1.�
Summary
United
States temperature and precipitation observations for the period 1950-2002 were
used to obtain High Frequency (HF) and Trend Adjusted (TA) composites by ENSO
phase.� ENSO events were chosen using a
historical sea-surface temperature data set and NOAA definitions of El Ni�o and
La Ni�a.� Years that do not satisfy the
NOAA definitions are ENSO-neutral.� The
gridded precipitation and surface air temperature data sets were developed at
NOAA's Climate Prediction Center as described.��� Results are presented for each 3-month season (i.e. JFM, FMA,
MAM, etc).� ENSO temperature and
precipitation percentages are expressed as departures from random chance
(33.3%) of the indicated category (based on a tercile distribution), for El
Ni�o, La Ni�a and ENSO-neutral events.�
Comparisons are made to results from Climate Division data for the same
period (1950-2002) and for a longer period (1895-2002).�
2. Data
Observed U.S. surface air temperature data are from
the analysis of Janowiak et al. 1999.�
Daily data were gridded to a horizontal resolution of 0.5� lat x 0.5�
lon for the period 1950-2002 (June).��
Observed U.S. precipitation data are from the Unified Precipitation
Reanalysis of Higgins et al. 2000 and Shi et al. 2002.� Daily data were gridded to a horizontal
resolution of 0.25� lat x 0.25� lon for the period 1950-2002 (June).� Both temperature anomalies and precipitation
anomalies for the basic composites�
(section IV of the webpage) are defined as departures from base period
(1971-2000) mean values.� Time series of
the seasonal anomalies for each 3-month season (JFM, FMA, MAM, AMJ,...) were
generated for the temperature and precipitation fields prior to the composite
analysis.�
3. High Frequency (HF) and
Trend Adjusted (TA) composites
There have been significant trends in precipitation
and surface air temperature at many locations in recent decades, so it is
worthwhile to examine the influence of trends on ENSO composites.� For this purpose, two basic types of
composites are examined:
�
�
High-frequency
(denoted HF) composites
�
Trend
adjusted (denoted TA) composites
HF (or detrended) composite anomalies (sections II
and III of the web page) are obtained after first removing an 11-year (15-year)
running mean from the raw temperature (precipitation) seasonal time
series.� At the beginning and end of the
time series the closest possible approximation to the low-frequency signal is
used. For 2000, for example, 1992-2002 is used.�� The HF composites are then computed directly.� The TA composites are obtained after adding
the most recent 11-year (15-year) mean value (computed from the raw data after
removing base period 1971-2000 means). In this way the HF time series are
�adjusted� to account for the most recent estimate of the low-frequency climate
signal.
In order to determine the percentage of events that
occur in a particular tercile class (expressed as percentage anomalies in
sections II and III on the webpage), it is necessary to determine class
limits.� Tercile class limits for the HF
composite are determined from ranked values of the HF time series for the
entire period of record. Class limits for the TA composite are determined from
the original (raw) time series using the period 1971-2000 fit to a normal
(gamma) distribution for temperature (precipitation).� The 1971-2000 base period is used so that results can be applied
for forecast operations at CPC, where the current base period is
1971-2000.� Thus, these HF and TA
composites can be viewed as the high-frequency and low-frequency components of
the seasonal forecasts.��
3.1� Anomalies (High Frequency and Trend
Adjusted)
HF and TA composites of U.S. temperature anomalies
in �C (precipitation anomalies in mm day-1) for El Ni�o,
La Ni�a and ENSO-neutral events during 1950-2002 (June) are available in
sections II and III of the webpage.�
3.2� Percentages (High Frequency
and Trend Adjusted)
HF and TA� temperature and precipitation percentages, expressed as departures from random chance (33.3%) of the indicated category, for El Ni�o, La Ni�a and ENSO-neutral events during 1950-2002 (June) are available in sections II and III of the webpage.� Percentages are based on the fraction of the total number of events that occur in the indicated category. Dashed lines are the 1971-2000 climatology in �C.
3.3 Clickable maps
Clickable maps showing the percentage of the total
number of El Ni�o, La Ni�a, or ENSO-neutral events in each temperature and
precipitation tercile for each season (JFM, FMA, MAM,�) are found in sections
II and III of the webpage.� The clickable maps are subdivided into 2�x2�
grid boxes, and results are based on the average of all points within the box
and on its edges.�
3.4 Temperature and precipitation anomalies (Basic Composites)
Composites of U.S. temperature
and precipitation anomalies in �C (precipitation anomalies in mm day-1)
for� and ENSO-neutral events during
1950-2002 (June) are available in section IV of the webpage.� Anomalies are computed with respect to base
period 1971-2000 mean values�
3.5 ENSO Composites Based on CPC's Consolidation Forecast for� Ni�o 3.4 SST
Seasonal
U.S. temperature and precipitation projections can be obtained at lead times
out to 12.5 months using CPC's official consolidation forecast for tropical
Pacific SST's.�� In particular, the
standardized anomaly version of the consolidated forecast for Ni�o 3.4 SST's is
used to obtain weights (i.e. a projection fraction) for El Ni�o, La Ni�a and
ENSO-neutral conditions at leads out to 12.5 months.� The weights are applied to the El Ni�o, La Ni�a and ENSO-neutral
composites and summed to obtain the projections at each lead.� At early leads the composites are dominated
by the HF component of the forecast (i.e. a particular ENSO phase), while at
longer leads the trend component (i.e. OCN) dominates.� These composites are used as an objective
tool for CPC's Official Seasonal Outlooks.�
����������� For most values of n, where n is the
number of ENSO events in a particular composite, the number of events required
to achieve 95% confidence is n/2.� This
translates to a percentage anomaly of ~17% (i.e. 50%-33.3%). Thus, for
simplicity, shaded areas on the maps at and above 20% can be considered
significant at the 95% confidence level.
4.0 References
Higgins, R. W., W. Shi and E. Yarosh, 2000: Improved United States
precipitation quality control� system
and analysis.� NCEP/Climate Prediction
Center Atlas Number 7, 40 pp.��
[Available from Climate Prediction Center, World Weather Building, Room
605, Camp Springs, MD, 20746].
Janowiak, J. E., G. D. Bell, and M. Chelliah, 1999: A gridded data base
of daily temperature maxima and minima for the conterminous United States:
1948-1993. NCEP/Climate Prediction Center ATLAS No. 6, 50 pp.� [Available from Climate Prediction Center,
World Weather Building, Room 605, Camp Springs, MD, 20746].�
Shi, W,� R. W. Higgins and E. Yarosh, 2002:� A Unified Raingauge Dataset and Multi-year Daily Precipitation Reanalysis for the United States.� J. Geophys. Res
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