In order to investigate the climate impact of ENSO and AO and the relationships between the phase of ENSO and the phase of the AO, it is necessary to first have a classification of cold, neutral and warm ENSO episodes. The classification of El Nino and La Nina events was compiled by scientists at CPC to provide a season-by-season breakdown (i.e., JFM, FMA, MAM, etc) of conditions in the tropical Pacific. The process of classification was primarily subjective using sea surface temperature reanalyses produced at the NCEP/CPC (Smith et al., 1996) and at the United Kingdom Meterological Office. It mainly based on a key region of the tropical Pacific (along the equator from 150W to the date line). Each season is classified as one of the following: (W+, W, W-, N, C-, C, C+). Table 1 shows the CPC compiled classification of ENSO from 1950 to 2000 .
The daily AO index in this work is constructed by projecting the daily 1000-hPa geopotential height (Z1000) anomalies poleward of 20oN onto the loading pattern of the AO, which is the leading EOF of monthly mean Z1000 for all the years.
Daily time series from the NCEP/NCAR CDAS/Reanalysis (Kalnay et al., 1996) for the period 1950-1999 were assembled for the following fields:
1. 500-hPa height
2. 500-hPa height tendency (magnitude and seasonal average)
3. 1000-5000-hPa thickness (mean and anomaly)
4. 300-hPa wind
In addition, daily time series of observed U.S. surface air temperature (Janowiak et al. ,1999) and precipitation (Higgins et al., 2000) for the period 1950-1999 were assembled. In order to define the anomalies, the annual cycle, which was defined as the sum of the time mean and the first four harmonics of the climatological daily mean data, was removed from the original time series. For precipitation anomlies, a 30-day running mean of daily precipitation climotology is used to represent the annual cycle. Daily time series for each 3-month running seasons (JFM, FMA, MAM, etc.) were then generated for these fields as well as the daily AO index.
The CPC classification of ENSO phase by season and daily AO index are used to obtain composites for ENSO only phases and AO only phases, as well as the following 9 combinations (Table 8):
(C, AO+) (N, AO+)
(C, AON) (N, AON) (W, AON)
(C, AO-) (N, AO-) (W, AO-)
where C, N, W refer to La Nina, ENSO-neutral and El Nino, respectively, and AO+, AON and AO- refer to the high, neutral and low index phases of the AO. Here the La Nina (El Nino) episodes includes those moderate and strong La Nina (El Nino) events as classfied by CPC , and the ENSO-neutral composites include the weak La Nina (C-), El Nino (W-) and neutral ones (N). The AO index is standardized and it is defined as positive (negative) phase when the index exceeds +0.5 (-0.5) standard deviations, otherwise a day is defined asAO-neutral. The composites are generated for each running season and each field mentioned above.
The ENSO only (Table 3) and AO only composites (Table 5) can be constructed by combining the above columns (for ENSO only composites) and rows (for AO only composites), respectively. It is our goal to use these composites to study the ENSO, AO and their combined effects on US weather and to use them as reference in the forecasting.