked as El Nino or La Nina events are
listed below for each season in order of increasing magnitude
(i.e. the first value is the year of the weakest warm or
cool anomaly and the last value is the strongest warm or cool
anomaly).
The maps show the proportions of years that experienced above-, near-,
and below-normal precipitation.  As an example, a data point with a 
value of 0.6 for above-normal precipitation, 0.3 for near-normal 
precipitation, and 0.1 for below-normal precipitation in a particular 
season, would indicate that for the years in which NINO3.4 reached its 
warmest values during 1950-1996, 6 out of 10 of those years experienced 
above-normal precipitation over that grid point in that season, 3 out 
of 10 experienced near-normal precipitation, and 1 out of 10 experienced 
below-normal precipitation.
For more information on this approach, see Mason and Goddard (2001).

ENSO Cases --
  JFM - El Nino: 1988, 1991, 1995, 1966, 1987, 1969, 1973, 1992, 1958, 1983
        La Nina: 1985, 1968, 1951, 1956, 1955, 1976, 1950, 1989, 1971, 1974

  FMA - El Nino: 1993, 1991, 1973, 1995, 1966, 1969, 1987, 1958, 1992, 1983
        La Nina: 1975, 1967, 1968, 1956, 1976, 1955, 1989, 1974, 1971, 1950

  MAM - El Nino: 1994, 1995, 1991, 1966, 1993, 1969, 1958, 1987, 1983, 1992
        La Nina: 1985, 1964, 1976, 1956, 1975, 1989, 1971, 1955, 1974, 1950

  AMJ - El Nino: 1994, 1958, 1993, 1969, 1972, 1982, 1991, 1987, 1983, 1992
        La Nina: 1956, 1985, 1989, 1988, 1974, 1964, 1971, 1975, 1955, 1950

  MJJ - El Nino: 1969, 1994, 1983, 1965, 1993, 1982, 1992, 1991, 1972, 1987
        La Nina: 1954, 1956, 1971, 1974, 1973, 1964, 1955, 1950, 1988, 1975

  JJA - El Nino: 1963, 1958, 1992, 1994, 1957, 1982, 1965, 1972, 1991, 1987
        La Nina: 1971, 1970, 1950, 1954, 1964, 1956, 1955, 1973, 1988, 1975

  JAS - El Nino: 1993, 1986, 1994, 1963, 1957, 1982, 1991, 1965, 1972, 1987
        La Nina: 1971, 1950, 1970, 1964, 1988, 1956, 1954, 1973, 1955, 1975

  ASO - El Nino: 1969, 1957, 1963, 1994, 1986, 1991, 1965, 1972, 1982, 1987
        La Nina: 1971, 1956, 1950, 1954, 1964, 1970, 1988, 1973, 1975, 1955

  SON - El Nino: 1976, 1963, 1957, 1994, 1986, 1991, 1965, 1987, 1972, 1982
        La Nina: 1974, 1971, 1954, 1950, 1964, 1970, 1988, 1973, 1975, 1955

  OND - El Nino: 1969, 1963, 1957, 1986, 1994, 1991, 1987, 1965, 1972, 1982
        La Nina: 1974, 1971, 1954, 1950, 1964, 1970, 1975, 1988, 1973, 1955

  NDJ - El Nino: 1969/70, 1968/69, 1986/87, 1987/88, 1994/95, 1965/66, 1991/92, 1957/58, 1972/73, 1982/83
        La Nina: 1971/72, 1984/85, 1964/65, 1950/51, 1954/55, 1955/56, 1970/71, 1988/89, 1975/76, 1973/74

  DJF - El Nino: 1990/91, 1987/88, 1968/69, 1986/87, 1994/95, 1965/66, 1991/92, 1972/73, 1957/58, 1982/83
        La Nina: 1962/63, 1984/85, 1964/65, 1950/51, 1954/55, 1955/56, 1988/89, 1975/76, 1970/71, 1973/74


Precipitation Data:
-------------------
 The precipitation data is that produced by the Climate Research Unit
of the University of East Anglia (New et al, 1999; New et al, 2000).
The data are based on rain gauge data, and interpolated spatially,
resulting in a dataset with 0.5x0.5 degree resolution and no
missing data points over land.

We also supply the analysis on a coarser resolution grid, where
we have upscaled the data by spatial averaging to 2.5x2.5 degree
resolution.

Significance Masking:
--------------------- 
 A version of this analysis is provided with significance masking.
The prior probabilities that any subset of years, drawn at random
from the 1950-1996 sample should have experienced above-, near-,
or below-normal precipitation are the climatological probabilities of
33.3% for each category. Thus, one would like to know how significantly
different from the climatological probabilities is that obtained from
a set of 10 ENSO years. Using the hypergeometric equation to estimate
confidence levels (as in Mason and Goddard, 2001), we find that

Confidence level    # out of 10 
---------------     -----------
    99.9%+           9 or more
    99.8%              >= 8
    98%                >= 7
    90%                >= 6
    98%                  0

 --> Thus 1-5 out of 10 cases would not be considered statistically
   significant at the 90% confidence level.


REFERENCES:
-----------
Mason, S.J. and L. Goddard, 2001. Probabilistic precipitation anomalies
associated with ENSO. Bulletin of the American Meteorological Society,
Vo. 82, 619-638.

New, M., M. Hulme, and P.D. Jones, 1999. Representing twentieth century
space-time climate variability. Part I: Development of a 1961-1990 mean 
monthly terrestrial climatology. J. Climate. Vol. 12, 829-856.

New, M., M. Hulme, and P.D. Jones, 2000. Representing twentieth century
space-time climate variability. Part II: Development of 1901-96 monthly 
grids of terrestrial climatology. J. Climate. Vol. 13, 2217-2238.