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.