Description
US MAPS
Two types of maps encompassing the United States
are provided for overlapping three-month periods,
the November/December period, and the December through
March period. Both types of charts are based on ranking
mean precipitation from the 102-year climate division
record for a particular period (like January through
March) from wettest (rank 1) to driest (rank 102)
year, and mean temperature from coldest (rank 1) to
warmest (rank 102) year.
One set of charts show the average ranks of temperature
and precipitation for each three-month or other period
for those years with strong El Nino episodes
in progress during the period. These are straight
forward to interpret.
The second set of maps contains more detailed information
than the first set, but requires additional description
to interpret. Specifically, these maps show the number
of times in the past during strong El Nino
episodes that the mean temperature (precipitation)
ranked among the warmest (wettest) or coldest (driest)
third of the 102-year climate division record dating
back to 1895. This information is depicted only for
those climate divisions where the distribution of
occurrences in the tercile classes of above, near,
and below normal departed sufficiently from a uniform
distribution that the odds of the departure being
an accident were less than about 10%.
Thus, this set of charts not only provides insight
into what kind of conditions El Nino favors in a specific
area for a specific time of year, but also how reliably
those conditions were observed in past strong El Nino
episodes.
The number of cases that fall in the tercile class
that the distribution is skewed towards is color coded;
for example on the seasonal precipitation charts when
most of the El Nino years at a location were wet the
number of years is denoted by a green shade covering
the climate division, but when most were dry the number
of cases is indicated by a brown to yellow shade.
For each colored division on a chart the number of
El Nino years that fall in the opposite tercile class
(for example the driest third for divisions that are
colored a green shade on the precipitation charts)
is denoted by a number. Thus for every three-month
or other period at every climate division the complete
distribution among temperature and precipitation terciles
for strong El Nino episodes can be determined from
the diagrams.
For specific examples refer to the [October
thru December] precipitation chart constructed
from 11 strong El Ninos: For coastal Southern California
seven of the El Nino years were among the wettest
third, three among the middle third, and only one
of the eleven among the driest third, while for Long
Island the distribution was almost the opposite, i.e.
two among the wettest, two among the middle, and seven
among the driest. Thus, suitably smoothed, the information
on the diagrams can be used to formulate a priori
probabilities of the different temperature or precipitation
classes conditional on a high-confidence El Nino forecast.
STATE INFORMATION
For each state, key periods have been selected to
highlight El Nino effects on the particular state
with two charts. The first type of chart is a map
of selected statistics, division by division, for
the state to contrast conditions during strong El
Nino years with normal conditions. The other is a
set of bar graphs for each division within the state
depicting the same information about the distribution
of temperature (precipitation) for strong El Nino
episodes among the warmest (wettest), middle, and
coldest (driest) thirds of the 102-year climate division
record.
The years representing strong El Ninos change from
period to period. This is because the part of the
year for which the central Equatorial Pacific sea
surface temperatures (SSTs) are well above normal
differs from episode to episode. The cases that were
included are those for which the average SST in a
prescribed area was close to or greater than one degree
Celsius above normal in all (two to four) months spanning
a particular period. The key area used for case selection
is bounded by the Dateline and 150 west longitude
and 5 north and 5 south latitudes. This area was used
because it approximates the region in the equatorial
Pacific where tropical convection and rainfall (the
major source of atmospheric energy in the tropics)
are the most sensitive to relatively small changes
in SST. Thus, the SST anomaly in this area should
be a good index of how strong an El Nino's impact
on the global atmosphere will be.
Note also that precipitation charts are constructed
from more years than the temperature charts; for the
former El Ninos prior to 1940 were included as candidates
for selection but not for the latter. This is because
there are obvious large-scale trends in the temperature
data but not in the precipitation data. Even with
this restriction to the 1940-1996 period the temperature
series are still likely to be somewhat nonstationary,
so the temperature distributions should be used with
more caution than the precipitation distributions.
In particular, we have observed that for the strong
El Nino cases from the 1980s and 90s the northern
United States has overall been considerably warmer
in the January through April period than earlier in
the post-1940 record. Thus temperature distributions
for these cases are more skewed towards the warm tercile
than shown here for JFM and FMA; areas where the full
period distributions are skewed towards the warm tercile
are more skewed towards warm for the recent period.
However, for FMA in the Southeast this difference
in skewness between pre- and post-1980 is only slight.
- Precipitation Distributions
The diagrams shown here for both precipitation and
temperature reflect, complement, and extend the information
recently presented by Livezey et al. (1997:
J. Climate, 10, 1787-1820; hereafter L97).
For example, the area centered on Montana in the precipitation
distributions for NDJ, DJF, and JFM was not highlighted
in the text of L97 and only hinted at in their Figs.
14 and 15a-b therein, yet emerges as a prominent teleconnection
in this analysis. Likewise, it was not possible from
L97 to fully appreciate the remarkable consistency
of some of the strong El Nino precipitation signals
shown here, particularly for the wetness for different
parts of the Southwest in a number of the charts,
the wetness in Florida for NDJ and JFM, the aforementioned
Montana dryness, and the dryness in the Ohio Valley
for JFM. With the benefit of the L97 maps it is apparent
that the OND precipitation signals are dominated by
November and December while the FMA signals are largely
a consequence of February and March. Thus, the probabilities
of OND precipitation terciles implied by the OND chart
should not be automatically applied to the month of
October, etc. A final point to note in the precipitation
charts is a modest tendency for bimodality in the
mean precipitation in the center of the country for OND.
- Temperature Distributions
Despite the caveat about the possible nonstationarity
of the temperature data series mentioned in the discussion
above about case selection, there is still very useful
information in the effected JFM and FMA charts that
is insensitive to this problem. Specifically, there
are no reported instances of average temperatures
in the above normal tercile during strong El Nino
episodes for extensive areas of the Southeast in either
period. In this same context, not only is there substantial
skewness towards the above normal tercile over the
north central United States in NDJ and DJF, there
are also a very large number of climate divisions
with no instances of average temperatures in the below
normal class. The economic implications of these observations
should be considerable. Lastly, the warning against
use of three-month precipitation tercile probabilities
for individual months within a period is equally applicable
here, especially with respect to the OND temperature
chart that is mostly dominated by November and, especially,
December (note that detailed ND information instead
of OND is provided for eastern states).
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